Norbornenes, hydrosilylation

Very recently, the yttrium hydride [2,2 -bis(tert-butyldimethylsilylamido)-6,6 -di-methylbiphenyl]YH(THF) 2 (36), conveniently generated in situ from [2,2 -bis(tert-butyldimethylsilylamido)-6,6 -dimethylbiphenyl]YMe(THF)2 (35) demonstrated its high catalytic activity in olefin hydrosilylation. This system represents the first use of a d° metal complex with non-Cp ligands for the catalytic hydrosilylation of olefins. Hydrosilylation of norbornene with PhSiHs gave the corresponding product (37) of 90% ee (Scheme 3-15) [43]. 

The Pd/MOP combination has proved active for the asymmetric hydrosilylation of cyclic alkenes and dienes. Thus treatment of norbornene with HSiCl3 at 0 °C for 24 h in the presence of 0.01 mol.% of M eO-MOIV[Pd(// -C3H5)Cl]2 gave quantitative yield of evo-2-(trichlorosilyl)norbornane oxidation produced the corresponding alcohol in 93% ee (Equation (12)). Lowering the temperature (to — 20 °C) increased this to a 96% ee. Both mono- and difunctionalization of nbd has proved possible, depending upon the quantity of trichlorosilane used (Scheme 22). In both reactions, extremely good enantioselectivities are observed 113... 

Table 6 Palladium-catalyzed asymmetric hydrosilylation of norbornene 64... 

Although it is not a catalytic asymmetric hydrosilylation, chirality transfer was reported in the palladium-catalyzed addition of an enantiomerically enriched hydrosilane to norbornene.76... 

Norbomanol with 96% ee was obtained by the asymmetric hydrosilylation of norbomene. Monofunctionalization of norbomadiene (565) was achieved with high chemo- and enantioselectivities to give uro-2-trich 1 orosilyl-5-norbornene (566) with one equivalent of trichlorosilane and converted to the alcohol 567 with 95% ee. With 2.5 equivalents, the chiral disilylnorbomane 568 was obtained rather than the me so isomer 569 (18 1). The disilylated product 568 was converted to the diol 570 with 99% ee [218],... 

Palladium-catalysed hydrosilylation of reactive alkenes, such as norbornene, with silanes R3SiH, chiral at the silicon atom, has been reported to occur with asymmetric amplification.80... 

The hydrosilylation-oxidation of simple unfunctionalized alkenes has not been widely used for the diastereoselective preparation of alcohols, probably because it would not in general be expected to give very different results to hydroboration oxidation. One example which has been reported is the exo-selective hydrosilylation of norbornene (1) with trichlorosilane and hexachloroplatinic acid6, followed by oxidation to c.vo-norbornanol (2)1. 

Early work involving simple alkenes employed the catalyst dichloro (y )-AUV-dimethyl-l-[(S)-2-(diphenylphosphino)ferrocenyl]ethylamine palladium(II), 1 (often abbreviated to Pd-Cl2[(i )-(S)-PPFA]). When applied to the hydrosilylation of norbornene with trichlorosilane, it yielded an adduct which was converted to a pentafluorosilicate and thence to the alcohol 2 in 25% overall yield and with 50% ee26. 

Asymmetric Hydrosilylation of Alkenes. The palladium complex PdCl2[(/ )-(5)-PPFA] catalyzes the asymmetric hydrosilylation of norbornene, styrene, and 1,3-dienes (eq 3). The hydrosilylation of 1-phenyl-1,3-butadiene with Trichlorosilane proceeds regioselectively in a 1,4-fashion to give (Z)-1-phenyl-1-silyl-2-butene of 64% ee. 

Few examples of asymmetric hydrosilylations of styrene derivatives with chiral lan-thanocene catalysts have been reported (13) [29], The cyclopentadienyl-free chiral yttrium diamidobiphenyl complex 8 was used for asymmetric hydrosilylation of norbornene with high enantioselectivity (14) [53],... 

Asymmetric synthesis through a selective monofunctionalization of enantiotop-ic positions is considered as being one of the most attractive strategies for the one-step construction of multiple chiral carbon centers [34,35]. Asymmetric hydrosilylation of norbornene (21) was first attempted by use of a palladium catalyst coordinated with ferrocenylmonophosphine, (1 )-(S)-PPFA (15a) [28]. The... 

Representative alkene hydrosilylation reactions carried out with the MOP ligands are identified in the following schemes. The low catalyst loading in many cases is noteworthy. Norbornene (2.145), dihydrofuran (2.146), styrene (2.147) and cy-clopentadiene (2.148) are all converted into the hydrosilylated derivatives with good to excellent enantioselectivity. In most cases, the trichlorosilanes were deriva-tised into the corresponding alcohols, although the trichloroallylsilane (2.152) was reacted with benzaldehyde to give the addition product (2.156). 

Asymmetric hydrosilylation of norbornene (2). This reaction can be effected in high regio- and enantioselectivity by reaction of 2 with ClySiH catalyzed by (allyl)chloropalladium dimer complexed with (R)-l. The product can be converted to (lS,2S,4R)-norbornanol (4) in 96% ee. 

Rhodium-catalysed intramolecular hydrosilylation of homoallyl silyl ethers (52), mediated by norbornene, has been shown to be controlled by ligands thus, with bis(diphenylphosphino)hexane (dpph), the reaction produces oxasilacyclohexanes (53), whereas frani-oxasilacyclopentanes (54) are formed in the presence of BINAR Norbornene is believed to serve as a hydride shuttle. " ... 

Hydrosilylation of norbornadiene (nbd) by chloro- or alkyl-hydrosilanes in the presence of Pd- or Mo-catalysts [178, 179] led to the corresponding silylnor-bornenes. Norbornenes with ethane spacer between bicyclic nuclear and silicon atom can be synthesized by hydrosilylation of 5-vinyl-2-norbomene [180],... 

Asymmetric hydrosilylation of alkenes, catalysed by ferrocenylphosphine-palladium complexes, can be utilized in a synthesis of optically active alcohols from alkenes (Scheme 1). The initial optically active silane adducts are converted to alkyl pentafluorosilicates (1) which are then cleaved oxidatively, with retention of configuration, using peracid (c/. 3,132). Complex (2) was the best of several hydrosilylation catalysts examined using this, alcohols with enantiomeric purities of approx. 50% were obtained from norbornene and styrene as the alkenes. 

The performance of normal and abnormal carbene eomplexes as in situ formed hydrosilylation catalysts was investigated by using [Pt(nbe)3] (nbe = norbornene) and the imidazolium salts 61 and 62 containing a... 

Pioda G, Togni A (1998) Highly enantioselectiye palladium-catalyzed hydrosilylation of norbornene with trichlorosilane using ferrocenyl ligands. Tetrahedron Asym 9 3903-3910... 

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