.
Angewandte
Communications
ponents, butane, was explored. To our delight, an excellent
catalyst TON of up to 505 was observed after 7 h under the
same photocatalytic reaction conditions. This result confirms
the extensive applicability of our efficient photocatalytic
method (Table 3; entry 10).
variety of alkanes, including various liquid organic hydrogen
carriers and shale gas components such as butane, can be
efficiently dehydrogenated under environmentally benign
conditions and at much lower temperatures than previously
reported. Importantly, highly challenging alkane dehydrogen-
ations can now be performed under more practical conditions,
making the use of alkanes as olefin feedstock a real alter-
native for further synthetic manipulations. Further improve-
ments towards the development of a second generation
catalyst, with a focus on preventing the deactivation of the
present catalyst, and detailed mechanistic studies are cur-
rently underway.
Recently, alkanes such as decalin, methylcyclohexane,
indoline, and in particular, carbazole, have been proposed as
liquid organic hydrogen carriers (LOHC) because of their
high energy density, facile energy storage, and their ability to
be safely transported.[17,18] Fully hydrogenated N-ethylcarba-
zole (H12NEC), for example, has a theoretical hydrogen
capacity of 5.8% and caloric value of 1.9 kWhkgÀ1 [18a]
.
Excellent dehydrogenation of tetralin, decalin, indoline, and
H12NEC was achieved after 7 h with catalyst TONs of 357,
154, 708, and 640, respectively (Table 3, entries 11–14). These Experimental Section
General procedure: The specific glass vessel was initially charged with
dehydrogenations operate in essentially solvent-free condi-
tions below 908C, which demonstrate the efficacy of the
current photocatalytic procedure. It should be noted that the
thermochemical dehydrogenation of H12NEC has previously
been shown to require temperatures of more than 1808C.[18]
In order to show the efficiency of the present photo-
catalytic method, selected reactions were performed using
3 mmol of substrate in the presence of 0.1 mol% of catalyst
(Scheme 1). Excellent yields (71% and 75%, respectively)
the [Rh(PMe3)2(CO)Cl] catalyst (0.004 mmol) and the required
additive (0.02 mmol), unless otherwise specified. The reaction
vessel and water-cooled condenser were then evacuated completely
under vacuum and backfilled with argon gas at least three times to
ensure complete removal of residual air. The substrate (30 mmol),
unless otherwise noted, was added to the vessel under an argon
atmosphere. The glass vessel was then connected to a condenser
under a strong flow of argon gas at the top of the condenser to remove
evolved hydrogen. The vessel was covered by aluminum foil and
irradiated with light from a Lumatec Superlite 400 source for
a specified time. A high stirring rate (1000 rpm) was used to facilitate
the removal of liberated hydrogen. After the reaction, the vessel was
cooled to room temperature. The product yield/turnover number
(TON) was then determined by GC or NMR spectroscopy with
respect to isooctane or mesitylene, respectively, as internal standard.
Turnover numbers represented in tables are expressed as [mmol of
product]/[mmol of catalyst] and TOF (turn over frequency) is
expressed as (TON/time in h). A more detailed experimental
procedure is provided in the Supporting Information.
Received: February 11, 2014
Revised: March 28, 2014
Published online: && &&, &&&&
Scheme 1. Catalytic dehydrogenation reactions of alkanes in the pres-
ence of 0.1 mol% catalyst. Reaction conditions: [RhCl(CO)(PMe3)2]
(0.1 mol%) and 4,4’-bipy (5 equiv; 4,4’-bipy=4,4’-bipyridine) in
a Schlenk tube (wall width 1.2 mm), irradiating at l=320–500 nm for
7 h.
Keywords: dehydrogenation · olefination · photochemistry ·
.
rhodium · homogeneous catalysis
were obtained from cyclooctane and H12-NEC as substrates.
To our knowledge, such high yields have not been obtained in
catalytic dehydrogenations without adding hydrogen accept-
ors. This is an important result for future practical applica-
tions. In the case of octane, slightly improved yields of octenes
(28% with similar product distribution) were observed due to
the higher enthalpy of dehydrogenation. In the case of the
dehydrogenation of H12NEC, the reaction yielded H8NEC as
the major product.[19] The detection of H8NEC as the major
component after dehydrogenation is typical. Further dehy-
drogenation from H8NEC is predicted to be difficult because
of its weak absorption enthalpy on the planar pyrrole-type
structure in the reverse process.[18b,c]
[1] C. Gunanathan, D. Milstein, Science 2013, 341, DOI: 10.1126/
science.1229712.
[2] J. Choi, A. H. Roy MacArthur, M. Brookhart, A. S. Goldman,
[3] a) R. P. OꢀConnor, E. J. Klein, D. Henning, L. D. Schmidt, Appl.
Catal. A 2003, 238, 29 – 40; b) K. J. Caspary, H. Gehrke, M.
Heinritz-Adrian, M. Schwefer in Handbook of Heterogeneous
Catalysis, 2nd ed., Wiley, New York, 2008.
[4] 2013 Annual Energy Outlook with projections to 2040 (U.S.
Energy Information Administration), U.S. Department of
Shi, T. Suguri, C. Dohi, H. Yamada, S. Kojima, Y. Yamamoto,
In conclusion, we have developed the most efficient
acceptorless catalytic alkane dehydrogenation methodology
known to date. By using light under optimal conditions, the
well-known trans-[Rh(PMe3)2(CO)Cl] complex provides high
catalyst turnover numbers and excellent product yields. A
4
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!