Starting from tricyclo [3.1.0.02,6]hexane 5 and 1‐ bromobicyclo[1.1.0]butane 6, a series of [1.1.1]propellanes 15 and 21 has been synthesized which carry alkyl, aryl, alkenyl, and alkynyl groups. Propellane formation proceeded via 1‐bromo‐1‐chloro‐3‐alkylidenecyclobutanes of type 13 and 19, which on treatment with methyllithium gave rise to the generation of carbenes 14 and 20 as short‐lived intermediates. For these carbenes, the most efficient path of stabilization is obviously the intramolecular cycloaddition. Ab initio MO calculations at the Becke3LYP/6‐31G* and MP2/6‐31G* level of theory indicated that 3‐alkylidenecyclobutylidenes 4 and 37a–d are not local energy minima but collapse to the corresponding [1.1.1]propellanes. On this basis, propellane formation should follow a carbenoid reaction path.

Synthesis of (1.1.1)Propellanes by Cyclization of 3-Alkylidenecyclobutylidenes.-Various kinds of propellanes such as (I) and (II) are obtained via carbene cyclization as key step. Mechanistical aspects based on ab initio calculations are discussed. -(KENNDOFF, M.; SINGER, A.; SZEIMIES, G.; J. Prakt. Chem./Chem.-Ztg. 339 (1997) 3, 217-232; Inst. Org. Chem.,