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.

本研究系统考察了(反式-2-苯基环丙基)乙炔（1a）、(反式,反式-2-甲氧基-3-苯基环丙基)乙炔（1b）以及(反式,反式-2-甲氧基-1-甲基-3-苯基环丙基)乙炔（1c）分别与硫酸水溶液、三(三甲基硅基)硅烷（tris(trimethylsilyl)silane）或三丁基氢化锡（tributyltin hydride）与偶氮二异丁腈（AIBN）的反应行为。反应过程中，质子化以及硅基自由基（silyl radical）或锡基自由基（stannyl radical）分别加成至炔烃的末端位点，对应生成α-环丙基取代的乙烯基正离子（vinyl cation）或自由基中间体。在两类反应条件下，1a均得到由环丙基环朝向苯基取代基一侧开环衍生的产物。与之不同的是，炔烃1b和1c的产物种类取决于反应路径为自由基路径还是阳离子路径：当采用自由基反应条件时，可分离得到区域选择性朝向苯基取代基一侧开环的产物；而当采用阳离子反应条件时，则得到区域选择性朝向甲氧基取代基一侧开环的产物。本研究还合成了对应的α-环丙基取代乙烯基锂衍生物，并证实其不易发生重排反应。此外，本研究对α-环丙基乙烯基正离子的开环速率常数进行了估算：由炔烃1a~1c末端碳原子质子化生成的乙烯基正离子，其开环速率常数介于10^10~10^12 s^-1之间。基于上述实验结果，环丙基乙炔类化合物1a~1c可作为高灵敏机理探针，用于检测环丙基环邻近生成的乙烯基自由基或阳离子中间体；针对1b和1c而言，还可通过产物分布区分反应究竟经由自由基中间体还是阳离子中间体路径。