Three Molecules of an Arylalkyne Reacting with a β-Amino-Substituted α,β-Unsaturated Fischer Carbene Complex to Give Highly Substituted Spiro[4.4]nonatrienes

A novel mode of reaction towards arylethynes is shown by the β-trimethylsilyl-substituted α,β-unsaturated Fischer carbene complex 10. A mixture of the isomeric, highly substituted spiro[4.4]nonatrienes 12 and 13 (ratios 1.6:1 to 4.5:1) in yields ranging from 34 to 62% is formed by the formal insertion of three alkyne molecules and concomitant or subsequent cyclization. Such selective triple reactions of alkynes with ethenylcarbene complexes have not previously been observed. It is believed that this reaction proceeds via an unprecedented domino sequence of a formal [3+2] cycloaddition, carbene reformation, and formal [2+2+1] cycloaddition process which occur in conjunction with a number of other additional in situ transformations of transient chromium π-complexes.