Silyl boryl complexes

Cross-coupling to form carbon heteroatom bonds occurs by oxidative addition of an organic halide, generation of an aryl- or vinylpalladium amido, alkoxo, tholato, phosphido, silyl, stannyl, germyl, or boryl complex, and reductive elimination (Scheme 2). The relative rates and thermodynamics of the individual steps and the precise structure of the intermediates depend on the substrate and catalyst. A full discussion of the mechanism for each type of substrate and each catalyst is beyond the scope of this review. However, a series of reviews and primary literature has begun to provide information on the overall catalytic process.18,19,22,23,77,186... 

In relation to the mechanistic proposal, an interesting reactivity of (boryl)(silyl)platinum(n) complex has been reported.223 The complex is prepared by the reaction of silylborane with Pt(cod)2 complex via oxidative addition (Scheme 46). The (boryl)(silyl)platinum complex undergoes insertion of alkynes at the B-Pt bond to give (/3-borylalkenyl)(silyl)platinum(n) complex in high yield. Importantly, the insertion takes place regioselectively, with Pt-G bond formation at the internal. -carbon atom. This result may indicate that the boron-transition metal bond is more prone to undergo insertion of unsaturated molecules. 

The use of these boryl complexes in catalytic, enantioselective additions to aldehydes by silyl ketene acetals has also been the subject of intense investigation by Yamamoto (Eq. 30) [108]. Although ethyl and benzyl acetate-derived enol silanes furnished racemic products, the phenyl acetate-derived trimethylsilyl ketene acetals proved optimal, giving adducts in up to 84% ee. Additionally, Yamamoto has documented the use of 184 in aldol addition reactions of propionate- and isobutyrate-derived enol silanes (Eqs. 31 and 32). Thus, the addition of the phenyl acetate derived (E)-enol silane afforded adducts as diastereomeric mixtures with the syn stereoisomer displaying up to 97% ee (Eq. 32). 

Fig. 22 Mesityloxyboryl complexes 8.55-8.59 (v) Silyl- and Germyl(halo)boryl Complexes...

Reaction of alkenes with boryl and silyl groups c/r-located at the C=C bond with alcohols is known to proceed in two different ways <1973S309, 1986JOM(310)151> by protolytic cleavage of the B-C bond and by complexation... 

The isomerization of multifunctionalized allyl silyl or allyl boryl ethers is efficiently catalyzed by a ruthenium complex in high yield (Scheme 38).23,24... 

Fig. 28 Rhodium complex 9.16 a system containing boryl, silyl and hydride ligands...

Addition of the silicon-boron bond aCTOss carbon-carbon triple bonds (i.e., silabora-tion) is most effectively catalyzed by a paUadium(0)-t rt-alkyl isocyanide complex to give (Z)-l-boryl-2-silyl alkenes with high regio- and stereoselectivity, which are useful for synthesis of stereodefined alkenylsilanes. 

More recently, a copper-bifluoride complex has been described, starting once again from the alkoxide species. Subsequently, [Cu(NHC)2] [HF2] complexes were developed (Figure 8.2) [91]. Both complexes were highly air stable in the solid state and moderately stable in solution compared to their analog [Cu(F) (IPr)] [92]. Remarkably, catalytic activity was observed in several transformations such as the reduction of ketones, the 1,4-conjugated borylation and silylation, and the diastereoselective allylation. 

Isocyanides, which have carbene-like divalent carbon atoms, also insert into Si-B bond of 1 in the absence of catalysts to give (boryl)(silyl)iminomethanes, which are isolable after conversion to borane-imine complex (eq 40). ... 

---