REACTIONS OF DI-TERT-BUTYLCYCLOPENTADIENES AND TRI-TERT-BUTYLCYCLOPENTADIENES WITH DIHALOCARBENES

Dihalocarbene additions to mixed 1,3- and 1,4-di-tert-butylcyclopentad ienes (1, 3) allow to prepare 1,3-di-tert-butyl-4-halobenzenes (9) in an efficient way. Only the 3,4 double bond is attacked regiospecifical ly in 1,3,5-tri-tert-butylcyclopentadiene (11) (and also in 1). Totall y unexpected main products from 11 are compounds 9 again being formed under fragmentation of a tert-butyl group. Only small amounts of 1,2,4 -tri-tert-butyl-5-halobenzenes (13, X = CI, Br) are obtained from 11. A labile primary dichlorocarbene adduct to 11 (12a) can be isolated. I ts thermal decomposition occurs with the loss of one tert-butyl group to yield 9a. Base catalyzed degradation of 12a gives 9a and 13a as by- product.