Diels-alder adducts of 5-alkynylcyclopentadienols with tetracyanoethylene and dimethyl acetylenedicarboxylate: An X-ray crystallographic study of unexpected rearrangement products |
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Authors: | Dunn Stradiotto McGlinchey |
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Institution: | Department of Chemistry, McMaster University, Hamilton, Ontario, L8S 4M1 Canada. |
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Abstract: | The Diels-Alder reaction of tetracyanoethylene (TCNE) with 1, 4-diethyl-5-(trimethylsilyl)ethynyl-2,3-diphenylcyclopentadien-5-ol (3a) occurs on the hydroxyl-bearing face of the diene and yields ultimately an imino lactone (5a), whereby the hydroxyl functionality has added across an exo nitrile linkage. TCNE and 5-(trimethylsilyl)ethynyl-1,2,3,4-tetraphenylcyclopentadien-5-ol (3b) behave analogously. In contrast, the 4 + 2] adduct of 3b with dimethyl acetylenedicarboxylate (DMAD) undergoes a dramatic skeletal rearrangement to generate the 1,4-cyclohexadiene (9) in which an alkynyl ketone moiety has migrated onto an ester-bearing carbon. The molecules 5a and 9 have been characterized by X-ray crystallography, and a mechanism for the skeletal rearrangement is proposed. |
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