Cyclic Carboalumination of Alkynylsilanes Forming Exocyclic Alkenes

The reaction of 6‐trimethylsilylhex‐1‐en‐5‐yne with i ‐Bu 3 Al (1.1 equivalents) and a catalytic amount of Cl 2 ZrCp 2 in 1,2‐dichloroethane or CH 2 Cl 2 at room temperature produces, after protonolysis, trimethylsilylmethylenecyclopentane in 85% yield. Upon iodinolysis of the reaction mixture, the corresponding iodide is obtained in 68% yield. Under the same reaction conditions, 7‐trimethylsilylhept‐1‐en‐6‐yne is converted into trimethylsilylmethylenecyclohexane in 72% yield upon protonolysis of the reaction mixture. Attempts to produce four‐ or seven‐membered ring compounds have not been successful. The i ‐Bu 3 Al–Cl 2 ZrCp 2 reaction of 6‐trimethylsilyl‐4‐methylhex‐1‐en‐5‐yne is nonstereoselective. However, the use of I 2 ZrCp 2 in place of Cl 2 ZrCp 2 improves the E/Z ratio to ca 80/20. The reaction of 1‐trimethylsilylhex‐5‐en‐1‐yn‐3‐ol with i ‐Bu 3 Al–Cl 2 ZrCp 2 gives the ca 90% pure Z isomer under “kinetic” conditions and the ca 97% E isomer under thermally equilibrating conditions in excellent yields.