91 research outputs found
Hydrogenation of Unsaturated Six-Membered Cyclic Hydrocarbons Studied by the Parahydrogen-Induced Polarization Technique
Parahydrogen-induced polarization
(PHIP) is an efficient technique
for mechanistic investigations of homogeneous and heterogeneous catalytic
hydrogenations. Herein, heterogeneous gas phase hydrogenation of six-membered
cyclic hydrocarbons (benzene, toluene, cyclohexene, 1,3-cyclohexadiene
and 1,4-cyclohexadiene) over Rh/TiO<sub>2</sub>, Pd/TiO<sub>2</sub>, and Pt/TiO<sub>2</sub> catalysts was studied using PHIP. As expected,
cyclohexene hydrogenation led to the formation of cyclohexane which
because of its symmetry should not exhibit any PHIP effects. However,
the presence of <sup>13</sup>C nuclei at natural abundance (1.1%)
breaks molecular symmetry, resulting in the observation of <sup>13</sup>C satellite signals exhibiting PHIP effects in the <sup>1</sup>H
NMR spectra. In experiments with cyclohexene, the reactantās
NMR signals were also polarized, demonstrating the possibility of
cyclohexene dehydrogenation to 1,3-cyclohexadiene and subsequent hydrogenation
to cyclohexene. In the hydrogenation of 1,3-cyclohexadiene and 1,4-cyclohexadiene,
all NMR signals of cyclohexene exhibited PHIP effects, implying migration
of Cī»C bonds in 1,4-cyclohexadiene and cyclohexene. At the
same time, upon hydrogenation of benzene and toluene the reaction
products were those with saturated cycles exclusively (cyclohexane
and methylcyclohexane, respectively), and their NMR signals were not
polarized. The absence of PHIP effects for arene hydrogenation can
be explained by a difference in the reaction mechanism compared to
cyclohexane and cyclohexadienes hydrogenations, along with the larger
extent to which hydrogen atoms undergo migration on the catalyst surface
facilitated by lower catalyst coverage with an adsorbed substrate
in case of arenes
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