Intersystem Crossing in Triplet 1,4-Biradicals: Conformational Memory Effects on the Stereoselectivity of Photocycloaddition Reactions

Abstract

Biradicals have been postulated as intermediates in many organic reactions and described in excellent articles.2 Even ground-state transformations allowed from the Wood war d-Hoffmann orbital symmetry rules have been discussed as proceeding via l,n-biradicals.3The latter assumptions usually originate from calculations rather than from experimental facts. In the

Spin‐dependent diastereoselectivity in the photocycloaddition of aldehydes to 2,2‐dimethyl‐2,3‐dihydrofuran

“…An extreme example was 1,2-dimethylcyclobutene: the Paternò-Büchi reaction of this substrate with benzaldehyde resulted solely in the exo-diastereomer. This is exactly predicted by the "ISC-geometry model" [12] which is depicted in Figure 1 and relates to an orthogonal approach of the radical centers in a triplet 1,4-biradical because of an optimized spin-orbit coupling (SOC) value [13].…”

Tracking a Paternò–Büchi Reaction in Real Time Using Transient Electronic and Vibrational Spectroscopies

“…The UV photophysics of benzaldehyde has been investigated extensively; 27-30 the consensus view is that initial photoexcitation to the first excited singlet (S1) state leads to rapid (~100 fs timescale) intersystem crossing (ISC) to either (or both of) the first (T1) or second (T2) triplet excited states. 29 Neither relating to PB reactions [6][7][8][9][10][11][12][13][14][15][16][17]19 suggests that these absorptions are signatures of the biradical intermediate(s) formed by reaction between benzaldehyde(T1) and cyclohexene. Once formed, this biradical would be expected to either undergo intersystem crossing prior to ring-closure (to form the final oxetane), or dissociation to reform benzaldehyde in its S0 state, it is not clear from these measurements whether ISC occurs before or after dissociation has occurred.…”

“…There is still much to explore about the detailed dynamical factors that influence (and determine) the branching between ring-closure and dissociation of the biradical intermediate 10 and, as this report shows, ultrafast transient absorption spectroscopies could be an important tool in advancing such understanding.…”

“…5 Since these initial works, numerous studies have been devoted to exploring mechanistic details of several PB reactions with differing substrates. [6][7][8][9][10][11][12][13][14][15][16][17] Reaction between the carbonyl and the alkene can proceed by attack at either the C or the O atom of the carbonyl group, forming one of two possible biradical intermediates. Which is formed is dependent on their relative stabilities, 18 and determines the regioselectivity of the resulting products.…”

Exploring Pathways of Photoaddition Reactions by Artificial Force Induced Reaction Method: A Case Study on the Paternò–Büchi Reaction

“…In this study, a photoaddition reaction between an aldehyde (or ketone) and an alkene known as Paternò-Büchi (PB) reaction [42][43][44][45] is considered. Its mechanism has been studied extensively both experimentally and theoretically [46][47][48][49][50][51][52][53][54]. Thus, this reaction is ideal for the present purpose of benchmarking the applicability to photochemical addition reactions.…”