Organic Electron Donors

Murphy, John A.

doi:10.1002/9781119953678.rad024

Cited by 2 publications

(2 citation statements)

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“…The most rational strategy is the reduction of the high-valent metal complexes by organic compounds, as the low-valent metal species can be easily purified by simple separation from the coproduced organic byproducts by filtration or evacuation. In fact, some early transition metal halides are reduced by organic compounds, such as benzene for MoCl 5 and norbornene for WCl 6 , to form the corresponding low-valent metal chlorides, from which the low-valent metal complexes are isolated by filtration.…”

Section: Introductionmentioning

confidence: 99%

Salt-Free Reduction of Transition Metal Complexes by Bis(trimethylsilyl)cyclohexadiene, -dihydropyrazine, and -4,4′-bipyridinylidene Derivatives

Tsurugi

Mashima

2019

Acc. Chem. Res.

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Conspectus Chemical reduction of transition metals provides the corresponding low-valent transition metal species as a key step for generating catalytically active species in metal-assisted organic transformations and is a fundamental unit reaction for preparing organometallic complexes. A variety of metal-based reductants, such as metal powders and organometallic reagents of alkali and alkaline-earth metals, have been developed to date to access low-valent metal species. During the reduction, however, reductant-derived metal salts are formed as reaction waste, some of which often interact with the reactive low-valent metal center, thereby disrupting the catalytic performance and hampering the isolation of organometallic complexes as a result of salt coordination to the coordinatively unsaturated vacant and active sites and the formation of thermally unstable ate complexes. In this Account, we emphasize the synthetic utility and versatility of organic reductants containing two trimethylsilyl groups, i.e., 1,4-bis(trimethylsilyl)cyclohexa-2,5-diene (1a) and its methyl derivative (1b), 1,4-bis(trimethylsilyl)dihydropyrazine (2a) and its dimethyl (2b) and tetramethyl (2c) derivatives, and 1,1′-bis(trimethylsilyl)-4,4′-bipyridinylidene (3), leading to the reduction of various kinds of metal compounds in a salt-free fashion by release of two electrons together with the coproduction of easily removable (hetero)aromatics and trimethylsilyl derivatives from these organic reductants 1–3. When homoleptic chlorides of group 5 and 6 metals are treated with 1a and 1b, in situ-generated highly reactive low-valent metal species react with redox-active molecules such as ethylene, α-diimines, and α-diketones to produce metallacyclopentane, (ene–diamido)metal, and (ene–diolato)metal complexes, respectively. The advantage of the salt-free protocol is further exemplified in the low-valent titanocene-catalyzed Reformatsky-type reaction when 2c is used as a reductant: the yield of the product using the organosilicon reductant is higher than that when manganese powder is used as the reductant for the catalytic Reformatsky-type reaction of ethyl 2-bromoisobutyrate and its derivatives with various aldehydes. Moreover, when halides, carboxylates, and acetylacetonate compounds of late transition metals and main-group elements are treated with the organosilicon reductant 2c, metal(0) particles are smoothly precipitated under mild conditions. Among them, metallic nickel(0) nanoparticles are applicable to reductive biaryl formation and reductive cross-coupling of aryl halides/aryl aldehydes. In addition, reduction of the heterogeneous catalysts on a solid supporting matrix was also achieved by this salt-free reduction method; volatile byproducts are easily removed from the catalyst surface without suppressing the catalytic performance. Thus, the salt-free reduction strategy is a very powerful synthetic method that can be extended to various metals throughout the periodic table.

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Section: Introductionmentioning

confidence: 99%

Salt-Free Reduction of Transition Metal Complexes by Bis(trimethylsilyl)cyclohexadiene, -dihydropyrazine, and -4,4′-bipyridinylidene Derivatives

Tsurugi

Mashima

2019

Acc. Chem. Res.

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“…Among them, those that can promote singleelectron reduction of aryl iodides are considered as ''super electron donors'' (SEDs). [1][2][3] When compared to conventional reducing agents such as alkali metals, alkali earth metals or metals in low oxidation states, SEDs have unique advantages in various aspects, which include tunable reduction power, excellent solubility in organic solvents, mild reaction conditions, and environmental friendliness. Electron-rich alkenes have been widely used as SEDs (Fig.…”

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confidence: 99%