Solar synthesis: prospects in visible light photocatalysis

Abstract

Chemists have long aspired to synthesize molecules the way that plants do-using sunlight to facilitate the construction of complex molecular architectures. Nevertheless, the use of visible light in photochemical synthesis is fundamentally challenging because organic molecules tend not to interact with the wavelengths of visible light that are most strongly emitted in the solar spectrum. Recent research has begun to leverage the ability of visible light-absorbing transition metal complexes to catalyze a broad range of synthetically valuable reactions. In this review, we highlight how an understanding of the mechanisms of photocatalytic activation available to these transition metal complexes, and of the general reactivity patterns of the intermediates accessible via visible light photocatalysis, has accelerated the development of this diverse suite of reactions.

Fig. 1. Converting solar energy into chemical potential

Certain transition metal complexes are strong absorbers of visible light and can thereby harness solar energy for chemical synthesis, particularly by driving radical-mediated transformations from their photoexcited states (10).

Fig. 2. Visible light photocatalysis

(A) Ruthenium and iridium complexes, such as 1–3, readily absorb visible light and can mediate numerous photochemical transformations (13, 14). (B) Photoexcited Ru*(bpy)₃²⁺ can act as an electron shuttle, interacting with sacrificial electron donors D (path i) or acceptors A (path ii) to yield either a strongly reducing or oxidizing catalyst toward organic substrates S. Ru*(bpy)₃²⁺ can also directly transfer energy to an organic substrate to yield electronically excited species (path iii). Abbreviations: bpy, 2,2'-bipyridine; bpz, 2,2’-bipyrazine; ppy, 2-phenylpyridine.

Fig. 3. Photoreduction of alkyl halides for radical reactions

(A) Photoexcited ruthenium or iridium complexes can reduce electron deficient alkyl halides to radical ions that readily undergo fragmentation to an electrophilic organic radical. (B) This reactivity was first explored by Fukuzumi in the dehalogenation of α-halocarbonyl compounds (21) and later revisited by MacMillan (C) in the context of an asymmetric α-alkylation of aldehydes through the merging of photo and organocatalysis (23). (D) The generation of radicals through visible light photoreduction of alkyl halides is mild and selective, as demonstrated in the synthesis of (+)-gliocladin C (30). (E) The range of alkyl an aryl halides susceptible to photoreduction can be extended by tuning the photoelectrochemical properties of the catalyst (31). Abbreviations: Acr-H₂, 9,10-dihydro-10-methylacridine; Boc, tert-butyloxycarbonyl; Bu, butyl; Cbz, carbobenzyloxy; LED, light-emitting diode; Ph, phenyl; t-Bu, tertiary-butyl.

Fig. 4. Applications of photocatalysis in polymerization and organometallic chemistry

(A) The ability to temporally control radical formation under photocatalytic conditions has important benefits in atom transfer radical polymerization reactions (33). Turning off the light source halts polymerization to allow a new monomer to be added (monomer B) before polymerization is reinitialized. (B) Likewise, photogenerated radical species can be intercepted with either copper or palladium organometallic complexes in co-catalytic transformations (34, 35). Abbreviations: Ar, aryl; Bn, benzyl; OAc, acetate; Ph, phenyl.

Fig. 5. Diverse reactivity of α-amino radical cations

(A) Tertiary amines readily undergo photooxidation to yield a highly versatile amine radical cation intermediate, which can be transformed into a nucleophilic (α-amino radical) or electrophilic (iminium ion) species. (B) Pandey and Reiser were able to functionalize tetrahydroisoquinolines through the formation of an α-amino radical that readily intercepted various electrophiles (41). (C) Through the utilization of a more strongly reducing iridium photocatalyst (2) MacMillan and coworkers were able to intercept α-amino radicals with cyanoarenes, overall providing a route for α-acylating amines (43). (D) Depending upon the reaction conditions, α-amino radicals can undergo further oxidation to electrophilic iminium ions that can subsequently be trapped with nucleophilic reagents (44). Abbreviations: dtbbpy, 4,4’-di-tert-butyl-2,2’-bipyridyl; LED, light-emitting diode; Me, methyl; Ph, phenyl; ppy, 2-phenylpyridine.

Fig. 6. Unique reactivity of photogenerated radical ions

Radical ions are underutilized reactive intermediates that can participate in otherwise inaccessible bond formations. (A) Yoon has demonstrated that the photocatalytic activation of enones produces radical anions that readily participate in [2+2] cycloadditions to afford cyclobutane products that are not generated upon UV irradiation (60). (B) Likewise, the photooxidation of electron-rich styrenes yields an electron-deficient radical cation that undergoes facile [4+2] cycloaddition with an electron rich diene, a reaction that is disfavored under thermal conditions (63). (C) Radical ion intermediates also afford products with atypical atom connectivities, such as the exclusive formation of the less common anti-Markovnikov product under photochemical conditions (64). Abbreviations: i-Pr, iso-propyl; Me, methyl; Mes, mesityl; n-Bu, normal-butyl; OTf, trifluoromethanesulfonate; Ph, phenyl; UV, ultraviolet.

Fig. 7. Visible light photocatalyzed triplet sensitization

(A) Visible light induced energy transfer sensitization involves coupling the generation of an electronically excited organic substrate to the relaxation of a photoexcited transition metal chromophore (13). (B) Iridium catalyst 8 mediates the facile [2+2] cycloaddition of styrenes (70). Triplet sensitization provides a means to access interesting and highly strained cyclobutane frameworks that are prevalent in natural products and would be difficult to construct through other methods. Abbreviations: dF(CF₃)ppy, 2-(2,4-difluorophenyl)-5-trifluoromethylpyridine, dtbbpy, 4,4’-di-tert-butyl-2,2’-dipyridyl; ISC, intersystem crossing; Me, methyl; Ph, phenyl; t-Bu, tertiary-butyl.