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

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• 作者：Teruhiko Saito ; Haruka Nishiyama ; Hiromasa Tanahashi ; Kento Kawakita ; Hayato Tsurugi ; Kazushi Mashima
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：April 2, 2014
• 年：2014
• 卷：136
• 期：13
• 页码：5161-5170
• 全文大小：545K
• 年卷期：v.136,no.13(April 2, 2014)
• ISSN：1520-5126

文摘

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鈥?bis(trimethylsilyl)-1,1鈥?dihydro-4,4鈥?bipyridine (4), served as versatile reducing reagents of group 4鈥? metal chloride complexes, such as Cp₂TiCl₂, Cp*₂TiCl₂ (Cp* = 畏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鈥?b>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鈥?b>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鈥?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(渭-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鈥揷 and 4 due to the suppression of an undesired reaction between 2c and ethyl 2-bromoalkanoates.