Cross-coupling reactions stereochemistry

The formation of a cyclic siloxane obtained by intramolecular hydrosilylation has been used to control the stereochemistry of a cross-coupling reaction (Equation (18)) 79... 

Bis(pinacolato)diboron 8 also reacts with 1-halo-l-lithioalkenes 10 (available from 1,1-dihaloalkenes or 1-haloalkenes by halogen-metal exchange or metalation reaction, respectively) to afford 1,1-bismetalated dienes 11. These are readily converted into polysubstituted dienes through various transition metal-catalyzed carbon-carbon bond formations (Scheme 4) [29]. However, these seemingly attractive methods must not hide the problems of the stereochemistry and regiospecificity of the subsequent cross-coupling reactions. 

The kind of a fluoride ion activator and the leaving group in electrophiles affects the stereochemistry in the cross-coupling reaction of allylsilanes as exemplified with 2-cyclohexenyl(difluoro)phenylsilane (Eq. 31) 135]. 

The Mechanism of the cross coupling reaction can be accommodated by an oxidative addition of 1-bromopropene to iron(l) followed by exchange with ethylmagnesium bromide and reductive elimination. Scheme 3 is intended to form a basis for discussion and further study of the catalytic mechanism. In order to maintain the stereospecificity, the oxidative addition of bromo-propene in step a should occur with retention. Similar stereochemistry has been observed in oxidative additions of platinum(O) and nickel(O) complexes.(32)(33) The metathesis of the iron(lll) intermediate in step b is ixp icted to be rapid in analogy with other alkylations.(34) The formation of a new carbon-carbon bond by the redilcTive elimination of a pair of carbon-centered ligands in step c has been demonstrated to occur... 

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. 

As the overall cross-coupling reaction proceeds with inversion of stereochemistry and reductive ehmination is well known to undergo retention of stereochemistry, the result imphes that transmetaUation in this reaction proceeds predominantly with retention of stereochemistry. In addition to this study, in 1998, Woerpel and Soderquist [102] independently studied the stereochemistry of transmetaUation for the Suzuki-Miyaura cross-coupling reactions of alkyl boranes with aryl or alkenyl hahdes. Their deuterium labehng study revealed that the transmetaUation of alkyl boranes 163 or 166 proceeds with retention of stereochemistry to give products 165 or 167. Soderquist proposed a closed four-membered cyclic transition state 168 to account for the retention of stereochemistry observed during the reaction. 

The cross-coupling reaction of an optically active alkylsilane gives us valuable information on the stereochemistry in transmetallation. The reaction of (5)-l-phenyl-1-(trifluorosilyl)ethane (34% ee) with 4-acetylphenyl triflate in the presence of 5 mol % of Pd(PPh3>4 and TBAF (2 equiv) at 50 °C gave (5)-1-phenyl-l-(4-acetylphenyl)ethane of 33% ee with nearly complete retention of configuration. At higher temperatures, ee of the product decreased linearly, and above 75 °C inversion of configuration predominated (Scheme 26). Similar temperature dependency was observed also in the reaction of 3-formylphenyl triflate. 

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