Visible Light‐Driven, Room Temperature Heck‐Type Reaction of Alkyl Halides with Styrene Derivatives Catalyzed by B 12 Complex

Aqueous Micellar Environment Impacts the Co-Catalyzed Phototransformation: A Case Study

In recent years, methodologies that rely on water as the reaction medium have gained considerable attention. The unique properties of micellar solutions were shown to improve the regio-, stereo-, and chemoselectivity of different transformations. Herein, we demonstrate that the aqueous environment is a suitable medium for a visible light driven cobalt-catalyzed reaction involving radical species. In this system, reduced vitamin B12 reacts with alkyl halides, generating radicals that are trapped by the lipophilic olefin present in the Stern layer. A series of NMR measurements and theoretical studies revealed the location of reaction components in the micellar system.

Green Molecular Transformation in Dual Catalysis: Photoredox Activation of Vitamin B12 Using Heterogeneous Photocatalyst

This concept focuses on dual-catalysis using metal complexes and heterogeneous photocatalysts. Vitamin B12 derivatives are sophisticated metal complexes that facilitate enzymatic reactions in the biological systems. The B12 enzymes inspired reactions catalytically proceed in dual-catalyst systems of B12 derivatives and heterogeneous photocatalysts, such as titanium oxide (TiO2) and metal-organic frameworks (MOFs), under light irradiation. The cobalt ions in B12 derivatives are effectively reduced by photoexcited photocatalysts, producing low-valent Co(I) species. The photoinduced nucleophilic Co(I) species react with an alkyl halide to form an organometallic complex with a Co-C bond. The Co-C bond dissociates during photolysis to generate alkyl radicals. Based on this mechanism, dual-catalysis effectively promotes various light-driven organic syntheses and light-driven dehalogenation reactions of toxic alkyl halides. The trends of the dual-catalyst system and recent progress in this field are discussed in this concept.

Photoinduced Cobalt Catalyzed Radical Heck-Type Reaction of Alkyl Electrophiles

We report here a general alkylation reaction of terminal alkenes with nucleophilic cobaloxime complex catalysis under visible light irradiation. A broad range of vinyl arenes and heteroarenes, as well as a diverse set of alkyl(pseudo)halides or strained rings as alkyl electrophiles, can efficiently undergo the transformation up to the gram scale. Mechanistic study reveals the radical nature of the reaction and corroborates our design involving alkylcobaloxime as the key intermediate.

Application of bioorganometallic B12 in green organic synthesis

Bioorganometallic structure found in coenzyme B₁₂ is a key component in B₁₂-dependent enzymatic reactions in natural enzymes. Cleavage of a cobalt-carbon bond in organometallic B₁₂ compound provide reactive intermediate for molecular transformations. Application of the bioorganometallic B₁₂ in organic synthesis have been developed using natural vitamin B₁₂ as well as synthetic vitamin B₁₂ derivatives as a bioinspired catalyst in organic solvent. Vitamin B₁₂ derivatives composed of corrinoid structure should form stable organometallic compound having a cobalt-carbon bond. Using the unique property of the organometallic vitamin B₁₂ derivatives, various catalytic reactions have been developed in synthetic organic chemistry. The dual catalytic system of vitamin B₁₂ derivatives and photocatalyst, such as Ru(II) polypyridyl complex or titanium oxide, could construct light-driven molecular transformations. The B₁₂-dependent enzymes mimic reactions, such as the dechlorination of organic halides and the radical mediated isomerization reactions, catalytically proceed in the dual catalyst system. Electroorganic syntheses mediated by the vitamin B₁₂ derivatives have been developed as green molecular transformations. The redox active vitamin B₁₂ derivatives shows a unique catalysis in the electroorganic synthesis, such as alkene and alkyne reductions.

Controlling Non-Native Cobalamin Reactivity and Catalysis in the Transcription Factor CarH

Vitamin B₁₂ derivatives catalyze a wide range of organic transformations, but B₁₂-dependent enzymes are underutilized in biocatalysis relative to other metalloenzymes. In this study, we engineered a variant of the transcription factor CarH, called CarH*, that catalyzes styrene C-H alkylation with improved yields (2-6.5-fold) and selectivity relative to cobalamin. While the native function of CarH involves transcription regulation via adenosylcobalamin (AdoCbl) Co(III)-carbon bond cleavage and β-hydride elimination to generate 4',5'-didehydroadenosine, CarH*-catalyzed styrene alkylation proceeds via non-native oxidative addition and olefin addition coupled with a native-like β-hydride elimination. Mechanistic studies on this reaction echo findings from earlier studies on AdoCbl homolysis to suggest that CarH* selectivity results from its ability to impart a cage effect on radical intermediates. These findings lay the groundwork for the development of B₁₂-dependent enzymes as catalysts for non-native transformations.

Cobalt Catalyst Determines Regioselectivity in Ring Opening of Epoxides with Aryl Halides

Ring-opening of epoxides furnishing either linear or branched products belongs to the group of classic transformations in organic synthesis. However, the regioselective cross-electrophile coupling of aryl epoxides with aryl halides still represents a key challenge. Herein, we report that the vitamin B₁₂/Ni dual-catalytic system allows for the selective synthesis of linear products under blue-light irradiation, thus complementing methodologies that give access to branched alcohols. Experimental and theoretical studies corroborate the proposed mechanism involving alkylcobalamin as an intermediate in this reaction.

Visible light-driven Giese reaction with alkyl tosylates catalysed by nucleophilic cobalt

The scope of the Giese reaction is expanded using readily available alkyl tosylates as substrates and nucleophilic cobalt(i) catalysts under visible-light irradiation. The reaction proceeds preferentially with less bulky primary alkyl tosylates. This unique reactivity enables the regio-selective Giese reaction of polyol derivatives.

We report an efficient approach for the Giese reaction with abundant alkyl tosylates as alkyl radical sources using a nucleophilic cobalt catalyst under visible-light irradiation.

Synthesis of a B12-BODIPY dyad for B12-inspired photochemical transformations of a trichloromethylated organic compound

A B₁₂ complex-BODIPY dyad was synthesized by peripheral modification of cobalamin derivatives. The photophysical properties of the dyad were investigated by UV-vis, PL, and transient absorption spectroscopy. A visible light-driven dechlorination reaction of a trichlorinated organic compound, DDT, was reported. The dyad showed efficient catalysis for dechlorination under N₂ with turnover numbers of over 220 for the reaction. One-pot syntheses of an ester and amide from DDT and benzotrichloride were also achieved using the dyad under air.

Visible-Light-Induced Pd-Catalyzed Radical Strategy for Constructing C-Vinyl Glycosides

A novel visible-light-induced palladium-catalyzed Heck reaction for bromine sugars and aryl olefins with high regio- and stereochemistry selectivity for the preparation of C-glycosyl styrene is described. This reaction takes place in one step at room temperature by using a simple and readily available starting material. This protocol can be scaled up to a wide range of glycosyl bromide donors and aryl olefin substrates. Mechanistic studies indicate that a radical addition pathway is involved.

Visible light-driven cross-coupling reactions of alkyl halides with phenylacetylene derivatives for C(sp3)–C(sp) bond formation catalyzed by a B12 complex

Visible light-driven cross-coupling reactions of alkyl halides with phenylacetylene and its derivatives catalyzed by the cobalamin derivative (B₁₂) with the [Ir(dtbbpy)(ppy)₂]PF₆ photocatalyst at room temperature are reported.