Pincer-Nickel Catalyzed Selective Guerbet-Type Reactions

Article abstract of DOI:10.1021/acs.organomet.1c00328

We report here the synthesis and characterization of a series of NNN pincer-nickel complexes of the type (R2NNN)NiCl2(CH3CN) (R = iPr, tBu, Cy, Ph, and p-F-C6H4) based on bis(imino)pyridine ligands. In solution, these complexes are found to be equilibrium mixtures containing one and two pincer ligands, respectively. While the crystal structure of the former was reported by us recently for R = iPr, we report the crystal structure of the latter in this study for R = p-F-C6H4. The considered NNN pincer-Ni complexes have been successfully employed to accomplish the catalytic β-alkylation of several secondary alcohols with a variety of benzyl alcohols at 140 °C with high yields and unprecedented turnovers. A maximum of 92% yield of the β-alkylated product at 18 ?400 TON was obtained in the reaction of benzyl alcohol with 1-(4-(trifluoromethyl)phenyl)ethane-1-ol in the presence of 0.005 mol % of (Ph2NNN)NiCl2(CH3CN) and 5 mol % of NaOtBu at 140 °C after 24 h. The reaction exhibits zero-order dependence of rate on catalyst concentration and first-order dependence on the concentration of base, benzyl alcohol, and 1-phenyl ethanol which points to the base-mediated aldol condensation as the rate-determining step. Most of the intermediates involved in catalysis have been identified by HRMS. To the best of our knowledge, this is the first report on a pincer-Ni catalyzed β-alkylation of alcohols and, hitherto, such unprecedented turnovers have not been reported with a homogeneous molecular nickel-based catalyst.

Full text of DOI:10.1021/acs.organomet.1c00328

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Pincer-Nickel Catalyzed Selective Guerbet-Type Reactions
Vinay Arora, Himani Narjinari, and Akshai Kumar*
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ABSTRACT: We report here the synthesis and characterization of
a series of NNN pincer-nickel complexes of the type (R2NNN)-
i
t
NiCl2(CH3CN) (R = Pr, Bu, Cy, Ph, and p-F-C6H4) based on
bis(imino)pyridine ligands. In solution, these complexes are found
to be equilibrium mixtures containing one and two pincer ligands,
respectively. While the crystal structure of the former was reported
i
by us recently for R = Pr, we report the crystal structure of the
latter in this study for R = p-F-C6H4. The considered NNN pincer-
Ni complexes have been successfully employed to accomplish the
catalytic β-alkylation of several secondary alcohols with a variety of
benzyl alcohols at 140 °C with high yields and unprecedented
turnovers. A maximum of 92% yield of the β-alkylated product at
18 400 TON was obtained in the reaction of benzyl alcohol with 1-(4-(trifluoromethyl)phenyl)ethane-1-ol in the presence of 0.005
mol % of (Ph2NNN)NiCl2(CH3CN) and 5 mol % of NaOtBu at 140 °C after 24 h. The reaction exhibits zero-order dependence of
rate on catalyst concentration and first-order dependence on the concentration of base, benzyl alcohol, and 1-phenyl ethanol which
points to the base-mediated aldol condensation as the rate-determining step. Most of the intermediates involved in catalysis have
been identified by HRMS. To the best of our knowledge, this is the first report on a pincer-Ni catalyzed β-alkylation of alcohols and,
hitherto, such unprecedented turnovers have not been reported with a homogeneous molecular nickel-based catalyst.
coupling of alcohols was recently reported by Lang and co-
workers (Figure 1) in which they utilized a Ni(II) 4,6-
dimethylpyrimidine-2-thiolate cluster catalyst toward the
synthesis of α,β-unsaturated ketones, α-alkylated ketones,
and β-alkylated alcohols by simple tuning of the reaction
conditions.14 Very recently, Balaraman reported excellent
yields (up to 90%) in the β-alkylation of various alcohols
using NiBr2 (5 mol %)/TMEDA (5 mol %) in the presence of
an equivalent of KOH using n-octane as solvent at 130 °C
(Figure 1).15 To the best of our knowledge, there are no other
homogeneous Ni systems (none with pincer-Ni,13r,16 in
particular) that have been reported for this reaction (Figure 1).
Encouraged by our recent success in obtaining unprece-
dented turnovers in the N-alkylation of alcohols using the
NNN pincer-nickel catalyst (iPr2NNN)NiCl2(CH3CN) (2a)
under solvent-free conditions,17 we wished to probe the
activity of these pincer-nickel systems toward the synthetically
valuable β-alkylation of alcohols. In this study, we report the
synthesis of a series of pincer-nickel complexes of the type
INTRODUCTION
■
The formation of C−C bonds plays an important role in
synthetic organic chemistry having varied applications in fuel,
fine chemicals, medicinal, agrochemicals, pharmaceuticals, and
many others.1 A plethora of reactions that lead to C−C bonds
majorly involve transition-metal catalyzed C-H or C-X (X = Cl,
Br, I) activation operating via radical2 or purely organometallic
pathways.3 The direct coupling of primary and secondary
alcohols to yield β-alkylated alcohols has been extensively
studied in the past decade via the hydrogen borrowing
pathway4 which has been pegged as one of the most important
green chemistry research areas recently.5 It is a useful and an
environmentally friendly process, in which alcohols are used as
an alkyl precursor and water is the sole byproduct.
In the absence of any metal-based catalysts, the β-alkylation
of alcohols6 requires a stoichiometric amount of base that
results in an equivalent amount of waste formation. The β-
alkylation of alcohols has been widely reported with catalysts
based on precious metals such as Ru,7 Rh,8 Ir,7a,m,n,9 and Pd.10
Recently, first-row transition-metal catalysts have also been
employed for these Guerbet-type reaction.11 Considering the
fact that one of the early and finest examples of the Guerbet
reaction came from the use of Raney nickel in the presence of a
strong base;12 it is interesting to note that the corresponding
reports with homogeneous Ni catalysts are very sparse and
mainly devoted to α-alkylation of alcohols and ketones apart
from related C−C bond forming reactions.13 The C-C cross-
i
t
(R2NNN)NiCl2(CH3CN) (2a, R = Pr; 2b, R = Bu; 2c, R =
Received: June 3, 2021
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Scheme 1. A General Pathway to the Synthesis of
a
(R2NNN)NiCl2(CH3CN) (2a−e)
Figure 1. Homogeneous Ni catalysts reported for β-alkylation of 1-
phenyl ethanol with benzyl alcohol.
a
The molecular structure of 3e is provided as an ORTEP drawn at
50% probability. All the hydrogen atoms and the aromatic groups on
two N atoms of one of the pincer fragment are omitted for the sake of
clarity. The molecular structure of previously reported 2a′, an aqua-
derivative of 2a, is also provided.
Cy; 2d, R = Ph; and 2e, R = p-F-C6H4) and their utility in
catalyzing the β-alkylation of alcohols (Figure 2). It is
gratifying to note that among 2a−2e, the catalyst 2d (0.005
mol %) is highly active toward the Guerbet-type reaction of
various benzyl alcohols with several 1-phenyl ethanols at 140
°C under neat conditions in the presence of only 5 mol %
NaOtBu. In particular, 18 400 TON (ca. 92% yield) was
obtained in the β-alkylation of benzyl alcohol with 1-(4-
(trifluoromethyl)phenyl)ethane-1-ol at 140 °C after 24 h.
(even at very low temperatures, Figures S95−S98) which is
typical of an octahedral d8 Ni(II) species.20
The single-crystal X-ray analysis of 2a′ (obtained by the
substitution of acetonitrile in 2a by a water molecule) reported
by us earlier17 showed Ni is in an octahedral environment
having the pincer ligand bound in a meridional fashion with
the two chlorides trans to each other. Surprisingly,
crystallization of 2e via slow evaporation from methanol led
to the isolation of a dicationic complex 3e (presumably formed
due to its equilibration with 2e in the mother liquor (Scheme
1)) where the Ni is in an octahedral environment with two
pincer ligands attached to it in a meridional geometry (Scheme
1, Tables S1 and S2). While the complex (iPr2NNN)-
NiCl2(H2O) (2a′) crystallized in the C2/c space group, the
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Pincer-Nickel
Complexes Based on Bis(imino)pyridine Ligands. The
NNN pincer ligands18 and their corresponding Ni complexes17
were synthesized according to our recently reported protocol.
Treatment of anhydrous NiCl2(DME) (DME = dimethoxy-
ethane) with (R2NNN) ligand in acetonitrile at room
temperature for 20 h, followed by washing with diethyl
ether, afforded the corresponding Ni(II) pincer complexes
(2a−e) in good yields (Scheme 1). While NMR analysis was
not possible for these paramagnetic complexes, the magnetic
susceptibility measurements of these complexes provided
evidence for their octahedral structure and paramagnetic
nature (2a, μeff = 3.21 μB; 2b, μeff = 3.20 μB; 2c, μeff = 3.28 μB;
2d, μeff = 3.01 μB; 2e, μeff = 4.24 μB).19 The EPR signals of the
octahedral complexes 2a−e were broad and hard to detect
̅
dicationic complex 3e crystallized in the triclinic P1 space
group. The Ni−N(pyridyl) bond distance was slightly longer
in the case of neutral complex 2a′ (2a′, 2.008(6) Å; 3e,
1.983(3) Å). On the other hand, the Ni−N(imine) bond
length was similar in both the complexes (2a′, 2.171(5) Å; 3e,
2.172(3) Å). The (pyridyl)N−Ni−N(imine) bond angles in
both the complexes were comparable (2a′: 77.60(5)°; 3e:
Figure 2. Pincer-nickel complexes investigated in the current study for the β-alkylation of 1-phenyl ethanol with benzyl alcohol.
B
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77.73(12)°). A detailed comparison of the crystallographic
parameters of 2a′ and 3e is provided in Tables S1 and S2.
Interesting observations were made from the HRMS analysis
that provided evidence for the presence of fragments
originating from both 2 and 3 (Figures 3 and S57−S79).
To arrive at the optimal conditions, the catalytic β-alkylation
reactions were initiated in the presence of 2a−e along with a
variety of bases using 1-phenyl ethanol and benzyl alcohol as a
model secondary and primary alcohol, respectively, at 140 °C
(Table 1 and Table S3). At a loading of 0.01 mol % of 2d, the
Table 1. Solvent-Free β-Alkylation of 1-Phenyl Ethanol with
a
Benzyl Alcohol under Varying Conditions
Figure 3. HRMS analysis of pincer-Ni complexes 2/3. Also see
Figures S57−S79.
For instance, the HRMS analysis of 2b showed peaks at m/z
values 274.1783 and 338.1178 that correspond to [3b − 2Cl]2+
and [2b − CH3CN − Cl]+, respectively. Similar observations
were made in the HRMS analysis of the other considered
complexes (Figures 3 and S57−S79). Single-crystal X-ray
analysis and HRMS studies thus clearly indicate that, in
solution, the complex 2 exists in equilibrium with 3 (Scheme
1), the extent of which is likely to be different for various
i
t
complexes based on the nature of the R group (R = Pr, Bu,
Cy, Ph, and p-F-C6H4). The pincer-Ni complexes (2a−e)/
(3a−e) demonstrated very good thermal stability as indicated
by the TGA analysis (Figure 4). The mass loss21 could be
correlated to loss of fragments either from 2 or from 3 (Figure
4 and Figure S56). The TGA profile of 2a has been previously
discussed.17
a
Reaction conditions: 4.14 mmol of 4, 4.14 mmol of 5, X mol % of
base, and Y mol % of Ni catalyst at 140 °C. For selectivity of 7, please
see Table S3. Yield determined from H NMR using toluene as
b
1
c
external standard. Yield reported as an average of two runs.
d
Reaction was performed at 120 °C.
Investigations on the Pincer-Nickel Catalyzed β-
Alkylation of 1-Phenyl Ethanol with Benzyl Alcohol.
reaction of 4 with 5 did not proceed when 5 mol % of either
Na2CO3 or K2CO3 was used (entries 1 and 2, Table 1). Poor
yields of β-alkylated product 7 was observed when the 2d
(0.01 mol %) catalyzed reaction was performed independently
with KOtBu (5 mol %) and KOH (5 mol %) (entries 3 and 4,
Table 1). On the other hand, use of 5 mol % of NaOH
provided moderate yields (68%) of 7 in the 2d (0.01 mol %)
catalyzed reaction (entry 5, Table 1). The yields of 7 dropped
steadily upon lowering the NaOH loading (entries 6 and 7,
Table 1). Employing Na (5 mol %) to generate the base in
situ22 (prior to addition of 2d) resulted in yields that are
comparable to that obtained with the use of NaOH (5 mol %)
(entry 5 vs entry 8, Table 1). The yield of 7 improved (75%)
with the use of NaOtBu (5 mol %) in the 2d (0.01 mol %)
catalyzed β-alkylation of 5 with 4 (entry 9, Table 1).
Similar to the observations made during the use of NaOH,
lowering the amounts of NaOtBu led to reduced yields (entries
10 and 11, Table 1). Lowering the loading of 2d to 0.005 mol
Figure 4. TGA analysis of pincer-Ni complexes 2/3. Also see Figure
S56.
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% while maintaining NaOtBu at 5 mol % resulted in about 82%
of 7 and 11% of 6 that amounts to a total of 18 600 TON (6 +
7) (entry 12, Table 1). Repeating the same reaction at a lower
temperature (120 °C) or with a different base (5 mol %
NaOH) resulted in a decrease in productivity of 7 (entries 13
and 15, Table 1). The β-alkylation of 5 with 4 did not proceed
either in the absence of a base (entry 14, Table 1) or in the
absence of a catalyst.7d Under the optimized conditions
comprising 0.005 mol % of Ni catalyst in the presence of
NaOtBu (5 mol %) at 140 °C, the total turnovers (6 + 7)
obtained were clearly lower with the other considered catalysts
(entries 16−21, Table 1) in comparison with 2d (entry 12,
Table 1).
Table 3. 2d Catalyzed Solvent-Free β-Alkylation of 1-Phenyl
Ethanol Derivatives with Benzyl Alcohol
a
The practical utility of this reaction was confirmed by
carrying out the 2d (0.005 mol %) catalyzed β-alkylation of 1 g
of 5 with 0.895 g of 4 in the presence of 5 mol % of NaOtBu at
140 °C to obtain 1.301 g of 7 in 74% isolated yield and 85%
selectivity. The synthetic utility of the optimized catalytic
system (entry 12, Table 1) was further investigated for the
catalytic β-alkylation of several 1-phenyl ethanol derivatives
with a variety of benzyl alcohols (Table 2, Table S4, Table 3,
and Table S5).
In most cases, good tolerance was observed for the electron-
withdrawing (−Cl, −F) and electron-donating (−Me, −OMe)
groups at the para and meta positions of the phenyl ring in
primary alcohols, affording good yields of the desired products
(7a−7g, Table 2) with high selectivity (up to 98%, Table S4).
a
Table 2. 2d Catalyzed Solvent-Free β-Alkylation of 1-Phenyl
Ethanol with a Variety of Benzyl Alcohols
Reaction conditions: 4.14 mmol of 4, 4.14 mmol of 5, 5 mol % of
a
b
NaOtBu, and 0.005 mol % of 2d at 140 °C (Table S5). Yield
determined from H NMR using toluene as external standard.
1
A decrease in product yield (7h−7l, Table 2) barring 7l was
observed when heteroaromatic primary alcohols were used as
alkylating agents presumably due to inhibition of Ni by the
heteroatoms. The inhibition affect appears to be more
pronounced in the case of 7h that is capable of forming a
chelate with the Ni center. The formation of 7l in good yields
points to the poor inhibition of the Ni(II) by the relatively soft
S. Aromatic primary alcohols consisting of naphthyl and
anthracyl groups could be used as alkylating agents with
moderate yields (7m,n, Table 2). Lower yields were observed
upon use of primary aliphatic alcohols (7v, 7o, and 7p, Table
2).
A general trend of good yields was observed across various
1-phenylethanol substrates using benzyl alcohol as alkylating
agent in the 2d (0.005 mol %) catalyzed β-alkylation at 140 °C
(Table 3 and Table S5). However, in particular, the presence
of electron-withdrawing groups in the meta position (−Cl and
CF3 in 7y and 7zd, respectively, Table 3) and in the para
position (−F, −OCF3, and NO2 in 7t, 7ze, and 7zf,
respectively, Table 3) leads to lower yields of products. A
lower yield was also obtained for the β-alkylation of an
aliphatic secondary alcohol (7zg, Table 3).
Mechanistic studies (vide infra) have indicated that the aldol
condensation of benzaldehyde 4′ with acetophenone 5′ is the
rate-determining step (RDS). Apparently, electron-withdraw-
ing groups on 5 are likely to have a detrimental effect on the
overall yield of the reaction. Accordingly, very poor yields of 7t
were obtained as a result of the highly electronegative fluoro
group in the para position that withdraws electrons by an
inductive effect. The compound 7u that had a less electro-
a
Reaction conditions: 4.14 mmol of 4, 4.14 mmol of 5, 5 mol % of
b
NaOtBu, and 0.005 mol % of 2d at 140 °C (Table S4). Yield
determined from H NMR using toluene as external standard.
1
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negative chloro group was obtained in better yields. The higher
yields of 7u in comparison with 7y can be traced to the fact
that the inductive effect of the chloro group is more
pronounced at the meta position as compared to the para
position considering the fact that the inductive effect of a
substituent is directly proportional to the distance.23 However,
upon replacement of chloro with a poorer electron-with-
drawing and less electronegative bromo substituent, the
inductive effect is hardly noticeable. Not surprisingly, the
yields of the corresponding para- and meta-substituted bromo
derivatives 7w and 7x are comparable. The −CF3 group
demonstrates an electron-withdrawing nature only by an
inductive effect which is more significant in the meta position
in comparison to the para position. Rightly, the yield of 7zd
was poorer than that of 7zc. However, in the case of 7zf, the
nitro group exhibits a very strong electron-withdrawing
character due to the involvement of both inductive and
resonance activities that result in a complete mitigation of
reactivity.
Scheme 3. Plausible Mechanism Involved in the 2/3
Catalyzed β-Alkylation of 5 with 4
Control Experiments and Mechanistic Insights. We
have recently demonstrated the hydrogen evolution in the 2a
catalyzed dehydrogenation of 4 with an associated formation of
benzaldehyde 4′ (eq 1, Scheme 2).17 Similarly, one could
hydride elimination from 9/8, followed by extrusion of 4′/5′,
results in the formation of a pincer Ni-H species 10 similar to
that reported by us recently (Scheme 3).17 The active species
9/8 is regenerated by the alcoholysis of 10 with 4/5 along with
the liberation of H2. In the presence of NaOtBu, the aldol
reaction of 4′ with 5′ results in the formation of α,β-
unsaturated ketone 6′.
Scheme 2. Control Experiments
Insertion7d of the C−C double bond in 6′ into the Ni−H
bond of 10 results in the formation of intermediate 11
(Scheme 3). The carbonyl compound 6 is obtained from 11
either by the hydrogenolysis with H2 or by the alcoholysis with
4/5 while regenerating 10/9/8. A similar insertion, followed
by a hydrogenolysis/alcoholysis pathway involving 10→12→
10/9/8, can account for the transformation of 6 to 7 (Scheme
3). We have previously shown that both hydrogenolysis and
alcoholysis contribute to the 2a catalyzed N-alkylation
reactions under open-vessel conditions.17 The fact that we
observed 7 as a major product in our current studies that are
performed in an open-vessel further fortifies the involvement of
alcoholysis.
Valuable information on the intermediates proposed in
Scheme 3 was obtained from the HRMS(ESI) analysis
performed by periodic sampling of the reaction between 4
and 5 in the presence of 0.5 mol % of 2d and 5 mol % of
NaOtBu. The HRMS analysis of the reaction mixture at t = 0
(Figure 5) contained several adducts of 4/5 with pincer-Ni
species such as [8a + CH3CN + K + H]2+, [8b + CH3CN +
H2O + H]+, [8b + CH3OH + CH3OK]+, and [8b + CH3CN +
CH3OK + H]+ corresponding to peaks at m/z 326.1919,
645.4985, 687.3524, and 697.3808, respectively. The HRMS
profile at t = 2 h (Figure 5) provided key evidence to the
intermediates proposed in Scheme 3 and demonstrated peaks
at m/z 681.3039, 764.5815, 771.3520, and 859.3856 that
correspond to [12b + CH3CN + 3H2O + H]+, [12a + CH3OH
+ H2O + K]+, [12a + 3CH3OH]+, and [12a + 5′ +
2CH3OH]+, respectively. The profile of HRMS analysis at t
= 4, 8, 16, and 24 h (Figures S80−S94) were similar to that
observed at t = 2 h, and some of them contained an additional
peak at m/z = 209.0986 corresponding to [6′ + H]+ (Figures
S89, S92, and S93).
anticipate formation of 5′ and hydrogen in the 2 catalyzed
dehydrogenation of 5. The reaction of 4′ and 5′ in the
presence of catalytic amounts of base yields the α,β-
unsaturated ketone 6′ (eq 2, Scheme 2).7d Furthermore, 6′
has been detected in the reaction mixture by HRMS
experiments (Figures S89, S92, and S93). Transfer hydro-
genation of 6′ with 4 results in the formation of a mixture of 6
(9% isolated yield with respect to 6′) and 7 (40% isolated yield
with respect to 6′) (eq 3, Scheme 2). The above observations
in addition to the fact that both 6 and 7 are isolated in minor
and major amounts, respectively (Table 1), form the basis of
the proposed mechanism (Scheme 3) for the 2 catalyzed β-
alkylation of 5 with 4 under open-vessel conditions.
Treatment of the NNN pincer-Ni complex with NaOtBu in
the presence of 4/5 results in the formation of 9/8 by the
dissociation of either CH3CN from 2 or the ligand 1 from 3
along with the formation of NaCl (Scheme 3).17 The β-
Kinetic studies were carried out for the 2d catalyzed reaction
of 4 with 5 in the presence of 5 mol % of NaOtBu at 140 °C
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X-axis, implying a zero-order dependence of rate on catalyst
concentration (Figure 6). On the other hand, the correspond-
ing plots of initial rate vs concentrations of base, 4, and 5 were
all linear and nearly passing through the origin. This indicates
that the rate has a first-order dependence on the concen-
trations of base and primary and secondary alcohols.
This can be explained only if one invokes the possibility of a
fast dehydrogenation (4/5→4′/5′ + H2) and hydrogenolysis
(6′ and 6 with H2)/alcoholysis (6′ and 6 with 4/5) steps.
Apparently, the base mediated coupling of benzaldehyde 4′
and acetophenone 5′ that gives α,β-unsaturated ketone 6′ is
the rate-determining step. Not surprisingly, 6′ (Figures S89,
S92, and S93) and its Ni-adducts 11a and 11b (Figures S89−
S94) are detected in HRMS analysis.
Figure 5. HRMS analysis of the reaction mixture containing 4 and 5
in the presence of 0.5 mol % 2d and 5 mol % NatBuO at t = 0 h at
room temperature and at t = 2 h at 140 °C. Also see Figures S80−
S94.
CONCLUSIONS
■
We have accomplished the synthesis of a series of NNN
pincer-nickel complexes of the type (R2NNN)NiCl2(CH3CN)
i
t
(R = Pr, Bu, Cy, Ph, and p-F-C6H4) based on bis(imino)-
pyridine ligands. Single-crystal, HRMS, and TGA analyses
reveal that these complexes exist as equilibrium mixtures of
neutral and dicationic pincer-nickel complexes containing one
and two pincer ligands, respectively. Among the five pincer-Ni
(Figures S99−S108). Notably, upon use of 0.005 mol % of 2d,
a very high initial rate of 3200 TOh−1 was observed (Figure
S99). Using the initial rate method, we observed that the plot
of initial rate vs [2d] was a straight line that ran parallel to the
Figure 6. Variation of initial rate of Guerbet reaction with concentration of (a) 2d, (b) NaOtBu, (c) benzyl alcohol, and (d) 1-phenyl ethanol.
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0.15 mmol) in anhydrous acetonitrile (2 mL) for 20 h at room
temperature. The solvent was evaporated under reduced pressure, and
the mustard solid was washed with diethyl ether (3 × 3 mL). The
solid was dried under vacuum and isolated as a light mustard solid
(0.039 g) in 61% yield. HRMS (ESI): m/z calculated for [3b −
2Cl]2+: 274.1569, found 274.1783; m/z calculated for [2b − Cl −
CH3CN]+: 338.0934, found 338.1178; m/z calculated for [(2b − 2Cl
− CH3CN) + HCOO ]+: 348.1222, found 348.1521; m/z calculated
for [3b + H]+: 619.2593, found 619.4700. m/z calculated for [3b + H
+ 2H2O + CH3OH + CH3CN]+: 728.3332, found 728.5434.
Magnetic susceptibility μeff = 3.20 μB
complexes that have been screened for the catalytic β-
alkylation in the presence of 5 mol % of NaOtBu at 140 °C,
(
Ph2NNN)NiCl2(CH3CN) (0.005 mol %) was the most
efficient catalyst, giving up to 92% yield (ca. 18 400 TON)
for a combination of benzyl alcohol and 1-(4-(trifluoro-
methyl)phenyl)ethane-1-ol. Kinetic studies on the (Ph2NNN)-
NiCl2(CH3CN) catalyzed β-alkylation of 1-phenyl ethanol
with benzyl alcohol revealed a first-order dependence of rate
on the concentration of base, first-order dependence on both
the alcohols, and zero-order dependence on catalyst concen-
tration. This is indicative of a base-mediated aldol
condensation as the rate-determining step. HRMS analysis
proved to be a useful tool in the identification of several
intermediates that are involved in the catalytic cycle. For the
first time, we have reported the application of a pincer-Ni
catalyst for the Guerbet-type reactions which gives the highest
TON reported hitherto among homogeneous Ni-based β-
alkylation systems.
Synthesis of Complex (Cy2NNN)NiCl2(CH3CN) (2c). The ligand
1c (0.077 g, 0.26 mmol) was stirred with NiCl2(DME) (0.057 g, 0.26
mmol) in anhydrous acetonitrile (2 mL) for 20 h at room
temperature. The solvent was evaporated under reduced pressure,
and the light green solid was washed with diethyl ether (3 × 3 mL).
The solid was dried under vacuum and isolated as a light green solid
(0.057 g) in 47% yield. HRMS (ESI): m/z calculated for [3c −
2Cl]2+: 326.1882, found 326.1894; m/z calculated for [2c − Cl −
CH3CN]+: 390.1247, found 390.1240; m/z calculated for [3c − Cl]+:
687.3452, found 687.3447. Magnetic susceptibility μeff = 3.28 μB
Synthesis of Complex (Ph2NNN)NiCl2(CH3CN) (2d). The ligand
1d (0.057 g, 0.20 mmol) was stirred with NiCl2(DME) (0.044 g, 0.20
mmol) in anhydrous acetonitrile (2 mL) for 20 h at room
temperature. The solvent was evaporated under reduced pressure,
and the light orange solid was washed with diethyl ether (3 × 3 mL).
The solid was dried under vacuum and isolated as a light orange solid
(0.041 g) in 89% yield. HRMS (ESI): m/z calculated for [3d −
2Cl]2+: 314.0943, found 314.0944; m/z calculated for [2d − Cl −
CH3CN]+: 378.0808, found 378.0276; m/z calculated for [3d − Cl]+:
663.1574, found 663.1526. Magnetic susceptibility μeff = 3.01 μB
Synthesis of Complex ((p‑F‑Ph)2NNN)2Ni]Cl2 (2e). The ligand 1e
(0.048 g, 0.15 mmol) was stirred with NiCl2(DME) (0.033 g, 0.15
mmol) in anhydrous acetonitrile (2 mL) for 20 h at room
temperature. The solvent was evaporated under reduced pressure,
and the orange solid was washed with diethyl ether (3 × 3 mL). The
solid was dried under vacuum and isolated as an orange solid (0.040
g) in 69% yield. Crystals suitable for X-ray analysis were obtained by
slow evaporation of a solution containing 2e (10 mg) in 1 mL of
methanol under noninert conditions. HRMS (ESI): m/z calculated
for [3e − 2Cl]2+: 350.0755, found 350.0788; m/z calculated for [2e −
Cl − CH3CN]+: 414.0120, found 414.0128; m/z calculated for [3e −
Cl]+: 735.1197, found 735.1196. Magnetic susceptibility μeff = 4.24
μB.
EXPERIMENTAL SECTION
■
General Procedure and Materials. All manipulations were
carried out under an Ar atmosphere in a glovebox or by using a
standard double manifold. The nickel precursor, NiCl2(DME), was
purchased from Sigma-Aldrich. Benzyl alcohol, acetonitrile, and
hexane were purchased from MERCK and were dried according to a
literature procedure prior to experiment.24 Other chemicals were
purchased from MERCK or Sigma-Aldrich and used as such. All
catalytic reactions were carried out under an argon atmosphere using
dried glassware. The ligands (1a−e)18 and complex 2a17 were
prepared according to literature procedures.
Physical Measurements. 1H, 13C{H}, and 19F NMR were
1
recorded on a Bruker ASCEND 600 operating at 600 MHz for H,
150 MHz for 13C{H}, and 565 MHz for 19F or on a Bruker AVANCE
1
400 operating at 400 MHz for H, 100 MHz for 13C{H}, 377 MHz
for 19F or on a Bruker AVANCE 500 operating at 500 MHz for H,
1
125 MHz for 13C{H}, 471 MHz for 19F. Spin−spin coupling
constants (J) are expressed in Hz; chemical shifts (δ) are reported in
ppm. Other data are reported as follows: s = singlet, d = doublet, t =
triplet, m = multiplet, q = quartet, and br s = broad singlet. HRMS
measurements were done using an Agilent Accurate-Mass Q-TOF
ESI−MS 6520. X-ray crystallographic data were acquired on a Bruker
D8 Venture single-crystal X-ray diffractometer using graphite-
monochromated Mo Kα radiation. The data refinement and cell
reductions were carried out by the Bruker SAINT program.25
Structures were further solved and refined by the full matrix least-
squares method using SHELXS-14.26 A JES-FA200 ESR spectrometer
was use to record the X-band EPR spectra. Thermogravimetric
analyses were performed using a thermal analyzer (SDTQ600) with a
simultaneous DTA/TGA system, under nitrogen with a heating rate
of 10 °C min−1. Solid-state magnetic susceptibilities of the complexes
at room temperature were recorded using a Sherwood Scientific
magnetic balance MSB-1.
General Procedure for the Pincer-Nickel Catalyzed β-
Alkylation of Alcohols. In a 10 mL two-neck round-bottom flask
was added NaOtBu (0.04 g, 0.416 mmol) inside the glovebox. This
was followed by addition of 0.005 mol % of 2d (0.0002 g, 0.44 μmol)
(from a stock solution in either benzyl alcohol or 1-phenyl ethanol)
under an Ar atmosphere. Subsequently, the reaction mixture as made
up with the required amounts of benzyl alcohol (4) and 1-phenyl
ethanol (5). Ultimately, the reaction mixture contained 0.430 mL of 4
(4.14 mmol) and 0.500 mL of 5 (4.14 mmol). The mixture was
heated at 140 °C for 24 h and was then cooled down to room
temperature. An aliquot (10 mg) was withdrawn from the reaction
mixture, and the NMR yield was determined by 1H NMR using
CDCl3 as solvent and toluene as external standard (10 μL added in
the NMR tube). The rest of the reaction mixture was quenched with
water, followed by extraction of the organic fraction with dichloro-
methane. The organic phase was separated and was dried over
anhydrous Na2SO4. The solvent was removed from the organic
fraction under reduced pressure. Silica gel column chromatography
using 0−5% ethyl acetate in hexane as eluent gave the product 7 in a
pure form.
Synthesis of (1E,1′E)-1,1′-(Pyridine-2,6-diyl)bis(N-(4-fluoro-
phenyl)methanimine) (1e). The ligand 1e was prepared according
to the procedure reported in literature.18 The reaction of pyridine-2,6-
dicarbaldehyde (0.1 g, 0.529 mmol) with 4-fluoroaniline (0.117 g,
1.06 mmol) in anhydrous dichloromethane containing molecular
sieves (4 Å) for 12 h at 40 °C, followed by filtration and removal of
solvent, afforded the ligand 1e as light-yellow powder (0.102 g) in
60% yield. 1H NMR (400 MHz, CDCl3): δ 8.66 (s, 2H), 8.27 (d, J =
7.8 Hz, 2H), 7.94 (t, J = 7.8 Hz, 1H), 7.35−7.29 (m, 4H), 7.12 (t, J =
8.6 Hz, 4H). 13C{H} NMR (101 MHz, CDCl3): δ 163.25, 160.81,
159.85, 154.71, 146.90, 137.52, 123.40, 122.98, 122.90, 116.34,
116.12. 19F NMR (377 MHz, CDCl3): δ −115.59. HRMS (ESI): m/z
calculated for [1e + 4H2O]+: 393.1500, found 393.1826.
1,3-Diphenylpropan-1-ol (7). 1H NMR (600 MHz, CDCl3): δ
7.39−7.36 (m, 4H), 7.32−7.29 (m, 3H), 7.22−7.20 (m, 3H), 4.69
(m, J = 6.0 Hz, 1H), 2.79−2.74 (m, 1H), 2.71−2.66 (m, 1H), 2.18−
2.12 (m, 1H), 2.07−2.02 (m, 1H). 13C{H} NMR (151 MHz,
CDCl3): δ 144.65, 141.88, 128.63, 128.55, 128.50, 127.75, 126.04,
Synthesis of Complex (tBu2NNN)NiCl2(CH3CN) (2b). The ligand
1b (0.037 g, 0.154 mmol) was stirred with NiCl2(DME) (0.034 g,
G
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Organometallics XXXX, XXX, XXX−XXX

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Article
125.96, 73.97, 40.55, 32.15. HRMS (ESI): m/z calculated for [M +
Na]+: 235.1099, found 235.0846.
1-Phenyl-3-(thiophen-2-yl)propan-1-ol (7l). 1H NMR (500 MHz,
CDCl3): δ 7.27−7.19 (m, 5H), 7.02−7.01 (m, 1H), 6.83−6.82 (m,
1H), 6.71 (s, 1H), 4.61 (s, 1H), 2.88−2.77 (m, 2H), 2.11−1.94 (m,
3H). 13C{H} NMR (126 MHz, CDCl3): δ 144.73, 144.46, 128.66,
127.81, 126.87, 126.00, 124.43, 123.20, 73.60, 40.78, 26.32.
3-(Naphthalen-1-yl)-1-phenylpropan-1-ol (7m). 1H NMR (600
MHz, CDCl3): δ 7.98 (d, J = 6.0 Hz, 1H), 7.85−7.81 (m, 1H), 7.71
(d, J = 12.0 Hz, 1H), 7.49−7.45 (m, 2H), 7.40−7.34 (m, 5H), 7.31−
7.28 (m, 1H) 4.81−4.80 (m, 1H), 3.28−3.23 (m, 1H), 3.14−3.09 (m,
1H), 2.29−2.23(m, 1H), 2.19−2.14 (m, 1H), 1.93 (d, J = 3.1 Hz,
1H). 13C{H} NMR (151 MHz, CDCl3): δ 144.65, 138.13, 134.04,
131.96, 128.90, 128.69, 127.84, 126.82, 126.09, 125.93, 125.68,
125.59, 123.91, 74.34, 39.97, 29.26.
1-Phenyl-3-(p-tolyl)propan-1-ol (7a). 1H NMR (400 MHz,
CDCl3): δ 7.44−7.38 (m, 4H), 7.34 (ddd, J = 8.6, 3.7, 2.1 Hz,
1H), 7.15 (brs, 4H), 4.72 (ddd, J = 8.1, 5.3, 3.1 Hz, 1H), 2.80−2.64
(m, 2H), 2.39 (s, 3H), 2.22−2.02 (m, 3H). 13C{H} NMR (151 MHz,
CDCl3): δ 144.72, 138.76, 135.42, 129.20, 128.63, 128.43, 127.74,
126.06, 74.02, 40.68, 31.72, 21.12.
1-Phenyl-3-(m-tolyl)propan-1-ol (7b). 1H NMR (400 MHz,
CDCl3) δ 7.25 (d, J = 4.3 Hz, 4H), 7.21−7.16 (m, 1H), 7.11−7.06
(m, 1H), 6.93−6.89 (m, 4H), 4.58 (ddd, J = 7.9, 5.3, 2.3 Hz, 1H),
2.67−2.48 (m, 2H), 2.23 (s, 3H), 2.08−1.92 (m, 2H), 1.91 (brs, J =
3.0 Hz, 1H). 13C{H} NMR (126 MHz, CDCl3) δ 144.71, 141.82,
137.98, 129.34, 128.56, 128.37, 127.66, 126.67, 126.03, 125.53, 73.99,
40.57, 32.06, 21.46.
3-(Anthracen-9-yl)-1-phenylpropan-1-ol (7n). 1H NMR (600
MHz, CDCl3): δ 8.33 (s, 1H), 8.18 (d, J = 8.6 Hz, 2H), 7.99 (d, J
= 8.1 Hz, 2H), 7.46 (dt, J = 16.6, 7.0 Hz, 6H), 7.39 (t, J = 7.5 Hz,
2H), 7.32 (t, J = 7.3 Hz, 1H), 4.94 (p, J = 3.7 Hz, 1H), 3.78 (ddd, J =
13.4, 11.2, 5.0 Hz, 1H), 3.65 (ddd, J = 13.8, 10.9, 6.0 Hz, 1H), 2.34−
2.26 (m, 1H), 2.22 (ddd, J = 13.9, 10.6, 5.4 Hz, 1H), 2.05 (brs, 1H).
13C{H} NMR (151 MHz, CDCl3) δ 144.60, 134.47, 131.76, 129.72,
129.33, 128.73, 127.92, 126.10, 125.92, 125.66, 124.96, 124.46, 74.57,
40.18, 24.12.
1
3-(4-Methoxyphenyl)-1-phenylpropan-1-ol (7c). H NMR (400
MHz, CDCl3): δ 7.43−7.35 (m, 5H), 7.20−7.18 (m, 2H), 6.93−6.91
(m, 2H), 4.70 (t, J = 4.0 Hz, 1H), 3.83 (s, 3H), 2.80−2.65 (m, 2H),
2.19−2.04 (m, 2H). 13C{H} NMR (151 MHz, CDCl3): δ 157.87,
144.73, 133.91, 129.44, 128.62, 127.72, 126.05, 113.91, 73.94, 55.37,
40.80, 31.24. HRMS (ESI): m/z calculated for [M + Na]+: 265.1204,
found 265.1055.
3-Phenyl-1-(p-tolyl)propan-1-ol (7q). 1H NMR (500 MHz,
CDCl3): δ 7.18−7.15 (t, J = 7.5 Hz, 2H), 7.12−7.10 (m, 2H),
7.08−7.03 (m, 5H), 4.50 (t, J = 5.0 Hz, 1H), 2.64−2.58 (m, 1H),
2.56−2.50 (m, 1H), 2.23 (s, 3H), 2.03−1.96 (m, 2H), 1.92−1.85 (m,
1H). 13C{H} NMR (151 MHz, CDCl3): δ 141.94, 141.65, 137.33,
129.24, 128.53, 128.44, 126.00, 125.88, 73.73, 40.41, 32.15, 21.21.
HRMS (ESI): m/z calculated for [M + Na]+: 249.1255, found
249.1276.
1
3-(3-Methoxyphenyl)-1-phenylpropan-1-ol (7d). H NMR (400
MHz, CDCl3): δ 7.36 (d, J = 4.3 Hz, 4H), 7.32−7.25 (m, 1H), 7.20
(td, J = 7.4, 1.7 Hz, 1H), 6.80 (d, J = 7.6 Hz, 1H), 6.74 (d, J = 7.0 Hz,
2H), 4.70 (ddd, J = 8.1, 5.3, 2.8 Hz, 1H), 3.79 (s, 3H), 2.79−2.61 (m,
2H), 2.20−1.98 (m, 2H), 1.93 (brs, J = 3.2 Hz, 1H). 13C{H} NMR
(101 MHz, CDCl3): δ 159.83, 144.70, 143.57, 129.47, 128.65, 127.77,
126.05, 120.99, 114.35, 111.35, 74.00, 55.27, 40.47, 32.24. HRMS
(ESI): m/z calculated for [M + H]+: 265.1204, found 265.1198.
3-(4-Fluorophenyl)-1-phenylpropan-1-ol (7e). 1H NMR (600
MHz, CDCl3): δ 7.37−7.34 (m, 4H), 7.30−7.28 (m, 1H), 7.15−
7.13 (m, 2H), 6.96 (t, J = 6 Hz, 2H), 4.67 (brs, 1H), 2.75−2.70 (m,
1H), 2.67−2.60 (m, 1H), 2.14−2.08 (m, 1H), 2.02−1.97 (m, 1H),
1.87 (brs, 1H). 13C{H} NMR (151 MHz, CDCl3): δ 162.21, 144.61,
137.48, 129.91, 129.86, 128.71, 127.88, 126.03, 115.31, 115.17, 73.92,
40.72, 31.37. 19F NMR (377 MHz, CDCl3): δ −117.65.
1-(4-Methoxyphenyl)-3-phenylpropan-1-ol (7r). 1H NMR (600
MHz, CDCl3): δ 7.28 (d, J = 5.0 Hz, 3H), 7.19 (d, J = 7.1 Hz, 3H),
6.89 (d, J = 8.6 Hz, 2H), 4.64 (brs, 1H), 3.81 (s, 3H), 2.68 (d, J =
79.0 Hz, 2H), 2.17−1.98 (m, 2H). 13C{H} NMR (101 MHz,
CDCl3): δ 159.28, 141.98, 128.57, 128.51, 127.35, 125.97, 114.05,
73.65, 55.44, 32.27. HRMS (ESI): m/z calculated for [M + Na]+:
265.1204, found 265.1384.
1
1-(3-Methoxyphenyl)-3-phenylpropan-1-ol (7s). H NMR (500
3-(4-Chlorophenyl)-1-phenylpropan-1-ol (7f). 1H NMR (600
MHz, CDCl3): δ 7.29−7.25 (m, 4H), 7.23−7.20 (m, 1H), 7.18−
7.16 (m, 2H), 7.04 (d, J = 6.0 Hz, 2H), 4.58 (t, J = 6.0 Hz, 1H),
2.66−2.61 (m, 1H), 2.59−2.54(m, 1H), 2.05−1.99 (m, 1H), 1.94−
1.88 (m, 1H), 1.85−1.82 (m, 1H). 13C{H} NMR (151 MHz,
CDCl3): δ 144.52, 140.34, 131.68, 129.92, 128.71, 128.59, 127.89,
126.01, 73.84, 40.44, 31.51. HRMS (ESI): m/z calculated for [M +
Na]+: 285.0448, found 285.1295.
MHz, CDCl3): δ 7.16 (q, J = 7.9 Hz, 3H), 7.09 (d, J = 7.7 Hz, 3H),
6.82 (d, J = 7.0 Hz, 2H), 6.72 (d, J = 8.4 Hz, 1H), 4.59−4.54 (m,
1H), 3.70 (s, 3H), 2.69−2.53 (m, 2H), 2.06−1.89 (m, 2H), 1.85 (brs,
1H). 13C{H} NMR (126 MHz, CDCl3): δ 159.95, 146.45, 141.90,
129.66, 128.57, 128.51, 125.98, 118.37, 113.22, 111.56, 73.94, 55.36,
40.52, 32.16.
1-(4-Bromophenyl)-3-phenylpropan-1-ol (7w). 1H NMR (600
MHz, CDCl3) δ 7.49−7.46 (m, 2H), 7.29 (t, J = 7.5 Hz, 2H), 7.24−
7.17 (m, 5H), 4.65 (ddd, J = 8.2, 5.1, 3.0 Hz, 1H), 2.77−2.63 (m,
2H), 2.13−1.96 (m, 2H), 1.95 (brs, J = 3.4 Hz, 1H). 13C{H} NMR
(151 MHz, CDCl3) δ 143.62, 141.56, 131.70, 128.58, 128.54, 127.78,
126.09, 121.46, 73.28, 40.57, 32.01.
3-(3-Chlorophenyl)-1-phenylpropan-1-ol (7g). 1H NMR (400
MHz, CDCl3): δ 7.30−7.17 (m, 5H), 7.14−7.07 (m, 3H), 6.99 (d, J =
4.0 Hz, 1H), 4.61−4.57 (m, 1H), 2.69−2.53 (m, 2H), 2.08−1.83 (m,
3H). 13C{H} NMR (151 MHz, CDCl3): δ 144.49, 143.99, 134.25,
129.76, 128.73, 127.92, 126.79, 126.20, 126.01, 73.85, 40.32, 31.86.
1-(3-Bromophenyl)-3-phenylpropan-1-ol (7x). 1H NMR (500
MHz, CDCl3): δ 7.42 (brs, 1H), 7.33−7.31 (d, J = 10.0 Hz, 1H),
7.22−7.16 (m, 3H), 7.14−7.09 (m, 4H), 4.55 (brs, 1H), 2.69−2.59
(m, 2H), 2.04−1.88 (m, 3H). 13C{H} NMR (126 MHz, CDCl3): δ
147.10, 141.57, 130.76, 130.21, 129.17, 128.59, 128.55, 126.11,
124.64, 122.77, 73.25, 40.60, 32.03.
1
1-Phenyl-3-(pyridin-3-yl)propan-1-ol (7i). H NMR (600 MHz,
CDCl3): δ 8.43−8.37 (m, 2H), 7.51−7.50 (m, 1H), 7.35−7.34 (m,
4H), 7.29−7.28 (m, 1H), 7.20−7.18 (m, 1H), 4.68−4.66 (m, 1H),
2.77−2.65 (m, 2H), 2.15−2.08 (m, 1H), 2.02−1.98 (m, 1H), 1.71 (s,
1H). 13C{H} NMR (151 MHz, CDCl3): δ 149.91, 147.30, 144.60,
137.34, 136.14, 128.70, 127.84, 125.99, 123.50, 73.50, 40.21, 29.29.
3-Phenyl-1-(thiophen-2-yl)propan-1-ol (7za). 1H NMR (400
MHz, CDCl3) δ 7.27−7.20 (m, 3H), 7.16 (d, J = 7.5 Hz, 3H),
6.97−6.90 (m, 2H), 4.88 (t, J = 6.7 Hz, 1H), 2.79−2.63 (m, 2H),
2.25−2.05 (m, 2H), 1.95 (s, 1H). 13C{H} NMR (101 MHz, CDCl3)
δ 148.67, 141.60, 128.63, 128.59, 126.82, 126.11, 124.83, 124.06,
69.69, 40.86, 32.16.
1
1-Phenyl-3-(pyridin-4-yl)propan-1-ol (7j). H NMR (500 MHz,
CDCl3): δ 8.32−8.30 (m, 2H), 7.27−7.26 (m 4H), 7.21−7.19 (m,
1H), 7.02−7.01 (m, 2H), 4.61−4.58 (m, 1H), 2.70−2.64 (m 1H),
2.62−2.56 (m 1H), 2.07−2.00 (m, 1H), 1.97−1.90 (m, 1H). 13C{H}
NMR (151 MHz, CDCl3): δ 151.16, 149.70, 144.43, 128.75, 127.95,
125.97, 124.08, 73.65, 39.38, 31.50. HRMS (ESI): m/z calculated for
[M + H]+: 214.1232, found 214.1273.
1
1-(Naphthalen-2-yl)-3-phenylpropan-1-ol (7zb). H NMR (500
MHz, CDCl3): δ 7.71−7.68 (m, 3H), 7.62 (brs, 1H), 7.37−7.32 (m,
3H), 7.18−7.15 (m, 2H), 7.09−7.07 (m, 3H), 4.68 (brs, 1H), 2.67−
2.60 (m, 1H), 2.58−2.53 (m, 1H), 2.12−2.06 (m, 2H), 2.01−1.94
(m, 1H). 13C{H} NMR (151 MHz, CDCl3) δ 141.93, 141.82, 133.31,
133.06, 128.56, 128.51, 128.48, 128.03, 127.80, 126.29, 125.98,
124.79, 124.14, 74.03, 40.38, 32.12. HRMS (ESI): m/z calculated for
[M + Na]+: 285.1255, found 285.1200.
3-(Furan-2-yl)-1-phenylpropan-1-ol (7k). 1H NMR (500 MHz,
CDCl3): δ 7.35 (d, 4H), 7.31−7.28 (m, 2H), 6.28 (brs, 1H), 6.06−
6.01 (s, 1H), 4.73−4.70 (m, 1H), 2.79−2.68 (m, 2H), 2.17−2.03 (m,
3H), 1.93 (brs, 1H). 13C{H} NMR (151 MHz, CDCl3): δ 155.66,
144.46, 141.10, 128.68, 128.65, 127.83, 126.02, 110.27, 105.17, 73.84,
37.30, 24.54.
H
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Article
3-Phenyl-1-(4-(trifluoromethyl)phenyl)propan-1-ol (7zc). 1H
NMR (600 MHz, CDCl3): δ 7.63 (d, J = 8.0 Hz, 2H), 7.49 (d, J =
8.0 Hz, 2H), 7.32 (t, J = 7.5 Hz, 2H), 7.22 (d, J = 7.8 Hz, 3H), 4.78
(b rs, J = 3.7 Hz, 1H), 2.83−2.70 (m, 2H), 2.17−2.03 (m, 2H), 2.03
(brs, J = 3.4 Hz, 1H). 13C NMR (151 MHz, CDCl3): δ 148.61,
141.42, 129.83 (q, J = 32.4 Hz), 128.62, 128.54, 126.27, 126.17,
125.57 (q, J = 3.7 Hz), 125.14, 123.34, 73.27, 40.69, 31.96. 19F NMR
(377 MHz, CDCl3) δ −62.45.
Technology (Grant No. DST-SERB CRG/2018/000607).
A.K. is also thankful to the Scheme for Transformational and
Advanced Research in Sciences, Ministry of Human Resource
Development, implemented by the Indian Institute of Science
(IISc), Bangalore (Grant No. STARS/APR2019/CS/629/FS).
The DST-FIST program, Department of Chemistry at IITG,
NECBH-IITG, and CIF-IITG are acknowledged for various
instrumental facilities.
3-Phenyl-1-(3-(trifluoromethyl)phenyl)propan-1-ol (7zd). 1H
NMR (500 MHz, CDCl3): δ 7.55 (s, 1H), 7.47−7.45 (m, 2H),
7.40−7.37 (m, 1H), 7.23−7.18 (m, 2H), 7.13−7.11 (m, 3H), 4.70−
4.67 (m, 1H), 2.73−2.60 (m, 2H), 2.06−1.91 (m, 3H). 13C{H} NMR
(126 MHz, CDCl3) δ 145.77, 141.48, 129.38, 129.09, 128.65, 128.57,
126.20, 124.53 (q, J = 3.8 Hz), 122.86 (q, J = 3.7 Hz), 73.38, 40.76,
32.08. 19F NMR (471 MHz, CDCl3): δ −62.60.
DEDICATION
■
Dedicated to Prof. Ashoka G. Samuelson (IISc, Bangalore,
India) on the occasion of his 65th birthday and his
superannuation from service.
3-Phenyl-1-(4-(trifluoromethoxy)phenyl)propan-1-ol (7ze). 1H
NMR (500 MHz, CDCl3): δ 7.30−7.29 (m, 2H), 7.22−7.18 (m,
2H), 7.12−7.11 (m, 5H), 4.63 (s, 1H), 2.71−2.57 (m, 2H), 2.07−
2.00 (m, 1H), 1.97−1.90 (m, 1H), 1.86 (s, 1H). 13C{H} NMR (126
MHz, CDCl3): δ 148.70, 143.42, 141.59, 128.61, 128.55, 127.44,
126.14, 121.13, 73.23, 40.71, 32.09. 19F NMR (471 MHz, CDCl3): δ
−57.88.
ABBREVIATIONS
TON,turnover number; THF,tetrahydrofuran; RDS,rate-deter-
mining step
■
REFERENCES
■
(1) Brahmachari, G. Design for carbon−carbon bond forming
reactions under ambient conditions. RSC Adv. 2016, 6, 64676−64725.
(2) Yi, H.; Zhang, G.; Wang, H.; Huang, Z.; Wang, J.; Singh, A. K.;
Lei, A. Recent Advances in Radical C−H Activation/Radical Cross-
Coupling. Chem. Rev. 2017, 117, 9016−9085.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.1c00328.
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X-ray crystallographic parameters, NMR spectra, and
computational data (PDF)
Accession Codes
CCDC 2071950 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
Akshai Kumar − Department of Chemistry, Indian Institute of
Technology Guwahati, Guwahati 781039 Assam, India;
Center for Nanotechnology, Indian Institute of Technology
Guwahati, Guwahati 781039 Assam, India; orcid.org/
0000-0002-3911-3630; Email: akshaikumar@iitg.ac.in
■
Authors
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Vinay Arora − Department of Chemistry, Indian Institute of
Technology Guwahati, Guwahati 781039 Assam, India;
orcid.org/0000-0003-0356-4680
Himani Narjinari − Department of Chemistry, Indian Institute
of Technology Guwahati, Guwahati 781039 Assam, India
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.1c00328
Author Contributions
V.A. performed the experiments. H.N. purified the derivatives
of 7 by column chromatography. A.K. conceptualized,
compiled the work, and wrote the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
A.K. is grateful for the grants received from the Science and
Engineering Research Board, Department of Science and
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