An argument for abandoning the "allowed" and "forbidden" classification of electrocyclic reactions.

Carpenter, Barry K. ORCID: https://orcid.org/0000-0002-5470-0278 2025. An argument for abandoning the "allowed" and "forbidden" classification of electrocyclic reactions. Chemical Science 10.1039/d4sc08748h

PDF - Published Version Available under License Creative Commons Attribution. Download (1MB)

Abstract

The division of electrocyclic reactions into "allowed" and "forbidden" classes carries the implication that reactions of the latter class are so energetically penalised that they will occur only if their "allowed" alternatives are rendered effectively impossible. The present work tests that assumption, using NEVPT2 and DFT calculations on a variety of cyclobutene ring openings and (Z)-1,3,5-hexatriene ring closures, and their benzannelated congeners. The results show the assumption to be incorrect. The potential energy differences between "forbidden" and "allowed" transition states are found to cover a wide range of values, with the smallest being less than half the classical barrier to internal rotation of ethane. It follows that planning a total synthesis on the presumption that electrocyclic reactions will always follow the "allowed" stereochemical course is an unreliable strategy because other commonly occurring factors, such as routine steric and electronic substituent effects, can easily outweigh the electronic penalty for following the nominally forbidden mechansim. A particular case involving a proposed synthetic route to a class of anticancer compounds is highlighted as an example.

Item Type:	Article
Date Type:	Published Online
Status:	In Press
Schools:	Schools > Chemistry
Additional Information:	License information from Publisher: LICENSE 1: Title: cc by, Type: cc by
Publisher:	Royal Society of Chemistry
ISSN:	2041-6520
Date of First Compliant Deposit:	3 March 2025
Date of Acceptance:	23 January 2025
Last Modified:	03 Mar 2025 10:15
URI:	https://orca.cardiff.ac.uk/id/eprint/176573

Actions (repository staff only)

Edit Item