A Pseudodearomatized PN3P?Ni-H Complex as a Ligand and σ-Nucleophilic Catalyst

Article abstract of DOI:10.1021/acs.joc.8b02205

In contrast to the conventional strategy of modifying the reactivities and selectivities of the transition metal and organocatalysts by varying the steric and electronic properties of organic substituent groups, we hereby demonstrate a novel approach that the sigma (σ) nucleophilicity of the imine arm can be significantly enhanced in a pseudodearomatized PN³P? pincer ligand platform to reach unprecedented N-heterocyclic carbene-like reactivity. Accordingly, the imine arm of the PN³P?Ni-H pincer complex efficiently catalyzes the hydrosilylation of aldehydes, cycloaddition of carbon dioxide (CO₂) to epoxides, and serves as a ligand in the Ru-catalyzed dehydrogenative acylation of amines with alcohols.

Full text of DOI:10.1021/acs.joc.8b02205

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Article
A Pseudo-Dearomatized PN3P*Ni-H Complex
as a Ligand and #-Nucleophilic Catalyst
Huaifeng Li, Théo P. Gonçalves, Jinsong Hu, Qianyi Zhao, Dirong
Gong, Zhiping Lai, Zhixiang Wang, Junrong Zheng, and Kuo-Wei Huang
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b02205 • Publication Date (Web): 24 Oct 2018
Downloaded from http://pubs.acs.org on October 25, 2018
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A Pseudo-Dearomatized PN3P*Ni-H Complex as a Ligand and σ-Nucleophilic Catalyst
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Huaifeng Li,†,‡ Théo P. Gonçalves,†,‡ Jinsong Hu,†,‡ Qianyi Zhao,†,‡ Dirong Gong,†,‡ Zhiping Lai,†,§
Zhixiang Wang,|| Junrong Zheng,# and Kuo-Wei Huang*,†,‡
†Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology,
Thuwal 23955-6900, Saudi Arabia.
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‡KAUST Catalysis Center, King Abdullah University of Science and Technology, Thuwal 23955-6900,
Saudi Arabia.
§Advanced Membranes & Porous Materials Center, King Abdullah University of Science and Technology,
Thuwal 23955-6900, Saudi Arabia.
||College of Chemistry and Chemical Engineering, Graduate University of the Chinese Academy of Sciences,
Beijing 100049, China.
#College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China.
*Correspondence: hkw@kaust.edu.sa
An imine N with similar reactivitiesas
to those of NHCs as a ligand and -
nucleophilic catalyst.
N
N
Ni
H
NH
PtBu2
tBu
2 P
Abstract
In contrast to the conventional strategy of modifying the reactivities and selectivities of the transition metal
and organocatalysts by varying the steric and electronic properties of organic substituent groups, we hereby
demonstrate a novel approach that the sigma (σ) nucleophilicity of the imine arm can be significantly
enhanced in a pseudo-dearomatized PN3P* pincer ligand platform to reach unprecedented N-heterocyclic
carbene-like reactivity. Accordingly, the imine arm of the PN3P*Ni-H pincer complex efficiently catalyzes
the hydrosilylation of aldehydes, cycloaddition of carbon dioxide (CO2) to epoxides, and serves as a ligand
in the Ru-catalyzed dehydrogenative acylation of amines with alcohols.
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Introduction
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Strong organic σ-donors such as carbenes and phosphines play an essential role in modern coordination and
organic chemistry as ligands or organocatalysts.1 While nitrogen σ-donor analogs (amine and imine) are
considered more abundant and easily accessible, with many of them being utilized in various catalytic
systems,2 they are in general not applicable to the reactions where the reactivity is unique to carbenes and
phosphines (Figure 1A).3 For example, in the process of hydrosilylation of carbon dioxide, N-heterocyclic
carbenes (NHCs) show superior catalytic activity,4 but the catalysts involving the nitrogen σ-donor groups
are extremely rare.5 To the best of our knowledge, there is no strategy to modify and enhance the reactivity
of the nitrogen donors to reach the same level of reactivity as those of carbenes and phosphines.
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A)
N
N
C
P
PCy3
IMes
N
N
N
N
N
N
N
H
DBU
DABCO
TBD
N
N
B)
resonance
N
N
Ln
Ln
M
N
tBu2P
HN
PtBu2 tBu2P
M
PtBu2
N
N
HN
N
resonance
aromatic zwitterionic form
dearomatized
Figure 1. A) Examples of organic σ-donors. B) Comparison of DBU with a pseudo-dearomatized PN3P*
pincer complex.
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In organometallic and coordination chemistry, the reactivity of the metal center can be manipulated by
modifying the steric and/or electronic properties of the surrounding ligands.6 Likewise, the activity and
selectivity of an organocatalyst or ligand can also be tuned by varying the steric and/or electronic factors
of the substituent groups.1a, 2c, 7 Since the coordination of a metal center could influence the property of the
ligand, it is conceivable that this interaction could in principle be utilized to offer new strategies for the
design and modifications of “organocatalysts” through changing the coordinated metal and its spectator
ligands. Such an idea, however, has not been comprehensively examined, presumably due to the highly
reactive nature of the metal atoms. Nevertheless, there are supporting observations showing the possibility
of modifying the property or enhancing the reactivity of the ligand through complexation with a metal.8 For
instance, Mayer and co-workers studied hydrogen atom transfer (HAT) organometallic reaction and found
that hydrogen donor abilities of ligands were influenced by the spectator metal.8b Ogasawara and co-
workers applied kinetic resolution of racemic (arene)chromium complexes via asymmetric ring-closure
metathesis (ARCM) and observed that the constructed planar-chiral environment onto chromium atom
reflects the significant change in the enantioselectivity with only a small modification of the ligand.8d In
some even more relevant cases, “cooperating” ligands can participate directly in the bond breaking and
forming steps together with the metal center to activate substrates to achieve metal-ligand cooperative
catalysis.9
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We have shown that the seemingly small change of replacing the CH2 group with an NH spacer in the
central pyridine ring, dearomatization of the PN3P-type pincer complexes has significantly changed the
thermodynamic and kinetic properties compared to their CH2 analogs.10 Furthermore, we have also
demonstrated the importance of the aromatic mesomeric form in the pseudo-dearomatized pyridine-based
PN3P* metal pincer complexes.10g Such a zwitterionic structure could exhibit a similar dual character of
base and nucleophile of the nitrogen atom as that of DBU (1,8-Diazabicyclo[5.4.0]undec-7-ene) through
the resonance effect (Figure 1B).11 We have recently developed the diverse catalytic reactivity of
dearomatized PN3P*Ni-H complex 1 for CO2 reduction, and formylation and methylation of amines.12 Very
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intriguingly, the insertion of CO2 into the Ni-H bond was not detected.13 These observations prompted us
to explore the nucleophilicity of the imine arm of PN3P*-Ni complexes and their potential applications in
various organic reactions.
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In this contribution, we demonstrate that the nucleophilicity of the imine arm of the pseudo-dearomatized
PN3P* complex can be significantly enhanced such that it becomes catalytically active to reach
unprecedented activities and plays a similar role of the N-heterocyclic carbene as a catalyst in the
hydrosilylation of aldehydes to alcohols, cyclic carbonates synthesis via cycloaddition of CO2 to epoxides,
and as a ligand in the dehydrogenative acylation of amines with alcohols.
Results and Discussion
structure
resonance forms contribution
N
N
N
N
N
N
...
...
DBU
62.0%
8.2%
N
N
N
N
N
N
DHIP
55.5%
21.5%
N
N
N
N
N
N
N
N
...
HN
HN
HN
HN
TBD
46.0%
15.7%
7.5%
H
H
Ni
N
H
Ni
N
tBu2P
Ni
N
tBu2P
tBu2P
PtBu2
NH
N
N
N
...
1
40.2%
39.9%
at M06/6-31G(d) level of theory
Figure 2. The contribution of resonance forms in DBU, DHIP, TBD and 1 from NBO analysis.
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N
S
N
N
N
frequen
N
N
N
N
N
N
P N
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HN
N
tBu
N
cy
NEt2
NC
CN
ImCN2
Pyridine
DABCO
TBD
Imidazole
ThN
BEMP
H
Ni
N
Cl
L
tBu2P
PtBu2
NH
tBu2P
Ni
N
PtBu2
NH
 - donating
P
N
N
N
N
N
N
PMe3
1
2
IMes
DHIP
Entry
Ligand (L)
ν (CO)a
ν (CO)b
NPAc
-0.47
-0.51
-0.62
-0.53
-0.56
1
2
Pyridine
DABCO
TBD
2075.0
2071.6
2067.3
2066.8
2066.8
2066.6
2065.9
2063.0
2063.95
2060.7
2051.4
2060.7
2070.5
3
4
Imidazole
DHIP
ImCN2
PMe3
ThN
2051.7
5
6
2066.2
2065.2
2061.0
7
8
9
2
-0.99
-1.00
10
11
12
1
IMes
BEMP
2050.5
-1.18
aFrequencies are in [cm−1] at M06/6-31G(d,p)/SDD scaled by 0.955. Values taken from the reference.14 cNatural
b
Population Analysis charge on the nucleophilic nitrogen atom.
Figure 3. Calculated TEP and NPA for σ-donating ligands.
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Our investigation started with a close look into the well-known nitrogen-based nucleophiles, such as
DBU and TBD (1,5,7-Triazabicyclo[4.4.0]dec-5-ene), and correlated their nucleophilicity to the mesomeric
effect. The investigation on resonance forms from Natural Bond Orbital Analysis (NBO-NRT)15 of DBU
shows that the molecule has 8.2% of the contribution from the zwitterionic forms (Figure 2). This value
rises to 21.5% for DHIP (2,3-dihydroimidazo[1,2-a]pyridine) as result of incorporating dearomatized
pyridine ring in bicyclic structure.16 As expected, the contribution from the zwitterionic structures is
increased to 23.3% when the iminic moiety is conjugated with one more nitrogen in TBD, consistent with
the fact the TBD is a stronger base and nucleophile than DBU.17 Importantly and interestingly, the
contribution of the zwitterionic structures of 39.9% in the pseudo-dearomatized PN3P*Ni-H complex 1 is
much higher than in DBU, DHIP and TBD Schiff bases. In order to undusted further the electron donating
ability of 1 we calculated Tolman Electronic Parameter (TEP) for the range of ligand L-Ni(CO)3 (Figure
3).18, 14 The IMes has the lowest frequencies of the CO stretching which is attributed to the strongest σ-
donating character of the ligand to the metal center. The complex 1 is the second most σ-donating ligand
from the presented study, even stronger than some of the carbenes. In addition, the NPA charges show
remarkably high negative charge on the nitrogen for the complexes 1 and 2.19 For the further comparison,
the phosphazenes, well know push-pull P=N bases, displays highly electron donative nitrogen (Figure 3,
S2 and S3).20 While BEMP has a greater NPA charge, it shows a similar TEP value compared to that of 1,
reflecting the difference between nucleophilicity and Lewis basicity presumably due to its steric bulk.
Moreover, 1 is expected to unveil enhanced nucleophilicity due to the α-effect of the adjacent phosphor
atom.21 Those results suggest that 1 may act as a very strong σ-electron donating nucleophile and encourage
us to explore the nucleophilic reactivity of such type of complexes.
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Initially, the hydrosilylation of aldehydes catalyzed by PN3P* complex 1 was investigated. Guan and co-
workers disclosed a POCOP-NiH system which showed excellent catalytic activity for the hydrosilylation
of aldehydes and moderate reactivity for ketones.22 The insertion of C=O double bond of carbonyl group
into the Ni-H was directly observed and this insertion reaction was revealed as one of the essential steps
involved in the catalytic cycle. Under the similar reaction conditions,22 benzaldehyde 3a-3h bearing either
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electron-rich or electron-poor functional groups could all react smoothly to give the corresponding alcohols
4a-4h in excellent yields in the presence of 1 (Table 1, entries 1-8). All aromatic and aliphatic (Table 1,
entries 9 and 10) aldehydes were also suitable substrates for hydrosilylation. Very surprisingly, trans-
cinnamaldehyde was converted to cinnamyl alcohol selectively at 60 °C with no sign of the formation of
1,4-reduction product (Table 1, entry 11), in sharp contrast to the results of similar reactions catalyzed by
Ni-H species22-23 or carbene-Cu-H complexes.24 Subsequently, hydrosilylation of a mixture of acetophenone
(1.0 equiv.) of and benzaldehyde (1.0 equiv.) catalyzed by 1 was examined to give benzyl alcohol 4a in a
quantitative yield with full recovery of acetophenone, showing a high chemoselectivity to aldehyde
hydrosilylation over ketone hydrosilylation (Scheme 1). As observed in the hydrosilylation of CO2, the
reaction of complex 1 with benzaldehyde 3a did not take place even when the mixture was heated in toluene
to 80 °C for 16 h. These observations also suggest that the “active site” for the catalytic reactions may not
involve the Ni-H group.
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Table 1. The hydrosilylation of aldehyde with PhSiH3 catalyzed by PN3P*Ni-H complex 1.a
5
6
7
8
O
OH
1
0.2 mol%
10% NaOH
+
PhSiH3
toluene, 80 oC
16 h
R
H
R
1 equiv
0.4 equiv
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Entry
R
Yield (%)b
CHO
X
3a
3b
3c
3d
3e
3f
4a
1
2
3
4
5
6
7
, X = H
, 99
4b
, X = 4-Me
, X = 3-Me
, X = 2-MeO
, X = 4-Cl
, 90
4c
, 85
4d
, 93
4e
, 90
4f
, X = 4-Br
, 96
3g
4g
, X = 2-CF3
, X = 2-OH
O
, 86
3h
4h
8
, 91
R
H
3i
3j
3k
4i
9
10
11c
, R = 2-pyridyl
, R = Cy
, 94
4j
, 83
4k
, R = PhCHCH
, 85
aUnless otherwise specified, all reactions were performed on a 1-mmol
1
scale in dry toluene (1 mL) under Ar in the presence of (0.2 mol%) at 80
oC. bIsolated yields after basic work-up. cThe reaction was performed at 60
oC for 48 h.
v
Scheme 1 Hydrosilylation of acetophenone and benzaldehyde catalyzed by PN3P*Ni-H complex 1.
O
O
O
OH
1
0.2 mol%
+
+
Ph
Ph
H
PhSiH3 (1.0 equiv)
toluene, 80 oC
Ph
Ph
H
Yield:
1.0 equiv
1.0 equiv
99%
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Table 2. Cycloaddition of CO2 to epoxide catalyzed by PN3P*Ni-H 1.a
7
8
9
O
O
1
2 mol%
CO2
O
O
+
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54
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57
58
59
60
R
100 oC, neat, 48 h
R
1 atm
Yield(%)b
73
Epoxide
O
Product
Entry
1
Ph
O
5a
5b
5c
5d
6a
6b
6c
6d
O
O
Ph
n-Bu
O
O
O
O
2
3
4
67
74
65
n-Bu
PhOH2C
O
O
O
PhOH2C
O
Et
O
O
O
O
Et
ClH2C
O
O
5e
6e
5
81
O
ClH2C
O
aReaction conditions: CO2 balloon, catalyst (9 mg, 20 µmol) , 1.0 mmol of
epoxide, and the reaction was carried out at 100 °C for 48h.
bIsolated yields.
In an effort to support the assumption of nucleophilic activity of the iminic nitrogen of 1, we continued
to examine other organic transformations where carbenes serve as catalysts or ligands and the conventional
reactivity of a Ni-H bond is unlikely to play any role. Under the similar reaction condition developed by
Lu and co-workers for NHCs-catalyzed coupling of CO2 with epoxides,25 PN3P*Ni-H 1 showed analogous
reactivity to offer the corresponding cyclic carbonates in good yields (Table 2). On the other hand, the Hong
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Table 3. Ruthenium catalyzed amide synthesis from alcohol and amine in the presence of PN3P*Ni-
H 1.a
9
1
5 mol%
10
11
12
13
14
15
16
17
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19
20
21
22
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60
O
R
2.5 mol% [Ru(p-cymene)Cl2]2
NH
+
Ph
OH
15 mol% KOtBu
Ph
NRR'
R'
toluene, reflux, 24 h
Yield(%)b
78
Amine
Product
O
Entry
1
NH2
7a
7b
7c
8a
8b
8c
Ph
N
H
O
O
NH2
2
3
85
83
Ph
Ph
N
H
NH2
N
H
Cl
Cl
O
NH2
7d
8d
4
73
Ph
N
H
MeO
OMe
O
O
7e
7f
8e
8f
5
6
NH2
68
71
Ph
Ph
N
H
O
NH
N
O
aReaction conditions: catalyst (0.025 mmol, 12 mg), [Ru(p-cymene)Cl2]2
(0.013 mmol, 8 mg), KOtBu (0.075 mmol, 8.4 mg), benzyl alcohol (0.50
mmol) and amine (0.55 mmol) were refluxed in 0.6 mL of toluene for 24h.
bIsolated yields.
group has demonstrated that the dehydrogenative acylation of amines with alcohols could be catalyzed by
a ruthenium catalyst precursor in the presence of a NHC ligand.26 We demonstrated that this reaction could
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be achieved with the same Ru precursor and PN3P*Ni-H 1 in satisfactory yields (Table 3). To the best of
our knowledge, the dehydrogenative acylation of amines with alcohols has never been achieved by
transition metal catalysts with imine/amine ligands. As we have previously reported that the Ru complex
with the same PN3P* ligand did not show any good activity in this type of dehydrogenative acylation
reaction,10c the role of metal exchange in the observed reactivity can be ruled out. PN3P*Ni-H 1 may serve
as a strong σ-donating ligand similar to the NHC during this process.
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benzene
ICu
tBu
N
N
Ni
H
NH
PtBu2
N
N
Ni
H
NH
PtBu2
+ CuI
tBu
2 P
2 P
1
9
Figure 4. Synthesis of complex 9 and the ORTEP diagram of complex 9 at 30% ellipsoid probability.
Hydrogen atoms (except for pincer arms and Ni-H) are omitted for clarity. Selected bond lengths (Å) Ni1-
N2 1.901(5); Ni1-P1 2.1645(16); Ni1-P2 2.1649(16); N3-C5 1.364(7); N1-C1 1.388(7).
The attempt to isolate the active species in the Ru catalyzed dehydrogenative acylation of amine to
support the ligand role of 1 was in vain. However, we were able to demonstrate that complex 1 can be
coordinated to copper iodide to give complex 9. The molecular structure of 9 was determined by the single
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crystal X-ray diffraction (Figure 4), analogous to the corresponding copper(I)–NHC complexes,27
suggesting that the electron donating ability of the imine arm of complex 1 is comparable to those of NHCs.
This result indeed supports that complex 1 can act as a strong σ-donating ligand and play a similar role as
a carbene in transition metal catalysis.
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Conclusion
In summary, while the conventional wisdom is to modify the reactivities and selectivities of the
coordination metal catalysts and organocatalysts through changing the steric and/or electronic properties of
organic substituent groups, we have demonstrated that the nucleophilicity of the imine group of the pseudo-
dearomatized PN3P*Ni-H complex 1 could be significantly increased to reach unprecedented carbene-like
reactivity in several organic catalytic transformations. These observations suggest a novel paradigm of
strategy for the design and modifications of “organocatalysts” and ligands through coordination to metal.
This study may open a new direction for the catalyst and ligand design.
Experimental Section
General Information
All manipulation of air- and/or moisture-sensitive compounds were carried out under an atmosphere of
purified argon in a Vacuum Atmospheres glovebox or using standard Schlenk techniques. All solvents were
distilled under Ar from appropriate drying agents. Unless otherwise statement, commercial reagents were
used as received without purification. Column chromatography was performed on silica gel (200–300
mesh). NMR spectra were recorded on Bruker Advance 400, Bruker Avance-500, Bruker Avance-600, or
Bruker Avance-700 NMR spectrometers in deuterated solvents. 1H NMR chemical shifts were referenced
to the residual hydrogen signals of the deuterated solvents, and the 13C NMR chemical shifts were
referenced to the 13C signals of the deuterated solvents. Data are reported as follows: chemical shift,
multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, p = pentet, s = sextet, h = heptet, m = multiplet,
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br = broad), coupling constants (Hz) and integration. Gas chromatography was performed on Agilent 5975C
GC inert XL EI/CI MSD with Triple-Axis MS Detector. The X-ray diffraction data were collected using
Bruker-AXS KAPPA-APEXII CCD diffractometer (CuKα, λ= 1.54178 Å). Indexing was performed using
APEX2 (Difference Vectors method). Data integration and reduction were performed using SaintPlus 6.01.
Absorption correction was performed by multi-scan method implemented in SADABS. Space groups were
determined using XPREP implemented in APEX2. Structures were solved using SHELXS-97 (direct
methods) and refined using SHELXL-97 (fullmatrix least-squares on F2). Elemental analyses were
conducted by Flash 2000-Thermo Scientific CHNO Analyzer.
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The DFT studies were carried out with Gaussian09 program.28 All the structures were fully optimized at
M06/6-31G(d,p) level of theory and SDD pseudo potential with corresponding basis set was used for Ni
atom .29,30 The A1-mode of the CO frequency was scaled by 0.955 to obtain more reliable values frequencies
corresponding Tolman Electronic Parameter. Post-processing visualization was carried out with the
ChemCraft.31 The Natural Bond Orbital (NBO) analysis was carried with the Natural Bond Orbital (NBO6)
program.32 The Natural Resonance Theory (NRT) calculations were calculated at M06/6-31G(d)/SDD
method level.
Procedures for synthesis of complex 9: To a solution of complex 1 (228 mg, 0.5 mmol) in dry benzene
(5.0 mL) was added CuI (95.2 mg, 0.5 mmol) under an argon atmosphere, and the mixture was stirred at
room temperature for 24 hours. The solution was filtered and the solid was washed with pentane (3×3.0
mL), then dried under vacuum. The solid 9 thus obtained as a brown solid (294 mg, 91%). mp: 213-215 °C.
Crystals of complex 9 suitable for X-ray diffraction were obtained by slow evaporation of its benzene
solution. 1H NMR (600 MHz, C6D6): 7.28 (t, J = 6.2 Hz, 1H), 6.76 (t, J = 8.1 Hz, 1H), 5.78 (s, 1H), 4.89
(s, 1H), 6.76 (t, J = 8.1 Hz, 1H), 1.38 (d, J = 14.2 Hz, 18H), 0.99 (d, J = 14.6 Hz, 18H), -16.50 (t, J = 58.6
Hz, 1H). 13C NMR (DMF-d7, 126 MHz): 171.2, 140.6, 125.3, 102.4 (d, J = 13.7 Hz), 93.5, 36.6, 36.5, 36.3,
31
36.1, 28.8, 28.7, 28.0, 27.9. P NMR (243 MHz, C6D6): 135.5 (dd, J = 228.4, 44.1 Hz), 125.4 (dd, J =
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229.9, 46.0 Hz). Elemental analysis (%) for C21H41CuIN3NiP2: Calc. C, 39.00; H, 6.39; N, 6.50. Found: C,
39.07; H, 6.44; N, 6.42.
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General procedures for hydrosilylation of aldehyde: To a flame-dried Schlenk flask was added a
solution of nickel complex PN3P*Ni-H 1 (1.0 mg, 2 µmol) in toluene (1.0 mL), an aldehyde substrate (1.0
mmol) and PhSiH3 (49 µL, 0.4 mmol) under an argon atmosphere. The resulting mixture was stirred at 80
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oC until there was no aldehyde left (monitored by withdrawing aliquots and analyzing their H NMR
spectra). The reaction was then quenched by 10% aqueous solution of NaOH (about 1.0 mL) with vigorous
stirring for more than 12 h. The solution containing the alcohol product was extracted with diethyl ether
three times, dried over anhydrous Na2SO4, and concentrated under vacuum. The desired alcohol was further
purified by flash column chromatography and petroleum ether/EtOAc as eluent.
General procedures for cycloaddition of CO2 to epoxide: The epoxide (1.0 mmol) and the complex
PN3P*Ni-H 1 (9 mg, 20 µmol) was charged in a dry flask. Then CO2 was introduced (1 bar) and the mixture
was heated at 100°C for 48 hours. The reaction mixture was then cooled to room temperature, methylene
chloride was added and concentrated under vacuum. The product was purified over silica gel column. The
desired product was further purified by flash column chromatography and petroleum ether/EtOAc as eluent.
General procedures for the amide synthesis from alcohols and amines: In an argon-filled glovebox, a
10.0 mL oven-dried Schlenk tube was charged with complex PN3P*Ni-H 1 (0.025 mmol, 12 mg), KOtBu
(0.075 mmol, 8.4 mg), and 0.6 mL of toluene. The alcohol (0.50 mmol) and amine (0.55 mmol) were then
added. The reaction mixture was heated at reflux under a flow of argon to facilitate the removal of hydrogen
for 24 h before being cooled to room temperature. All the volatiles were removed under vacuum.
Purification of the crude product by flash chromatography afforded the amide product.
Benzyl alcohol 4a.33 This compound was purified by flash chromatography (petroleum ether/ethyl acetate
= 10:1) to afford a colorless oil (107 mg, 99%). 1H NMR (500 MHz, CDCl3,): 7.39-7.29 (m, 5H), 4.62 (s,
2H), 2.79 (br, 1H); 13C NMR (CDCl3, 126 MHz): 141.0, 128.6, 127.6, 127.1, 65.1.
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4-Methylbenzyl alcohol 4b.34 This compound was purified by flash chromatography (petroleum
ether/ethyl acetate = 10:1) to afford a clear liquid (110 mg, 90%). 1H NMR (500 MHz, CDCl3): 7.27 (d, J
= 7.8 Hz, 2H), 7.20 (d, J = 7.8 Hz, 2H), 4.65 (s, 2H), 2.39 (s, 3H), 2.00 (br, 1H); 13C NMR (CDCl3, 126
MHz): 138.1, 137.5, 129.4, 127.3, 65.3, 21.3.
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3-Methylbenzyl alcohol 4c.34 This compound was purified by flash chromatography (petroleum ether/ethyl
acetate = 10:1) to afford a light yellow oil (104 mg, 85%). 1H NMR (500 MHz, CDCl3): 7.28-7.25 (m, 1H),
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7.18-7.12 (m, 3H), 4.61 (s, 2H), 2.66 (br, 1H), 2.38 (s, 3H); C NMR (CDCl3, 126 MHz): 140.9, 138.2,
128.5, 128.4, 127.8, 124.1, 65.2, 21.5.
2-Methoxybenzyl alcohol 4d.35 This compound was purified by flash chromatography (petroleum
ether/ethyl acetate = 10:1) to afford a colorless oil (128 mg, 93%). 1H NMR (400 MHz, CDCl3): 7.30-7.26
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(m, 2H), 6.95 (t, J = 7.6 Hz, 1H), 6.89 (d, J = 8.0 Hz, 1H), 4.69 (s, 2H), 3.87 (s, 3H), 2.37 (br, 1H); C
NMR (CDCl3, 100 MHz): 157.4, 129.0, 128.9, 128.7, 120.6, 110.1, 62.1, 55.2.
4-Chlorobenzyl alcohol 4e.34 This compound was purified by flash chromatography (petroleum ether/ethyl
1
acetate = 10:1) to afford a white solid (128 mg, 90%). H NMR (500 MHz, CDCl3): 7.33-7.31 (m, 2H),
7.28-7.26 (m, 2H), 4.64 (s, 2H), 2.02 (br, 1H); 13C NMR (CDCl3, 126 MHz): 139.4, 133.5, 128.8, 128.4,
64.7.
4-Bromobenzyl alcohol 4f.34 This compound was purified by flash chromatography (petroleum ether/ethyl
acetate = 10:1) to afford a yellow solid (180 mg, 96%). 1H NMR (500 MHz, CDCl3): 7.47 (d, J = 8.4 Hz,
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2H), 7.21 (d, J = 8.1 Hz, 2H), 4.62 (d, J = 5.7 Hz, 2H), 2.05 (t, J = 5.8 Hz, 1H); C NMR (CDCl3, 126
MHz): 139.9, 131.8, 128.7, 121.6, 64.7.
2-Trifluoromethylbenzyl alcohol 4g.36 This compound was purified by flash chromatography (petroleum
ether/ethyl acetate = 10:1) to afford an orange liquid (151 mg, 86%). 1H NMR (400 MHz, CDCl3): 7.68 (d,
J = 7.6 Hz, 1H), 7.63 (d, J = 7.6 Hz, 1H), 7.55 (t, J = 7.6 Hz, 1H), 7.38 (t, J = 7.6 Hz, 1H), 4.85 (s, 2H),
2.37 (br, 1H); 13C NMR (CDCl3, 100 MHz): 139.1, 132.1, 128.7, 128.4, 127.2 (q, J = 31 Hz), 125.7 (q, J =
31 Hz), 124.4 (q, J = 272.2 Hz), 61.2 (q, J = 2.6 Hz); 19F NMR (376 MHz, CDCl3): -60.0 (3F, s).
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2-Hydroxybenzyl alcohol 4h.37 This compound was purified by flash chromatography (petroleum
ether/ethyl acetate = 10:1) to afford a yellow solid (113 mg, 91%). 1H NMR (500 MHz, CDCl3): 7.40 (bs,
1H), 7.22-7.19 (m, 1H), 7.04-7.03 (m, 1H), 6.88-6.84 (m, 2H), 4.83 (s, 2H); 13C NMR (CDCl3, 126 MHz):
156.1, 129.7, 128.1, 124.9, 120.3, 116.6, 64.6.
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2-Pyridinemethanol 4i.38 This compound was purified by flash chromatography (petroleum ether/ethyl
acetate = 10:1) to afford a light yellow oil (103 mg, 94%). 1H NMR (500 MHz, CDCl3): 8.37-8.36 (m, 1H),
7.58-7.55 (m, 1H), 7.29-7.27 (m, 1H), 7.07-7.05 (m, 1H), 5.27 (br, 1H), 4.66 (s, 2 H); 13C NMR (CDCl3,
126 MHz): 160.2, 148.3, 136.9, 122.2, 120.8, 64.4.
Cyclohexanemethanol 4j.34 This compound was purified by flash chromatography (petroleum ether/ethyl
acetate = 10:1) to afford a light yellow oil (95 mg, 83%). 1H NMR (500 MHz, CDCl3): 3.34 (bs, 1H), 3.30-
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3.29 (m, 2H), 1.69-1.56 (m, 5H), 1.40-1.33 (m, 1H), 1.20-1.03 (m, 3H), 0.86-0.78 (m, 2H); C NMR
(CDCl3, 126 MHz): 68.3, 40.4, 29.7, 26.6, 25.9.
Cinnamyl alcohol 4k.34 This compound was purified by flash chromatography (petroleum ether/ethyl
acetate = 10:1) to afford a light yellow oil (114 mg, 85%). 1H NMR (500 MHz, CDCl3): 7.42-7.40 (m, 2H),
7.36-7.33 (m, 2H), 7.29-7.26 (m, 1H), 6.65-6.62 (m, 1H), 6.41-6.36 (m, 1H), 4.34-4.33 (m, 2H), 2.07 (br,
1H); 13C NMR (CDCl3, 126 MHz): 136.8, 131.2, 128.7, 128.6, 127.8, 126.6, 63.7.
4-Phenyl-1,3-dioxolan-2-one 6a.39 This compound was purified by flash chromatography (petroleum
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ether/ethyl acetate = 4:1) to afford a white solid (120 mg, 73%). H NMR (400 MHz, CDCl3): 7.41-7.42
(m, 3H), 7.34-7.36 (m, 2H), 5.67 (t, J = 8.0 Hz, 1H), 4.79 (t, J = 8.0 Hz, 1H), 4.31 (t, J = 8.0 Hz, 1H); 13C
NMR (CDCl3, 100 MHz): 155.1, 135.9, 129.7, 129.2, 126.0, 78.1, 71.2.
4-Butyl-1,3-dioxolan-2-one 6b.40 This compound was purified by flash chromatography (petroleum
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ether/ethyl acetate = 4:1) to afford a colorless oil (97 mg, 67%). H NMR (400 MHz, CDCl3): 4.65-4.72
(m, 1H), 4.51 (t, J = 8.0 Hz, 1H), 4.02-4.06 (m, 1H), 1.62-1.79 (m, 2H), 1.30-1.46 (m, 4H), 0.89 (t, J = 8.0
Hz, 3H); 13C NMR (CDCl3, 100 MHz): 155.2, 77.1, 69.4, 33.5, 22.2, 13.7.
4-(Phenoxymethyl)-1,3-dioxolan-2-one 6c.41 This compound was purified by flash chromatography
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(petroleum ether/ethyl acetate = 4:1) to afford a light yellow solid (144 mg, 74%). H NMR (400 MHz,
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CDCl3): 7.30-7.34 (m, 2H), 7.01-7.04 (m, 1H), 6.91-6.93 (m, 2H), 5.01-5.06 (m, 1H), 4.61 (t, J = 8.0 Hz,
1H), 4.51-4.54 (m, 1H), 4.22-4.26 (m, 1H), 4.11-4.15 (m, 1H); 13C NMR (CDCl3, 100 MHz): 157.8, 154.8,
129.7, 121.9, 114.6, 74.3, 66.9, 66.2.
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4-Ethyl-1,3-dioxolan-2-one 6d.42 This compound was purified by flash chromatography (petroleum
ether/ethyl acetate = 4:1) to afford a yellow oil (75 mg, 65%). 1H NMR (400 MHz, CDCl3): 4.70-4.63 (m,
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1H), 4.53 (t, J = 8.3 Hz, 1H), 4.10-4.06 (m, 1H), 1.87-1.69 (m, 2H), 1.02 (t, J = 7.4 Hz, 3H); C NMR
(CDCl3, 100 MHz): 155.2, 78.1, 69.0, 26.9, 8.5.
4-(Chloromethyl)-1,3-dioxolan-2-one 6e.43 This compound was purified by flash chromatography
(petroleum ether/ethyl acetate = 4:1) to afford a yellow oil (111 mg, 81%). 1H NMR (400 MHz, CDCl3):
5.03-4.97 (m, 1H), 4.58 (t, J = 8.7 Hz, 1H), 4.39-4.36 (m, 1H), 3.38-3.79 (m, 1H), 3.73-3.69 (m, 1H); 13C
NMR (CDCl3, 100 MHz): 154.5, 74.5, 67.0, 44.1.
N-Benzylbenzamide 8a.44 This compound was purified by flash chromatography (petroleum ether/ethyl
acetate = 4:1) to afford a white solid (165 mg, 78%). 1H NMR (500 MHz, CDCl3): 7.78-7.80 (m, 2H), 7.48-
7.52 (m, 1H), 7.41-7.44 (m, 2H), 7.34-7.36 (m, 4H), 7.29-7.32 (m, 1H), 6.49 (br, 1H), 4.64 (d, J = 5.0 Hz,
2H); 13C NMR (CDCl3, 125 MHz): 167.5, 138.3, 134.5, 131.8, 129.0, 128.8, 128.1, 127.8, 127.1, 44.3.
N-Benzyl-3-methylbenzamide 8b.44 This compound was purified by flash chromatography (petroleum
ether/ethyl acetate = 4:1) to afford a yellow solid (191 mg, 85%). 1H NMR (500 MHz, CDCl3): 7.81 (d, J
= 5.0 Hz, 2H), 7.48-7.51 (m, 1H), 7.41 (t, J = 10.0 Hz, 2H), 7.22-7.25 (m, 1H), 7.10-7.16 (m, 3H), 6.79
(br, 1H), 4.58 (d, J = 5.0 Hz, 2H), 2.34 (s, 3H); 13C NMR (CDCl3, 125 MHz): 167.5, 138.5, 138.2, 134.4,
131.6, 128.7, 128.6, 128.4, 127.1, 125.0, 44.1, 21.5.
N-Benzyl-4-chlorobenzamide 8c.44 This compound was purified by flash chromatography (petroleum
ether/ethyl acetate = 4:1) to afford a white solid (204 mg, 83%). 1H NMR (500 MHz, CDCl3): 7.78 (d, J =
10.0 Hz, 2H), 7.48-7.51 (m, 1H), 7.37-7.40 (m, 2H), 7.21-7.27 (m, 3H), 7.01 (br, 1H), 4.53 (d, J = 10.0 Hz,
2H); 13C NMR (CDCl3, 125 MHz): 167.7, 137.0, 134.1, 133.3, 131.8, 129.2, 128.9, 128.7, 127.1, 43.3.
N-(4-Methoxybenzyl)benzamide 8d.45 This compound was purified by flash chromatography (petroleum
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ether/ethyl acetate = 3:1) to afford a white solid (176 mg, 73%). H NMR (600 MHz, CDCl3): 7.78-7.77
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(m, 2H), 7.47-7.44 (m, 1H), 7.38-7.35 (m, 2H), 7.24-7.23 (m, 2H), 6.84-6.83 (m, 3H), 4.51 (d, J = 5.7 Hz,
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2H), 3.76 (s, 3H); C NMR (CDCl3, 150 MHz): 167.5, 159.1, 134.5, 131.5, 130.5, 129.3, 128.5, 127.1,
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114.1, 55.3, 43.6.
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N-Benzhydrylbenzamide 8e.46 This compound was purified by flash chromatography (petroleum
ether/ethyl acetate = 4:1) to afford a pale yellow solid (195 mg, 68%). 1H NMR (600 MHz, CDCl3): 7.83
(d, J = 7.6 Hz, 2H), 7.52 (t, J = 7.3 Hz, 1H), 7.43 (t, J = 7.6 Hz, 2H), 7.38-7.29 (m, 10H), 6.85 (d, J = 6.2
Hz, 1H), 6.47 (d, J = 7.8 Hz, 1H); 13C NMR (CDCl3, 150 MHz): 166.2, 141.1, 133.8, 131.3, 128.4, 128.3,
127.2, 127.1, 126.7, 57.1.
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Morpholino(phenyl)methanone 8f.47 This compound was purified by flash chromatography (petroleum
ether/ethyl acetate = 8:1) to afford a yellow oil (136 mg, 71%). 1H NMR (600 MHz, CDCl3): 7.36 (s, 5H),
3.70-3.39 (m, 8H); 13C NMR (CDCl3, 150 MHz): 170.4, 135.3, 129.8, 128.6, 127.1, 77.3, 66.9.
Supporting Information
NMR spectra for all products, crystallographic data and computational details. This material is available
free of charge via the Internet at http://pubs.acs.org.
Notes
The authors declare no competing financial interest.
Acknowledgements
Financial support was provided by King Abdullah University of Science and Technology (KAUST).
References
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