The addition of 2-oxido-2-cyclopenten-1-ylium to some olefins and dienes in 2,2,2-trifluoroethanol

The title reaction has been studied with 2-methoxypropene, (E)-cyclooctene, ethoxyethene, 1,3-cyclopentadiene, 3-methylenecyclohexene, styrene and iso prene as the olefinic substrates. This sequence is one of decreasing reacti vity of the substrates towards 2-oxido-2-cyclopenten-1-ylium (1) if [4 + 3] cycloaddition (the prevailing reaction in the case of cyclopentadiene and isoprene) is ignored. This reactivity encompasses: (a) the formation of int ermediate 1,5-dipoles, which in the majority of cases give rise to a multit ude of products including many of higher molecular mass, and (b) two types of ene reactions, dubbed "ene-type 1" and "ene-type 2". Type 1, in which th e migrating hydrogen atom is abstracted from 1, is ubiquitous (except with 2-methoxypropene) but is always a minor reaction; type 2, in which the migr ating hydrogen atom is transferred to 1, was encountered only in the cases of 3-methylenecyclohexene, in which it is a major reaction path, and isopre ne. Both types were found to be slightly concerted. In this context, we obs erved an effect which we have dubbed "dipolar diversion of a concerted reac tion". In general, those substrates incapable of forming [4 + 3] cycloadduc ts gave complicated mixtures, with the exception of two compounds, which we re found to give one predominant product with 1. The first of these is the transfixed 1,3-diene 3-methylenecyclohexene, which gave the ene-type 2 addu ct 10d1, while the second is the most nucleophilic substrate, 2-methoxyprop ene, which gave a mixture of four adducts (two pairs of epimers), all of wh ich gave the diketone 17 on hydrolysis.