Photocycloadditionen mit α‐ und β‐Naphthaldehyd: Vollständige Umkehr der Diastereoselektivität als Konsequenz unterschiedlich konfigurierter elektronischer Zustände

Abstract

Photocycloadditions with α‐ and β‐Naphthaldehyde: Complete Inversion of Diastereoselectivity as a Consequence of Differently Configurated Electronic States The diastereoselectivity of the [2 + 2] photocycloaddition reaction between aromatic aldehydes and 2,3‐dihydrofuran ( 3 ) is completely inverted when changing the electronic configuration of the excited triplet from 3 (nη*) to 3 (nπ * ). Mesitylenaldehyde, which corresponds to the first case, reacts endo ‐ selectively, whereas α‐ and β‐naphthaldehyde (with low‐lying ππ * triplets and reactive nπ * singlets) react exo ‐selectively. The configuration of the α‐naphthaldehyde adduct is proven by means of X‐ray structure analysis. This type of spin‐correlation effect can be rationalized by comparing the conformation of the reactive biradical intermediates for rapid inter‐system crossing and/or bond closure. In the furan ( 1 ) case, this effect cannot be observed. All carbonyl compounds investigated here react exo ‐selectively. A possible explanation for this pronounced selectivity is the assumption of a secondary orbital effect.