Cyclopropyl Alkynes as Mechanistic Probes To Distinguish between Vinyl Radical and Ionic Intermediates

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 α-cyclopropyl-substituted vinyl cation or radical, respectively. Under
both reaction conditions, 1a yielded products derived from ring opening toward the phenyl
substituent. Alkynes 1b and 1c, however, gave different products depending on whether radical or
cationic conditions were used. When radical conditions were employed, products derived from
regioselective ring opening toward the phenyl substituent were obtained. In contrast, when cationic
conditions were employed, products derived from selective ring opening toward the methoxy
substituent were isolated. The corresponding α-cyclopropyl-substituted vinyllithium derivatives
were also synthesized and were found to be stable toward rearrangement. An estimate of the rate
constants for ring opening of the α-cyclopropylvinyl cations was also made:  values of 1010−1012 s-1
were found for the vinyl cations derived from protonation of the terminal carbon of alkynes 1a−c.
Based on these results, cyclopropyl alkynes 1a−c can be classified as hypersensitive mechanistic
probes for the detection of vinyl radical or cationic intermediates generated adjacent to the
cyclopropyl ring and, in the case of 1b and 1c, the distinction between a radical or cationic
intermediate is possible.