Intra- and intermolecular reaction selectivities of γ-substituted adamantanylidenes.

A study of adamantanylidenes having a γ-substituent (R) was undertaken to gauge how inductive and steric effects of remotely positioned functional groups influence intra- and intermolecular product selectivity. 3H-Diazirines were thermolyzed or photolyzed to generate the corresponding carbenes. On rapid heating, the resulting carbenes isomerized to 2,4-didehydroadamantanes by intramolecular 1,3-CH insertions. When R was an electron donor (R(D)) mostly asymmetric 1-substituted derivatives were produced but when it was an electron acceptor (R(A)) the symmetric 7-substituted ones were formed. When solutions were exposed to UV-A light, intermolecular adducts from the carbenes and solvent predominated with lesser amounts of intramolecular product being formed. Valence isomerization of 3H-diazirines also afforded diazo compounds. In methanol, protonation of diazo compounds to give the corresponding 2-adamantyl cations exceeds their coupling. This diversion was controlled with fumaronitrile by trapping the diazo compounds. The adducts possessed mostly anti configurations with R = R(D) and syn arrangements with R = R(A). The connection between as- and anti-product formation and that of s- and syn-products was deemed to be the consequence of a rapid equilibrium between two distinct carbene conformations. This was qualified and quantified using ab initio calculations and NBO analyses.