Ruthenium-catalyzed β-alkylation of secondary alcohols with primary alcohols

Article abstract of DOI:10.1016/j.ica.2015.03.023

The catalytic properties of a series of ruthenium complexes for β-alkylation of secondary alcohols with primary alcohols were studied. The catalytic activities of the ruthenium complexes were found to be dependent on the auxiliary ligands. The most active catalytic precursor found in this study is the ruthenium complex RuCl₂(PPh₃)₂(2-NH₂CH₂Py) [2-NH₂CH₂Py = 2-aminomethyl pyridine], which effectively catalyzed the β-alkylation of both aryl- and alkyl-substituted secondary alcohols with benzylic and alkyl primary alcohols.

Full text of DOI:10.1016/j.ica.2015.03.023

Inorganica Chimica Acta 431 (2015) 234–241
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Ruthenium-catalyzed b-alkylation of secondary alcohols with primary
alcohols
⇑
Wei Bai, Guochen Jia
Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
a r t i c l e i n f o
a b s t r a c t
Article history:
The catalytic properties of a series of ruthenium complexes for b-alkylation of secondary alcohols with
primary alcohols were studied. The catalytic activities of the ruthenium complexes were found to be
dependent on the auxiliary ligands. The most active catalytic precursor found in this study is the ruthe-
nium complex RuCl2(PPh3)2(2-NH2CH2Py) [2-NH2CH2Py = 2-aminomethyl pyridine], which effectively
catalyzed the b-alkylation of both aryl- and alkyl-substituted secondary alcohols with benzylic and alkyl
primary alcohols.
Received 13 January 2015
Received in revised form 14 March 2015
Accepted 17 March 2015
Available online 7 April 2015
Keywords:
Ruthenium
Ó 2015 Elsevier B.V. All rights reserved.
Alcohol
Alkylation
Transfer hydrogenation
1. Introduction
promoted by strong bases such as alkaline hydroxide at high tem-
peratures [6]. However, the method works better for reactions
Guerbet-type self- and cross-coupling of alcohols represents a
highly attractive route to produce higher alcohols because alcohols
are readily available and the reaction is highly atom efficient with
water as the sole by-product [1]. The Guerbet reaction, which
involves self-coupling of primary and secondary alcohols is known
for more than a century [2]. The original procedure to carry out the
reaction is to heat the reaction mixture in the presence of a strong
base at high temperatures (>220 °C). Under the reaction condition,
the desired coupled alcoholic products were formed usually in low
yields along with side products. There has been much interest in
developing catalysis for carrying out the reactions at lower
reaction temperatures, minimizing side products and expanding
the scope of the reaction [3–5].
between aryl-substituted secondary alcohols and primary benzylic
alcohols and can give coupled ketones as side products. Base-pro-
moted reactions involving aryl-substituted secondary alcohols and
primary alkylalcohols were found to proceed slowly and no reac-
tion was observed between alkyl-substituted secondary alcohols
RCH(OH)Me (R = nPr, Et) and benzylic alcohols. To overcome the
difficulty, there has been much interest in the development of cat-
alytic version of the reaction. In the area of heterogeneous cataly-
sis, cross-coupling reactions of secondary benzylic alcohols with
primary alcohols mediated by Pd/C [7], Ag/Al2O3 [8] and AuꢀPd
(hydrotalcite supported) [9] have been investigated. However,
the reactions produced coupled ketones as the major products.
Homogeneous catalysis based on soluble transition metals com-
plexes [10] has also been actively explored for the transformation,
for example, with complexes of metals such as Ir [11–13], Rh
[4l,14] Pd [15], Cu [16], Ni [17], and Fe [18]. Another class of com-
plexes that have been attracted much attention as potential cata-
lysts for the transformation are ruthenium complexes. In 2003,
Cho reported that the ruthenium complex RuCl2(PPh3)3 (5 mol%,
300% KOH, 80 °C, 40 h) can catalyze the direct b-alkylation of sec-
ondary alcohols with primary alcohols in the presence of the
hydrogen acceptor 1-dodecene in dioxane [19]. Since then, a num-
ber of the ruthenium complexes have been tested for the reaction,
OH
OH
R"
+ H2O
(1)
+
R
H
R"
OH
R
R'
C
R'
A
B
ð1Þ
This work concerns Guerbet-type cross-coupling between a
secondary alcohol (A) and a primary alcohol (B) to give a coupled
alcoholic product (C) resulting from b-alkylation of the secondary
alcohol with the primary alcohol (Eq. (1)). The reaction can be
including RuCl2(@CHPh)(PCy3)2 [20], [(
RuCl2(DMSO)4 [21], [(
6-C6H6)Ru(o-C6H4CH2NR2(NCMe)]PF6 [22],
[(
6-cymene)RuCl(pyrimidine-NHC)]PF6 [13], RuCl2(PPh3)(2,20:60,
200-terpyridine) [12], RuCl3(2,20:60,200-terpyridine) [12], Cp⁄RuCl2
g
6-cymene)RuCl2]2 [12,20],
g
g
⇑
Corresponding author. Tel.: +852 23587361.
E-mail address: chjiag@ust.hk (G. Jia).
http://dx.doi.org/10.1016/j.ica.2015.03.023
0020-1693/Ó 2015 Elsevier B.V. All rights reserved.

W. Bai, G. Jia / Inorganica Chimica Acta 431 (2015) 234–241
235
[12], (
g
6-cymene)ruthenium complexes supported by triazolidene
complex RuH2(CO)(PPh3)3 are essentially inactive, giving the
expected product 1,3-diphenyl-1-propanol (3a) in less than 7%
yields (entries 1–5, Table 1). The complexes RuHCl(CO)(PPh3)3,
[23] and cymene-ruthenium NHC [24], (NHC@imidazolin-2-yli-
dene, imidazolin-4-ylidene, and pyrazolin-3-ylidene) ligands,
[CpRu(CH3CN)(PPh3)2]BF4 [25], CpRuCl(PPh3)2 [25], CpRuCl(dppm)
[25], [TpRu(CH3CN)(PPh3)2]BF4 [25], TpRuCl(PPh3)2 [25] and cis-
[Ru(H2O)2(6,6’Cl2bpy)2](OTf)2 [25].
{[(g
6-cymene)RuCl(NH2CH2CH2O)]2H}Cl,
(g
6-cymene)RuCl(NH2
CH2CH2NTs)Cl and RuCl2(2,20-bipyridine)(PPh3)2 are marginally
active, giving the expected product 1,3-diphenyl-1-propanol (3a)
in 12–19% yields (entries 6–8 and 11, Table 1). The complexes
These previous studies have revealed that b-alkylation of aryl-
substituted secondary alcohols ArCH(OH)R with primary alcohols
can be accomplished with many ruthenium complexes, although
the activity varies with ligands. However, catalytic reactions
involving alkyl-substituted secondary alcohols RCH(OH)R’ have
been less studied and b-alkylation of alkyl-substituted secondary
alcohols RCH(OH)R’ with primary alkyl alcohols RCH2OH in partic-
ular has met with limited success. In fact, there are only two
reports concerning Ru-catalyzed b-alkylation of alkyl-substituted
secondary alcohols RCH(OH)R’ with primary alkyl alcohols
RCH2OH. Cho showed that the ruthenium complex RuCl2(PPh3)3
(5 mol%, 300% KOH, 80 °C, 40 h) catalyzed b-alkylation of
RCH(OH)Me (R = CH3(CH2)4, Ph(CH2)2, Me2CH) with PhCH2CH2OH
in the presence of the hydrogen acceptor 1-dodecene to give
RCH(OH)CH2CH2CH2Ph in low or moderate yields (23–58%) [19].
Lau et al. showed that only trace amounts of the expected coupling
products were obtained in the reactions of RCH(OH)Me
(R = CH3(CH2)4, Ph(CH2)2) with PrCH2OH catalyzed by complexes
such as CpRuCl(PPh3)2, CpRuCl(dppm) and cis-[Ru(H2O)2(6,60
Cl2bpy)2](OTf)2 (0.4 mol%, 120 °C, 20 h) [25]. Previous studies have
also revealed that the ligand environment around ruthenium can
play a significant role in determining the catalytic activity of
ruthenium complexes. For example, while RuCl2(PPh3)(2,20:60,
200-terpyridine) effectively catalyzed the coupling reaction between
(g
6-cymene)RuCl(PPh3) and Ru(OAc)2(PPh3)2 are moderately
active, giving the expected product 1,3-diphenyl-1-propanol in
55% and 36% respective yields (entries 9 and 10, Table 1).
Ruthenium complexes containing 2-aminomethylpyridine (2-
NH2CH2Py) such as RuCl2(dppb)(2-NH2CH2Py), RuCl2(dppf)
(2-NH2CH2Py) and RuCl2(PPh3)2(2-NH2CH2Py) showed good activ-
ity for the reaction, giving the desired secondary alcohol product in
64–91% yields with RuCl2(PPh3)2(2-NH2CH2Py) being most active
(entries 13–15, Table 1). Further study shows that the reaction
catalyzed by RuCl2(PPh3)2(2-NH2CH2Py) also went smoothly with
1 mol% loading of RuCl2(PPh3)2(2-NH2CH2Py) in toluene at 105 °C
in the presence of one equivalent of KOtBu (entry 17, Table 1).
For comparison, we have also carried out catalytic reactions
with complexes RuCl2(PPh3)2 [25], RuCl2(DMSO)4 [25],
CpRuCl(PPh3)2 [25] and RuCl2(PPh3)(2,20:60,200-terpyridine) [12],
which were previously reported to be catalytically active for the
reaction. As shown in entries 18–21, Table 1, these complexes
are less effective than RuCl2(PPh3)2(2-NH2CH2Py) under similar
reaction condition.
2.2. Scope of substrates
After finding out that the complex RuCl2(PPh3)2(2-NH2CH2Py) is
the most active catalyst for b-alkylation of PhCH(OH)Me with
PhCH2OH, we have explored the substrate scope of the reactions
catalyzed by RuCl2(PPh3)2(2-NH2CH2Py). In these catalytic
experiments, a mixture of a secondary alcohol, two equivalents
of a primary alcohol, 1 mol% of the ruthenium complex and one
equivalent of KOtBu in toluene was heated for a period time. The
PhCH(OH)Me with PhCH2OH, the complexes [(g
6-cymene)RuCl2]2
and Cp⁄RuCl2 are essentially inactive for the reaction [12]. These
observations promoted us to investigate the catalytic properties
of other ruthenium complexes with the intention of finding
efficient catalytic systems for b-alkylation of alkyl-substituted
secondary alcohols with primary alcohols.
In this work, we report (i) ligand effect on the catalytic activities
of ruthenium complexes for b-alkylation of secondary alcohols
with primary alcohols, and (ii) new catalytic systems that work
for b-alkylation of both aryl- and alkyl-substituted secondary alco-
hols with benzylic and alkyl primary alcohols.
Table 1
Catalytic b-alkylation of 1-phenyl ethanol with benzyl alcohol catalyzed by selected
ruthenium complexes.a
Entry
Catalyst
NMR yield (%)
1
2
3
4
CpRu(dppe)Cl
<5
<5
7
2. Results and discussion
CpRu(dppe)H
Cp⁄Ru(COD)Cl
Cp⁄Ru(PPh3)2Cl
<5
<5
16
19
19
55
36
12
56
64
77
91
84b
>99c
13
24
19
49
2.1. Selection of catalysts
5
6
7
8
RuH2(CO)(PPh3)3
RuHCl(CO)(PPh3)3
In our initial effort in searching for catalysts for b-alkylation of
secondary alcohols with primary alcohols, we have studied the cat-
alytic activities of a series of ruthenium complexes for the reaction
of 1-phenylethanol (1a) with benzyl alcohol (2a) to give 1,3-diphe-
nyl-1-propanol (3a) (Eq. (2)).
{[(p-cymene)RuCl(NH2CH2CH2O)]2H}Cl
(p-cymene)RuCl(NH2CH2CH2NTs)Cl
(p-cymene)RuCl2(PPh3)
Ru(OAc)2(PPh3)2
RuCl2(PPh3)2(2,20-bipyridine)
RuCl2(PPh3)2(NH2CH2CH2NH2)
RuCl2(dppb)(2-NH2CH2Py)
RuCl2(dppf) (2-NH2CH2Py)
RuCl2(PPh3)2(2-NH2CH2Py)
RuCl2(PPh3)2(2-NH2CH2Py)
RuCl2(PPh3)2(2-NH2CH2Py)
RuCl2(PPh3)3
9
10
11
12
13
14
15
16
17
18
19
20
21
OH
OH
[Ru]
OH
+
KOH
3a
1a
2a
RuCl2(DMSO)4
CpRu(PPh3)2Cl
RuCl2(PPh3)(2,20:60,200-terpyridine)
ð2Þ
a
In these screening experiments, a mixture of 1-phenyl ethanol,
two equivalents of benzyl alcohol, 5 mol% of ruthenium complex
and one equivalent of KOH in dioxane was heated at 80 °C for
5 h. The reaction mixture was then analyzed by 1H NMR. The
results are shown in Table 1.
A mixture of 5 mol% of a ruthenium complex, 35 mg of KOH (0.624 mmol, 1
equiv.) 75 L of 1-phenyl ethanol (0.620 mmol, 1 equiv.) and 130 L of benzyl
l
l
alcohol (1.256 mmol, 2 equiv.) in 1 mL of dioxane was heated at 80 °C for 5 h.
b
2 mol% of the ruthenium complex was used, without change of other reagents
a
or conditions as stated in
.
c
1 mol% of the ruthenium complex and one equivalent of KOtBu in 1 mL of
toluene were used. The mixture was stirred at 105 °C for 5 h. No signal of 1-phenyl
ethanol was detected at the end of the reaction.
The cyclopentadienyl complexes CpRuCl(dppe), CpRuH(dppe),
Cp⁄RuCl(COD) and Cp⁄RuCl(PPh3)2 as well as the dihydride

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W. Bai, G. Jia / Inorganica Chimica Acta 431 (2015) 234–241
Table 2
RuCl2(PPh3)2(2-NH2CH2Py) catalyzed b-alkylation of secondary alcohols with and primary alcohols.a
Entry
1
Secondary alcohol
Primary alcohol
Product (isolated yield)
OH
OH
OH
2a
1a
3a (82.0%)
2
OH
OH
OH
OH
MeO
2b
OMe
1a
3b (84.8%)
3
4
OH
OH
2c
1a
3c (68.9%)
OH
OH
OH
1a
2d
3d (66.0%)
5
6
7
8
OH
OH
OH
2e
1a
3e (66.0%)
OH
OH
OH
2f
1a
3f (81.5%)
OH
OH
OH
2g
2a
2f
1a
3g (66.0%)
OH
OH
OH
Br
1b
Br
3h (66.2%)
9
OH
OH
OH
Br
1b
Br
3i (54.6%)
a
A mixture of 1 mol% of the ruthenium complex, one equivalent of KOtBu, one equivalent of a secondary alcohol, two equivalents of a primary alcohol in 1 mL of toluene
was stirred at 80–105 °C for 8–24 h (see Section 4 for details). The product was isolated by chromatography.
reaction products are purified by chromatography and analyzed by
1H NMR. The results are shown in Table 2.
1-octanol (2h) to give the corresponding coupling products 3j-l
in moderate to excellent isolated yields (64–93%) (entries 1–3,
Table 3). Under the same condition, 2-propanol (1d) reacted with
two equivalents of benzyl alcohol to produce the bis-alkylated pro-
duct 4a in 88.4% isolated yields (entry 4, Table 3). When 2-propa-
nol (1d) was allowed to react with one equivalent of benzyl
alcohol, the reaction produced the mono-alkylated product 3m
and the bis-alkylated product 4a, which can be isolated in 30.5%
and 19.7% yields, respectively (Scheme 1).
As shown in Table 2, RuCl2(PPh3)2(2-NH2CH2Py) effectively cat-
alyze the b-alkylation of PhCH(OH)Me (1a) with both benzylic and
alkyl primary alcohols, giving desired products (3a-g) in 66–85%
isolated yields (entries 1–6, Table 2). Our in situ 1H NMR showed
that more than 99% conversions were achieved (i.e. no secondary
alcohol left) and the side products ketones were not produced in
all the reactions. Thus the true yields of the reactions should be
higher than those listed in Table 2. 4-Bromobenzylic alcohol (1b)
similar reacted with primary benzylic and alkyl alcohols, giving
desired products 3h-i in 55–66% isolated yields (entries 7 and 8,
Table 2).
2.3. Reaction mechanism
It has been proposed previously that b-alkylation of secondary
We have also carried out catalytic reactions involving secondary
alkyl alcohols with primary alcohols. As shown in Table 3, in the
presence of RuCl2(PPh3)2(2-NH2CH2Py), 2-pentanol (1c) reacted
with two equivalents of benzyl alcohol (2a), 1-butanol (2f), and
alcohols with primary alcohols proceed via a ‘‘borrowing
hydrogen’’ process [1]. A similar mechanism can be proposed for
the present b-alkylation reactions promoted by RuCl2(PPh3)2
(2-NH2CH2Py) (5).

W. Bai, G. Jia / Inorganica Chimica Acta 431 (2015) 234–241
237
Table 3
RuCl2(PPh3)2(2-NH2CH2Py) catalyzed b-alkylation of secondary alkyl alcohols with primary alcohols.a
Entry
1
Secondary alcohol
Primary alcohol
Product (isolated yield)
OH
OH
OH
2a
1c
3j (92.6%)
2
OH
OH
OH
2f
1c
3k (75.3%)
3
4
1-octanol
2h
OH
OH
1c
3l (64.0%)
OH
OH
OH
2a
1d
4a (88.4%)
a
A mixture of 1 mol% of the ruthenium complex, one equivalent of KOtBu, one equivalent of secondary alcohol, two equivalents of primary alcohol in 1 mL of toluene or
dioxane was stirred at 100 °C for 24 h. The product was isolated by chromatography.
OH
OH
OH
OH
100 oC, 24 h
+
+
RuCl2(PPh3)2(2-NH2CH2Py)
1d
2a
4a
19.7%
3m
30.5%
Scheme 1. Alkylation of 2-propanol with benzyl alcohol.
PPh3
H
O
H2
H2
H
PPh3
PPh3
N
NaOCHMe2
- acetone
N
Ru
Ru
Cl
Ph
Me
N
N
H
OH
PPh3
7
6
Ph
Me
H
Ph
Me
O
N
PPh3
PPh3
O
H
H
H2
H
N
H
Ru
H
H
Ru
N
H
Ru
H
PPh3
PPh3
N
Ph
Me
PPh3
PPh3
H
N
8
N
OH
9
10
Ph
Me
Scheme 2. Reported reactions of ruthenium hydride complexes supported by 2-aminomethyl pyridine.
It is known that the monohydride complex RuHCl(PPh3)2
with PhC(O)Me via transition state 10 to give PhCH(OH)Me and
to re-generate the five coordinated amido complex 8.
Based on these previous work, a plausible mechanism for the
present b-alkylation of secondary alcohols with primary
alcohols catalyzed by RuCl2(PPh3)2(2-NH2CH2Py) (5) is proposed
in Scheme 3.
In the presence of base, RuCl2(2-H2NCH 2Py)(PPh3)2 (5) can
react with RCH(OH)Me and R’CH2OH to give the carbonyl
compounds RC(O)Me and R’CH@O, and the dihydride complex
c,t-RuH2(2-H2NCH2Py)(PPh3)2 (7) which may react with RC(O)Me
or R’CH@O to give the five coordinated amido complex RuH
(2-HNCH2Py)(PPh3)2 (8). Complex 8 may react with RCH(OH)Me
and R’CH2OH via a transition state like 10 shown in Scheme 2
(an out-sphere mechanism) to give the dihydride complex
t,c-RuH2(2-H2NCH2Py)(PPh3)2 (9) and the carbonyl compounds
RC(O)Me and R’CH@O. The carbonyl compounds RC(O)Me and
(2-NH2CH2Py) (6) can reacts with NaOCHMe2 to give the dihydride
complex c,t-RuH2(PPh3)2(2-NH2CH2Py) (7) (Scheme 2) [26]. It has
also been reported that the dichloro complex RuCl2(PPh3)2(2-NH2
CH2Py) (5) and the monohydride complex RuHCl(PPh3)2
(2-NH2CH2Py) (6) can be used as catalytic precursors for transfer
hydrogenation of PhC(O)Me with isopropanol in the presence of
NaOH [26], and that the monohydride complex RuHCl(PPh3)2
(2-NH2CH2Py) (6) (in the presence of KOiPr) and the dihydride
complex c,t-RuH2(PPh3)2(2-NH2CH2Py) (7) are catalytically active
for hydrogenation of PhC(O)Me [27]. In the catalytic hydrogenation
reaction, it was proposed that the dihydride complex c,t-RuH2
(PPh3)2(2-NH2CH2Py) (7) is converted to the five coordinated
amido complex 8 through its reaction with PhC(O)Me. The catalytic
cycle is completed by the reaction of complex 8 with H2 to give the
dihydride complex t,c-RuH2(PPh3)2(2-NH2CH2Py) (9), which react

238
W. Bai, G. Jia / Inorganica Chimica Acta 431 (2015) 234–241
O
O
OH
OH
Cl
Ru
Cl
+
Me
+
Me
PPh3
R'
H2
N
R'
H2
N
R
R
H
H
H
PPh3
Ru
N
N
PPh3
base
OH
PPh3
7
5
OH
O
O
+
R'
or
Me
R
Me
R'
R
H
H
N
HO
PPh3
PPh3
OH
OH
Ru
H
+
Me
N
R'
R
R
R'
8
H
N
H
H
PPh3
PPh3
Ru
H
N
9
O
O
O
+
aldol condensation
base
R'
R
Me
H
R
R'
Scheme 3. A proposed mechanism for the alkylation of secondary alcohols with primary alcohols.
R’CH@O can undergo base-catalyzed aldol condensation to form
a
,
Solvents were distilled under nitrogen from sodium benzophenone
(hexane, ether, THF), or calcium hydride (CH2Cl2). Other solvents
were purged with nitrogen for 10 min before use. CpRuCl(dppe)
[28], CpRuH(dppe) [29], Cp⁄RuCl(COD) [30], Cp⁄RuCl(PPh3)2 [31],
RuHCl(CO)(PPh3)3 [32], RuH2(CO)(PPh3)3 [32], {[(p-cymene)
RuCl(NH2CH2CH2O)]2H}Cl [33], (p-cymene)RuCl(NH2CH2CH2NTs)
Cl [34], (p-cymene)RuCl2(PPh3) [35], Ru(OAc)2(PPh3)2 [36], RuCl2
(PPh3)2(2,20-bipyridine) [37], RuCl2(NH2CH2CH2NH2)(PPh3)2 [38],
RuCl2(dppf)(2-PyCH2NH2) [39], RuCl2(dppb)(2-NH2CH2Py) [26],
RuCl2(2-H2NCH2Py)(PPh3)2 [26], RuCl2(PPh3)3 [40], RuCl2(DMSO)4
[41], CpRuCl(PPh3)2 [42], and RuCl2(PPh3)(2,20:60,200-terpyridine)
[43] were prepared following the procedures described in the
literature. All other reagents were used as purchased from
Aldrich Chemical Co., Acros Organics, International Laboratory
(USA), or Kodak. Microanalyses were performed by M-H-W
Laboratories (Phoenix, AZ). 1H, 13C{1H} and 31P{1H} NMR spectra
were collected on a Bruker-400 spectrometer (400.1 MHz) or a
Bruker ARX-300 spectrometer (300.1 MHz). 1H and 13C NMR shifts
are relative to TMS, and 31P chemical shifts are relative to 85%
H3PO4.
b-unsaturated carbonyl compounds RC(O)CH@CHR’, which then
reacts with the dihydride complex t,c-RuH2(2-H2NCH2Py)(PPh3)2
(9) to give the product RCH(OH)CH2CH2R’ and regenerate the five
coordinated amido complex RuH(2-HNCH2Py)(PPh3)2 (8) to start
another catalytic cycle.
We have tried to detect the active species involved in the reac-
tion of PhCH(OH)Me with PhCH2OH catalyzed by RuCl2(PPh3)2
(2-NH2CH2Py) (5) using in situ NMR spectroscopy. In agreement
with our proposed mechanism, the dihydride complex c,t-RuH2
(PPh3)2(2-NH2CH2Py) (7) was detected in the early stage of the
reaction. After a 1:1 mixture of PhCH(OH)Me and PhCH2OH in
C6D6 in the presence of 10 mol% of RuCl2(PPh3)2(2-NH2CH2Py)
and one equivalent of KOtBu was heated at 60 °C for 10 min., the
1H NMR spectrum of the mixture showed the 1H NMR signals of
the dihydride complex c,t-RuH2(PPh3)2(2-NH2CH2Py) (7) at ꢀ18.2
and ꢀ16.3 ppm, and the 31P NMR spectrum showed a singlet
31P{1H} signal of the dihydride complex c,t-RuH2(PPh3)2
(2-NH2CH2Py) (7) at 66.4 ppm. It should be noted that the in situ
1H NMR spectrum also showed minor hydride signals of
RuH2(CO)(PPh3)3 at ꢀ8.3 and ꢀ6.5 ppm and unidentified hydride
signals at ꢀ7.0 (br) ppm, ꢀ12.7 ppm.
4.1. General procedure for ruthenium catalyzed alkylation reactions
between 1-phenyl ethanol and benzyl alcohol in the screening
experiments (Table 1)
3. Summary
We have investigated the catalytic properties of a series of
ruthenium complexes for b-alkylation of secondary alcohols with
primary alcohols. The catalytic activities of the ruthenium
complexes were found to be dependent on the auxiliary ligands.
The ruthenium complexes RuCl2(PPh3)2(2-NH2CH2Py) was found
to be the most active catalytic precursor, which effectively
catalyzed the b-alkylation of both aryl- and alkyl-substituted sec-
ondary alcohols with benzylic and alkyl primary alcohols.
A mixture of 5 mol% of a ruthenium complex, 35 mg of KOH
(0.624 mmol, 1 equiv.), 75
lL of 1-phenyl ethanol (0.620 mmol, 1
equiv.) and 130 L of benzyl alcohol (1.256 mmol, 2 equiv.) in
l
1 mL of dioxane was stirred at 80 °C for 5 h. The reaction mixture
was then analyzed by NMR.
4.2. General procedure for RuCl2(PPh3)2(2-NH2CH2Py) catalyzed
b-alkylation reactions of secondary alcohols with primary alcohols
(Tables 2 and 3)
4. Experimental section
A
mixture of RuCl2(PPh3)2(2-NH2CH2Py) (1 mol%), KOtBu
All manipulations were carried out under a nitrogen atmo-
sphere using standard Schlenk techniques unless otherwise stated.
(1 equiv.), a secondary alcohol (1 equiv) and a primary alcohol
(2 equiv.) in 1 mL of toluene or dioxane was heated for a period

W. Bai, G. Jia / Inorganica Chimica Acta 431 (2015) 234–241
239
of time. The product was extracted with diethyl ether, purified by
b-CH2), 1.26–1.47 (m, 2H,
c
-CH2), 0.93 (t, J = 7.0 Hz, 3H, Me). The
column chromatography on silica gel using petroleum ether/
diethyl ether 6:1 and 4:1 as the eluents, and identified by NMR.
NMR data are consistent with the reported ones [44].
4.8. Reaction of 1-phenyl ethanol with butanol (Table 2, entry 6)
RuCl2(PPh3)2(2-NH2CH2Py), 7 mg (0.0087 mmol); KOtBu, 93 mg
4.3. Reaction of 1-phenyl ethanol with benzyl alcohol (Table 2, entry 1)
RuCl2(PPh3)2(2-NH2CH2Py), 7 mg (0.0087 mmol); KOtBu, 93 mg
(0.829 mmol); 1-phenyl ethanol, 100
lL (0.827 mmol); butanol,
(0.829 mmol);
1-phenyl
ethanol,
100
lL
(0.827 mmol);
151 (1.650 mmol); toluene, 1 mL; temperature, 105 °C;
lL
benzyl alcohol, 171
l
L (1.652 mmol); toluene, 1 mL; temperature,
reaction time, 22 h. Yield of 3f (1-phenyl-1-hexanol): 120 mg,
105 °C; reaction time, 22 h. Yield of 3a (1,3-diphenyl-propanol):
81.5%. 1H NMR (400 MHz, CDCl3): d 7.30–7.38 (m, 5H, Ph), 4.63
144 mg, 82.0%. 1H NMR (400 MHz, CDCl3): d 7.15–7.36 (m, 10H,
(t, J = 8.00 Hz, 1H,
a-CH), 2.59 (br s, 1H, OH), 1.69–1.84 (m, 2H,
Ph), 4.66 (t, J = 8.0 Hz, 1H,
a-CH), 2.61–2.77 (m, 2H,
c
-CH2),
b-CH2), 1.29–1.44 (m, 6H, other CH2), 0.94 (t, J = 8.00 Hz, 3H, Me).
1.96–2.16 (m, 3H, b-CH2 and OH). The NMR data are consistent
The NMR data are consistent with the reported ones [45].
with the reported ones [44].
4.9. Reaction of 1-phenyl ethanol with 3-methylbutanol (Table 2, entry
7)
4.4. Reaction of 1-phenyl ethanol with 4-methoxyphenyl methanol
(Table 2, entry 2)
RuCl2(PPh3)2(2-NH2CH2Py), 7 mg (0.0087 mmol); KOtBu, 93 mg
RuCl2(PPh3)2(2-NH2CH2Py), 7 mg (0.0087 mmol); KOtBu, 93 mg
(0.829 mmol);
1-phenyl
ethanol,
100
lL
(0.827 mmol);
(0.829 mmol); 1-phenyl ethanol, 100
l
L (0.827 mmol); 4-methox-
3-methylbutanol, 180 lL (1.652 mmol); toluene, 1 mL; tempera-
yphenyl methanol, 228 mg (1.650 mmol); toluene, 1 mL;
temperature, 105 °C; reaction time, 22 h. Yield of 3b (3-(4-methox-
yphenyl)-1-phenyl-1-propanol): 170 mg, 84.8%. 1H NMR
(400 MHz, CDCl3): d 6.80–7.35 (m, 9H, Ph), 4.67 (dd, J1 = 8 Hz,
ture, 105 °C; reaction time, 22 h. Yield of 3g (5-methyl-1-phenyl-
hexanol): 105 mg, 66.0%. 1H NMR (400 MHz, CDCl3): d 7.24–7.36
(m, 5H, Ph), 4.64 (t, J = 7.5 Hz, 1H, a-CH), 1.18–1.79(m, 8H), 0.850
(d, J = 6.5 Hz, 3H, Me), 0.846 (d, J = 6.5 Hz, 3H, Me). The NMR data
J2 = 5.6 Hz, 1H,
a
-CH), 3.80 (s, 3H, OMe), 2.55–2.75 (m, 2H,
are consistent with the reported ones [44].
c-CH2), 2.00–2.15 (m, 2H, b-CH2), 1.95 (br s, 1H, OH). The NMR data
are consistent with the reported ones [21].
4.10. Reaction of 4-bromo-1-phenyl ethanol with benzyl alcohol
(Table 2, entry 8)
4.5. Reaction of 1-phenyl ethanol with 2-naphthyl methanol (Table 2,
entry 3)
RuCl2(PPh3)2(2-NH2CH2Py), 6 mg (0.0075 mmol); KOtBu, 81 mg
(0.722 mmol); 4-bromo-1-phenyl ethanol, 100
lL (0.726 mmol);
RuCl2(PPh3)2(2-NH2CH2Py), 5 mg (0.0062 mmol); KOtBu, 70 mg
benzyl alcohol, 150 L (1.450 mmol); toluene, 1 mL; temperature,
l
(0.624 mmol); 1-phenyl ethanol, 75
l
L (0.620 mmol); 2-naphthyl
90 °C; reaction time, 8 h. Yield of 3h (1-(4-bromophenyl)-3-
methanol, 98 mg (0.619 mmol); toluene, 1 mL; temperature,
100 °C; reaction time, 22 h. Yield of 3c (3-(2-naphthyl)-1-phenyl-
1-propanol): 112 mg, 68.9%. 1H NMR (400 MHz, CDCl3): d 7.25–
8.00 (m, 12H, Ph and naphthyl), 4.74 (dd, J1 = 8 Hz, J2 = 5.6 Hz,
phenylpropan-1-ol): 140 mg, 66.2%. 1H NMR (300 MHz, CDCl3): d
7.15–7.46 (m, 9H, Ph), 4.64 (dd, J1 = 7.2 Hz, J2 = 6 Hz, 1H,
2.63–2.71 (m, 2H, -CH2), 1.95–2.09 (m, 3H, b-CH2 and OH). The
NMR data are consistent with the reported ones [11c].
a-CH),
c
1H,
a-CH), 3.05–3.30 (m, 2H, c-CH2), 2.10–2.30 (m, 2H, b-CH2),
1.95 (br s, 1H, OH). The NMR data are consistent with the reported
ones [21].
4.11. Reaction of 4-bromo-1-phenyl ethanol with butanol (Table 2,
entry 9)
4.6. Reaction of 1-phenyl ethanol with 1-pyrenyl methanol (Table 2,
entry 4)
RuCl2(PPh3)2(2-NH2CH2Py), 6 mg (0.0075 mmol); KOtBu, 81 mg
(0.722 mmol); 4-bromo-1-phenyl ethanol, 100
lL (0.726 mmol);
benzyl alcohol, 140 L (1.530 mmol); toluene, 1 mL; temperature,
l
RuCl2(PPh3)2(2-NH2CH2Py), 5 mg (0.0062 mmol); KOtBu, 70 mg
(0.624 mmol); 1-phenyl ethanol, 75 lL (0.620 mmol); 1-pyrenyl
100 °C; reaction time, 23 h. Yield of 3i (1-(4-bromophenyl)-hex-
anol): 102 mg, 54.6%. 1H NMR (400 MHz, CDCl3): d 7.17–7.43
methanol, 173 mg (0.745 mmol); toluene, 1 mL; temperature,
80 °C; reaction time, 21 h. Yield of 3d (3-(1-pyrenyl)-1-phenyl-1-
propanol): 138 mg, 66.0%. 1H NMR (400.1 MHz, CDCl3): 8.23–7.88
(m, 9H, pyrenyl ring), 7.42–7.30 (m, 5H, Ph), 4.82 (dd, J1 = 7.6 Hz,
(m, 4H, Ph), 4.56 (t, J = 6.8 Hz, 1H, a-CH), 2.38 (br s, 1H, OH),
1.58–1.75 (m, 2H), 1.20–1.38 (m, 6H), 0.86 (t, J = 7.0 Hz, 3H, Me).
The NMR data are consistent with the reported ones [46].
J2 = 4.8 Hz, 1H,
a-CH), 3.52–3.39 (m, 2H,
c-CH2), 2.38–2.25 (m,
4.12. Reaction of pentan-2-ol with benzyl alcohol (Table 3, entry 1)
2H, b-CH2), 1.86 (br s, 1H, OH). 13C{1H} NMR (100.62 MHz,
CDCl3): 143.91, 135.55, 130.79, 130.28, 129.24, 128.05, 127.95,
127.10, 126.87, 126.66, 126.60, 126.00, 125.33, 125.18, 124.48,
124.37, 124.22, 124.09, 122.73 (pyrenyl and phenyl rings), 73.43
RuCl2(PPh3)2(2-NH2CH2Py), 7 mg (0.0087 mmol); KOtBu,
103 mg (0.918 mmol); pentan-2-ol, 100
lL (0.921 mmol); benzyl
alcohol, 95 L (0.918 mmol); toluene, 1 mL; temperature, 100 °C;
l
(
a
-C), 40.19 (b-C), 28.97 (
c
-C). Anal. Calc. for C25H20O: C, 89.25;
reaction time, 24 h. Yield of 3j (1-phenyl-3-hexanol): 101 mg,
H, 5.99. Found: C, 89.17; H, 6.16%.
61.5%. 1H NMR (300 MHz, CDCl3): d 7.33–7.18 (m, 5H, Ph),
3.66–3.63 (m, 1H,
a-CH), 2.82–2.67 (m, 2H,), 1.83–1.72 (m, 2H),
4.7. Reaction of 1-phenyl ethanol with ethanol (Table 2, entry 5)
RuCl2(PPh3)2(2-NH2CH2Py), 7 mg (0.0087 mmol); KOtBu, 93 mg
1.68 (br s, 1H), 1.49–1.35 (m, 4H), 0.99 (t, J = 6.4 Hz, 3H, Me). The
NMR data are consistent with the reported ones [11c].
(0.829 mmol); 1-phenyl ethanol, 100
96 L (1.646 mmol); toluene, 1 mL; temperature, 105 °C; reaction
time, 22 h. Yield of 3e (1-phenyl-1-butanol): 82 mg, 66.0%. 1H
NMR (400 MHz, CDCl3): 7.27–7.36 (m, 5H, Ph), 4.63
(t, J = 6.4 Hz, 1H, -CH), 2.45 (br s, 1H, OH), 1.64–1.84 (m, 2H,
l
L (0.827 mmol); ethanol,
4.13. Reaction of pentan-2-ol with 1-butanol (Table 3, entry 2)
l
RuCl2(PPh3)2(2-NH2CH2Py), 7 mg (0.0075 mmol); KOtBu,
d
103 mg (0.918 mmol); pentan-2-ol, 100
lL (0.921 mmol); 1-bu-
a
tanol, 170 L (1.858 mmol); dioxane, 1 mL; temperature, 100 °C;
l

240
W. Bai, G. Jia / Inorganica Chimica Acta 431 (2015) 234–241
reaction time, 24 h. Yield of 3k (nonan-4-ol): 100 mg, 75.3%. 1H
NMR (400 MHz, CDCl3): d 3.66–3.56 (m, 1H, -CH), 1.57–1.23 (m,
13H, CH2 and OH), 0.93 (t, J = 7.1 Hz, 3H, Me), 0.89 (t, J = 7.0 Hz,
[4] Examples of work on self-coupling of alcohols with hetergeneous catalysts. Pt
based catalysts (a) C. Chaudhari, S.M.A. Hakim Siddiki, K. Shimizu, Top Catal.
57 (2014) 1042;
a
. Cu based catalysts(b) G. Torres, C.R. Apesteguia, J.I.D. Cosimo, Appl. Catal. A
317 (2007) 161;
(c) S.A. El-Molla, Appl. Catal. A 298 (2006) 103;
3H, Me). The NMR data are consistent with the reported ones [47].
(d) C. Carlini, C. Flego, M. Marchionna, M. Noviello, A.M.R. Galletti, G. Sbrana, F.
Basile, A. Vaccari, J. Mol. Catal. A: Chem. 220 (2004) 215;
(e) J.I.D. Cosimo, G. Torres, C.R. Apesteguia, J. Catal. 208 (2002) 114;
(f) C. Carlini, M.D. Girolamo, A. Macinai, M. Marchionna, M. Noviello, A.M.R.
Galletti, G.J. Sbrana, Mol. Catal. A 200 (2003) 137;
(g) M.N. Dvornikoff, M.W. Farrar, J. Org. Chem. 22 (1957) 540;
(h) C. Weizmann, E. Bergmann, M. Sulzbacher, J. Org. Chem. 15 (1950) 54;
. Ni-based catalysts(i) K. Shimura, K. Kon, S.M.A.H. Siddiki, K. Shimizu, Appl.
Catal. A 462 (2013) 137;
(j) E.F. Pratt, D.G. Kubler, J. Am. Chem. Soc. 76 (1954) 52;
. Fe based catalysts(k) M. Leon, E. Diaz, A. Vega, S. Ordonez, A. Auroux, Appl.
Catal. B 102 (2011) 590;
. Rh based catalysts(l) P.L. Burk, R.L. Pruett, K.S. Campo, J. Mol. Catal. 33 (1985)
15;
4.14. Reaction of pentan-2-ol with 1-octanol (Table 3, entry 3)
RuCl2(PPh3)2(2-NH2CH2Py), 7 mg (0.0087 mmol); KOtBu,
103 mg (0.918 mmol); pentan-2-ol, 100
tanol, 300 L (1.905 mmol); dioxane, 1 mL; temperature, 100 °C;
reaction time, 24 h. Yield of 3l (tridencan-4-ol): 60 mg, 118 mg,
64.0%. 1H NMR (300 MHz, CDCl3): d 3.60 (m, 1H,
-CH), 1.10–
lL (0.921 mmol); 1-oc-
l
a
1.60 (m, 21 H, CH2 and OH), 0.93 (t, J = 3.6 Hz, 3H, Me), 0.87 (t,
J = 6.8 Hz, 3H, Me) The NMR data are consistent with the reported
ones [48].
. Acid-basic bifunctional metal oxides(m) W. Ueda, T. Kuwabara, T. Ohshida, Y.
Morikawa, J. Chem. Soc., Chem. Commun. (1990) 1558;
(n) J.I.D. Cosimo, C.R. Apesteguia, M.J.L. Gines, E. Iglesia, J. Catal. 190 (2000)
261;
4.15. Reaction of isopropanol with two equivalents of benzyl alcohol
(Table 2, entry 4)
(o) A.S. Ndou, N. Plint, N.J. Coville, Appl. Catal., A 251 (2003) 337;
(p) A.S. Ndou, N.J. Coville, Appl. Catal. A 275 (2004) 103;
(q) T. Tsuchida, S. Sakuma, T. Takeguchi, W. Ueda, Ind. Eng. Chem. Res. 45
(2006) 8634;
RuCl2(PPh3)2(2-NH2CH2Py), 10 mg (0.0124 mmol); KOtBu,
147 mg (1.310 mmol); isopropanol, 100
lL (1.308 mmol); benzyl
(r) T. Tsuchida, J. Kubo, T. Yoshioka, S. Sakuma, T. Takeguchi, W. Ueda, J. Catal.
259 (2008) 183;
(s) M. Leon, E. Diaz, S. Ordonez, Catal. Today 164 (2011) 436;
(t) S. Ogo, A. Onda, K. Yanagisawa, Appl. Catal. A 402 (2011) 188;
(u) S. Ogo, A. Onda, Y. Iwasa, K. Hara, A. Fukuoka, K. Yanagisawa, J. Catal. 296
(2012) 24.
alcohol, 270 (2.610 mmol); toluene, 1 mL; temperature,
l
L
100 °C; reaction time, 24 h. Yield of 4a (1,5-diphenyl-3-pentanol):
278 mg, 88.4%. 1H NMR data of 4a (400 MHz, CDCl3): d 7.11–7.32
(m, 10H, Ph), 3.68–3.70 (m, 1H,
a-CH), 2.78–2.85 (m, 2H, c-CH2),
2.66–2.73 (m, 2H, -CH2), 1.79–1.86 (m, 5H, b-CH2 and OH). The
c
[5] Examples of work on cross-coupling of primary alcohols. For homogeneous
catalysis, see: Pd-based catalysts (a) C. Carlini, M.D. Girolamo, A. Macinai, M.
Marchionna, M. Noviello, A.M.R. Galletti, G. Sbrana, J. Mol. Catal. A: Chem. 204–
205 (2003) 721;
NMR data are consistent with the reported ones [49].
4.16. Reaction of isopropanol with one equivalent of benzyl alcohol
(Scheme 2)
(b) C. Carlini, A. Macinai, A.M.R. Galletti, G. Sbrana, J. Mol. Catal. A: Chem. 212
(2004) 65;
. For heterogeneous catalysis, see:(c) C. Carlini, M.D. Dirolamo, M. Marchionna,
M. Noviello, A.M.R. Galletti, G. Sbrana, J. Mol. Catal. A: Chem. 184 (2002) 273;
(d) C. Carlini, A. Macinai, M. Marchionna, M. Noviello, A.M.R. Galletti, G.J.
Sbrana, Mol. Catal. A 206 (2003) 409;
RuCl2(PPh3)2(2-NH2CH2Py), 10 mg (0.0124 mmol); KOtBu,
147 mg (1.310 mmol); isopropanol, 100
lL (1.308 mmol); benzyl
(e) C. Carlini, M. Marchionna, M. Noviello, A.M.R. Galletti, G. Sbranaa, F. Basiled,
A.J. Vaccarid, Mol. Catal. A 232 (2005) 13;
alcohol, 135 (1.305 mmol); toluene, 1 mL; temperature,
l
L
100 °C; reaction time, 24 h. Yield of 3m, 62 mg 19.7%; yield of 4a,
(f) R. Cano, M. Yus, D.J. Ramon, Chem. Commun. 48 (2012) 7628.
[6] L.J. Allen, R.H. Crabtree, Green Chem. 12 (2010) 1362.
[7] C.S. Cho, W.X. Ren, S.C. Shim, Bull. Korean Chem. Soc. 26 (2005) 1611.
[8] K. Shimizu, R. Sato, A. Satsuma, Angew. Chem., Int. Ed. 48 (2009) 3982.
[9] X. Liu, R.-S. Ding, L. He, Y.-M. Liu, Y. Cao, H.-Y. He, K.-N. Fan, ChemSusChem 6
(2013) 604.
60 mg, 30.5%. 1H NMR data of 4a (400 MHz, CDCl3): d 7.15–7.32
(m, 5H, Ph), 3.83 (m, 1H,
a-CH), 2.62–2.82 (m, 2H, c-CH2), 1.70–
1.85 (m, 2H, b-CH2), 1.58 (br s, 1H, OH), 1.23 (d, J = 6.0 Hz, 3H,
Me). The NMR data are consistent with the reported ones [50].
[10] It has been reported recently that arylaldehydes can also catalyze the reactions
of aryl- and alkyl-substituted secondary alcohols with primary alcohols Q. Xu,
J. Chen, Q. Liu, Adv. Synth. Catal. 355 (2013) 697.
Acknowledgements
[11] (a) D. Wang, K. Zhao, C. Xu, H. Miao, Y. Ding, ACS Catal. 4 (2014) 3910;
(b) D. Wang, X.-Q. Guo, C.-X. Wang, Y.-N. Wang, R. Zhong, X.-H. Zhu, L.-H. Cai,
Z.-W. Gao, X.-F. Houa, Adv. Synth. Catal. 355 (2013) 1117;
(c) X. Gong, H. Zhang, X. Li, Tetrahedron Lett. 52 (2011) 5596;
(d) C. Xu, L.Y. Goh, S.A. Pullarkat, Organometallics 30 (2011) 6499;
(e) C. Segarra, E. Mas-Marz, J.A. Mata, E. Peris, Adv. Synth. Catal. 353 (2011)
2078;
This work was supported by the Hong Kong Research Grant
Council (Project No.: 602611, 601812, 602113, CUHK7/CRF/12G-2).
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