Halogenoborane mediated allene cyclooligomerization.

The halogenoboranes XB(C6F5)2 (X: Cl, Br) react with allene to give a mixture of the cyclotrimer 1,3,5-trimethylenecyclohexane (1) and the related halogenoborylated cyclotetramerization products 3a and 3b. Alkyl-substituted allenes were catalytically cyclotrimerized metal-free by XB(C6F5)2 (X: Cl, Br) to give cis,trans-2,4,6-trialkyl-1,3,5-trimethylenecyclohexanes under mild conditions.