Selective Catalytic Styrene Dimerization through the Combined Action of a Strong Sn(IV)-Porphyrin Lewis Acid and a Weak Brønsted Base

Complexes of the form Sn(TPP)X₂ (where TPP^2- = tetraphenylporphyrinate and X^- = Cl^-, OTf^-, ClO₄^-, and NTf₂^-) were investigated as catalysts for the activation of the nonpolar C═C bond of styrene. Sn(TPP)(OTf)₂ was able to dimerize styrene with 100% selectivity for 1,3-diphenyl-1-butene. Other catalysts based on p-block metals have been used to activate styrene in the past, but they produced mixtures of oligomerization products. Pd catalysts have been used to selectively yield 1,3-diphenyl-1-butene, and prior mechanistic studies suggest that those reactions proceed through a hydride transfer mechanism. In this present work, our data, particularly those from experiments with isotopically labeled molecules, suggest a mechanism that requires the combined action of the strong Lewis acidity of a transiently formed [Sn(TPP)(OTf)]⁺ and the weak Brønsted basicity of free OTf^- to perform a rate limiting deprotonation step. The reaction does not proceed when X^- = ClO₄^- or NTf₂^- suggesting that OTf^- has a Brønsted basicity just strong enough to perform a deprotonation of an activated C-H bond while still being a weak enough Lewis base to easily dissociate from the Sn center.