Room Temperature Dehydrogenation of Ethane, Propane, Linear Alkanes C4–C8, and Some Cyclic Alkanes by Titanium–Carbon Multiple Bonds

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Abstract

The transient titanium neopentylidyne, [(PNP)Ti≡C(t)Bu] (A; PNP(-)≡N[2-P(i)Pr2-4-methylphenyl]2(-)), dehydrogenates ethane to ethylene at room temperature over 24 h, by sequential 1,2-CH bond addition and β-hydrogen abstraction to afford [(PNP)Ti(η(2)-H2C═CH2)(CH2(t)Bu)] (1). Intermediate A can also dehydrogenate propane to propene, albeit not cleanly, as well as linear and volatile alkanes C4-C6 to form isolable α-olefin complexes of the type, [(PNP)Ti(η(2)-H2C═CHR)(CH2(t)Bu)] (R = CH3 (2), CH2CH3 (3), (n)Pr (4), and (n)Bu (5)). Complexes 1-5 can be independently prepared from [(PNP)Ti═CH(t)Bu(OTf)] and the corresponding alkylating reagents, LiCH2CHR (R = H, CH3(unstable), CH2CH3, (n)Pr, and (n)Bu). Olefin complexes 1 and 3-5 have all been characterized by a diverse array of multinuclear NMR spectroscopic experiments including (1)H-(31)P HOESY, and in the case of the α-olefin adducts 2-5, 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-H coupled and decoupled (1)H-(13)C multiplicity edited gHSQC, (1)H-(31)P HMBC, and dqfCOSY experiments. Heavier linear alkanes (C7 and C8) are also dehydrogenated by A to form [(PNP)Ti(η(2)-H2C═CH(n)Pentyl)(CH2(t)Bu)] (6) and [(PNP)Ti(η(2)-H2C═CH(n)Hexyl)(CH2(t)Bu)] (7), respectively, but these species are unstable but can exchange with ethylene (1 atm) to form 1 and the free α-olefin. Complex 1 exchanges with D2C═CD2 with concomitant release of H2C═CH2. 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(t)Bu)] 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 °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═CPh2; 2 equiv) and the diazoalkane, N2CHtolyl2. For complexes 3-7, oxidation with N2O also liberates the α-olefin.

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