Reductive cleavage of the Csp2-Csp3 bond of secondary benzyl alcohols: Rhodium catalysis directed by N-containing groups

Article abstract of DOI:10.1002/anie.201204338

Cutting loose: 1,1-Biarylmethanol substrates undergo reductive cleavage of the C-C bond in the presence of a cationic Rh^III catalyst and H ₂ (see scheme; DG=directing group). Various functional groups are tolerated in the reaction system. Preliminary studies indicate that a five-membered rhodacycle intermediate, which then converts into a Rh ^III hydride species for the reduction, is involved in the catalytic cycle. Copyright

Full text of DOI:10.1002/anie.201204338

Angewandte
Chemie
DOI: 10.1002/anie.201204338
Synthetic Methods
ꢀ
Reductive Cleavage of the Csp2 Csp3 Bond of Secondary Benzyl
Alcohols: Rhodium Catalysis Directed by N-Containing Groups**
Kang Chen, Hu Li, Zhi-Quan Lei, Yang Li, Wen-He Ye, Li-Sheng Zhang, Jian Sun, and Zhang-
Jie Shi*
for cleaving and degrading synthetic polymers[3] and bio-
mass[4] to produce platform chemicals, as well as provide
a solution to diminish the “white pollution” from synthetic
polymers.
ꢀ
Oxidative coupling of two nucleophiles to construct a C C
bond (Scheme 1) is gradually becoming a prestigious and
powerful method relative to the traditional cross-coupling
Unfortunately, this field has been neglected by chemists
and only limited examples have been reported. Alkylalumi-
num was often used as the reductant in transition-metal-
catalyzed deallylation reactions.[5] It was also applied to the
reductive cleavage of cycloalkanes in the presence of rare-
earth-metal catalysts.[6] Other reductants, such as active
metals,[7] metal hydrides,[8] and hydrosilanes[9] were also
reported to realize the goal of the reductive cleavage of
ꢀ
C C bonds. Compared with the above reductants, H2 is
Scheme 1. Oxidative coupling versus reductive cleavage.
a superior choice for reductive cleavage. H2 as a source is
extremely abundant, and more importantly, no extra waste is
introduced into the reaction system. Early efforts to utilize H2
as the reductant mainly focused on relatively active strained
reactions.[1] Specifically, recent developments in cross-dehy-
drogenative coupling/arylation made such a transformation
more popular and “greener” and produces valuable com-
pounds by avoiding the preactivation of easily available and
inexpensive chemicals.[2] In contrast, less attention has been
paid to its reverse reaction, that is the reductive cleavage of
molecules.[10] However, for unstrained C C bonds, harsh
ꢀ
reaction conditions using heterogeneous catalytic systems[11]
or the assistance of a specially designed pincer ligand[12] were
required. To date, only a few examples on the cleavage of
ꢀ
activated C C bonds adjacent to a carbonyl group or
ꢀ
a C C bond (Scheme 1). Such a reductive cleavage is
aromatic ring under mild reaction conditions have been
reported.[13] In our previous studies we observed that N-
containing heterocycles were successful directing groups for
transition-metal-catalyzed C C bond cleavage.
basis, we demonstrate the first successful example of the
challenging, but important for a number of reasons. First of
all, such investigations are of theoretical importance for
[14]
ꢀ
ꢀ
understanding the reactivity of C C bonds, which are
On this
abundant in nature and the synthetic world. Secondly, this
method offers the potential to make valuable chemicals from
easily available starting materials and even from chemical
waste. Finally, such a method also offers a new and useful tool
ꢀ
reductive C C bond cleavage of benzyl alcohols with H2
through rhodium catalysis.[15]
To prove our concept of reductive cleavage, we chose the
substrate 1a as a model to investigate the hydrogenative
ꢀ
cleavage of C C bonds (Table 1). First of all, we tried some
[*] K. Chen, H. Li, Y. Li, W.-H. Ye, Li.-S. Zhang, Prof. Dr. Z.-J. Shi
Beijing National Laboratory of Molecule Science (BNLMS) and Key
Laboratory of Bioorganic Chemistry and Molecular Engineering of
Ministry of Education, College of Molecular Engineering and Green
Chemistry Center, Peking University
commonly used hydrogenation catalysts such as Pd/C, RhI,
and IrI complexes in our system. Unfortunately, none of them
worked for this transformation (Table 1, entries 1–4). The
RhIII catalyst [{Cp*RhCl2}2], which was successfully applied in
our previous studies,[14a] also failed to deliver the desired
product. To our delight, the cationic RhIII catalyst [Cp*Rh-
(CH3CN)3][SbF6]2 exhibited excellent catalytic activity
(entries 5 and 6). Further screening of solvents indicated
that EtOH and DCE gave the best results for this trans-
formation (entries 6–10). We choose EtOH for additional
studies because it is environmentally friendly. Further studies
unveiled that the reactivity was highly dependent upon the
reaction temperature. Systematic studies indicated that the
highest yield of 3a was obtained at 808C (entries 6, 11, and
12). Increasing the scale to 0.3 mmol led to a slight decrease of
the yield of benzyl alcohol as well as the appearance of a small
amount of benzaldehyde in the crude reaction mixture. This
Beijing 100871 (China)
E-mail: zshi@pku.edu.cn
Prof. Dr. Z.-J. Shi
State Key Laboratory of Organometallic Chemistry
Chinese Academy of Science
Shanghai 200032 (China)
Z.-Q. Lei, Prof. J. Sun
Chengdu Institute of Biology, Chinese Academy of Science
Chengdu, Sichuan 610068 (China)
[**] Support of this work by the “973” Project from the MOST of China
(2009CB825300) and NSFC (Nos. 20925207 and 21002001) is
gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201204338.
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Table 1: Optimization of the reaction conditions.
Entry[a]
Catalyst
Solvent
T [8C]
Yield[b]
1
2
3
4
5
6
7
8
Pd/C
[{Ir(cod)Cl}2]
Rh(cod)2BF4
Rh(PPh3)3Cl
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
MeOH
tBuOH
DCE
toluene
EtOH
EtOH
EtOH
EtOH
EtOH
80
80
80
80
80
80
80
80
80
80
70
90
80
80
80
n.d.
n.d.
n.d.
n.d.
[{Cp*RhCl2}2]
n.d.
[Cp*Rh(CH3CN)3][SbF6]2
[Cp*Rh(CH3CN)3][SbF6]2
[Cp*Rh(CH3CN)3][SbF6]2
[Cp*Rh(CH3CN)3][SbF6]2
[Cp*Rh(CH3CN)3][SbF6]2
[Cp*Rh(CH3CN)3][SbF6]2
[Cp*Rh(CH3CN)3][SbF6]2
[Cp*Rh(CH3CN)3][SbF6]2
[Cp*Rh(CH3CN)3][SbF6]2
[Cp*Rh(CH3CN)3][SbF6]2
96%
20%
70%
99%
88%
52%
92%
91%
93%
96%
9
10
11
12
13
14
15
[a] 0.1 mmol sacle in 1.0 mL solvent for 24 h unless otherwise noted.
[b] Yield of 3a as determined by 1H NMR spectroscopy using benzyl
methyl ether as the internal standard. [c] 0.3 mmol scale, 3.0 mL EtOH,
24 h. [d] 0.3 mmol scale, 2.0 mL EtOH, 24 h. [e] 0.3 mmol scale, 2.0 mL
EtOH, 36 h. cod=1,5-cyclooctadiene, Cp*=C5Me5, DCE=1,2-
dichloroethane.
problem could be addressed by simply increasing the
concentration of the substrate and increasing the reaction
time.
With the best reaction conditions in hand, we set out to
test the reactivity of other substrates. To extend the potential
application of the reaction, different directing groups were
tested (Scheme 2). We found that the substrates bearing
a removable pyrazolyl group (1a–c) could undergo reductive
ꢀ
Scheme 2. Substrate scope of 1,1- biarylmethanols for C C bond
reductive cleavage. The yields given in brackets refer to products 2 and
3, respectively. [a] Alcohol 1 (0.3 mmol), [Cp*Rh(CH3CN)3][SbF6]2
(0.015 mmol), EtOH (2.0 mL), H2 balloon, 808C, 36 h. [b] 48 h.
[c] Yield determined by 1H NMR spectroscopy using benzyl methyl
ether as the internal standard.
ꢀ
cleavage of the C C bond smoothly, thus giving moderate to
good yields of the corresponding products. The pyridinyl
group also proved to be an efficient directing group.
Substituents, having various electronic properties, on the
ring bearing the directing group (1d–g) were compatible in
this catalyst system. To our delight, Cl (1h) was tolerated
under these reductive conditions, and thus provides oppor-
tunities for additional functionalization.[16] The substrate
bearing a tertiary alcohol substituent (1i) led to a low yield
of the corresponding arylpyridine derivatives. Importantly,
the low yield of the desired product was not a result of the
of the alcohol 3. However, an electron-rich aryl group (1p)
did not perform well, thus giving moderate yield even after
a longer reaction time. Again, halogens (1q–s) survived under
such reaction conditions. We also tested alkyl-substituted
secondary alcohols (1t and 1u), which could undergo the
reductive cleavage albeit with lower efficiency.
A possible mechanism is proposed in Scheme 3. First, the
C C bond of 1 is cleaved with the assistance of RhIII and thus
ꢀ
ꢀ
ꢀ
cleavage of the C C bond at the tertiary alcohol moiety, but
the C Rh species 6 is generated. The intermediate 6 could
to the nucleophilic substitution of the tertiary hydroxy group
by an ethoxy group.[17] This result indicated that the directing
group played a key role in controlling the regioselectivity of
then be cleaved by H2 to generate 2 and the RhIII hydride
species,[18] which reduces the aldehyde to the alcohol. Alter-
natively, 6 could undergo protonation with H+ to regenerate
the C C bond cleavage.
the cationic RhIII species for the C C bond cleavage of next
ꢀ
ꢀ
Differently substituted aryl groups on the right-hand side
of the substrate alcohols were additionally surveyed
(Scheme 2). Substrates with a naphthyl ring or electron-
neutral phenyl ring (1j–m) exhibited excellent reactivity.
Electron-withdrawing substituents, such as ester group or
trifluoromethyl group (1n and 1o) led to slightly lower yields
catalytic cycle. At this stage, we cannot exclude the con-
version of the cationic RhIII precursor into the RhIII hydride
species. The RhIII hydride species could then promote both
ꢀ
the C C bond cleavage and reduction of the aldehyde in the
same catalytic cycle.
2
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membered rhodacycle 6’ was a possible intermediate in the
catalytic cycle. Importantly, when we subjected 6’ to the
reaction conditions in the presence of H2 but in the absence of
1k, we observed the formation of phenylpyridine and the
1
[20]
signal for Rh H in the H NMR spectrum. Thus, the RhIII
hydride species might be generated from the five-membered
rhodacycle intermediate 6’ under an H2 atmosphere.
ꢀ
To explain the sources of the protons on both the alcohol
and 2-phenylpyridine, D2 was used under the same reaction
ꢀ
conditions [Eq. (4)]. The results showed that both the C H
Scheme 3. Proposed mechanisms for the reductive cleavage of the
ꢀ
C C bond.
To understand the catalytic cycle, preliminary studies
were conducted. As observed in previous studies, we found
that the RhIII precatalyst could indeed catalyze the cleavage
ꢀ
of the C C bond of 1k in the absence of H2 [Eq. (1)]. Thus, we
bond at the ortho position of the pyridinyl group and a-
position of alcohol were partially deuterated. However, when
we used [D6]-EtOH instead of EtOH, more than one
ortho position of the pyridinyl group was deuterated, and
might be attributed to both the direct deuteration of the five-
ꢀ
membered rhodacycle intermediate and the reversible C H
bond cleavage of the aromatic ring by the RhIII species.
Interestingly, the deuteration ratio at the a-position of the
alcohol was even higher than that obtained when the reaction
was run under a D2 atmosphere [Eq. (5)]. The above results
ꢀ
indicated that the Rh H bond was involved in the reduction
of the aldehyde and the H/D exchange between the rhodium
hydride species and the protic solvent indeed existed. The
difference in the deuteration ratio between Equations (4) and
(5) may arise from the different concentration of deuterium
source in the reaction systems. When we used the nonprotic,
deuterated solvent [D2]-CD2Cl2, no H/D exchange took place
with either of the two products [Eq. (6)], thus confirming our
above conclusion.
To gain further insight into the mechanism, we used
1H NMR spectroscopy to monitor the reaction progress
(Figure 1). The formation of the alcohol 3c was observed
within 0.5 hours, and the consumption of 1k was not
complete. When all of 1k was consumed, the yield of 3c
was still increasing with time and the aldehyde 10 also
remained in the reaction system. The data indicate that the
formation of 2c is faster than 3c. According to these results, it
is not possible for the RhIII hydride species to catalyze both
hypothesized that a five-membered rhodacycle intermediate
ꢀ
was also involved in the C C bond cleavage. At this point, we
surveyed the reactivity of the RhIII complex 6’.[19] We found
that 6’ could catalyze both the reductive cleavage of 1k and
the reduction of 1-naphthaldehyde (10) into 3c smoothly
[Eqs. (2) and (3)]. These results suggested that the five-
ꢀ
the C C bond cleavage and the reduction of the aldehyde in
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&
Figure 1. Reductive cleavage of 1k ( ) and the formation of 2c ( )
1
*
and 3c ( ) as monitored by H NMR spectroscopy. Reaction condi-
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ꢀ
reductive cleavage of the Csp2 Csp3 bond of unstrained 1,1-
biaryl methanols in the presence of H2 as the reducing agent
under mild reaction conditions. Various functional groups are
tolerated albeit under a reductive atmosphere. Both pyridinyl
and pyrazolyl groups can serve as directing groups. Prelimi-
nary studies indicate that the RhIII hydride species is
generated from a five-membered rhodacycle intermediate.
Additional studies into the catalytic pathway and extention of
this concept to other systems are underway.
Experimental Section
The secondary alcohol substrate
1 (0.3 mmol) and [Cp*Rh-
(CH3CN)3][SbF6]2 (12.5 mg, 0.015 mmol) were added to the reaction
tube. The system was then purged with H2 (in balloon) three times.
Anhydrous EtOH (2.0 mL) was injected into the tube. The mixture
was heated at 808C in a parallel reactor for 36 h. The solvent was then
removed in vacuo and the residue purified by flash chromatography
(eluent: petroleum ether/EtOAc 20:1!5:1) to afford compounds 2
and 3.
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Received: June 4, 2012
Published online: && &&, &&&&
ꢀ
Keywords: C C activation · cleavage reactions · hydrogenation ·
reaction mechanisms · rhodium
.
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Communications
Synthetic Methods
K. Chen, H. Li, Z.-Q. Lei, Y. Li, W.-H. Ye,
Li.-S. Zhang, J. Sun,
Z.-J. Shi*
&&&& — &&&&
Cutting loose: 1,1-Biarylmethanol sub-
strates undergo reductive cleavage of the
system. Preliminary studies indicate that
a five-membered rhodacycle intermedi-
ate, which then converts into a RhIII
hydride species for the reduction, is
involved in the catalytic cycle. DG=
directing group.
ꢀ
ꢀ
Reductive Cleavage of the Csp2 Csp3 Bond
of Secondary Benzyl Alcohols: Rhodium
Catalysis Directed by N-Containing
Groups
C C bond in the presence of a cationic
RhIII catalyst and H2 (see scheme; DG=
directing group). Various functional
groups are tolerated in the reaction
6
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Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!