Spectroscopic and product studies of the effect of para substituents on the reactivity of triplet bis(2,6-dimethylphenyl)carbenes

A series of diazobis(2,6-dimethylphenyl)methanes (1) bearing eight symmetrical para di-substituents have been prepared and photolyzed to generate the corresponding carbenes (2). Product analysis studies showed that carbenes (2) decay mainly either by dimerization to form tetra(aryl)ethylene (3) or by attack at an o-methyl group to afford 1,2-dihydrobenzocyclobutenes (4) by way of o-quinodimethanes (6) in solution. The zero-field splitting parameters, D and E, were measured in matrices of different viscosities and are analyzed in terms of a sigma-dot (sigma*) scale of spin-delocalization substituent constants. Fairly good correlation with sigma* was found for the D values of (3)2 in its minimum energy geometry. Stabilities of (3)2 were estimated either by measuring the temperature at which the triplet carbene signals disappeared upon thawing the matrix or by analyzing the decay kinetics of (3)2 in a degassed solution at room temperature. They are examined in terms of the D values in matrix at low temperature and in terms of product distributions in solution at room temperature.