Photo-catalyzed -cycloaddition

Irradiation of the reaction mixture with UV-Vis light (350-500 nm), or sunlight enabled the reaction to be carried out at room temperature. The photochemistry process provided improved chemoselectivity by avoiding the homocyclotrimerization for the alkynes. For example, photo-catalyzed [2 + 2 + 2]-cycloaddition of various nitriles with acetylene at room temperature in the presence of CpCo(COD)2 provided the corresponding 2-pyridines such as 271 in 3-4 h in good to excellent yields. ... 

Asymmetric synthesis of the rocaglamides was accomplished by employing [3+2] photo-cycloaddition mediated by functionalized TADDOL based chiral Brpnsted acids. The synthesis consisted of a [3+2] dipolar cycloaddition, a base-mediated a-ketol rearrangement and a hydroxyl-directed reaction <06JA7754>. Asymmetric synthesis of 1,2-dihydrobenzo /j]furans was achieved by adamantylglycine derived dirhodium tetracarboxylate catalyzed C-H insertion <06OL3437>. 

Olefins are also susceptible to cycloaddition reactions (Fig 105) (150). In particular, some olefin-containing APIs can dimerize with another molecule of API to form a 2+2 cycloaddition product under photo conditions (151). A classic example of such a 2+2 cycloaddition catalyzed by UV radiation is that of the nucleoside thymidine (Fig. 106) (152,153). These reactions are proposed to go through more than one mechanism concerted, diradical, electron transfer, and radical ion pairs. 

The photo-cycloaddition of ethylene to 3-alkoxy chromones such as 315 has been applied to the synthesis of marine sesquiterpene filiformin and congeners (Scheme 43) <1996JOC4391>. Tandem [2+2] 7i-photo-cycloaddi-tion and y-hydrogen abstraction provided tetracyclic intermediate 316 which was converted to terpene 317 by subsequent oxetane ring reduction and acid-catalyzed rearrangement. 

Despite the fact that the [2 + 2] -photocycloaddition reaction of enones has a history of more than 100 years, it has remained a vital and attractive reaction. The continuing interest and many applications to increasingly more complex targets not only bear testimony to its utility but also contradict the myth that photochemical reactions are nonselective and unpredictable. It would be desirable if this most useful chemistry could also be appreciated in the life sciences industry. The first blockbuster drug to be synthesized via a [2 + 2]-photocycloaddition is yet to be developed. Apart from the conventional evolution of the reaction, which involves an increase in scope and an improvement in its practical execution [154], it is expected that sensitization - as a means of catalyzing photochemical reactions in general [155-157] - will become a dominant factor in the development of catalytic enantioselective [2 + 2]-photo-cycloaddition variants. 

Stimulated by the early work of Bard et al. on the Ti02-catalyzed photo-Kolbe reaction [36], many papers appeared in subsequent years dealing with photocatalytic organic reactions [115] in the presence of colloidal or suspended semiconductor particles. They include cis-trans isomerizations [68, 93, 116-119], valence isomer-izations [120, 121], substitution and cycloaddition reactions [73, 80, 122-125], oxidations [126, 127], and reductions [128-130]. Characteristic of all these reactions is that in almost all cases well known compounds were formed, which were not isolated but only characterized by spectroscopic methods. The nature of the products can be rationalized within the mechanistic scheme of semiconductor photocatalysis type A, which means that at least one reduced and one oxidized compound are... 

Photo-cross-linkable oligophenols were synthesized by the HRP-catalyzed polymerization of cinnamoyl-hydroquinone-ester and cinnamoyl-4-hydroxyanilide in an aqueous 1,4-dioxane.40 UV irradiation of the oligomer films induced the cross-linking by the photochemical [2+2]-cycloaddition of the cinnamoyl function. 

As with other diazo compounds, cyclopropanation with 1 or 2 can proceed either via a photo-chemically or thermally generated carbene, in a transition-metal-catalyzed reaction, or by [3 -I- 2] cycloaddition of the diazo function to the C-C double bond followed by nitrogen extrusion from the thermally unstable cycloadduct. 

A full account of the Ru(PyBox)-catalyzed enantioselective 13-dipolar cycloaddition between 2,3-dihydrofuran and diazopyruvates, first described in 2004, was reported <05HCA101Q>. 2,3-Dihydrofurans having a 3-acetyl group, e.g. benzocycloalla[l,2-fojfurans and spiro[furan-2(37/),r-benzocycloalkane], underwent benzannulation via photo-induced cleavage of the dihydrofuran ring <05rL7303>. An example that produced a helicene-type compound is shown below. 

Some syntheses and reactions of benzotriazoles were reported. Copper-catalyzed C-N coupling of 2-iodoaryltriazines 163 with sodium azide and intramolecular addition of nitrene to N=N bonds afforded N-aryl-2H-benzotriazoles 164 (14CEJ1825). Photo-initiated benzyne-based [3 + 2] cycloaddition of triazene benzoic acid 165 afforded N-substituted-lH-benzotriazoles 166 (140L2003). An efficient synthetic... 

Chiral l-aryl-5,6,7,8-tetrahydroquinolines (136) were synthesized by photo induced chiral cobalt-catalyzed [2-1-2-1-2] cycloaddition reaction of l-aryl-l,7-octadiynes (134) and nitriles (135). ... 

F. 10 Schematic representation for the photo-affinity-based ABPP strategy. The structure of HDAC photo-crosslinking probe (SAHA-BPyne) is shown in the middle [115]. ABPP activity-based protein profiling, HDAC histone deacetylase, SAHA suberoylanilide hydroxamic acid, CuAAC copper(l)-catalyzed azide-alkyne cycloaddition, LC-MS/MS liquid chromatography-tandem mass spectrometry... 

Generation of chemical patterns with photoreactive polymers photo-Diels-Alder surface anchoring followed by azide-alkyne click reaction to immobilize fluorescent dyes, (a) SPAAC strain-promoted azide-alkyne cycloaddition and (b) CuAAC Cu(l)-catalyzed azide-alkyne cycloaddition. (Source Adapted with permission from Reference [36d].)... 

In 2015, organic solvent soluble AuNPs stabilized by mono-sulfhydryl functionalized pillar[5]arene (SH-P5) were prepared by Zhou et al. [31] Quaternary ammonium salt functionalized anthracene has reversible noncovalent host-guest interactions with SH-P5, which can lead to reversible assembly of AuNPs. Based on photo [4-f 4] cycloaddition of anthracene, they can reversibly aggregate and disassemble by variation of the temperature and/or photo-irradiation, and can catalyze borohydride reduction of 2,6-dichloro-4-nitrophenol. 

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