ChemComm
Communication
and deactivated (i.e. 20 and 21, entries 3 and 4) aromatic
aldehydes could be converted to the corresponding methyl
esters in excellent isolated yield in the presence of 18, DBU,
methanol and air. Activated benzaldehydes provided interest-
ing results: m-chlorobenzaldehyde (22) proved an excellent
substrate, while the esterification of its para-substituted isomer
8 J. Guin, S. De Sarkar, S. Grimme and A. Studer, J. Am. Chem. Soc.,
2
010, 132, 1190.
For examples of related reactions not involving ester formation see:
a) S. De Sarkar and A. Studer, Org. Lett., 2010, 12, 1992; (b) S. De Sarkar
9
(
and A. Studer, Angew. Chem., Int. Ed., 2010, 49, 9266; (c) A. Biswas, S. De
Sarkar, R. Fr ¨o hlich and A. Studer, Org. Lett., 2011, 13, 4966.
1
1
0 (a) H. Inoue and K. Higashiura, J. Chem. Soc., Chem. Commun., 1980,
49; (b) C. Noonan, L. Baragwanath and S. J. Connon, Tetrahedron Lett.,
5
23 and a m-methoxy-substituted variant (i.e. 24) proceeded in
2008, 49, 4003; (c) C. A. Rose and K. Zeitler, Org. Lett., 2010, 12, 4552.
1 For related NHC-mediated oxidations see: (a) J. Castells, H. Llitj ´o s and
M. Moreno-Ma n´ as, Tetrahedron Lett., 1977, 18, 205; (b) H. Inoue and
S. J. Tamura, J. Chem. Soc., Chem. Commun., 1985, 141; (c) S. Shinkai,
T. Yamashita, Y. Kusano and O. Manabe, J. Org. Chem., 1980, 45, 4947;
77–92% yields (entries 5–7). Ortho-substitution is not well
tolerated: o-tolualdehyde (25) and o-chlorobenzaldehyde (26)
underwent conversion to 37 and 38 in low to moderate yields
respectively (entries 8 and 9), while o-trifluoromethyl-benzaldehyde
was inert (entry 10).
(d) S. Shinkai, T. Yamashita, Y. Kusano and O. Manabe, Tetrahedron
Lett., 1980, 21, 2543; (e) S. Shinkai, Y. Hara and O. Manabe, Bull. Chem.
Soc. Jpn., 1983, 56, 770; ( f ) Y. Yano, Y. Hoshino and W. Takagi, Chem.
Lett., 1980, 749; (g) Y. Yano and Y. Tsukagoshi, J. Chem. Res., Synop.,
Substrates incorporating oxidisable heterocyclic functionality
1
984, 406; (h) W. H. Rastetter, J. Adams, J. W. Frost, L. J. Nummy,
J. E. Frommer and K. B. Roberts, J. Am. Chem. Soc., 1979, 101, 2752;
i) W. H. Rastetter and J. Adams, J. Org. Chem., 1981, 46, 1882;
( j) D. Hilvert and R. Breslow, Bioorg. Chem., 1984, 12, 206;
k) L. Jimenez and F. Diederich, Tetrahedron Lett., 1989, 30, 2759.
2 (a) S.-W. Tam, L. Jimenez and F. Diederich, J. Am. Chem. Soc., 1992,
14, 1503; (b) T. Uno, T. Inokuma and Y. Takemoto, Chem. Commun.,
(
i.e. 28 and 29) could be converted to their methyl ester analo-
2
5
gues in good-excellent yields (entries 11 and 12). In contrast,
the aliphatic aldehyde 30 resulted in a poor yield of the ester 42
and a complex product mixture (entry 13).
(
(
1
In summary, we have developed an efficient NHC-catalysed
esterification of aldehydes involving alcohols and air (i.e. oxygen)
as the oxidant. No other added stoichiometric oxidants or
catalysts to activate molecular oxygen are required. Unhindered
1
2012, 48, 1901; (c) Rovis et al. have noted the application of catalytic
oxidant where imine substrates are supposed to re-oxidize riboflavin
tetraacetate: X. Zhao, K. E. Ruhl and T. Rovis, Angew. Chem., Int. Ed.,
2012, 51, 12330.
aromatic aldehydes (including heterocyclic analogues) can be 13 E. E. Finney, K. A. Ogawa and A. J. Boydston, J. Am. Chem. Soc., 2012,
1
34, 12374.
converted to the corresponding methyl esters in good to excellent
yields at ambient temperature.
Financial support from IRCSET, Science Foundation Ireland
and the DFG is gratefully acknowledged.
1
1
4 L. Lin, Y. Li, W. Du and W.-P. Deng, Tetrahedron Lett., 2010, 51, 3571.
5 B. Maji, S. Vedachalan, X. Ge, S. Cai and X.-W. Liu, J. Org. Chem.,
2011, 76, 3016.
6 Y.-C. Xin, S.-H. Shi, D.-D. Xie, X.-P. Hui and P.-F. Xu, Eur. J. Org.
Chem., 2011, 6527.
1
1
7 For a related process involving intramolecular cyclisation of the
carboxylate ion onto an alkyne see: J. H. Park, S. V. Bhilare and
S. W. Youn, Org. Lett., 2011, 13, 2228.
Notes and references
1
(a) J. Otera, Esterification: Methods, Reactions and Applications, Wiley, 18 Recent claims that CO
New York, 2003; (b) R. C. Larock, Comprehensive Organic Transforma-
tions, VCH, New York, 1989.
2
can be involved in NHC-mediated oxidations
of aldehydes as an oxidant (ref. 18a and b) were subsequently
investigated by Bode et al. (ref. 18c), who ascertained that O is
the actual oxidant in these processes, with CO serving an ancillary
2
2
For reviews on NHC-catalysed oxidative transformations see: (a)
C. E. I. Knappke, A. Imami and A. Jacobi von Wangelin, Chem-
CatChem, 2012, 4, 937; (b) S. DeSarkar, A. Biswas, R. C. Samanta and
A. Studer, Chem.–Eur. J., 2013, 19, 4664.
2
role as a suppressant of side-reactions. (a) L. Gu and Y. Zhang, J. Am.
Chem. Soc., 2010, 132, 914; (b) V. Nair, V. Varghese, R. R. Paul,
A. Jose, C. R. Sinu and R. S. Menon, Org. Lett., 2010, 12, 2653;
(c) P.-C. Chiang and J. W. Bode, Org. Lett., 2011, 13, 2422; for a
NHC-catalysed aerobic oxidation of ArCHO to their acids using
‘‘Bode-type’’ conditions:; (d) W. Yang, G.-Z. Gou, Y. Wang and
W.-F. Fu, RSC Adv., 2013, 3, 6334.
3
Recent reviews concerning various aspects of NHC organocatalysis:
(
a) H. U. Vora and T. Rovis, Aldrichimica Acta, 2011, 44, 3; (b) A. T.
Biju, N. Kuhl and F. Glorius, Acc. Chem. Res., 2011, 44, 1182;
c) V. Nair, R. S. Menon, A. T. Biju, C. R. Sinu, R. R. Paula,
(
A. Josea and V. Sreekumar, Chem. Soc. Rev., 2011, 40, 5336; 19 Du and Deng (ref. 14) have proposed an alternative mechanism for
(
(
d) P.-C. Chiang and J. W. Bode, RSC Catal. Ser., 2010, 6, 399;
e) C. D. Campbell, K. B. Ling and A. D. Smith, in Catalysis by Metal
oxygenative esterification involving O-alkylation of the Breslow
intermediate by the alkyl halide electrophile.
Complexes, 2010, vol. 32, p. 263; ( f ) J. L. Moore and T. Rovis, Top. 20 Intermediate 7 has also been proposed to for a recent related
Curr. Chem., 2009, 291, 77; (g) E. M. Phillips, A. Chan and K. A.
Scheidt, Aldrichimica Acta, 2009, 42, 55; (h) T. Rovis, Chem. Lett.,
2
NHC-catalysed esterification under O -atmosphere: I. N. C. Kiran,
K. Lalwani and A. Sudalai, RSC Adv., 2013, 3, 1695.
2
(
008, 1; (i) K. Zeitler, Ernst Schering Found. Symp. Proc., 2007, 2, 183; 21 (a) M. S. Kerr, J. Read de Alaniz and T. Rovis, J. Org. Chem., 2005,
j) D. Enders, O. Niemeier and A. Henseler, Chem. Rev., 2007, 107, 5506; 70, 5725; (b) H. U. Vora, S. P. Lathrop, N. T. Reynolds, M. S. Kerr,
J. Read de Alaniz and T. Rovis, Org. Synth., 2010, 87, 350; (c) The pK
of catalyst 14 (H O) has been determined to be 17.7: C. D. Campbell,
(
k) N. Marion, S. Diez-Gonzalez and S. P. Nolan, Angew. Chem., Int. Ed.,
007, 46, 2988; (l) K. Zeitler, Angew. Chem., Int. Ed., 2005, 44, 7506.
a
2
2
4
5
(a) A. Miyashita, Y. Suzuki, M. Kobayshi, N. Kuriyama and
T. Higashino, Heterocycles, 1996, 43, 509; (b) A. Miyashita,
Y. Suzuki, I. Nagsaki, C. Ishiguro, K.-I. Iwamoto and T. Higashino,
Chem. Pharm. Bull., 1997, 45, 1254; (c) J. Castells, F. Pujol, H. Llitj o´ s
and M. Moreno-Ma n´ as, Tetrahedron, 1982, 38, 337.
(a) B. E. Maki and K. A. Scheidt, Org. Lett., 2008, 10, 4331; (b) B. Maki, 22 S. Iwahana, H. Iida and E. Yashima, Chem.–Eur. J., 2011, 17, 8009.
A. Chan and K. A. Scheidt, Synthesis, 2008, 1306; (c) H. Kim, Y. Park 23 For a recent flavin-mediated aerobic oxidation of 12 in a Dakin-type
N. Duguet, K. A. Gallagher, J. E. Thomson, A. G. Lindsay, A. C.
O’Donoghue and A. D. Smith, Chem. Commun., 2008, 3528; (d) For a
recent compilation of more pK values of triazolium salts: R. S.
a
Massey, C. J. Collett, A. G. Lindsay, A. D. Smith and A. C.
O’Donoghue, J. Am. Chem. Soc., 2012, 134, 20421.
and J. Hong, Angew. Chem., Int. Ed., 2009, 48, 7577; (d) B. Maki,
A. Chan, E. M. Philips and K. A. Scheidt, Tetrahedron, 2009, 65, 3102; 24 For a recent report concerning the use of Fe ions as a co-catalyst
process see: S. Chen and F. W. Foss Jr., Org. Lett., 2012, 14, 5150.
2+
(
e) K. Lee, H. Kim and J. Hong, Angew. Chem., Int. Ed., 2012, 51, 5735.
and coupling with phenols see: R. S. Reddy, J. N. Rosa, L. F. Veiros,
S. Caddick and P. M. P. Gois, Org. Biomol. Chem., 2011, 9, 3126.
6
7
Related reactions mediated by MnO : (a) B. E. Maki, A. Chan,
E. M. Phillips and K. A. Scheidt, Org. Lett., 2007, 9, 371; 25 In this context it is interesting to note that Goswami and Hazra
2
(
b) M. Rueping, H. Sund ´e n, L. Hubener and E. Sugiono, Chem.
had previously reported the aerobic esterification of heterocyclic
aromatic aldehydes (only) involving thiamine hydrochloride and
Commun., 2012, 48, 2201.
J. Guin, S. De Sarkar, S. Grimme and A. Studer, Angew. Chem., Int. Ed.,
3
NEt (2.0 equiv.) in MeOH (heating under reflux was required):
2
008, 47, 8727.
S. Goswami and A. Hazra, Chem. Lett., 2009, 484.
This journal is c The Royal Society of Chemistry 2013
Chem. Commun.