Mechanism of the Solution-Phase Reaction of Alkyl Sulfides with Atomic Hydrogen. Reduction via a 9-S-3 Radical Intermediate.

The low selectivity of benzyl alkyl sulfide fragmentation subsequent to its reaction with atomic hydrogen is indicative of a reaction that proceeds via an early transition state. The competitive reduction of a series of substituted-benzyl alkyl sulfides was insensitive to the substituent on the aromatic ring (rho = -0.13, r = 0.99). The relative rates of fragmentation of a series of the substituted-benzyl alkyl sulfides gave a V-shaped Hammett plot. Both electron-donating and electron-withdrawing groups destabilized the transition state (rho = +0.99, r = 0.999; rho = -0.82, r = 0.992). Since the relative rates of disappearance of the alkyl benzyl sulfides are not substituent dependent, but the relative rates of fragmentation are, a 9-S-3 intermediate is preferred as the structure leading to products.