Regiocontrolled Hydrosilation of .alpha.,.beta.-Unsaturated Carbonyl Compounds Catalyzed by Hydridotetrakis(triphenylphosphine)rhodium(I)

Electrophilic Activation of Silicon–Hydrogen Bonds in Catalytic Hydrosilations

“…For example, a mechanism proposed in 1995 involves the coordination of a ketone substrate to the silicon center of a [Rh]-SiR2H ligand to generate an intermediate featuring a 5-coordinate silicon center. [24] This particular mechanism has not been experimentally supported, but some of its key features have since been implicated in other electrophilic hydrosilation mechanisms. For example, transition metal silylene complexes bind and activate substrates at the metal-bound silicon center, which has Lewis acidic character (Scheme 2, B).…”

“…The Zheng-Chan mechanistic proposal involves coordination of the ketone to the silicon center of a silyl ligand to generate a 5-cooridinate silicon center. [24] When utilizing a secondary silane, the 5-coordinate silicon intermediate would possess an Si-H bond, and it was proposed that the carbonyl group could insert into this Si-H bond (Scheme 48). The availability of this mechanism was proposed to account for rate accelerations observed using secondary silanes.…”

“…In this regard, the silylene mechanism for alkene hydrosilation is reminiscent of the Zheng-Chan mechanism for ketone hydrosilation, which proposed insertion of a ketone substrate into a terminal Si-H bond. [24] Subsequent studies have observed this type of reactivity in stoichiometric transformations of the silylene complex 42 and ketones such as benzophenone (Scheme 53). [85d] Note that this latter reactivity was confirmed to involve direct insertion into the Si-H bond of 42 since analogues of 42 that lack a free Si-H bond (i.e.…”

Dirhodium(II) Tetrakis(perfluorobutyrate)-Catalyzed 1,4-Hydrosilylation of .ALPHA.,.BETA.-Unsaturated Carbonyl Compounds

“…1,2) The catalytic 1,4-hydrosilylation of a,b-unsaturated carbonyl compounds is generally recognized as a powerful method for the direct and regioselective synthesis of silyl enol ethers. Consequently, a number of transition metal complexes based on rhodium, [3][4][5] platinum, 6) and copper 7) as well as Lewis acids such as B(C 6 F 5 ) 3 8) have been developed for that purpose. Aside from their effectiveness in diazo decomposition, 9,10) dirhodium(II) complexes are also recognized as Lewis acid catalysts 11,12) as well as catalysts for hydrosilylation of terminal alkynes, 13) terminal alkenes, 14) and enamides, 15) silylformylation of terminal alkynes, 16) and silane alcoholysis.…”

“…Although other solvents such as benzene, toluene, and THF were also suitable, the reaction times in these solvents were extended (entries 3-5). Using dichloromethane as the solvent, we next evaluated the performance of other dirhodium(II) complexes, Rh 2 (OAc) 4 (1b), Rh 2 (oct) 4 (1c), Rh 2 (tpa) 4 (1d), and Rh 2 (cap) 4 (1e) (entries 6-9). Although these dirhodium(II) complexes could also be used for this process, 0.1 mol% of catalyst was required for completion of the reaction.…”