Cross-coupling reaction between alcohols through sp3 C-H activation catalyzed by a ruthenium/lewis acid system

Article abstract of DOI:10.1002/chem.200801317

A study was conducted to investigate the Ru-catalyzed/acid mediated C-C cross-coupling reaction between alcohols. It was observed during the experiment that functionalized alcohols can be synthesized by sp³ C-H bond activation of primary alcohols. The study also investigated the stereoselectivity of this coupling reaction with tertiary alcohols. The study used 1,1-diphenylethanol and ethanol as substrate and transition metal catalysts with various acid and solvents. The study also found that the RhCl(PPh ₃)₃ and RuCpCl(PPh₃)₂ catalysts accelerate the catalyze of the reactions with a lower yield. The study also observed that the ruthenium-catalyzed Lewis acid can enhance the cross-coupling reaction between ethanol and the alkene 1,1-diphenylethene. Lewis acid can promote the C-H bond activation and removal of hydroxy in the reaction.

Full text of DOI:10.1002/chem.200801317

COMMUNICATION
DOI: 10.1002/chem.200801317
3
À
Cross-Coupling Reaction between Alcohols through sp C H Activation
Catalyzed by a Ruthenium/Lewis Acid System
Shu-Yu Zhang, Yong-Qiang Tu,* Chun-An Fan, Yi-Jun Jiang, Lei Shi, Ke Cao, and
En Zhang[a]
À
The C C bond formation is pivotal and fundamental to
molecular architecture and complexity in organic chemistry;
drogen autotransfer process” was proposed on the basis of
the tandem oxidation-condensation-reduction processes.[9]
However, most recently we found that the coupling reaction
of the same type of above-mentioned alcohols catalyzed by
À
the direct C C cross-coupling using alcohols is one of ideal
reactions for achieving this goal, since it not only makes
[1–6]
À
new C C bonds,
but also displays atom-economic and
[RuCl2ACHTUNGTERNNU(G PPh3)3] in the presence of Lewis acid BF3·OEt2, as
environmentally benign chemistry.[7]
shown in Equation (2), mainly delivered a totally different
isomeric product 4-phenyl-2-butanol, despite in moderate
yield of 42%,[10] instead of the above-obtained b-alkylation
product in Equation (1). To our knowledge, this constitutes
the first example concerning the chemoselective intermolec-
ular a-alkylation of primary alcohols 2 with aliphatic alco-
Some pioneering work on the transition-metal-catalyzed
À
C C cross-coupling directly utilizing alcohols has been
made extensively under basic conditions.[6] For example,
ruthenium-catalyzed[8]/base-mediated cross-coupling of sec-
ondary alcohols with primary alcohols exclusively afforded
b-alkylated secondary alcohols [Eq. (1)],[6a] wherein the “hy-
À
hols 1 through transition-metal-catalyzed C C cross-cou-
pling (Scheme 1). In comparison with our previous report
À
Scheme 1. a-Alkylation of primary alcohols through catalytic C C cross-
coupling reaction with aliphatic alcohols.
concerning the olefin/alcohol coupling reaction,[4] the signifi-
cant difference and advantage of the present sequence in-
clude the following: 1) employment of easily available alco-
hols, especially tertiary alcohols, as green materials instead
of alkenes; 2) discovery of new effective catalyst, [RuCl2-
AHCTUNTGRENNG(UN PPh3)3], for the cross-coupling between alcohols, but which
is less effective in previous alkene/alcohol coupling; 3) for-
mation of functionalized secondary alcohols as product in
good to excellent yields. This Ru-catalyzed/Lewis acid pro-
moted cross-coupling demonstrates a novel synthetic trans-
formation directly from simple alcohols to advanced alco-
hols in an environmentally benign fashion. Herein, we pres-
ent our experimental results in detail.
Initially to widely explore the above novel transformation
commencing from easily available and handled alcohols, we
selected 1,1-diphenylethanol (1a) and ethanol (2a) as sub-
strates for our initial investigation with various transition-
[a] S.-Y. Zhang, Y.-Q. Tu, C.-A. Fan, Y.-J. Jiang, L. Shi, K. Cao, E. Zhang
State Key Laboratory of Applied Organic Chemistry and
Department of Chemistry, Lanzhou University
Lanzhou 730000 (P. R. China)
Fax : (+86)931-8915557
E-mail: tuyq@lzu.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801317.
Chem. Eur. J. 2008, 14, 10201 – 10205
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10201

metal catalysts under different acid and solvent conditions
(Table 1); we found that catalytic amounts of [RuCl2A(PPh3)3]
in the presence of 1.2 equivalents of BF3·OEt2 as a nonme-
nary experimental facts imply that the gradual in situ gener-
ation of water by dehydration of tertiary alcohols might
have the subtle influence on this ruthenium catalyst system.
Following the above optimized conditions, the generality
and scope of this cross-coupling were then investigated. As
listed in Table 2, a series of primary alcohols 2b–2h in the
presence of 1a or 1b were subjected to the current protocol
(entries 1–8), smoothly giving the desired a-alkylation prod-
ucts in good to excellent yields. It should be noted that an
enantiopure primary alcohol 2d with a chiral b-methyl
group was investigated in the presence of 1a (entry 3), and
the chiral a-alkylation product 3ad was readily afforded in
70% yield with a pair of epimers (2:1). From the results ob-
tained, it could be seen that this cross-coupling reaction, to
some extent, was independent upon the steric hindrance of
CTHUNGTRENNUNG
Table 1. Optimization of reaction conditions.[a]
Entry
Catalyst
Acid
Solvent
Yield[%][b]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
IrCl3
RuCl3
T
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
InCl3
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
N
trace
trace
72
22
82
R
E
R
N
64
trace
N
G
R
15
[c]
N
TsOH
SnCl4
Table 2. a-Alkylation of various primary alcohols with tertiary alcohols.[a]
U
62
R
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
toluene
THF
CH3NO2
CH3CN
68
[c]
G
R
60
[c]
U
[d]
Entry
Substrates
Product
Yield
[%][b]
N
Cl
42
[a] For entries 1–14, the general reaction condition: 1,1-diphenylethanol
1a (0.5 mmol) reacted with ethanol 2a (0.75 mmol) in the presence of
catalyst (0.0125 mmol) and acid (0.6 mmol) at 508C for 5 h. [b] Yield of
isolated product. [c] No desired product. [d] BF3·OEt2 (0.3 equiv), 4 days.
1
2
1a
1a
93
91
tallic Lewis acid in Cl
yield, entry 5). Among the catalysts screened (entries 1–7),
[RhCl(PPh3)3] (entry 3) or [RuCpCl(PPh3)2] (entry 6) could
also catalyze this reaction despite a slightly lower yield, but
the other transition-metal catalysts, such as [Pd(OAc)2PPh3],
IrCl3, RuCl3, and [AuCl(PPh3)3] were not effective. The role
of acid in this reaction was also investigated (entries 8–10),
and surprisingly SnCl4 as an alternative Lewis acid could
give the desired product in 62% yield. The solvent was ex-
amined further, and it was found that toluene (entry 11) and
nitromethane (entry 13) were good reaction media. Howev-
er, the current reaction did not proceed at all in either THF
(entry 12) or CH3CN (entry 14). Notably, employing substoi-
chiometric amounts of BF3·OEt2 (0.3 equiv, entry 15) also
gave rise to the formation of 3aa in 42% yield, albeit in a
less efficient manner, and this provides a potential access to
an effective catalytic system for this reaction.
The current ruthenium-catalyzed/Lewis acid-promoted
cross-coupling reaction between ethanol 2a and the alkene
1,1-diphenylethene (Ph2C=CH2), instead of the tertiary alco-
hol 1a, with the addition of a stoichiometric amount of
water has been investigated. Surprisingly, the coupling reac-
tion proceeded without apparent accelerating effect, and the
desired product 3aa was only afforded in a lower yield than
that obtained by directly using tertiary alcohol 1a (entry 5
of Table 1). In addition, the use of 4 ꢁ molecule sieves as
water scavenger was also subjected to our standard coupling
reaction of 1a and 2a, but there is no evident influence on
the isolated yield of 3aa or the reaction rate. These prelimi-
ACHTUGNTERN(NUNG CH2)2Cl gave the best result (82%
70
2:1
U
ACHTUNGTRENNUNG
3
1a
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
4
5
6
1a
1a
1a
92
71
98
7
8
2g
93
96
1b
[a] The general reaction conditions: (0.5 mmol) was treated with
(0.75 mmol) in the presence of [RuCl2CATHNUGTRNE(NUG PPh3)3] (0.0125 mmol), and Lewis
acid (0.6 mmol) at 508C. [b] Yield of isolated product based on the tertiary
alcohol 1 used.
1
2
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À
C H Activation
COMMUNICATION
Table 3. a-Alkylation of primary alcohols with various tertiary alcohols.[a]
b- or g-branched chain primary alcohols, such as 2c
(entry 2), 2d (entry 3), 2h (entry 8), and 2e (entry 4). For
most of the examples in Table 2, the reaction time needed
was around 5 h, but the a-alkylation was accomplished by
prolonging reaction time to 16 h when using long-chain pri-
mary alcohol 2 f (entry 5). In addition, some primary benzyl-
ic alcohols 2 (e.g., benzylic alcohol and 2,6-dichlorophenyl-
methanol) were also tested, but no positive results were ob-
tained. We also examined the cross-coupling reaction of
some secondary benzylic alcohols 1 (e.g., 1-phenylethanol
and 1,2,3,4-tetrahydro-1-naphthol) with primary alcohols 2
(e.g., 2a, 2b, and 2g), but the isolated yields of desired
products were generally lower (around 30–40% yields) than
using tertiary alcohols (1a and 1b in Table 2).
Entry
1
Substrates
Product[b]
Yield [%][c]
92 (7:1)
2g
2g
2g
2
3
92 (3:2)
93 (1:1)
À
To probe the stereoselectivity in the current C C cross-
coupling reactions, some tertiary alcohols 1c–1i with differ-
ent quaternary centers were subsequently investigated (en-
tries 1–8, Table 3). Clearly, the level of diastereocontrol was
found to be dependent to the relative bulkiness of R1 and
R2 in 1. While both R1 and R2 in 1 of Table 3 were aromatic
groups, the ortho-substituent effect on benzene ring was ob-
served. For example, the coupling reaction of tertiary alco-
hol 1c, bearing an ortho-methylphenyl substituent, with 2g
gave the expected product 3cg with a diastereomeric ratio
(syn:anti) of 7:1 in 92% yield (entry 1, Table 3); the relative
configuration of major product was assigned by comparison
with a pure sample prepared by a known method.[11] When
1d was used, with a slightly less hindered ortho-methoxyl-
phenyl group compared to 1c, the syn:anti ratio dramatically
decreased to 3:2 (entry 2), although the reaction yield re-
mained almost unchanged. Certainly, no diastereomeric con-
trol could be achieved while employing the starting tertiary
alcohol with para-substituted phenyl group such as 1e
(entry 3).
In addition, tertiary alcohols 1 containing the quaternary
center attached by two aliphatic groups (i.e., R1 and R2)
were also subjected to the current reaction under standard
condition (entries 4–7). When 3-phenyl-3-pentanol (1 f) and
3-phenyl-1-propanol (2g) were used in this coupling
(entry 4), a moderate diastereoselectivity (syn:anti=5:1)
and yield (55%) were obtained. Remarkably, however, only
the syn diastereoisomer product was isolated for the case of
1g (entry 5) and 1h (entries 6,7), for which the high diaste-
reoselectivity (syn:anti>99:1) demonstrated here presents a
clue for mechanistically understanding the stereo process in
this selective a-alkylation of primary alcohols through ruthe-
nium-catalyzed/Lewis acid mediated cross-coupling with ter-
tiary alcohols.
4
5
2g
2g
55[d] (5:1)
76 (>99:1)
6
2g
62 (>99:1)
7
8
1h
2h
2g
88 (>99:1)
32 (8:1)
[a] Reaction conditions: 1 (0.5 mmol) was treated with 2 (0.75 mmol) in
the presence of [RuCl2A(PPh3)3] (0.0125 mmol), and Lewis acid (0.6 mmol)
CTHUNGTRENNUNG
at 508C. [b] Major syn diastereoisomers with relative configuration were
shown. [c] Yield of isolated product based on the tertiary alcohol 1 used.
The syn:anti ratio of the two diastereomers is given in parentheses, and
the assignment of relative configuration (see Scheme 1 in the Supporting
Information. [d] Only two diastereomers were isolated, and the stereo-
chemistry remains unknown currently.
benzyl-1-methylethene, could be slowly converted into the
final cross-coupling product 3ig presumably due to the
steric effect of thermodynamic trisubstituted olefin.
Moreover, one extending example of reaction of a tertiary
alcohol with the quaternary center attached by three alkyl
substituents (e.g., 1i in the entry 8 of Table 3) was included,
but the reaction proceeded slowly (16 h) and the expected
product 3ig, with a diastereomeric ratio of 8:1, was generat-
ed merely in 32% yield. In this case, the lower reaction
yield might result from the fact that two olefin intermediates
were generated in situ through the dehydration of 1i under
current conditions, but only the kinetic terminal olefin, 1-
À
To elucidate the current C C cross-coupling process, a
supporting experiment by reaction of 1a with ethanol-1,1-D2
([D2]2a) was conducted. As demonstrated in Scheme 2, the
dideuterated coupling product [D2]3aa with a deuterium
purity of 98% was exclusively obtained in 84% yield, which
clearly rules out the previously known “oxidation–hydroacy-
lation–reduction”[2] or “transfer-hydrogenative-coupling”[3]
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Y.-Q. Tu et al.
metal hydride ([Ru]H) undergoes a stereoselective reductive
attack through a less steric conformation in transition state
D,[15] preferentially giving the diastereoisomer syn-3 as
major product by releasing the ruthenium catalyst for the
next catalytic cycle.
From the proposed mechanism mentioned above, some
explanations to our experimental results could be addressed.
For example, while employing the secondary alcohols [e.g.,
Eq. (2)], the chemical yields were generally lower than
using tertiary alcohols (Tables 2 and 3), probably due to the
fact that the generation of secondary radical species from B
to C (Scheme 3) is less favorable than that of the tertiary
one. Analogously, the formation of a less stable nonbenzylic
radical species in C might be one of reasons for the longer
reaction time of 16 h and the low yield of 32% (entry 8,
Table 3).
Scheme 2. Deuterium-labeled cross-coupling experiment.
sequences for our reaction.[13] Additionally, when some radi-
cal scavengers such as 1,4-benzoquinone and TEMPO
(2,2,6,6-tetramethyl-1-piperidinyloxy) were added to the
system of 1a with 2a, the formation of the desired coupling
product was inhibited completely, which confirmed the pres-
ence of a radical reaction mechanism.
In summary, we have developed a new Ru-catalyzed/acid-
À
mediated C C cross-coupling reaction between alcohols as
green starting materials, and we have shown that various
functionalized alcohols could be straightforwardly synthe-
3
À
sized by sp C H bond activation of primary alcohols. The
On the basis of the above experimental results and our
previous protocol promoted by Wilkinson catalyst [RhCl-
stereoselectivity in this kind of coupling was investigated for
the first time by using different tertiary alcohols. The cur-
rent protocol provides a new strategy for the efficient access
to a range of secondary alcohols by selective a-alkylation of
primary alcohols using aliphatic alcohols, which can be
simply performed in good yields. The widespread investiga-
tion on its asymmetric version as well as the study toward
its mechanistic details is underway in our laboratory.
3
À
N
tion is proposed in Scheme 3. The Lewis acid promoted, ki-
netically controlled dehydration of tertiary alcohol 1 was
Experimental Section
Typical procedure: Cl
ACHTUGNERTN(NUNG CH2)2Cl (4 mL), primary alcohol 2 (0.75 mmol)
and [RuCl2A(PPh3)3] (12 mg, 0.0125 mmol) were sequentially added to a
CTHUNGTRENNUNG
flame-dried 25 mL flask. The resulting mixture was stirred and heated
from room temperature to 408C over a period of 10 min. The alcohol 1
(0.5 mmol) was added and stirring was continued at 408C for 10 min
under an argon atmosphere; then freshly distilled BF3·OEt2 (0.02 mL,
0.15 mmol) was introduced into the reaction mixture. The resulting mix-
ture was stirred at 408C for 20 min, and another portion of BF3·OEt2
(0.06 mL, 0.45 mmol) was added. The reaction was heated over an oil
bath to 508C, and stirred at 508C for 5 h. After that, it was cooled to
room temperature, and diluted with ethyl acetate (3 mL) followed by ad-
dition of saturated aqueous NaHCO3 solution (2 mL). The organic layer
was separated, and the aqueous phase was re-extracted with ethyl acetate
(3ꢂ5 mL). The combined organic extracts were washed with H2O
(20 mL), and dried over anhydrous Na2SO4. After removal of the solvent,
the residue was purified by the flash chromatography to afford the de-
sired separable product 3.
Scheme 3. Proposed catalytic mechanism.
first reacted in situ to generate the alkene intermediate, and
then the ruthenium-catalyzed/Lewis acid mediated activa-
[4,14]
À
tion of the a-C H bond adjacent to the oxygen atom
in
primary alcohol 2 took place. Compared with our previous
Acknowledgements
coupling system,[4] the Lewis acid displays pivotal dual roles
We thank the NSFC (Nos. 20621091, 20672048, and 20732002) and the
Chang Jiang Scholars Program for financial support.
À
for not only accelerating the C H bond activation, but also
promoting the elimination of hydroxy in current reaction.
3
À
After this rate-determined sp C H activation in A, the for-
À
À
mation of radical pair B followed by simultaneous free-radi-
cal addition results in the generation of C. Subsequently, the
Keywords: alcohols · C C coupling · C H activation · cross-
coupling · ruthenium
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À
C H Activation
COMMUNICATION
À
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À
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À
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À
[2] For recent examples of transition-metal-catalyzed C C cross-cou-
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À
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À
À
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À
[4] For Rh- and Pd-catalyzed C C cross-coupling between alkenes and
À
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[15] For the preferred conformation of the radical intermediate D, see
Scheme 3 in the Supporting Information.
À
[5] For C C cross-coupling of alkenes with supercritical alcohols, see:
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Received: July 1, 2008
Published online: October 8, 2008
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10205