metal basicity vs nucleophilicity and reductivityy
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ORGANOMETALLIC COMPOUNDS IN ORGANIC SYNTHESIS
CATALYTIC
Advantages = 'Difficult Reactions'
Disadvantages = Cost, Enviromental
Low Cost Metals
STOICHEIOMETRIC
Advantages = Cost, Enviromental
Disadvantages = May not be possible
High Cost Metals / ComplexesTransition Metals
This presentation looks at the synthesis, reactivity and STOICHIOMETRIC use of...
MAIN GROUP ORGANOMETALLICS AND ORGANOTRANSITIONCOMPOUNDS WHICH ARE ALSO HIGHLY NUCLEOPHILIC (OR BASIC)SOURCES OF "R-"
in the order....
LiR, NaR, KR RMgX RTiX 3
RAlX2 ZnR2 Organoceriums Organosameriums
Organomanganese
RX excess Li
-LiXRLi
RHBuLi
-BuHRLi
ArBr BuLi
-BuHArLi
All Common !!
Also Grignards etc.
THINK !! - Don’t discount the unusual, e.g.........
O SPh
NMe2
Li
"LIDMAN"
O Li
T. Cohen and T. R. Matz, Synth. Commun., 1980, 10 , 311.S. Bank, M. Platz, Tetrahedron Lett. 1973, 14 , 2097
Structures. Hydrocarbons C6H6 Et2O THF
(RLi)n
# Solvent Polarity
# Steric
# S -hybridization
# delocalisation
EtLi
BuLi
Me3SiCH2Li
i-PrLi
sec-BuLi
tert-BuLi
6 4 4
6 4 4
6 4
4
4
4 2 2 / 1
Solvent Effects
BunLi Et2O
OO
O
O
Temp. for t1/2 (dec.)= 100 h
0C -50C -100C
Li O
NR2
X
X = C(O)NR2
BuLi
BuLi
X = H
N/R unless
NMe2Me2N
Li
V. Snieckus, Chem.Rev. 1990, 90 , 879-933.
Metal Basicity vs Nucleophilicity and Reductivityy
Metal Basicity vs Nucleophilicity and Reductivityy
"CH3 M +
" +R
O
R
H Hpka 4.2
R
O
R
H
R
H H
O
M
Me
ROR
• Gas-phase Basicity
• CH3- CH4 0 KCH3 CH4 20 NaCH3 CH4 40 LiCH3 CH4 60 CH3MgBr CH4 80
• in K cal mol-1
Metal Effects - IMetal Effects - I
K 100 : 0
Na 94 : 6
Li 84 : 16
MgBr 0 : 100
PhMMe
O
MePh
Ph
O
M
+
O
Ph
M
Metal Effects - IIMetal Effects - II
BuLi + But OK- 40 C
o
BuK
Strong Base
+ LiOBut
K
LiBr
Li
- 35 Co
Metal Effects - IIIMetal Effects - III
CO2
+
CO2MM
CO2
EMg
X
Strong
low e -density
K 10 : 90
MgBr 99 : 1
Metal Acidity vs. Reactivity and Selectivity
Metal Acidity vs. Reactivity and Selectivity
H
CO
O
OMeMBr
H
C
OO
Me
MeLi
N/R
via H
O
O
Mg
Me
BrO
+
_
Affects * Substrates that form carbocation
* Strong Lewis acid metal complexes (Zn, Mg,Ti, Al etc.)
TITANIUMTITANIUM
RMTransmetallation
Cl TiX3
R TiX3
X = Cl More Lewis Acidic
= OR, NR2 Less Lewis Acidic
M = Li, Mg, Zn
* TiCl4 limited to MeLi, ZnMe2, C3H5Li, resonance stabilisedcarbanions
* X3Ti-R, R is not stable if - hydrogen(s) present( e.g. Et )
Problems
MeTi(OPri)3 < Me2Ti(OPr
i)2 < Me3Ti(OPr
i)
TiCl4 Ti(OPri)4+ 3
RM
Cl Ti(OPri)3
R Ti(OPri)3R
O
H
O
C
R
OHC
RO
Selective for Aldehydes
Cram Selectivity :Cram Selectivity :
M S
L
CHO
Favoured
MR
R
OH
H
H
Ph
Me
R
OH
H
H
Ph
Me+
MeMgBr 66 : 34
MeTi(OPri)3 90 : 10
PhSO2CH2Ti(OPri)3 65 : 35
Enantioselectivity :Enantioselectivity :
R
OH
68-98 % e.e.
O
Ph Ph
O
PhPh
H
H
O
O
TiR
X
Ar H
O
R = Me
X = OPri
R H
O
R = allyl
X = Cp
Ar
OH
Me
H
70-95 % e.e.
Olefination Reaction :Olefination Reaction :
O
CH2Br2 / Zn / TiCl4
83 %
( 10 % with Ph3P=CH2 )+
iPr Pri
OTiCl3(DME)1.5
Pri
Pri
Pri
Pri
87 %
( 12 % with TiCl4 / LAH )
Zn
But what is the mechanism .... ?
ALUMINIUMALUMINIUM
MeLi 79 21MAD/MeLi 1 99
S. Saito and H.Yamamoto, JCS Chem. Commun., 1997, 1585.
2
OH
AlMe3O
AlO
Me
'MAD'
OOH
Me
Me
OH+
OAlX3
R
ZINC I : Synthesis
ZINC I : Synthesis
Klement et al. Tetrahedron Lett., 1994, 35 , 1177-1180.P. Knochel and R. D. Singer Chem. Rev., 1993, 93 , 2117-2188.
Source Problem
ZnEt2 Comercial only one !
ZnX2 + RM Getting ZnX2 dry !
ZnEt2 + RI Getting rid of EtI
R MLn
Br
ZnEt2
RZnBrMLnEt
EtH2C=CH2+
EtH
MLn
RBr
CuCl + MnBr2CuX
M = Mn, Cu
ZINCII : UseZINC
II : Use
R. Noyori, Chem. Soc. Rev., 1989, 18 , 187-208.
ZnEt2
Ar H
O
R2
N
OH
ZnEt2 +
N
O
R2
Zn
O
Et
Ar
H
Slow
NR2
Zn
O
Et
Ar
H
ZnOO
Et2
NR2
Zn
OAr
H
ZnOEt2
99 % ee
OZnEt
HAr
Et
CERIUMHydride Reductions - I
CERIUMHydride Reductions - I
NaBH4-CeCl3
O HO H O
VS.
Sole Product
Via.
(MeO)nHmB H C O H O Me
Ce3+
CERIUMHydride Reductions - II
CERIUMHydride Reductions - II
LiAlH4 CeCl3
RX
RH
X = Cl, F
R = Alkyl, Aryl
NOH NH2
NaBH4PR3
O
BH3
PR3
PR3
O
PR3
..
CERIUMOrganocerium Reagent
CERIUMOrganocerium Reagent
BuCeCl2
BuLi+ CeCl3OH Bu
Ph Ph
O
Ph Ph
96 %(33 %)Si
H
.
O
Si
O
Bu
91 %
Me
Bu
HO
MeO
NH2
BuBu
Ph
PhCN
57 %(< 10 %)
SAMARIUMSAMARIUM
SmI2
RX
RH
O
R
R'R'
OR2
O
R
R'R'
H2
R Cl
O
R' R'
OR
O
R'R'
OH
RCN
R NH2
RCO2H
R OH
R R
O
R'I
R R'R
OH
ROH
( )n
I R
O
( )n
RX
XYI--NC
R SmX2
N
XYI
R R
O
OH
RR
OH
RR
SAMARIUMSAMARIUM
H. Yasuda et al. J. Am. Chem. Soc., 1992, 114 , 4908
XSCO2Me
Me
Me
CO2Me
CO2Me
Me
O
MeO CH2
Me
H
Cp2Sm*
O
MeO
O
MeCH2 H
CH2
OMe
O
Cp2Sm*
Sm
HCp*
HCp*
Sm
Cp*
Cp*
OMe
O
2
O
OMe
MANGANESEOrganomanganese - I
MANGANESEOrganomanganese - I
G. Catiez and M. Alami Tetrahedron , 1989, 45 , 4163-4176.
Mn + I2 MnI2
LiRLiMnR3
10 to 30 C
R2MnRMnX
5 C
Li2MnCl4LiCl +MnCl2
~
RLi Or RMgX
MANGANESEOrganomanganese - II
MANGANESEOrganomanganese - II
RMnX
CuCl
CHO
R
CHO
R' O R'
O O
R R'
O
Cl R'
O
R R'
O
Me COCl
OH
O
Me
OH
O
R
O
CuX
O
O
R
84% c.y.
98% e.e.
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