Cyclopropyl alkynes as mechanistic probes to distinguish between vinyl radical and ionic intermediates.

[reaction: see text] The reactions of (trans-2-phenylcyclopropyl)ethyne, 1a, (trans,trans-2-methoxy-3-phenylcyclopropyl)ethyne, 1b, and (trans,trans-2-methoxy-1-methyl-3-phenylcyclopropyl)ethyne, 1c, with either aqueous sulfuric acid or tris(trimethylsilyl)silane (or tributyltin hydride) and AIBN have been investigated. Protonation and addition of the silyl (or stannyl) radical occurred at the terminal position of the alkyne giving an alpha-cyclopropyl-substituted vinyl cation or radical, respectively. Under both reaction conditions, 1a yielded products derived from ring opening toward the phenyl substituent.

Determination of the rate constant for ring opening of an alpha-cyclopropylvinyl radical.

The rate constant for ring opening of the 1-(trans-2-phenylcyclopropyl)ethen-1-yl radical, 4, generated by photolysis of the corresponding vinyl iodide 2, is reported. The value of the rate constant was determined by the tin hydride method and was found to be (1.6+/-0.