Chiral metal complexes hydrosilylation

Reactions of 2-alkenes, 3-alkenes, etc., with monohydrosilanes lead predominantly to alkylsilanes with terminal silyl group, which means that in the presence of transition metal complexes, hydrosilylation is accompanied by the isomerization of olefins. The formation of adducts with an internal silyl group is also possible, especially in the presence of chiral platinum and palladium complexes (8). 

The discussion of the activation of bonds containing a group 15 element is continued in chapter five. D.K. Wicht and D.S. Glueck discuss the addition of phosphines, R2P-H, phosphites, (R0)2P(=0)H, and phosphine oxides R2P(=0)H to unsaturated substrates. Although the addition of P-H bonds can be sometimes achieved directly, the transition metal-catalyzed reaction is usually faster and may proceed with a different stereochemistry. As in hydrosilylations, palladium and platinum complexes are frequently employed as catalyst precursors for P-H additions to unsaturated hydrocarbons, but (chiral) lanthanide complexes were used with great success for the (enantioselective) addition to heteropolar double bond systems, such as aldehydes and imines whereby pharmaceutically valuable a-hydroxy or a-amino phosphonates were obtained efficiently. 

Owing to the high Lewis acidity the group 14 organometallic cations are polymerization catalysts par excellence. so Silanorbonyl cations and triethylsilyl arenium have been shown to be efficient catalysts for metal-free hydrosilylation reactions. Chiral silyl cation complexes with acetonitrile have been applied as cata -lysts in Diels Alder-type cyclization reactions °792 intramolecularly stabilized tetracoordinated silyl cations have been successfully used as efficient catalysts in Mukaiyama-type aldol reactions. 

Alkenes. Most Group VIII metals, metal salts, and complexes may be used as catalyst in hydrosilylation of alkenes. Platinum and its derivatives show the highest activity. Rhodium, nickel, and palladium complexes, although less active, may exhibit unique selectivities. The addition is exothermic and it is usually performed without a solvent. Transition-metal complexes with chiral ligands may be employed in asymmetric hydrosilylation 406,422... 

It should be pointed out that asymmetric reactions other than hydrogenation have been carried out with chiral phosphine complexes of rhodium (and a few other metals). For example, asymmetric hydrosilylations (addition of Si—H across C=C, C=0, and C=N bonds) have been catalyzed by such complexes... 

Addition of the elements of Si—H to a carbonyl group produces silyl ethers which are synthetically equivalent to chiral secondary alcohols since the silyl groups are easily hydrolyzed. Hydrosilylation can be catalyzed by acids or transition metal complexes. Enantioselective hydrosilylation of prochiral ketones has been extensively studied using platinum or rhodium complexes possessing chiral ligands such as BMPP (86), DIOP (87), NORPHOS (88), PYTHIA (89) and PYBOX (90)." ... 

Many metal complexes with chiral ligands such as phosphines, phosphites, or imidates will catalyze asymmetric hydrogenations, hydrosilylations, and the like. Extremely high enantioselectivities have been achieved with a wide variety of substrates. 

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. 

This review deals with recent advances in catalytic asymmetric hydrosilylation of olefins, carbonyl and imino compounds in the presence of transition metal complexes of chiral phosphine ligands with particular emphasis on the asymmetric reduction of prochiral carbonyl compounds, which has been extensively studied in the last few years by several research groups and proved to provide an effective reduction method for organic syntheses. 

In the same year, a series of Ci-symmetric chiral triazolium Pd(II) complexes were prepared by Enders et al. As typically reported with the use of Ci-symmet-ric ligands, their NHC-metal complexes were obtained as diastereomeric mixtures due to the restricted rotation around the carbene-metal bond [6]. Without further elaboration, the authors stated that these complexes were used in an enantioselective Heck-type reaction achieving low asymmetric inductions. Soon thereafter, the authors investigated the coordination behavior of chiral triazolium salts 35 with [Rh(COD)Cl]2 and obtained a mixture of axially chiral complexes 36 with a diastereomeric excess of up to 94% (Scheme 3.20). These complexes were used as catalysts in asymmetric hydrosilylation reactions, achieving up to 44% ee for aromatic and ahphatic ketones [38,39]. 

Hydrosilanes undergo addition to carbon-carbon multiple bonds under catalysis by transition metal complexes. Nickel, rhodium, palladium, and platinum were used as catalytically active metals. By incorporating chiral ligands into the metal catalyst, the hydrosilylation can be performed analogously to other addition reactions with double bonds, for example, asymmetric hydrogenation to obtain optically active alkylsilanes. 

---