Hexacoordinated phosphates: how to teach old chiral anions new asymmetric tricks

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  • 275 2007 The Japan Chemical Journal Forum and Wiley Periodicals, Inc.

    Hexacoordinated Phosphates: How toTeach Old Chiral Anions NewAsymmetric Tricks

    JEROME LACOUR, DAVID LINDERDepartment of Organic Chemistry, University of Geneva, Quai Ernest Ansermet 30, CH-1211Geneva 4, Switzerland

    Received 8 March 2007; Revised 8 May 2007; Accepted 2 April 2007

    ABSTRACT: Chemical reactions and processes often involve chiral, yet racemic, cationic reagents,intermediates, or products. To afford instead nonracemic or enantiopure compounds, an asymmet-ric ion pairing of the cations with enantiopure anions can be consideredthe counter ions behav-ing as asymmetric auxiliaries, ligands, or reagents. Detailed herein is a short review of our approachtoward gaining reliable and predictable control over stereoselective ion pairing phenomena throughthe synthesis and use of novel configurationally stable hexacoordinated phosphate anions. 2007The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 7: 275285; 2007: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.20124

    Key words: asymmetric ion pairing; chiral anion; hexacoordinated phosphorus; resolution;supramolecular stereocontrol

    The Chemical Record, Vol. 7, 275285 (2007)

    T H EC H E M I C A L

    R E C O R D

    Correspondence to: Jrme Lacour; e-mail:jerome.lacour@chiorg.unige.ch

    Introduction

    Cationic species, omnipresent in coordination, organic,organometallic, and supramolecular chemistry, are ofteninvolved in chemical reactions and processes as reagents, inter-mediates, or products. These cationic species can be prochiralor chiral, and many of the resulting products are, unfortu-nately, racemic molecular structures or supramolecular assem-blies. To afford instead nonracemic or enantiopure adducts,and to benefit from new possible applications, a stereoselectiveion pairing of these cations with enantiopure anions can beconsideredthe counter ions behaving as asymmetric auxil-iaries, ligands, or reagents.1

    A wealth of evidence suggests that an ion electrostaticallyremoved from its counterion is never formed in low-polaritysolvents but, instead, an ion pair is produced. The association

    of racemic cations with enantiopure counterions thereforeleads to the formation of diastereomeric ion pairs.2 As a result,large chemical and physical differences can happen among thesalts of the tightly associated ions.

    In early approaches, chiral anions issued or derived fromthe chiral pool have been essentially considered. Numerousapplications have been developed, especially in the field ofenantiomeric resolutions.3 Today, these anions are still usedwith much success.1 Recent developments in this field have,however, also made use of new synthetic anions that are (i)

  • 2007 The Japan Chemical Journal Forum and Wiley Periodicals, Inc.

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    BINOL-based phosphoric acid derivatives,4 (ii) borate anionsmade from amino-acids,5 and (iii) chiral hexacoordinatedphosphate anions; the latter category of anions being particu-larly studied in our group.6 This review will thus survey ourefforts toward the preparation and the use of chiral hexacoor-dinated phosphates as anionic auxiliaries and reagents in par-ticularother anions and their use being detailed recently inanother review.1 Applications of these moieties as NMR chiralsolvating reagents, as resolving agents for organic and inorganiccations, and as chiral auxiliaries for stereoselective processeswill be presented (Scheme 1).

    Chiral Hexacoordinated Phosphate Anions

    The octahedral geometry of pentavalent hexacoordinatedphosphorus allows indeed the formation of chiral anions by

    complexation of a central phosphorus atom with three identi-cal dianionic bidentate ligands. As early as 1965, Hellwinkelreported the synthesis of hexacoordinated phosphate anion 1,which was shown to be chiral through a resolution procedure.7

    The enantiomers exist as or antipodes with a left- andright-handed propeller shape (M or P helicity), respectively.8 Adecade later, Wolf and Koenig studied the chiral tris(benzene-diolato)phosphate anion 2, of particular interest for its easyone-step preparation from catechol, PCl5, and an amine. It wasthen shown that anion 2 unfortunately racemizes rapidly insolution as an ammonium salt.9 This (negative) observation ofa poor configurational stability for 2 in solution resulted in anoverall lack of interest for this type of compounds from thestereochemical community, and this for more than twodecades.

    However, this chemistry of hexacoordinated phosphateanions was recently rejuvenated as chiral tris(tetrachloroben-

    David Linder was educated at the Ecole Nationale Suprieure de Chimie de Clermont-Ferrandand obtained his Masters from University Paris VI (Jussieu). He is currently studying towardshis Ph.D. in the group of Professor Lacour. His current research interests concern the useof organoruthenium complexes for the development of enantioselective CC bond-formingreactions.

    Jrme Lacour was educated at the Ecole Normale Suprieure (Ulm, Paris) and in 1993obtained his Ph.D. in Chemistry at the University of Texas at Austin under the supervision of Pro-fessor Philip D. Magnus. After postdoctoral studies in the laboratory of Professor David A. Evansat Harvard University, he joined the Organic Chemistry Department of the University of Genevain 1995. Since 2004, he holds a full Professor position in the department. In 2001, he receivedthe Werner Prize and Medal of the Swiss Chemical Society and in 2005, the Grammaticakis-Neuman Prize of the French Academy of Sciences.

  • C h i r a l H e x a c o o r d i n a t e d P h o s p h a t e s

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    zenediolato)phosphate(v) anion 3 was shown to be configura-tionally stable at room temperature in all common organic solvents.10 This D3-symmetric anion, known as TRISPHAT(Scheme 2), can be resolved by association with N-H-cinchonidinium as cationic counterion. Both and enan-tiomers can be isolated on large scale as [cinchonidinium][-3] and [Bu3NH][-3] salts, respectively.

    The synthesis of functionalized /mixed hexacoordinatedphosphate anions containing two tetrachlorocatechols and athird different 1,2-diolato ligand can also be achieved.However, the preparation of such P(VI) anions is not a trivialmatter.11 A practical and general synthetic procedure had to be developed for the making of such novel hexacoordinatedphosphates; anion Bis(tetrachlorobenzenediolato)mono([1,1]binaphthalenyl-2,2-diolato) phosphate(V) anion (BINPHAT)4 (Scheme 3), which includes a BINOL moiety, is the arche-type of such derivatives.12 The procedure for the making ofthese advanced anions involves the in situ formation of a

    spirophosphorane intermediate (5, Scheme 3), which reactswith essentially any diol to generate, in one pot, mixed hexa-coordinated phosphate anions. For instance, BINPHAT anion4 can be prepared in good yields (7085% yield) from tetra-chlorocatechol, P(NMe2), o-chloranil, and BINOL. This pro-tocol was recently streamlinedall reagents being added asessentially one equivalent. It can be scaled up to multigramquantities.13 Starting from enantiopure R- or S-BINOL, thediastereoselectivity of the reaction is high (>96%) and yieldsare reproducible in favor of -4 or -4, respectively; the stere-oselectivity being most probably of kinetic rather than ther-modynamic origin.14

    It is feasible, although with some risks, to change theoxidant (o-chloranil) in this process for another ortho-quinone,but original phosphate anions such as phenanthroline deriva-tive 6 can result from the operation (see Scheme 4).11 The one-pot procedure can be extended to the making of other mixedphosphates by varying the nature of the diol added last in theprotocol (e.g., mannose, tartrate, dihydrobenzoin derivatives79) (Scheme 4).13,15

    Two novel anions containing fluorine substituents atselected positions were reported recently (10a and 10b). Forthe resolution of these two anions, a novel and general proto-col was developed using N-benzyl-cinchonidium chloride as aresolving agent; the and enantiomers of the anions beingafforded in very high chemical and enantiomeric purity.Anions 10a and 10b display unique NMR properties that canbe used for the determination of exact (absolute) ion pairingstructures in solution (Scheme 5).16

    Finally, the synthesis of a novel nitrogen-containing hexa-coordinated phosphate anion 11, named TRISPHATN, wasvery recently reported. This unique anion, through its Lewis

    2007 The Japan Chemical Journal Forum and Wiley Periodicals, Inc.

    -2

    O

    O

    O

    O

    O

    O

    PP

    -1

    Scheme 1. Historical examples of chiral hexacoordinated phosphates.

    O

    O

    O

    O

    O

    O

    P

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    a,b

    Bu3NH

    Cl

    Cl

    Cl

    Cl

    OH

    OHrac-3

    O

    O

    O

    O

    O

    O

    P

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    O

    O

    O

    O

    O

    O

    P

    Cl ClBu3NH

    61%

    cinchonidinium

    Cl Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl-3 -3

    c

    Scheme 2. Synthesis and resolution of TRISPHAT anion 3. (A) PCl5 (0.33equiv), Ph-Me, 70C. (B) nBu3N, CH2Cl2/n-Hexane, 20C. (C) cinchonidine(0.50 equiv), CH2Cl2, 20C.

    HO

    HO

    Cl

    Cl

    Cl

    Cl

    PO

    O

    Cl

    Cl

    Cl

    Cl

    P

    O

    O

    O

    O

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    O

    OP

    O

    O

    O

    O

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    > 90%

    -4

    Me2N

    Me2N

    85%

    (S)-BINOL

    de > 96%

    a

    b

    Me2NH2

    5

    Scheme 3. One-pot synthesis of BINPHAT anion 4. (A) P(NMe2), toluene,reflux. (B) o-chloranil (1.0 equiv), BINOL (1.0 equiv), CH2Cl2, 20C.

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    basic nitrogen atom, can interact directly with metal centersand allow the stereocontrol of molecular events that previousnoncoordinating anions of the TRISPHAT family could notachieve. It can, for instance, control the absolute P or M con-figuration of axially chiral tropos ligands bound to metal centersor the configuration of metal centers themselves (diastere-omeric ratio up to 96 :4) (Scheme 6).17

    Successful NMR Chiral Solvating Agents

    As mentioned, chiral cations are involved in many areas ofchemistry and, unfortunately, only few methods are availableto determine with precision their optical purity. In the lastdecades, NMR has evolved as one of the methods of choicefor the measurement of the enantiomeric purity of chiral

    species.18 Over the past decade, we could demonstrate thatanions 3 and 4 are very effective NMR chiral solvating agents.6

    They form tightly associated diastereomeric ion pairs withchiral cations, and the short-range interactions that occur leadto efficient NMR enantiodifferentiations. An arbitrary selec-tion of six cationic systems (1217) that have been analyzedwith successout of a collection of more than 30 speciesisrepresented in Figure 1.

    The cationic moieties can be of organic, organometallic,or metalloorganic nature. The stereogenic elements can be ofcentral, axial, planar, or helical chirality. 1H, 13C, 15N, 19F, and31P NMR spectroscopy can be used in these studies (e.g., Figs. 2 and 3). TRISPHAT 3 is overall more efficient withcationic metallo-organic and organometallic substrates,19 whileBINPHAT 4 has often-superior chiral solvating propertieswhen associated with organic cations.12,20 Anions 3 and 4 canalso be used to determine the enantiomeric purity of planarchiral chromium and palladium complexes such as 18 and 19.This result broadens the fields of application of the anionicreagents to neutral species.21

    Very recently, two classes of original chiral objects were studied in conjunction of anions 3 or 4. One class is

    O

    OMe

    O O

    Ph

    O

    O

    O

    O

    P

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    O

    O

    O

    O

    O

    O

    O

    O

    P

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    RO2C

    RO2C

    O

    O

    O

    O

    O

    O

    P

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    O

    O

    O

    O

    O

    O

    P

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    N

    N

    6 -7

    8 9

    Cl

    Scheme 4. Functional mixed phosphate anions.

    O

    O

    O

    O

    O

    O

    P

    Cl

    Cl

    Cl

    Cl

    F

    Cl

    Cl

    Cl

    Cl

    Cl-10a

    O

    O

    O

    O

    O

    O

    P

    Cl Cl

    Cl

    FCl

    Cl

    Cl Cl-10b

    Scheme 5. Fluoro-phosphate anions for detailed structural NMR studies.

    O

    O

    O

    O

    O

    O

    P

    N

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl

    Cl Cl11

    Cl

    Scheme 6. Lewis basic phosphate anion.

    2

    NN

    NN

    N

    NRu

    P

    CD3

    CH3O

    MeN

    Me

    MeMe

    OMe

    Me

    Mn(CO)3

    S

    Ph

    MeMe

    12 14

    15 16

    O

    N N

    O

    Pr Pr

    13

    17

    Fig. 1. Some chiral cations analyzed efficiently with anions 3 or 4.

  • C h i r a l H e x a c o o r d i n a t e d P h o s p h a t e s

    279

    Efficient Resolving Agents

    The lipophilicity of TRISPHAT anion 3 confers to its salts anaffinity for organic solvents and, once dissolved, the ion pairsdo not partition in aqueous layers. This rather uncommonproperty was used to develop a practical resolution procedureof racemic cationic substrates by preferential extraction of oneenantiomer from water into immiscible organic solvents.[Ru(Me2bpy)3]2+ 28 and [Ru(Me2phen)3]2+ 29 were selected asracemic substrates for their ease of synthesis and high watersolubility as chloride salts (Me2bpy = 4,4-dimethyl-2,2-bipyridine; Me2phen = 4,7-dimethyl-1,10-phenanthroline).

    2007 The Japan Chemical Journal Forum and Wiley Periodicals, Inc.

    5.05.25.45.65.8 5.05.25.45.65.8

    H(3)H(6)

    (a)

    (b)

    (c)

    (d)

    18 19

    CH3

    SiMe3

    Cr(CO)3

    Pd

    Cr(CO)3

    N

    Me2NMeO

    MeO

    Fig. 2. 1H NMR spectra (parts, 400MHz, C6D6/2% DMSO-d6) of (A) rac-18; (B) rac-18 + 2.3 equiv of [n-Bu4N][-3]; (C) (+)-(1S)-18 (87% e.e.) + 3.7equiv of [n-Bu4N][-3]; (D) (+)-(1S)-18 (>99% e.e.) + 2.3 equiv of [n-Bu4N][-3].

    NN

    N

    N NN

    L

    1H

    [Co4(L)6(BF4)] [BF4]7

    (a)

    19F

    246245244243455055

    (b)

    21

    Fig. 3. 1H and 19F NMR spectra (5% CD3NO2 in CDCl3) of[Co4(L)6(BF4)][BF4]7 21 with (A) 0 and (B) 8.0 equiv of [Bu4N][-3].

    NN

    N

    N

    N

    N

    Ru

    Ru

    N

    N

    N

    N

    N

    Ru

    Ru

    Ru

    Ru

    6+

    N

    O O

    O O

    O O

    O O

    O O

    O O

    R

    R

    R

    R

    R

    R

    20

    Scheme 7. Chiral triangular metalloprisms.

    constituted of cationic triangular metalloprisms of type[Ru6(arene)6(tpt)2(C2O4)3]6+ 20 (arene = C6Me6 and p-iPrC6H4Me; tpt = 2,4,6-tripyridyl-1,3,5-triazine). Thanks tothe presence of anion 4, it was shown that the oxalato deriva-tives 20 possess a double helical chirality induced by (i) a twistof the tpt units and (ii) a concerted tilt of pyridyl moieties;something not so obvious at first sight (Scheme 7).22

    The second class of interesting derivatives is that of chiralpseudo-tetrahedral cationic cages of type [Co4(L)6(BF4)]7+ (21)made of four metal ions at each corner of an approximate tetra-hedron, and of six bis-bidentate bridging ligand L spanningeach edge. The central cavity is occupied by one tetrahedralcounterion (BF4) which is tightly bound and does notexchange with external anions on the NMR timescale. Thetetranuclear complexes are chiral, having T symmetry in solu-tion with all four metal centers in each complex having thesame tris-chelate configuration. Addition of enantiopureanions -3 to complexes 21 led to the enantiodifferentiationof the ligands of the racemic salts and, more interestingly, ofthe achiral tetrafluoroborate anion trapped inside in the cage.The discrimination of the inside BF4 anion was even easierthan that of the surrounding chiral cages ( 2.0 and 0.4ppmin 19F and 1H NMR, respectively).23

    Several reports independent from our group have con-firmed the efficiency of these NMR chiral solvating agents andthose of Amouri, Andraud, Cordier, Gruselle, LeBozec,Lemercier, Maury, Mikami, Nitschke, Rose-Munch,...