Umlagerung von Vinyl‐Cyclopropan‐Carbaminalen

Rearrangement of Vinyl‐Cyclopropane‐Carbaminals Both ( c ‐2, t ‐3‐diphenyl‐ r ‐1‐cyclopropyl)methylene‐dipyrrolidine ( 4 ) and its ( t ‐2, t ‐3)‐isomer 10 underwent a thermal rearrangement to ( E )‐ N ‐2‐benzylidene‐1‐indanyl‐pyrrolidine ( 5 ). Under the conditions of the rearrangement, 5 was partially converted into 2‐benzyl‐1‐indanone ( 6 ) in a base catalysed reaction. The structures of 5 and 6 were derived from spectroscopic data and from degradation reactions.‐ N , N ′‐( t ‐2‐Vinyl‐ r ‐1‐cyclopropyl)methylene‐dipyrrolidine ( 11 ) rearranged thermally to N ‐(2‐methylidene‐3‐cyclopenten‐1‐yl)pyrrolidine ( 12 ), the structure of which was established from spectroscopic evidence and from a hydrogenation to N ‐(2‐methylcyclopentyl)pyrrolidine ( 13 , cis / trans mixture 3:2). The aminal 4 was reduced with formic acid to give N ‐( c ‐2, t ‐3‐diphenyl‐ r ‐1‐cyclopropyl)methyl‐pyrrolidine ( 14 ). If perdeuterio formic acid was used, the mixture product 14‐d/14‐d 2 was obtained which contained exactly one deuterium atom in its methylene group and about half a deuterium atom on C(1). This labeling pattern is mechanistically explained with the existence of a fast equilibrium between the iminium ion 19 and the enamine 18 , so that 18 and 19 are considered to be plausible reactive intermediates in the above mentioned thermal rearrangement. ‐ Based on this, several mechanisms for the rearrangements 4 → 5, 10 → 5 and 11 → 12 were considered: A Pictet ‐ Spengler ‐ or Mannich ‐type reaction, which starts from the iminium ion 23 and is followed by a cyclopropylmethyl‐homoallylic rearrangement and by deprotonation (path a, Scheme 5 ), was judged to be improbable because the postulated intermediates could lead more easily to other stable products than the observed ones. If the reaction is formulated as a [3,3]‐sigmatropic shift occurring on exclusively the ( E )‐isomer 5 suggests a concerted process whose steric course is predominantly controlled by strain factors. Alternatively, the reaction could be formulated via a dipolar ( 27 ) or a diradical ( 26 ) species derived from the enamine 22 (paths c and d, Scheme 5 ); attempts to trap such species by a number of agents were unsuccessful. ‐ The previously unknown aminals 10 and 11 were synthesized by standard methods.