Organometallic reagents for catalytic cross-coupling
Phosphine complexes of nickel and palladium provide the best catalysts for the homogeneous catalysed carbon-carbon bond forming reaction between an organometallic nucleophile and an organic electrophile. Use of a homochiral ligand on the catalyst can lead to stereoselectivity in the cross-coupling reaction, with high ee's of coupled product being obtained. The processes of selectivity in the transmetalation step of the catalytic cycle have not been elucidated and the initial aim of the project was to study these processes. Initial experiments using organotin derivatives as the organometallic nucleophile highlighted the problems of selectivity and the forcing conditions needed in the attempted transfer of a benzyl group from the tin to the palladium catalyst. The compounds [8- (dimethylamino)-1-naphthyl]methyldiphenyltin (60) and [2-((dimethylamino)methyl) phenyl]methyldiphenyltin (70) were prepared and their reactivity in the palladium catalysed cross-coupling with 2-furoyl chloride, to give 2-benzoylfuran, was investigated. These molecules were found to undergo facilitated transfer of a phenyl group compared to transfer from control molecules. The effect was tested and attributed to the internal nucleophilic attack at the tin atom by the lone pair on the nitrogen atom. The compound [2- ((dimethylamino)methyl)-3-trimethylsilylphenyl]methyldiphenyltin (79) was prepared to test the effects of steric buttressing within the molecule, but was found to be of the same order of magnitude of reactivity as (60) and (70). All three molecules showed a reactivity of at least an order of magnitude greater than control compounds. The effect did not prove applicable to the transfer of a benzyl group or in other coupling reactions. The effect of palladium ligation was tested and the ligand triphenyl arsine found to increase the rate of the coupling reaction. The two facilitating processes were found to work in a co-operative fashion, giving a rate enhancement of ca. one hundredfold over control reactions. The nickel catalysed cross-coupling of α-substituted sp3 hybridised Grignard reagents with allylic esters was attempted, but proved unsuccessful. Stoichiometric reactions with nickel complexes were also found to be unsatisfactory in yielding coupled products. The synthesis of α-substituted diorganozinc reagents was attempted, but furnished only homocoupled products. The reaction of dibenzylzinc with aldehydes was found to proceed in the absence of catalyst, highlighting the reactivity of a benzylzinc moiety.