Formal [4 + 3], [4 + 2], [4 + 1] cycloadditions and acid‐base reaction of 2‐methyl‐1,3‐dimorpholino‐1,3‐butadiene with Fischer carbene complexes

Abstract

Abstract 2‐Methyl‐1,3‐dimorpholino‐1,3‐butadiene 1 reacted with α,β‐unsaturated Fischer carbene complexes to give a wide range of different products depending on the substitution pattern. Thus, sevenmembered rings ( 4, 5 and 6 ) could be obtained from chromium complexes 2 with aromatic or vinylic groups at the β position. Similar results were observed when α‐methyl‐substituted carbene complex 7 a was used. Six‐membered carbocycles (derivatives of cycloadducts 12 and 13 ) were isolated after reaction with both chromium and tungsten complexes bearing one or two alkyl groups at the β position ( 10 and 11 ). Moreover, cyclopentenones 20 were the main products when the starting carbene complexes were alkyl‐substituted at both α and β positions ( 19a, b ) or when aromatic ( 19c, d ) instead of vinylic complexes were used. A bicyclo[4.1.0]heptene system 18 has also been obtained in the special case of reaction with β,β‐dimethylvinylchromium complex 13b ; its formation could be explained as a formal carbene insertion into a C–H bond. The behaviour of diene 1 towards alkoxymethylcarbene complexes 22 was unusual. The different reaction products (cyclopentadienes 23 , bicyclo[3.1.0]hexenes 24 , aromatic amine 25 and metallatrienes 26 ) imply a mechanism in which the deprotonation of the carbene complex by the diene is followed by Michael addition to the iminium salt formed.