Abstract
The simple (non‐induced) diastereoselectivity of the photocycloaddition of aliphatic as well as aromatic aldehydes to 2,2‐dimethyl‐2,3‐dihydrofuran (1) was analyzed as a function of the substrate concentration (spin mapping) and rationalized in terms of optimal spin‐orbit coupling controlled triplet biradical geometries.
Highlights
The technique of “spin-mapping” of diastereoselectivities of bimolecular photochemical reactions is of special relevance in reactions of electronically excited molecules and decisive in controlling and manipulating the stereochemistry of photocycloaddition reactions [1]
In a series of publications, we have recently described the effect of substrate concentration on the spin-selectivity [2] of the Paternò-Büchi reactions of aliphatic aldehydes with 2,3-dihydrofuran [3], allylic alcohols [4], and cyclooctene [5], as well as temperature and viscosity dependence of these reactions [6]
Paternò-Büchi reactions with singlet excited carbonyls most probably proceed via conical intersections, the reactions between triplet excited carbonyls and alkenes pass through triplet 1,4-biradical intermediates [8]
Summary
The technique of “spin-mapping” of diastereoselectivities of bimolecular photochemical reactions is of special relevance in reactions of electronically excited molecules and decisive in controlling and manipulating the stereochemistry of photocycloaddition reactions [1]. Whereas benzaldehyde and derivatives are applied as reactive triplet species, naphthaldehydes can be used in Paternò-Büchi reactions as reactive singlet excited components [9]. The Paternò-Büchi reaction of β-naphthaldehyde with 2,3-dihydrofuran is a singlet process (as shown by—non productive-triplet sensitization experiments) and gave predominantely the thermodynamically favoured exo-diastereoisomer. In order to expand our recent studies on the spin-mapping of endo/exo-diastereoselectivities, we envisaged to modify the cycloalkene part in such a way that the thermodynamic preference of the exodiastereoisomer is strongly enhanced. From such an approach we expected a more distinct effect on the.
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