Leaving groups of the coupling partner

Coupling of PhSiF3, Ph2SiF2, or Ph3SiF with 4-/er/-butyl-1 -cyclohexenyl triflate afforded 4-terAbutyl-l-phenylcyclohexene in 57% (2.5 h), 62% (2.5 h), or 0% (24 h) yield, respectively. Here again, the difluoro, i y group was the most effective [26]. 

Furthermore, 4-acetylphenyl triflate could couple with alkyltrifluorosilane [Eqs.(30)] [26]. 

There has been controversy on the stereochemistry of the transmetallation of alkylmetals. Transmetallation from Hg to Pd is reported to proceed in THF with retention of configuration [27]. By contrast, cross-coupling reactions of organotin reagents are assumed to take place with inversion in HMPA [28] but with retention in toluene [29]. We consider that the organosilicon-based coupling reaction would be a useful model for examining the stereochemistry of the transmetallation process. 

When (S )-l-phenylethyltrifluorosilane of 34% ee was allowed to react with 4-acetylphenyl triflate (a), optically active I-phenyl-l-(4-acetylphenyl)ethane was isolated. At 50 °C, almost complete retention of configuration resulted, but at higher temperatures the degree of retention dropped linearly and finally inversion resulted above 75 °C (a in Fig. 10-1). A similar temperature dependency was also observed in the case of 3-formylphenyl triflate (b) [30]. 

A significant solvent effect was observed. The reaction of 4-acetylphenyl triflate with PhCH(Me)SiF3 (38% ee, S) at 60 °C resulted in retention (31% ee, S) in THF, but inversion (8% ee, R) in HMPA-THF (1 20). In DMF-THF (1 10) or DMSO-THF (1 10) retention (16% S in both cases) was still observed, but at a low level. Thus, in a nonpolar solvent, retention of configuration resulted, whereas in a polar solvent system the reaction proceeded by inversion [30]. 

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