Addition of Cyclopropyl Alkynes to a Brook Silene: Definitive Evidence for a Biradical Intermediate

The addition of three newly developed mechanistic probes, (trans-2-phenylcyclopropyl)ethyne,
(trans,trans-2-methoxy-3-phenylcyclopropyl)ethyne, and (trans,trans-2-methoxy-1-methyl-3-phenylcyclopropyl)ethyne, 1a−c, to a Brook silene, 2-tert-butyl-2-trimethylsiloxy-1,1-bis(trimethylsilyl)-1-silene, 10, was
examined. When alkyne 1a was added to silene 10 products derived from a formal ene reaction were
obtained. When alkynes 1b−c were added to silene 10, in addition to the typical silacyclobutenes, a variety
of silacycloheptenes were obtained in which the cyclopropyl ring had clearly opened. Formal ene-addition
products were also produced from the addition of 1b to 10. Based on the relative positions of the phenyl
and methoxy substituents within the seven-membered ring of the silacycloheptenes and the known behavior
of the alkyne probes under both radical and ionic conditions, it was concluded that a biradical intermediate
was formed during the addition of alkynes 1b−c to silene 10. In the addition of alkynes 1a−b to silene 10,
the ene products are most likely formed by a competitive pericyclic reaction. We also present a
straightforward method for the unambiguous determination of the regiochemistry of silacyclobutenes derived
from the cycloaddition of terminal alkynes to silenes.

将三种新开发的机理探针（mechanistic probes）——（反式-2-苯基环丙基）乙炔、（反式,反式-2-甲氧基-3-苯基环丙基）乙炔以及（反式,反式-2-甲氧基-1-甲基-3-苯基环丙基）乙炔（标记为1a~1c）——加入到布鲁克硅烯（Brook silene）2-叔丁基-2-三甲基硅氧基-1,1-双（三甲基硅基）-1-硅烯（标记为10）中，对其反应进行了考察。当将炔烃1a加入硅烯10中时，得到了源自形式ene反应的产物。当将炔烃1b~1c加入硅烯10中时，除得到典型的硅杂环丁烯（silacyclobutene）外，还获得了多种环丙烷环明显开环的硅杂环庚烯（silacycloheptene）；同时将1b加入10中时也生成了形式ene加成产物。基于硅杂环庚烯七元环内苯基与甲氧基取代基的相对位置，以及炔烃探针在自由基与离子条件下的已知行为，可推断出炔烃1b~1c与硅烯10发生加成反应时生成了双自由基中间体（biradical intermediate）。而在炔烃1a~1b与硅烯10的加成反应中，ene产物极有可能通过竞争性周环反应（pericyclic reaction）生成。本研究还提出了一种简便方法，可明确确定端炔烃与硅烯发生环加成反应所得到的硅杂环丁烯的区域化学（regiochemistry）。