Ion and Radical Rearrangements as a Probe of the Mechanism of a Surface Reaction:  The Desulfurization of Cyclopropylmethanethiol and 3-Butene-1-thiol on Mo(110)

Rearrangement reactions were used to probe the transient intermediates in thiol desulfurization induced by Mo(110) by studying cyclopropylmethanethiol and 3-butene-1-thiol. Thiolate intermediates were identified in both cases using vibrational spectroscopy, which indicates facile S−H bond scission on Mo(110). Heterolytic C−S bond scission, leading to a cationic intermediate, is excluded based on the lack of rearrangement products in the reactions of 3-butene-1-thiolate and the absence of cyclobutane or cyclobutene in the reaction of cyclopropylmethyl thiolate on Mo(110). Hydrogenolysis without rearrangement is the primary pathway for both thiols investigated. The lack of rearrangement in the 3-butene-1-thiolate indicates that C−S bond scission and C−H bond formation occur nearly simultaneously. Evidence for the radical pathway is obtained from the production of 1,3-butadiene formed via the rearrangement of cyclopropylmethyl group following C−S bond scission in the cyclopropylmethyl thiolate and by related studies of cyclopropylmethyl bromide. The investigation of the cyclopropylmethyl bromide also demonstrates that trapping of the cyclopropylmethyl radical is favored over selective β-dehydrogenation. This is the first study in which radical rearrangements have been used to obtain detailed information about the nature of extremely short-lived reactions in a surface process.