Siloles Display Surprising Chemistry

Chemistry professor Robert West of the University of Wisconsin, Madison, has seen his share of unusual chemical behavior in the organosilicon compounds he’s been studying for many years. On Dec. 17, during a symposium at Pacifichem, he described his latest observations, which focused on “the surprising reactions of silole dianions.” These silicon heterocycles are so reactive that their lithium cations are reduced to lithium metal during a variety of unique addition reactions, he reported.

Siloles are five-membered aromatic rings that exist as doubly charged anions, with the charge centered on the lone silicon atom. Silafluorenes are extended versions in which the silole ring is flanked on both sides by a benzene ring.

In Hawaii, West provided evidence from X-ray structure bond lengths and theoretical calculations that the rings are more aromatic than one might guess. Siloles and their germanium analogs have six π electrons and equal C–C bond lengths in the rings, as expected, he said. But in the fluorene compounds, the π electrons in the two benzene rings are only partially delocalized (leading to unequal C–C bond lengths), while those in the central silole ring remain fully delocalized. Thus, the silole moiety appears to be more aromatic than the benzene rings. “Siloles want to be aromatic so bad, they steal aromaticity from benzene,” West quipped.

The strong aromaticity leads the compounds to be very reactive, he said, including having the ability to function as silylenes—silicon analogs of carbenes. For example, addition reactions of the lithium salts of tetraphenylsilole or silafluorene with 1,3-dienes lead to spiro compounds in which the silicon atom is shared by two rings. The remarkable part of the reaction, West emphasized, is that the lithium ions are reduced to metallic lithium, which is unprecedented. The lithium precipitates out and helps drive the reaction to completion, he noted.

One reaction that “surprised me the most,” West said, was that tetraphenylsilole readily adds substituted acetylenes, rather than deprotonating them, to form a compound with a pendant three-membered ring. Silole also adds ethylene, which attaches to silicon and starts to form a polyethylene chain, he reported.

The reactivity of the silole, particularly the failure to abstract a hydrogen from acetylene, is “amazing and quite unexpected,” commented chemistry professor Akira Sekiguchi of the University of Tsukuba, in Japan.