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

The
transient titanium neopentylidyne, [(PNP)­TiCtBu] (A; PNP–N­[2-PiPr2-4-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(η2-H2CCH2)­(CH2tBu)] (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(η2-H2CCHR)­(CH2tBu)] (R = CH3 (2), CH2CH3 (3), nPr (4), and nBu (5)). Complexes 1–5 can be independently prepared from [(PNP)­TiCHtBu­(OTf)] and the corresponding alkylating reagents,
LiCH2CHR (R = H, CH3(unstable), CH2CH3, nPr, and nBu). Olefin complexes 1 and 3–5 have all been characterized by a diverse array
of multinuclear NMR spectroscopic experiments including 1H–31P 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 1H–13C multiplicity edited gHSQC, 1H–31P HMBC, and dqfCOSY experiments. Heavier linear alkanes (C7 and C8) are also dehydrogenated by A to form [(PNP)­Ti­(η2-H2CCHnPentyl)­(CH2tBu)] (6) and [(PNP)­Ti­(η2-H2CCHnHexyl)­(CH2tBu)] (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(CH2tBu)] 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.