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Relevant academic research and scientific papers

The direct arylation of allylic sp3 C-H bonds via organic and photoredox catalysis

The direct functionalization of unactivated sp3 C-H bonds is still one of the most challenging problems facing synthetic organic chemists. The appeal of such transformations derives from their capacity to facilitate the construction of complex organic molecules via the coupling of simple and otherwise inert building blocks, without introducing extraneous functional groups. Despite notable recent efforts, the establishment of general and mild strategies for the engagement of sp3 C-H bonds in C-C bond forming reactions has proved difficult. Within this context, the discovery of chemical transformations that are able to directly functionalize allylic methyl, methylene and methine carbons in a catalytic manner is a priority. Although protocols for direct oxidation and amination of allylic C-H bonds (that is, C-H bonds where an adjacent carbon is involved in a C = C bond) have become widely established, the engagement of allylic substrates in C-C bond forming reactions has thus far required the use of pre-functionalized coupling partners. In particular, the direct arylation of non-functionalized allylic systems would enable access to a series of known pharmacophores (molecular features responsible for a drug's action), though a general solution to this long-standing challenge remains elusive. Here we report the use of both photoredox and organic catalysis to accomplish a mild, broadly effective direct allylic C-H arylation. This C-C bond forming reaction readily accommodates a broad range of alkene and electron-deficient arene reactants, and has been used in the direct arylation of benzylic C-H bonds.

Preparation of Functionalized Diorganomagnesium Reagents in Toluene via Bromine or Iodine/Magnesium-Exchange Reactions

A wide range of polyfunctionalized di(hetero)aryl- and dialkenyl-magnesium reagents are prepared in toluene within 10 to 120 minutes between 78 C and 25 C via an I/Mg- or Br/Mg-exchange reaction using reagents of the general formula R2Mg (R = sBu, Mes). Highly sensitive functional groups, such as triazene or nitro, are tolerated in these exchange reactions, enabling the synthesis of various functionalized (hetero)arene and alkene derivatives after quenching with several electrophiles including allyl bromides, acyl chlorides, aldehydes, ketones, and aryl iodides.

Synthesis of Polyfunctional Diorganomagnesium and Diorganozinc Reagents through In Situ Trapping Halogen–Lithium Exchange of Highly Functionalized (Hetero)aryl Halides in Continuous Flow

We report a halogen–lithium exchange performed in the presence of various metal salts (ZnCl2, MgCl2?LiCl) on a broad range of sensitive bromo- or iodo(hetero)arenes using BuLi or PhLi as the exchange reagent and a commercially available continuous-flow setup. The resulting diarylmagnesium or diarylzinc species were trapped with various electrophiles, resulting in the formation of polyfunctional (hetero)arenes in high yields. This method enables the functionalization of (hetero)arenes containing highly sensitive groups such as an isothiocyanate, nitro, azide, or ester. A straightforward scale-up was possible without further optimization.

Radical ions in photochemistry, 15. The photosubstitution reaction between dicyanobenzenes and alkyl olefins

The photosubstitution (electron transfer) reaction between 1,4-dicyanobenzene (1) and 2,3-dimethyl-2-butene (2), which gives 1-(4-cyanophenyl)-2,3-dimethyl-2-butene (3) and 3-(4-cyanophenyl)-2,3-dimethyl-1-butene (4), has been extended to other dicyanobenzene-olefin mixtures.Substitution of cyano group occurs when both 1 or 1,2-dicyanobenzene (5) are irradiated in acetonitrile solution, in the presence of 2 or cyclohexane (16).Under comparable conditions 1,3-dicyanobenzene (6) failed to react.Little or no substitution was observed in any case when the olefin was methylpropene (19).The results for 1 and 5 are in agreement with empirical free energy calculations (Weller equation) for the electron transfer process which, howover, fail to explain the general lack of reactivity of 1,3-dicyanobenzene.Phenanthrene (11) has been shown to photosensitize the photosubstitution reaction between dicyanobenzenes and 2.Under these conditions the olefin reacts with 6 predominantly at the 4-position, resulting in overall substitution of a hydrogen atom.This reaction occurs regiospecifically, resulting in the formation of only one of the two possible isomeric side chains.The mechanistic details of these reactions have been substantiated by means of deuterium labelling studies.The aromatic nitriles also undergo photosubstitution by 2, in acetonitrile-methanol solution, resulting in methanol-incorporated products.Whereas reaction with 1 or 5 results in substitution of a cyano group, 6 was observed to give isomeric dicyanocyclohexenes, resulting from initial reaction at the 4-position, followed by reduction.A detailed mechanism for this secondary photoreduction has been substantiated by deuterium labelling studies.The anomalous behaviour of 1,3-dicyanobenzene has been attributed to a difference in the reactivity of the radical anion.