Lewis acid catalysis of the ene addition of chloral and bromal to olefins; stereochemical and mechanistic studies

Asymmetric ene reactions in organic synthesis

“…Diastereoselectivity (see "Internal Asymmetric Induction") in the thermal and AlCl3-catalyzed ene reactions of chloral has been exploited (Scheme 20). 25 Reaction of 2-methyl-2-butene at 130 °C gives the anti isomer in 84% diastereoselectivity. By contrast, Lewis acid-catalyzed reaction affords syn diastereomer in 85 % selectivity.…”

“…23 The A1C13-and SnCl4-promoted ene reactions of chloral were also examined. 23 Recently, Gill has reported a complete account on the chloralene reaction.24 25 Lewis acid-catalyzed reactions are best carried out with 2 mol % A1C13 in CH2CI2 or CCL for reactive alkenes and 6-20 mol % A1C13 for less reactive alkenes. The order in reactivity of alkenes is as follows : 1,1-disubstituted > trisubstituted > monosubstituted > cis-l,2-disubstituted > alkynes > trans-l,2-disubstituted.…”

Ene reactions of indane-1,2,3-trione (a super-enophile) and related vicinal tricarbonyl systems

“…The reaction of hexafluorocydobutanone benefits from the sp2 --, sp3 change at the reaction centre and other aIdehydes and ketones that form stable hydrates also show enhanced reactivity. The catalysis of ene reactions by Lewis acids 2 7 7 has also proven to be of benefit in certain cases where the enophile is of only moderate reactivity. It is necessary for the Lewis acid to co-ordinate an electron pair close to the reactibn site in order to affect the LUMO energy significantly; such sites, for example, are available in chloral and in acrylate esters8…”

The Prins and Carbonyl Ene Reactions

“…The regioselectivity is most likely due to steric interaction between the fOffilaldehyde-BF3 complex and the alkenyl methyl group in the transition state for proton abstraction from the carbon anti to the vinylic hydrogen. Reaction occurs with high facial selectivity from the less hindered~-face of (31) to give an 86: 10 mixture of (33): (34). Reaction with (32) shows little facial selectivity.…”