Intramolecular Cyclopropanation of Alkali-Metal-Substituted Silylene with the Aryl Substituent of an N-Heterocyclic Framework.

Reaction of the bulky N-heterocyclic silylene Si[NB(NArCH)2CH2]2 (3; Ar = 2,6-iPr2C6H3) with BCl3 resulted in ring expansion with formation of the six-membered C2N2SiB heterocycle 4. Reduction of 4 with lithium afforded the first silanorcaradienyllithium 5, which presumably formed by intramolecular cyclopropanation of a lithium-substituted borylsilylene intermediate with one of the aryl C═C bonds. Treatment of 5 with MeOTf and Et3NHCl led to electrophilic substitution accompanied by the SiC2 ring expansion to give silacycloheptatrienes (silepins) 6 and 7, respectively. The addition of 1,3,4,5-tetramethylimidazol-2-ylidene (NHCMe) to silepin 7 yielded the NHC-stabilized hydrosilylene 8 with the silicon atom free from the silepin ring. The formation of 5 could be attributed to the unique electronic structure of the lithium-substituted silylene, which has been predicted to be in a triplet ground state with a relatively large singlet-triplet energy gap by theoretical calculations.