Regio- and stereoselectivity in the Paternò–Büchi reaction on furan derivatives

Abstract

The photochemical coupling reaction between 2,3-dihydrofuran and benzaldehyde was studied by using DFT/B3LYP/6 − 31G + (d, p) method. The regiocontrol of the attack of the benzaldehyde on the double bond is related to the different stabilities of the biradical intermediates. The endo stereoselectivity of the reaction depends on the superposition between HSOMO and LSOMO in the biradical intermediate. In the photochemical reaction between furan and benzaldehyde also the regiocontrol depends on the relative stability of the possible biradical intermediates. The exo stereoselectivity of the coupling reaction depends on the superposition between the HSOMO and LSOMO of the biradical intermediate. The reaction of chiral phenylglyoxylates with furan gave the corresponding adducts with de = 15-95%. The stereocontrol can be explained considering the energy gap between the biradical intermediates in the coupling reaction. When the reaction was performed in the presence of zeolite, the diastereoisomeric excess increased. The reaction of benzoin and 2-phenylpropiophenone with furan gave the cycloadduct with high diastereocontrol. All the products were obtained with de > 98%. The Paterrnò-Büchi reaction between 2-furylmethanols with aromatic carbonyl compounds also showed high regio-and stereocontrol. On the contrary, when 5-methyl derivatives were used, a lack of regiocontrol was observed. Furthermore, with aliphatic carbonyl compounds, no diastereoselectivity was observed. These results were explained assuming the attack of the excited carbonyl compound on the same side as the hydroxyl group, through the formation of a hydrogen bond or of a complex. This type of attack gave the biradical intermediate in preferential conformations. The relative energies of these conformers account for the observed diastereoselectivity.

Structural Dependence of Electronic Properties in A-A-D-A-A-Type Organic Solar Cell Material

“…The dispersion-corrected B3LYP functional was chosen because our previous study [20] on CPs has shown that it yields accurate geometry and torsional potentials. The robustness of B3LYP functional to produce accurate geometries and energies of SCMs and CPs has also been validated in similar studies [21][22][23][24][25].…”

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.…”

Paternò-Büchi reaction between aromatic carbonyl compounds and 1-(3-furyl)alkanols

“…The regioselectivity of the reaction depended on the relative stability of the biradical intermediate. 1 In this case, addition of an excited carbonyl compound to C2 forms a tertiary radical site at C3, while such addition to C5 would generate only a secondary radical at C4. Unfortunately, the reaction of 11 in the presence of benzaldehyde did not occur (Scheme 3, Table 1).…”

“…The stereoselectivity of the Patern ò-B üchi reaction on allylic alcohols has been studied in the last years. 1 Adam and co-workers showed that the allylic alcohol 1 reacted with benzophenone to give the corresponding adducts 2 and 3 with high regio-and stereoselectivity (Scheme 1). 2 Scheme 1 Patern ò-B üchi reaction on allylic alcohols.…”