ChemComm
Communication
listed in Table 1. For the aliphatic alcohol, there is no p–p conjugation such as p-COOMe also gave the corresponding esters in a high yield
from the carbonyl of the intermediate aldehyde; hence addition of (entry 2). Benzylic alcohols bearing meta-substituents, such as m-Me or
alcohol to the carbonyl of the corresponding aliphatic aldehyde is m-OMe (entries 3 and 4), afforded their corresponding esters in good
facile. Therefore, selective esterification was observed by using desyl Cl yields. The ortho-C–Cl bond was well tolerated and the ester product
as the oxidant.
was obtained in an excellent yield. The steric hindrance did not have
any negative effect on the yield (entry 6). It is worth noting that other
aromatic arenes such as thiophene-2-methanol could also be applied
in this oxidative esterification and gave the corresponding esters in a
good yield (entry 7). Moreover, alkyl alcohol is more suitable for
oxidative esterification, which has been shown in other reaction
systems. We subjected an alkyl alcohol to these oxidative conditions
and quantitative amounts of the desired ester were obtained (entry 8).
We have demonstrated an oxidant controlled palladium catalysed
selective oxidation of primary alcohols to aldehydes or esters. The
electronic properties of benzylic alcohols and the structure of the
oxidant are the important factors that control the selectivity between
aldehydes and esters. Electron-withdrawing groups promote the ester
formation by using desyl chloride as the oxidant. The covalent benzyl
ligand, derived from BnCl, is the key for the selective oxidative
esterification of primary alcohols. The Z3 coordination of benzyl
group to palladium might be essential for the ester formation.
This work was supported by the 973 Program (2012CB725302),
the National Natural Science Foundation of China (21025206 and
21272180) and the China Postdoctoral Science Foundation funded
project (2012M521458). We thank Dr Andrew G. Crawford for his
help in revising the manuscript.
(3)
Comparing BnCl with desyl chloride and the oxidants in
Table S1 (ESI†), it was found that the Bn–Pd moiety (X = Bn,
Scheme 2) is essential to the selective oxidative esterification, as
the Bn group could be considered as a covalent carbon anion
ligand which could easily form Z3 coordination to palladium
(eqn (1)).11 This Z3 coordination might facilitate the facile
dissociation of PPh3 and will favour the coordination of alde-
hyde to the palladium centre. This claim could be proved by
using a rigid bidentate dppf ligand in the reaction system,
which resulted in a worse selectivity (see ESI,† Table S2).
Compared with selective aldehyde formation by using desyl
Cl as the oxidant, the selective oxidative esterification of
primary alcohols is more interesting when BnCl is used as
the oxidant. Previously, very few examples have demonstrated
the selective formation of esters directly from the alcohol in the
presence of a palladium catalyst.5 Moreover, benzyl chloride is
a fundamental industrial chemical, which is cheap and easily
obtained. The side product toluene is a widely used organic
solvent and can easily be removed. There is strong potential to
use benzyl Cl as the oxidant in both academia and industry.
Therefore, further application of BnCl in the palladium catalysed
oxidative esterification of more primary alcohols was investigated
(Table 2). p-F substituted benzyl alcohol gave excellent yields of the
corresponding ester (entry 1). Strong electron-withdrawing groups
Notes and references
1 R. A. Sheldon and J. K. Kochi, Metal-catalyzed oxidations of organic
compounds: mechanistic principles and synthetic methodology including
biochemical processes, Academic Press, New York, 1981.
2 (a) M. S. Sigman and D. R. Jensen, Acc. Chem. Res., 2006, 39, 221–229;
(b) R. A. Sheldon, I. W. C. E. Arends, G.-J. ten Brink and A. Dijksman,
Acc. Chem. Res., 2002, 35, 774–781; (c) J. Muzart, Tetrahedron, 2003, 59,
5789–5816; (d) M. J. Schultz and M. S. Sigman, Tetrahedron, 2006, 62,
8227–8241; (e) S. S. Stahl, Angew. Chem., Int. Ed., 2004, 43, 3400–3420;
( f ) W. Kroutil, H. Mang, K. Edegger and K. Faber, Adv. Synth. Catal.,
2004, 346, 125–142; (g) T. Mallat and A. Baiker, Chem. Rev., 2004, 104,
3037–3058; (h) S. Krishnan, J. T. Bagdanoff, D. C. Ebner, Y. K.
Ramtohul, U. K. Tambar and B. M. Stoltz, J. Am. Chem. Soc., 2008,
130, 13745–13754; (i) T. Nishimura, T. Onoue, K. Ohe and S. Uemura,
J. Org. Chem., 1999, 64, 6750–6755.
Table 2 Pd-catalysed selective oxidative esterification of primary alcoholsa
3 G. Cainelli and G. Cardillo, Chromium oxidations in organic
chemistry, Springer-Verlag, Berlin, New York, 1984.
Entry
Product
3
Yieldb (%)
4 M. Beller and C. Bolm, Transition metals for organic synthesis:
building blocks and fine chemicals, WILEY-VCH, Weinheim, 2nd
rev. and enl. edn, 2004.
1
2
R0 = F
3g
3h
91
78
R0 = COOMe
5 (a) S. Gowrisankar, H. Neumann and M. Beller, Angew. Chem., Int.
Ed., 2011, 50, 5139–5143; (b) C. Liu, J. Wang, L. Meng, Y. Deng, Y. Li
and A. Lei, Angew. Chem., Int. Ed., 2011, 50, 5144–5148; (c) D. Zhang
and C. Pan, Catal. Commun., 2012, 20, 41–45; (d) F. Luo, C. Pan,
J. Cheng and F. Chen, Tetrahedron, 2011, 67, 5878–5882.
6 C. Liu, S. Tang, L. Zheng, D. Liu, H. Zhang and A. Lei, Angew. Chem.,
Int. Ed., 2012, 51, 5662–5666.
R0 = Me
3i
3j
89
80
3
4
R0 = OMe
5c
R0 = Me
R0 = Cl
3k
3l
>99
94
6d
7 Y. Tamaru, Y. Yamada, K. Inoue, Y. Yamamoto and Z. Yoshida,
J. Org. Chem., 1983, 48, 1286–1292.
8 R. Liu, X. Liang, C. Dong and X. Hu, J. Am. Chem. Soc., 2004, 126,
4112–4113.
9 S. L. Marquard and J. F. Hartwig, Angew. Chem., Int. Ed., 2011, 50,
7119–7123.
10 M. A. Zuideveld, P. C. J. Kamer, P. W. N. M. van Leeuwen, P. A. A.
Klusener, H. A. Stil and C. F. Roobeek, J. Am. Chem. Soc., 1998, 120,
7977–7978.
11 (a) P. Fitton, J. E. McKeon and B. C. Ream, Chem. Commun., 1969,
370–371; (b) R. Ros, M. Lenarda, T. Boschi and R. Roulet, Inorg.
Chim. Acta, 1977, 25, 61–64.
7
3m
3n
83
8c
>99
a
Reaction conditions: 1 (0.50 mmol), PdCl2(PPh3)2 (0.025 mmol),
K2CO3 (0.50 mmol), BnCl (0.50 mmol) in 2 mL of THF at 65–70 1C for
b
c
d
20 h. Isolated yields. 1.0 mmol scale. 30 h.
c
1326 Chem. Commun., 2013, 49, 1324--1326
This journal is The Royal Society of Chemistry 2013