[2 + 2]‐Cycloadditions to Strained Bridgehead Olefins. II. Diphenylketene

Abstract

SummaryThe strained bridgehead olefins bicyclo [3.3. llnon-1-ene (l), bicyclo [4.2. Ilnon-1 (8)-ene (2), and bicyclo [4.2. llnon-1-ene (3), and the comparable monocyclic (E)-1-methylcyclooctene (4) react with diphenylketene (6) to give a single cycloadduct 7, 8, 9 and 10, respectively, in which the diphenyl-substituted C-atom is bound to the bridgehead. The structure of the cyclobutanone 8 has been determined by X-ray analysis of a twin crystal obtained by crystallization with spontaneous enrichment of enantiomers.

Strained bridgehead double bonds

“…This was explained in terms of a concerted "x2a + T2a" transition state. 123 The reactions with l,l-dichloro-2,2-difluoroethylene were accounted for in the usual fashion, i.e., as proceeding via diradicals. On the basis of the product distribution, it was concluded that biradicals 248 and 249 are of similar stability.…”

Can Twisted Double Bonds Facilitate Stepwise [2 + 2] Cycloadditions?

“…21 12 and 14 also gave facile concerted [2 + 2] cycloaddition reactions with a ketene. 22 The twisted double bonds of trans-cycloalkenes (n = 7−10, n is the number of ring atoms) exhibit higher reactivity compared to their unstrained cis-analogues. 21 In these examples, the reactivity of the twisted double bond mimics that of a twisted alkene (cf.…”