Cyclic Carboalumination of Alkynylsilanes Forming Exocyclic Alkenes

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Abstract
The reaction of 6‐trimethylsilylhex‐1‐en‐5‐yne with i‐Bu3Al (1.1 equivalents) and a catalytic amount of Cl2ZrCp2 in 1,2‐dichloroethane or CH2Cl2 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‐Bu3Al–Cl2ZrCp2 reaction of 6‐trimethylsilyl‐4‐methylhex‐1‐en‐5‐yne is nonstereoselective. However, the use of I2ZrCp2 in place of Cl2ZrCp2 improves the E/Z ratio to ca 80/20. The reaction of 1‐trimethylsilylhex‐5‐en‐1‐yn‐3‐ol with i‐Bu3Al–Cl2ZrCp2 gives the ca 90% pure Z isomer under “kinetic” conditions and the ca 97% E isomer under thermally equilibrating conditions in excellent yields.