1,4-Bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadienes as Strong Salt-Free Reductants for Generating Low-Valent Early Transition Metals with Electron-Donating Ligands

Abstract

Electron-rich organosilicon compounds, such as 1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene (2a), 2,5-dimethyl-1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene (2b), 2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene (2c), and 1,1'-bis(trimethylsilyl)-1,1'-dihydro-4,4'-bipyridine (4), served as versatile reducing reagents of group 4-6 metal chloride complexes, such as Cp2TiCl2, Cp*2TiCl2 (Cp* = η(5)-C5Me5), Cp*TiCl3, Cp*TaCl4, and WCl4(PMe2Ph)2, to generate the corresponding low-valent metal species in a salt-free manner. Nitrogen-containing reductants, such as 2a-c and 4, had stronger reducing ability than the parent organosilicon reductants, 3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (1a) and 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (1b), as well as a pyridine-derived reductant, 1,4-bis(trimethylsilyl)-1-aza-2,5-cyclohexadiene (3). These greater effects of 2a-c and 4 are likely due to their negative one-electron redox potentials, as typically demonstrated in the reduction of Cp2TiCl2, for which compounds 2a and 4 gave the corresponding one-electron reduced products, pyrazine-bridged and 4,4'-bipyridyl-bridged dimeric Ti(III) complexes 5 and 6, and compounds 2b and 2c afforded the same double chloride-bridged dimeric Ti(III) complex, [Cp2Ti]2(μ-Cl)2 (7), though 1a and 1b could not reduce Cp2TiCl2. Application of the organosilicon compounds as reducing agents for catalytic reactions revealed that the combination of 2c and a catalytic amount of Cp2TiCl2 assisted a Reformatsky reaction of nonanal and ethyl 2-bromoisobutyrate and its derivatives to give ethyl 3-hydroxy-2,2-dimethylundecanoate and its derivatives. In this coupling reaction, 2c served as the best reductant among 2a-c and 4 due to the suppression of an undesired reaction between 2c and ethyl 2-bromoalkanoates.

Crystal structure of (η 5-pentamethylcyclopentadienyl)titanium(III)dichloride (THF), C14H23Cl2OTi

“…Only a few mononuclear structures of monocyclopentadienyltitanium(III) halides are known (CpTiCl 2 (THF)1.5 [3], η 5 -tetrabenzo uorenetitaniumdichloride(THF) [4] [7]. Recently, Mashima et al described the formation of Cp*TiCl 2 (THF) as a side product during the reaction of Cp*TiCl 3 with electron-rich organosilicon compounds [8]. Even though the title compound is well known and described in literature, there is hitherto no published crystal structure.…”

One-Pot Synthesis of Heavier Group 14 N-Heterocyclic Carbene Using Organosilicon Reductant

“…The milder organosilicon reductants have been well-known to efficiently reduce early transition metals without the formation of reductant-derived metal salts and over-reduced impurities [19,20]. Very recently, 2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene have been employed for metal salt-free reduction of dibromobismuthine and dibromostibine to dibismuthine and distibine, respectively [21].…”

“…Worth mentioning, there are few discrete examples where a NHE (E = Si, Ge, and Sn) has been synthesized under metal-free conditions: Dehydrochlorination of cyclic diaminohydrochlorosilane using bulky NHC [22], dehydrogenation of dihydrogermane by frustrated Lewis pair [23], and transamination of Sn{N(SiMe 3 ) 2 } 2 with α-amino-aldimines [24,25], respectively. In this study, we have developed a simple one-pot synthetic route under milder conditions to synthesize NHGe and NHSn, respectively, free from reductant-derived metal salts using 1,4-bis-(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene [19,20] as the reductant.…”

“…All chemicals were used as purchased. Diazabutadiene [30] ligand, and the reducing agents 1,4-bis-(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene, 1,1 -bis(trimethylsilyl)-1, 1 -dihydro-4,4 -bipyridine [19,20] were synthesized according to reported literature procedure. Photochemical reactions were performed in Peschl Photoreactorsystem (Peschl Ultraviolet GmbH, Mainz, Germany).…”

Salt‐Free Reduction of Nonprecious Transition‐Metal Compounds: Generation of Amorphous Ni Nanoparticles for Catalytic C–C Bond Formation

“…[1,2] Nanocatalysts have ar elatively wide reactive surface and ac ontrolled nanoscale size and shape.I ng eneral, crystalline nanoparticles (cNPs) of noble metals,such as Pd and Au,exhibit high catalytic performances in several C À Cbond-forming reactions. [9][10][11][12] Ther elatively poor catalytic performance of these nonprecious transition-metal nanocatalysts is due mainly to their larger particle size and smaller reactive surface,a sw ell as their sensitivity to oxidation damage.H erein, we report that 2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene (1) [13] reduced halides,c arboxylates,a nd acetylacetonates (acac) of some nonprecious metals to produce the corresponding metallic particles under mild conditions.Additionally,a morphous Ni 0 nanoparticles (Ni aNPs), derived from [Ni(acac) 2 ]a nd 1,w ere effective catalysts for reductive CÀCbond-forming reactions.We began by examining the potential of 1 to act as ar eductant for some typical nonprecious transition metals along with typical noble transition metals (Table 1). Although several cNPs of these nonprecious metals have been prepared, they exhibited poor catalytic activity toward CÀCb ond-forming reactions.…”