Umlagerungen von 1,4,4‐ und 2,2,5‐Trimethylbicyclo[3.2.1]oct‐6‐yl‐Kationen

Abstract

Rearrangements of 1,4,4‐ and 2,2,5‐Trimethylbicyclo[3.2.1]oct‐6‐yl Cations1,4,4‐Trimethylbicyclo[3.2.1]oct‐6‐yl cations (18) are generated from the tosylhydrazone 7, by nitrous acid deamination of the amines 6 and 8, and by acetolysis of the brosylates 21 and 24. The product distributions are but slightly affected by the configuration (exo vs. endo) of the leaving groups. The predominant product, 1,4,4‐trimethylbicyclo[3.2.1]octan‐exo‐6‐ol (11) is formed without significant redistribution of a 6‐2H label. The degenerate Wagner‐Meerwein rearrangement of the parent bicyclo[3.2.1]oct‐6‐yl cation is virtually eliminated by the presence of two methyl groups at C‐4. Enhanced conformational strain, raising the barrier to ring flipping, accounts for these observations. Minor products (12, 13) arise from a 7,6‐hydride shift of 18, followed by Wagner‐Meerwein rearrangement. When the 2,2,5‐trimethylbicyclo[3.2.1]oct‐6‐yl cation (19) is generated directly from the tosylhydrazone 17, the tertiary alcohol 13 is obtained as the major product. The conformational barrier to Wagner‐Meerwein rearrangement is compensated by the incipient stabilization of the tertiary carbocation 20.