Room Temperature Dehydrogenation of Ethane, Propane, Linear Alkanes C4鈥揅8, and Some Cyclic Alkanes by Titanium鈥揅arbon Multiple Bonds

详细信息    查看全文

• 作者：Marco G. Crestani ; Anne K. Hickey ; Xinfeng Gao ; Balazs Pinter ; Vincent N. Cavaliere ; Jun-Ichi Ito ; Chun-Hsing Chen ; Daniel J. Mindiola
• 刊名：Journal of the American Chemical Society
• 出版年：2013
• 出版时间：October 2, 2013
• 年：2013
• 卷：135
• 期：39
• 页码：14754-14767
• 全文大小：797K
• 年卷期：v.135,no.39(October 2, 2013)
• ISSN：1520-5126

文摘

The transient titanium neopentylidyne, [(PNP)Ti鈮^tBu] (A; PNP^{鈥?/sup>鈮[2-PiPr₂-4-methylphenyl]₂鈥?/sup>), dehydrogenates ethane to ethylene at room temperature over 24 h, by sequential 1,2-CH bond addition and 尾-hydrogen abstraction to afford [(PNP)Ti(畏2-H₂C鈺怌H₂)(CH₂tBu)] (1). Intermediate A can also dehydrogenate propane to propene, albeit not cleanly, as well as linear and volatile alkanes C₄鈥揅₆ to form isolable 伪-olefin complexes of the type, [(PNP)Ti(畏2-H₂C鈺怌HR)(CH₂tBu)] (R = CH₃ (2), CH₂CH₃ (3), nPr (4), and nBu (5)). Complexes 1鈥?b>5 can be independently prepared from [(PNP)Ti鈺怌HtBu(OTf)] and the corresponding alkylating reagents, LiCH₂CHR (R = H, CH₃(unstable), CH₂CH₃, nPr, and nBu). Olefin complexes 1 and 3鈥?b>5 have all been characterized by a diverse array of multinuclear NMR spectroscopic experiments including 1H鈥?sup>31P HOESY, and in the case of the 伪-olefin adducts 2鈥?b>5, formation of mixtures of two diastereomers (each with their corresponding pair of enantiomers) has been unequivocally established. The latter has been spectroscopically elucidated by NMR via C鈥揌 coupled and decoupled 1H鈥?sup>13}C multiplicity edited gHSQC, ¹H鈥?sup>31P HMBC, and dqfCOSY experiments. Heavier linear alkanes (C₇ and C₈) are also dehydrogenated by A to form [(PNP)Ti(畏²-H₂C鈺怌HⁿPentyl)(CH₂^tBu)] (6) and [(PNP)Ti(畏²-H₂C鈺怌HⁿHexyl)(CH₂^tBu)] (7), respectively, but these species are unstable but can exchange with ethylene (1 atm) to form 1 and the free 伪-olefin. Complex 1 exchanges with D₂C鈺怌D₂ with concomitant release of H₂C鈺怌H₂. In addition, deuterium incorporation is observed in the neopentyl ligand as a result of this process. Cyclohexane and methylcyclohexane can be also dehydrogenated by transient A, and in the case of cyclohexane, ethylene (1 atm) can trap the [(PNP)Ti(CH₂^tBu)] fragment to form 1. Dehydrogenation of the alkane is not rate-determining since pentane and pentane-d₁₂ can be dehydrogenated to 4 and 4-d₁₂ with comparable rates (KIE = 1.1(0) at 29 掳C). Computational studies have been applied to understand the formation and bonding pattern of the olefin complexes. Steric repulsion was shown to play an important role in determining the relative stability of several olefin adducts and their conformers. The olefin in 1 can be liberated by use of N₂O, organic azides (N₃R; R = 1-adamantyl or SiMe₃), ketones (O鈺怌Ph₂; 2 equiv) and the diazoalkane, N₂CHtolyl₂. For complexes 3鈥?b>7, oxidation with N₂O also liberates the 伪-olefin.