Manganese-Catalyzed Dual-Deoxygenative Coupling of Primary Alcohols with 2-Arylethanols

Article abstract of DOI:10.1002/anie.201809333

Reported herein is a general and efficient dual-deoxygenative coupling of primary alcohols with 2-arylethanols catalyzed by a well-defined Mn/PNP pincer complex. This reaction is the first example of the catalytic dual-deoxygenation of alcohols using a non-noble-metal catalyst. Both deoxygenative homocoupling of 2-arylethanols (17 examples) and their deoxygenative cross-coupling with other primary alcohols (20 examples) proceeded smoothly to form the corresponding alkenes by a dehydrogenation and deformylation reaction sequence.

Full text of DOI:10.1002/anie.201809333

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201809333
German Edition: DOI: 10.1002/ange.201809333
Cross-Coupling
Manganese-Catalyzed Dual-Deoxygenative Coupling of Primary
Alcohols with 2-Arylethanols
Yujie Wang+, Zhihui Shao+, Kun Zhang, and Qiang Liu*
Dedicated to Prof. Xiyan Lu on the occasion of his 90th Birthday
Abstract: Reported herein is a general and efficient dual-
deoxygenative coupling of primary alcohols with 2-aryletha-
nols catalyzed by a well-defined Mn/PNP pincer complex.
This reaction is the first example of the catalytic dual-
deoxygenation of alcohols using a non-noble-metal catalyst.
Both deoxygenative homocoupling of 2-arylethanols (17
examples) and their deoxygenative cross-coupling with other
primary alcohols (20 examples) proceeded smoothly to form
the corresponding alkenes by a dehydrogenation and defor-
mylation reaction sequence.
stocks. Such transformations, which involve simultaneous
[10]
À
cleavage of two inert C O bonds, are very challenging,
although the corresponding mono-deoxygenation of alcohols
with primary alcohols by a hydrogen-borrowing strategy has
been well developed.[11] To the best of our knowledge, only
two examples of the full deoxygenative coupling of two
alcohol-containing molecules have been reported. In 2011,
Ishii, Obora, and co-workers developed an Ir-catalyzed
homocoupling of w-arylalkanols to a,w-diarylalkanes by
a two-step method with the use of two different Ir catalysts
(Scheme 1a).[12]
D
eoxygenation of alcohols provides an efficient and envi-
ronmentally benign technique for using sustainable biomass-
derived feedstocks to produce biofuels and platform chem-
icals.[1] For the last 40 years, the best-known method for the
deoxygenation of alcohols has been the Barton–McCombie
reaction, which uses nBu3SnH as the hydrogen source.[1d,2] To
avoid the use of toxic organotin reagents, alternative reduc-
tants such as silanes,[3] H2,[4] HCOOH,[5] and isopropanol[6]
have been used in catalytic versions of this transformation.
À
Metal-catalyzed C O bond homolysis is also promising for
alcohol deoxygenation.[7] However, the substrate scope is
restricted to benzylic and allylic alcohols. Recently, Li and co-
workers reported a more efficient deoxygenation of alcohols
by a tandem reaction that involves dehydrogenation and
Wolff–Kishner reduction, catalyzed by either a Ru or Ir
catalyst.[8] Later on, Milsteinꢀs group developed a more
sustainable version of the same transformation by using
a non-noble-metal catalyst.[9]
Despite significant advances such as those mentioned
above, the alcohol deoxygenation reaction is still limited to
À
C O cleavage of one alcohol-containing molecule. Direct
coupling of two alcohol molecules by a dual-deoxygenation
process could be an ideal approach for the production of high-
energy-density hydrocarbons from oxygen-rich biomass feed-
Scheme 1. Catalytic tandem dual-deoxygenative coupling of alcohols
according to Obora et al. (2011) and Johnson et al. (2018).
[*] Y. Wang,[+] Z. Shao,[+] Prof. Dr. Q. Liu
Center of Basic Molecular Science (CBMS)
Department of Chemistry
While we were performing the work reported in this
paper, the group of Johnson reported a novel Ru-catalyzed
deoxygenative homocoupling of 2-arylethanols (Scheme
1b).[13] The two reported examples were both catalyzed by
noble-metal catalysts and limited to the production of diaryl-
substituted hydrocarbon chains by homocoupling reactions. A
general and sustainable full deoxygenative coupling of
alcohols is still highly desirable. On the basis of recent
achievements in alcohol dehydrogenation enabled by Mn
Tsinghua University, Beijing 100084 (China)
E-mail: qiang_liu@mail.tsinghua.edu.cn
Prof. K. Zhang, Prof. Dr. Q. Liu
School of Biotechnology and Health Sciences, Wuyi University
Jiangmen, Guangdong Province 529090 (China)
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201809333.
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catalysts,[14] herein we report the first non-noble metal
catalyzed dual-deoxygenative coupling of primary alcohols
with 2-arylethanols by using a well-defined Mn catalyst
(Scheme 1c).[15] Notably, this method was applicable to both
homocoupling of 2-arylethanols and their cross-coupling
reactions with various primary alcohols, revealing a broad
substrate scope and excellent functional-group tolerance.
Mechanistic studies showed that this transformation proceeds
by a dehydrogenation and deformylation process, with gen-
eration of formate as a side-product.
During our study of Mn-catalyzed dehydrogenative
coupling of alcohols with hydroxide,[16] we found that 1,3-
diphenylpropene (2a) rather than the target carboxylate was
obtained when 2-phenylethanol (1a) was used as the sub-
strate. This aroused our interest because of the significant
value of the reaction in alcohol deoxygenation. We therefore
started this work by systematic evaluation of the effects of
various catalysts, bases, and solvents on this transformation
(see Tables S1 and S2 in the Supporting Information). 2-
Phenylethanol (1a; 5 mmol), NaOH (0.5 equiv), xylene
(0.6 mL), and 0.2 mol% [Mn]-1 (Scheme 2) at reflux were
native homocoupling of 2-arylethanols (1; Scheme 2). Various
functional groups with different electronic and steric effects
were well tolerated at the ortho-, meta-, and para-positions.
Specifically, arylethanols with reducible moieties such as
À
À
C Br and C I bonds were successfully converted into the
corresponding diarylpropenes (2 f, 2i, and 2j) without deha-
logenation under these reductive conditions. A sulfur-con-
taining substrate was also compatible and afforded the
deoxygenative coupling product 2o in 93% yield. However,
the substrate bearing a free amino group gave a much lower
yield (2l), which was likely caused by the catalyst position
through N coordination. In addition, the low yield of 2k from
the trifluoromethyl-substituted arylethanol is the result of
formation of a dehydration by-product, 1-(trifluoromethyl)-4-
vinylbenzene, because of the enhanced acidity of the benzylic
À
C H bond. The reactions of sterically hindered substrates 2-
(naphthalen-1-yl)ethan-1-ol and 2-(naphthalen-2-yl)ethan-1-
ol proceeded smoothly in the presence of 0.2 mol% Mn
catalyst to give the desired products 2p (60%) and 2q (76%),
respectively. These results are better than those reported for
the Ru catalytic system, which gave only 12% and 26%
yields, respectively, from the same substrates with 1 mol%
catalyst.[13]
Furthermore, the deoxygenative coupling of 2-aryletha-
nols (1) with other primary alcohols (3) was also investigated
(Scheme 3). This reaction is more challenging because of
homocoupling side-reactions. Gratifyingly, moderate to good
yields of cross-coupling products were obtained by refining
Scheme 2. Mn-catalyzed deoxygenative homocoupling of the 2-aryl-
ethanols 1.
the optimum reaction conditions for full conversion of 1a,
resulting in 93% yield of 2a. Remarkably, 1,3-diphenylpro-
pane, a possible reduction product of 2a by a potential
hydrogen-autotransfer process, was not detected. It revealed
that H2 evolution was more rapid than alkene hydrogenation
in this reaction system. The use of stronger (NaOEt and
NaOtBu) or weaker (Na2CO3, NaOAc, and K3PO4) bases led
to much lower yields or even prevented this transformation
(see Table S2, entries 2–6). It is also worth noting that the use
of at least 0.5 equivalents of NaOH was needed to attain
a high reaction efficiency (see Table S2, entries 1 and 11).
These results indicated the pivotal role of the base.
Scheme 3. Mn-catalyzed deoxygenative cross-coupling of the 2-aryl-
ethanols 1 with primary alcohols 3. [a] 1 (1.25 mmol), 3 (3.75 mmol).
[b] 1 (3.75 mmol), 3 (1.25 mmol). The overall yield of the isolated
product for two reaction steps is shown.
Under the optimized reaction conditions, a number of
substituted 1,3-diarylpropenes (2a–q) were obtained in
moderate to excellent yields by the Mn-catalyzed deoxyge-
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the ratio of the two alcohol substrates. In the Mn-catalyzed
which enables slow release of 5a and thus ensures high
reaction, a mixture of alkene products was obtained as a result
of the non-selective aldol condensation of the two aldehyde
intermediates generated by alcohol dehydrogenation
(Scheme 1). Nevertheless, the subsequent Ni-catalyzed
hydrogenation of the mixed alkenes gave only the reduced
alkanes 4 (Scheme 3). Awide range of functional groups were
compatible in this reaction sequence, and various unsym-
metrical 1,3-diarylpropanes (4a–i) and functionalized 1-
arylalkanes (4j–s) were obtained in good yields. However,
deoxygenative coupling of the benzyl-substituted arylethanol
gave a low yield of 4t because steric hindrance decelerated
the aldol condensation step. Compared with the reported
stepwise catalytic homocoupling of arylalkanols to diaryl-
alkanes with the use of two different Ir catalysts,[12] this
protocol catalyzed by earth-abundant metals (Mn and Ni) is
more economical and broadly applicable.
efficiency of the following condensation step.
Further mechanistic insights were gained by performing
kinetic studies (Figure 1). The kinetic profile of the standard
To shed light on the reaction mechanism, control experi-
ments and kinetic studies were conducted. It is noteworthy
that Stoermer and co-workers reported the condensation of
phenylacetaldehyde (5a) in basic ethanol solution to produce
1,3-diphenylpropene (2a) in high yield.[17] Nevertheless, this
protocol is highly sensitive to various substituents on the
phenyl ring,[18] which highlights the advantages of our
reported catalytic transformation (see Table S3). On the
basis of this known reaction, we hypothesized that the Mn-
catalyzed dehydrogenation of 1a produced the reaction
intermediate 5a, which underwent a base-mediated conden-
sation to give the final product 2a. To verify this hypothesis,
we first investigated the reactivity of 5a under standard
reaction conditions with or without the Mn catalyst (Scheme
4a,b). Interestingly, 2a was obtained with the same yield
(50%) in these two reactions, and supported aldehyde
intermediate 5a as an intermediate for this Mn-catalyzed
transformation, as well as excluded the assistance of Mn
Figure 1. Kinetic studies.
reaction of 1a to 2a is shown in Figure 1a. No induction
period was observed, and neither intermediate 5a nor 6a was
detected by GC analysis during the entire reaction process. It
illustrated that the dehydrogenation of 1a was much slower
À
catalyst for the condensation of 5a and the following C C
À
bond cleavage process. In addition, another possible reaction
intermediate, 6a, generated by aldol reaction of 5a, was
smoothly transformed into the desired product 2a in 84%
yield without Mn catalyst (Scheme 4c), indicating 6a to be
a key intermediate in this reaction. Based on this observation,
we postulated that this catalytic deoxygenative coupling
reaction is induced by Mn-catalyzed dehydrogenation of 1a,
than the subsequent aldol condensation and C C bond
cleavage steps. This outcome is consistent with our hypothesis
regarding the mechanism for selectivity control as mentioned
above. For comparison, the kinetic behavior of the conden-
sation reaction of 5a to 2a was also studied under catalyst-
free conditions (Figure 1d). Full conversion of 5a and a 38%
yield of 2a were obtained, with a much higher initial rate.
Interestingly, the selectivity and yield of this transformation
could be increased by enhancing the amount of base from 0.5
to 2 equivalents, albeit with similar reaction rate (Figure 1c).
Because of the poor solubility of NaOH in xylene and larger
number of undefined side-reactions of 5a at high temper-
atures, kinetic studies of the known base-mediated conden-
sation reactions in ethanol at lower temperatures were
performed.[17] Specifically, a low reaction rate and yield
were obtained with 0.5 equivalents of NaOH, the same
amount of base as in the catalytic reaction (Figure 1e). In
contrast, when 2 equivalents of base were used, the reaction
rate and yield were significantly improved because of the
better solubility of NaOH in ethanol (Figure 1b). Given these
results, we concluded that the concentration of NaOH must
be much higher than that of 5a to avoid side-reactions during
the condensation process. This condition is achieved by the
Scheme 4. Control experiments to identify reaction intermediates.
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slow release of 5a by dehydrogenation during the catalytic
reaction.
intermediate originated from isomerization of the a,b-unsa-
turated aldehyde to the b,g-unsaturated aldehyde.
Although the base-mediated condensation of 5a to 2a has
Based on the mechanistic studies, a plausible reaction
pathway is proposed for the Mn-catalyzed deoxygenative
coupling of 1a to 2a (Scheme 6). Firstly, Mn-catalyzed
dehydrogenation of 1a gives 5a with concomitant release of
hydrogen. Then 5a is converted into 6a by a base-promoted
aldol condensation. 6a is in equilibrium with the b,g-
unsaturated aldehyde 7a under basic conditions. The inter-
mediate 7a further undergoes deformylation by nucleophilic
attack on the carbonyl group by hydroxide to afford the allyl
anionic species 12a, which is protonated to deliver the final
product 2a.
À
been known for many years, the mechanism of the C C
cleavage process is still unclear. Recently, metal- or base-
promoted decarbonylation or decarboxylation reaction path-
ways were proposed along with the release of CO2 or CO
gas.[12,13] However, such speculated mechanisms were not
verified experimentally. In our scenario, trace amounts of
gases other than hydrogen were monitored by GC analysis of
the gaseous phase (see Figure S1). Instead, sodium formate
was produced in 89% yield, which indicates a different
deformylation mechanism (Scheme 5a). Based on these
Scheme 6. Proposed reaction pathway.
In summary, a novel dual-deoxygenative coupling of 2-
arylethanols with primary alcohols, catalyzed by an earth-
abundant metal catalyst, was developed. Both the homocou-
pling of 2-arylethanols (17 examples) and their cross-coupling
with various primary alcohols (20 examples) proceeded
smoothly with excellent functional-group tolerance in the
presence of a well-defined Mn catalyst. Mechanistic studies
uncovered that this transformation underwent a cascade
reaction process, involving catalytic dehydrogenation, aldol
condensation, and base-promoted deformylation.
Scheme 5. Control experiments for clarification of deformylation
process.
observations, we propose that the base-mediated isomeriza-
tion of the intermediate 6a to b,g-unsaturated aldehyde 7a
was a key step in this deformylation process.[19] The inter-
mediate 7a underwent further nucleophilic attack by hydrox-
À
ide to accomplish C C bond cleavage and generate formate
and allylic anionic species. Under basic conditions, the a,b-
unsaturated aldehyde 8a, bearing two different aryl-substi-
tuted groups, was converted into a mixture of two 1,3-
diarylpropene isomers, 8b and 8c. This conversion supports
participation of the allylic anionic intermediate (Scheme 5b).
Moreover, independently prepared 8b could not be converted
into the isomer 8c under the same basic conditions (Scheme
5c). This outcome suggests that the allylic anion could only be
generated from the a,b-unsaturated aldehyde rather than 1,3-
diarylpropene. To further support our speculation, the g-
methyl-substituted a,b-unsaturated aldehyde 9a was reacted
with base, giving the deformylative products 2,4-diphenyl-2-
butene (9b) and 1,3-diphenyl-1-butene (9c) in 53% and 12%
yields, respectively (Scheme 5d). In contrast, blocking the
allylic position with two methyl groups completely prevented
Acknowledgements
We are grateful for the financial supports from the National
Natural Science Foundation of China (21822106), the Foun-
dation of the Department of Education of Guangdong
Province (2017KZDXM085), and the 111 project.
Conflict of interest
The authors declare no conflict of interest.
Keywords: alcohols · dehydrogenation · deoxygenation ·
manganese · reaction mechanisms
À
the deformylative C C cleavage reaction (Scheme 5e). These
results strongly support the proposal that the allylic anionic
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Manuscript received: August 15, 2018
Version of record online: && &&, &&&&
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Communications
Cross-Coupling
Y. Wang, Z. Shao, K. Zhang,
Q. Liu*
&&&& — &&&&
Manganese-Catalyzed Dual-
Deoxygenative Coupling of Primary
Alcohols with 2-Arylethanols
“Mn” of action: A general and efficient
dual-deoxygenative coupling of primary
alcohols with 2-arylethanols, catalyzed by
a well-defined manganese/PNP pincer
complex, is reported. Mechanistic studies
indicate that this transformation under-
goes a reaction process involving cata-
lytic dehydrogenation, aldol condensa-
tion, and base-promoted deformylation.
6
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