ALKENYL FISCHER CARBENE COMPLEXES AND ALPHA,BETA-UNSATURATED IMINE DERIVATIVES - SYNTHESIS OF AZEPINES AND MECHANISTIC NMR-STUDIES

4-Amino-1-azadienes 1 react with alpha,beta-unsaturated Fischer carben e complexes at -40 degrees C to give stereoselectively a variety of su bstituted 3H-4,5-di-hydroazepines 3; similarly, 1-hydroxy-1-azadienes (alpha,beta-unsaturated oximes) 6 afforded the corresponding azepine d erivatives 7. Chiral, nonracemic carbene complexes 11 gave azepines 12 -13 (d.e. = 40-44%) upon reaction with oxime 6a; the major isomers wer e obtained in a diastereomerically and enantiomerically pure form (45- 50% overall yield) after crystallization. An X-ray structure of 12a al lowed assignment of the absolute stereochemistry. The acid hydrolysis of azepines synthesized provided racemic and enantiomerically pure 1,6 -dicarbonyl compounds (+/-)-5, (+/-)-9, and (-)-14, as well as diol(-) -15. The mechanism of the reaction of 1 and 2 was investigated by mult inuclear (H-1,C-13, N-15, and W-183) NMR characterization of four inte rmediates (A, B, C, and D) at low temperature. The experimental sequen ce of events involves: i) 1,2-nucleophilic addition of the unsubstitut ed imine nitrogen of 1 to the metal carbene function (zwitterion A, -6 0 degrees C), ii) cyclization to the seven-membered ring with 1,2-migr ation of the pentacarbonyl metal (zwitterion B, -40 degrees C), iii) r eductive elimination and coordination of the metal to the amine nitrog en (intermediate C, -40 degrees C), and iv) thermal decomplexation and tautomerization (intermediate D and compound 3, above - 20 degrees C) .