Group 6 Heteroatom- and Non-Heteroatom-Stabilized Carbene Complexes. β,β‘- and α,β,β‘-Annulation Reactions of Cyclic Enamines

Cyclization reactions of group 6 Fischer carbene complexes with cyclopentanone and cyclohexanone enamines are described. Enamine 3a undergoes thermal α,β,β‘-annulation with alkenylcarbene complexes 1 and 2 (THF, 60 °C), affording semibullvalenes 5. The metalate intermediates 6, resulting from β,β‘-annulation of the enamines 3a and 4a, were quantitatively formed by running the reaction in hexane at room temperature. Acid-promoted demetalation of 6 afforded endo-2-bicyclo[3.2.1]octen-8-ones 7 and endo/exo-2-bicyclo[3.3.1]nonen-9-ones 8 (endo/exo = 5:1). Using (S)-methoxymethylpyrrolidine-derived enamines 3b and 4b,c allowed highly enantioenriched cycloadducts endo-(+)-7 as well as endo-(−)-8 and exo-(−)-8 to be accessed. The non-heteroatom-stabilized carbene complex 10 was formed from complex 6 by Me₃SiOTf-promoted elimination of the methoxy group, characterized by ¹³C NMR, and transformed into the organic compounds 7, 7-d, and 11 as well as into bicyclo[3.2.1]octan-2,8-diones 14 and cycloheptanones 15. On the basis of this sequence, enantioenriched cycloheptanones (+)-15 were efficiently prepared in one pot from carbene complexes 2 and enamine 3b (51−55% yield, 91−96% ee). Extension of this work to simple Fischer carbene complexes 16 allowed an appropriate way to generate the nonstabilized pentacarbonyl[(phenyl(alkyl)carbene]tungsten complex 17 to be designed, for which the thermal and chemical behavior leading to compounds 18−21 is described.