Catalytic carbon-carbon σ-bond hydrogenation with water catalyzed by rhodium porphyrins

Article abstract of DOI:10.1021/ja3047415

The catalytic carbon-carbon σ-bond activation and hydrogenation of [2.2]paracyclophane with water in a neutral reaction medium is demonstrated. The hydrogen from water is transferred to the hydrocarbon to furnish hydrogen enrichment in good yields.

Full text of DOI:10.1021/ja3047415

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Communication
Catalytic Carbon-Carbon #-Bond Hydrogenation
with Water Cata-lyzed by Rhodium Porphyrins
Ching Tat To, Kwong Shing Choi, and Kin Shing Chan
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja3047415 • Publication Date (Web): 28 Jun 2012
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Catalytic Carbon-Carbon σ-Bond Hydrogenation with Water
Catalyzed by Rhodium Porphyrins
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Ching Tat To, Kwong Shing Choi and Kin Shing Chan*
Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, The
People’s Republic of China
Supporting Information Placeholder
Scheme 1. Catalytic Hydrogenation of Carbon-
ABSTRACT: The catalytic carbon-carbon σ-bond acti-
Carbon σ-Bond using Water
vation and hydrogenation of [2.2]paracyclophane with
water in neutral reaction medium is demonstrated. The
hydrogen from water is transferred to the hydrocarbon
to furnish hydrogen enrichment in good yields.
Table 1. Catalytic Carbon-Carbon Bond Hydro-
genation of PCP with Water
Selective intermolecular, catalytic carbon-carbon ac-
tivation1 (CCA) with transition metal complexes is chal-
lenging. Despite the generally weaker carbon-carbon
bond than the carbon-hydrogen bond, carbon-
hydrogen activation2 (CHA) occurs predominantly due
to the statistically more abundant and sterically more
accessible natures of C-H bonds in organic com-
pounds.3 Examples of successful stoichiometric CCA in
homogenous solutions take advantages of ring strain
relief,4a,b chelation assistance1b or photochemical activa-
Entry
X
I
Additive
KOH (1 equiv)
--
Time / h
25
Yield 1 / %
tion.4c Similar strategies are also employed to the cata-
lytic CCA of strained carbonyls5 or simple carbonyls
assisted with chelation.6 Hydrogenolysis of C-C bond
catalyzed by transition metals has also been reported.7
However, these examples are limited to functionalized
substrates.
1
83
78
2a
Me
54
a 79% CH4 was formed w.r.t. RhIII(ttp)Me.
Replacing the hydrogen gas with water as the hydro-
gen donor in hydrogenation process is benefited from
the reduced cost and the safety issue. When coupled
with the CCA of hydrocarbon, catalytic hydrogenation
of carbon-carbon σ-bond in neutral medium can be
achieved (Scheme 1).
Initially, a benzene-d6 solution of PCP, in the pres-
ence of H2O, was reacted in the dark10 with RhIII(ttp)I
and KOH as a convenient source of RhII(ttp) via the
RhIII(ttp)OH intermediate, which undergoes rapid elim-
ination to give H2O2 as well.9 Surprisingly, no benzylic
CHA product of PCP was obtained.11 Instead, PCP was
catalytically converted to 4,4’-dimethylbibenzyl 1 in
83% yield (Table 1, eq 1, entry 1). The benzylic carbon-
carbon σ-bond of PCP was catalytically hydrogenated.
As RhIII(ttp)Me can undergo hydrolysis in neutral con-
ditions to give RhIII(ttp)OH, which rapidly gives
Recently, we have reported the stoichiometric CCA of
cyclooctane,8 and water splitting by RhII porphyrin.9 We
like to couple these two reactions for the catalytic car-
bon-carbon σ-bond hydrogenation with water to pro-
vide hydrogen enriched fuels. Herein, we report our
initial success using [2.2]paracyclophane (PCP) as the
organic substrate.
RhII (ttp)2,12 we tested its catalytic ability as well. To our
2
delight, PCP was catalytically hydrogenated to give 78%
yield of 1 in 54 h in neutral conditions when 10 mol% of
RhIII(ttp)Me was used (Table 1, eq 1, entry 2).13 79%
1
yield of CH4 was also detected by H NMR spectrosco-
py. The hydrolysis of RhIII(ttp)Me to RhIII(ttp)OH was
further ascertained.
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To test whether water is the hydrogen source, label-
monomeric RhII(tmp) was chosen to react with PCP,
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ing experiments using D2O were carried out. In a ben-
zene-d6 solution with excess D2O and 10 mol% of
RhIII(ttp)Me, PCP was catalytically converted to deuter-
without the complication due to the thermal equilibri-
um of RhII (ttp)2 with RhII(ttp).16 RhII(tmp) reacted
2
o
smoothly with PCP at 150 C for 23 h to yield 85% of 3
1
ium enriched 1-d in 64% yield in 3 d (eq 2). H NMR
(eq 4), establishing the high yielding stoichiometric
reaction.
analysis of 1-d confirmed the deuterio percentage at the
terminal and internal benzylic positions were 45% and
47%, respectively.
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The kinetic studies of reaction 4 were conducted at
[RhII(tmp)] = 0.24–0.48 mM, [PCP] = 4.8–14.4 mM,
temperatures of 140–170 oC and monitored by 1H NMR
spectroscopy. With at least 20-fold excess of PCP, the
disappearance of RhII(tmp) exhibited pseudo second
order kinetics over at least 4 half-lives (Figure 1). The
rate of disappearance of RhII(tmp) increased with in-
creasing [PCP] from 4.8 mM to 14.4 mM, giving a line-
ar pseudo first order kinetic plot (Figure 2). The kinetic
measurements indicated an overall third order reac-
tion, with second order in RhII(tmp) and first order in
PCP. Therefore, the rate of reaction can be expressed as
rate = kobs[RhII(tmp)]2[PCP].
The deuterium incorporation via exchange of D2O
with terminal benzylic hydrogens of 1 was very minor.
As a control experiment, when 1 was heated with 10
mol% of RhIII(ttp)Me and excess D2O to produce a deu-
terium enriched 1-d', only 8% of the terminal benzylic
hydrogens underwent deuterium exchange in a long
reaction time over 3 d (eq 3). The extensive benzylic
H/D exchanges at the internal positions were much
faster as reported in the more rapid benzylic CHA in
PhCH2CH3 by Wayland.14 Therefore, water was con-
firmed to be the hydrogen source for the catalytic hy-
drogenolysis of PCP to 1.
To investigate whether benzylic CHA is an intermedi-
ate for benzylic CCA of PCP,15 the reaction of PCP with
RhIII(ttp)Cl, KOH and D2O in benzene-d6 at lower reac-
Figure 1. Second order kinetic plots for the reactions of
RhII(tmp) with 9.6 mM PCP in C6D6 at 150 oC.
Analysis of the temperature dependent rate constants
1
tion temperatures were closely monitored by H NMR
‡
o
(140-170 C) yielded the activation parameters ꢀH =
spectroscopy (SI, Table S1, eq S1, Figure S1). Initially,
37.9±4.7 kcal mol-1, ꢀS‡ = 38.4±11.1 cal mol-1 K-1 and
ꢀG‡ = 21.7±4.7 kcal mol-1 (Figure 3). The ꢀH‡ agrees
very well with that of the thermal ring-opening of PCP
RhIII(ttp)Cl was completely consumed to give 71% yield
of RhII (ttp)2 only after heating at 60 oC for 4 d and 120
2
oC for 8 d. When the temperature was further raised to
‡
o
o
at 188 C measured by Roth et al (ꢀH = 37.7±0.5 kcal
130 C, benzylic CCA of PCP began to occur slowly to
mol-1).17 Hence, the benzylic CCA of PCP occurs via the
carbon-carbon bond homolysis. The positive and large
magnitude of ꢀS‡ suggests a dissociative nature of the
transition state. The second order dependence of
RhII(tmp) indicates a possible formation of a compact
encounter complex between two RhII(tmp) and a PCP
molecules prior to the carbon- carbon bond cleav-
age.18,19
afford 6% of the di-rhodium benzyl 2 (Scheme 2) in 1 d,
which was confirmed structurally by an independent
synthesis of 2 (SI, eq S2). No benzylic CHA products
were observed even with ~0.4 mM RhII (ttp)2 existing
2
in the reaction mixture, which should be very reactive
towards benzylic hydrogens. Moreover, the ratio of the
benzylic protons to the aromatic protons of PCP re-
mained unchanged. These observations suggested that
benzylic CCA of PCP occurred directly without passing
through any CHA intermediate.
To gain further knowledge on the benzylic CCA step
of PCP with rhodium porphyrins by kinetic studies, the
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RhIII(ttp)Bn to give toluene in 66% yield as a model
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study (eq 5).
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Figure 2. Plot of k’obs against [PCP]. [RhII(tmp)] = 0.24
mM, [PCP] = 4.8-14.4 mM.
Based on the above findings, scheme 2 illustrates the
proposed catalytic cycle. RhIII(ttp)OH is first generated
by ligand substitution of RhIII(ttp)I with KOH,9 or hy-
drolysis of RhIII(ttp)Me with H2O.12 At 200 oC,
RhIII(ttp)OH is unstable and rapidly decomposes to
produce RhII(ttp) and H2O2. H2O2 rapidly dispropor-
tionates to give H2O and O2.20 Two RhII(ttp) combine to
Figure 3. Determination of activation parameters for the
reactions of RhII(tmp) with PCP in C6D6 at 140-170 oC.
[RhII(tmp)] = 0.24 mM, [PCP] = 9.6 mM.
form RhII (ttp)2 as a non-productive equilibrium. PCP
2
reacts productively with RhII(ttp) to furnish benzylic
CCA with the formation of 2. Subsequently, hydrolysis
of 2 with H2O completes the hydrogenation process to
give 1 and RhIII(ttp)OH is thus recycled for the cataly-
sis. This process is supported by the hydrolysis of
In summary, the catalytic carbon-carbon σ-bond ac-
tivation and hydrogenation of PCP with water is
demonstrated. The hydrogen from water is transferred
to the hydrocarbon. Further studies are ongoing.
Scheme 2. Proposed Catalytic Cycle.
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(15) Rybtchinski, B.; Milstein, D. Angew. Chem. Int. Ed.
1999, 38, 870-883.
(16) (a) Wayland, B. B.; Sherry, A. E.; Poszmik, G.; Bunn, A.
G.; J. Am. Chem. Soc. 1992, 114, 1673-1681. (b) Wayland, B. B.
Polyhedron, 1988, 7, 1545–1558.
(17) Roth, W. R.; Hopf. H.; de Meijere, A.; Hunold, F.;
Börner, S.; Neumann, M.; Wasser, T.; Szurowski, J.; Mlynek, C.
Liebigs Ann. 1996, 2141-2154.
(18) Alternatively, the thermolysis of benzylic C-C bond of
PCP generates the radical which is bis-benzylic.
(19) Examples of 4-centered transition states: (a) Sherry, A.
E.; Wayland, B. B. J. Am. Chem. Soc. 1990, 112, 1259-1261. (b)
Wayland, B. B.; Ba, S.; Sherry, A. E. J. Am. Chem. Soc. 1991,
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Chem. 1992, 31, 148-150. (d) Cui, W.; Zhang, X. P.; Wayland, B.
B. J. Am. Chem. Soc. 2003, 125, 4994-4995. (e) Cui, W.; Way-
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(20) (a) Walling, C. Acc. Chem. Res. 1975, 8, 125-131. (b)
Kato, S.; Ueno, T.; Fukuzumi, S.; Watanabe, Y. J. Bio. Chem.
2004, 279, 52376-52381. (c) Kumar, A.; McCluskey, R. J. Ind.
Eng. Chem. Res. 1987, 26, 1323-1329. (d) Harned, H. S. J. Am.
Chem. Soc. 1918, 40, 1461-1481.
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ASSOCIATED CONTENT
Supporting Information.
Experimental procedures, kinetic data, H NMR and mass
spectra. This material is available free of charge via the In-
ternet at http://pubs.acs.org.
1
AUTHOR INFORMATION
Corresponding Author
9
ksc@cuhk.edu.hk
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ACKNOWLEDGMENT
We thank the Direct Grant of Chinese University of Hong
Kong for financial support.
ABBREVIATIONS
PCP, [2.2]paracyclophane; ttp, tetratolylporphyrinato dian-
ion; tmp, tetramesitylporphyrinato dianion.
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