Photochemical [2 + 2] cycloaddition reactions of 6-alkenyl-3-phenylcyclohex-2-en-1-ones: using biradical conformation control to account for exceptions to the “rule of five”

Abstract

A series of 6-alkenyl-3-phenylcyclohex-2-enones has been synthesised and the structures of the products obtained from them on irradiation have been determined. The 6-propenyl compounds afforded a tricyclic 'parallel' [2 + 2] cycloaddition product and a bicyclic enone resulting from hydrogen abstraction in the biradical intermediate. The 6-butenyl and 6-pentenyl analogues gave 'crossed' cycloaddition products only. Although the regiochemistry of these cycloaddition reactions cannot be explained in terms of the 'rule of five', it is compatible with the concept of 'biradical conformation control' which is based on a consideration of the energy and structure of the possible 1,4-biradical intermediates.

Tandem RCM–Claisen Rearrangement–[2+2] Cycloaddition of O,O'-(But-2-en-1,4-diyl)-bridged Binaphthols

“…Transformations of 5a – d into 6a – d and 7a – d , as well as the formation of 11 from epi- 5a , are triggered by light and exited states are obviously involved. Due to their sufficient energy content various diradical structures may be readily derived therefrom and have to be considered as feasible intermediates [ 45 , 46 , 47 ]. A detailed investigation for better understanding of these processes is presently under way and will be published in near future.…”

Recent Advances in the Synthesis of Cyclobutanes by Olefin [2 + 2] Photocycloaddition Reactions

“…The 1,4-diradical intermediates have been invoked to influence the regioselectivity by “biradical conformation control”. 281 …”

“…The 1,4-diradical intermediates have been invoked to influence the regioselectivity by "biradical conformation control". 281 The intermediacy of a triplet 1,4-diradical en route to the cyclobutane product is consequential for the relative configuration of the former olefin carbon atoms. Free rotation around the single bond in the diradical intermediate renders the reaction nonstereospecific and often stereoconvergent.…”

“…Exceptions are known, in particular if the tether is not attached to the α- or β-position of the enone. The 1,4-diradical intermediates have been invoked to influence the regioselectivity by “biradical conformation control” …”

Investigations of an Unexpected [2+2] Photocycloaddition in the Synthesis of (−)-Scabrolide A from Quantum Mechanics Calculations

“…Surprisingly, the desired [4–5] ring system ( 2 ) was not formed in this process; rather, a cis -fused [4–4] product 6 was produced as the exclusive product (Scheme B). The observed selectivity contrasts the “rule of five” generally attributed to enone–olefin cycloadditions, wherein the formation of five-membered rings is considered to be kinetically facile . As a consequence of the undesirable reactivity, three additional steps to protect and subsequently deprotect the isopropenyl olefin had to be incorporated into the synthesis.…”