Photochemical α-Aminonitrile Synthesis Using Zn-Phthalocyanines as Near-Infrared Photocatalysts

Abstract

While photochemical transformations with sunlight almost exclusively utilize the UV–vis part of the solar spectrum, the majority of the photons emitted by the sun have frequencies in the near-infrared region. Phthalocyanines show high structural similarity to the naturally occurring light-harvesting porphyrins, chlorins, and mainly bacteriochlorins and are also known for being efficient and affordable near-infrared light absorbers as well as triplet sensitizers for the production of singlet oxygen. Although having been neglected for a long time in synthetic organic chemistry due to their low solubility and high tendency toward aggregation, their unique photophysical properties and chemical robustness make phthalocyanines attractive photocatalysts for the application in near-infrared-light-driven synthesis strategies. Herein, we report a cheap, simple, and efficient photocatalytic protocol, which is easily scalable under continuous-flow conditions. Various phthalocyanines were studied as near-infrared photosensitizers in oxidative cyanations of tertiary amines to generate α-aminonitriles, a synthetically versatile compound class.

Zinc(II) Complexes with Triplet Charge-Transfer Excited States Enabling Energy-Transfer Catalysis, Photoinduced Electron Transfer, and Upconversion

“…With their photoactive 3 ILCT excited states, both [Zn­( m -L) 2 ] and [Zn­( p -L) 2 ] should in principle be able to sensitize triplet–triplet annihilation upconversion, which is an attractive process for the conversion of low-energy input light into higher energy output radiation. ,− In particular, upconversion from the visible to the ultraviolet spectral region has received increasing interest lately, − and our Zn II complexes with their relatively high 3 ILCT energies seemed promising for this purpose. Previous upconversion studies with Zn II sensitizers focused on red-to-blue upconversion and on near-infrared sensitization of photochemical reactions. − We identified the combination of [Zn­( m -L) 2 ] and (TMS) 2 napht (Figure e) as particularly promising for blue to UV upconversion. Unsubstituted naphthalene has a triplet energy of 2.62 eV, which is too high for both our Zn II complexes (Figure ).…”

“…Previous upconversion studies with Zn II sensitizers focused on red-to-blue upconversion and on near-infrared sensitization of photochemical reactions. 103 − 106 We identified the combination of [Zn( m -L) 2 ] and (TMS) 2 napht ( Figure 8 e) as particularly promising for blue to UV upconversion. Unsubstituted naphthalene has a triplet energy of 2.62 eV, 84 which is too high for both our Zn II complexes ( Figure 4 ).…”

Sensitizer-controlled photochemical reactivityviaupconversion of red light

“…The number of photocatalysts with suitable absorption properties in the red spectral range is somewhat limited, 31,[69][70][71][72] and we chose a well-known polypyridyl complex of osmium(II) from as class of robust sensitizers that have been used successfully for triplet-triplet annihilation previously. [73][74][75][76][77][78][79] In combination with DCA, the change from [Cu(dap) 2 ] + to [Os(bpy) 3 ] 2+ as a primary light absorber completely alters the photochemical reactivity of the system.…”

Porphyrins as Promising Photocatalysts for Red-Light-Induced Functionalizations of Biomolecules

“…16 Consequently, they have been widely studied as photosensitizers in photodynamic therapy; however, they are only occasionally used as photocatalysts in red-light-induced processes. 13 Along this line, subphtalocyanines and phtalocyanines proved effective in the perfluoroalkylation of alkenes and alkynes, 17−19 the cyanation of tertiary amines, 19 and the photoreductive dehalogenation of α-bromo ketones. 20 The advantage of using porphyrinoids as photoredox catalysts was recently demonstrated by the MacMillan group, who developed a proximity labeling platform based on a red-lightexcited Sn(IV) chlorin e6 that enabled the generation of aminyl radicals both in vitro and in cellulo.…”