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

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* = η5-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.