Bifunctional RuII-Complex-Catalysed Tandem C?C Bond Formation: Efficient and Atom Economical Strategy for the Utilisation of Alcohols as Alkylating Agents

Article abstract of DOI:10.1002/chem.201603557

Catalytic activities of a series of functional bipyridine-based Ru^IIcomplexes in β-alkylation of secondary alcohols using primary alcohols were investigated. Bifunctional Ru^IIcomplex (3 a) bearing 6,6’-dihydroxy-2,2’-bipyridine (6DHBP) ligand exhibited the highest catalytic activity for this reaction. Using significantly lower catalyst loading (0.1 mol %) dehydrogenative carbon?carbon bond formation between numerous aromatic, aliphatic and heteroatom substituted alcohols were achieved with high selectivity. Notably, for the synthesis of β-alkylated secondary alcohols this protocol is a rare one-pot strategy using a metal–ligand cooperative Ru^IIsystem. Remarkably, complex 3 a demonstrated the highest reactivity compared to all the reported transition metal complexes in this reaction.

Full text of DOI:10.1002/chem.201603557

DOI: 10.1002/chem.201603557
Full Paper
&
Catalysis
Bifunctional RuII-Complex-Catalysed Tandem CÀC Bond
Formation: Efficient and Atom Economical Strategy for the
Utilisation of Alcohols as Alkylating Agents
Bivas Chandra Roy, Kaushik Chakrabarti, Sujan Shee, Subhadeep Paul, and Sabuj Kundu*[a]
Abstract: Catalytic activities of a series of functional bipyri-
dine-based RuII complexes in b-alkylation of secondary alco-
hols using primary alcohols were investigated. Bifunctional
RuII complex (3a) bearing 6,6’-dihydroxy-2,2’-bipyridine
(6DHBP) ligand exhibited the highest catalytic activity for
this reaction. Using significantly lower catalyst loading
(0.1 mol%) dehydrogenative carbonÀcarbon bond formation
between numerous aromatic, aliphatic and heteroatom sub-
stituted alcohols were achieved with high selectivity. Nota-
bly, for the synthesis of b-alkylated secondary alcohols this
protocol is a rare one-pot strategy using a metal–ligand co-
operative RuII system. Remarkably, complex 3a demonstrat-
ed the highest reactivity compared to all the reported transi-
tion metal complexes in this reaction.
and IrIII complexes for b-alkylation of secondary alcohols with
primary alcohols in which terpyridine-based complexes
showed enhanced catalytic activity compare to other com-
plexes.[8] Ruthenacycles derived from phenylmethanamine, N-
methylphenylmethanamine, N,N-dimethylphenylmethanamine,
and naphthalen-1-ylmethanamine ligands were also used for
the same purpose.[9] Recently, RuIII-promoted b-alkylation of
secondary alcohols and RuIINHC-catalysed a-alkylation of meth-
ylene ketones was revealed by the groups of Yu and Glorious,
respectively.[10] Iridium complexes such as [Ir(cod)Cl]2,[11]
[Cp*IrCl2]2,[12] iridium-NHC,[13] pincer-(PCP)Ir[14] and benzoxazolyl
iridium(III) complexes,[15] were also reported for this alkylation
reaction. Despite significant advancements, these systems still
have many limitations, and in most cases it requires high cata-
lyst loading, more than stoichiometric amount of bases, high
temperature, and long reaction time.
Introduction
The efficient synthesis of long chain alcohols as biofuels from
renewable sources is an indispensable component of alterna-
tive energy production and has lately received much attention
as a viable, greener alternative to conventional fuels such as
petrol and diesel.[1] Considering the demand of environmental-
ly benign processes, modified Guerbet-type coupling of alco-
hols using various transition-metal catalysts has recently been
explored by many groups.[2] Synthesis of b-alkylated secondary
alcohols by using primary alcohols following the hydrogen-
borrowing or hydrogen-autotransfer strategy has significant
advantages over conventional methods, which typically require
a multistep process and produces stoichiometric amount of
halide wastes.[3]
In 2003, the Cho group reported [RuCl2(PPh3)3]-catalysed
direct b-alkylation of secondary alcohols using primary alcohols
in the presence of sacrificial hydrogen acceptors.[4] Inspired by
this result, significant efforts have been made to explore this
transformation using several transition metal complexes.[5]
Thus far, the most frequently used homogeneous catalysts are
based on ruthenium and iridium and they have shown higher
reactivity and selectivity compared to other systems. For this
transformation, Peris and co-workers found that pyrazolin-3-yli-
dene containing RuII was more active in b-alkylation compare
to other RuIINHC complexes.[6] Ru complexes bearing cyclopen-
tadienyl (Cp), trispyrazolylborate (Tp), and bipyridine ligands
were also found to be effective for this reaction.[7] Crabtree
et al. reported terpyridine and NHC pyrimidine containing RuII
Metal–ligand cooperation has become a powerful strategy
in activation of various bonds and catalysis.[16] For efficient mo-
lecular transformations, ligands in bifunctional catalysts directly
take part in substrate bond activation and subsequent bond-
formation steps. Among the numerous cooperative ligands,
6,6’-dihydroxy-2,2’-bipyridine (6DHBP) have received a great
deal of attention and various transition-metal complexes con-
taining this ligand are known.[17] 6DHBP-based iridium com-
plexes have been reported to catalyse CO2 hydrogenation and
formic acid dehydrogenation,[18] dehydrogenative oxidation of
alcohols,[19] dehydrogenation and hydrogenation of heterocy-
cles,[20] hydrogen production from a methanol/water solution
under weakly basic conditions,[21] water oxidation,[22] and che-
moselective dehydrogenation of alcohols in lignin model com-
pounds.[23] Recently, Cp*Ir(6DHBP)-promoted synthesis of a-al-
kylated ketones from ketones and primary alcohols, and from
secondary alcohols and primary alcohols were demonstrated
by Li and co-workers.[24] They also reported the synthesis of
quinazolinones and quinolines following acceptorless dehydro-
[a] B. C. Roy, K. Chakrabarti, S. Shee, S. Paul, Dr. S. Kundu
Department of Chemistry, IIT Kanpur, Kanpur 208016, UP (India)
E-mail: sabuj@iitk.ac.in
Supporting information and the ORCID identification number for the
author of this article can be found under http://dx.doi.org/10.1002/
chem.201603557.
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genative coupling protocol using the same catalyst.[25] Surpris-
ingly, only few examples of 6DHBP-based RuII catalysed reac-
tions such as transfer hydrogenation of ketones[26] and alcohol
oxidation[23] have been reported, which showed moderate to
poor activity.
Final turnover frequencies of the recently reported catalysts
in cross-coupling reaction of alcohols are listed in Figure 1. De-
spite considerable efforts that have been devoted to this reac-
tion, the efficiency of these catalytic systems is still significantly
low. Hence, development of a new, more effective and sustain-
able process for CÀC bond formation reaction using alcohols is
highly desirable. Inspired by the pioneering work by Fujita and
Yamaguchi with a series of Cp*Ir complexes bearing hydroxy-
pyridine motif in the acceptorless dehydrogenation of alco-
hols,[27] we envisioned that Ru-complexes containing bifunc-
tional 6,6’-dihydroxy-2,2’-bipyridine ligand may have the po-
tential to promote the cross-coupling reaction of alcohols. Our
proposed strategy for the utilisation of alcohols as alkylating
agents is listed in Scheme 1.
Scheme 1. Proposed bifunctional RuII-promoted strategy for the utilisation
of alcohols as alkylating agents.
ing the reported literature.[26] Treatment of 6DMeOBP with
[RuHCl(CO)(PPh3)3] in DCM at room temperature afforded com-
plex 2a in 64% yield (Scheme 2). Due to poor solubility of the
6DHBP in DCM, reaction of equimolar amount of [RuHCl(-
CO)(PPh3)3] with 6DHBP was carried out in DMF at 608C which
produced air stable RuII complex 3a in 81% yield. Complexes
2b and 3b were also synthesised in good yield by reacting
[RuCl2(PPh3)3] with the corresponding ligands in DCM and DMF
in similar fashion.
Scheme 2. General scheme for the synthesis of bipyridine-based Ru com-
plexes.
Figure 1. Final turnover frequency (TOF) of recently reported complexes in
cross-coupling reaction of alcohols.
1
In the H NMR spectrum, the hydride of complexes 2a and
3a appeared as a triplet at d=À11.11 ppm (JHP =19.9 Hz) and
À13.19 ppm (JHP =15.5 Hz), respectively. The 31P NMR resonan-
ces of 2a and 3a appeared at d=46.64 and 46.59 ppm, re-
spectively; this indicates the presence of one kind of phospho-
rous environment around the Ru centres. The nCO of complexes
2a and 3a appeared at 1938 and 1947 cmÀ1, respectively, sug-
gesting greater electron density over the ruthenium centre in
complex 2a compared to 3a. The molecular structure of com-
plexes 2a and 3a were established by single X-ray crystal
structure analysis (Figure 2). The solid state structure and 1H
and 31P NMR spectra of 2a and 3a further confirmed the for-
mation of an RuII hydride carbonyl complex where two PPh3
molecules were attached to the octahedral Ru centre in trans
This is the first example of 6,6’-dihydroxy-2,2’-bipyridine-
based ruthenium complex catalysed alcohol coupling reaction.
In this study, we compare catalytic activities of a series of func-
tional bipyridine-based ruthenium complexes and report a re-
markably efficient, atom economical bifunctional RuII-based
catalytic system for the cross-coupling between a range of dif-
ferent alcohols with high selectivity.
Results and Discussion
6,6’-Dimethoxy-2,2’-bipyridine (6DMeOBP) and 6,6’-dihydroxy-
2,2’-bipyridine (6DHBP) were synthesised in good yields follow-
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geometry containing one bidentate NN ligand. The P-Ru-P
bond angles of complexes 2a and 3a were 170.738 and
170.388, respectively, indicating axial arrangement of the two
PPh3 molecules and a longer RuÀP bond length compared to
the other four RuÀligand bonds (RuÀN, RuÀCO and RuÀH); this
further suggests a distorted geometry around the Ru centre.
The chloride ion was present outside the primary coordination
sphere for both complexes.
carried out for 45 min in refluxing toluene, employing various
RuII precatalysts (0.1 mol%) and the results are summarised in
Table 1. With [RuCl2(PPh3)3] and [RuHCl(CO)(PPh3)3] complex
yields as well as selectivity for the b-alkylated alcohol product
were poor (Table 1, entries 1 and 2). Significant improvement
in both conversion and selectivity were achieved with isolated
RuII complexes bearing various 2,2’-bipyridine derived ligands
(Table 1, entries 4–9). Among these catalysts, complex 3a bear-
ing bifunctional 6,6’-dihydroxy-2,2’-bipyridine exhibited the
maximum reactivity with 70% conversion of 1-phenylethanol
within 45 min (Table 1, entry 6), and after 1 h it showed 94%
conversion with 92% selectivity for 1,3-diphenylpropan-1-ol
(Table 1, entry 7). But, complexes 3b and 6 showed only 11
and 21% conversion, respectively (Table 1, entries 3 and 10).
Most of these substituted bipyridine-RuII complexes were not
fully soluble in toluene at room temperature. However, under
the reaction condition in the presence of base, they produced
a homogenous solution except with 3b as indicated by the
Hg-poisoning test. Complex 3b showed significantly lower cat-
alytic activity compare to other complexes probably due to
poor solubility (Table 1, entry 10). The same reactions with
these RuII complexes were carried out in more polar solvents
like dioxane and tert-amyl alcohol, which showed significantly
lower conversion of 1-phenylethanol compared to toluene,
probably due lower boiling point (Supporting Information,
Table S3). This suggests that the a-hydroxyl substituent in the
2,2’-bipyridine ligand as well as other ligands bound to the
ruthenium centre were crucial for the high catalytic per-
formance. Importantly, in this cross-coupling reaction, rutheni-
um-based complex 3a displayed significantly higher activity
compare with all the previously reported catalysts including
many iridium complexes (Figure 1).
Initially, b-alkylation of 1-phenylethanol with benzyl alcohol
was picked as a model reaction to screen the performance of
a series of RuII-bipyridine-based complexes. The reactions were
Next, b-alkylation of 1-phenylethanol with benzyl alcohol
was carried out with 0.1 mol% catalyst 3a using various bases
to optimize the reaction condition, and the results are listed in
Table 2. Carbonate bases showed poor conversion in the given
reaction condition (Table 2, entries 1–3). Based on the superior
performance, KOtBu was selected as base for this reaction, al-
though KOH and NaOH also worked effectively. Subsequently,
the 3a-catalysed cross-coupling reaction of 1-phenylethanol
with benzyl alcohol was carried out in the presence of different
equivalent KOtBu to determine the ideal amount of base re-
quired for this reaction (Table 2, entries 6–11). Based on the
conversion of 1-phenylethanol, 0.5 equiv KOtBu was found to
be optimum. As anticipated, no b-alkylation was observed
without the base or the catalyst (Table 2, entries 12 and 13).
Probably due to the higher boiling point of toluene, the yield
of the CÀC coupling product was much higher in toluene com-
pare to dioxane as solvent (Table 2, entry 8).
b-Alkylation of 1-phenylethanol with a range of primary al-
cohols was conducted under the optimised conditions to de-
termine the substrate scope and the results are presented in
Table 3. The reaction of para-substituted benzyl alcohols with
both electron-withdrawing atoms (chloro, bromo, and fluoro)
and electron-donating groups (methyl and methoxy) proceed-
ed smoothly to give the corresponding b-alkylated secondary
alcohols in excellent yields with high selectivity (Table 3, en-
Figure 2. Molecular structure of [6DMeOBP)Ru(H)(CO)(PPh3)2]Cl (2a; top) and
[6DHBP)Ru(H)(CO)(PPh3)2]Cl (3a; bottom); 30% thermal ellipsoids; counter
chloride anion of both complexes were omitted for clarity.
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tries 1–5). Substituents in ortho- and meta-positions of the
benzyl alcohols were also converted to the desired secondary
alcohols in good to high yields (Table 3, entries 6–8).
Table 1. b-Alkylation of 1-phenylethanol with benzyl alcohol using vari-
ous RuII complexes.[a]
In addition, b-alkylation of naphthalen-1-ylmethanol and
heteroatom-substituted 2-thiophenemethanol and cyclohexyl-
methanol generated the desired products in moderate to ex-
cellent yields with slightly longer heating (Table 3, entries 9–
11). Notably, this method was also successfully applied to chal-
lenging aliphatic long-chain alcohols such as 1-butanol and 1-
hexanol which selectively afforded the corresponding alcohol
products in good yields within 4 h (Table 3, entries 12 and 13).
The substrate scope for the b-alkylated secondary alcohols
with primary alcohols was further expanded by treating benzyl
alcohol with various secondary alcohols. These results are sum-
marised in Table 4. Similar to the alcohol cross coupling reac-
tions listed in Table 3, reaction of substituted 1-phenylethanols
bearing both electron-withdrawing atoms (chloro, bromo, and
fluoro) and electron-donating groups (methyl and methoxy)
progressed efficiently to provide the corresponding long-chain
secondary alcohols in excellent yields with high selectivity
(Table 4, entries 2–7). 1-(Naphthalen-2-yl)ethanol was also
smoothly coupled with benzyl alcohol (Table 4, entry 8). In the
case of 1-tetralinol, conversion and selectivity towards the al-
cohol products was moderate (Table 4, entry 9). In the same
fashion, treatment of 1-phenyl-1-propanol with benzyl alcohol
in the presence of 3a resulted in 1,5-diphenylpentan-3-ol in
61% yield with high selectivity (Table 4, entry 10). The reaction
of aliphatic secondary alcohols with benzyl alcohols required
a slightly longer time and 2 equivalent secondary alcohol to
afford high conversion and selectivity (Table 4, entry 11–13).
Entry
Catalyst
Conversion[b]
A/B ratio[c]
76:24
1
2
3
4
5
6
7[d]
8
4
5
6
1a
2a
3a
3a
1b
2b
3b
45
29
21
48
49
70
94
52
54
11
58:42
92:8
86:14
86:14
90:10
92:8
86:14
85:15
81:19
9
10
[a] Reaction condition: catalyst (0.1 mol%), 1-phenylethanol (1.25 mmol),
benzyl alcohol (1.25 mmol) and KOtBu (0.625 mmol) in reflux condition in
toluene for 45 min. [b] Determined by GC analysis based on secondary al-
1
cohol. [c] Determined by H NMR spectroscopy. [d] 1 h reflux.
Reaction mechanism
A plausible mechanism for this tandem b-alkylation of secon-
dary alcohols with primary alcohols is shown in Scheme 3. A
similar mechanism was proposed by Fujita, Yamaguchi and Li
with (DHBP)Ir complexes for the dehydrogenation alcohols and
a-alkylation of ketones, respectively.[19,24] Initially, the precata-
lyst 3a was transformed to active catalyst P bearing a bipyrido-
nate ligand by extrusion of one molecule of HCl in the pres-
ence of KOtBu. Next, complex P promoted oxidation of the
secondary and primary alcohols by accepting the alcoholic
proton with its bipyridonate ligand moiety to afford an alkoxy
ruthenium species Q. Then, b-hydride elimination from com-
plex Q would generate a dihydride RuII species R and corre-
sponding carbonyl compounds. Subsequently, base catalysed
cross-aldol condensation between the ketones and aldehydes
afforded the a,b-unsaturated ketones. Finally, the cooperation
of the Ru-hydride and the ligand hydroxyl proton facilitated
the hydrogenation of the C=C bond of the a,b-unsaturated ke-
tones to generate the ketone B and the regenerated complex
P. Following similar cooperative catalysis, ketone B was then
hydrogenated to afford the corresponding alcohol A.
Table 2. b-Alkylation of 1-phenylethanol with benzyl alcohol in the pres-
ence of different bases.[a]
Entry
Base [equiv]
Conversion [%][b]
1
2
3
4
5
6
7
8[c]
9
10
11
12
13[d]
Cs2CO3(0.5)
K2CO3 (0.5)
Na2CO3 (0.5)
NaOH (0.5)
KOH (0.5)
KOtBu (0.5)
KOtBu (0.7)
KOtBu (0.5)
KOtBu (0.4)
KOtBu (0.2)
KOtBu (0.1)
No base
14
2
4
90
92
94
95
40
75
38
28
0
KOtBu (0.5)
0
To obtain more information about this proposed mecha-
nism, some test reactions were carried out. In order to find out
which unsaturated bond in the a,b-unsaturated ketone (C=C
or C=O) would be hydrogenated first, two control experiments
[a] Reaction condition: catalyst 3a (0.1 mol%), 1-phenylethanol
(1.25 mmol), benzyl alcohol (1.25 mmol) and base (0.625 mmol) in reflux
condition in toluene for 60 min. [b] Determined by GC analysis based on
secondary alcohol. [c] Dioxane as solvent. [d] No catalyst.
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Table 3. b-Alkylation of 1-phenylethanol with diverse substituted benzyl alcohol.[a]
Entry
1
Primary alcohol
Product
Conv.[b] [%]
100
A/B ratio[b]
91:9
90:10
89:11
89:11
2
3
4
100
100
78
5
6
98
96
98:2
91:9
7
8
52
71
94:6
89:11
9[c]
99
92:8
10[d]
11[e]
12[d]
13[d]
55
93
84
83
99:1
89:11
98:2
96:4
[a] Reaction condition: catalyst 3a (0.1 mol%), secondary alcohol (1.25 mmol), benzyl alcohol (1.25 mmol) and KOtBu (0.625 mmol) in reflux condition in
1
toluene for 75 min. [b] Determined by H NMR spectroscopy with respect to the secondary alcohol. [c] 3 h reflux. [d] 4 h reflux. [e] 6 h reflux.
were carried out following the standard catalytic condition.
The reaction of 1-phenylethanol with benzaldehyde in the
presence of 3a showed 62% conversion of 1-phenylethanol
after 30 min and produced acetophenone, 1,3-diphenylpropan-
1-one (B) and 1,3-diphenyl-2-propan-1-ol (A) in a 1:9:4 ratio.
After the reaction no unreacted benzaldehyde was detected as
it was converted to benzyl alcohol and benzoic acids by base-
promoted Cannizzaro reaction. Under similar conditions, reac-
tion of acetophenone with benzyl alcohol afforded chalcone,
compound B and compound A in 1:40:25 ratio (100% conver-
sion of acetophenone). In both reactions, selective C=C bond
hydrogenated product of chalcone that is, compound B was
identified as the major and 1,3-diphenylpropan-1-ol (A) as
minor product. Importantly, in both cases, only the C=O bond
hydrogenated product of chalcone, that is, 1,3-diphenylprop-2-
en-1-ol was not observed. These results indicate that the re-
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Table 4. b-Alkylation of 1-phenylethanol with different substituted benzyl alcohol.[a]
Entry
1
Secondary alcohol
Product
Conv.[b] [%]
99.7
A/B ratio[b]
93:7
90:10
93:7
2
3
4
5
100
91
86
81:19
97
96:4
6
7
95
98
85:15
91:9
8
95
65
61
54
99
99
93:7
72:28
99:1
99:1
99:1
99:1
9
10[c]
11[c,d]
12[c,d]
13[c,d]
[a] Reaction condition: catalyst 3a (0.1 mol%), secondary alcohol (1.25 mmol), benzyl alcohol (1.25 mmol) and KOtBu (0.625 mmol) in reflux condition in
toluene for 75 min. [b] Determined by 1H NMR spectroscopy with respect to the secondary alcohol. [c] 4 h reflux. [d] 2.5 mmol secondary alcohol and
1.25 mmol benzyl alcohol was used and conversion was determined with respect to benzyl alcohol.
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of alcohols (P to Q) and in the b-hydride elimination from the
alkoxy ruthenium species (Q to R), as both of these intermedi-
ates are six-coordinated 18-electron species. In order to exam-
ine the importance of PPh3 dissociation, b-alkylation of 1-phe-
nylethanol with benzyl alcohol was carried out in the presence
of excess PPh3 (2–8 equiv), which is shown in Figure 4. Conver-
sion of 1-phenylethanol decreased significantly when excess of
PPh3 was added to the reaction mixture. This suggests that in
the catalytic cycle, elimination of PPh3 from the active complex
was essential. However, based on this observation we cannot
conclude whether in the rate-determining step dissociation of
PPh3 was involved or not. To isolate the proposed intermedi-
ates mentioned in the catalytic cycle, we treated complex 3a
with various bases such as KOtBu, KOH, K2CO3 and Cs2CO3 in
DCM in the presence and absence of 1-phenylethanol. Howev-
er, all these attempts to isolate the intermediates were unsuc-
cessful.[29] The homogeneous nature of this catalytic system
was proved by the Hg-poisoning test.
Scheme 3. Proposed reaction mechanism.
duction of the C=C bond of the a,b-unsaturated ketone is
much faster than the reduction of the C=O bond, which is con-
sistent with prior reports.[13c,14,28]
We also studied complex 3a-catalysed time-dependent
product distribution of b-alkylation of 1-phenylethanol with
benzyl alcohol (Figure 3). The selectivity for the 1,3-diphenyl-
propan-1-ol (A) gradually increased with time and reached
maximum at the end of the reaction. However, the concentra-
tion of 1,3-diphenylpropan-1-one (B) was nominal and did not
change significantly during the course of the reaction.
Dissociation of PPh3 might be involved in two separate
steps of the proposed catalytic cycle: in the oxidative addition
Figure 4. Effect of externally added PPh3 on catalyst 3a in b-alkylation of 1-
phenylethanol with benzyl alcohol.
Conclusions
In summary, employing a series of RuII complexes bearing bi-
pyridine-based functional ligands, b-alkylation of secondary al-
cohols using primary alcohols was investigated. Complex 3a
having 6,6’-dihydroxy-2,2’-bipyridine ligand was found to be
a highly effective and versatile precatalyst which displayed the
highest reactivity among all the complexes. The present proto-
col provides efficient, atom economical and a greener strategy
for the synthesis of a variety of b-alkylated secondary alcohols
with high selectivity. Control experiments suggested that this
system preferentially reduced the C=C bond of the a,b-unsatu-
rated ketone over the C=O bond. Notably, this is a rare exam-
ple of an effective and versatile bifunctional RuII catalysed
tandem b-alkylation reaction of secondary alcohols under mild
conditions. Importantly, compared to all the previously report-
ed catalysts, complex 3a exhibited the highest reactivity and
this study revealed the unique potential of this system in CÀC
Figure 3. Time-dependence product distribution of b-alkylation of 1-phenyl-
ethanol with benzyl alcohol catalysed by catalyst 3a.
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predicted: 871.1792). Elemental analysis calcd (%) for
C49H43ClN2O3P2Ru: C 64.93, H 4.78, N 3.09; found C 64.68, H 4.56, N
2.87.
bond forming reactions by utilising alcohols as alkylating
agents.
Synthesis of [(6,6’-dimethoxy-2,2’-bipyridine)RuCl2(PPh3)2]
(2b)
Experimental Section
General procedures and materials
A mixture of 6,6’-dimethoxy-2,2’-bipyridine (20 mg, 0.092 mmol),
[RuCl2(PPh3)3] (88.7 mg, 0.092 mmol) and DCM (6 mL) was stirred at
room temperature under argon for 12 h. Work-up procedure was
All reactions were carried out under an inert atmosphere by using
standard Schlenk-line techniques. Glassware was dried in a 1008C
oven, overnight, before use. Solvents were dried by distillation
under argon according to standard literature methods and deoxy-
genated prior to use. RuCl3·nH2O (39% Ru) was purchased from
Arora Matthey, India. All the chemicals were purchased from
Sigma–Aldrich, Alfa Aesar, SD Fine and Spectrochem. 6,6’-dihy-
droxy-2,2’-bipyridine,[26] 6,6’-dimethoxy-2,2’-bipyridine,[26] [RuHCl(-
CO)(PPh3)3],[30] [RuCl2(PPh3)3],[31] [(2,2’-bipyridine)Ru(H)(CO)(PPh3)2]Cl
(1a)[32] and [(2,2’-bipyridine)RuCl2(PPh3)2] (2a)[33] were synthesised
1
similar to 2a. Yield: 52.6 mg (63%). H NMR (500 MHz, CDCl3): d=
8.00 (d, J=7.45 Hz, 2H), 7.69–7.65 (m, 14H), 7.56–7.52(m, 6H),
7.47–7.44 (m, 12H), 6.74 (d, J=8.15 Hz, 2H), 4.03 ppm (s, 3H).
13C NMR (125 MHz, CDCl3): d=163.49, 153.52, 139.33, 133.05,
132.15, 128.63, 113.71, 110.98, 53.30 ppm. 31P{1H} NMR (202 MHz,
CDCl3): d=29.79 ppm. Elemental analysis calcd (%) for
C48H42Cl2N2O2P2Ru: C 63.16, H 4.64, N 3.07; found: C 62.91, H 4.42,
N 2.88.
1
according to previously reported literature procedures. H, 13C and
31P NMR spectra were recorded on Jeol 400 and 500 MHz spec-
trometer. Elemental analysis was performed on a Thermoquest
EA1110 CHN analyser. The crystallised compounds were powdered,
washed several times with dry diethyl ether and dried under
vacuum for at least 48 h prior to elemental analyses. ESI-MS were
recorded on a Waters Micromass Quattro Micro triple-quadrupole
mass spectrometer. All the GC analysis were carried out by using
a Perkein Elmer Clarus 600 gas chromatograph and GC-MS were
taken by using an Agilent 7890 A gas chromatograph equipped
with Agilent 5890 triple-quadrupole mass system.
Synthesis of [(6,6’-dihydroxy-2,2’-bipyridine)Ru(H)(-
CO)(PPh3)2]Cl (3a)
A mixture of 6,6’-dihydroxy-2,2’-bipyridine (100 mg, 0.053 mmol)
and [Ru(H)(CO)(Cl)(PPh3)3] (506.1 mg, 0.053 mmol) was heated at
608C for 24 h in 6 mL dry DMF under argon. After cooling the solu-
tion slowly, the pale yellow precipitate was filtrated off and
washed with ether and hexane. Yield: 380 mg (81%). 1H NMR
(500 MHz, CDCl3): d=7.02 (t, J=6.65 Hz, 2H, bpy-H), 6.90 (d, J=
8.85 Hz, 2H, bpy-H), 6.86–6.74 (m, 30H, Ph-H), 6.46 (d, J=6.65 Hz,
1H, bpy-H), 5.66 (d, J=8.9 Hz, 1H, bpy-H), À13.19 ppm (t, J=
15.55 Hz, 1H). 31P{1H} NMR (202 MHz, [D6]DMSO): d=46.59 ppm.
ESI-MS: m/z 843.1479 ([MÀCl]+, predicted: 843.1479). Elemental
analysis calcd (%) for C47H39ClN2O3P2Ru: C 64.27, H 4.48, N 3.19;
found: C 63.98, H 4.31, N 2.96.
General procedure for b-alkylation of secondary alcohols
with primary alcohol
The catalytic b-alkylation of secondary alcohol reaction was carried
out in Schlenk tube under closed argon conditions. Initially catalyst
3a (0.1 mol%) and KOtBu (0.5 equiv) were taken as solid and then
under argon conditions secondary alcohol (1 equiv), primary alco-
hol (1 equiv) and toluene (2 mL) were added and the resulting mix-
ture was heated at 1308C (oil bath temperature) for 75 min. After
it cooled to room temperature, the toluene was evaporated under
reduced pressure and the resulting mixture was purified by silica
gel column chromatography using ethyl acetate and hexane as
eluent to afford the desire product.
Synthesis of [(6,6’-dihydroxy-2,2’-bipyridine)RuCl2(PPh3)2]
(3b)
A mixture of 6,6’-dihydroxy-2,2’-bipyridine (85 mg, 0.045 mmol)
and [RuCl2(PPh3)3] (433 mg, 0.045 mmol) was heated at 608C for
24 h in 6 mL dry DMF under argon. After cooling the solution
slowly, the bright yellow precipitate was filtrated off and washed
with ether and hexane to remove free triphenylphosphine. Yield:
280 mg (70%). Due to very poor solubility in any common solvent
1
such as DCM, DMSO, DMF and MeOH etc., H and 13C NMR spectra
Synthesis of [(6,6’-dimethoxy-2,2’-bipyridine)Ru(H)(-
CO)(PPh3)2]Cl (2a)
could not be recorded for this complex. Elemental analysis calcd
(%) for C46H38Cl2N2O2P2Ru: C 62.45, H 4.33, N 3.17; found: C 62.38,
H 4.18, N 2.98.
A mixture of 6,6’-dimethoxy-2,2’-bipyridine (15 mg, 0.069 mmol),
[Ru(H)(CO)(Cl)(PPh3)3] (66.1 mg, 0.069 mmol) and DCM (6 mL) was
stirred at room temperature under argon condition for 12 h. Then
diethyl ether was added to precipitate the product and resulting
bright yellow solid was washed carefully with diethyl ether and
hexane to remove free triphenylphosphine. Yield: 40 mg (64%).
1H NMR (500 MHz, CDCl3): d=8.22 (d, J=7.65 Hz, 1H, bpy-H), 8.15
(d, J=7.65 Hz, 1H, bpy-H), 7.94 (t, J=9.2 Hz, 1H, bpy-H), 7.73 (t,
J=7.65 Hz, 1H, bpy-H), 7.32–7.00 (m, 30H, Ph-H), 6.63 (d, J=
7.65 Hz, 1H, bpy-H), 5.91 (d, J=9.15 Hz, 1H, bpy-H), 3.73 (s, 3H,
-CH3), 3.08 (s, 3H, -CH3), À11.11 ppm (t, J=19.9 Hz, 1H, Ru-H).
Acknowledgements
We are grateful to the Science and Engineering Research
Board, India, Council of Scientific and Industrial Research, New
Delhi, and Indian Institute of Technology Kanpur for financial
support. B.C.R. and S.S. thank the CSIR and K.C. thanks the
UGC, India, for fellowships.
3
13C NMR (125 MHz, CD2Cl2): d=206.31 (t, Jcp=14.3 Hz, CO), 164.64
(s, bpy-C), 163.93 (s, bpy-C), 154.90 (s, bpy-C), 153.81 (s, bpy-C),
142.06 (s, bpy-C), 141.52 (s, bpy-C), 133.17 (Ph-C), 128.08 (s, Ph-C),
117.90 (s, Ph-C), 117.90 (s, bpy-C), 117.66 (s, bpy-C), 107.59 (s, bpy-
C), 107.39 (s, bpy-C), 55.83 (s, CH3), 55.58 ppm (s, CH3). 31P{1H} NMR
(202 MHz, CDCl3): d=46.64 ppm. ESI-MS: m/z 871.1793 ([MÀCl]+,
Keywords: atom economical
catalysis · CÀC bond formation · ruthenium
·
bifunctional catalysis
·
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Received: July 27, 2016
Published online on && &&, 0000
Chem. Eur. J. 2016, 22, 1 – 10
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&
&
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Full Paper
FULL PAPER
Alcohols as alkylating agents: A rare
example of highly efficient, atom eco-
nomical and a greener strategy for the
synthesis of a variety of b-alkylated sec-
ondary alcohols with high selectivity is
presented.
&
Catalysis
B. C. Roy, K. Chakrabarti, S. Shee, S. Paul,
S. Kundu*
&& – &&
Bifunctional RuII-Complex-Catalysed
Tandem CÀC Bond Formation:
Efficient and Atom Economical
Strategy for the Utilisation of Alcohols
as Alkylating Agents
&
&
Chem. Eur. J. 2016, 22, 1 – 10
www.chemeurj.org
10
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!