Boosting activity of molecular oxygen by pyridinium-based photocatalysts for metal-free alcohol oxidation

Abstract

Pyridinium molecules are developed as electron- and energy-transfer mediators to boost the activity of air oxygen and thus achieve highly efficient photocatalytic oxidation of alcohols without any noble metal and additional co-catalysts/additives under mild conditions.

Visible Light-Induced Aerobic Oxidative Dehydrogenation of C–N/C–O to C=N/C=O Bonds Using Metal-Free Photocatalysts: Recent Developments

“…The commonly accepted mechanism for the aerobic photocatalytic oxidative dehydrogenation of alcohols is shown in Scheme 16 [148]. Firstly, the photocatalyst (PC) is excited by irradiation with visible light to form its excited state (PC*), which is converted to its free radical state (PC • ) by an electron transfer process, and the cation radical of the alcohol is formed.…”

“…This group includes a tetrazine-based catalyst (pytz, 75) using two methodologies [171], a dicyanopyrazine-derived chromophore (DPZ, 76) used in cooperation with N-hydroxysuccinimide (NHS) and dibenzyl phosphoric acid (DBPA) [172], and an iminoquinone-based catalyst (PA, 77) [173], enabling all of them to obtain aldehydes and ketones 69 from alcohols in moderate to excellent yields (Figure 21). In addition, a carbazole-based promoter (4CzIPN, 78) with quinuclidine and tetrabutylammonium phosphate (TBAP) as additives [174] and a pyridinium derivative (TETPY•3Br, 79) [148] were successfully used in the same mentioned transformation (Figure 22). The carbazole photocatalyst 78 has also been employed in a novel strategy of oxidizing alco-hols to aldehydes and coupling them in situ with amines to generate the corresponding amides [175].…”

“…In addition, a carbazole-based promoter (4CzIPN, 78) with quinuclidine and tetrabutylammonium phosphate (TBAP) as additives [174] and a pyridinium derivative (TETPY•3Br, 79) [148] were successfully used in the same mentioned transformation (Figure 22). The carbazole photocatalyst 78 has also been employed in a novel strategy of oxidizing alcohols to aldehydes and coupling them in situ with amines to generate the corresponding amides [175].…”

Redox-active and Brønsted basic dual sites for photocatalytic activation of benzylic C–H bonds based on pyridinium derivatives

“…[11][12][13][14][15][16][17][18][19][20][21] Since direct catalytic oxidation by molecular oxygen is often kinetically unfavorable due to the spin-restriction of its triplet ground state, 22 an innovative alternative has been developed by first abstracting hydrogen from the C-H bond to form a carbon radical, which subsequently reacts with molecular oxygen to produce oxygenated compounds. 23,24 The representative approaches include the employment of N-oxyl radicals for H abstraction from the C-H bond in the presence of radical initiators, 25,26 decatungstatemediated photocatalysis with the assistance of acids or metal salts, 27,28 cercosporin or benzoquinone derivative inspired photooxidation with additives or cocatalysts, 29 semiconductor photocatalytic oxidation in which photogenerated holes drive the dissociation of the C-H bond to form alkyl radical intermediates, etc. [30][31][32] However, finding ways to overcome the disadvantages such as the requirement of additives or co-catalysts, substrate limitations, high catalyst loading and low efficiency is still a tough nut to crack.…”

“…[37][38][39][40] By using pyridinium-mediated electron transfer and energy transfer for the activation of molecular oxygen to produce • O 2 − (superoxide radical) and 1 O 2 (singlet oxygen), we have achieved selective photocatalytic oxidation of alcohols to ketones or aldehydes using air at room temperature without any metals and co-catalysts/additives. 25 In continuation of our goal to develop sustainable protocols for green organic synthesis by using pyridinium-based photocatalysts, we find that the pyridinium deriva-tive containing a neutral N-heterocyclic unit may provide a concerted pathway for C-H bond activation through protoncoupled electron transfer (PCET) 26 based on the combination of the strong electron-accepting ability of the pyridinium units and Brønsted basic site of the N-heterocyclic unit (Scheme 1), thus promoting hydrogen abstraction that is an essential step for activating the C-H bond. 41 The current photocatalytic system is effective for toluene, ethylbenzene, p-xylene and other hydrocarbons containing benzylic C-H bonds under mild conditions without the assistance of any additives and cocatalysts.…”