Bifunctional Ru(II) complex catalysed carbon-carbon bond formation: an eco-friendly hydrogen borrowing strategy

Article abstract of DOI:10.1039/C6OB02010K

The atom economical borrowing hydrogen methodology enables the use of alcohols as alkylating agents for selective C-C bond formation. A bifunctional 2-(2-pyridyl-2-ol)-1,10-phenanthroline (phenpy-OH) based Ru(ii) complex (2) was found to be a highly efficient catalyst for the one-pot β-alkylation of secondary alcohols with primary alcohols and double alkylation of cyclopentanol with different primary alcohols. Exploiting the metal-ligand cooperativity in complex 2, several aromatic, aliphatic and heteroatom substituted alcohols were selectively cross-coupled in high yields using significantly low catalyst loading (0.1 mol%). An outer-sphere mechanism is proposed for this system as exogenous PPh₃ has no significant effect on the rate of the reaction. Notably, this is a rare one-pot strategy for β-alkylation of secondary alcohols using a bifunctional Ru(ii)-complex. Moreover, this atom-economical methodology displayed the highest cumulative turn over frequency (TOF) among all the reported transition metal complexes in cross coupling of alcohols.

Full text of DOI:10.1039/C6OB02010K

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Bifunctional Ru (II) Complex Catalysed Carbon–Carbon Bond Formation:
An Ecoꢀ friendly Hydrogen Borrowing Strategy
Kaushik Chakrabarti, Bhaskar Paul, Milan Maji, Bivas Chandra Roy, Sujan Shee and Sabuj
Kundu*
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India
Abstract: Atom economical borrowing hydrogen methodology enables the use of alcohols as
alkylating agents for selective CꢀC bond formation. Bifunctional 2ꢀ(2ꢀpyridylꢀ2ꢀol)ꢀ1,10ꢀ
phenanthroline (phenpyꢀOH) based Ru(II) complex (2) was found to be a highly efficient
catalyst for the oneꢀpot βꢀalkylation of secondary alcohols with primary alcohols and double
alkylation of cyclopentanol with different primary alcohols. Exploiting the metalꢀligand
cooperativity in complex 2, several aromatic, aliphatic and heteroatom substituted alcohols
were selectively crossꢀcoupled in high yields using significantly low catalyst loading (0.1
mol%). An outer-sphere mechanism is proposed for this system as exogenous PPh3 has no
significant effect in rate of the reaction. Notably, this is a rare oneꢀpot strategy for βꢀ
alkylation of secondary alcohols using a bifunctional Ru(II)ꢀcomplex. Moreover, this atomꢀ
economical methodology displayed the highest cumulative turn over frequency (TOF) among
all the reported transition metal complexes in cross coupling of alcohols.
Keywords: CꢀC bond formation, Bifunctional, Ruthenium, Hydrogen Borrowing Strategy,
Outerꢀsphere mechanism.
Introduction
CꢀC bond forming reactions are one of the most crucial and fundamental reactions in
organic chemistry to synthesize functionalized molecules with increasing complexity,
rationally and predictability.1ꢀ3 To construct a new CꢀC bond following the conventional
synthetic approach requires toxic, expensive alkyl halides and strong bases which generate
stoichiometric amounts of salts as waste.4, 5 This led chemists to develop cleaner and greener
methodologies for this chemical transformation. In this regard, substituting alkyl halides with
cheap and easy to handle alcohols as electrophiles has a great advantage.6ꢀ8
Scheme
1 Proposed strategy for βꢀalkylation of secondary alcohol by using bifunctional Ru
complex
Selfꢀcoupling of alcohols using strong bases and heterogeneous transition metal
catalysts at very high temperature is known for more than a century as the Guerbet reaction.9
In the last decade, significant effort has been invested in this field to make this reaction

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practically viable.10ꢀ15 Several transition metals such as Ir,16ꢀ25 Rh,26 Pd,27 Ru,21, 23, 24, 28ꢀ37 Ni,38
Fe,39 and Cu40, 41 based complexes were tested for the CꢀC bond formation via alcohol
activation following the “borrowing hydrogen” strategy. This methodology consist mainly of
three steps; starting with dehydrogenation of alcohols to generate the corresponding carbonyl
compounds followed by base catalyzed aldol condensation to afford α,βꢀunsaturated ketones
which are subsequently hydrogenated to afford the longer chain alcohols. This atom
economical borrowing hydrogen approach is highly attractive as it provides a wide range of
valuable organic molecules with loss of only environment friendly water.42ꢀ46
Fig. 1 Cumulative TOF of the reported Ru and Ir catalysts in βꢀalkylation of alcohols
Catalytic activities of recently reported various complexes in alcohol crossꢀcoupling
are listed in Fig. 1. Although, noteworthy progress have been made in this field, still the
cumulative TOF of these catalysts are significantly low and far from any realꢀworld
applicability. From industrial and sustainable chemistry perspectives, it is essential to develop
a new catalytic system for the efficient synthesis of βꢀalkylated alcohols with greater
selectivity and yield under more environment friendly conditions. Carefully designed
bifunctional ligand has the potential to craft better performing catalysts to enable the
advancement of this reaction.
2ꢀHydroxypyridine (2ꢀHP) emerged as an exciting fragment in bifunctional ligand
design in both tautomeric forms depending on the reaction conditions (Scheme 2).47, 48 The
presence of the pendent ꢀOH group in these metal complexes enhances their catalytic ability.
Yamaguchi et al. demonstrated dehydrogenation of various alcohols and diols bearing a
49ꢀ51
bifuncitional iridium complexes containing 6,6′ꢀdihydroxyꢀ2,2′ꢀbipyridine ligand.3,
Recently, Li and coꢀworkers reported bipyridonate iridium catalyzed αꢀalkylation of ketones
using primary alcohols.18 They also showed βꢀalkylation of secondary alcohols with primary
alcohols in two steps using the same catalysts (TOF= 7.5 hꢀ1).16 Dehydrogenative crossꢀ
coupling of alcohols were also reported by Gelman et al. by using bifunctional PC(sp3)P
pincer Ir (TOF= 7.8 hꢀ1) and Ru (TOF= 1.9 hꢀ1) complexes.24

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Scheme 2 Metalꢀligand cooperativity in phenpyꢀOH based Ru complex (2)
We have recently demonstrated that bifunctional RuCl(phenpyꢀOH)(PPh3)2PF6 (2)
catalyst was highly effective in transfer hydrogenation of ketones and nitriles.52 Two 2ꢀHP
containing similar Ru(II)ꢀ6,6´ꢀdihydroxy terpyridine complex showed very poor activity in
transfer hydrogenation with bulkier ketones as the extra pendent orthoꢀOH may hinder the
approach of the substrate to the active site.53, 54 To exhibit the metalꢀligand cooperativity only
one 2ꢀHP unit is theoretically sufficient. However, all the reported 2ꢀHPꢀderived catalysts in
dehydrogenation of alcohols2, 3, 49ꢀ51 as well as αꢀalkylation16, 18 contain at least two 2ꢀHP
units.
These observations encouraged us to investigate the catalytic properties of this
bifunctional ruthenium complex 2 in direct βꢀalkylation of secondary alcohols with primary
alcohols. Fascinated by the high catalytic activity of 2ꢀHP based Irꢀcomplexes in “ligandꢀ
promoted dehydrogenation” of alcohols,2 we envisioned that Ruꢀcomplexes bearing
bifunctional tridentate ligand containing one 2ꢀHP unit may facilitate the βꢀalkylation of
secondary alcohols (Scheme 1). However, to the best of our knowledge no such cooperative
complex has been reported for the oneꢀpot synthesis of βꢀalkylated secondary alcohols.
Herein, we report a new, highly efficient bifunctional Ru(II) complex catalyzed tandem βꢀ
alkylation of secondary alcohols with primary alcohols without any sacrificial hydrogen
acceptors.
Result and Discussion
In the initial experiment, reaction of 1ꢀphenylethanol with benzyl alcohol was selected
as the benchmark reaction to evaluate the catalytic activities of a series of NNNꢀpincer Ru(II)
complexes for the βꢀalkylation of secondary alcohols with primary alcohols. The progress of
1
the reaction was monitored by HꢀNMR spectroscopy and the results are shown in Table 1.
Complexes 1, 3 and 4 were found to be moderately active, whereas, complex 2 bearing
bifunctional 2ꢀ(2ꢀpyridylꢀ2ꢀol)ꢀ1,10ꢀphenanthroline (phenpyꢀOH) ligand exhibited
significantly higher activity among all the complexes (Table 1). By using 0.1 mol% of
catalyst 2, 73% conversion of 1ꢀphenylethanol was detected after 1 hour of heating with
highest 1,3ꢀdiphenylpropanꢀ1ꢀol to 1,3ꢀdipheynlpropanꢀ1ꢀone ratio (A:B = 97:3) and within
90 minutes it presented 99% conversion. To the best of our knowledge, the highest reported
cumulative TOF value in similar reaction was 198 hꢀ1, which was reported by Crabtree et al.
using terpyridine based Ir complex (Fig. 1).32 To our delight, in comparison to this Ir
complex, catalyst 2/NaOH exhibited much higher activity (TOF= 640 hꢀ1). Notably, using
very low catalyst loading (0.002 mol%) this system displayed remarkably high 31500 TON
after 16 h.
Table 1 βꢀalkylation of 1ꢀphenylethanol with benzyl alcohol catalysed by different Ru(II)
NNN pincer complexesa

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PF6
PF6
PPh3
PPh3
N
N
N
N
N
N
Ru
Ru
OH
OMe
Ph3P
Cl
Ph3P
Cl
1
2
Conversion 73% (99%)*
Conversion 22%
A:B = 77:23
97:3
A:B =
PF6
PF6
PPh3
N
PPh3
N
N
N
N
N
Ru
Ru
Me
Ph3P
Cl
Ph3P
Cl
3
4
Conversion 23%
A:B = 78:22
Conversion 27%
A:B = 79:21
Reaction Condition: catalyst (0.1 mol%), 1ꢀphenylethanol (0.654 mmol), benzyl alcohol
°
(0.654 mmol) and NaOH (0.327 mmol) at 125 C for 60 min (*90 min heating). Conversion
and selectivity were determined by 1H NMR based on secondary alcohol.
In order to optimize the reaction conditions and to investigate the influence of base
and solvent, βꢀalkylation of 1ꢀphenylethanol with benzyl alcohol was carried out with 0.1
mol% of complex 2 and the results are summarized in Table 2. Conversion of 1ꢀ
phenylethanol was much higher in toluene than dioxane probably due to higher boiling point
of toluene (Table 2 entry 6). Although complex 2 was not fully soluble in toluene at room
temperature; under the reaction conditions in the presence of base it produced a homogenous
solution which was confirmed by Hg0 poisoning test. Among the various bases tested in this
reaction, NaOH exhibited the highest efficiency. To determine whether the amount of base
influenced the generation of the active catalyst via deprotonation of the orthoꢀOH group of
the pyridine moiety which affects the reaction rate; we inspected the dependence of the base
in the βꢀalkylation of 1ꢀphenylethanol with benzyl alcohol using complex 2.54 Conversion of
1ꢀphenylethanol increased steadily with increasing amounts of NaOH up to 0.5 eq., as can be
seen in Fig. 2. However, higher than 0.5 eq. of base did not significantly increase the
conversion and saturation behaviour was observed. As expected, no conversion was
accomplished without the base or the ruthenium complex (Table 2, entries 12 and 13).
Fig. 2 Dependence of NaOH (with respect to 1ꢀphenylethanol) in βꢀalkylation of 1ꢀ
phenylethanol with benzyl alcohol catalysed by complex 2

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Table 2 Effect of the base and solvent in the βꢀalkylation of 1ꢀphenylethanol with benzyl
alcohol catalysed by complex 2a
Entry
Base (eq.)
Solvent
Conv.
(%)b
A/B
ratioc
1
2
KOtBu (0.5)
KOH (0.5)
Toluene
Toluene
Toluene
Toluene
Toluene
Dioxane
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
51
50
73
52
77
13
65
66
<1
<1
<2
0
92:8
87:13
97:3
93:7
97:3
94:6
90:10
91:9
ND
3
NaOH (0.5)
NaOH (0.3)
NaOH (1.0)
NaOH (0.5)
NaOiPr (0.5)
NaOtBu (0.5)
Na2CO3 (0.5)
K2CO3 (0.5)
Cs2CO3 (0.5)
No base
4
5
6
7
8
9
10
11
12
13d
ND
ND
ND
NaOH (0.5)
0
ND
aReaction Condition: catalyst (0.1 mol%), 1ꢀphenylethanol (0.654 mmol), benzyl alcohol
°
b
(0.654 mmol) and base (0.327 mmol) at 125 C for 60 min. Determined by GC analysis
c
1
d
based on secondary alcohol. Determined by H NMR analysis. No catalyst. ND = Not
Determined.
To evaluate the scope of the present catalytic system, βꢀalkylation of 1ꢀphenylethanol
with a variety of primary alcohols was conducted (Table 3). Benzylic alcohols bearing both
electronꢀdonating and electronꢀwithdrawing groups at para- position, such as methoxy,
methyl, chloro, bromo, and fluoro groups afforded corresponding long chain βꢀalkylated
secondary alcohols selectively in high yields (Table 3, entries 1, 4ꢀ7). Substrates with
electronꢀdonating and electronꢀwithdrawing groups at ortho- and meta- positions also gave
moderate to good yields with high selectivity (Table 3, entries 2, 3, and 8). Furthermore, 1ꢀ
naphthylmethanol, heteroaromatic substrate 2ꢀthiophenemethanol and cyclohexylmethanol
also successfully acted as a βꢀalkylating agents and generating the desire products selectively
in moderate to good yields (Table 3, entries 9ꢀ11). Catalyst 2 showed excellent reactivity in βꢀ
alkylation of 1ꢀphenylethanol with more challenging straight chain alcohols, such as 1ꢀ
butanol, 1ꢀhexanol and 1ꢀoctanol (Table 3, entries 12, 13 and 14). Although the reaction rate
was slightly slower compared with benzylic alcohols, interestingly no selfꢀcoupling products
were detected.55

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Table 3 Variation of primary alcohols in βꢀalkylation with 1ꢀphenylethanola
Entr Primary alcohol
y
Product
Yieldb
89
A/B
ratiob
1
94:6
2
96
93:7
OH
3
55
88
90
92
87
65
82
70
99:1
93:7
94:6
92:8
97:3
99:1
93:7
99:1
Cl
4
5
6
7
8
9
10

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11
68
88
86
94:6
12c
13c
87:13
84:16
14c
81
83:17
aReaction Condition: catalyst (0.1 mol%), secondary alcohol (0.654 mmol), benzyl alcohol
°
1
(0.654 mmol) and NaOH (0.327 mmol) at 125 C for 90 min. bDetermined by H NMR with
respect to secondary alcohol. cHeating for 4 hours.
The generality of this catalytic reaction was further expanded by reacting benzyl
alcohol with different secondary alcohols (Table 4). 1ꢀphenylethanol bearing both electronꢀ
donating and electronꢀwithdrawing groups at different positions, such as methoxy, methyl,
chloro, bromo, and fluoro groups afforded the corresponding βꢀalkylated alcohols in excellent
yields with high selectivity (Table 4, entries 1ꢀ8). 1ꢀ(naphthalenꢀ2ꢀyl)ethanol and 1ꢀtetralinol
were successfully converted to the coupled alcohol products with high yields (Table 4, entries
9 and 10). The βꢀalkylation of 1ꢀphenylꢀ1ꢀpropanol with benzyl alcohol gave moderate yield
but selectivity was high (Table 4, entry 11). Aliphatic secondary alcohols, such as 1ꢀ
cyclopropylethanol, 3ꢀmethylbutanꢀ2ꢀol and 2ꢀheptanol were also converted to the desired
products in good yields, although 2 equiv. of secondary alcohol was required (Table 4, entries
12ꢀ14). In addition, heteroaromatic 1ꢀ(pyridinꢀ3ꢀyl)ethanol also gave moderate conversion
with high selectivity in this catalytic condition (Table 4, entry 15).
Table 4 Variation of secondary alcohols in βꢀalkylation with benzyl alcohola
Entry
Secondary alcohol
Product
Yieldb
99
A/B
ratiob
1
2
99:1
93
51
94:6
93:7
3

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4
5
6
7
8
92
93
96
91
93
95:5
93:7
88:12
87:13
92:8
9
96
95
52
70
72
88
69
85:15
84:16
99:1
93:7
97:3
93:7
99:1
10
11
12c
13c
14c
15d
aReaction Condition: catalyst (0.1 mol%), secondary alcohol (0.654 mmol), benzyl alcohol
°
1
(0.654 mmol) and NaOH (0.327 mmol) at 125 C for 90 min. bDetermined by H NMR with
c
respect to secondary alcohol. 2 mmol secondary alcohol and 1 mmol benzyl alcohol were
used. d 0.3 mol% catalyst was used and heated for 5 hr.
To exploit the versatility of this catalytic system, the oneꢀpot double alkylation
reaction was tested by treatment of cyclopentanol with various primary alcohols in the
presence of complex 2. To our delight, this catalytic system afforded the desired double
alkylated secondary alcohol products in moderate to good yields (Table 5). Electronic
withdrawing group at para position increased the yield of this reaction (Table 5, 5b) where
reverse was true for electron donating group (Table 5, 5d and 5e). Heteroaromatic 2ꢀ
thiophenemethanol was also successfully utilized as double alkylating agent which showed
moderate yield (Table 5, 5f).

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Table 5 Oneꢀpot double alkylation reaction of cyclopentanol using different primary
alcoholsa
aReaction Condition: catalyst (0.5 mol%), cyclopentanol (0.696 mmol), benzyl alcohol (1.46
°
1
mmol) and NaOH (0.696 mmol) at 125 C for 10 h. Yield determined by H NMR with
respect to primary alcohol (isolated yields are given in parenthesis).
Reaction Mechanism
Based on the bifunctional nature of the catalyst 2 and related literature reports,2, 3, 16, 18,
we proposed a probable mechanism for the βꢀalkylation of secondary alcohols with
54
primary alcohols as shown in Scheme 3. In the initial step, in presence of base complex 2 is
converted to Ru(II)ꢀbipyridonate complex (P) and addition of alcohol following concerted
outer-sphere pathway would generate the ruthenium hydride species (R) and the
corresponding carbonyl compound. This outer-sphere pathway is more favoured over the
inner-sphere mechanism as indicated by many recent reports.56ꢀ59 However, addition of
alcohol to P followed by βꢀhydrogen elimination to afford R cannot be completely ruled out.
Next, baseꢀmediated crossꢀaldol condensation between the resulting aldehydes and ketones
produces the α,βꢀunsaturated ketones. Finally, metalꢀligand cooperativity enables the
hydrogenation of the double bond of the α,βꢀunsaturated ketone by simultaneous transfer of
hydroxyl proton from the ligand and the hydride on the ruthenium centre to produce the
ketone (B), which then further reduces to alcohol (A) following similar outer-sphere
mechanism.

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Scheme 3 Proposed mechanism for βꢀalkylation of secondary alcohol with primary alcohol
catalysed by complex 2
To confirm the possible steps in this proposed mechanism several control experiments
were carried out. The reaction of 1ꢀphenylethanol with benzaldehyde in presence of complex
2 under optimum catalytic conditions provided a mixture of products comprising 72% of 1,3ꢀ
diphenylpropanꢀ1ꢀone (B), 11% of 1,3ꢀdiphenylpropanꢀ1ꢀol (A), and 17% of 1,3ꢀ
1
diphenylpropenꢀ1ꢀone (chalcone) (91% conversion of 1ꢀphenylethanol based on H NMR
analysis) (Scheme 4, Eq. 1). Similar results were observed when the reverse reaction i.e.
reaction of acetophenone with benzyl alcohol was performed (Scheme 4, Eq. 2). In this case,
the product distribution was 75% of B, 9% of A and 16% of chalcone (93% conversion of 1ꢀ
1
phenylethanol based on H NMR analysis). In both of these control experiments among the
product mixtures, saturated ketone 1, 3ꢀdiphenylpropanꢀ1ꢀone (B) was identified as the major
and 1,3ꢀdiphenylpropenꢀ1ꢀone (C, chalcone) as a minor product which clearly indicates that
the transfer of hydrogen from starting alcohols to the unsaturated aldol condensation product
(C) happened smoothly. The absence of 1,3ꢀdiphenylpropꢀ2ꢀenꢀ1ꢀol (only ketone
hydrogenated product) indicates that the reduction of C=C bond of the α,βꢀunsaturated ketone
was much faster than the reduction of C=O bond under the catalytic conditions.39

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Scheme 4 Control experiments for mechanistic studies of βꢀalkylation of alcohols
To obtain more information regarding the hydrogenation mechanism of the α,βꢀ
unsaturated ketones, two control experiments with chalcone were executed. The transfer
hydrogenation of chalcone in the presence of 1ꢀphenylethanol afforded exclusively B in 87%
yield (Scheme 5, Eq. 3). Under similar conditions, reaction of benzyl alcohol with chalcone
produced 76% of B and 3 % of A (Scheme 5, Eq. 4). These results further suggest that
complex 2 can effectively catalyze the transfer of hydrogen from the starting alcohols to the
C=C bond of chalcone and the reduction of the C=C bond is preferred over the C=O bond.24,
60, 61
Scheme 5 Transfer hydrogenation of chalcone using 1 equiv. alcohol
To inspect if the Ru metal center was involved in the crossꢀaldol condensation step,
reaction of acetophenone and benzaldehyde was performed in presence and absence of
complex 2 using the optimum catalytic conditions. In the presence of complex 2, 1ꢀ
phenylethanol was quantitatively converted to chalcone (C) within 90 minutes (Scheme 6,
Eq. 5). However, in the absence of 2, only 74% conversion of 1ꢀphenylethanol was observed
(Scheme 6, Eq. 6). In this reaction small amount of benzoic acid and benzyl alcohol were
generated from base induced Cannizzaro reaction of benzaldehyde and trace amounts of 1,3ꢀ
diphenylbutꢀ2ꢀenꢀ1ꢀone were also produced from selfꢀcondensation of acetophenone. Results
from these experiments may indicate the involvement of the ruthenium catalyst in the crossꢀ
aldol condensation reaction. With the NHCꢀIr complex a similar result was also reported by
Oro et al.62
Scheme 6 Cross aldol condensation in presence and absence of cat. 2

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In order to probe the selectivity in βꢀalkylation of secondary alcohols with primary
alcohols, we studied time dependent product distribution of this catalytic reaction. As
reported in Fig. 3, the concentrations of 1ꢀphenylethanol and benzyl alcohol gradually
decreased and concurrently, the concentration of 1,3ꢀdiphenylpropanꢀ1ꢀol (A) increased.
During the whole course of reaction, the concentration of 1,3ꢀdiphenylpropanꢀ1ꢀone (B) was
minimal and also did not change significantly. On the other hand, we never detected a trace
amount of crossꢀaldol product. This result advocated that with increasing reaction time the
selectivity of alcohol (A) increases; whereas throughout the process the concentration of keto
(B) remain the same.62
Fig. 3 Time dependent product distribution in βꢀalkylation of 1ꢀphenylethanol with benzyl
alcohol catalysed by complex 2
In the hydrogenation reaction excess of PPh3 significantly reduces the reaction rate for
a catalytic system following inner-sphere mechanism as ligand dissociation is considered as
an initiation step.63ꢀ68 In contrast, an excess of PPh3 has no significant effect on the catalytic
activity of a system following outer-sphere mechanism.69 In order to inspect which
mechanism our system was following, βꢀalkylation of 1ꢀphenylethanol with benzyl alcohol
was carried out using both the complexes 1 and 2 in presence of excess of PPh3 (2ꢀ8 eq.). The
rate of βꢀalkylation of secondary alcohol was significantly affected when excess of PPh3 was
added to the reaction mixture containing complex 1 (Fig. 4). After 3 h, in absence of
additional PPh3 conversion of 1ꢀphenylethanol was 77% with complex 1. But, under similar
reaction conditions with 8 eq. of PPh3 conversion of 1ꢀphenylethanol was decreased
drastically to 28%. However, with catalyst 2 exogenous PPh3 had no significant effect (Fig.
4). This result clearly validates that in the rate determined step dissociation of PPh3 did not
occur with the 2/NaOH system and probably it was following an outer-sphere mechanism.

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Fig. 4 Effect of externally added PPh3 (with respect to cat.) on cat. 1 and cat. 2 in βꢀalkylation
of 1ꢀphenylethanol with benzyl alcohol. Conditions: 0.1 mmol% cat., 1ꢀphenylethanol (0.654
mmol), benzyl alcohol (0.654 mmol), NaOH (0.327 mmol), toluene, 125 °C. Time for Cat. 1
(180 mins.) and Cat 2 (90 mins.).
Conclusion
A ruthenium(II) complex bearing a bifunctional phenpyꢀOH ligand was found to be a
highly efficient catalyst for the atom economical βꢀalkylation of secondary alcohols with
primary alcohols. This work demonstrates a new, practical and greener methodology to
construct CꢀC bonds using readily available, less toxic alcohols and produces only H2O as
byproduct. The metalꢀligand cooperativity in complex 2 accounts for the remarkably high
catalytic activity in coupling of a variety of secondary alcohols with numerous primary
alcohols to yield the corresponding βꢀalkylated alcohols with high selectivity. Notably,
double alkylation of cyclopentanol with various primary alcohols also proceeded smoothly.
Control experiments indicated involvement of the ruthenium catalyst in crossꢀaldol
condensation and that hydrogenation of the C=C bond of the α,βꢀunsaturated ketone was
much faster than the hydrogenation of the ketone with this system. Mechanistic studies with
excess of PPh3 in βꢀalkylation of 1ꢀphenylethanol with benzyl alcohol revealed that the
2/NaOH system was following an outer-sphere mechanism. To the best of our knowledge,
this present protocol is a rare oneꢀpot atom economical synthetic strategy for βꢀalkylation of
secondary alcohols with primary alcohols using a bifunctional Ru(II)ꢀcomplex which
exhibited highest reactivity among all the reported catalysts.
Experimental Section
General Procedures and Materials: All reactions were carried out under an inert
atmosphere using standard Schlenkꢀline techniques. Glasswares were flameꢀdried under
vacuum prior to use. Solvents were dried according to literature methods, distilled under
argon and deoxygenated prior to use. RuCl3.nH2O (39% Ru) and PdCl2 (60% Pd) were
purchased from Arora Matthey, India. 2ꢀbromo phenanthroline, 5ꢀmethoxyꢀ2ꢀ
(tributylstannyl)pyridine, 5ꢀmethylꢀ2ꢀ(tributylstannyl)pyridine, 2ꢀ(tributylstannyl)pyridine
were synthesized following the literature procedures.70ꢀ74 Synthesis of complexes 1 and 2
were already reported from our group.52 All the chemicals were purchased from Sigmaꢀ
Aldrich, Alfa Aesar, SDFCL and Spectrochem. 1H, 13C, 31P NMR spectra were recorded on
JEOL 400 and 500 MHz spectrometers. Elemental analyses was performed on a Thermoquest
EA1110 CHNS/O analyser. The crystallized compounds were powdered, washed several
times with dry diethyl ether and dried under vacuum for at least 48 h prior to elemental
analyses. ESIꢀMS were recorded on a Waters Micromass Quattro Micro tripleꢀquadrupole
mass spectrometer. All the GC analysis were done using Perkein Elmer Clarus 600 Gas

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Chromatograph and GCꢀMS were taken using Agilent 7890 A Gas Chromatograph equipped
with Agilent 5890 tripleꢀquadrupole mass system.
Synthesis of 2ꢀ(6ꢀmethylpyridinꢀ2ꢀyl)ꢀ1, 10ꢀphenanthroline: A mixture of 2ꢀbromo
phenanthroline (0.772 mmol, 0.20 g), 5ꢀmethylꢀ2ꢀtributhylstannylpyridine (1.544 mmol,
0.589 g) and Pd(PPh3)4 (0.077 mmol, 0.089 g) in 30 mL of toluene was heated at the reflux
temperature for 4 days. The mixture was allowed to cool to room temperature and the solvent
was removed in vacuum. The final product was purified by column chromatography (using
neutral alumina) with hexaneꢀethyl acetate as eluent. A light yellow product was obtained
1
(0.170 g, 81%). H NMR (400 MHz, CDCl3): δ = 9.21 (dd, JH,H = 4.12 Hz, JH,H = 2.28 Hz,
1H), 8.79 (d, JH,H = 8.24 Hz, 1H), 8.72 (d, JH,H = 7.76 Hz, 1H), 8.33 (d, JH,H = 8.24 Hz, 1H),
8.24 (dd, JH,H = 8.02 Hz, JH,H = 6.40 Hz, 1H), 7.82ꢀ7.75 (m, 3H), 7.63 (dd, JH,H = 8.24 Hz,
JH,H = 3.64 Hz, 1H), 7.21 (d, JH,H = 7.32 Hz, 1H), 2.67 (s, 3H) . 13C{1H} NMR (500 MHz,
CDCl3) : 157.9, 156.6, 155.5, 150.4, 146.4, 145.4, 137.2, 136.9, 136.2, 129.1, 128.7, 126.6,
123.8, 122.9, 121.0, 119.8, 24.7. ESIꢀMS: m/z = 273.1182 (100%, MH+).
Synthesis of 2ꢀ(pyridinꢀ2ꢀyl)ꢀ1,10ꢀphenanthroline : A mixture of 2ꢀbromo phenanthroline
(0.772 mmol, 0.2 g), 2ꢀtributhylstannylpyridine (1.543 mmol, 0.568 g) and Pd(PPh3)4 (0.077
mmol, 0.089 g) in 30 mL of toluene was heated at the reflux temperature for 3 days. The
mixture was allowed to cool to room temperature and the solvent was removed in vacuum.
The final product was purified by column chromatography (using neutral alumina) with
hexaneꢀethyl acetate as eluent. A cream white product was obtained (0.163 g, 82%). 1H NMR
(400 MHz, CDCl3): δ = 9.22 (dd, JH,H = 4.58 Hz, JH,H = 3.08 Hz, 1H), 8.98 (d, JH,H = 7.96
Hz, 1H), 8.79 (d, JH,H = 8.56 Hz, 1H), 8.72 (d, JH,H = 4.92 Hz, 1H), 8.36 (d, JH,H = 8.56 Hz,
1H), 8.26 (dd, JH,H = 8.24 Hz, JH,H = 6.72 Hz, 1H), 7.90 (dt, JH,H = 7.94 Hz, JH,H = 1.84 Hz,
1H ), 7.78 (d, JH,H = 8.56 Hz, 1H), 7.64 (dd, JH,H = 7.94 Hz, JH,H = 3.68 Hz, 1H), 7.36 (dt,
JH,H = 5.04 Hz, JH,H = 1.24 Hz, 1H ) . 13C{1H} NMR (500 MHz, CDCl3), δ = 156.2, 156.1,
150.4, 149.1, 146.3, 145.7, 137.1, 137.0, 136.3, 129.1, 128.8, 126.8, 126.6, 124.2, 123.0,
122.8, 120.9. ESIꢀMS: m/z = 258.1030 (100%, MH+).
Synthesis of transꢀRuCl(phenpyMe)(PPh3)2PF6 (3) : In a Schlenk flask 2ꢀ(6ꢀmethylpyridinꢀ
2ꢀyl)ꢀ1, 10ꢀphenanthroline (0.020 g; 0.074 mmol) and RuCl2(PPh3)3 (0.071 g; 0.074 mmol)
was taken and 40 mL dry methanol was added. The mixture was refluxed for 24 hours under
argon atmosphere. It was allowed to cool to room temperature and solid ammonium
hexafluorophosphate (0.240 g; 1.474 mmol) was added to the solution. The solution was
stirred at room temperature overnight, during which a red precipitate emerged. The
precipitate was filtered, washed with dry diethyl ether and hexane and dried under vacuum to
1
provide the title compound as a brick red powder. (0.041 g; 70 %). H NMR (500 MHz,
CD2Cl2), δ = 9.23 (d, JH,H = 5.2 Hz, 1H), 8.35 (d, JH,H = 8.19 Hz, 1H), 8.26 (d, JH,H = 8.75
Hz, 1H), 8.15 (d, JH,H = 8.75 Hz, 1H), 7.98 (d, JH,H = 7.55 Hz, 1H), 7.94 (bs, 2H), 7.86ꢀ7.77
(m, 2H), 7.45 (d, JH,H = 7.6 Hz, 1H), 7.35 (t, JH,H = 4.3 Hz, 6H), 7.28ꢀ7.15 (m, 12H), 6.92ꢀ
6.65 (m, 12H), 2.68 (s, 3H). 13C{1H} NMR (125 MHz, CD2Cl2) : δ = 157.72, 137.10, 135.36,
134.54, 134.15, 132.89, 132.72, 130.46, 130.42, 130.11, 129.11, 129.64, 128.82, 128.69,
128.21, 127.91, 127.86, 126.91, 125.26, 121.46, 120.84, 116.33, 28.21. 31P{1H} NMR, (202
ꢀ
MHz, CD2Cl2), δ = 17.90 (s, PPh3), ꢀ145.06 (sep., J = 703.6, PF6 ). ESIꢀMS: m/z = 932.1780
([MꢀPF6]+). Anal. Calculated (C54H43ClF6N3P3Ru) (Found): C, 60.20 (59.85); H, 4.02 (3.81);
N, 3.90 (3.62).
Synthesis of transꢀRuCl(phenpy)(PPh3)2PF6 (4) : In a Schlenk flask 2ꢀ(pyridinꢀ2ꢀyl)ꢀ1,10ꢀ
phenanthroline (0.020 g; 0.077 mmol) and RuCl2(PPh3)3 (0.075 g; 0.077 mmol) was taken

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and 40 mL dry methanol was added. The mixture was refluxed for 24 hours under argon
atmosphere. It was allowed to cool to room temperature and solid ammonium
hexafluorophosphate (0.253 g; 1.554 mmol) was added to the solution. The solution was
stirred at room temperature overnight, during which a red precipitate emerged. The
precipitate was filtered, washed with dry diethyl ether and hexane and dried under vacuum to
1
provide the title compound as a crimson red powder. Yield: (0.037 g; 74 %). H NMR (500
MHz, CD2Cl2): δ = 9.46 (d, JH,H = 4.60 Hz, 1H), 9.12 (d, JH,H = 5.00 Hz, 1H), 8.17 (d, JH,H
=
8.05 Hz, 1H), 7.90 (d, JH,H = 8.00 Hz, 1H), 7.79ꢀ7.69 (m, 4H), 7.56ꢀ7.50 (m, 2H), 7.19 (t,
JH,H = 5.9 Hz, 1H), 7.13 (t, JH,H = 7.30 Hz, 6H), 7.55ꢀ7.59 (m, 2H), 7.14 (t, JH,H = 7.2 Hz,
6H), 6.93 (t, JH,H = 7.6 Hz 12H), 6.82ꢀ6.86 (m, 12H), 7.07ꢀ7.04 (m, 12H), 6.96 (t, JH,H = 7.55
Hz, 12H). 13C{1H} NMR (100 MHz, CD2Cl2) : δ = 157.75, 156.99, 156.70, 156.10, 147.76,
136.72, 135.01, 132.58, 129.90, 129.52, 129.36, 129.20, 129.07, 128.35, 128.05, 127.98,
126.99, 126.93, 125.40, 122.68, 120.93. 31P{1H} NMR, 202 MHz (CD2Cl2), δ (ppm): 19.45
ꢀ
(s, PPh3), ꢀ145.08 (sep., J = 704.4, PF6 ). ESIꢀMS: m/z = 918.1582 ([MꢀPF6]+). Anal.
Calculated (C53H41ClF6N3P3Ru) (Found): C, 59.86 (59.57); H, 3.89 (3.73); N, 3.95 (3.57).
General Procedure for the Catalytic βꢀAlkylation of Alcohols: The catalyst solution was
prepared by dissolving complex 2 in CH3CN (1 mL) in an argon filled glovebox. Then red
catalyst solution (0.1 mol%) was taken into a Schlenk tube and solvent was removed under
vacuum. After that secondary alcohol (0.654 mmol), primary alcohol (0.654 mmol), NaOH
(0.327 mmol) and 3.0 ml toluene were added under an argon atmosphere. Then, the tube was
dipped at oil bath (the red color immediately changed to purple) and heated for 90 minutes at
°
125 C (oil bath temperature). It was cooled to room temperature and 10 ꢁL solution was
syringed out for GC analysis (faint purple color). The reaction mixture was concentrated
under reduced pressure and submitted crude for NMR to calculate the conversion and product
selectivity. The final desired alcohol (A, major) and ketone (B, minor) products were isolated
and purified by column chromatography (silica) using hexaneꢀethyl acetate as eluent.
General Procedure for the Catalytic Double βꢀAlkylation of Cyclopentanol: The catalyst
solution was prepared by dissolving complex 2 in CH3CN (1 mL) in an argon filled glovebox.
Then red catalyst solution (0.5 mol %) was taken into a Schlenk tube and solvent was
removed under vacuum. After that cyclopentanol (0.696 mmol), primary alcohol (1.46
mmol), NaOH (0.696 mmol) and 3.0 mL toluene were added under argon atmosphere. Then,
the tube was dipped at oil bath (the red color immediately changed to purple) and heated for
°
10 hours at 125 C (oil bath temperature). It was cooled to room temperature (faint purple
color), concentrated under reduced pressure and submitted crude for NMR to calculate the
conversion. The final desired products were isolated and purified by column chromatography
(silica) using hexaneꢀethyl acetate as eluent.
Associated Content
Supporting Information. General procedures for PPh3 dependence studies, control
experiments procedure, Hg0 poising experiment, characterization data and NMR spectra of
the products are available. This material is available free of charge via the Internet at
http://pubs.rsc.org.
Author Information
Corresponding Author
*Email: sabuj@iitk.ac.in; Tel.: +91ꢀ512ꢀ2597425.

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Notes
The authors declare no competing financial interests.
Acknowledgement
We are grateful to DSTꢀINSPIRE, Science and Engineering Research Board India and Indian
Institute of Technology Kanpur for financial support. K. C., B.P. thanks UGC India and
M.M., B.C.R., S.S. thanks CSIR India, for fellowships.
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Bifunctional Ru(II) complex (0.1 mol%) catalysed oneꢀpot βꢀalkylation of secondary alcohols
with primary alcohols and double alkylation of cyclopentanol were carried out successfully
following the atom economical borrowing hydrogen methodology. Notably, this greener
methodology displayed significantly higher reactivity among all the reported complexes
which exhibited remarkably high 31500 TON.