Alkenyl Fischer Carbene Complexes and α,β‐Unsaturated Imine Derivatives: Synthesis of Azepines and Mechanistic NMR Studies

Abstract

4-Amino-I -azadienes 1 react with a,&unsaturated Fischer carbene complexes at -40 "C to give stereoselectively a variety of substituted 3H-4,S-dihydroazepines 3; similarly, 1 -hydroxy-1azadienes (cc,#?-unsaturated oximes) 6 afforded the corresponding azepine derivatives 7. Chiral, nonracemic carbene ~~

Facile and Versatile Annulation of the Imidazole Ring: Single and Sequential Cyclization Reactions of Fischer Carbene Complexes with 1,4‐Diazafulvenes

“…A likely mechanistic rationale for this [6+3] cycloaddition reaction of alkenylcarbenes and fulvenes is based on the accepted sequence of nucleophilic addition and metal-migration-promoted cyclization (Figure 1). [23] Thus, the 1,2-addition of the ring nitrogen atom of the diazafulvene to the carbene complex would form the intermediate I, which would then undergo a [1,2]-M(CO) 5 -shift and stereoselective cyclization through the more favorable conformation to give the intermediate II. The last step involves metal elimination to generate the enolether functionality.…”

[6+3] Cycloaddition of Fischer aminocarbene complexes: An efficient annulation of fulvenes to indene derivatives

“…A suitable mechanism that would account for this [6 + 3] carbocyclization reaction is based on the well recognized capability of the M(CO) 5 fragment of metallate species to undergo 1,2-migration thus inducing unusual cyclization processes (Scheme 3) [10]. The initial 1,2-nucleophilic addition of fulvene to the electrophilic carbene carbon would lead to the zwitterionic intermediate I.…”

New heterocyclization reactions with Fischer carbene complexes

“…9 This reaction requires one equivalent of a simple carbene complex, which acts as a reducing agent to remove the oxygen of the oxime functionality at some point during the process. Significantly, the major diastereoisomers were found to crystallize from methanol, allowing the isolation of both azepines 33 and 34 in a diastereomerically pure form.…”

“…Hydrolysis of chiral azepine 33a with 3M HCl in THF led to formyl ester 35 in 93% yield, accompanied by a small amount of azepinone 36 (<5% yield). 9 The entire protocol formally represents the enantioselective Michael addition of ester homoenolates to α,β-unsaturated aldehydes, in which two chiral centers are created. Finally, the chiral auxiliary was efficiently removed by reduction of 35 with LiAlH 4 at room temperature, obtaining chiral diol 37 in 95% yield.…”