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

High Resolution Image Download MS PowerPoint Slide Complexes of the form Sn(TPP)X 2 (where TPP 2– = tetraphenylporphyrinate and X – = Cl –, OTf –, ClO 4 –, and NTf 2 – ) 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 – = ClO 4 – or NTf 2 – 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.