Chemoselective aerobic oxidation of unprotected diols catalyzed by Pd-(NHC) (NHC = N-heterocyclic carbene) complexes

Article abstract of DOI:10.1016/j.molcata.2010.02.016

Neutral Pd(X)(η³-allyl) (X = Cl, OAc (acetate)) complexes bearing mono-coordinating NHC ligands have been synthesized, characterized and employed to catalyze the aerobic oxidation of unprotected 1,2- and 1,3-diols selectively to hydroxy ketones. A comparison of the catalytic performance of these precursors with a reference system has shown that the precursor with the ligands N,N′-bis(adamantyl)imidazol-2-ylidene and chloride is the most efficient for the chemoselective oxidation of 1,2-diols is concerned. High-pressure ¹H NMR (HPNMR) experiments in combination with catalytic batch reactions have provided valuable information on the activation of the precursor as well as on the stability of the catalysts.

Full text of DOI:10.1016/j.molcata.2010.02.016

Journal of Molecular Catalysis A: Chemical 322 (2010) 63–72
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Chemoselective aerobic oxidation of unprotected diols catalyzed by Pd–(NHC)
(NHC = N-heterocyclic carbene) complexes
Lorenzo Bettuccia, Claudio Bianchinia, Werner Oberhausera,∗, Tsun-Hung Hsiaob, Hon Man Leeb,∗∗
a Istituto di Chimica dei Composti Organometallici (ICCOM - CNR), Area di Ricerca CNR di Firenze, via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy
b Department of Chemistry, National Changhua University of Education, 50058 Changhua, Taiwan
a r t i c l e i n f o
a b s t r a c t
Neutral Pd(X)(␩3-allyl) (X = Cl, OAc (acetate)) complexes bearing mono-coordinating NHC ligands have
been synthesized, characterized and employed to catalyze the aerobic oxidation of unprotected 1,2- and
1,3-diols selectively to hydroxy ketones. A comparison of the catalytic performance of these precursors
with a reference system has shown that the precursor with the ligands N,Nꢀ-bis(adamantyl)imidazol-2-
ylidene and chloride is the most efficient for the chemoselective oxidation of 1,2-diols is concerned. High-
pressure 1H NMR (HPNMR) experiments in combination with catalytic batch reactions have provided
valuable information on the activation of the precursor as well as on the stability of the catalysts.
© 2010 Elsevier B.V. All rights reserved.
Article history:
Received 11 November 2009
Received in revised form 6 February 2010
Accepted 8 February 2010
Available online 13 February 2010
Keywords:
Chemoselective oxidation
Diols
Palladium
N-heterocyclic carbenes
1. Introduction
alcohol oxidation. In this respect, the most significant catalytic
systems applied up to now for the aerobic oxidation of alcohols
The oxidation of alcohols to carbonyl compounds is an
important functional group transformation in organic synthe-
sis. Among the synthetic protocols developed so far, the Omura
and Swern [1a] and the Dess–Martin [1b] oxidation have been
extensively used to convert primary and secondary alcohol func-
tional groups into the corresponding aldehyde or ketone. These
methods, however, are often scarcely selective, and therefore
alternative synthetic protocols have been developed to obtain a
higher selectivity for the oxidation of secondary alcohols [2a],
using stoichiometric amounts of chemical oxidizing reagents
[2b–d]. An attractive and more sustainable oxidant is molec-
ular oxygen because it is readily available, inexpensive and
can produces either H2O and/or H2O2 as by-products of the
oxidation reactions. The most applied metals for the aerobic
oxidation of alcohols are Mn [3], Fe [4], Ru [5], Co [6], Cu
[7], Pt [8], Zn [9], V [10], Ni [11] and Pd [12]. In particular,
palladium-based oxidation catalysts have been extensively used
for aerobic alcohol oxidations since Blackburn and Schwartz [13]
reported the homogeneous oxidation of secondary alcohols to
ketones by molecular oxygen under mild reaction conditions.
Since then, increasing research efforts have been made in an
attempt of developing new Pd-based catalysts for the aerobic
are: (i) the Pd(OAc)2/DMSO system [14] developed by Peterson
and Larock, (ii) the Uemura Pd(OAc)2/pyridine system [15] and
its NEt3-modified version introduced by Sigman [16], (iii) the
Sheldon Pd(OAc)2/modified-phenanthroline system [17], (iv) the
Pd–sparteine system [18], (v) the neutral cyclopalladate com-
plexes [19] applied by Moberg and (vi) the Sigman Pd–NHC
complex of the formula Pd(OAc)2(H2O)L with L being 1,3-di-(2,6-
iso-propyl)phenyl-imidazol-2-ylidene [20]. The latter complex
represents the first example of a Pd–NHC complex utilized for the
aerobic oxidation of alcohols under rather mild reaction conditions
[20b] exploiting the exceptional stability towards air and moisture
of the NHC ligands [21].
From several mechanistic studies of aerobic oxidation reactions
of alcohols catalyzed by ligand-stabilized palladium complexes has
clearly emerged the requirement of: (i) a monodentate ligand that
stabilizes palladium and lowers significantly the energy barrier
for the ␤-hydride elimination reaction of the palladium alkoxide
species (rate-determiningstep ofthe catalytic oxidation cycle) [22];
(ii) a base to convert the alcohol into the corresponding alkoxide,
that subsequently undergoes a ␤-hydride elimination reaction to
give a Pd-hydride species and the oxidized substrate.
Alternatively to the Sigman complex, aerobic alcohol oxidation
reactions can be promoted by neutral Pd(␩3-allyl)(NHC) com-
plexes, largely employed in C–C and C–N coupling reactions such as
the Suzuki–Miyaura coupling [23], the Buchwald–Hartwig amina-
tion [23a,b], the allylic alkylation reaction [24] the telomerization
reaction of 1,3-butadienes with alcohol [25] and the chemoselctive
anaerobic oxidation of secondary alcohols [26].
∗
Corresponding author. Tel.: +39 055 5225384; fax: +39 055 5225203.
Co-corresponding author. Tel.: +886 4 7232105 3523; fax: +886 4 7211190.
E-mail addresses: werner.oberhauser@iccom.cnr.it (W. Oberhauser),
∗∗
leehm@cc.ncue.edu.tw (H.M. Lee).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.02.016

64
L. Bettucci et al. / Journal of Molecular Catalysis A: Chemical 322 (2010) 63–72
Herein we report the synthesis of neutral Pd(II) complexes
0.50 ␮m film thickness) CP-WAX 52 CB WCOT-fused silica col-
umn.
of the type Pd(X)(␩3-allyl)(NHC) (X = Cl, OAc) (Scheme 1) bear-
ing the NHC ligands N,Nꢀ-bis(benzyl)-imidazol-2-ylidene (L1)
(1, 5), N,Nꢀ-bis(adamantyl)-imidazol-2-ylidene (L2) (2, 6), N,Nꢀ-
bis(mesityl)-imidazol-2-ylidene (L3) (3, 7) and N,Nꢀ-bis(di(2,6-iso-
propyl)phenyl)-imidazol-2-ylidene (L4) (4, 8) and their application
in the aerobic oxidation of unprotected 1,2- and 1,3-diols in a sol-
vent mixture of toluene and DMSO.
2.2. Syntheses
2.2.1. Preparation of PdCl(ꢀ3-allyl)(ꢁ1-C-L1) (1)
The imidazolium salt (200.0 mg, 0.705 mmol) was reacted with
K2CO3 (100.0 mg, 0.724 mmol) and [Pd(␩3-allyl)Cl]2 (130.0 mg,
0.350 mmol) in dry CH3CN (15 mL) under a nitrogen atmosphere
at room temperature for 1 day. The solvent was then com-
pletely removed under vacuum, followed by the addition of
water (20 mL) and dichloromethane (20 mL). After vigorous stir-
ring of the emulsion, the organic layer was separated, dried
with MgSO4, filtered and the remaining solution was concen-
trated to dryness. The residue was washed with dry hexane (3×
15 mL). The pale yellow product was dried under vacuum. Yield:
142.8 mg (47%). Anal. calcd. for C20H21Cl1N2Pd: C, 55.72; H, 4.87;
N, 6.49. Found: C, 55.03; H, 4.67; N, 6.30. 1H NMR (ı, 400.13 MHz,
2. Experimental
2.1. Materials and equipments
All synthetic reactions and manipulations were carried out
under a nitrogen atmosphere by using standard Schlenk tech-
niques. Solvents were either distilled or passed through columns
containing dehydrating agents. Reagents were used as received
from Aldrich, unless stated otherwise. Compounds PdCl2(␩4-
COD) (COD = 1, 5-cyclooctadiene) [27a], [PdCl(␩3-allyl)]2 [27b],
PdCl(␩3-allyl)(␬1-C-L2) (2) [28], PdCl(␩3-allyl)(␬1-C-L3) (3) [28],
PdCl(␩3-allyl)(␬1-C-L4) (4) [28] and Pd(␬1-O-OAc)2(H2O)(␬1-C-L4)
(9) [22a] and (HL1)Cl [29] were prepared according to litera-
ture methods. Deuterated solvents for routine NMR measurements
3
CD2Cl2, 21 ◦C) 2.02 (d, JHH = 12.0 Hz, 1H, allyl-H), 3.13 (m, 2H,
3
allyl-H), 4.16 (d, JHH = 7.2 Hz, 1H, allyl-H), 5.18 (m, 1H, allyl-H),
2
2
5.42 (d, JHH = 13.6 Hz, 2H, ArCHaHb), 5.50 (d, JHH = 13.6 Hz, 2H,
ArCHaHb), 6.99 (s, 2H, imi-H), 7.32–7.41 (m, 10H, Ar). 13C{ H}
1
NMR (ı, 100.62 MHz, CD2Cl2, 21 ◦C) 48.76 (s, allyl-C), 54.68 (s,
ArCHaHb), 71.62 (s, allyl-C), 114.87 (s, allyl-C), 121.60 (s, imi-
C), 127.95 (s, Ar), 128.02 (s, Ar), 128.71 (s, Ar), 136.85 (s, Ar),
181.51 (s, NCN). 1H NMR (ı, 400.13 MHz, CD2Cl2, −60 ◦C) 1.90
were dried with activated molecular sieves. 1H and 13C{ H} NMR
1
spectra were obtained with a Bruker Avance DRX-400 spectrom-
eter (400.13 and 100.62 MHz, respectively). HMQC spectra were
acquired with the same NMR spectrometer. Chemical shifts are
reported in ppm (ı) with reference to either TMS as an internal
3
3
(d, JHH = 12.4 Hz, 1H, allyl-H), 3.07 (d, JHH = 13.6 Hz, 1H, allyl-H),
3
3
3.12 (d, JHH = 6.4 Hz, 1H, allyl-H), 4.13 (d, JHH = 7.6 Hz, 1H, allyl-
1
standard (1H and 13C{ H} NMR spectra). 1H HPNMR experi-
3
H), 5.19 + 5.35 (m, 4H, allyl-H + ArCHaHb), 5.68 (d, JHH = 14.8 Hz,
ments were carried out on a Bruker Avance II-200 spectrometer
equipped with a 10 mm BB probe using a 10 mm sapphire tube
(Sahikon, Milford, NH), equipped with a titanium high-pressure
charging head constructed at ICCOM-CNR [30]. Microanalyses
were performed using a Carlo-Erba Model 1106 elemental ana-
lyzer. Oxidation reactions were performed with 65 mL stainless
steel autoclaves, constructed at ICCOM-CNR, equipped with mag-
netic stirring, oil bath heating and a temperature and pressure
controller. GC analyses were performed on a Shimadzu 2010
gas chromatograph equipped with a flame ionization detector
and a 30 m (0.25 mm i.d., 0.25 ␮m film thickness) VF-WAXms
capillary column. GC/MS analyses were performed on a Shi-
madzu QP 5000 apparatus equipped with 30 m (0.32 mm i.d.,
1H, ArCHaHb), 6.97 (s, 1H, imi-H), 7.02 (s, 1H, imi-H), 7.27–7.37
1
(m, 10H, Ar). 13C{ H} NMR (ı, 100.62 MHz, CD2Cl2, −60 ◦C)
49.48 (s, allyl-C), (ArCHaHb, overlapped signal), 71.65 (s, allyl-
C), 115.34 (s, allyl-C), 121.69 (s, imi-C), 122.26 (s, imi-C), 127.81
(s, Ar), 128.13 (s, Ar), 128.86 (s, Ar), 136.93 (s, Ar), 180.41 (s,
NCN).
2.2.2. Preparation of Pd(ꢁ1-O-OAc)(ꢀ3-allyl)(ꢁ1-C-L1) (5)
Compound 1 (150.0 mg, 0.348 mmol) was dissolved in CH2Cl2
(15 mL). To this solution was added AgOAc (58.1 mg, 0.348 mmol)
under vigorous stirring at room temperature. The suspension was
allowed to stir at the latter temperature for 1 h. Afterwards the
suspension was passed through a plug of celite and the clear
solution was concentrated to a small volume (2 mL). On addi-
tion of diethyl ether (10 mL) the product precipitated as yellow
micro-crystalline compound, which was separated by filtration and
dried in a stream of nitrogen. Yield: 126.6 mg (80%). Anal. calcd.
for C22H24N2O2Pd: C, 58.12; H, 5.28; N, 6.16. Found: C, 58.22;
H, 5.52; N, 6.30. 1H NMR (ı, 400.13 MHz, CD2Cl2, 21 ◦C) 1.91 (s,
3H, CH3), 2.12 (brs, 1H, allyl-H), 2.38 (brs, 1H, allyl-H), 3.34 (d,
3
3JHH = 13.6 Hz, 1H, allyl-H), 4.17 (d, JHH = 7.6 Hz, 1H, allyl-H), 5.24
3
(quintet, JHH = 6.8 Hz, 1H, allyl-H), 5.50 (s, 2H, ArCH2), 6.96 (s,
1
4H, imi-H), 7.32–7.40 (m, 10H, Ar). 13C{ H} NMR (ı, 100.62 MHz,
CD2Cl2, 21 ◦C) 23.46 (s, CH3), 43.08 (s, allyl-C), 54.55 (s, ArCH2),
70.41 (s, allyl-C), 114.76 (s, allyl-C), 121.38 (s, imi-C), 128.02 (s,
Ar), 128.10 (s, Ar), 128.60 (s, Ar), 137.03 (s, Ar), 176.31 (s, COCH3),
181.60 (s, NCN). 1H NMR (ı, 400.13 MHz, CD2Cl2, −60◦ C) 1.88 (s, 3H,
CH3), 1.94 (d, 3JHH = 12.0 Hz, 1H, allyl-H), 2.87 (d, 3JHH = 6.0 Hz, 1H,
3
3
allyl-H), 3.32 (d, JHH = 13.6 Hz, 1H, allyl-H), 4.11 (d, JHH = 7.2 Hz,
3
1H, allyl-H), 5.27 (quintet, JHH = 6.0 Hz, 1H, allyl-H), 5.40 (s, 2H,
ArCH2), 5.44 (s, 2H, ArCH2), 6.97 (s, 2H, imi-H), 7.27–7.39 (m,
1
10H, Ar). 13C{ H} NMR (ı, 100.62 MHz, CD2Cl2, −60 ◦C) 23.95 (s,
CH3), 43.98 (s, allyl-C), (ArCH2, overlapped signal), 70.34 (s, allyl-
C), 115.20 (s, allyl-C), 121.80 (s, imi-C), 127.80 (s, Ar), 128.08 (s,
Ar), 128.87 (s, Ar), 137.14 (s, Ar), 176.92 (s, COCH3), 180.44 (s,
NCN).
Scheme 1.

L. Bettucci et al. / Journal of Molecular Catalysis A: Chemical 322 (2010) 63–72
65
2.2.3. Preparation of Pd(ꢁ1-O-OAc)(ꢀ3-allyl)(ꢁ1-C-L2) (6)
sion was passed through a plug of celite and the obtained clear
solution was concentrated to a small volume (2 mL), followed by
the addition of diethyl ether (10 mL), which caused the precip-
itation of the product as an off-white micro-crystalline powder
that was separated by filtration and dried in a stream of nitro-
gen. Yield: 124.1 mg (70%). Anal. calcd. for C32H44N2O2Pd: C, 64.62;
H, 7.40; N, 4.71. Found: C, 64.74; H, 7.53; N, 4.82. 1H NMR (ı,
Compound 2 (120.0 mg, 0.231 mmol) was dissolved in CH2Cl2
(10 mL). To this solution was added AgOAc (38.6 mg, 0.231 mmol)
under vigorous stirring at room temperature. The suspension was
allowed to stir at the latter temperature for 1.5 h. The suspension
was then passed through a plug of celite and the obtained clear
solution was concentrated to a small volume (2 mL). Then diethyl
ether (10 mL) was added to the solution, causing the precipita-
tion of the product as yellow semi-crystalline powder that was
separated by filtration and dried in a stream of nitrogen. Yield:
87.7 mg (75%). Anal. calcd. for C28H40N2O2Pd: C, 61.97; H, 7.37;
N, 5.16. Found: C, 62.14; H, 7.51; N, 5.23. 1H NMR (ı, 400.13 MHz,
CD2Cl2, 21 ◦C) 1.79 (brs, 10H, adamantyl-H), 1.84 (s, 3H, CH3), 1.99
(d, 3JHH = 15.6 Hz, 1H, allyl-H), 2.26 (brs, 8H, adamantyl-H), 2.43 (m,
6H, adamantyl-H), 2.55 (m, 6H, adamantyl-H), 2.82 (brs, 1H, allyl-
3
400.13 MHz, CD2Cl2, 21 ◦C) 1.52 (d, JHH = 6.8 Hz, 12H, CH(CH3)2),
3
1.36 (d, JHH = 6.8 Hz, 12H, CH(CH3)2), 1.73 (s, 3H, COCH3), 2.21
3
(brs, 2H, allyl-H), 2.89 (septet, JHH = 6.8 Hz, 4H, CH(CH3)2), 3.10
3
3
(d, JHH = 13.6 Hz, 1H, allyl-H), 4.08 (d, JHH = 7.6 Hz, 1H, allyl-H),
3
4.87 (quintet, JHH = 6.0 Hz, 1H, allyl-H), 7.21 (s, 2H, imi-H), 7.35
3
3
(d, JHH = 7.6 Hz, 4H, Ar-m-H), 7.52 (t, JHH = 7.6 Hz, 2H, Ar-p-H).
1
13C{ H} NMR (ı, 100.62 MHz, CD2Cl2, 21 ◦C) 22.35 (s, CH(CH3)2),
23.53 (brs, COCH3), 25.61 (s, CH(CH3)2), 28.49 (s, CH(CH3)2), 44.21
(s, allyl-C), 71.82 (s, allyl-C), 113.38 (s, allyl-C), 123.79 (s, Ar-m-C),
124.44 (s, imi-C), 129.87 (s, Ar-p-C), 135.88 (s, Ar-ipso-C), 145.99 (s,
Ar-o-C), 175.90 (s, COCH3), 185.26 (NCN). 1H NMR (ı, 400.13 MHz,
3
3
H), 3.45 (d, JHH = 13.2 Hz, 1H, allyl-H), 4.33 (d, JHH = 7.6 Hz, 1H,
allyl-H), 5.31 (quintet, 3JHH = 6.0 Hz, 1H, allyl-H), 7.30 (s, 2H, imi-H).
13C{ H} NMR (ı, 100.62 MHz, CD2Cl2, 21 ◦C) 23.94 (s, CH3), 30.16 (s,
1
CD2Cl2, −60 ◦C) 1.04 (d, JHH = 6.4 Hz, 6H, CH(CH3)2), 1.13 (d,
3
adamantyl-C), 35.91 (s, adamantyl-C), 43.63 (s, adamantyl-C), 46.73
(s, allyl-C), 58.96 (s, adamantyl-C), 67.88 (s, allyl-C), 111.17 (s, allyl-
C), 116.73 (s, imi-C), 175.56 (s, COCH3), 176.37 (s, NCN). 1H NMR
(ı, 400.13 MHz, CD2Cl2, −60 ◦C) 1.70 (m, 10H, adamantyl-H), 1.84
(s, 3H, CH3), 2.19 (m, 11H, adamantyl-H), 2.42 (m, 6H, adamantyl-
3JHH = 6.4 Hz, 6H, CH(CH3)2), 1.25 (d, JHH = 6.4 Hz, 6H, CH(CH3)2),
3
1.32 (d, 3JHH = 6.4 Hz, 6H, CH(CH3)2), 1.76 (s, 3H, COCH3), 1.78 (allyl-
3
H, overlapped signal), 2.64 (septet, JHH = 6.8 Hz, 2H, CH(CH3)2),
2.81 (septet, 3JHH = 6.8 Hz, 2H, CH(CH3)2), 2.91 (d, 3JHH = 5.2 Hz, 1H,
3
3
3
allyl-H), 3.15 (d, JHH = 13.2 Hz, 1H, allyl-H), 3.86 (d, JHH = 7.2 Hz,
1H, allyl-H), 4.92 (quintet, JHH = 6.0 Hz, 1H, allyl-H), 7.22 (s, 2H,
H), 2.53 (m, 3H, adamantyl-H), 3.20 (d, JHH = 5.2 Hz, 1H, allyl-H),
3
3.42 (d, 3JHH = 13.2 Hz, 1H, allyl-H), 4.21 (d, 3JHH = 6.8 Hz, 1H, allyl-
3
3
imi-H), 7.35 (d, JHH = 7.6 Hz, 4H, Ar-m-H), 7.54 (t, JHH = 7.6 Hz,
H), 5.29 (m, 1H, allyl-H), 7.22 (s, 1H, imi-H), 7.27 (s, 1H, imi-H).
2H, Ar-p-H). 13C{ H} NMR (ı, 100.62 MHz, CD2Cl2, −60 ◦C) 21.92
1
1
13C{ H} NMR (ı, 100.62 MHz, CD2Cl2, −60 ◦C) 24.37 (s, CH3), 29.79
(s, CH(CH3)2), 22.43 (s, CH(CH3)2), 23.90 (s, COCH3), 25.64 (s,
CH(CH3)2), 26.35 (s, CH(CH3)2), 28.54 (s, CH(CH3)2), 28.65 (s,
CH(CH3)2), 45.02 (s, allyl-C), 70.99 (s, allyl-C), 113.87 (s, allyl-C),
123.92 (s, imi-C), 124.05 (s, imi-C), 124.86 (s, Ar-m-C), 130.10 (s,
Ar-p-C), 135.62 (s, Ar-ipso-C), 145.91 (s, Ar-o-C), 145.98 (s, Ar-o-C),
176.66 (s, COCH3), 184.17 (NCN).
(s, adamantyl-C), 29.84 (s, adamantyl-C), 29.92 (s, adamantyl-C),
35.62 (s, adamantyl-C), 43.10 (s, adamantyl-C), 43.51 (s, adamantyl-
C), 44.29 (s, adamantyl-C), 47.95 (s, allyl-C), 57.85 (s, adamantyl-C),
58.81 (s, adamantyl-C), 67.52 (s, allyl-C), 111.28 (s, allyl-C), 116.65
(s, imi-C), 117.39 (s, imi-C), 175.52 (s, COCH3), 176.08 (s, NCN).
2.2.4. Preparation of Pd(ꢁ1-O-OAc)(ꢀ3-allyl)(ꢁ1-C-L3) (7)
2.2.6. Preparation of trans-[PdCl2(ꢁ1-C-L1)2] (10)
To a solution of 3 (140.0 mg, 0.287 mmol) in CH2Cl2 (10 mL)
was added AgOAc (47.9 mg, 0.287 mmol) under vigorous stirring
at room temperature. After a reaction time of 1 h, the suspension
was passed through a plug of celite and the obtained clear solu-
tion was concentrated to a small volume (3 mL). Then diethyl ether
(15 mL), was added, causing the precipitation of the product as a
brownish micro-crystalline powder that was separated by filtra-
tion and dried in a stream of nitrogen. Yield: 95.3 mg (65%). Anal.
calcd. for C26H32N2O2Pd: C, 61.15; H, 6.27; N, 5.48. Found: C, 61.23;
H, 6.32; N, 5.53. 1H NMR (ı, 400.13 MHz, CD2Cl2, 21 ◦C) 1.74 (brs, 3H,
COCH3), 1.95 (brs, 1H, allyl-H), 2.17 (s, 12H, Ar-o-CH3), 2.39 (s, 6H,
Ar-p-CH3), 2.85 (br d, 1H, allyl-H), 2.99 (d, 3JHH = 13.6 Hz, 1H, allyl-
H), 3.95 (d, 3JHH = 7.6 Hz, 1H, allyl-H), 4.86 (m, 1H, allyl-H), 7.04 (s,
The imidazolium salt (HL1)BF4 (100.0 mg, 0.300 mmol) was
reacted with KtOBu (40.0 mg, 0.300 mmol) and PdCl2(␩4-COD)
(40.0 mg, 0.150 mmol) in dry CH3CN (15 mL) under a nitrogen
atmosphere at room temperature for 12 h. Afterwards, the solvent
was removed completely under vacuum. To the resulting residue
was added water (20 mL) and CH2Cl2 (20 mL). The organic layer was
separated and dried over MgSO4. The inorganic salt was removed
from the solution by filtration and the resulting solution was con-
centrated to dryness, obtaining a solid that was suspended in THF,
filtered off, washed with THF (3× 15 mL) and then dried under
vacuum. Yield: 54.6 mg (27%). Anal. calcd. for C34H32Cl2N4Pd: C,
60.59; H, 4.79; N, 8.31. Found: C, 60.33; H, 4.79; N, 7.89. 1H NMR
(ı, 400.13 MHz, DMF-d7, 21 ◦C) ı 5.86 (s, 4H, CH2), 7.31 (m, 8H,
CH2 + Ar), 7.70 (m, 4H, imi-H), 8.02 (m, 20H, Ar). Due to the low
solubility of 10 in all common organic solvents including DMF-d7,
1
4H, Ar), 7.13 (s, 2H, imi-H). 13C{ H} NMR (ı, 100.62 MHz, CD2Cl2,
21 ◦C) 15.10 (s, Ar-o-CH3), 20.87 (s, Ar-p-CH3), 23.20 (s, COCH3),
43.65 (s, allyl-C), 70.09 (s, allyl-C), 113.57 (s, allyl-C), 122.84 (s,
imi-C), 128.80 (s, Ar), 135.53 (s, Ar), 135.99 (s, Ar), 138.85 (s,
Ar), 169.42 (s, COCH3), 182.87 (s, NCN). 1H NMR (ı, 400.13 MHz,
1
we failed in acquiring a reliable 13C{ H} NMR spectrum.
2.2.7. Selected NMR data for [Pd(ꢂ-OH)(ꢁ1-O-OAc)
3
CD2Cl2, −60 ◦C) 1.54 (br d, JHH = 12.0 Hz, 1H, allyl-H), 1.72 (s, 3H,
(ꢁ1-C-L2)]2 (11)
COCH3), 2.10 + 2.11 (s, 12H, Ar-o-CH3), 2.34 (s, 6H, Ar-p-CH3), 2.95
To a solution of 6 (46.5 mg, 0.085 mmol) in wet THF (4 mL)
was added K-tOBu (10.1 mg, 0.09 mmol) at room temperature. The
obtained suspension was allowed to stir for 1 h. Afterwards, the
solvent was completely removed by vacuum and the remaining
solid was suspended in CH2Cl2 (4 mL). The latter suspension was
filtered through a plug of celite and the obtained yellow solution
was concentrated to half of its original volume (2 mL). Diethyl ether
was then slowly added in order to obtain a yellowish powder, that
was dried under nitrogen. The 1H NMR spectrum of the obtained
powder acquired in CD2Cl2 revealed that the isolated powder was
a mixture of compounds, containing 11 in a low amount (15%).
Several attempts to obtain 11 as a pure compound failed due to
3
(m, 2H, allyl-H), 3.87 (d, JHH = 6.4 Hz, 1H, allyl-H), 4.89 (m, 1H,
1
allyl-H), 7.01 + 7.02 (s, 4H, Ar), 7.14 (s, 2H, imi-H). 13C{ H} NMR
(ı, 100.62 MHz, CD2Cl2, −60 ◦C) 17.82 (s, Ar-o-CH3), 21.21 (s, Ar-p-
CH3), 23.70 (s, COCH3), 44.35 (s, allyl-C), 69.47 (s, allyl-C), 114.05
(s, allyl-C), 122.93 (s, imi-C), 128.84 (s, Ar), 135.62 (s, Ar), 135.64 (s,
Ar), 138.93 (s, Ar), 176.32 (s, COCH3), 181.29 (s, NCN).
2.2.5. Preparation of Pd(ꢁ1-O-OAc)(ꢀ3-allyl)(ꢁ1-C-L4) (8)
To a solution of 4 (170.0 mg, 0.298 mmol) in CH2Cl2 (15 mL)
was added AgOAc (49.7 mg, 0.298 mmol) under vigorous stirring
at room temperature. After a reaction time of 1 h, the suspen-

66
L. Bettucci et al. / Journal of Molecular Catalysis A: Chemical 322 (2010) 63–72
the similar solubility property of all compounds present in the iso-
to 10 ◦C by means of a water-ice bath, followed by the release of the
air pressure from the autoclave. Afterwards, the catalytic solution
was subjected to GC and GC/MS analyses.
1
lated mixture. As a consequence, only selected 1H and 13C{ H} NMR
chemical shifts for 11 are reported.
1H NMR (ı, 400.13 MHz, CD2Cl2, 21 ◦C) −4.01 (s, 2H, OH), 1.40
(s, 6H, COCH3), 7.26 (3JHH = 2.4 Hz, 2H, imi-H), 7.30 (3JHH = 2.4 Hz,
2.4.2. Blank reaction
1
2H, imi-H). 13C{ H} NMR (ı, 100.62 MHz, CD2Cl2, 21 ◦C) 22.96 (s,
A 19:1 (v:v) solvent mixture of toluene and DMSO (20 mL) was
heated in a 65 mL stainless steel autoclave equipped with a home-
made temperature and pressure controller and a magnetic stirrer
at 80 ◦C in the presence of 20 bar of air for 2 h. Afterwards the auto-
clave was cooled to room temperature, the air pressure released
and the solution was analyzed by GC and GC/MS.
COCH3), 116.75 (s, imi-C), 117.29 (imi-C), 175.66 (s, COCH3), 185.61
(s, NCN).
2.3. Variable-temperature 1H HPNMR experiments with 2
A solution of 2 (0.015 mmol) in toluene-d8 (2 mL) was trans-
ferred into a 10 mm sapphire tube followed by the acquisition of a
1H NMR spectrum at room temperature. Afterwards the NMR tube
was removed from the NMR probe and successively charged with a
1:1 mixture of meso/rac-2,4-pentanediol (82.0 ␮L, 0.75 mmol) and
air (20 bar). The sapphire tube was then placed again into the NMR
probe, followed by the acquisition of a 1H NMR spectrum at room
temperature. The NMR probe was then gradually heated from room
temperature to 80 ◦C in a temperature interval of 10 ◦C. At each tem-
perature a 1H NMR spectrum was acquired. The sapphire tube was
then heated for 2 h at 80 ◦C, acquiring 1H NMR spectra in a time
interval of half an hour. Afterwards the NMR probe was cooled to
room temperature and a final 1H NMR spectrum was acquired. The
gas pressure was then released and the NMR solution was subjected
to a GC/MS analysis.
3. Results and discussion
3.1. Synthesis of complexes
Compound 1 has been obtained by the reaction of N,Nꢀ-
bis(benzyl)-imidazolium chloride with K2CO3 and [Pd(␩3-allyl)Cl]2
in acetonitrile in 47% yield (Scheme 2a), while the known related
complexes 2–4 (Scheme 2b) have been synthesized following a
synthetic procedure reported by Nolan and co-workers [28]. The
synthetic protocol for the latter compounds comprises the deproto-
nation of the corresponding imidazolium salt with K-tOBu in THF at
room temperature (2) or at 50 ◦C (3 and 4), followed by the reaction
of the “in situ” generated imidazol-2-ylidene ligand with [PdCl(␩3-
allyl)]2 in the same solvent at room temperature (Scheme 2b).
Unlike complexes 2–4 that are stable compounds even when
stored in the presence of air and moisture, 1 is converted by mois-
ture into the palladium bis-carbene complex trans-PdCl2(␬1-C-L1)2
(10). This compound has been also obtained by an independent syn-
thesis in relatively low yield (27%), which involves the reaction of
PdCl2(␩4-COD) with the corresponding “in situ” synthesized car-
bene in acetonitrile. The low yield of 10 is due to the concomitant
formation of 10 and its cis-isomer along with not yet identified
compounds. Due to its low solubility, complex 10 has been charac-
terized in solution only by 1H NMR spectroscopy, and in the solid
state by elemental analysis and a single crystal X-ray structure
analysis (vide infra).
The chloride complexes 1–4 have been converted into the cor-
responding acetate complexes 5–8 by reaction of the former with
silver acetate in dichloromethane at room temperature as shown
in Scheme 3.
All acetate complexes have been isolated as micro-crystalline
compounds in good yield (65–80%). The crystal structure of 8, the
synthesis of which has been recently reported by Pregosin and co-
workers [32] confirms unambiguously the ꢁ1-O coordination of the
acetate group to palladium (vide infra).
2.4. Crystallography
Single crystals of 1, 8, 10 and 11·2C7H8 suitable for a single
crystal X-ray structure analysis have been obtained by diffusion
of a toluene-layered dichloromethane solution of the correspond-
ing compounds at room temperature. Typically, a suitable crystal
was mounted on a glass fiber with silicon grease and placed in
the cold nitrogen stream. Diffraction data for 1 and 10 were col-
lected on a Bruker APEX II diffractometer, while diffraction data
for 8 and 11·2C7H8 were collected on an Oxford Diffraction CCD
diffractometer, using Mo K␣ radiation (ꢃ = 0.71069 Å) with both
diffractometers. The diffraction data were corrected for Lorentz
and polarization effects and an absorption correction was per-
formed using the SADABS program [31a]. All structures were solved
by direct methods using SHELXS-97 and refined by full-matrix
least squares methods against F2 using either the SHELXTL [31b]
or WINGX [31c] software package. All non-hydrogen atoms were
refined anisotropically, while hydrogen atoms were added at calcu-
lated positions and refined applying a riding model with isotropic U
values depending on the Ueq of the adjacent carbon atom. Crystallo-
graphic data (excluding structure factors) were deposited with the
Cambridge Crystallographic Data Centre. CCDC-742450 (1); CCDC-
742449 (8); CCDC-742448 (10); CCDC-742447 (11·2C7H8). Copies
of the data can be obtained free of charge on application to CCDC,
12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223/336033;
e-mail: deposit@ccdc.cam.uk).
2.4.1. Catalytic oxidation reactions
Typically, the amount of precursor (0.01 mmol) was added at
room temperature to a round bottom-flask that contained a 19:1
(v:v) solvent mixture of toluene and DMSO (20 mL). To the latter
solution was added the substrate (1.00 mmol) and the obtained
clear solution was transferred into a 65 mL stainless steel autoclave
equipped with a home-made temperature and pressure controller
and a magnetic stirrer. The autoclave was then heated to 80 ◦C
by means of an oil bath. Once the desired temperature had been
reached the autoclave was pressurized with 20 bar of air and stir-
ring of the catalytic solutions was started. After the desired reaction
time the autoclave was successively removed from oil bath, cooled
Scheme 2.

L. Bettucci et al. / Journal of Molecular Catalysis A: Chemical 322 (2010) 63–72
67
the typical chemical shift range as observed for related Pd(␩3-allyl)
complexes [32,34].
3.3. Crystal structure determination of 1, 8, 10 and 11·2C7H8
Crystals, suitable for a single crystals structure analysis have
been obtained by slow diffusion of toluene into a CH2Cl2 solution
of the corresponding compound at room temperature. Crystallo-
graphic data for 1, 8, 10 and 11·2C7H8 are summarized in Table 1,
while selected bond distances and angles are reported in the cap-
tion of the ORTEP plot of each crystal structure.
Scheme 3.
The crystal structure of 1 (Fig. 1) shows one molecule of 1 in the
asymmetric unit.
An attempt of crystallizing 6 from a (1:1) solvent mixture of
toluene and CH2Cl2 in air at room temperature led to the forma-
tion of a small amount of both crystals (i.e. 5%) and a powdered
material that 1H NMR spectroscopy showed to be the corre-
sponding imidazolium salt and 6. A single crystal X-ray structure
analysis of the crystalline material proved it to be the binuclear
OH-bridged palladium carbene complex [Pd(␮-OH)(␬1-O-OAc)(␬1-
C-L2)]2 (11) (vide infra). Consistently, the 1H NMR spectrum of the
latter compound exhibits a singlet centred at −4.01 ppm that is
characteristic for the [Pd-(␮-OH)]2 core in 11 [33]. In addition,
the presence of two 1H doublets centred at 7.26 and 7.30 ppm
(3JHH = 2.4 Hz) for the imidazol-2-ylidene hydrogen atoms shows
their chemical inequivalence due to the hinged structure of 11.
An attempt to synthesize 11 by an independent synthesis from 6
and K-tOBu in wet THF gave a mixture of compounds that con-
tained 11 in only 15% yield. K-tOBu yields in the presence of traces
of water hydroxide ions and t-BuOH. The latter alcohol may be
involved in the protonation reaction of the coordinated allyl unit
yielding free propene. The major side reaction is the nucleophilic
attack of K-tOBu on the coordinated allyl moiety, giving allyl t-
butyl ether and a non-identified Pd(0) carbene compound, that
decomposes in part to palladium black. Any attempt to increase
the yield of 11 and to isolate it as a pure compound failed so
far.
The palladium atom, that exhibits a square planar coordination
geometry in 1, deviates from the best coordination plane defined by
C(1), C(18), C(20) and Cl(1) of 0.0593(25) Å in direction of C(13). The
Pd–C(1) bond length of 2.035(4) Å is comparable with that found for
related Pd(␩3-allyl)(NHC)-complexes [28,32,34]. The reason for the
slightly different Pd(1)–C(18), Pd(1)–C(20) and Pd(1)–Cl(1) bond-
ing distances when compared to those of the related structure
bearing N,Nꢀ-bis(1-S,S-phenylethylimidazol)-2-ylidene is mainly
steric in nature [28]. Both, the N,Nꢀ-di-substituted imidazol-2-
ylidene unit and the ␩3-allyl group are tilted with respect to the
coordination plane by 72.38(21)◦ and 60.90(53)◦, respectively.
The crystal structure of 8 (Fig. 2) exhibits one molecule of the
latter compound in the asymmetric unit.
The palladium atom is coordinated by the carbene carbon atom
C(1), a ␩3-allyl group and a monodentate-coordinating acetate
anion which is disordered over two equally occupied positions
sharing the carboxylate carbon atom C(31). The acetate oxygen
atom O(2a) shows a small intra-molecular distance to the allyl-
hydrogen atom H(28b) of 2.793 Å.
The palladium atom in 8 deviates form the best coordina-
tion plane defined by the atoms C(1), C(28), C(30), O(1a), O(1b)
3.2. NMR characterization of complexes
Solutions of the new synthesized Pd(␩3-allyl) complexes in
1
CD2Cl2 have been characterized by 1H and 13C{ H} NMR spec-
troscopy. The NMR signal assignment was based on HMQC
experiments. The 1H NMR spectra of the isolated compounds
acquired at room temperature are in accordance with a flux-
ional behaviour of the metal coordinating allyl unit do to a rapid
3
␩3–␩1–␩ allyl rearrangement [32,34]. As a consequence, the 1H
NMR signals of H(1a) and H(1s) (a = anti, s = syn with respect to H(2)
)
(Scheme 4) are not resolved at room temperature, while analo-
gous 1H spectra acquired at −60 ◦C in CD2Cl2 exhibited for H(1a)
and H(1s) doublets with a 3JHH of around 13 and 7 Hz, respectively.
The 1H NMR signals assigned to H(3a) and H(3s) (Scheme 4) are well
resolved doublets even at room temperature.
1
The 13C{ H} NMR spectra of the Pd(␩3-allyl) compounds stud-
ied herein showed for the allyl-carbon atoms C(1), C(2) and C(3)
(Scheme 4) at room temperature as well as singlets at −60 ◦C in
Fig. 1. ORTEP plot of 1. Thermal ellipsoids are drawn at the 30% probabil-
ity level and hydrogen atoms are omitted for clarity. Selected bond distances
(Å) and angles (◦): Pd(1)–Cl(1) 2.4028(14), Pd(1)–C(1) 2.035(4), Pd(1)–C(18)
2.084(5), Pd(1)–C(19) 2.124(6), Pd(1)–C(20) 2.222(5), C(18)–C(19) 1.346(8),
C(19)–C(20) 1.367(8), Cl(1)–Pd(1)–C(1) 95.30(12), C(1)–Pd(1)–C(20) 162.38(19),
Cl(1)–Pd(1)–C(18) 170.06(16).
Scheme 4.

68
L. Bettucci et al. / Journal of Molecular Catalysis A: Chemical 322 (2010) 63–72
Table 1
Crystallographic data for 1, 8, 10 and 11·2C7H8.
1
8
10
11·2C7H8
Empirical formula
Formula weight
Crystal color, shape
Crystal size (mm)
Temperature (K)
Crystal system
Space group
a (Å)
C20H21ClN2Pd
431.24
Yellow, plate
0.24 × 0.15 × 0.11
150(2)
Monoclinic
C2/c
23.983(8)
11.764(4)
13.788(4)
C32H44N2O2Pd
595.09
Yellow, plate
0.30 × 0.20 × 0.10
170(2)
Monoclinic
P21
9.1960(2)
17.5190(3)
9.9456(2)
C34H32Cl2N4Pd
673.94
Yellow, prism
0.25 × 0.20 × 0.10
150(2)
Monoclinic
P21/n
10.966(7)
12.534(9)
11.432(8)
C64H88N4O6Pd2
1222.18
Yellow, plate
0.20 × 0.20 × 0.10
170(2)
Triclinic
P-1
12.2299(4)
13.9907(3)
16.7472(4)
82.052(2)
85.085(2)
84.234(2)
2816.14(13)
2
b (Å)
c (Å)
˛ (◦)
ˇ (◦)
98.99(2)
109.266(2)
106.174(13)
ꢄ (◦)
Volume (Å3)
Z
3842(2)
8
1512.55(5)
2
1509.1(18)
2
F(000)
1744
1.491
1.108
1.72–26.75
−29 ≤ h ≤ 30,
−14 ≤ k ≤ 14,
−17 ≤ l ≤ 17
22614
4068
205
0.918
0.0422
624
1.307
0.643
3.86–37.39
−15 ≤ h ≤ 15,
−29 ≤ k ≤ 27,
−16 ≤ l ≤ 16
20713
12372
364
1.035
0.0284
688
1.483
0.822
2.28–28.99
−13 ≤ h ≤ 14,
−16 ≤ k ≤ 16,
−15 ≤ l ≤ 15
16253
3950
187
1.043
0.0370
1280
1.441
0.695
3.69–32.48
−17 ≤ h ≤ 17,
−20 ≤ k ≤ 21,
0 ≤ l ≤ 24
17879
17879
689
0.814
0.0574
ꢅcalc (g cm−3
)
ꢂ (Mo K␣) (mm−1
)
ꢆ range (◦)
Index range
Reflections collected
Independent reflections
Refined parameters
Goodness-of-fit on F2
R1 (2ꢇ(I))
R1 (all data), wR2 (all data)
Largest diff peak and hole (eÅ−3
0.0646, 0.1222
1.627, −0.836
0.0348, 0.0719
1.017, −0.526
0.0602, 0.0980
0.654, −0.781
0.1488, 0.1157
1.412, −0.812
)
sion angle of 81.70(0.36)◦ and 83.76(0.19)◦ with respect to the
imidazol-2-ylidene plane, defined by the atoms C(1), N(1) and N(2).
The coordinating allyl moiety is tilted with respect to the best
coordination plane of 52.03(0.25)◦, which is in agreement with
analogous torsion anglers found for related neutral and cationic
Pd(␩3-ally)(NHC) complexes [32]. Due to the significantly different
trans influence of the carbene carbon atom C(1) and the acetate
oxygen atoms O1(1a) and O(1b), two different Pd-allyl-carbon
atom distances of 2.2195(13) Å (Pd(1)–C(28)) and of 2.065(18) Å
(Pd(1)–C(30)) have been observed.
of 0.1197(29) Å in direction of C(25). The Pd–C(1) bonding dis-
tance of 2.0399(12) Å and the almost perpendicular orientation
of the imidazol-2-ylidene unit with respect to the best coor-
dination plane defined by the atoms C(1), Pd(1), C(29), O(1a)
and O(1b) of 85.96(0.24)◦ is comparable to analogous struc-
tural parameters found for the related chloride [28] and iodide
[32] structure. The 2,6-di-iso-propylphenyl moieties show a tor-
The crystal structure of 10 (Fig. 3) confirms the trans arrange-
ment of both chloride and NHC ligands with respect to the
palladium atom. The palladium atom is square planar coordinated,
occupying a centre of inversion. As a consequence, half of the
molecule (i.e. labeled part of Fig. 3) constitutes the asymmetric unit.
The imidazol-2-ylidene units coordinated to palladium are tilted
with respect to the palladium coordination plane defined by Pd(1),
Cl(1) and C(1) of 71.05(0.12)◦. Both the Pd(1)–C(1) and Pd(1)–Cl(1)
distances of 2.036(3) and 2.3204(16) Å, respectively, are compa-
rable with analogous distances found in related Pd-bis(carbene)
structures [35].
The crystal structure of 11·2C7H8 (Fig. 4) exhibits compound 11
and two molecules of toluene in the asymmetric unit.
Both palladium atoms in the dimeric structure of 11·2C7H8
are bridged by two OH-groups. As a consequence, the cen-
tral [Pd(␮-OH)]2 core is folded showing an intra-planar angle
between both Pd-coordination planes of 21.44(6)◦. The differ-
ent Pd-bridging oxygen distances [33] clearly reflect the higher
trans influence of the carbene carbon atom when compared
to the monodentate-coordinating acetate group. Both trans-
coordinating NHC ligands are almost perpendicular (85.06(0.16)◦)
with respect to the corresponding palladium coordination plane.
The palladium–carbene–carbon bonding distances of 1.977(3) Å
(Pd(1)–C(1)) and 1.979(4) Å (Pd(2)–C(24)) are in agreement with
related distances found in other NHC-modified palladium com-
plexes [32,34].
Fig. 2. ORTEP plot of 8. Thermal ellipsoids are drawn at the 30% probability
level and hydrogen atoms are omitted for clarity. The indices
a and b are
referred to the two equally occupied acetate positions. Selected bond distances
(Å) and angles (◦): Pd(1)–C(1) 2.0399(12), Pd(1)–C(28) 2.2195(13), Pd(1)–C(29)
2.1403(17), Pd(1)–C(30) 2.0651(18), Pd(1)–O(1a) 2.108(6), Pd(1)–O(1b) 2.132(7),
C(28)–C(29) 1.370(3), C(29)–C(30) 1.419(3), C(1)–Pd(1)–C(28) 165.09(6),
C(1)–Pd(1)–C(29) 129.19(8), C(1)–Pd(1)–C(30) 96.79(8), C(1)–Pd(1)–O(1a)
91.87(16), C(1)–Pd(1)–O(1b) 98.66(17), C(28)–C(29)–C(30) 120.1(3).

L. Bettucci et al. / Journal of Molecular Catalysis A: Chemical 322 (2010) 63–72
69
Scheme 5.
(A) and 2R, 3R-butanediol (B)) and 1,3-diols (i.e. ( ) 1,3-butanediol
(C) and a 1:1 mixture of meso/rac-2,4-pentanediol (D)) as shown
in Scheme 5.
The catalytic reactions were carried out in a 19:1 (v:v) solvent
mixture of toluene and DMSO, in order to fully dissolve the sub-
strates, and in the presence of 20 bar of air. The catalytic activity as
well as the chemoselectivity provided by 1–8 have been compared
with that of the reference precursor 9 (Scheme 1) by carrying out
catalytic reactions under identical experimental conditions.
It is important to mention at this stage that the air pressure
(i.e. 20 bar) and the reaction temperature (i.e. 80 ◦C) are optimized
reaction parameters. The results of this comparative catalytic study
are summarized in Table 2.
Fig. 3. ORTEP plot of 10. Thermal ellipsoids are drawn at the 30% probability level
and hydrogen atoms are omitted for clarity. Only non-hydrogen atoms belonging to
the asymmetric unit have been labeled. Selected bond distances (Å) and angles (◦):
Pd(1)–Cl(1) 2.3204(16), Pd(1)–C(1) 2.036(3), Cl(1)–Pd(1)–C(1) 91.26(8).
Irrespective of the precursor employed, hydroxy ketones (Aꢀ–Dꢀ,
Scheme 5) have been chemoselectively obtained, confirming
the preference of Pd(II)-based catalysts to dehydrogenate sec-
3.4. Catalytic study
Table 2
3.4.1. Catalytic aerobic oxidation of diols
Aerobic oxidation of unprotected 1,2- and 1,3-diols with complexes 1–9.a.
Compounds 1–8 were applied as precursors for the selective
aerobic oxidation of unprotected 1,2-diols (i.e. ( ) 1,2-propanediol
Entry
Precursor
Substrate
Conv. (%)b
1 h
2 h (3 h) (5 h)
1
2
1
2
2
2
2
3
5
6
7
8
8
8
9
2
8
8
9
2
8
8
9
2
8
8
9
A
A
A
A
A
A
A
A
A
A
A
A
A
B
B
B
B
C
C
C
C
D
D
D
D
24
30
26
56 (60) (74)
14
24
8
18
22
18
22
26
32
26
24
50 (60) (73)
34
60
33
22 (24)
38
34
36
26 (36)
52
3c
4d
5e
6
7
8
5
9
20
10
16
23
29
25
24
27
29
50
32
11
33
31
32
14
44
40
47
9
10
11f
12g
13
14
15
16f
17
18
19
20f
21
22
23
24f
25
50
52
Fig. 4. ORTEP plot of 11·2C7H8. Thermal ellipsoids are drawn at the 30%
probability level. Hydrogen atoms and solvent molecules are omitted for
clarity. Selected bond distances (Å) and angles (◦): Pd(1)–C(1) 1.977(3),
Pd(1)–O(1) 2.058(2), Pd(1)–O(2) 2.013(3), Pd(1)–O(3) 2.004(3), Pd(2)–C(24)
1.979(4), Pd(2)–O(1) 2.010(3), Pd(2)–O(2) 2.054(2), Pd(2)–O(5) 2.028(3),
C(1)–Pd(1)–O(3) 92.80(13), O(1)–Pd(1)–O(2) 82.06(10), O(1)–Pd(1)–C(1)
a
Catalytic conditions: precursor 0.01 mmol, 19:1 (v:v) toluene/DMSO 20 mL, sub-
strate 1.00 mmol, p(air) 20 bar, T 80 ◦C.
b
Conversion given as average value of 3 catalytic runs.
Addition of Cs2CO3 0.02 mmol.
Addition of H2O2 (35% water solution) 20 ␮L.
Addition of hydroxyacetone 1.00 mmol.
Addition of NaOAc 0.02 mmol.
Addition of acetic acid (HOAc) 0.02 mmol.
c
d
e
171.02(12),
O(2)–Pd(1)–O(3)
175.94(10),
C(24)–Pd(2)–O(5)
90.85(14),
f
O(1)–Pd(2)–O(2) 82.24(10), O(1)–Pd(2)–O(5) 175.41(10), O(2)–Pd(2)–C(24)
172.70(13).
g

70
L. Bettucci et al. / Journal of Molecular Catalysis A: Chemical 322 (2010) 63–72
ondary over primary alcohol functional groups [12]. Indeed,
1,3-propanediol and 1,2-ethanediol were not oxidized under the
catalytic conditions employed in this study. The selective oxidation
of a primary alcohol group in the presence of a secondary alcohol
within the same molecule is feasible only by a synthetic protocol
that comprises a protecting–deprotecting synthetic strategy of the
secondary alcohol group [20b].
d8 were carried out in the presence of substrate D (Scheme 5) and
20 bar of air. Fig. 5 shows a sequence of selected 1H NMR spectra
acquired in the course of the NMR study carried out with 2.
The 1H NMR spectrum of a solution of 2 and substrate D in
toluene-d8 in the presence of air (20 bar) shows unambiguously
the presence of signals stemming from the coordinating allyl-unit
(Fig. 5, trace a). By heating the latter NMR solution to 80 ◦C, cor-
responding to the reaction temperature employed in the catalytic
batch reactions, the characteristic signals of the coordinating allyl
unit disappeared (Fig. 5, trace b) and new NMR signals appeared due
to the starting catalytic conversion of the substrate. After a reac-
tion time of 2 h, the 1H NMR spectrum, acquired at 80 ◦C, showed a
NMR singlet of low intensity at around 9.50 ppm, that was assigned
to the 2-imidazolium-hydrogen atom. The GC/MS analysis of the
NMR solution revealed a substrate conversion of 32% into Dꢀ and
the presence of propene in solution.
It is important to mention that a non-deareated solution of 2
in dry or wet toluene-d8 is stable (i.e. no propene release) even
on heating it at 80 ◦C for 10 min, whereas an analogous solution of
8, leads under identical experimental conditions to the concomi-
tant formation of palladium black and allyl acetate, the presence of
which in solution has been proved by GC/MS analysis. The absence
of activation of 4 under the present catalytic conditions is most
likely due to a strong chloride coordination to palladium. Accord-
ingly, the crystal structures of 4 exhibits a Pd–Cl bond length of
(2.370(5) Å) which is significantly shorter when compared to that
of 2 (2.391(14) Å) [28].
A comparison of the catalytic performance of the precursors by
employing 1,2-propanediol (i.e. A, Scheme 5) as substrate, showed
that 2 is the most efficient catalyst precursor, outperforming even
the reference system 9 in terms of catalytic activity and stability
with time (Table 2, entry 2 vs. 13). An attempt of increasing fur-
ther the catalytic efficiency of 2 by adding a base such as Cs2CO3
in order to foster the Pd-alkoxide formation failed (Table 2, entry
3 vs. 2) most likely due to coordination of carbonate to the pal-
ladium centre, which would hinder the access of the substrate
to palladium. Precursor 4 was under identical catalytic conditions
completely inactive, which is ascribed to the lacking activation of
the precursor and not to its decomposition to palladium black. In
fact, 4 was recovered unchanged after the catalytic reaction. Pre-
cursor 1 showed a rapid deactivation in the second hour of reaction
due to the formation of 10 in the course of the catalytic reaction.
A parallel catalytic batch reaction with the latter compound in the
presence of 1,2-propanediol gave no substrate conversion, confirm-
ing the requirement of coordinating only one carbene to the metal
centre.
Within the series of the palladium-acetate precursors 5–8, com-
plex 8 was the most promising one, showing a catalytic activity
that resembles that of the reference system (Table 2, entry 10 vs.
13). Importantly, the catalytic activity of 8 increased by adding
2 mol equivalents of NaOAc to the reaction solution when 1,2-diols
(i.e. substrates A and B) were employed (Table 2, entries 11/16 vs.
10/15). In contrast, analogous catalytic reactions carried out with
1,3-diols in the presence of NaOAc showed no beneficial effect
of the latter base (Table 2, entries 20/24 vs. 19/23). The same
applies to catalytic reactions carried out with 8 in the presence of
2 mol equivalent of HOAc (entry 12 vs. 10). Both NaOAc and HOAc
are known to have a stabilizing effect on palladium in the course of
the catalytic cycle [22]. The experimental fact, that this observation
applies for the oxidation of 1,2-diols and not 1,3-diols may be due to
the difference in palladium coordination property of the latter sub-
strates, which may be higher for 1,3-diols. All catalytic reactions
carried out in the presence of the palladium-acetate precursors
showed, regardless of the substrate employed, the formation of a
significant amount of palladium black in the course of the catalytic
reaction. As a consequence, the catalytic activity dropped signifi-
cantly in the second hour of reaction (Table 2), whereas 2 always
gave a transparent yellow solution with only a trace amount of
palladium black after catalysis. Nevertheless, 2 showed a drop of
the catalytic activity with substrates A and B during the third hour
of reaction (Table 2, entries 2 and 14). In order to rationalize this
experimental result, a catalytic reaction with 2 in the presence of
a 1:1 molar mixture of 1,2-propanediol and hydroxyacetone (i.e.
reaction product) was carried out. As a result, the catalytic activ-
ity of 2 dropped significantly when compared to that obtained by
carrying out the analogous reaction in the absence of added hydrox-
yacetone (Table 2, entry 5 vs. 2). From this experimental result,
we can thus infer that the oxidation product (i.e. hydroxy ketone)
formed in the course of the catalytic reaction competes with the
substrate for the coordination to palladium, slowing down thus the
kinetics of catalytic cycle.
It is thus reasonable to assume that anion dissociation (i.e. Cl,
OAc) from palladium is important for the propene release from pre-
cursor A (Scheme 6). In consequence of the anion dissociation, the
substrate may form the palladium-hydrido-allyl species C [36], that
releases propene by a reductive elimination reaction forming the
Pd(0) species D (Scheme 6) [22a].
The latter species undergoes either decomposition to palladium
black and carbene or enters the catalytic cycle upon reversible
coordination of oxygen, yielding intermediate E (Scheme 6),
which is most likely protonated by the free acid to yield the
palladium-hydroperoxide species (F) [37]. The latter intermediate
is then transformed into the neutral palladium–alkoxide species
(G) (Scheme 6) with concomitant release of H2O2. The forma-
tion of H2O2 in the course of the catalytic reactions is in line
with the oxygen consumption (i.e. measured by a flow meter) and
with the formation of trace amounts of dimethyl sulfone, that has
been detected after the catalytic reactions. In fact, a blank reac-
tion carried out with the solvent mixture proved a H2O2-mediated
Fig. 5. Selected 1H NMR spectra of the variable-temperature HPNMR study (sap-
phire tube, toluene-d8, 200.13 MHz) of the aerobic oxidation of substrate D with 2:
(a) solution of 2 in toluene-d8 in the presence of substrate D and air (20 bar) at room
temperature (x = 1H NMR signal assigned to the coordinating allyl moiety); (b) after
3.4.2. Mechanistic considerations
In order to shed light on the mechanism operative in the
catalytic oxidation reaction catalyzed by the precursors studied
herein, variable-temperature 1H HPNMR studies with 2 in toluene-
heating the NMR solution to 80 ◦C; (c) after heating the latter solution for 2 h at 80 ◦
C
(y = 1H NMR signal assigned to the 2-imidazolium-hydrogen atom).

L. Bettucci et al. / Journal of Molecular Catalysis A: Chemical 322 (2010) 63–72
71
Scheme 6. Proposed catalytic cycle.
oxidation of DMSO to dimethyl sulfone (i.e. 3% of dimethyl sul-
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Acknowledgments
This work was financially supported by the Italy-Taiwan Bilat-
eral Cooperation Project (NSC 97-2923-M-018-001-MY2). Thanks
are also given to the European Commission for financing the project
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