2 research outputs found
Room Temperature Dehydrogenation of Ethane, Propane, Linear Alkanes C4–C8, and Some Cyclic Alkanes by Titanium–Carbon Multiple Bonds
The
transient titanium neopentylidyne, [(PNP)ÂTiî—¼C<i><sup>t</sup></i>Bu] (<b>A</b>; PNP<sup>–</sup>î—¼NÂ[2-P<i><sup>i</sup></i>Pr<sub>2</sub>-4-methylphenyl]<sub>2</sub><sup>–</sup>), dehydrogenates ethane to ethylene at room temperature
over 24 h, by sequential 1,2-CH bond addition and β-hydrogen
abstraction to afford [(PNP)ÂTiÂ(η<sup>2</sup>-H<sub>2</sub>Cî—»CH<sub>2</sub>)Â(CH<sub>2</sub><i><sup>t</sup></i>Bu)] (<b>1</b>). Intermediate <b>A</b> can also dehydrogenate propane to
propene, albeit not cleanly, as well as linear and volatile alkanes
C<sub>4</sub>–C<sub>6</sub> to form isolable α-olefin
complexes of the type, [(PNP)ÂTiÂ(η<sup>2</sup>-H<sub>2</sub>Cî—»CHR)Â(CH<sub>2</sub><i><sup>t</sup></i>Bu)] (R = CH<sub>3</sub> (<b>2</b>), CH<sub>2</sub>CH<sub>3</sub> (<b>3</b>), <i><sup>n</sup></i>Pr (<b>4</b>), and <sup><i>n</i></sup>Bu (<b>5</b>)). Complexes <b>1</b>–<b>5</b> can be independently prepared from [(PNP)ÂTiî—»CH<i><sup>t</sup></i>BuÂ(OTf)] and the corresponding alkylating reagents,
LiCH<sub>2</sub>CHR (R = H, CH<sub>3</sub>(unstable), CH<sub>2</sub>CH<sub>3</sub>, <i><sup>n</sup></i>Pr, and <sup><i>n</i></sup>Bu). Olefin complexes <b>1</b> and <b>3</b>–<b>5</b> have all been characterized by a diverse array
of multinuclear NMR spectroscopic experiments including <sup>1</sup>H–<sup>31</sup>P HOESY, and in the case of the α-olefin
adducts <b>2</b>–<b>5</b>, 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 <sup>1</sup>H–<sup>13</sup>C multiplicity edited gHSQC, <sup>1</sup>H–<sup>31</sup>P HMBC, and dqfCOSY experiments. Heavier linear alkanes (C<sub>7</sub> and C<sub>8</sub>) are also dehydrogenated by <b>A</b> to form [(PNP)ÂTiÂ(η<sup>2</sup>-H<sub>2</sub>Cî—»CH<i><sup>n</sup></i>Pentyl)Â(CH<sub>2</sub><i><sup>t</sup></i>Bu)] (<b>6</b>) and [(PNP)ÂTiÂ(η<sup>2</sup>-H<sub>2</sub>Cî—»CH<sup><i>n</i></sup>Hexyl)Â(CH<sub>2</sub><i><sup>t</sup></i>Bu)] (<b>7</b>), respectively, but these
species are unstable but can exchange with ethylene (1 atm) to form <b>1</b> and the free α-olefin. Complex <b>1</b> exchanges
with D<sub>2</sub>Cî—»CD<sub>2</sub> with concomitant release
of H<sub>2</sub>Cî—»CH<sub>2</sub>. 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 <b>A</b>, and in the case of cyclohexane, ethylene (1 atm) can trap
the [(PNP)ÂTiÂ(CH<sub>2</sub><i><sup>t</sup></i>Bu)] fragment
to form <b>1</b>. Dehydrogenation of the alkane is not rate-determining
since pentane and pentane-<i>d</i><sub>12</sub> can be dehydrogenated
to <b>4</b> and <b>4</b>-<i>d</i><sub>12</sub> 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 <b>1</b> can
be liberated by use of N<sub>2</sub>O, organic azides (N<sub>3</sub>R; R = 1-adamantyl or SiMe<sub>3</sub>), ketones (OCPh<sub>2</sub>; 2 equiv) and the diazoalkane, N<sub>2</sub>CHtolyl<sub>2</sub>. For complexes <b>3</b>–<b>7</b>, oxidation with
N<sub>2</sub>O also liberates the α-olefin
Room Temperature Dehydrogenation of Ethane, Propane, Linear Alkanes C4–C8, and Some Cyclic Alkanes by Titanium–Carbon Multiple Bonds
The
transient titanium neopentylidyne, [(PNP)ÂTiî—¼C<i><sup>t</sup></i>Bu] (<b>A</b>; PNP<sup>–</sup>î—¼NÂ[2-P<i><sup>i</sup></i>Pr<sub>2</sub>-4-methylphenyl]<sub>2</sub><sup>–</sup>), dehydrogenates ethane to ethylene at room temperature
over 24 h, by sequential 1,2-CH bond addition and β-hydrogen
abstraction to afford [(PNP)ÂTiÂ(η<sup>2</sup>-H<sub>2</sub>Cî—»CH<sub>2</sub>)Â(CH<sub>2</sub><i><sup>t</sup></i>Bu)] (<b>1</b>). Intermediate <b>A</b> can also dehydrogenate propane to
propene, albeit not cleanly, as well as linear and volatile alkanes
C<sub>4</sub>–C<sub>6</sub> to form isolable α-olefin
complexes of the type, [(PNP)ÂTiÂ(η<sup>2</sup>-H<sub>2</sub>Cî—»CHR)Â(CH<sub>2</sub><i><sup>t</sup></i>Bu)] (R = CH<sub>3</sub> (<b>2</b>), CH<sub>2</sub>CH<sub>3</sub> (<b>3</b>), <i><sup>n</sup></i>Pr (<b>4</b>), and <sup><i>n</i></sup>Bu (<b>5</b>)). Complexes <b>1</b>–<b>5</b> can be independently prepared from [(PNP)ÂTiî—»CH<i><sup>t</sup></i>BuÂ(OTf)] and the corresponding alkylating reagents,
LiCH<sub>2</sub>CHR (R = H, CH<sub>3</sub>(unstable), CH<sub>2</sub>CH<sub>3</sub>, <i><sup>n</sup></i>Pr, and <sup><i>n</i></sup>Bu). Olefin complexes <b>1</b> and <b>3</b>–<b>5</b> have all been characterized by a diverse array
of multinuclear NMR spectroscopic experiments including <sup>1</sup>H–<sup>31</sup>P HOESY, and in the case of the α-olefin
adducts <b>2</b>–<b>5</b>, 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 <sup>1</sup>H–<sup>13</sup>C multiplicity edited gHSQC, <sup>1</sup>H–<sup>31</sup>P HMBC, and dqfCOSY experiments. Heavier linear alkanes (C<sub>7</sub> and C<sub>8</sub>) are also dehydrogenated by <b>A</b> to form [(PNP)ÂTiÂ(η<sup>2</sup>-H<sub>2</sub>Cî—»CH<i><sup>n</sup></i>Pentyl)Â(CH<sub>2</sub><i><sup>t</sup></i>Bu)] (<b>6</b>) and [(PNP)ÂTiÂ(η<sup>2</sup>-H<sub>2</sub>Cî—»CH<sup><i>n</i></sup>Hexyl)Â(CH<sub>2</sub><i><sup>t</sup></i>Bu)] (<b>7</b>), respectively, but these
species are unstable but can exchange with ethylene (1 atm) to form <b>1</b> and the free α-olefin. Complex <b>1</b> exchanges
with D<sub>2</sub>Cî—»CD<sub>2</sub> with concomitant release
of H<sub>2</sub>Cî—»CH<sub>2</sub>. 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 <b>A</b>, and in the case of cyclohexane, ethylene (1 atm) can trap
the [(PNP)ÂTiÂ(CH<sub>2</sub><i><sup>t</sup></i>Bu)] fragment
to form <b>1</b>. Dehydrogenation of the alkane is not rate-determining
since pentane and pentane-<i>d</i><sub>12</sub> can be dehydrogenated
to <b>4</b> and <b>4</b>-<i>d</i><sub>12</sub> 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 <b>1</b> can
be liberated by use of N<sub>2</sub>O, organic azides (N<sub>3</sub>R; R = 1-adamantyl or SiMe<sub>3</sub>), ketones (OCPh<sub>2</sub>; 2 equiv) and the diazoalkane, N<sub>2</sub>CHtolyl<sub>2</sub>. For complexes <b>3</b>–<b>7</b>, oxidation with
N<sub>2</sub>O also liberates the α-olefin