Unimolecular net heterolysis of symmetric and homopolar σ-bonds.

ABSTRACT

The unimolecular heterolysis of covalent σ-bonds is integral to many chemical transformations, including SN1-, E1- and 1,2-migration reactions. To a first approximation, the unequal redistribution of electron density during bond heterolysis is governed by the difference in polarity of the two departing bonding partners1-3. This means that if a σ-bond consists of two identical groups (that is, symmetric σ-bonds), its unimolecular fission from the S0, S1, or T1 states only occurs homolytically after thermal or photochemical activation1-7. To force symmetric σ-bonds into heterolytic manifolds, co-activation by bimolecular noncovalent interactions is necessary4. These tactics are only applicable to σ-bond constituents susceptible to such polarizing effects, and often suffer from inefficient chemoselectivity in polyfunctional molecules. Here we report the net heterolysis of symmetric and homopolar σ-bonds (that is, those with similar electronegativity and equal leaving group ability3) by means of stimulated doublet-doublet electron transfer (SDET). As exemplified by Se-Se and C-Se σ-bonds, symmetric and homopolar bonds initially undergo thermal homolysis, followed by photochemically SDET, eventually leading to net heterolysis. Two key factors make this process feasible and synthetically valuable (1) photoexcitation probably occurs in only one of the incipient radical pair members, thus leading to coincidental symmetry breaking8 and consequently net heterolysis even of symmetric σ-bonds. (2) If non-identical radicals are formed, each radical may be excited at different wavelengths, thus rendering the net heterolysis highly chemospecific and orthogonal to conventional heterolyses. This feature is demonstrated in a series of atypical SN1 reactions, in which selenides show SDET-induced nucleofugalities3 rivalling those of more electronegative halides or diazoniums.