Hydrogen-transfer reduction of α,β-unsaturated carbonyl compounds catalyzed by naphthyridine-functionalized N-heterocyclic carbene complexes

Article abstract of DOI:10.1002/aoc.3673

Substitution of silver complex of 2-chloro-7-(mesitylimidazolylidenylmethyl)naphthyridine (NpNHC) with palladium(II), rhodium(I) and iridium(I) metal precursors provided [Pd(C,N-NpNHC)(η³-allyl)](BF₄) (5), RhCl(COD)(C-NpNHC) (6a) and IrCl(COD)(C-NpNHC) (6b), respectively. Abstraction of chloride from 6a and 6b with AgBF₄ provided the chelation complexes [Rh(COD)(C,N-NpNHC)](BF₄) (7a) and Ir(COD)(C,N-NpNHC)(BF₄) (7b), respectively. All complexes were characterized using NMR and elemental analyses and the structural details of 5 and 6a were further confirmed using X-ray crystallography. In catalytic activity studies, complex 5 was found to be an effective catalyst in the hydrogen-transfer reduction of α,β-unsaturated carbonyl compounds into the corresponding saturated carbonyl compounds.

Full text of DOI:10.1002/aoc.3673

Received: 7 September 2016
DOI 10.1002/aoc.3673
Revised: 1 October 2016
Accepted: 3 October 2016
F U L L PA P E R
Hydrogen‐transfer reduction of α,β‐unsaturated carbonyl
compounds catalyzed by naphthyridine‐functionalized N‐
heterocyclic carbene complexes
Hsiao‐Ching Huang | Mani Ramanathan | Yi‐Hong Liu | Shie‐Ming Peng | Shiuh‐Tzung Liu
Department of Chemistry, National Taiwan
Substitution of silver complex of 2‐chloro‐7‐(mesitylimidazolylidenylmethyl)
University, Taipei, Taiwan, 106
naphthyridine (NpNHC) with palladium(II), rhodium(I) and iridium(I) metal
Correspondence
S.‐T. Liu, 1, Sec. 4, Roosevelt Road, Department
of Chemistry, National Taiwan University, Taipei,
precursors provided [Pd(C,N‐NpNHC)(η3‐allyl)](BF4) (5), RhCl(COD)(C‐NpNHC)
(6a) and IrCl(COD)(C‐NpNHC) (6b), respectively. Abstraction of chloride from 6a
and 6b with AgBF4 provided the chelation complexes [Rh(COD)(C,N‐NpNHC)]
Taiwan 106.
Email: stliu@ntu.edu.tw
(BF4) (7a) and Ir(COD)(C,N‐NpNHC)(BF4) (7b), respectively. All complexes were
characterized using NMR and elemental analyses and the structural details of 5 and
Funding information
Ministry of Science and Technology, Taiwan,
Grant/Award Number: (MOST103‐2113‐M‐002‐
002‐MY3).
6a were further confirmed using X‐ray crystallography. In catalytic activity studies,
complex 5 was found to be an effective catalyst in the hydrogen‐transfer reduction of
α,β‐unsaturated carbonyl compounds into the corresponding saturated carbonyl
compounds.
KEYWORDS
hydrogen transfer, naphthyridine, N‐heterocyclic carbene, palladium, reduction
1
|
INTRODUCTION
Accordingly, the incorporation of a 1,8‐naphthyridine ring in
NHC offers multiple binding sites and coordination
flexibility for metal ions. Bera and co‐workers demonstrated
that a rhodium(I) complex with a naphthyridine NHC ligand
could act as an excellent catalyst for the hydration of nitriles
to provide the corresponding amides under mild conditions
(Scheme 2).[8] Density functional theory studies suggested
that the hydrogen‐bonding interaction of a water nucleophile
with the naphthyridine nitrogen center facilitates the hydra-
tion process, showing a typical example of bifunctional
catalysis.
In our earlier investigation, complexation of 2,7‐bis
(mesitylimidazolylidenyl)naphthyridine with Pd(II) ions in
the presence ofexcess KI gave a dimeric species (I), consisting
of two trans‐PdI2 fragments bridged bytwo NHC ligands, not a
dimetallic complex hosted by the ligand (II) (Scheme 3).[5] In
the present contribution, we report the coordination of the new
hybrid ligand 2‐chloro‐7‐(mesitylimidazolylidenylmethyl)
naphthyridine (NpNHC) with Rh(I), Ir(I) and Pd(II) metal ions
and exploit the catalytic activity of the resulting complexes in
transfer hydrogenation reactions.
N‐Heterocyclic carbenes (NHCs) have been proven to be
strong σ‐donors. Thus, the bond between NHC and the
transition metal center of a complex is usually described a
σ‐dative metal–carbon single bond, with the result that the
strong metal–carbenic bond disfavors ligand dissociation.[7]
Hence, the transition metal complexes of NHCs have been
widely employed as catalysts in various organic processes.
One of the recent research efforts has focused on the devel-
opment of NHC‐based heterotopic multi‐dentate ligands,
which may have the capability to fine‐tune the property of
the metal center, thus facilitating catalytic reactions.[11] In
this regard, heteroarene‐functionalized NHCs are frequently
considered in ligand design to offer a wide variety of coor-
dination environments for metal ions and catalytic
properties.
Compared to the well‐documented pyridine‐ and pyrazole‐
substituted NHCs, studies of 1,8‐naphthyridine NHCs
(Scheme 1) are limited.[5,6,8,14,15] The 1,8‐naphthyridine ring
can act as a monodentate, a bidentate or a binuclear ligand.
Appl Organometal Chem 2016; 1–9
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
1

2
HUANG ET AL.
(d, J = 8.4 Hz, 1H, NP‐H), 4.71 (s, 2H, CH2). 13C NMR
(100 MHz, δ, ppm): 161.5, 154.6, 154.6, 138.9, 138.0,
124.1, 122.6, 120.5 (aromatic‐C), 33.4. ESI‐HRMS for
[M + H]+: calcd m/z = 256.9481 (C9H7N2ClBr). Found:
256.9483.
SCHEME 1 Naphthyridine‐based NHC ligands
2.3 | Preparation of 2‐Chloro‐7‐[(1‐mesitylimidazol‐3‐
ium)methyl]‐1,8‐naphthyridine Bromide (3)
A mixture of 2 (500 mg, 1.9 mmol) and 1‐mesitylimidazole
(380 mg, 2.0 mmol) in tetrahydrofuran (THF) was heated
under refluxing temperature for 24 h. After cooling to room
temperature, the desired product was precipitated from
solution as a white solid (818 mg, 95%). 1H NMR
(400 MHz, CDCl3, δ, ppm): 10.40 (s, 1H, Im‐H), 8.39
(d, J = 8.1 Hz, 1H, NP‐H), 8.33 (d, J = 8.1 Hz, 1H, NP‐
H), 8.25 (s, 1H, Im‐H), 8.20 (d, J = 8.4 Hz, 1H, NP‐H),
7.52 (d, J = 8.4 Hz, 1H, NP‐H), 7.11 (s, 1H, Im‐H), 6.98
(s, 2H, Ar‐H), 6.45 (s, 2H, CH2), 2.33 (s, 3H, ─Me), 2.06 (s,
6H, ─Me). 13C NMR (100 MHz, δ, ppm): 157.7 (NP‐C7),
154.8 (NP‐C), 154.7, 141.5 (NP‐C2), 139.7, 139.2, 138.3
(Im‐C2), 134.3, 130.6, 129.9 (Im‐C3), 124.4, 124.3, 123.4,
122.6, 121.4 (aromatic‐C), 53.9 (CH2), 21.1 (CH3), 17.7
(CH3). ESI‐HRMS for [M − Br]+: calcd m/z = 363.1376
(C21H20N4Cl). Found: 363.1377.
SCHEME
2
Rh–naphthyridine‐based NHC complex as catalyst in
hydration of nitriles
SCHEME
3
Palladium complexes of 2,7‐bis(mesitylimidazolylidenyl)
naphthyridine
2.4 | Preparation of silver Carbene complex (4)
A solution of 3 (100 mg, 0.27 mmol) and AgNO3 (47 mg,
0.27 mmol) in dry CH2Cl2 (10 ml) was stirred at ambient
temperature for 5 min. K2CO3 (633 mg, 4.6 mmol) was then
added and the resulting mixture was stirred for another 18 h.
The reaction mixture was passed through celite and the fil-
trate was concentrated. The residue was re‐precipitated from
CH2Cl2–hexane to give the desired complex as an orange
2
| EXPERIMENTAL
2.1 | General
All reactions and manipulations were performed under a
nitrogen atmosphere. Dichloromethane and acetonitrile were
dried over CaH2 and distilled under nitrogen. Other
chemicals and solvents were of analytical grade and were
used after degassing. NMR spectra were recorded in CDCl3
or acetone‐d6 with a Bruker AVANCE 400 spectrometer.
Chemical shifts are given in parts per million relative to
1
solid (105 mg, 85%). H NMR (400 MHz, CDCl3, δ, ppm):
8.30 (d, J = 8.2 Hz, 1H, NP‐H), 8.21 (d, J = 8.4 Hz, 1H,
NP‐H), 7.68 (d, J = 8.2 Hz, 1H, NP‐H), 7.57 (s, 1H, Im‐
H), 7.44 (d, J = 8.4 Hz, 1H, NP‐H), 6.85 (s, 1H, Im‐H),
6.78 (s, 2H, Ar‐H), 5.73 (s, 2H, CH2), 2.27 (s, 3H, ─Me),
1.70 (s, 6H, ─Me). 13C NMR (100 MHz, δ, ppm): 182.6
(Ag═C), 160.1, 154.6, 154.4, 139.8, 139.0, 135.2, 134.6,
129.1, 128.9, 124.0, 122.8, 122.6, 121.7, 121.0 (aromatic‐
C), 57.2 (CH2), 21.0 (CH3), 17.4 (CH3). ESI‐HRMS for
[M − AgBr2]+: calcd m/z = 831.1642 (C42H38N8Cl2Ag).
Found: 831.1640.
Me4Si for H NMR and 13C NMR. 2‐Chloro‐7‐methyl‐1,8‐
1
naphthyridine was prepared according to a method reported
previously.[3]
2.2 | Preparation of 2‐Chloro‐7‐bromomethyl‐1,8‐
naphthyridine (2)
A mixture of 1 (1.0 g, 5.6 mmol) and N‐bromosuccinimide
(1.2 g, 6.8 mmol) in acetonitrile (30 ml) was irradiated with
light from a bulb (250 W) for 24 h. After completion, the
reaction mixture was extracted with CH2Cl2–H2O. The
organic extracts were dried and concentrated. The residue
was chromatographed on silica gel with elution using
CH2Cl2–ethyl acetate (1:10) to give the desired compound
as a white solid (0.6 g, 42%). H NMR (400 MHz, CDCl3,
δ, ppm): 8.20 (d, J = 8.4 Hz, 1H, NP‐H), 8.12 (d, J = 8.4 Hz,
1H, NP‐H), 7.70 (d, J = 8.4 Hz, 1H, NP‐H), 7.49
2.5 | Preparation of [Pd(C,N‐NpNHC)(η3‐allyl)]
(BF4) (5)
A mixture of 4 (100 mg, 0.1 mmol), [Pd(π‐C3H5)Cl]2 (33 mg,
0.1 mmol) and AgBF4(37 mg, 0.2 mmol) in a 25 ml round‐
bottom flask was degassed and then syringed in CH2Cl2
(10 ml). The resulting mixture was stirred at room tempera-
ture for 3 h. The mixture was filtered through celite and the
filtrate was concentrated. The residue was re‐precipitated
1

HUANG ET AL.
3
from CH2Cl2–diethyl ether. The desired complex was
obtained as an orange solid (75 mg, 70%). 1H NMR
(400 MHz, CDCl3, δ, ppm): 8.66 (d, J = 8.2 Hz, 1H, NP‐
H), 8.47–8.38 (m, 2H, NP‐H), 7.91 (s, 1H, Im‐H), 7.62
(d, J = 8.2 Hz, 1H, NP‐H), 6.99 (s, 2H, Ar‐H), 6.90 (s, 1H,
Im‐H), 6.03 (d, J = 15.4 Hz, 1H, CH2), 5.60 (d,
J = 15.4 Hz, 1H, CH2), 5.31–5.14 (m, 1H, allyl‐H), 4.45
(d, J = 6.4 Hz, 1H, allyl‐H), 3.88 (d, J = 14.1 Hz, 1H,
allyl‐H), 3.02 (d, J = 6.4 Hz, 1H, allyl‐H), 2.35 (s, 3H,
─Me), 1.98 (s, 3H, ─Me), 1.95 (s, 1H, allyl‐H), 1.89
(s, 3H, ─Me). 13C NMR (100 MHz, CD2Cl2, δ, ppm):
178.8 (Pd═C), 160.1, 155.4, 153.4, 141.7, 140.6, 139.6,
135.9, 135.4, 134.8, 129.2, 129.0, 125.6, 124.8, 123.2, 122.2
(aromatic‐C), 117.1 (allyl), 77.2 (allyl), 55.5 (CH2), 47.3
(allyl), 20.9, 17.8, 17.6. Anal. Calcd for C24H24BClF4N4Pd
(%): C, 48.27; H, 4.05; N, 9.38. Found (%): C, 48.03;
H, 4.11; N, 9.34.
J = 2.0 Hz, 1H, Im‐H), 7.01 (s, 1H, Ar‐H), 6.90 (s, 1H, Ar‐
H), 6.75 (d, J = 2.0 Hz, 1H, Im‐H), 6.31 (d, J = 15.0 Hz,
1H, ─CH2), 6.08 (d, J = 15.0 Hz, 1H, ─CH2), 4.53–4.38
(m, 2H, COD), 2.88 (br, 1H, COD), 2.74 (br, 1H, COD),
2.34 (s, 3H, ─Me), 2.33 (s, 3H, ─Me), 2.06–1.95 (m, 2H,
COD), 1.92 (s, 3H, ─Me), 1.87–1.72 (m, 2H, COD), 1.57–
1.45 (m, 2H, COD), 1.45–1.32 (m, 2H, COD). 13C NMR
(100 MHz, δ, ppm): 181.0 (Ir═C), 161.5, 154.9, 154.3,
139.3, 138.7, 137.8, 136.7, 135.7, 134.3, 129.5, 128.2,
123.9, 123.5, 123.0, 121.3, 120.8 (aromatic‐C), 84.0
(COD), 83.9 (COD), 56.6 (CH2), 52.3 (COD), 51.4 (COD),
34.3 (COD), 32.5 (COD), 29.4 (COD), 28.7 (COD), 21.0,
19.5, 18.0. ESI‐HRMS for [M − Cl]+ m/z = calcd 663.1867
(C29H31N4ClIr). Found: 663.1866. Anal. Calcd for
C29H31Cl2IrN4 (%): C, 49.85; H, 4.47; N, 8.02. Found (%):
C, 49.53; H, 4.12; N, 7.82.
2.8 | Preparation of [Rh(COD)(C,N‐NpNHC)](BF4)
(7a)
2.6 | Preparation of [RhCl(COD)(C‐NpNHC)] (6a)
A mixture of 4 (105 mg, 0.1 mmol) and [Rh(COD)Cl]2
(55 mg, 0.1 mmol) in CH2Cl2 (10 ml) was stirred at ambient
temperature overnight. The reaction mixture was filtered
through celite and the filtrate was concentrated. The residue
was re‐precipitated from CH2Cl2–hexane. The desired com-
plex was obtained as a yellow solid (111 mg, 95%). 1H
NMR (400 MHz, CDCl3, δ, ppm): 8.24 (d, J = 8.2 Hz, 1H,
NP‐H), 8.15 (d, J = 8.4 Hz, 1H, NP‐H), 8.11 (d,
J = 8.2 Hz, 1H, NP‐H), 7.49 (d, J = 8.4 Hz, 1H, NP‐H),
7.16 (s, 1H, Im‐H), 7.07 (s, 1H, Ar‐H), 6.90 (s, 1H, Ar‐H),
6.75 (s, 1H, Im‐H), 6.56 (d, J = 15.0 Hz, 1H, ─CH2), 6.18
(d, J = 15.0 Hz, 1H, ─CH2), 4.86 (br, 1H, COD), 4.80 (br,
1H, COD), 3.28 (br, 1H, COD), 3.00 (br, 1H, COD), 2.42
(s, 3H, ─Me), 2.35 (s, 3H, ─Me), 2.17–2.04 (m, 2H,
COD), 2.04–1.92 (m, 2H, COD), 1.85 (s, 3H, ─Me), 1.77–
1.63 (m, 2H, COD), 1.58–1.38 (m, 2H, COD). 13C NMR
(100 MHz, δ, ppm): 183.2 (d, JRh─C = 51.6 Hz, Rh═C),
161.5, 154.7, 154.1, 139.1, 138.5, 137.7, 136.8, 135.7,
134.1, 129.4, 128.1, 123.7, 123.6, 123.0, 121.4, 120.7 (aro-
matic‐C), 97.6 (br, COD‐C), 68.8 (d, JRh─C = 14.7 Hz,
COD‐C), 67.7 (d, JRh─C = 14.2 Hz, COD‐C), 57.1 (CH2),
34.0 (COD‐C), 31.6 (COD‐C), 29.1 (COD‐C), 28.1 (COD‐
C), 21.2 (CH3), 19.9 (CH3), 18.1 (CH3). HR‐HRMS for
[M − Cl]+ m/z = calcd 573.1292 (C29H31N4ClRh). Found:
573.1286. Anal. Calcd for C29H31Cl2N4Rh(CHCl3) (%): C,
49.44; H, 4.43; N, 7.69. Found (%): C, 49.71; H, 4.46; N,
7.79.
A mixture of 6a (111 mg, 0.2 mmol) and AgBF4 (49 mg,
0.2 mmol) in CH2Cl2 (10 ml) was stirred under nitrogen
atmosphere at ambient temperature overnight. The reaction
mixture was filtered through celite and the filtrate was con-
centrated. The residue was re‐precipitated from CH2Cl2–hex-
ane. The desired complex was obtained as an orange solid
1
(118 mg, 98%). H NMR (400 MHz, CDCl3, δ, ppm): 8.65
(d, J = 8.2 Hz, 1H, NP‐H), 8.47 (d, J = 8.5 Hz, 1H, NP‐
H), 8.32 (d, J = 8.2 Hz, 1H, NP‐H), 7.71 (d, J = 1.6 Hz,
1H, Im‐H), 7.58 (d, J = 8.5 Hz, 1H, NP‐H), 7.08 (s, 1H,
Ar‐H), 6.95 (s, 1H, Ar‐H), 6.69 (d, J = 1.6 Hz, 1H, Im‐H),
6.31 (d, J = 14.9 Hz, 1H, ─CH2), 6.19 (d, J = 14.9 Hz,
1H, ─CH2), 5.80–5.69 (m, 1H, COD), 4.45–4.31 (m, 1H,
COD), 3.99 (br, 1H, COD), 3.10 (br, 1H, COD), 2.64–2.41
(m, 1H, COD), 2.36 (s, 3H, ─Me), 2.29–2.16 (m, 3H,
COD), 2.15–2.03 (m, 2H, COD), 1.97 (s, 3H, ─Me), 1.95
(s, 3H, ─Me), 1.75–1.61 (m, 2H, COD). 13C NMR
(100 MHz, δ, ppm): 179.0 (d, JRh─C = 51.5 Hz, Rh═C),
161.2, 154.6, 153.7, 141.3, 140.9, 139.1, 135.6, 135.0,
134.1, 129.8,129.5, 129.1, 125.2, 124.6, 123.0, 122.2 (aro-
matic‐C), 100.9 (COD), 99.6 (COD), 74.1 (d,
JRh─C = 14.9 Hz, COD), 71.0 (d, JRh─C = 12.7 Hz, COD),
56.3 (CH2), 34.0 (COD), 30.7 (COD), 29.7 (COD), 27.2
(COD), 21.1 (CH3), 18.9 (CH3), 17.9 (CH3). ESI‐HRMS
for [M]+ m/z = calcd 573.1292 (C29H31N4ClRh). Found:
573.1297. Anal. Calcd for C29H31BCl F4N4Rh(CH2Cl2)
(%): C, 48.32; H, 4.46; N, 7.51. Found (%): C, 48.41; H,
4.67; N, 7.81.
2.7 | Preparation of [IrCl(COD)(C‐NpNHC)] (6b)
2.9 | Preparation of [Ir(COD)(C,N‐NpNHC)](BF4) (7b)
The procedure for the preparation of 6b was similar to that for
6a, affording a yellow solid (132 mg, 96%). 1H NMR
(400 MHz, CDCl3, δ, ppm): 8.24 (d, J = 8.3 Hz, 1H, NP‐
H), 8.15 (d, J = 8.4 Hz, 1H, NP‐H), 8.07 (d, J = 8.3 Hz,
1H, NP‐H), 7.51 (d, J = 8.4 Hz, 1H, NP‐H), 7.24 (d,
The procedure for the preparation of 7b was similar to that for
7a, affording an orange solid (78 mg, 93%). 1H NMR
(400 MHz, CDCl3, δ, ppm): 8.69 (d, J = 8.3 Hz, 1H, NP‐
H), 8.52 (d, J = 8.3 Hz, 1H, NP‐H), 8.31 (d, J = 8.3 Hz,

4
HUANG ET AL.
1H, NP‐H), 7.71 (s, 1H, Im‐H), 7.60 (d, J = 8.3 Hz, 1H, NP‐
H), 7.02 (s, 1H, Ar‐H), 6.95 (s, 1H, Ar‐H), 6.77 (s, 1H, Im‐
H), 6.07 (d, J = 15.1 Hz, 1H, ─CH2), 5.93 (d, J = 15.1 Hz,
1H, ─CH2), 5.49–5.31 (m, 1H, COD), 4.15–4.03 (m, 1H,
COD), 3.60–3.56 (m, 1H, COD), 3.05–2.89 (m, 1H, COD),
2.35 (s, 3H, ─Me), 2.30–2.19 (m, 3H, COD), 2.12–2.05
(m, 3H, COD), 2.03 (s, 3H, ─Me), 1.94–1.86 (m, 2H,
COD), 1.83 (s, 3H, ─Me). 13C NMR (100 MHz, CDCl3, δ,
ppm): 174.9 (Ir═C), 159.8, 154.5, 153.6, 141.5, 141.2,
139.2, 135.5, 134.7, 134.2, 129.5, 129.0, 125.5, 124.8,
122.9, 122.8, 122.3 (aromatic‐C), 87.4 (COD), 86.5 (COD),
60.6 (COD), 57.1 (COD), 55.5 (CH2), 35.5 (COD), 31.2
(COD), 30.3 (COD), 27.2 (COD), 21.0 (CH3), 19.0 (CH3),
17.9 (CH3). ESI‐HRMS for [M]+ m/z = calcd 663.1867
(C29H31N4ClIr). Found: 663.1868.
acetate–water. The organic extracts were dried and concen-
trated. The residue was chromatographed on a silica gel
column with elution using hexane–ethyl acetate. The
following obtained compounds were characterized by
1
comparing their H NMR and 13C NMR spectral data (see
supporting information) with those reported in the literature:
4‐phenylbutan‐2‐one, 1‐phenylbutan‐1one, methyl 3‐pheny-
lpropanoate, 3‐phenylpropanoic acid, 3‐phenylpropionamide,
phenyl 3‐phenylpropanoate, 4‐(naphthalen‐1‐yl)butan‐2‐one,
4‐(4‐cyanophenyl)butan‐2‐one, 1‐(4‐methylphenyl)butan‐3‐
one, 4‐(4‐aminophenyl)‐butan‐2‐one, 4‐(3‐hydroxyphenyl)
butan‐2‐one, 4‐pyridin‐3‐ylbutan‐2‐one, tridecan‐2‐one, 6‐
phenylhexan‐2‐one and methyl 5‐phenylpentanoate.
3
| RESULTS AND DISCUSSION
2.10 | Crystallography
3.1 | Synthesis and characterization of Palladium
complex 5
Crystals of 5 and 6a⋅(CHCl3) suitable for X‐ray determina-
tion were obtained by recrystallization from CH2Cl2–ether
and CHCl3–hexane, respectively. Cell parameters were deter-
mined using an Oxford Diffraction Gemini A diffractometer.
The structure was solved using the SHELXS‐97 program[9]
by full‐matrix least‐squares on F2 values. ORTEP plot
program[4a] and data reduction software[4b] were from avail-
able sources. CCDC numbers for 5 and 6a are 1502091 and
1502090, respectively. Crystal data for 5 and 6a are as
follows.
The synthesis of the ligand is shown in Scheme 4. The pre-
cursor 1 was prepared according to a previously reported
method.[3] Bromination of 1 followed by substitution with
imidazole gave the carbene precursor in good yield. ESI‐
HRMS of 3 shows m/z at 363.1377, corresponding to the
composition of C21H20N4Cl [M − Br]+, which confirms the
molecular formula.
The preparation of the desired palladium carbene com-
plex from the direct reaction of 3 with Pd(OAc)2 was unsuc-
cessful. We then pursued the carbene transfer method as an
alternative approach. Reaction of 3 with equimolar amount
of AgNO3 in the presence of K2CO3 gave the silver carbene
complex 4 as an orange solid in 85% yield. Characterization
of the silver complex 4 was achieved using NMR and MS
methods. The disappearance of the proton at C‐2 position
of the imidazolium in the 1H NMR spectrum and the appear-
ance of a downfield shift at 182.6 ppm corresponding to
Ag─C(carbene) clearly indicate the formation of the complex.
Typical ESI‐HRMS spectrum of the complex provides the
highest mass peak at m/z 831.1640, corresponding to the
mass of the cationic portion of the complex
[C42H40AgCl2N8]+ (calcd m/z 831.1642). These observations
confirm the structure of 4, which has the same stoichiometry
as previously reported species.[1]
Crystal data for 5: C24H24BClF4N4Pd, Fw = 597.13, temp.
= 150(2) K, triclinic, P‐1, a = 8.1823(3) Å, b = 12.9603(5) Å,
c = 13.0762(5) Å, α = 66.960(4)°, β = 78.251(3)°, γ = 88.334
(3)°, V = 1247.35(8) Å3, Z = 2, Dcalcd = 1.590 Mg m−3, F
(000) = 600, crystal size =0.20 × 0.15 × 0.10 mm3, θ
range = 2.87–27.50°, 11790 reflections collected, 5551 [R
(int) = 0.0354], final R indices [I > 2σ(I)] R1 = 0.0422,
wR2 = 0.0914, for all data R1 = 0.0614, wR2 = 0.1016, good-
ness‐of‐fit on F2 = 1.031.
Crystal data for 6a: C30H32Cl5N4Rh, Fw = 728.76, temp.
= 150(2) K, triclinic, P‐1, a = 10.3264(5) Å, b = 12.1585(6)
Å, c = 13.7168(4) Å, α = 68.924(4)°, β = 86.195(4)°,
γ
=
74.043(4)°,
V
=
1544.05(12) Å3,
Z
=
2,
Dcalcd = 1.567 Mg m−3, F(000) = 740, crystal size
=0.25 × 0.15 × 0.10 mm3, θ range = 2.85–25.00°, 12 012
reflections collected, 5447 [R(int) = 0.0518], final R indices
[I > 2σ(I)] R1 = 0.0382, wR2 = 0.0946, for all data
R1 = 0.0460, wR2 = 0.1024, goodness‐of‐fit on F2 = 1.042.
Several attempts at the reaction of 4 with various palla-
dium sources such as Pd(PhCN)2Cl2, Pd(COD)Cl2 and Pd
(PPh3)2Cl2 were not quite successful, providing a mixture
2.11 | General procedure for catalysis
A reaction tube was loaded with a mixture of α,β‐unsaturated
carbonyl compound (0.5 mmol), triethylamine (2.0 mmol),
formic acid (1.5 mmol) and metal complex (0.005 mmol,
1 mol%) in isopropanol (1.6 ml). The reaction vessel was
degassed and flashed with nitrogen. The mixture was stirred
for 16 h. After reaction, the mixture was extracted with ethyl
SCHEME 4 Preparation of NpNHC ligand

HUANG ET AL.
5
of various products in each case. Fortunately, the transfer
reaction of 4 with [(π‐allyl)PdCl]2 in the presence of AgBF4
proceeded smoothly to give the desired palladium complex
5 as the single product (Scheme 5). ESI‐HRMS analysis of
5 gave a signal at m/z = 509.0736, which is consistent with
the formulation of the cationic portion of the complex, [Pd
(C,N‐NpNHC)(η3‐allyl)]+. 1H NMR shifts at 5.20, 4.45,
3.88 and 3.02 ppm of 5 correspond to the protons of coordi-
nating π‐allyl moiety. The 13C NMR spectrum of 5 exhibits a
signal corresponding to the palladium–carbene carbon, which
appears at 178.8 ppm. In addition to the above information,
the detailed structure of 5 was further confirmed using X‐
ray crystallography.
Suitable crystals of 5 for X‐ray analysis were grown by
slow evaporation from a mixed CH2Cl2–ether solvent.
Selected bond distances and angles are summarized in
Table 1, while an ORTEP plot for the cationic part of 5 is
illustrated in Figure 1. Pd(1) is in a plane with the NHC
and naphthyridine rings. The NpNHC unit binds to the
palladium center in a bidentate fashion with a bite angle of
87.4(1)°. Considering the allyl being bidentate, the palladium
center exhibits a slightly distorted square planar geometry.
The distances from palladium to all carbons of the allyl group
are in the range 2.093(4)–2.183(4) Å, which are typical as
compared to those of related palladium species. The bond
length of Pd(1)─C(24) (2.183(4) Å) is slightly longer than
that of Pd(1)─C(26) (2.093(4) Å) by about 0.1 Å, which is
presumably due to the trans influence.[12]
FIGURE 1 ORTEP plot of cationic portion of 5 (30% probability level)
(symmetry operation A: x, y, z)
SCHEME 6 Preparation of NpNHC Rh(I) and Ir(I) complexes
The 13C NMR spectrum of 6a exhibits a characteristic
signal at 183.2 ppm with a coupling constant JRh─C = 51.2 Hz
due to the NpNHC carbene carbon resonance, while the
carbene carbon shift of 6b appears at 181.0 ppm. These data
are comparable to those of the related pyridinyl NHC species
reported in the literature such as 8a (181.1 ppm,
JRh─C = 51 Hz) and 8b (179.6 ppm).[13] The 1H NMR spectra
of 6a and 6b are also consistent with the corresponding
proposed structures and quite similar to those of complexes
8a and 8b (Scheme 7). Due to the diastereotopic nature of
3.2 | Preparation and characterization of Rhodium and
Iridium complexes
Treatment of [Rh(COD)Cl]2 with 4 in CH2Cl2 at room
temperature readily gave the desired carbene complex 6a,
whereas reaction of [Ir(COD)Cl]2 with 4 behaved similarly
to yield 6b (Scheme 6). Abstraction of a chloride ligand from
6a and 6b by AgBF4 gave the corresponding chelation
complexes 7a and 7b, respectively.
1
benzylic protons in the structures of 6a and 6b, H NMR
spectra of both 6a and 6b give two sets of doublets at 6.56
and 6.18 ppm with Jgem = 15 Hz for 6a, and 6.32 and
6.08 ppm with Jgem = 15 Hz for 6b, which are similar to
those in complexes 8a and 8b. The molecular structure
of 6a was unequivocally determined by means of X‐ray
diffraction study. Figure 2 shows the ORTEP diagram of
6a and Table 2 summarizes some structural parameters.
The geometry around the rhodium(I) center is square
planar with two coordination sites occupied by carbene and
SCHEME 5 Preparation of complex 5
TABLE 1 Selected bond distances (Å) and bond angles (°) for complex 5
Pd(1)–N(2)
2.137(3)
N(2)–Pd(1)–C(12)
87.4(1)
Pd(1)–C(12)
Pd(1)–C(24)
Pd(1)–C(25)
Pd(1)–C(26)
2.029(3)
2.183(4)
2.133(9)
2.093(4)
N(2)–Pd(1)–C(26)
C(12)–Pd(1)–C(24)
C(26)–Pd(1)–C(24)
C(12)–Pd(1)–C(26)
169.44(14)
170.04(16)
68.56(17)
102.69(15)
SCHEME 7 Pyridinyl NHC complexes

6
HUANG ET AL.
1
of chelation complexes 7a and 7b. In addition, H NMR
signals corresponding to the methylene units for 7a and
7b are shifted upfield by ca 0.12 ppm presumably due
to the ring effect. These shifts gave an indication of the
chelation towards the metal center. All these spectral data
suggest the chelation structures of the complexes.
3.3 | Catalysis
Palladium complexes of NHCs have received increasing
attention as catalysts for a number of reactions particularly
cross‐coupling reactions.[2] Compared to coupling reactions,
Pd–NHC‐catalyzed hydrogen‐transfer reductions are less
explored.[10] We studied the selective reduction of C═C in
α,β‐unsaturated carbonyl compounds catalyzed by 5–7 using
formate as the hydrogen‐transfer agent.
FIGURE 2 ORTEP plot of 6a (30% probability level) (symmetry operation
A: −x, −y, −z)
First, we searched the optimal conditions for the reduc-
tion of benzylideneacetone with 5 as the catalyst. All obser-
vations are summarized in Table 4. The optimized
conditions for the reduction are as follows: a mixture of
benzylideneacetone (0.5 mmol), complex 5 (1 mol%),
triethylamine and formic acid in isopropanol was heated
under reflux for 16 h (Table 4, entry 10). The use of hydrogen
gas as reducing agent under basic conditions does not provide
a good result. Alcohols as hydrogen‐transfer source provide
poor reduction of C═C bonds. It appears that the use of
formic acid as the hydrogen‐transfer source does assist the
reactions under basic conditions. With several attempts, we
determined learned that formic acid and triethylamine in a
ratio of 1.5:2 is the best combination for this reduction.
It is worth mentioning that the reaction proceeds in vari-
ous organic solvents except dichloromethane. Next we
ran a further series of trial reactions in the presence of
various complexes to compare the activity of these pre‐cat-
alysts (Table 5). On the basis of the product analysis, most
complexes have no catalytic activity in the reduction of
benzylideneacetone under these hydrogen‐transfer condition.
While rhodium complex 6a shows poor activity in the
reduction of C═C bond of the substrate, the corresponding
iridium complex 6b exhibits a different reactivity, i.e. two
reducing products are obtained. Other rhodium and iridium
complexes show some activity in reduction of the C═C bond,
but not as good as 5.
TABLE 2 Selected bond distances (Å) and bond angles (°) for 6a
Rh(1)–C(10)
2.035(4)
C(10)–Rh(1)–Cl(1)
87.13(11)
Rh(1)–C(24)
Rh(1)–C(23)
Rh(1)–C(28)
Rh(1)–C(27)
Rh(1)–Cl(1)
N(3)–C(9)
2.104(4)
2.110(4)
2.190(4)
2.216(4)
2.4141(10)
1.453(6)
C(23)–Rh(1)–Cl(1)
C(24)–Rh(1)–Cl(1)
C(10)–Rh(1)–C(23)
C(10)–Rh(1)–C(24)
C(10)–Rh(1)–C(27)
C(10)–Rh(1)–C(28)
161.32(11)
159.79(12)
94.69(16)
93.15(15)
165.17(16)
158.53(16)
chloride in a cis arrangement. It is quite clear that the
naphthyridinyl nitrogen donors remain uncoordinated. The
average distances of Rh─C(COD) cis to carbene donor
(2.107 Å) appear to be shorter than those in trans arrange-
ment (2.203 Å), indicating that the σ‐donation nature of
NHC carbene is stronger than that of a chloride ligand. The
imidazol‐2‐ylidene ring is bisected with the coordination
plane by 68.03°, which is quite similar to that of complex
8a (73.2°).[13] Other structural parameters including bond
distances and angles are in the normal range.
The formulations of 7a and 7b were determined using
elemental analysis and confirmed by NMR observation. It
is observed that the 13C NMR signals of carbene carbon
shift upfield upon chelation in the pyridinyl NHC iridium
or rhodium complexes, i.e. 8a versus 9a and 8b versus 9b
(Table 3).[13] A similar trend is observed in the complexes
6a versus 7a and 6b versus 7b, indicating the formation
With the optimized conditions in hand, the reduction of a
variety of α,β‐unsaturated carbonyl compounds catalyzed by
TABLE 3 Partial chemical shifts for 6–9 for comparison
Non‐chelation complexes
C,N‐chelation complexes
Compound
13C shift of Ccarbene (ppm)
1H shifts of CH2 (ppm)
Compound
13C shift of Ccarbene (ppm)
1H shifts of CH2 (ppm)
6a
6b
8a
8b
183.2
181.0
182.0
179.6
6.56, 6.18
6.31, 6.08
6.39, 5.92
6.21, 5.71
7a
7b
9a
9b
179.0
174.9
174.2
171.3
6.31, 6.19
6.07, 5.93
6.38, 6.17
6.07, 5.97

HUANG ET AL.
7
TABLE 4 Reduction of benzylideneacetone catalyzed by 5a
Entry
Reducing reagent (mmol)
Solvent
Temp. (°C)
Yield (%)b
1
H2/KOtBu (0.15)
CH3OH
CH3OH
i‐PrOH
i‐PrOH
i‐PrOH
i‐PrOH
i‐PrOH
i‐PrOH
i‐PrOH
i‐PrOH
CH2Cl2
CH3OH
Toluene
THF
r.t.
<10
42
2
H2/Et3N (0.15)
r.t.
3
NaOCH2CH3
reflux
r.t.
Trace
10
4
Et3N
5
HCOONa (1.5)
83
16
6
HCOONa–HCOOH (1.5 /1.5)
Et3N–HCOOH (1/1)
Et3N–HCOOH (2/1)
Et3N–HCOOH (1/2)
Et3N–HCOOH (2/1.5)
Et3N–HCOOH (2/1.5)
Et3N–HCOOH (2/1.5)
Et3N–HCOOH (2/1.5)
Et3N–HCOOH (2/1.5)
Et3N–HCOOH (2/1.5)
Et3N–HCOOH (2/1.5)
Et3N–HCOOH (2/1.5)
83
68
7
83
95
8
83
89
9
83
94
10
11
12
13
14
15
16
17
83
99
40
Trace
30
65
110
65
89
63
H2O
100
120
100
12
DMF
70
Dioxane
63
aReaction conditions: benzylideneacetone (0.5 mmol), complex 5 (5 × 10−3 mmol), formate and additives in solvent (1.5 ml) for 16 h under N2 atmosphere.
bNMR yield.
TABLE 5 Reduction of benzylideneacetone catalyzed by various com-
5 was investigated (Table 6). Most of the α,β‐unsaturated
carbonyl compounds are found to undergo C═C bond reduc-
tion selectively to afford the corresponding saturated car-
bonyl compounds in good to excellent yields (Table 6,
entries 1–18). For α,β‐unsaturated ester, carboxylic acid and
amide, the reduction proceeds smoothly to give the corre-
sponding products (Table 6, entries 3–6 and 18). However,
3‐methoxycyclohex‐2‐enone and 4‐(2‐thienyl)‐3‐buten‐2‐
one do not provide the corresponding reduction product.
Interestingly, we have found that this catalytic system can
be successfully applied to the reduction of aryl halides
and nitro compounds. Thus, 4‐(4‐bromophenyl)‐3‐buten‐2‐
one and the chloro‐substituted analogue are reduced to yield
4‐phenylbutan‐2‐one (Table 6, entries 9–10), whereas 4‐(4‐
nitrophenyl)‐3‐buten‐2‐one gives 4‐(4‐aminophenyl)‐3‐
buten‐2‐one as the exclusive product (Table 6 entry 12). It
is noticed that the extended conjugated systems are also
reduced to yield the saturated products (Table 6, entries
17 and 18).
plexes.a
Product yield (%)b
PhCH═CHCH(OH)
Entry
Catalyst
PhCH2CH2COCH3
CH3
1
5
99
16
29
0
0
0
2
6a
3
6b
21
0
4
7a
5
7b
0
0
6
Pd(OAc)2
[Pd(π‐allyl)Cl]2
Pd(COD)Cl2
Pd2(dba)3
Pd(PPh3)2Cl2
0
0
7
0
0
8
0
0
9
0
0
10
11
0
0
0
0
Based on the pseudo first‐order approximation, the
reaction rate constants of various substrates were obtained
(Table 7). It is found that the reduction rates of the sub-
strates with hydrogen‐bonding functionality are faster than
those of substrates without. For example, the reduction of
cinnamic acid proceeds at a faster rate as compared to that
of benzylideneacetone. Presumably, the hydrogen‐bonding
interaction of ─OH moiety with the naphthyridine append-
age makes a complementary interaction between the
substrate and the catalyst, which readily assists the reduc-
tion (Scheme 8).
12
13
14
[Rh(COD)Cl]2
[Ir(COD)Cl]2
15
38
55
0
0
0
[Rh(COD)Cl]2/
PPh3
15
16
[Ir(COD)Cl]2/
PPh3
61
56
0
0
[Rh(COD)Cl]2/
dppe
aReaction conditions: benzylideneacetone (0.5 mmol), complex (5 × 10−3 mmol),
formic acid (2 mmol) and Et3N (1.5 mmol) in i‐PrOH (1.5 ml) at refluxing tem-
perature for 16 h under N2 atmosphere.
bNMR yield.

8
HUANG ET AL.
TABLE 6 Reduction of various α,β‐unsaturated carbonyl compoundsa
Entry
Reactant
Product
Yield (%)b
1
2
3
4
5
6
7
PhCH═CHCOCH3
PhCOCH═CHCH3
PhCH═CHCOOCH3
PhCH═CHCOOH
PhCH═CHCONH2
PhCH═CHCOOPh
PhCH2CH2COCH3
PhCH(OH)CH2CH2CH3
PhCH2CH2COOCH3
PhCH2CH2COOH
97
96
93
96
98
47
74
PhCH2CH2CONH2
PhCH2CH2COOPh
8
p‐(NC)C6H4CH═CHCOCH3
p‐BrC6H4CH═CHCOCH3
p‐ClC6H4CH═CHCOCH3
p‐MeC6H4CH═CHCOCH3
p‐O2NC6H4CH═CHCOCH3
m‐HOC6H4CH═CHCOCH3
Cyclopent‐2‐enone
p‐(NC)C6H4CH2CH2COCH3
C6H5CH2CH2COCH3
C6H5CH2CH2COCH3
p‐MeC6H4CH2CH2COCH3
p‐H2NC6H4CH2CH2COCH3
m‐HOC6H4CH2CH2COCH3
Cyclopentanone
99
40
81
68
49
76
67
60
93
63
84
—
—
9
10
11
12
13
14
15
16
17
18
19
20
Cyclohex‐2‐enone
Cyclohexanone
CH3(CH2)8CH═CHCOCH3
PhCH═CHCH═CHCOCH3
PhCH═CHCH═CHCOOCH3
3‐Methoxy‐cyclohex‐2‐enone
CH3(CH2)10COCH3
Ph(CH2)4COCH3
Ph(CH2)4COOCH3
—
—
aReaction conditions: substrate (0.5 mmol), complex 5 (5 × 10−3 mmol), formic acid (2 mmol) and Et3N (1.5 mmol) in i‐PrOH (1.5 ml) at refluxing temperature for 16 h
under N2 atmosphere.
bIsolated yield.
TABLE 7 Reaction rates of some α,β‐unsaturated carbonyl compoundsa
excellent catalyst for the selective reduction of α,β‐unsatu-
Rate constant (h−1
)
Relative rate
rated carbonyl compounds into the corresponding saturated
carbonyl compounds. Both aryl halides and nitroarenes are
also reduced using this catalytic system. Although the chloro
substituent in the ligand appears to have no role in this
chemistry, it is a functionality allowing further modification
of new donors. More studies concerning this matter are
currently in progress.
Substrate
PhCH═CHCOCH3
PhCH═CHCOOH
0.096
0.156
0.132
0.124
0.070
1.00
1.62
1.37
1.29
0.73
PhCH═CHCONH2
PhCH═CHCH═CHCOOH
PhCH═CHCONMe2
aReaction conditions: as described in the footnote to Table 6.
ACKNOWLEDGMENTS
This work was financially supported by the Ministry of
Science and Technology, Taiwan (MOST103‐2113‐M‐002‐
002‐MY3).
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