Asymmetric catalysis hydrosilylation

A review8 with more than 186 references discusses the synthesis of Rh and Pd complexes with optically active P,N-bidentate ligands and their applications in homogeneous asymmetric catalysis. The effect of the nature of the P,N-bidentate compounds on the structure of the metal complexes and on enantioselectivity in catalysis was examined. Allylic substitution, cross-coup-ling, hydroboration and hydrosilylation catalyzed by Rh or Pd complexes with optically active P,N-bidentate ligands are considered. 

Nishiyama H (1999) Hydrosilylation of carbonyl and imino groups. In Jacobsen EN, Pfaltz A, Yamamoto H (eds) Comprehensive asymmetric catalysis, vol 1, chap 6.3. Springer, Berlin Heidelberg New York... 

The chiral organolanthanides have been especially designed for asymmetric catalysis. Thus far several enantioselective olefin transformations (hydrogenation, hydroamination/cyclization, hydrosilylation) as well as the polymerization of methyl methacrylate mediated by these chiral organolanthanide metallocenes have been investigated. 

The third part of this chapter reviews previously described catalytic asymmetric reactions that can be promoted by chiral lanthanoid complexes. Transformations such as Diels-Alder reactions, Mukaiyama aldol reactions, several types of reductions, Michael addition reactions, hydrosilylations, and hydroaminations proceed under asymmetric catalysis in the presence of chiral lanthanoid complexes. 

First tests of the ligand in the hydrosilylation of ketones catalyzed by [(41)RhI(COD)] only yielded racemic mixtures of the secondary alcohols, and no further application in asymmetric catalysis of 41 has been reported to date. 

Labande et al. tested the rhodium(I) complexes of diphenylphosphinoferrocenyl functionalised NHC ligands (Cp,Cp and Cp,Cp substitution) in the hydrosilylation of ketones finding these complexes of only moderate activity [186]. As no attempt was made for the chiral resolution of the catalysts prior to use in catalysis, the prochiral acetophenone could not be tested in asymmetric catalysis. 

The asymmetric catalytic reduction of ketones (R2C=0) and imines (R2C=NR) with certain organohydrosilanes and transition-metal catalysts is named hydrosilylation and has been recognized as a versatile method providing optically active secondary alcohols and primary or secondary amines (Scheme 1) [1]. In this decade, high enantioselectivity over 90% has been realized by several catalytic systems [2,3]. Therefore the hydrosilylation can achieve a sufficient level to be a preparative method for the asymmetric reduction of double bond substrates. In addition, the manipulative feasibility of the catalytic hydrosilylation has played a major role as a probe reaction of asymmetric catalysis, so that the potential of newly designed chiral ligands and catalysts can be continuously scrutinized. 

Our results demonstrate that during the course of a heterogeneous hydrosilylation, rhodium can move on the support (if free phosphino groups are still available on the support). It is very probable that part of the reaction could occur with soluble species. In the case of asymmetric catalysis, they are achiral and will lower the optical yield. It also is not clear at present if in homogeneous hydrosilylation similar phenomena do occur, which would increase the number of catalytically actives species. 

Especially noteworthy is the field of asymmetric catalysis. Asymmetric catalytic reactions with transition metal complexes are used advantageously for hydrogenation, cyclization, codimerization, alkylation, epoxidation, hydroformylation, hydroesterification, hydrosilylation, hydrocyanation, and isomerization. In many cases, even higher regio- and stereoselectivities are required. Fundamental investigations of the mechanism of chirality transfer are also of interest. New chiral ligands that are suitable for catalytic processes are needed. 

Asymmetric catalysis Initial attempts to develop non-racemic catalysts for the enantioselective hydrosilylation of alkenes have not been successful with C2-symmetric (5)-Ph-pybox and p-diketiminate calcium amide complexes catalyzing the addition of phenylsilane to styrene in the absence of solvent at 50 °C in, at best, 9 % e.e [69]. As with intramolecular hydroaminatiOTi catalysis, these results have been explained in terms of the loss of ligand control due to facile ligand redistribution reactions under the catalytic reaction conditions. 

Chiral Phenanthrolines in Asymmetric Catalysis. Synthetic modifications on the Phenanthroline core can provide access to highly valuable chiral phenanthroline derivatives with important applications in asymmetric catalysis (Scheme 1). For example, LI has been utilized in copper-catalyzed allylic oxidations, L2 in palladium-catalyzed allylation reactions, and L3-type ligands in rhodium-catalyzed enantioselective hydrosilylation reactions of acetophenone. ... 

Cyano compounds liquid crystals, 12, 278 in silver(III) complexes, 2, 241 Cyanocuprates, with copper, 2, 186 Cyano derivatives, a-arylation, 1, 361 Cyanosilanes, applications, 9, 322 Cyclic acetals, and Grignard reagent reactivity, 9, 53 Cyclic alkenes, asymmetric hydrosilylation, 10, 830 Cyclic alkynes, strained, with platinum, 8, 644 Cyclic allyl boronates, preparation, 9, 196 Cyclic allylic esters, alkylation, 11, 91 Cyclic amides, ring-opening polymerization, via lanthanide catalysis, 4, 145... 

Michael-aldol reaction as an alternative to the Morita-Baylis-Hillman reaction 14 recent results in conjugate addition of nitroalkanes to electron-poor alkenes 15 asymmetric cyclopropanation of chiral (l-phosphoryl)vinyl sulfoxides 16 synthetic methodology using tertiary phosphines as nucleophilic catalysts in combination with allenoates or 2-alkynoates 17 recent advances in the transition metal-catalysed asymmetric hydrosilylation of ketones, imines, and electrophilic C=C bonds 18 Michael additions catalysed by transition metals and lanthanide species 19 recent progress in asymmetric organocatalysis, including the aldol reaction, Mannich reaction, Michael addition, cycloadditions, allylation, epoxidation, and phase-transfer catalysis 20 and nucleophilic phosphine organocatalysis.21... 

As pointed out in the introduction, a particular feature of hydrosilylation reactions is that they require catalysis. Arguably the most valuable of enantioselective synthetic methods are those in which asymmetric induction occurs from small quantities of enantiomerically pure catalysts. It is natural, therefore, that considerable effort has been directed towards the catalytic enantioselective hydrosilylation-oxidation of C —C double bonds. Some degree of success has been met in the hydrosilylation of simple alkenes and 1,3-dienes, and in intramolecular hydrosilyla-tions. Also, as discussed at end of this section, a catalytic enantioselective disilylation (effectively the same as a hydrosilylation) has been developed for a,)3-unsaturated ketones. 

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