Stereoselectivity of Triplet Photocycloadditions:¹Diene−Carbonyl Reactions and Solvent Effects

Abstract

The diastereoselectivity of the photocycloaddition of benzaldehyde to furan was determined (exo/ endo ) 212:1) and compared with the reaction of other carbonyl compounds and carbonyl analogues. These results were compared with Paternò-Bü chi reactions of cycloalkenes and cyclic enol ethers. An increase in steric demand of the R-substituent in benzoyl compounds led to a change in exo/ endo-selectivity for furan cycloadditions that was not observed for cycloalkenes or cyclic enol ethers. Different ISC-reactive conformers with enhanced spin-orbit coupling are postulated as a reasonable explanation for the stereoselectivities observed. Additionally, solvent effects were studied, demonstrating the influence of photoinduced electron-transfer steps on the regio-and diastereoselectivity of Paternò-Bü chi reactions with 2,3-dihydrofuran in polar solvents. Two bicyclic oxetanes (8 and 10) were characterized by X-ray structure analysis.

Paterno-Büchi reaction on 5-methyl-2-furylmethanol derivatives

“…In our previous work 14 we formulated the hypothesis that the photochemical reaction of 2-furylmethanols with aromatic ketones involves a single electron transfer from the furan derivatives to the ketone followed by radical coupling with the formation of zwitterionic species. In contrast with current opinions in this field, 1 we thought that the most probable structures for these zwitteronic species would be those represented in the Figure. Our point of view can be supported by results of theoretical calculations showing that stability of intermediates explains the regiochemical behavior of the reaction. 14 We tested this hypothesis trying to explain the observed regiochemical behavior on 4 by respective calculatons.…”

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.…”

“…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.…”

Synthetic Applications of Photoinduced Electron Transfer Decarboxylation Reactions

“…the preparation of macrocyclic molecules. 4 The chemistry which we are describing in this report is connected to previous experiments in the field of electron transfer reactions with triplet excited carbonyl components as electron acceptors and thioethers 5 , alkenes 6 , alcohols 7 and arenes 7 as the electron donating groups in inter-and intramolecular photoadditions or photo-cyclizations, respectively. The principle behind these reactions is shown in Scheme 1.…”