A mechanistic study of certain cyclopropylidene and cyclopropylidenoid rearrangements

The rearrangement of 7,7-dibromonorcar-2-ene to syn-7-bromo-7-methylnorbornene upon treatment with MeLi has been proposed to involve a carbene-carbene rearrangement. We have studied the analogous rearrangement of exo-4-methoxy-10,10-dibromotricyclo4.3.1.0(\u271,6)deca-2-ene (192a), where 1,2-trimethylene-exo-5-methoxy-syn-7-bromo-7-methylnorborn-2-ene (218a) is the product of said rearrangement. Based on salt and crown ether effects, we have shown that Li must remain attached to the carbenoid formed from 192a in order for 218a to be produced. Thus the rearrangement does not involve free carbenes;The crown ether experiments also suggest the DPIBF trapping products (219a) were formed via a separate pathway, probably involving a free carbene. Further evidence to support this comes from pyrolytic decomposition of exo-4-methoxy-10-bromo-10-trimethyltintricyclo4.3.1.0(\u271,6)deca-2-ene (207) to give 219a in the presence of DPIBF, where the corresponding tricyclic cyclopropylidene (258a) is a logical intermediate, but where a (alpha)-bromoanion at C(,10) (222a) is excluded;The formation of two isomeric DPIBF adducts (245) from 11,11-dichloro-1,6-methano10annulene has been proposed to involve an allene intermediate. Our studies of the pyrolysis of two epimeric (at C(,4)) (alpha)-bromotin compounds (207, 208) in the presence of DPIBF show no crossover products. This stereospecificity precludes the intermediacy of a standard linear planar allene in the formation of the trapping products (219a, 219b);Treatment of exo-10,10-dibromotricyclo4.3.1.0(\u271,6)deca-2-ene-4-ol (192c) with MeLi at either -78(DEGREES)C or room temperature affords only one dimer, which is a norbornenyl-type dimer (i.e., a formal dimer of a 7-norbornenylidene). However, the epimer (at C(,4), 192d) affords four dimers. An explanation for this apparently divergent behavior involves control via lithium bridging in the exo-alkoxide (related to 192c);Lastly, dibromo4.3.1- and dibromo4.4.1propellanes have been reported to yield only intramolecular C-H insertion products upon treatment with MeLi. We have discovered that both intra- and intermolecular reactions take place at room temperature. Mechanisms for these reactions are discussed