Carbon-Carbon σ-Bond Transfer Hydrogenation with DMF Catalyzed by Cobalt Porphyrins

Article abstract of DOI:10.1021/acs.organomet.6b00434

Cobalt porphyrins were found to catalyze the transfer hydrogenation of the carbon-carbon σ bond of [2.2]paracyclophane (PCP) with the solvent DMF serving as the hydrogenating agent. Successful trapping experiments with benzene solvent and the kinetic isot

Full text of DOI:10.1021/acs.organomet.6b00434

Communication
pubs.acs.org/Organometallics
Carbon−Carbon σ‑Bond Transfer Hydrogenation with DMF Catalyzed
by Cobalt Porphyrins
Chun Meng Tam, Ching Tat To, and Kin Shing Chan*
Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, People’s Republic of
China
S
* Supporting Information
ABSTRACT: Cobalt porphyrins were found to catalyze the transfer hydrogenation of
the carbon−carbon σ bond of [2.2]paracyclophane (PCP) with the solvent DMF
serving as the hydrogenating agent. Successful trapping experiments with benzene
solvent and the kinetic isotope effect (4.9) suggested the presence of benzyl radical
intermediates in undergoing hydrogen atom transfer from DMF as the rate-limiting step.
The rate law was established by initial rate measurements to be rate = kobs[CoII(ttp)]-
[PCP].
atalytic carbon−carbon bond activation (CCCA) is the
Initially, we envisioned that CoII(ttp) (ttp = 5,10,15,20-
tetratolylporphyrinato dianion) could react with PCP similarly
to the rhodium and iridium analogues. A 50 mol % catalyst
loading was used to compensate the expected low reactivity of
CoII(ttp). Unfortunately, only a trace amount of the hydro-
genation product 4,4′-dimethylbibenzyl (2) was formed
together with other unknown organic products when the
reaction was carried out in benzene-d6 solvent (eq 1).
C
key chemical transformation in hydrocracking, turning
crude oil into petroleum.1 CCCA holds the potential to convert
heavy polymeric residues and biomass into lighter, econom-
ically valuable chemicals.2 Despite the usefulness of CCCA,
examples with transition-metal complexes in homogeneous
media remain limited.3 The small number of literature reports
on CCCA reflects the inertness of the C−C σ bond relative to
the C−H bond.4 CCCA with transition-metal complexes in
homogeneous media mainly employs strategies such as
chelation assistance,5 ring strain relief,6 and carbonyl function-
ality7 to generate organometallic intermediates, followed by
subsequent rearrangement of the carbon skeleton8 or M−C σ-
9
bond hydrogenation with H2 to complete the catalytic cycle.
Our group has been interested in carbon−carbon bond
activation (CCA) of organic substrates and has reported several
stoichiometric examples.10 Recently, we have developed
rhodium and iridium metalloporphyrin (M(por), M = Rh, Ir)
catalyzed C−C σ-bond hydrogenation of [2.2]paracyclophane
(1) with water as the hydrogenating agent.11 In light of these
successes, we wish to extend the catalysis to a much less
reactive but more easily accessible and cheaper cobalt
porphyrin catalyst. Co(II) porphyrin is expected to have a
lower reactivity than the corresponding rhodium and iridium
porphyrin analogues since (1) Co−C bonds are generally
weaker12 and (2) CoII(por) metalloporphyrin radical has a
lower SOMO energy level.13 As a result of low reactivity, CCA
by cobalt complexes remains scarce in the literature.14
Stoichiometric CCA with cobalt complexes includes ring
expansion of allyl or cyclopentadienyl with alkyne to give
seven-membered ring cobalt complexes15 and C−CN bond
cleavage via oxidative addition with electron-rich Co(I)
complexes.16 Catalytic CCA was recently reported for the
cleavage of secondary and tertiary benzyl alcohols bearing a
pyridinyl directing group.17 Here, we report our success on
Co(II) porphyrin catalyzed C−C σ-bond transfer hydro-
genation of PCP in DMF solvent as the transfer hydrogenating
agent.
We suspected that the hydrolysis of CoII(ttp) benzyl
intermediates was slow in nonpolar benzene solvent; the
more polar solvent DMF to facilitate hydrolysis of inter-
mediates was then examined with 10 mol % catalyst, as in the
case of rhodium and iridium analogues.11 To our delight, 2 was
obtained selectively in DMF in a good yield of 81% (Table 1,
entry 2). In order to promote hydrogenation, we then
Table 1. Water Loading Effect
yield (%)
entry
H2O (equiv)
time (days)
1
2
1 + 2
1
2
0
6
6
86
81
92
99
100
15
17
Received: May 30, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.6b00434
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Organometallics
Communication
investigated the water loading effect. To our surprise, catalytic
hydrogenation of 1 proceeded at a faster rate in the absence of
water to give an 86% yield of 2 in 6 days (Table 1, entry 1).18
We then investigated the effect of catalyst loading in DMF
without H2O added. A control experiment showed no
background reaction in the absence of catalyst (Table 2,
that the solvent DMF served as the hydrogenating agent.
Indeed, DMF has been used as a hydrogen atom transfer agent
for radical reactions.21
On the basis of our reported bimetalloradical activation of
PCP with Rh porphyrin11a and on the well-known metal-
loradical chemistry of Co(II) porphyrin, we attempted to trap
radical intermediates, most likely benzyl radicals, that may exist
in the catalysis. TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy)
failed to yield any trapping products, likely due to the known
thermal instability of TEMPO-Bn at temperatures higher than
93 °C.22 The benzyl radical intermediate was then successfully
trapped by radical substitution of benzene solvent as 6 (eq 5).23
The poor mass balance observed was attributed to the CoII(ttp)
end-capped oligomerization of 1.24
Table 2. CoII(ttp) Catalyst Loading Effect
yield (%)
entry
CoII(ttp) (mol %)
time (days)
1
2
1 + 2
1
2
3
4
0
10
20
30
5
6
3
2
99
6
0
99
92
95
73
86
95
73
0
0
entry 1). Increased CoII(ttp) loading from 10 to 20 mol %
enhanced the product yield from 86% to 95% and shortened
the reaction time from 6 to 3 days (Table 2, entries 2 and 3). A
higher loading of 30 mol % gave a faster hydrogenation but
slightly lower yield of 73% (Table 2, entry 4).
To establish the hydrogen source for the hydrogenation,
deuterium labeling experiments were conducted. Hydro-
genations with both full and partial conversion conducted in
deuterated DMF (DMF-d7)19 gave the deuterated product 2-d
and 2'-d in 75% and 5% yield and deuterium incorporation on
terminal benzylic positions in 25% and 33% yields, respectively,
as measured by 1H NMR (eqs 2 and 3).20 Deuterium
To gain mechanistic insight into CoII(ttp)-catalyzed transfer
hydrogenation of PCP in DMF, kinetic studies by initial rate
measurements were conducted with at least a 10-fold excess of
[PCP] over [CoII(ttp)]. Conversion of PCP to 2 was
monitored by GC-MS ([CoII(ttp)] = 0.060−0.48 mM, [PCP]
= 1.2−9.6 mM, and T = (180−205) 0.2 °C). The initial rate
of formation of hydrogenation product 2 increased linearly with
[CoII(ttp)] and [PCP], respectively (Figure 1), giving a
pseudo-first-order kinetic plot for both species and an overall
second-order reaction represented by rate = kobs[CoII(ttp)]-
[PCP].
The temperature-dependent rates constants measured at
(180−205)
0.2 °C yielded the following values for the
activation parameters: ΔH⧧ = +26.9 2.3 kcal mol−1, ΔS⧧ =
−12 5 cal mol−1 K−1, and ΔG⧧ = +32.5 3.3 kcal mol−1
incorporation on unreacted PCP was not observed; thus, no
prior C−H activation and exchange on 1 occurred before CCA.
A control experiment also indicated no deuterium exchange
between 2 and DMF-d7 (eq 4). These experiments confirmed
■
Figure 1. Plots of the initial rate of formation of [2] at 200 °C: ( )
II
▲
)
against [Co (ttp)] = 0.060−0.48 mM with [PCP] = 4.8 mM; (
against [PCP] = 1.2−9.6 mM with [CoII(ttp)] = 0.12 mM.
B
DOI: 10.1021/acs.organomet.6b00434
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Organometallics
Communication
(Figure 2).25 The small but negative ΔS⧧ suggests an
associative transition state.
In summary, the carbon−carbon σ-bond transfer hydro-
genation of [2.2]paracyclophane with DMF catalyzed by cobalt
porphyrin was achieved. The metalloradical Co(II) porphyrin
attacks the C−C σ-bond of PCP, and the resultant benzyl
radical abstracts a hydrogen atom from DMF to give the
hydrogenation product.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00434.
Experimental procedures and NMR and MS spectra
(PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for K.S.C.: ksc@cuhk.edu.hk.
Notes
Figure 2. Eyring plot for determination of activation parameters
([CoII(ttp)] = 0.48 mM, [PCP] = 4.8 mM) at 180−205 °C.
The authors declare no competing financial interest.
Further mechanistic insight into the transition state came
from kinetic isotope effect (KIE) studies. The initial rates of
catalysis in DMF and DMF-d7 were measured separately from
two parallel reactions to be kH = 0.083 mM h−1 and kD = 0.017
mM h−1, respectively ([CoII(ttp) = 0.48 mM, [PCP] = 4.80
mM at 200 °C). The KIE (kH/kD) was then determined to be
4.9. The KIE from a competition experiment with equimolar
amounts of DMF-d0 and DMF-d7 was determined to 6.5. Both
values point to a rate-limiting hydrogen atom transfer step of
the benzyl radical intermediate with the DMF molecule in a
linear transition state.
ACKNOWLEDGMENTS
■
We thank the GRF (No. 400212) of Hong Kong for financial
support. This work was supported in part by a special
equipment grant (No. SEG/CUHK09) from the University
Grants Committee of Hong Kong.
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