Intramolecular Carbene Chemistry. Evidence for Exclusive C-H Insertion in 8-Methylene-2-noradamantylidene

Article abstract of DOI:10.1021/jo00256a028

8-Methylene-2-noradamantylidene (5) reacts by intramolecular γ-C,H insertion to produce 6-methylene-2,4-didehydronoradamantane (4) as the sole product (92percent).Specifically labeled carbene 8-methylene-2-noradamantylidene-4-d (5a) produces 4 with the label located at only one position.This rules out the possible formation of the unstable olefin-cycloaddition product, 2,8-methano-2,8-didehydronoradamantane (6), followed by retro carbene ring opening to give 5.The higher homologue of 5, 4-methylene-2-adamantylidene (2), is known to react exclusively by intramolecular olefin cycloaddition to give 2,4-methano- 2,4-didehydroadamantane, a <3.1.1>propellane, although the carbenic center and the olefinic bond in this carbene are less favorably arranged for cycloaddition than those in carbene 5.This difference in behavior suggests a relatively late and high activation energy transition state between carbene 5 and the resultant intramolecular olefin-cycloaddition product 6, a highly strained <2.1.1>propellane.