recent advances in bismuth-catalyzed organic …...recent advances in bismuth-catalyzed organic...

Current Organic Synthesis, 2008, 5, 1-27 1

1570-1794/08 $55.00+.00 © 2008 Bentham Science Publishers Ltd.

Recent Advances in Bismuth-Catalyzed Organic Synthesis

Ruimao Hua*

Department of Chemistry, Tsinghua University, Beijing 100084, China

Abstract: Bismuth(III) compounds (BiX3, X = Cl, Br, NO3 and OTf etc.) are moisture- and air-tolerant Lewis acids, which have been widely applied as green catalyst in diverse organic synthesis in the past few years. This review illustrates significant advances in this field

over recent five years, and mainly focuses on the bismuth-catalyzed formation of carbon-carbon, carbon-nitrogen, carbon-sulfur bonds, as well as oxidation reaction, protection and deprotection of alcohols and carbonyl compounds, and organic reactions in aqueous media.

Keywords: Bismuth(III) compound, catalytic reaction, organic synthesis.

INTRODUCTION

Development of non-transition-metal-catalyzed organic synthesis is one of the most important, interesting and challenging research topics in catalytic synthesis. In the past two decades, bismuth(III) compounds such as BiCl3, BiBr3, Bi(OTf)3 and Bi(NO3)3 etc. have attracted growing interests as versatile catalysts in diverse organic synthesis owing to their remarkable chemical and physical properties such as relevant stability, air- and moisture-tolerance, low toxicity. Some excellent, comprehensive reviews on the bismuth(III)-catalyzed organic reactions have been published [1]. This review does not present a complete, historical coverage of bismuth-catalyzed organic reactions, but updates the most significant progress of the last five years, with emphasis on the following topics: 1) bismuth-catalyzed formation of carbon-carbon, carbon-nitrogen, carbon-sulfur bonds, 2) oxidation reaction, 3) protection and deprotection of alcohols and carbonyl compounds, 4) organic reactions in water and 5) multi-component coupling reactions.

1. BISMUTH-CATALYZED CARBON-CARBON BOND FORMATION

Friedel-Crafts alkylation and acylation of aromatic rings are important reactions for constructing carbon-carbon bond in organic

synthesis [2]. The reactions usually require the catalysts such as AlCl3, BF3, HF and H2SO4 etc. However, the use of these corrosive catalysts is undesirable from an environmental point of view, since the procedure generates a large amount of waste materials during isolation of products. Therefore, recently developed Friedel-Crafts reactions using other Lewis acid catalysts have attracted increasing attention. Among these research work, the bismuth(III) salts have been demonstrated to be the efficient catalysts for Friedel-Crafts reactions as summarized in previous reviews [1(d)].

The application of ionic liquids (ILs) as green reaction medium in organic reactions has been extensively studied in the last decade [3]. Recently, the use of bismuth(III) derivatives in ionic liquids as novel and recyclable catalytic systems for Friedel-Crafts acylation

*Address correspondence to this author at the Department of Chemistry, Tsinghua University, Beijing 100084, China; E-mail: [email protected]

of aromatic compounds with benzoyl chloride has been developed by Vaultier and co-workers [4]. They found that Bi(OTf)3, Bi2O3, BiCl3 and BiOCl immobilized in room temperature ionic liquids showed good catalytic activity for the benzoylation of electron-rich, neutral and electron-deficient aromatic compounds. Compared to organic solvents, ionic liquids can greatly improve the catalytic activity of the bismuth(III) derivatives. For example, BiCl3, Bi2O3, and BiOCl were ineffective catalysts for the acylation of toluene reported by Desmurs and co-workers [5], but as shown in Scheme 1, in the presence of [emim][NTf2] [emim = 1-ethyl-3-methylimidazolium; NTf2 = bis-(trifluoromethanesulfonyl)amide] (10 mol%), Bi2O3 could catalyze the benzoylation of toluene to give the corresponding methyl-substituted benzophenones in 89% isolated yield. Furthermore, the catalysts could be easily recovered and reused.

The Lewis acid-catalyzed dehydrative acylation of aromatic compounds with carboxylic acids to afford aromatic ketones is a much more attractive Friedel-Crafts reaction, since the relatively stable and halide-free carboxylic acid is used as acylating reagent in place of acyl halide. Shimada and co-workers reported that 1-tetralones could be prepared in good to high yields by direct dehydrative cyclization of 4-arylbutyric acids in the presence of bismuth(III) salts (Scheme 2) [6]. This is a green Friedel-Crafts acylation process, since the intramolecular dehydrative cyclization

reaction of 4-arylbutyric acids produces only water as a by-product. They also disclosed that although both Bi(NTf2)3 and Bi(OTf)3 could efficiently catalyze the cyclization of 4-arylbutyric acids, Bi(NTf2)3 showed higher catalytic activity than Bi(OTf)3, and AlCl3 did not work at all.

Another application of Bi(OTf)3 has been recently reported as co-catalyst in intermolecular acylation of aromatic compounds with carboxylic acids. It was found that in the presence of trifluoroacetic anhydride (TFAA), a catalytic amount of Bi(OTf)3·4H2O could efficiently improve the acylation of electron-rich aromatic compounds such as anisole, mesitylene, xylene and toluene with acetic acid or benzoic acid at 30

oC under solvent-free conditions to

give the aromatic ketones in good to high yields. The combined use of heptafluorobutyric anhydride (HFBA) and Bi(OTf)3·4H2O showed much higher catalytic activity, which could not only catalyze the acylation of electron-rich aromatic compounds at 30 oC, but also catalyze the acylation of neutral and even deactivated

Me

+Cl

O

2:1

Bi2O3 (10 mol%)[emim][NTf2] (10 mol%)

150 oC (oil temp.) for 6 h

O

Me

89% (isolated)

(para:ortho:meta = 79:19:2)

Scheme 1.

2 Current Organic Synthesis, 2008, Vol. 5, No. 1 Ruimao Hua

substrate. For example, benzene and chlorobenzene reacted with benzoic acids at 75 ~ 100

oC to give the corresponding aromatic

compounds in satisfactory yields. Furthermore, the bismuth(III) catalyst could be easily recovered and repeatedly reused after the reaction [7]. Scheme 3 and Scheme 4 show some typical results of the acylation in the presence or absence of Bi(OTf)3·4H2O.

Recently, Rueping and co-workers have demonstrated that Bi(OTf)3·4H2O is an efficient catalyst for benzylation of arenes, heteroarenes with phenylethyl alcohol, benzyl alcohol, benzyl

acetate or 3-hydroxy-3-phenylpropanoate as benzylating reagents [8]. Their further investigation found that Bi(III) salts such as BiCl3, BiBr3, Bi(NO3)3·5H2O and Bi(OTf)3·4H2O could be applied to intermolecular dehydration of 1,3-dicarbonyl compounds with 1-phenylethanol derivatives or allylic alcohols to afford 2-benzylated or allylated 1,3-dicarbonyl compounds in moderate to high yields [9]. As examples, Schemes 5 and 6 show the results of the reaction of anisole with different benzylating reagents and acetyl acetone with benzyl alcohols or cinnamyl alcohol in the presence of Bi(OTf)3·4H2O, respectively.

OH

O

R

R

O

Bi(NTf2)3 (1.0 mol%)

solvent

180 ~ 200 oC, 7 ~ 20 h

R = Ph, 4-methyl, 2,5-dimethyl, 4-methoxy,

3,4-dimethoxy, 4-fluoro, 4-iodo, 3,4-dichloro,

4-trifluoro etc.

GC yield: 46 ~ 100%

+ H2O

Scheme 2.

COOH

Ar H

Ar H

Ar

O

+

2.0 equiv

Bi(OTf)3

(mol%) Temp (oC) Isolated yield (%)

TFAA or HFBA(1.5 equiv)

anisolea

mesitylenea

benzeneb

chlorobenzeneb

0

3.3

0

10

10

10

30

30

30

30

75

100

12 h

41

98

36

99

90

65 (p-:o- = 94:6)

a TFAA, b HFBA

Scheme 3.

CH3COOH Ar H

Ar H

Ar

O

+

2.0 equiv

Bi(OTf)3 (mol%) GC yield (%)

HFBA (1.5 equiv)

0

3.3

0

10

100 oC, 3 h

19

100

1

32

(p-:o- = 84:16)

m-xylene

toluene

Scheme 4.

Recent Advances in Bismuth-Catalyzed Organic Synthesis Current Organic Synthesis, 2008, Vol. 5, No. 1 3

In the presence of Lewis acid, the cross-coupling reaction of acetals with allyltrimethylsilane is an efficient method for the synthesis of homoallyl ethers. Ten years ago, BiBr3 had been proven to be an efficient catalyst for this type of reaction at room temperature [10]. Recently, Mohan and co-workers reported that Bi(OTf)3·xH2O could also catalyze this cross-coupling reaction at room temperature, which provides an alternative method for the synthesis of homoallyl ether in a efficient, simple and mild manner (Scheme 7) [11]. Their up-to-date work modified this catalytic procedure to synthesize homoallyl ether from a one-pot, three component cross-coupling reaction of aldehydes, trimethylortho- formate or alkoxytrimethylsilane and allyltrimethylsilane in the presence of Bi(OTf)3·xH2O. By a similar protocol, homoallyl acetates could also be obtained by the one-pot reaction of aldehydes, acetic anhydride and allyltrimethylsilane (Scheme 8) [12]. The advantages of these catalytic systems include excellent

selectivity for monoallylated product and avoidance of the strictly anhydrous reaction conditions.

The use of Bi(OTf)3 can also be extended to the substitution reaction of various -acetoxy lactams with allyltrimethylsilanes (1.2 equiv) and silyl enol acetals (1.5 equiv), as well as the intramolecular arylation of -acetoxy lactams in MeCN at room temperature for 1 ~ 12 h [13]. The typical substitution reaction and intramolecular arylation are shown in Schemes 9 and 10, respectively.

As reported by Mohan [11], a similar strategy has been used by De and co-workers in a more recent research. They studied the bismuth(III)-catalyzed direct allylation of sec- and tert-benzylic alcohols with allyltrimethylsilane [14]. BiCl3 was found to show higher catalytic activity than Bi(OTf)3, and CH2Cl2 proved to be the optimal solvent for BiCl3-catalyzed substitution of benzhydrols with allyltrimethylsilane to give 4-diaryl-substituted alkenes in

OMe

R'O Ph

R

R

Me CH2Cl2

H MeNO2

H MeNO2

H

H

Ac

Ph

R

MeO

OMe

Ph

RBi(OTf)3

.4H2O

(0.5~1.0 mol%)

+

3.0 equiv.

+

p-isomer o-isomer

Solvent Temp (oC)/Time (h)Isolated

yield(%)

Isomer

p-:o-

55/1 95 4:1

100/2 91 1.4:1

100/5 92 1.4:1

R'

Scheme 5.

O O

Bi(OTf)3.4H2O

(1.0 mol%)

CH3NO2, 60 oC, 5 h

Ar OH

Ar O

O

O

O

Ph

Bi(OTf)3.4H2O

(1.0 mol%)

CH3NO2, 100 oC, 2 ~ 4 h

3.0 equiv.

Isolated yield:

62%

Ar = Ph, p-, m-BrC6H4, o-tolyl,

3,4,5-trimethoxyphenyl,

-naphthalenyl, p-phenylC6H4

Isolated yields:

69 ~ 91%

Ph OH

Scheme 6.

R OR'

OR'Me3Si

R

OR'

R' = CH3, C2H5

R = Ar, alkyl, benzyl, styryl

Isolated yield: 69-96%

+

1.3 equiv.

CH2Cl2, r.t.5 min - 1.5 h

Bi(OTf)3.xH2O

(1.0 mol%)

Scheme 7.


good to high yields. For sec-benzylic alcohols, the substitution took place smoothly at 80

oC. In addition, all the reactions proceeded

under a non-inert atmosphere (Scheme 11).

The intramolecular cyclic carbonyl-ene reaction of citronellal catalyzed by a variety of catalysts to produce isopulegol and neoiso-pulegol is a well-documented reaction. Although BiCl3- catalyzed cyclization of citronellal has been reported previously [15], Mohan and co-workers recently demonstrated that Bi(OTf)3·xH2O is a much more efficient catalyst for this cyclization [16]. In the presence of a low loading amount of Bi(OTf)3·xH2O (0.1 mol%), the cyclization reaction, in CH2Cl2 at room temperature for 25 min, afforded isopulegol and neoisopulegol in 35% and 11% yields, respectively (Scheme 12). This catalytic system could be applied to the synthesis of substituted piperidines via the similar cyclization reaction of the corresponding nitrogen-containing carbonyl-ene substrate.

Also in the same group, Bi(OTf)3 has been applied as catalyst in the condensation of aromatic and aliphatic aldehydes with resorinol to prepare resorcinarenes [17]. When benzaldehyde was used, a mixture of two diastereomers of the tetrameric cyclic products was obtained, and the ratio of the diastereomers depended on the reaction time. A prolonged reaction time led to the formation of the thermodynamically favored all-cis diastereomer. But in the cases of aliphatic aldehydes used, only all-cis diastereomer was formed (Scheme 13).

BiCl3 displayed the high catalytic activity to catalyze the electrophilic substitution of indole with aryldehydes affording bis(indolyl)methane derivatives reported by Xia and co-workers. As shown in Scheme 14, in the presence of BiCl3, the reaction of indole with 4-chlorobenzaldehyde produced 3,3’-bis(indoly)-4- chlorophenylmethane in 88% yield in a very short reaction time under solvent-free and irradiation conditions. The advantages of

Isolated yield:

77 ~ 82%

SiMe3

2.0 equiv.CH3CN, r.t.

R

OMe

R CHO + +

2.0 equiv.

R = aryl, alkyl, alkenyl

Isolated yield:

42 ~ 82%

SiMe3

1.2 ~ 2.0 equiv.CH2Cl2, r.t.

R

OMe

R CHO + R'OSiMe3 +

1.2 equiv.

R = aryl, alkyl

R' = Me, Et, PhCH2, allyl

Isolated yield:

57 ~ 66%

SiMe3

2.0 equiv.CH3CN, r.t.

R

OAc

R CHO + +

3.0 equiv.

R = Ph, p-ClC6H4, p-tolyl, m-MeOC6H4

Bi(OTf)3.xH2O

(1.0 mol%)

Bi(OTf)3.xH2O

(0.1~1.0 mol%)

Bi(OTf)3.xH2O

(5.0 mol%)

HC(OMe)3

(CH3CO)2O

Scheme 8.

N

O

O

O

O

SiMe3

OSiMe3

NO

O

NO

O

O

64%

84%

1.2 equiv

1.5 equiv

Bi(OTf)3 (1.0 mol%)

MeCN, r.t., for 1-12 h

Scheme 9.


this catalytic procedure include the rapid reaction rate and high yield [18].

Described by Yadav and co-workers, Bi(OTf)3 was also an efficient catalyst for the electrophilic substitution of indole or pyrroles with isatin to produce 3,3-diindolyl- or 3,3-dipyrrolyl oxindoles in high yields (Scheme 15) [19].

Michael reaction is one of the important carbon-carbon bond formation reactions [20]. Bismuth(III) salts have been recently found to be a good co-catalyst in the palladium(II)-catalyzed Michael-type hydroarylation of nitroalkenes with tetraphenyltin compounds (Scheme 16) [21]. Considerable improvement of the yields of the products could be achieved by a catalytic amount of

N

O

O

O

X Bi(OTf)3 (1.0 mol%)

MeCN, r.t., for 1-12 h

N

O

X

X = CH2, 72%

O, 65%

Scheme 10.

+SiMe3

BiCl3 (5 mol%)

CH2Cl2r.t., 0.5 ~ 1.0 h

1.5 equiv

Ar OH

Ar'

R

Ar

Ar'R

Isolated yield: 84 ~ 95%

+SiMe3

BiCl3 (5 mol%)

CH2Cl280 oC, 2.0 ~ 3.0 h

2.0 equiv

Ar OH

R'

R

Ar

R'R


Ar, Ar' = Ph, o-, p-tolyl, p-ClC6H4, p-MeOC6H4

R = H, Ph, Me

Ar = Ph, p-MOC6H4, 1-naphthyl

R, R' = H or alkyl

sec-benzylic

alcohol

Scheme 11.

N

O

TsBi(OTf)3

.xH2O (0.1 mol%)

CH2Cl2, r.t., 15 min

NTs

OH

NTs

OH

+

Isolated yield: 48% 20%

O

Bi(OTf)3.xH2O (0.1 mol%)

CH2Cl2, r.t., 25 min OH

+


OH

Scheme 12.


BiCl3. Similar effect was also observed when Bi2O3 or Bi(NO3)3·5H2O was used. However, in the cases of other aryltins employed, the combination catalyst system of PdCl2/BiCl3 showed no higher activity than PdCl2 only.

Bi(OTf)3 has been demonstrated to be the efficient catalyst for the conjugate addition of indoles to , -enones at room temperature in CH3CN (Scheme 17) [22]. Various , -unsaturated ketones including cyclic enones, chalcones and naphthoquinone could react with indole, 2-methylindole and 5-methoxyindole to give the

RCHO +

HO

HO

Bi(OTf)3.4H2O (5.0 mol%)

ethanol, 80 oC,

HO OH

OH

OH

OHHO

HO

HOR R

RR

HO OH

OH

OH

OHHO

HO

HOR R

RR

all-cis cis-trans-trans

+

time isolated yield(%)

all-cis : cis-trans-trans

(by 1H NMR)

75 min

8 days

66

89

91

84

n-C9H19 24 h

4 h

0.78 : 1.0

1.0 : 0

1.0 : 0

1.0 : 0

R

Ph

n-C5H11

Ph

Scheme 13.

N

Cl

CHO

+

microwave

solvent-free

BiCl3 (20 mol%)

2.5 min

N N

Cl

0.5 equiv

88%

Scheme 14.

NH

N

+

NH

O

O

Bi(OTf)3 (2 mol%)

MeCN

r.t., 2.5 ~ 4.0 h

NH

NH

HN

O

N N

HN

O

Isolated yields:

82 ~ 95%

RR

R

R

R R

R = H, isolated yield: 87%

R = Me, isolated yield: 80%

Scheme 15.


corresponding Michael adducts 3-alkylated indoles in excellent yields. At the same time, Yadav and co-workers reported Bi(OTf)3-catalyzed conjugate addition of indoles to p-quinones to develop a facile synthesis of 3-indolyl quinines in good to high yields [23]. Scheme 18 shows the results of the reaction of indole with 2,5-dichloro-p-benzoquinone in the presence of Bi(OTf)3 to afford 3-indolyl-2,5-dichlorohydroquinone in 85% yield.

In addition, other Bi(III) salts such as Bi(NO3)3 and BiCl3 have also been demonstrated to be efficient catalysts for the Michael reactions. For example, Banik and co-workers reported that Bi(NO3)3 could catalyze the Michael addition of amines, imidazoles, thiols, indoles and carbamates to enones to afford the corresponding Michael adducts in moderate to good yields [24]. Zhan and co-workers reported the BiCl3-catalyzed reaction of pyrroles with electron-deficient olefins, various , -unsaturated ketones to produce 2-C-alkylated pyrroles in high yields with high regioselectivity [25].

The aldol addition is also an important reaction for stereoselective construction of carbon-carbon bonds. Catalyzed addition of silyl enol ethers to aldehydes, the so-called Mukaiyama aldol reaction, has been extensively investigated [26]. In particular, various bismuth-mediated aldol reactions have been reported in the literature [27]. Although Bi(OTf)3·nH2O was found to be the efficient catalyst for Mukaiyama aldol reaction in CH2Cl2 [28], recently it has been examined again by Ollevier and co-workers for the same reaction in ionic liquid. The products -hydroxy carbonyl compounds could be achieved in moderate to high yields [29]. Mild reaction conditions, green solvent and no formation of by-products are the advantages of this procedure over the existing catalyst systems. Scheme 19 represents the results of the reactions of aldehydes with (1-phenylvinyloxy)trimethylsilane. Moreover, an efficient synthetic method of -hydroxy-1,3-dioxin-4-ones has also been developed by the same group through the reaction of 2,2-dimethyl-6-methylene-4-(trimethylsiloxy)-1,3-diox-4-ene with

RNO2 +

1.0 mmol 0.25 mmol

PdCl2 (0.05 mmol) / LiCl (2.0 mmol)

AcOH, 25 oC, 20 h

BiCl3 (0.1 mmol)

RNO2

Ph

Yield: 77 ~ 88%

(Without BiCl3, yield: 37 ~ 68%)R = Ph, p-MeC6H4, p-MeOC6H4, p-ClC6H4,

p-BrC6H4, m-O2NC6H4, n-C7H16

Ph4Sn

Scheme 16.

NH

H

R

R'

R = R' = H

R = H, R' = OMe

R = Me, R' = H

+

R2

O

R1

, -enone

Bi(OTf)3 (3.0 mol%)

CH3CN

25 oC, 1 ~ 2 h NH

R

R'

R2R1

O


Scheme 17.

+

2.0 equiv

Bi(OTf)3 (2 mol%)

CH3CN

r.t., 0.5 h

isolated yield: 85%

N

O

O

Cl

Cl

N

Cl

Cl

HO

OH

H

H

Scheme 18.

R = Ph, p-O2NC6H4, p-CF3C6H4, o-, p-FC6H4,

p-MeOC6H4, n-C3H7, COOEt

Isolated yield:

47 ~ 92%

R H

O

+Ph

OSiMe3

Bi(OTf)3.nH2O

(10 mol%)

[Bmim]BF4

25 oC, 6 ~ 40 h

R Ph

OH O

[Bmim]BF4: 1-butyl-3-methylimidazolium tetrafluoroborate

Scheme 19.


aromatic aldehydes in the presence of Bi(OTf)3·4H2O in diethyl ether under mild conditions (Scheme 20) [30].

Furthermore, Bi(OTf)3-chiral bipyridine complexes as water-compatible chiral Lewis acid catalysts could catalyze the asymmetric hydroxymethylation of sily enol ethers with aqueous formaldehyde solution [31]. For example, in the presence of Bi(OTf)3 and chiral bipyridine ligand, the reaction of (1-phenyl-1- propenyl-oxy)trimethylsilane with aqueous formaldehyde (5.0 equivalent) in H2O/DME (DME = 1,2-dimethyoxyethane) at 0

oC

for 21 h afforded 3-hydroxy-2-methyl-1-phenyl-propan-1-one in high isolated yield with high e.e. value (Scheme 21). This is the first example of highly enantioselective reactions using a chiral bismuth catalysts in aqueous media.

Using the same chiral bipyridine ligand (20 mol%) and Bi(OTf)3 (10 mol%), they developed a catalyzed asymmetric

ring-opening reactions of meso-epoxide with equivalent amount of aromatic amines in the presence of sodium dodecylbenzene sulfonate (SDBS) in water at room temperature to give the corresponding -amino alcohols in good yields with high enantioselectivities [32].

Sabitha and co-workers reported the BiCl3-catalyzed intramolecular hetero-Diels-Alder reaction of aldimines which were generated in situ from the reaction of aromatic amines with N-allyl derivatives of o-aminobenzaldehyde derivatives [33], or from the reaction of anilines with S-allyl derivatives of pyrazole aldehydes [34]. This method provides a simple approach to the synthesis of hexahydrodibenzo[b.h][1,6]naphthyridines (Scheme 22), and hexa- hydropyrazolo[4’,3’:5,6] thiopyrano[4,3-b]quinolines (Scheme 23) in good to high yields. In the former case, the products were obtained as a mixture of trans and cis diastereoisomers in 1:1 ratio.

O O

OSiMe3

H

O

R +

1.5 equiv.

1) Bi(OTf)3.4H2O (1.0 mol%)

Et2O, -78 oC, 1~4 h

2) HCl, THF/H2O

22 oC, 2 h

R

OH O O

O

Isolated yield: 74 ~ 98%R = H, p-, o-Me, o-, p-, m-MeO,

p-Cl, p-CF3, p-NO2

Scheme 20.

Ph

OSiMe3

+ HCHO (aq.)

5.0 equiv.

Bi(OTf)3 (1.0 mol%)

ligand (3.0 mol%)

Bipy (5.0 mol%)

H2O/DME = 1/4

0 oC, 21 h

HO Ph

O

Isolated yield: 93%

e.e.: 91%

Ligand: N N

HOOH

H

Scheme 21.

R

R'

NH2

+

NTs

OHC

BiCl3 (10 mol%)

CH3CN, reflux

1.5 ~ 2.0 hN

NTsR

R'

H

H

+

N

NTsR

R'

H

H

R = H, CH3, OCH3, Cl, F, amine

R' = H, CH3, Br, OH, 1-Naphthyl Isolated yield: 90 ~ 96%

1 : 1

Scheme 22.

R

R'

NH2

+

BiCl3 (5 mol%)

CH3CN, reflux, 1h N

SR

R'

H

H

R = H, CH3, Cl, F, OEt,

R' = H, Br R" = Me, PhIsolated yield: 70 ~ 85%

N

N

Ph

R"

N

N

Ph

R"

S

OHC

Scheme 23.


But in the later case, the cycloaddition reaction proceeded with high diastereoselectivity and only the cis products were isolated. The same catalytic system was also applied to the similar cycloaddition reaction of anilines with O-allyl derivative of the sugar derived aldehyde [35].

1,1-Diaryl alkenes are an important class of organic compounds. Development of simple and efficient methods to the synthesis of these compounds is an interesting and significant subject in organic chemistry. Our recent research work has disclosed that BiCl3 is an efficient catalyst to prepare 1,1-diaryl alkenes via Friedel-Crafts reaction of acyl chloride or vinyl chloride with aromatics [36]. In the presence of a catalytic amount of BiCl3, the reaction of acyl chlorides with aromatics afforded 1,1-diaryl alkenes in fair to good yield, instead of normal Friedel-Crafts acylation products (Scheme 24). It is proposed that the formation of 1,1-diaryl alkenes is resulted from the Friedel-Crafts type vinylation of aromatics with vinyl chloride, which is generated in situ from the reaction of aryl ketone with HCl or acyl chloride (Scheme 25). Thus the reaction of vinyl chlorides with aromatics has been also

investigated in details (Scheme 26). This work has developed an unprecedented Friedel-Crafts type vinylation of aromatics in which the C-Cl bond of vinyl chlorides is catalytically cleaved by BiCl3.

Le Roux and co-workers reported that BiCl3 was an efficient catalyst for the Friedel-Crafts acylation of electron-rich aromatic compounds using acid chlorides and acid anhydrides as acylating reagents [1d]]. In their later studies, it was found that Bi(III) could be recovered as BiOCl after an aqueous work-up. Therefore, they developed the BiOCl-catalyzed Friedel-Crafts acylation of aromatic compounds with acid chlorides. In this catalytic system, BiOCl was regarded as a procatalyst for the reaction involving the catalyst of BiCl3 formed by the reaction of BiOCl with acyl chloride. Scheme 27 presents the results of BiCl3-catalyzed benzoylation of mesitylene with benzoyl chloride and the recovery of bismuth. The advantage of the catalytic system is the easy recovery of BiOCl in near quantitative yield after an aqueous work-up due to its water-insensitive [37]. One year later, this catalyst system was applied to the acylation of heteroatoms with acetyl chlorides reported by Ghosh and co-workers [38].

R COCl

R'

R'R

+BiCl3 (10 mol%)

120 oC

10 ~ 16 h2 : 1


RR' = cyclohexyl R = H, CH3

R' = CH3, n-propyl

Scheme 24.

R" +

R COCl

R' cat. BiCl3

R"

O

R'

R R"

R'

R

OH

R"

R'

R

Cl

R"

R" R"

R R'

acyl ketone

Cl

HCl

OH

HCl

Scheme 25.

Cl

+

BiCl3 (10 mol%)

120 oC

6 ~ 24 h

Ar H

Ar

Ar H: benzene, naphthalene,

electron-donating group(s) bearing benzene

1 : 4Isolated yield: 29 ~ 80%

Scheme 26.


In recent efforts we have developed an efficient and selective hydroarylation of styrenes with electron-rich aromatics in the presence of BiCl3 [39]. The hydroarylation of styrenes with excess amount of electron-rich aromatics afforded Markovnikov adducts selectively in good to high yields (Scheme 28). It was also found that in the absence of aromatics, using n-octane as solvent, the intermolecular hydroarylation of -substituted styrenes and subsequent intramolecular hydroarylation occurred to produce the cyclic dimers of -substituted styrenes in good to high yields (Scheme 29).

It is however, surprising that in cyclohexane at 100 oC, BiCl3

(0.5 mol%) could not catalyze the hydroarylation of styrene with anisole at all reported by Rueping and co-workers. Under their reaction conditions, it was found that only Bi(OTf)3 showed high

catalytic activity for the hydroarylation of styrenes with electron-rich aromatic compounds [40].

Yadav and co-workers reported that in the presence of Bi(OTf)3, the allylation of p-quinones with allyltrimethylsilane readily occurred in CH2Cl2 at room temperature to give the corresponding allylated products in good to high yields with high selectivity. The reactions afforded allyl substituted benzenes, p-allylquinols or allyl substituted 1,4-naphthoquinones, depending on the structures of the reactant used. Thus, this method provides an easy access to allyl substituted aromatic compounds (Scheme 30) [41]. Moreover, under the similar reaction conditions, they examined the Bi(OTf)3-catalyzed allylation of epoxides via the reaction of epoxides with tetraallyltin to develop a facile synthesis of homoallylic alcohols in high yields. The catalytic system was

COCl+

O

0.5 equiv

BiCl3 (10 mol%)

120 oC for 4 h

isolated yield: 90%

after an aqueous

work-up

99.7% recovery of bismuth

BiOCl

Scheme 27.

OR

+

OR

RO

+BiCl3 (0.2 mmol)

R'100 oC, 10 h

R' R'

R = Me, Et R' = H, 4-Cl, 2-Cl, 2-Me

20.0 mmol 2.0 mmol


(99 ~ 65 : 1 ~ 35)

R" R" R"

R" = H, Me, Ph

R

+BiCl3 (0.2 mmol)

100 oC, 10 h

R = OMe, Me R' = H, Me

R" = H, Cl

20.0 mmol 2.0 mmolSelectivity: > 95%


R"R' R"

R

R'

Scheme 28.

R'

R'

R'

BiCl3 (0.1 mmol)

110 oC, 24 h

2.0 mmol

R

RR

R = H, Me, Cl; R' = Me, Ph Isolated yield: 56 ~ 92%

Scheme 29.


O

O

R

R' R"

+SiMe3

Bi(OTf)3 (2.5 mol%)

CH2Cl2, r.t., 10 ~ 25 min

2.0 equiv

OH

OH

R

R' R"

Isolated yield: 85 - 91%

O

O

+SiMe3

Bi(OTf)3

(2.5 mol%)

CH2Cl2r.t., 10 min

2.0 equiv


+

O

O

R

R = H, Me, OMe

+SiMe3

Bi(OTf)3 (2.5 mol%)

CH2Cl2, r.t., 15 ~ 18 min

2.0 equiv

Isolated yield: 85 - 90%

O

O

R

O

O

+SiMe3

Bi(OTf)3 (2.5 mol%)


2.0 equiv

Isolated yield: 75%

HO

O

R = Me, R' = R" = H

R = H, R' = R" = Me

R = R" = Me, R' = H

R = R' = Me, R" = H

R = OMe, R' = R" = H

R = H, R' = R" = OMe

O

O

+SiMe3

Bi(OTf)3 (2.5 mol%)


2.0 equiv

Isolated yield: 82%

HO

O

OH

OH

OH

OH

Scheme 30.

Bi(OTf)3 (2.0 mol%)

CH2Cl2r.t., 0.5 ~ 2.5 h


O

Ar R

epoxide

N

Ar

Ts

+ Sn 4 Ar

R

OH

Ar

NHTs


Ar = Ph, p-ClC6H4, p-MeC6H4

Epoxide = styrene oxide, 4-methylstyrene oxide,

4-chlorostyrene oxide, 3-chlorostyrene oxide,

1,2-dihydronaphthalene oxide, indene oxide,

trans-, cis-stilbene oxide, 2-naphthyl oxirane

Bi(OTf)3 (2.0 mol%)

CH2Cl2r.t., 2.0 ~ 2.5 h

+ Sn 4

Scheme 31.


also efficient for the reaction of aryl aziridines with tetraallyltin to give -amino olefin derivatives in good yields with excellent regioselectivity (Scheme 31) [42].

The same group has also demonstrated that Bi(OTf)3 showed good catalytic activity for the condensation of -hydroxy- , - unsaturated aldehydes with aryl amines. In the presence of Bi(OTf)3, in CH3CN at 80

oC, -hydroxy- , -unsaturated

aldehydes and aryl amines underwent the tandem Michael and intramolecular Friedel-Crafts type cyclization to produce a new class of chiral tetrahydroquinolines in good yields with high stereoselectivity. This method is quite simple and straightforward and constructs unusual benzo-fused heterobicycles in a single-step operation (Scheme 32) [43]. Among the examined various metal triflates such as In(OTf)3, Ce(OTf)3, Sc(OTf)3, Yb(OTf)3 and Bi(OTf)3, Bi(OTf)3 was found to be the most effective catalyst for this transformation.

Another application of Bi(OTf)3 as catalysts can be seen in the synthesis of 2,3,4-trisubstituted quinolines in high yields through a sequential condensation/annulation reaction of 2-aminoaryl ketones and -methylene ketones under mild conditions [44]. Scheme 33 shows the results from the reaction of 2-aminoaryl ketones with acetyl acetone. Besides acetyl acetone, other 1,3-dicarbonyl compounds such as ethyl acetoacetate, 1,3-cyclohexanedione, 5,5-dimethylcyclohexanedione and acyclic ketones including 2-butanone, 3-pentanone are also the good substrates for this

transformation. In addition, the use of cyclic ketones such as cyclopentanone and cyclohexanone resulted in the formation of tricyclic quinolines.

2. BISMUTH-CATALYZED CARBON-NITROGEN BOND FORMATION

As shown in Scheme 17, Bi(OTf)3 is the efficient catalyst for the Michael-type conjugate addition of indoles to , -enones at room temperature through activation of C-H bond of indoles to construct the C-C bond [22]. Interestingly, Banik and coworker reported that Bi(NO3)3 could efficiently catalyze versatile Michael addition reactions of electron-deficient olefins with substituted amines, imidazoles, thio compounds, indoles and carbamates at room temperature through activation of N-H and S-H bonds [45]. Compared to the existing acidic catalysts, the features of Bi(NO3)3-catalyzed procedure include its generality, high-yield and environmental benignity. Scheme 34 shows the results of Michael addition reactions of enones with amines to afford the protected

-aminocarbonyl compounds. On the other hand, the protected -aminocarbonyl compounds could also be obtained in very good

yields by the Mannich-type reaction of silyl enolates with N-alkoxycarbonylamino sulfones in the presence of Bi(OTf)3·4H2O (0.5~1.0 mol%) under mild conditions [46].

Bi(OTf)3 was found to accelerate the Michael addition of , -unsaturated esters with amines under microwave irradiation,

Isolated yield:

72 ~ 86%

RO

OH

OR

CHO+

H2N R1

R2

R3

1.5 equiv

Bi(OTf)3 (5.0 mol%)

CH3CN

80 oC, 4.0 ~ 7.0 h

NR1

R2

R3

H

HO

OR

OR

H

R = Ac, Me, BenzylR1 = H, Me, Ph, F, Cl, Br

R2 = H, Me, Cl

R3 = H, Me, CN, F, Br

Scheme 32.

R

NH2

O

R'

R = H, R' = Ph; R = H, R' = Me;

R = Cl, R' = Ph; R = Cl, R' = 2-FC6H4;

R = Cl, R' = 2-ClC6H4

+

O O

1 : 1.2

Bi(OTf)3 (5 mol%)

ethanol

r.t., 3.0 ~ 5.5 h N

R

R' O

Isolated yields:

77 ~ 91%

Scheme 33.

R R'

O

enone

+ H NR"2

amine

1 : 1

Bi(NO3)3 (10 mol%)

CH2Cl2r.t., 12 ~ 15 h

R R'

O NR"2

Yield: 64 ~ 79%

amine: piperidine, dibenzylamine, thiomorpholine, imidazole

enone: 2-cyclohexen-1-one, methyl vinyl ketone,

trans-1-phenyl-2-buten-1-one

Scheme 34.


but amines were limited to piperidine and unhindered primary amines. The use of Bu2NH led to the formation of adduct in trace amount, and t-BuNH2 could not undergo the addition reaction with , -unsaturated ester [47].

Bi(OTf)3 has also been employed for the intermolecular 1:1 hydroamination of 1,3-dienes with carbamates, sulfonamides and carboxamides to prepare allylic amines. The addition of catalytic amount of Cu(CH3CN)4PF6 and/or dppe [dppe = bis(diphenyl- phosphino)ethane] could suppress the polymerization of dienes, and effectively promotes the addition reaction under mild reaction conditions to afford allylic amines in good to high yields [48]. Scheme 35 shows the representative results. The same group has further developed the hydroamination of vinyl arenes, and the direct substitution of the hydroxy group in alcohols with carbamates, sulfonamides or carboxamides in the presence of Bi(OTf)3 and additives [49].

Ollevier and co-worker have developed an efficient method for the ring opening of cyclopentene oxide and cyclohexene oxide with

aromatic amines at 20 oC in the presence of BiCl3 to afford the

-amino alcohols in good yields. This BiCl3-catalyzed preparation of -amino alcohols is compatible with deactivated and sterically hindered aromatic amines (Scheme 36) [50].

Recently, Cunha and co-workers have reported the first Bi(III)-catalyzed guanylation of thioureas. In the presence of BiI3 or Bi(NO3)3, the reactions of primary and secondary amines with N-benzoyl or N-phenylthioureas in CH3CN under reflux conditions afforded the polysubstituted guanidines in good yields (Scheme 37) [51]. Compared to the stoichiometric HgCl2 protocol, this Bi(III)-catalyzed guanylation of thioureas uses non-toxic reagent, requires no excess amount of amines and is easy to handle.

Yadav and co-workers first reported that Bi(OTf)3/[bmim]BF4 could be used as novel and reusable catalytic system for the synthesis of furan, pyrrole and thiophene derivatives in high yields from 1,4-diketones under mild conditions. Through this catalytic system, various substituted furan, pyrrole and thiophene derivatives could be obtained by the cyclization of substituted 1,4-diketones

+

Bi(OTf)3 (10 mol%)

Cu(CH3CN)4PF6 (10 mol%)

dppe (4 mol%)

1,4-dioxane

25 ~ 100 oC, 3 ~ 24 h2 equiv.

NHR

Isolated yield: 75 ~ 96 %

R = COOCH2Ph, COOCH3, COO(n-Bu)

SO2Ph, SO2(p-toly), SO2(p-MeOC6H4), SO2(p-CF3C6H4)

SO2(o-NO2C6H4, COPh, CO(p-CF3C6H4)

H2NR

Scheme 35.

+ H2NAr

1.1 equiv

On

BiCl3 (10 mol%)

cyclohexane

20 oC, 6 ~ 24 h

n = 1, 2

n

OH

NHAr


Ar = Ph, o-, p-MeC6H4, o-, p-MeOC6H4, p-CF3C6H4, p-BrC6H4

Scheme 36.

N N

S

R1 R2

H H

+N

R3

R4

HBiI3 (5 mol%)

NaBiO3 (1.0 equiv)

Et3N (2.0 equiv)

CH3CN, reflux, 3 ~ 12 h1 : 1

R1

N N

NH

R2

R3

R4

Isolated yield:

69 ~ 97%

R1, R2 = Bz, p-MeOC6H4; Bz, Ph; Bz, p-tolyl; Bz, c-C6H11;

Bz, o-ClC6H4; Ph, p-MeOC6H4; Ph, c-C6H11

R3, R4 = p-MeOC6H4, H; c-C6H11, H; c-C6H11, c-C6H11;

CH2CH2OCH2CH2

Scheme 37.

RR'

O

OR"

R = n-Bu, p-FC6H4, n-C6H13; R' = Ph, 3-HOC6H4

R" = Ph,

S

Bi(OTf)3 (5.0 mol%)

[bmim]BF4 (3 mL)

90 oC, 4.0 ~ 4.5 h1.0 mmol

O

R"

R R'

Isolated yield:

80 ~ 85%

Scheme 38.


(Scheme 38), the reaction of substituted 1,4-diketones with aryl amines (Scheme 39), or the reaction of substituted 1,4-diketones with Lawesson’s reagent [2,4-bis(4-methoxyphenyl)-1,3,2,4-dithia- diphosphetane-2,4- disulfide] (Scheme 40) [52].

On the other hand, Banik and co-workers have developed the application of Bi(NO3)3·5H2O in the synthesis of substituted pyrroles from the reaction of amines with 1,4-diones [53]. This method of pyrrole formation proceeded very well with several less nucleophilic polycyclic aromatic amines to produce N-multicyclic aryl substituted pyrroles in high yields (Scheme 41). In addition, under identical conditions, the reactions of 1,2-dibenzoylethane and

1-phenyl-1,4- pentanedione with aniline or p-methoxyaniline also afforded the corresponding pyrroles in moderate to good yields. Compared to previously reported synthetic methods of pyrroles, the present Bi(NO3)3·5H2O-catalyzed reaction took place at room temperature smoothly. Thus Bi(NO3)3·5H2O has been proven to be a highly efficient catalyst for a straightforward synthesis of pyrroles. The same group then applied Bi(NO3)3·5H2O as catalyst to the reaction of isatins with 4-hydroxyproline to develop an expeditious synthesis of pyrrole-substtituted indolinones [54].

The enamination of -dicarbonyl compounds is one of the most important transformations in organic reactions. Khodaei and

RR'

O

OR"

Bi(OTf)3 (5.0 mol%)

[bmim]BF4 (3 mL)

90 oC, 4.0 ~ 5.0 h1.0 mmol

N

R"

R R'

Isolated yield:

82 ~ 90%

ArNH2 = aniline, p-fluoroaniline

+ ArNH2

1.5 mmol Ar

R = n-Bu, p-FC6H4, n-C6H13,

R' = Ph, 3-HOC6H4

R" = Ph, 2-pyridinyl,

O

S

Scheme 39.

RR'

O

OR"

Bi(OTf)3 (5.0 mol%)

[bmim]BF4 (3 mL)

90 oC, 4.5 ~ 5.0 h

1.0 mmol

S

R"

R R'

Isolated yield:

80 ~ 87%

+

1.5 mmol

R = n-Bu, n-C6H13,

R' = Ph, 3-HOC6H4; R" = Ph,

O

S

Lawesson's

Scheme 40.

H2NNH2

O

O

NN

85% (10 h)

PhCH2NH2

NPhCH2

95% (10 h)Reaction conditions:

amine:dione = 1:1.1

Bi(NO3)3.5H2O (5 ~ 10mol%)

CH2Cl2, r.t.

Ar NH2

NAr

70 ~ 96% (10 ~ 25 h)

Ar-NH2: aniline, p-anisidine

1-aminonaphthalene

1-aminoanthracene

6-aminochrysene 2-aminopyridine

Scheme 41.


co-workers recently reported that Bi(TFA)3 (TFA = trifluoro- acetate), as an efficient and water-tolerant Lewis acid, could catalyze the enamination of -dicarbonyl compounds with various primary amines in water at room temperature to afford

-enaminones in good to high yields with high regio- and chemoselectivity [55]. One of the advantages of this catalytic system is the recyclability of catalyst, and Bi(TFA)3 could be recovered at the end of the reaction and reused without any loss of activity. Some results are shown in Scheme 42. Their further investigation developed a modified catalyst system for the same reactions using Bi(TFA)3 immobilized on molten tetrabutyl- ammonium bromide (TBAB) as inexpensive ionic liquid at room temperature [56]. In all cases, the reactions under ionic liquid were more rapid and afforded excellent yields, compared with aqueous media.

3. BISMUTH-CATALYZED CARBON-OXYGEN BOND FORMATION AND CLEAVAGE

Catalytic formation and cleavage of carbon-oxygen bond are important transformations in organic synthesis, particularly in the selective protection and deprotection of hydroxy and carbonyl

groups in the multistep synthesis. Bi(III)-catalyzed such type of transformation has been intensively studied and great development has been achieved in the recent years.

3.1. Bismuth-Catalyzed Protection of Alcohols and Their

Reversible Reactions

Dihydropyran (DHP), dimethoxymethane (DMM) and triphenylmethyl (trityl) alcohol etc. are the most commonly used protecting reagents for alcohols. Mohan and co-workers reported that Bi(OTf)3 was an efficient catalyst for the formation and deprotection of tetrahydropyranyl (THP) ether [57]. In the presence of Bi(OTf)3, a variety of hydroxy compounds could react with DHP at room temperature under solvent-free conditions or in CH2Cl2 to afford the corresponding THP ethers in good to high yields. Interestingly, the same catalyst also showed the high catalytic activity for the deprotection of THP ether at an elevated temperature (110

oC) in a mixed solvent of DMF/CH3OH (9:1, v/v)

(Scheme 43). Bi(NO3)3·5H2O was then found to have the similar catalytic activity for both tetrahydropyranylation and depyranyla- tion of alcohols and phenols [58].

Bi(TFA)3 (5 mol%)

H2O

r.t., 5 ~ 60 min

Isolated yield:

63 ~ 97%R = benzyl, Ph, o-, p-tolyl

R' R"

O O

+

1 : 1

HN O

R"R'

R

R' = Me, R" = OEt

R' = Me, R" = Ph

RNH2

Scheme 42.

R OH

O

+


solvent-free or CH2Cl2 r.t., 0.5 ~ 25 h

ORO

1.3 ~ 2.6 equiv.

R = aliphatic, aromatic, allylic, cyclic

Isolated yield:

63 ~ 87%


DMF-CH3OH (9:1)

110 oC

Scheme 43.

R OH + MeO CH2 OMe

8.0 equiv.

Bi(OTf)3 (5 mol%)

CHCl3reflux, 1 ~ 8 h

O CH2 OMeR

Isolated yields:

40 ~ 95%

R = aromatic, cyclic and acyclic

Scheme 44.


Sreedhar and co-workers found that Bi(OTf)3 was an efficient catalyst for methoxymethylation (MOM) of alcohols and carboxylic acids with DMM in MeCN under reflux conditions to give the corresponding methoxymethyl (MOM) ethers and carboxylic esters in good to excellent yields (Scheme 44) [59]. The recyclable immobilized Bi(OTf)3 catalyst, microencapsulated Bi(OTf)3 (MC Bi(OTf)3) was prepared by the same group and has been applied as catalyst for a variety of organic transformations [60], which displayed somewhat lower catalytic activity compared to Bi(OTf)3

for the same transformation.

A few years ago, Sabitha and co-workers developed the BiCl3-catalyzed selective cleavage of acetals and ketals in methanol to give the parent aldehydes and ketones in high yields. The groups of silyl, benzyl and tetrahydropyranyl ether in acetal remained intact [61]. Recently, they demonstrated that BiCl3 is also an effective catalyst for the selective detritylation of trityl ethers in MeCN at room temperature to give the parent alcohols in excellent yields, and the reaction bore a variety of acid/base sensitive groups and substrates such as carbohydrates, terpenes and amino acids (Scheme 45) [62].

ROTrBiCl3 (5 mol%)

MeCNr.t., 3 ~ 10 min

R = alkyl, aryl,

terpenoid, carbohydrate units

Isolated yields:

86 ~ 95%

ROH

Scheme 45.

R

NR'

O

BiBr3 (10 ~ 20%)

MeCN

r.t., 1 ~ 8 h

RHO

NHR'

R = OMOM, OAc, OTBDPS,

OTBDMS, ester, cyc-O,O-acetals

R' = Boc, Cbz

Isolated yield:

85 ~ 99%

Scheme 46.

BiBr3 showed highly catalytic activity for the selective deprotection of cyclic N, O-aminals in MeCN at room temperature to produce the corresponding amino alcohols in high yields, and the moieties of OMOM, OAc, OTBDPS (TBDPS = t-butyldiphenyl- silyl), OTBDMS (TBDMS = t-butyldimethylsilyl), ester, cyc-O, O-acetals, Boc, Cbz were intact after the reaction (Scheme 46) [63].

3.2. Bismuth-Catalyzed Protection of Carbonyl Compounds

and Their Reversible Reactions

Catalytic conversions of carbonyl compounds to the corresponding acetals, acylals (geminal diesters) and their reversible reactions are important transformations in organic synthesis for protection of carbonyl groups. The previous work of

Mohan group disclosed that the formation of acetals or ketals from the reactions of aldehydes or ketones with trialkyl orthoformates or alcohols, and their reversible reactions (deprotection of acetals and ketals) could be achieved in the presence of a catalytic amount of Bi(OTf)3·4H2O under different reaction conditions [64]. Recently, Mohan and co-workers have developed Bi(NO3)3·5H2O-catalyzed synthesis of acylals from the reactions of aromatic aldehydes and Ac2O at room temperature [65]. Scheme 47 shows the representative results from the reaction of aromatic aldehyde with Ac2O catalyzed by Bi(NO3)3·5H2O.

Bi(NO3)3·5H2O has been found to be an efficient catalyst for the protection of carbonyl compounds as acetal, ketal, thioketal, mixed ketal and thioketal at room temperature in high to excellent yields. Using this procedure, a facile diastereoselective synthesis of dioxolanones and oxathiolanones has also been achieved (Schemes

48 and 49) [66].

3.3. Bismuth-Catalyzed Carbon-Oxygen Bond Formation And

Cleavage In Other Reactions

Transcarbamoylation reaction has important industrial application in the field of polyurethane chemistry. Bi(OTf)3 was

reported to be a very efficient catalyst in the transcarbamoylation reaction of N-alkyl O-alkyl carbamates. For example, in the

presence of Bi(OTf)3 (1.0 mol%), the reaction of N-hexyl O-methyl carbamate with equivalent amount of n-octanol at 160

oC for 2 h

afforded N-hexyl O-octyl carbamate in 92% GC yield (Scheme 50) [67]. Bi(OTf)3 shows higher catalytic activity than BiCl3 and other

Lewis acid catalysts such as Bu2Sn(OAc)2, Sc(OTf)3, Sm(OTf)3, Yb(OTf)3, La(OTf)3.

Sulfuric acid-catalyzed reaction of p-benzoquinones with acetic anhydride is a well-known procedure for the synthesis of the tri-

acetoxyaromatic precursors of hydroxyquinones. Recently, Bi(OTf)3 was employed in place of sulfuric acid to efficiently

catalyze the same reaction at room temperature [68]. For example, the reaction of p-benzoquinone with acetic anhydride (4 equiv) in

the presence of 2 mol% of Bi(OTf)3 in CH3CN at room temperature for 45 min resulted in the formation of 1,2,4-triacetoxybenzne in

91% isolated yield (Scheme 51). Substituted p-benzoquinones such as 2-methyl, 2-ethoxy, 2-methoxy, 2,3-dimethyl, 2,5-dimethyl,

2,6-dimethyl, and 2,5-dichlorobenzoquinone also underwent the same reaction smoothly to afford the corresponding 1,4-diacylated-

2-acetoxylated hydroquinones in good to high yields with high selectivity. The use of 1,4-naphthoquinone and 2-methylnaphtho-

quinone also produced triacetoxy derivatives under the same conditions.

Mohan and co-workers reported the Bi(OTf)3·xH2O-catalyzed

cycloaddition of substituted salicylaldehydes with 2,2-dimethoxypropane to afford substituted 3,4-dihydro-2H-1-benzopyrans in

moderate yields with high selectivity (Scheme 52) [69].

Bi(OTf)3 also showed high catalytic activity for the condensation of 2,2-dimethoxypropane with aryl anilines, or with o-diamino- benzenes under solvent-free conditions at room

RH

O

+

3.0 equiv.

Bi(NO3)3.5H2O

(3 ~ 10 mol%)

CH3CNr.t., 1.5 ~ 19.0 h

ROCOCH3

OCOCH3

Isolated yields:

57 ~ 91%

R = H, p-, m-Cl, p-Me, m-NO2,

m-MeO, p-COOMe, p-MeCO, p-Me2(t-Bu)SiO

(CH3CO)2O

Scheme 47.


temperature to produce high yields of 1,2-dihydroquinolines and 1,5-benzodi- azepines, respectively (Scheme 53) [70].

Another interesting Bi(OTf)3-catalyzed cyclization were the Prins reactions of styrenes with paraformaldehyde, and homoallyl alcohols with aldehydes to afford the corresponding 1,3-dioxanes and tetrahydropyran-4-ol in good yields (Schemes 54 and 55) [71].

Protection of heteroatoms is also an important transformation in organic synthesis. Previous reports disclosed that Bi(OTf)3·4H2O was an effective catalyst for the acetylation of alcohols and phenols

with acetic anhydride [72]. Chakraborti and co-workers recently studied the catalytic activity of various bismuth(III) salts such as Bi(OTf)3, BiOClO4·xH2O, BiCl3, Bi(OAc)3, BiOCl, BiONO2, (BiO)2CO3 and BiF3 in the acetylation of alcohols and phenols, and found that both Bi(OTf)3 and BiOClO4·xH2O were ideal catalysts for such type of transformation. The investigation of reaction scope and limitations found that BiOClO4·xH2O could catalyze the acetylation of structurally diverse phenols, alcohols, thiols and amines with equivalent of Ac2O at room temperature under

O

R

R'

Bi(NO3)3.5H2O

(0.1 mol%)

r.t., 4 ~ 6 hIsolated yield:

60 ~ 92%

R

R'

OMe

OMe

O

O

R

R'

S

S

R

R'

S

O

R

R'

Carbonyl compound:

Benzaldehyde, 2-bromobenzaldehyde, trans-cinnamaldehyde, p-anisaldehyde, 3-

bromobenzaldehyde, cyclohexanone, 4-methylcyclohexanone, 4-tert-butylcyclohexanone

MeOH

HOCH2CH2OH

HSCH2CH2SH

HSCH2CH2OH

Scheme 48.

O

R

H

Bi(NO3)3.5H2O

(0.1 mol%)

THF

r.t., 6 ~ 10 h

Isolated yield:

70 ~ 95%

E

O

R

H O

R'

HO O

HE R'

+

E = O, S

Carbonyl compounds:

Pivaldehyde, dihydrocinnamaldehyde

Acids:

L(+)-Lactic acid. mandellic acid, thiolactic acid

Scheme 49.

n-C6H13NH OMe

O

+ n-C8H17OH

Bi(OTf)3

(1.0 mol%)

160 oC, 2 hn-C6H13NH O(n-C8H17)

O

GC yield: 92%

Scheme 50.

O

O

+

1 : 4

Bi(OTf)3 (2.0 mol%)

CH3CN

r.t., 45 min

OAc

OAc

OAc

91%

Ac2O

Scheme 51.


solvent-free conditions to afford the corresponding acetylated compounds in high to excellent yields [73]. For instance, the results

of the acetylation of phenol, thiophenol and 1-nitroaniline are shown in Scheme 56.

OH

CHO

+

MeO OMe

Me Me

Bi(OTf)3.xH2O

(0.1 or 2.0 mol%)

CH3CN

0 oC, 2 ~ 24 h

O

OMe

Me

OMe

Isolated yield: 56-72%

Selectivity: > 90%

R R

R = H, Br, Cl, Me

OH

CHO

+

MeO OMe

Me MeCH3CN

0 oC, 18 h

O

OMe

Me

OMe

OMe

60%

OMe

Bi(OTf)3.xH2O

(0.1 mol%)

Scheme 52.

Bi(OTf)3 (5.0 mol%)

r.t., 2 ~ 4 h+

MeO OMe

Me Me

Isolated yield:

80 ~ 90%

R

NH2 NH

Bi(OTf)3 (5.0 mol%)

r.t., 2 ~ 4 h+

MeO OMe

Me MeR

NH2

NH2

NH

N

Isolated yield:

81 ~ 92%

R

R

R = H, 5-Me, 5-NO2, 4,5-dimethyl

R = H, o-, p-Me, o-, p-Cl, o-Et, o-MeO, p-NO2, 3,4-dimethoxy,

2,3,4-trimethoxy, 3,4-methylenedioxy

Scheme 53.

R

+ (CH2O)n

R

O

O

Bi(OTf)3 (5 mol%)

10.0 equiv.

MeCN

reflux, 4 ~ 10 h

Isolated yield:

77 ~ 90%R = H, Me, OMe, Cl, AcO

Scheme 54.

R

OH

+

R' H

O

O

OH

R R'

Bi(OTf)3 (5 mol%)

MeCN, reflux

1.0 equiv.Isolated yield:

65 ~ 82%R = Ph, p-O2NC6H4, p-MeOC6H4, p-ClC6H4

R' = Cyclohexyl, phenyl

Scheme 55.


It is noticeable that Bi(OTf)3 could catalyze further transformation of phenyl acetates at an elevated temperature to afford 2-acetyl phenols through Fries rearrangement [74]. For example, in the presence of Bi(OTf)3, aryl acetates in toluene heated at 110

oC resulted in the formation of the corresponding

hyroxyaryl ketones in fair to good yields (Scheme 57) [74b)]. It was found that the reaction was efficient for naphthyl acetates and phenyl acetates bearing electron-rich group at meta-position of benzene ring.

Recently, the efficient Bi(OTf)3-catalyzed [1,3] rearrangement of aryl 3-methylbut-2-enyl ethers to para- and ortho-prenylated phenols and naphthols, and the Claisen rearrangement of allyl naphthyl ethers to ortho-allyl naphthol have been developed by the same group [75].

In addition, Bi(OTf)3-catalyzed Claisen and Fries rearrangements of allyl ethers and phenyl esters were also reported by other groups [76].

In the past five years, some procedures for deprotection of other groups catalyzed by Bi(III) compounds have also been developed. For example, the deprotection of thioacetals to their carbonyl compounds underwent smoothly in the presence of Bi(OTf)3·4H2O in CH2Cl2/H2O (8:2) at room temperature, and this method was effectively employed in the preparation of DNA-binding pyrrolo[2,1-c] [1,4]benzodiazepine and its dimers [77]. The selective deprotection of ketoximes was achieved in the presence of BiBr3 (20~40 mol%) and Bi(OTf)3·4H2O (5 mol%) in CH3CN/ acetone/H2O (3:6:1) under reflux conditions [78]. Moreover, BiCl3

was applied in the chemoselective deprotection of N-Boc group from tryptophan containing peptides, which is an important transformation in the synthesis of peptides and biologically active natural products [79].

4. BISMUTH-CATALYZED CARBON-SULFUR BOND FORMATION

The chlorosulfination of aromatic compounds has been achieved by using Bi(OTf)3·xH2O as the efficient Lewis acid catalyst in the reactions of aromatic compounds with thionyl chloride at low temperature (-5 ~ 5

oC) (Scheme 58) [80]. BiCl3 can

also catalyze these reactions, but shows slightly lower catalytic activity compared to Bi(OTf)3·xH2O. Bi2O3 and BiOCl display high catalytic activity in the reaction of anisole with thionyl chloride under mild conditions (60

oC) , while BiI3 and BiOI show only

moderate activity. Although the scope of this reaction is restricted to highly electron-rich aromatic compounds, it has provided a convenient method for the synthesis of aryl sulfinyl chlorides.

Both Trifluoromethanesulfonic acid (TfOH) and BiCl3 are poor catalysts for Friedel-Crafts arylsulfonylation of aromatic compounds, but the total synergistic effect between TfOH and BiCl3 was observed by the same group in the reaction of aromatic compounds with methanesulfonyl chloride [81]. It has been disclosed that the combined catalysts of BiCl3 and TfOH shows good catalytic activity for the Friedel-Crafts arylsulfonylation of aromatic compounds with methanesulfonyl chloride to afford aryl methyl sulfones in good to high yields. This catalytic system works

Isolated yield (%)

OH

SH

NH2

NO2

BiOClO4.xH2O

(1.0 mol%)

Ac2O (1.0 equiv)

r. t.

OAc

SAc

NHAc

NO2

85 (0.5 h)

95 (0.5 h)

80 (2 h)

Scheme 56.

O

O

Bi(OTf)3.4H2O (10 mol%)

toluene, 110 oC, 6 h

OH O

Isolated yield: 80%

O

O

Bi(OTf)3.4H2O (10 mol%)

toluene, 110 oC, 6 ~ 33 h

OH O


R

R

R = H, m-Me, m-OMe, p-COOCH3

Scheme 57.


well with some electron-deficient aromatic compounds (Scheme 59). The proposed mechanism involves a sequential reactions of BiCl3 with TfOH giving BiCln(OTf)3-n; BiCln(OTf)3-n with

CH3SO2Cl producing MeSO2OTf, and MeSO2OTf is considered to be the real methanesulfonylating reagent, which reacts with aromatic compounds (Scheme 60).

Ar H +

Bi(OTf)3.xH2O

(2 mol%)

- 5 ~ 5 oC

2.5 ~ 24 h

Ar H

10 equiv

anisole

phenetole

veratrole

2-methylanisole

3-methylanisole

mesitylene

3, 4-dimethoxybenzenesulfiny chloride

3-methyl-4-methoxybenzenesulfiny chloride

2-methyl-4-methoxybenzenesulfiny chloride

2,4,6-trimethylbenzenesulfiny chloride

4-methoxybenzenesulfiny chloride

4-ethoxybenzenesulfiny chloride

> 99%

SOClAr

SOClArSOCl2

Scheme 58.

Ar H +

BiCl3 (10 mol%)

TfOH (10 mol%)

105 ~ 130 oC

0.7 ~ 7 days

Ar H

3 equiv

toluene methyl methylphenyl sulfone (mixture)

SO2CH3Ar

SO2CH3Ar Yield (%)

92

mesitylene methyl 2,4,6-trimethylphenyl sulfone 80

o-xylene dimethylphenyl methyl sulfone (mixture) 87

m-xylene dimethylphenyl methyl sulfone (mixture) 31

p-xylene dimethylphenyl methyl sulfone (mixture) 91

benzene methyl phenyl sulfone 94

chlorobenzene chlorophenyl methyl sulfone 78

o-dichlorobenzene dichlorophenyl methyl sulfone (mixture) 19

fluorobenzene 0

CH3SO2Cl

Scheme 59.

Ar H

SO2CH3Ar

BiCln(OTf)3-n

CH3SO2Cl

BiCl3TfOH

CH3SO2OTf

HCl

BiCl3

Scheme 60.


5. BISMUTH-CATALYZED OXIDATION REACTION

One of the most significant current interests in organic synthesis is to develop the oxidation reaction of organic compounds with hydrogen peroxide or molecular oxygen as oxidant. The applications of bismuth compounds as catalysts for oxidative reaction or oxidants have been reported for a long time [1a]. Recently, as their continuing interest in bismuth-catalyzed oxidation, Duñach and co-workers reported the oxidative cleavage of mandelic acid derivatives in the presence of either Bi(0) or Bi(III) compounds in a DMSO/O2 system to give a mixture of carboxylic acid and aldehyde [82]. Scheme 61 shows the typical results of the oxidation of mandelic acid derivatives with various substituents on the aromatic ring. It can be found that the ratio of benzaldehyde and benzoic acid derivatives greatly depends on the nature of the substitutent(s) on the aromatic ring. For example, when mandelic acid derivatives bear hydroxy groups at 2- or 4-position of the aromatic ring, the formation of benzaldehyde derivatives is highly favoured. In contrast, the oxidation of mandelic acids with electron-withdrawing groups on the aromatic ring resulted in the formation of benzoic acids in excellent chemoselectivity. In the cases of electron-donating groups such as methoxy, the oxidation reactions afforded a less selective mixture of benzaldehydes and benzoic acids. The predominate formation of aldehydes or acids is resulted from the different reaction mechanisms. The direct oxidative decarboxylation of mandelic acids affords aldehydes, while the formation of acids involves the corresponding keto acid intermediates [83].

Reported by Barrett and co-workers, Bi(0), combined with picolinic acid, has been applied as catalyst in the benzylic oxidations with tert-butyl hydroperoxide as oxidant [84]. In the presence of Bi(0) and picolinic acid, the oxidation of alkyl and cycloalkylarenes gave the corresponding benzylic ketones in

moderate to high yields. As exemplified by tetrahydronaphthalene, diphenylmetane and 4-ethylanisole, the oxidation proceeded at 100 oC for 16 h to give the corresponding ketones in 77%, 95% and

93% isolated yields, respectively (Scheme 62). However, in order to obtain the corresponding arenecarboxylic acids in moderate to good yields from the oxidation of methylarenes, a prolonged time (48 h) was required, along with 40 mol% of Bi(0) as catalyst. Therefore, a more efficient catalyst system using Bi(OTf)3 (20 mol%) and picolinic acid (10 mol%) for the oxidation of methylarenes to the arenecarboxylic acids has been developed (Scheme 63).

Bi(OTf)3 (20 mol%)

picolinic acid (10 mol%)

t-BuOOH (70% aq. 7.0 equiv)

pyridine:AcOH (ca. 9:2)

110 oC, 20 h

Ar CH3 Ar COOH

GC yield: 54 ~ 74%

Ar CH3:

1,2,4,5-tetramethylbenzene

1,3,5-trimethylbenzene

2,5-dimethoxytoluene

-methylnaphthalene

4-phenyltoluene

4-bromotoluene

Scheme 63.

At the same time, Salvador and co-workers reported Bi(III) salts such as BiCl3, Bi(NO3)3·5H2O, Bi2O3, Bi(OAc)3, BiOCl and BiO(NO3) could catalyze the allylic oxidation of both unsaturated steroids and valencene in good yields with high selectivity using t-butyl hydroperoxide as oxidant [85]. Among the examined catalysts, BiCl3 showed the best catalytic activity. Scheme 64 shows the examples of the oxidation of

5-steroids. As described

before [37], after work-up of the reaction, Bi(III) could be recovered and reused as BiOCl.

R'

R

COOH

OH

R'

R

COOH

R'

R

O

H

+

aldehyde acid

aldehyde:acid = 94:6

R =H, R' = Cl

R = R' = OH

total yield: 93% (24 h) aldehyde:acid = 3:97

total yield: 92% (1h)

R =OMe, R' = H total yield: 75% (6 h) aldehyde:acid = 38:62

Bi(0) (10 mol%), O2 (1 atm)

DMSO, 125 oC

1.5 equiv of aq AcOH (50%)

Scheme 61.

MeO

MeO

O

O

O

77%

93%

95%

(Bi2O3 + NaBH4)

Bi(0) (~20 mol%)

picolinic acid (20 mol%)

t-BuOOH (70% aq. 6.0 equiv)

pyridine:AcOH (9:1)

100 oC, 16 h

in situ

Scheme 62.


Interestingly, the use of recoverable heterogeneous catalysts BiCl3/K-10 could considerably increase the oxidation reaction rate.

In addition, recently the applications of bismuth compounds as oxidants such as Ar3BiCl2 (Ar = o-, p-MeC6H4, o-, p-MeOC6H4, Ph) [86], [Ph3BiAr]BF4 (Ar = mesityl, o-, p-MeC6H4, Ph) [87] have been also reported in the oxidation of alcohols to carbonyl compounds, Bi(NO3)3·5H2O in the oxidation of thiols to disulfides [88], and Ph3BiCO3 in the oxidation of urazoles to triazolinediones [89].

6. BISMUTH-CATALYZED MULTI-COMPONENT COUP- LING REACTION

In recent years, development of multi-component coupling reactions is of growing interest in synthetic chemistry, since such type of reactions can provide a rapid and efficient protocol for the synthesis of highly functionalized molecules from simple and diverse starting materials in one-pot reactions [90]. Bi(III) salts have been demonstrated to be an efficient water-, oxygen-tolerant catalysts for the multi-component coupling reactions including carbon-carbon, carbon-nitrogen bond formation.

Choudary and co-workers reported the allylation of aldehydes with allyltributylstannane in MeCN at room temperature catalyzed by Bi(OTf)3·4H2O in the presence of equivalent benzoic acid (Scheme 65). When the reactions were conducted using aryl aldehydes, allyltributylstannane and anilines, the three-component coupling reaction afforded homoallylic amines by direct allylation and subsequent amination of aryl aldehydes. Scheme 66 shows the

representative results of this three-component coupling reaction [91].

Two years later, Kim and co-workers reported that the same allylation of aldehydes could take place smoothly by using a recoverable silica gel grafted bismuth triflate catalyst [silica-Bi(OTf)2] under the similar conditions [92].

The similar products of protected homoallylic amines could be obtained by an another three-component reaction of aldehydes, primary carbamates and allytrimethylsilanes in the presence of Bi(OTf)3·nH2O (Scheme 67) [93]. An alternate efficient method using N-alkoxycarbonylamino sulfones was also reported by the same authors [94]. Therefore, it has been demonstrated that Bi(OTf)3·nH2O is also an efficient catalyst for the allylation of a variety of imines which are generated in situ from the reaction of aldehydes with primary carbamates. Compared to the procedure shown in Scheme 66, without any additives, using low quantity of Bi(OTf)3·nH2O (1.0 mol%) are the advantages of this procedure. Bi(OTf)3·4H2O were also applied by the same group to the three-component Mannich-type reaction of aldehydes, anilines and silyl enol ethers to establish a method for the preparation of protected -amino ketones. This method was applicable to a variety of substituted substrates bearing electron-donating and electron-withdrawing groups to afford the corresponding products in good to high yields [95]. Scheme 68 represents the results from the reactions of aldehydes with (1-phenylvinyloxy)trimethylsilane and aniline at 25

oC in MeCN.

Two years later, Ollevier’s group employed silyl ketene acetals instead of silyl enol ethers to perform the three-component

BiCl3 (10 mol%)

t-BuOOH (ca. 10.0 equiv)

AcO

RR'

CH3CN

70 oC, 18 ~ 24 h AcO

RR'

O

Isolated yield:

60 ~ 88%

Scheme 64.

R CHO + SnBu3

Bi(OTf)3 (2.0 mol%)

PhCOOH (1.0 equiv)

MeCN

25 oC, 5 ~ 30 min.

R

OH

1 : 1.2

R = p-ClC6H4, Ph, 2-furyl, 3,4,5-trimethoxyphenyl

2-phenylethyl, p-O2NC6H4, trans-styryl, cyclohexyl

NMR yield:

65 ~ 95%

Scheme 65.

Ar CHO + SnBu3 + Ar' NH2

Bi(OTf)3 (2.0 mol%)

PhCOOH (1.0 equiv)

MeCN, 25 oC

Ar' Yield (%, NMR)Time (min)

20

60p-ClC6H4 p-O2NC6H4

o-ClC6H4 60

70

Ar

NHAr'

1 : 1.2 : 1

Ar

Ph

Scheme 66.


Mannich-type reaction of aldehydes, anilines and silyl ketene acetals at room temperature in the presence of Bi(OTf)3·4H2O, which provided a convenient and direct synthetic method for the protected -amino ester in good to high yield (Scheme 69) [96].

De and co-workers have also demonstrated that BiCl3 is an efficient catalyst for the three-component coupling reaction of aldehydes, amines and trimethylsilyl cyanide to develop a one-pot synthesis of -aminonitriles in high yields (Scheme 70) [97].

The three-component coupling reaction of 1,3-dicarbonyl compound, aldehyde and urea, so-called Biginelli reaction, is one of the most prominent cyclocondensation reactions in organic synthesis, because Biginelli reaction provides an efficient and one-pot synthesis of 3,4-dihydropyrimidin-2(1H)-ones, which are widely used in pharmaceutical industry and synthetic chemistry

[98]. Rajitha and co-workers reported that BiO(ClO4)·xH2O as a new and efficient Lewis acid catalyst could catalyze the Biginelli reaction smoothly. Compared with conventional procedures, this method afforded 3,4-dihydropyrimidin-2(1H)-ones in high yields in a shorter time and at lower temperature. Thus this is a much improved modification of classical Biginelli reaction (Scheme 71) [99].

Zhan and co-workers have reported another three-component coupling reaction of carbonyl compounds, amines and dialkyl phosphates in the presence of BiCl3. On the basis of this three-component coupling reaction, -amino phosphonates could be conveniently prepared in high yields (Scheme 72) [100]. Besides aldehydes, ketones such as acetone, cyclohexanone, and cyclopentanone etc. also proceeded the similar coupling reaction at

R H

O

+PhCH2O NH2

O

+ SiMe3

Bi(OTf)3.nH2O

(1.0 mol%)

CH3CN,

r.t., 3 ~ 8 h

1 : 1 : 1.1

R

NHCOOCH2Ph

Isolated yield:

27 ~ 82%

R = alkyl, aryl, Ph

Scheme 67.

R CHO +Ph

+PhNH2

OSiMe3 Bi(OTf)3.4H2O (1.0 mol%)

MeCN, 25 oC, 0.5 ~ 1.5 h R

NH

1 : 1 : 1.2

Ph

Ph

R = Ph, p-ClC6H4, p-CF3C6H4, p-O2NC6H4,

p-MeOC6H4, p-MeC6H4, 2-furyl, cyclohexyl, trans-styryl

Isolated yield:

72 ~ 89%

O

Scheme 68.

R1 CHO + OR4+PhNH2

OSiMe3

Bi(OTf)3.4H2O

(2.0 mol%)

THF, -78 ~ 25 oC,

0.8 ~ 3.0 h

R1

NH

1 : 1 : 1.2

OR4

Ph

Isolated yield:

38 ~ 85%

R2

R3

R1 = Ph, p-O2NC6H4, p-MeOC6H4, cyclohexyl,

Ph

R4 = Ph, Me, Et

R2, R3 = H, H; Me, Me; H, Ph; H, Me; cyclohexylidene

O

R2 R3

Scheme 69.

R CHO + +

BiCl3 (10 mol%)

CH3CN,

r.t., 5 ~ 10 h

R CN

NHR'

1 : 1 : 1.5 Isolated yield:

81 ~ 91%

R = Ph, p-MeOC6H4, p-ClC6H4, n-C5H11, n-C4H9,

R' NH2 Me3Si CN

E

(E = O, S)

R'NH2 = aniline, benzylamine, morpholine, pyrroline,

furfurylamine, 3-methoxybenzylamine

Scheme 70.


a lower temperature (35 ~ 45 oC) to give the corresponding

-amino phosphonates in good yields.

7. BISMUTH-CATALYZED ORGANIC REACTIONS IN AQUEOUS MEDIA

During the last decade, the use of water instead of organic solvent as reaction medium has attracted much interest of organic chemists, since it is an environmentally friendly manner. Among the catalytic organic reactions investigated in aqueous medium, Bi(III)-catalyzed reactions have recently been widely studied and well developed, because some of Bi(III) compounds are water-tolerant, and show unexpected catalytic activity.

As described in section 3.2, Bi(III) salts have been widely applied in the deprotection of acetals and ethers through the cleavage of carbon-oxygen bond. Rao and co-workers reported that Bi(OTf)3 could efficiently catalyze deprotection of methoxymethyl (MOM) group from its ethers and esters at room temperature to

give the corresponding alcohols and carboxylic acids in high yields in a mixture solvents of THF and water (1:1) [101]. As shown in Scheme 73, the catalytic reaction showed high chemoselectivity, and other protecting groups such as t-butyldimethylsilyl (TBDMS), benzyl (Bn), and allyl remained intact.

As described in section 2, Ollevier and co-workers have reported that BiCl3 is an efficient catalyst for the nucleophilic opening of epoxides with anilines to afford good yields of the corresponding trans- -amino alcohols [50]. Recently, they have examined the same type of reactions using Bi(OTf)3 as catalyst under aqueous conditions instead of BiCl3 in an organic solvent [102]. As shown in Scheme 74, in the presence of Bi(OTf)3, the ring-opening of cyclohexene oxide with anilines bearing a variety of substituents worked well at room temperature. After 7~9 h, the corresponding trans- -amino alcohols could be obtained in high yields. However, in the case of cyclopentene oxide used, the corresponding trans- -amino alcohols were obtained in relative low yields after longer reaction time.

+

BiO(ClO4).xH2O (5.0 mol%)

MeCN, 50 oC, 2 ~ 4 h

R = Me, Et

Ar = Ph, p-O2NC6H4, 2-furyl Yield: 91 ~ 95%

RO

O O

H2N NH2

O

Ar CHO +

N

N

Ar

O

RO

O

Scheme 71.

+

BiCl3 (10 mol%)

MeCN, reflux, 5 ~ 14 h

Yield: 81 ~ 95%

R H

O

R' NH2 + HOP(OR")2

R NHR'

P(OR")2

O

R = Ph, p-MeOC6H4, o-MeOC6H4, 2,4-di-Cl-C6H3,

o-HOC6H4, p-O2NC6H4, cyclohexyl, trans-styryl

R' = Ph, Benzyl

R" = Me, Et, iPr

Scheme 72.

CH2OMOM

OMe

MOMO

OMe

Bi(OTf)3 (1-2 mol%)

THF:H2O (1:1), r.t.

CH2OH

OMe

HO

OMe

90% (30 min)

CH2OMOM

TBDMSO

OMe

CH2OH

TBDMSO

OMe

COOMOM

BnO

OMe

COOH

BnO

OMe

95% (30 min)

95% (15 min)

AllylO OMOM AllylO OH

90% (55 min)

Scheme 73.


Very recently, as a part of their ongoing interest in Bi(OTf)3-catalyzed transformation, Ollevier and co-workers have reported the first Bi(OTf)3·4H2O-catalyzed direct three-component Mannich reaction of cyclohexanone, aromatic aldehydes and aromatic amines in water to give the corresponding -amino carbonyl compounds [103]. As shown in Scheme 75, comparable yield and selectivity could be obtained in the reaction of benzaldehyde, aniline and cyclohexanone in water instead of organic solvents, albeit with a longer reaction time. The studies of scope and limitation disclosed that the reactions of electron-rich and electron-deficient benzaldehydes with various substituted anilines underwent the transformation smoothly to afford -amino carbonyl compounds in good yields with moderate to good diastereoselectivity.

8. BISMUTH-CATALYZED OTHER ORGANIC REACTIONS

In the past five years, bismuth salts have also been found to be the efficient catalysts in some other novel transformation. For example, the combined Bi(NO3)3·5H2O and BiCl3 shows high catalytic activity for the iodination of electron-rich arenes with iodine under air atmosphere at room temperature to give aryl iodides in good to high yields (Scheme 76) [104]. Recent studies by Barrett and co-workers implicated Bi(OTf)3 to be the efficient Lewis acid catalyst in the reaction of various nitriles with tertiary

alcohols to afford amides in good to excellent yields (Scheme 77) [105].

Bi(III) hexanoate has been used as the initiator for the co-polymerization of -caprolactone and L-lactide [106].

In addition, bismuth salts have been applied as catalysts in the key step of total synthesis of natural products such as (-)-centrolobine [107], (-)-Mucocin [108], trans- and cis-(6-methyl- tetrahydropyran- 2-yl)acetic acid [108].

Bismuth salts as the inexpensive and relatively non-toxic reagents have been used not only as catalysts in organic reactions, but also as the new versatile reagents in diverse transformation. For example, Bi(NO3)3·5H2O and montmorillonite/Bi(NO3)3 have been used as the efficient nitrating reagents for conversion of phenolic compounds to nitrophenolic compounds [110], substituted anisoles to the corresponding nitro compounds [111], and for the ring opening of oxiranes and aziridines to form -(nitrooxy)-substituted alcohols and amines [112]. Bi(NO3)3·5H2O has also been described as a convenient and selective reagent for the conversion of thiocarbonyls to the corresponding carbonyl compounds [113].

Furthermore, Bi(0)-mediated activation of bromine-carbon bond in organic transformation has been developed such as in the reductive debromogenation of -bromocarbonyl compounds [114], in the reaction of allylation of aldehydes with allyl bromide [115],

O +

Bi(OTf)3 (10 mol%)

H2O

r.t., 7 ~ 9 h

OH

NHAr

Isolated yield:

77 ~ 86%Ar = Ph, o-, p-tolyl, p-CF3C6H4,

p-MeOC6H4, p-BrC6H4

O +

Bi(OTf)3 (10 mol%)

H2O

r.t., 21 ~ 24 h

OH

NHAr

Isolated yield: 63% (Ar = p-tolyl)

69% (Ar = 69%)

1 : 1

1 : 1

ArNH2

ArNH2

Scheme 74.

H

O

+ +

O

Bi(OTf)3 4H2O

(5 mol%)

.NH

PhO

25 oC

1 : 1 : 2

Solvent Time (h)

anti:syn

(1H NMR) Isolated yield(%)

7

2

4

6

5.5

86:14

70:30

69:31

87:13

82:18

84

89

73

89

89

PhNH2

H2O

CH3CN

CH2Cl2

Et2O

PhMe

Scheme 75.


and the reaction of 5-benzyloxy-4-methylpent-2-enyl bromides with aldehydes [116].

9. CONCLUSION

As summarized in this paper and other previous reviews, bismuth(III) salts have been successfully applied as green catalysts in diverse organic transformations. The examples included in this review clearly demonstrated the importance and advantages of bismuth(III) compounds in organic reactions. It is reasonable to assume that development of bismuth(III)-catalyzed organic reactions is still an interesting and challenging research topic in the future. I hope and anticipate that this review will provide added stimulus for further development of this research field.

ACKNOWLEDGEMENTS

Our work included in this review was supported by the National Natural Science Foundation of China (20590360) and Specialized Research Fund for the Doctoral Program of Higher Education (20060003079).

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R Bi(NO3)3.5H2O (2.5 mol%)

BiCl3 (2.5 mol%)

air, CH3CN, r.t., 6 h

R

I

R = electron-donating group(s)

(0.5 equiv)

+ I2

Scheme 76.

R CN + R' OH

Bi(OTf)3 (20 mol%)

R = Ph, Me, t-Bu, cyclopropyl, o-tolyl, m-ClC6H4, p-HOC6H4,

S

R NH

R'

O

OH

HOPh

OH

t-Bu OHR' OH =

H2O, 100 oC, 17 h

Scheme 77.


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Received: February 3, 2007 Revised: February 21, 2007 Accepted: March 5, 2007

recent advances in bismuth-catalyzed organic …...recent advances in bismuth-catalyzed organic...

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