Reaction Mechanism of Transmetalation between Tetraorganostannanes and Platinum(II) Aryltriflate Complexes. Mechanistic Model for Stille Couplings

The complexes trans-[PtPh(OTf)(PMe2Ph)2] (1) and trans-[PtPh(OTf)(PPh3)2] (4) were synthesized
from the corresponding chloro complexes in moderate yields. Complex 1 is slowly hydrolyzed in solution,
giving the dinuclear hydroxo-bridged complex [Pt(μ-OH)(PMe2Ph)2]2(CF3SO3)2 (6), which was characterized by X-ray crystallography. In solution 1 and 4 undergo fast solvolysis to give the corresponding
solvento cations. The reactivities of 1 and 4 with Me3SnPh were investigated in different solvents, and
with 1 two products, trans-[PtPh2(PMe2Ph)2] (8) and cis-[PtPh2(PMe2Ph)2] (9), were always formed
simultaneously. In THF an intermediate, trans-[PtPhMe(PMe2Ph)2] (14), was characterized on the path
to 9. The kinetics for these reactions were evaluated numerically, and on the basis of rate laws, activation
parameters, and reactivity trends a mechanism involving parallel equilibria to 8 and 9 with associative
activation in all steps is proposed. In this mechanism the initial attack always takes place trans to the
phenyl group, giving 8 and 14, respectively, via an open transition state. In a subsequent reaction, 14
reacts with another molecule of stannane, giving 9 as the final product via a cyclic transition state. Complex
4 gives exclusive formation of cis products:  cis-[PtPhMe(PPh3)2] (12) and cis-[PtPh2(PPh3)2] (13). These
results are discussed in relation to the reaction mechanism of the transmetalation step in the Stille reaction.