1,3-Dimethyl-2-phenylbenzimidazoline (DMPBI)-Acetic Acid: An Effective Reagent System for Photoinduced Reductive Transformation of α,β-Epoxy Ketones to β-Hydroxy Ketones

Abstract

A combination of 1,3-dimethyl-2-phenylbenzimidazoline (DMPBI) and acetic acid has been utilized for photoinduced reductive transformation of a,b-epoxy ketones to b-hydroxy ketones. Study on photoreactions using several proton donors revealed that acetic acid is superior to other proton donors such as HCl, p-TsOH, MeOH, and water. 1,3-Dimethyl-2-phenylbenzimidazolium was produced in the reaction with acetic acid while N-benzoyl-N,N¢-dimethyl-o-phenylenediamine was formed in aqueous solvents. When THF solutions containing aryl carbonyl possessing a,b-epoxy ketones and DMPBI and acetic acid were irradiated (l > 280 nm), b-hydroxy ketones were isolated in good to excellent yields. Photosensitized conditions (l > 340 nm) were employed for the reactions of alkyl carbonyl possessing a,b-epoxy ketones.During several decades of the last century, photoreactions of three-membered ring compounds gained much attention due to their interesting reactivities; among these compounds are epoxides. 1 In particular, photoreaction of a,bepoxy ketones were extensively investigated, which demonstrated that several types of photochemical processes are open to these compounds. 2 Selectivity of the bond cleavage of epoxy rings such as C a -O, C b -O, C a -C b usually depends on the substitution and the nature of the reactive excited states. However, if a single electron is injected to carbonyl substituents to generate their radical anions (ketyl radicals), these intermediates would undergo selective C a -O bond cleavage to give distonic radical anions followed by formal two-proton and single-electron transfer (SET) to become b-hydroxy ketones (Scheme 1). Therefore, irradiation of a,b-epoxy ketones in the presence of electron-donating compounds would become a new way to achieve this synthetically important process. 3 About ten years ago, we first examined this concept by conducting photoreactions of several aromatic epoxy ketones in the presence of triethylamine and other electron donors. 4 About the same time, Cossy and co-workers reported photoreactions of several aliphatic as well as aromatic epoxy ketones with triethylamine. 5 In these experiments, while the yields of hydroxy ketones were modest, several problems were also encountered: other compounds such as b-diketones were the major products in some cases 4a and the reactions could not be completed since prolonged irradiation caused decomposition of bhydroxy ketones. 5 Our subsequent mechanistic study demonstrated that selectivity of the reaction pathway either to produce b-diketones or b-hydroxy ketones depends on the nature of the radical cations of amines used. 6 Now, we report herein a method to achieve photoinduced transformation of a,b-epoxy ketones to b-hydroxy ketones by using a combination of 1,3-dimethyl-2-phenylbenzimidazoline (DMPBI) 7 and acetic acid, which is superior to the previously reported DMPBI-based method. 8Irradiation of trans-1,3-diphenyl-2,3-epoxypropane-1-one (chalcone epoxide) (trans-1a) with DMPBI (1.2-1.5 equivalents) in either aqueous THF or wet benzene ...

Sterically Regulated α-Oxygenation of α-Bromocarbonyl Compounds Promoted Using 2-Aryl-1,3-dimethylbenzimidazolines and Air

“…Colorless plates; mp > 300 °C; 1 H NMR (400 MHz, CD 3 CN): δ 3.91 (s, 6H, 2× N -Me), 7.77–7.84 (m, 6H, 5/6-2H of benzimidazolium ring and o -2H and m -2H of 2-Ph), 7.87–7.91 (m, 1H, p -1H of 2-Ph), 7.94–7.97 (m, 2H, 4/7-2H of benzimidazolium ring). These peak patterns are similar to those of 1 H NMR of known salts 1a + X – ,, For comparison, information of iodide 1a + I – , perchlorare 1a + ClO 4 – , and tetrafluoroborate 1a + BF 4 – is also reported (see the Supporting Information).…”

“…Preparation of 1,3-Dimethyl-2-phenylbenzimidazolium Bromide (1a + Br − ). 1,3-Dimethyl-2-phenylbenzimidazoliums 1a + possessing different counter ions, such as iodide, 6a perchlorate, 6a sulfinate, 8a and acetate, 53 are known. In order to measure its reduction potential, bromide salt 1a + Br − was prepared by stirring a mixture of 1,3-dimethyl-2-phenylbenzimidazoline 1a (112.2 mg, 0.50 mmol) and αbromoacetophenone 2c (153.5 mg, 0.77 mmol) in THF (10 mL) at room temperature for 3 h under air.…”

Photochemistry in synthesis: Where, when, and why

“…However, the same process can be carried out under mild conditions provided that the reaction occurs via the excited state, much easier to reduce than the corresponding ground state. In fact, the excited state is reduced by weak electron donors, such as aliphatic amines [29] or benzimidazolines [30]. The EATOS analysis carried out on the thermal process with Ti III and on the hν/NEt 3 systems shows that the main contributions are essentially due to the reagents in the first method and to the solvent in the latter one; indeed, the indexes for the photoprocess are higher, but only because of the solvent (>95 %, see Fig.…”

10.1007/s11178-008-2001-z

“…A photoinduced transformation of α,β-epoxyketones 6 into β-hydroxyketones occurred in the presence of dimethyl-2-phenylbenzimidazoline and acetic acid [24] (Scheme 5 …”