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

Abstract

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.

Olefin Metathesis Catalysts Generated In Situ from Molybdenum(VI)‐Oxo Complexes by Tuning Pendant Ligands

“…We therefore decided to generate these putative species in‐situ (in the presence of an olefin) from a series of compounds of the form MoOCl 2 (OC(CH 3 ) 3‐x (CF 3 ) x ) 2 (Et 2 O) ( 1 ) combined with a molecularly‐defined reducing agent (Scheme 1b). We opted for the family of compounds 1 Fx (x=3, 6 and 9) because they contain two Cl ligands that are amiable to be removed upon reduction with organosilicon reducing agent, [26] thus generating low valent and low coordinate Mo(IV) species while also giving us an opportunity to evaluate the influence of the anionic OR Fx ligand(s) on the formation of alkylidene [19] …”

α- and β-Functionalized Ketones from 1,3-Dienes and Aldehydes: Control of Regio- and Enantioselectivity in Hydroacylation of 1,3-Dienes

“…( R , R )- i -Pr-DUPHOS complexes were used in this study (Table ) even though they gave less selective reactions compared to ( S,S )-Ph-BPE ligands (Table ) because the former set gave highly crystalline intermediates. The Co­(I) complex 26 , a known compound previously prepared by Chirik by an alternate route from 25a , is most conveniently prepared by reduction of 25a by 1,4-bis-trimethyl­silyl­pyrazine . Unlike the corresponding ( S,S )-BDPP Co­(I) complex, which forms a ligand-bridged dimer, the DUPHOS complex is formed as a chloride-bridged dimer ( 26 ).…”

“…The Co(I) complex 26, a known compound previously prepared by Chirik by an alternate route from 25a, 73 is most conveniently prepared by reduction of 25a by 1,4-bistrimethylsilylpyrazine. 74 Unlike the corresponding (S,S)-BDPP Co(I) complex, 75 which forms a ligand-bridged dimer, 60 the DUPHOS complex is formed as a chloridebridged dimer (26). This dimer 26 itself is not a catalyst for the reaction (Table 5, entry 1), but upon addition of NaBARF, a reaction ensues (entry 2) giving the same products as with our in situ-generated catalyst (Table 3, entry 7, product 7a).…”

Recent advances in metal-mediated nitrogen oxyanion reduction using reductively borylated and silylated N-heterocycles

“…Silyl substituted reduced N-heterocycles were first synthesized by Kaim, 33,34 and later pioneered by Mashima, 35 and are used for salt free reduction of transition metal complexes, [36][37][38] cyclopropanations, 39 and a variety of metal free deoxygenations of organic substrates. 40 Likewise, pioneered by Suginome, 41 the boryl substituted reduced N-heterocycles are employed in organocatalytic deoxygenations, 42 reductions of alkynes, 43 and de-aromatization of substituted pyrazines (Fig.…”