The Paternò–Büchi reaction: importance of triplet states in the excited-state reaction pathway

Abstract

The Paternò-Büchi (PB) reaction between an excited carbonyl compound and an alkene has been widely studied, but so far little is known about the excited-state dynamics of the reaction. In this investigation, we used a compound in which a formyl and a vinyl group are attached to a [2.2]paracyclophane in order to obtain a model system in pre-reactive conformation for the PB reaction. We studied the excited-state dynamics of the isolated molecule in a molecular beam using femtosecond time-resolved photoelectron spectroscopy and ab initio calculations. The results show that inter-system crossing within two picoseconds competes efficiently with the reaction in the singlet manifold. Thus, the PB reaction in this model system takes place in the triplet state on a time scale of nanoseconds. This result stresses the importance of triplet states in the excited-state pathway of the PB reaction involving aromatic carbonyl compounds, even in situations in which the reacting moieties are in immediate vicinity.

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.…”

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

“…When the reactants attached to each other on the S 1 PES, S 1 /T 1 -MSXs may again play a key role, since spin-orbital coupling values at these S 1 /T 1 -MSXs (∼ 50 cm −1 ) are large enough to cause a ultrafast ISC, according to a recent surface hopping simulation study [63]. Recent combined spectroscopic and ab initio studies did observed a ultrafast S 1 to T 1 ISC for some intramlecular PB reactions [54]. This ISC was explained by a crossing point between S 1 and a high-lying (repulsive) T 3 state outside the potential well of biradical intermediate found along a linear interpolation path between a S 0 /S 1 -CI and the Franck-Condon point, although no crossing with T 1 was further studied [54].…”

“…Recent combined spectroscopic and ab initio studies did observed a ultrafast S 1 to T 1 ISC for some intramlecular PB reactions [54]. This ISC was explained by a crossing point between S 1 and a high-lying (repulsive) T 3 state outside the potential well of biradical intermediate found along a linear interpolation path between a S 0 /S 1 -CI and the Franck-Condon point, although no crossing with T 1 was further studied [54]. We suggest alternately that S 1 /T 1 -MSXs similar to those obtained in this study would have a significant role also in the ultrafast ISC observed for these intramlecular PB reactions.…”

“…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.…”