Decatungstate Anion for Photocatalyzed “Window Ledge” Reactions

Abstract

The majority of organic reactions are commonly carried out inside a lab, under a fume hood. A particular case is that of photochemical reactions, a field where the pioneering experiments by Giacomo Ciamician demonstrated more than one century ago that different processes can be carried out outdoors, for example, on the balcony of his own department, upon exposure of the reacting mixtures to sunlight. The main problem related to this chemistry of the "window ledge" is that most organic compounds are colorless and their absorption in the solar light region is in most cases negligible. Recently, the impressive development in the use of visible light absorbing photocatalysts (e.g., Ru or Ir complexes, as well as organic dyes) made light-induced processes convenient even for non-photochemistry practitioners. It is thus possible to easily perform the reactions by simply placing the reaction vessel in a sunny place outside the lab. However, most of these processes are based on single electron transfer (SET) reactions (photoredox catalysis). Other photocatalysts able to activate substrates via alternative paths, such as hydrogen atom transfer (HAT), are emerging. In the last years, we were deeply involved in the use of the decatungstate anion ([WO], a polyoxometalate) in synthesis. Indeed, such a versatile species is able to promote the photocatalytic C-H activation of organic compounds via either SET or HAT reactions. Interestingly, though the absorption spectrum of [WO] does not extend into the visible region, it shows an overlap with solar light emission. In this Account, we provide an overview on the application of decatungstate salts as photocatalysts in window ledge chemistry. We initially discuss the nature of the photogenerated species involved in the mechanism of action of the anion, also supported by theoretical simulations. The first-formed excited state of the decatungstate anion decays rapidly to the active species, a dark state tagged wO, featuring the presence of electron-deficient oxygen centers. Next, we describe the main applications of decatungstate chemistry. A significant part of this Account is devoted to photocatalyzed synthesis (C-X bond formation, with X = C, N, O, and oxidations) carried out by adopting sunlight (or simulated solar light). This synthetic approach is versatile, and most of the reactions involved C-H activation in cycloalkanes, alkylaromatics, amides, ethers (1,4-dioxane, oxetane, benzodioxole, and THF), aldehydes, nitriles, and cyclopentanones, and the ensuing addition of the resulting radicals onto electron-deficient olefins. Finally, the increasing role of the decatungstate anion in water depollution and polymerization is briefly discussed.

Base Catalysis by Mono- and Polyoxometalates

“…into the vacant sites has been used to synthesize several metal-substituted POMs with controlled multinuclear active sites in both aqueous and organic media [7,11,23]. These molecular catalysts exhibit significant activity and selectivity for a wide range of reactions [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] [92][93][94].…”

Selective C(sp3)−H Aerobic Oxidation Enabled by Decatungstate Photocatalysis in Flow

“…Initial reaction optimization was carried out in a cylindrical batch reactor, which allows maximization of the gas–liquid interfacial area and irradiation surface in batch 20a. The reaction mixture was subjected to irradiation generated by a solar simulator.…”

Vinylpyridines as Building Blocks for the Photocatalyzed Synthesis of Alkylpyridines

“…By contrast, the direct generation of radicals by cleavage of a C−H bond still represents a difficult challenge and has not been exploited for vinylpyridine derivatization so far. We surmised that the formation of (carbon‐centered) radicals could be attained by a homolytic C−H cleavage through a hydrogen‐atom transfer (HAT) by using a decatungstate salt {TBADT, ( n Bu 4 N) 4 [W 10 O 32 ]} as the photocatalyst (Scheme , path d) …”

“…As for the mechanism, the formation of carbon‐centered radicals is promoted by excited TBADT through a HAT reaction (Scheme ) . Vinylpyridines behave as radical traps, and regioselective addition at the β position proceeded smoothly.…”

“…By contrast, the direct generation of radicals by cleavage of aC ÀH bond still represents ad ifficult challenge and has not been exploited for vinylpyridine derivatization so far.W es urmised that the formation of (carbon-centered) radicals could be attained by ah omolytic CÀHc leavage through ah ydrogen-atom transfer (HAT) [12] by using ad ecatungstate salt {TBADT, (nBu 4 N) 4 [W 10 O 32 ]} as the photocatalyst (Scheme 1, path d). [13,14] Firstly,w eo ptimized the reaction conditions by studying the photocatalyzed addition of cyclohexane (1g)o nto 4-vinylpyridine (2a)t oa fford 4-(2-cyclohexylethyl)pyridine (9)a samodel reaction. As reported in Ta ble S1 in the Supporting Information, the highest yield of 9 (68 %) was obtained by irradiating (l IRR centered at 310 nm;1 0 15 Wp hosphor-coated lamps were used) aM eCN solution of 2a (0.1 m)f or 16 hi nt he presence of TBADT (2 mol %; l MAX = 323 nm) and an excess of 1g (5 equiv).…”