Scheme 1 shows a plausible mechanism that excludes the
involvement of enol ether A0 as a reaction intermediate based
on a control experiment in which formic acid and aldehyde C0
are not reaction intermediates.7 This proposed mechanism is
distinct from those proposed by previous workers;2d,e it is
supported by our isolation of side product 7g, from which we
infer the participation of acylgold species C. We envisage that
oxygen activation is likely to occur before hydrodeauration of
gold enol ether A,9 because the gold fragment of species A is
more electron-rich than cationic PPh3AuNTf2 in the reduction
of O2 to form cyclic peroxide intermediates B. This O2-
activation process presumably proceeds through a radical
process, because the catalytic reaction is inhibited by a radical
scavenger such as 2,6-di-tert-butyl-p-cresol according to
control experiment.10 Cyclic peroxide intermediates B allow
cleavage of the carbon–carbon bond to form methyl acetate
and acylgold intermediate C. We propose that the enol form,
D, is equally active as species A for the second reduction of O2,
giving gold-substituted formic acid via cyclic peroxide E. As
metal-containing formic acid is the postulated intermediate for
the water–gas shift reaction,11 we speculate that the decompo-
sition of species F is expected to give CO and H2O, or CO2 and
H2, which were identified by GC analysis. Accordingly, side
product 7g was presumably generated from the decarbonyl-
ation of gold intermediate C, followed by hydrodeauration.
In summary, we report the gold-catalyzed oxidative
cleavage of aryl-substituted alkynyl ethers12 using molecular
oxygen under ambient conditions; the transformation involves
a remarkable cleavage of C–H, C–C and CRC bonds
simultaneously. This catalysis is mechanistically novel because
the mechanism, on the basis of control experiments and
product analysis, reveals that gold-containing enol ethers are
the active species for the activation of oxygen. This new
information enhances the use of gold complexes in the activa-
tion of molecular oxygen in organic synthesis under ambient
conditions.
2 For metal-catalyzed cleavage of a CRC bond excluding enyne
cycloisomerizations and metathesis reactions, see: (a) C.-H. Jun,
C.-H. Lee, C.-W. Moon and H.-S. Hong, J. Am. Chem. Soc., 2001,
123, 8600; (b) T. Shimada and Y. Yamamoto, J. Am. Chem. Soc.,
2003, 125, 6646; (c) S. Datta, C.-L. Chang, K.-L. Yeh and
R.-S. Liu, J. Am. Chem. Soc., 2003, 125, 9294; (d) Y.-H. Liu,
F.-J. Song and S.-H. Guo, J. Am. Chem. Soc., 2006, 128, 11332;
(e) W. Wang and H. Jiang, J. Am. Chem. Soc., 2008, 130, 5030;
(f) I. Nakamura, T. Araki and M. Terada, J. Am. Chem. Soc.,
2009, 131, 2804.
3 For metal-catalyzed cleavage of C–C and CQC bonds using O2,
see selected examples: (a) K. Kaneda, T. Itoh, N. Kii, K. Jitsukawa
and S. Teranishi, J. Mol. Catal. A: Chem., 1982, 15, 349;
(b) K. Kaneda, S. Haruna, T. Imanaka and K. Kawamoto,
J. Chem. Soc., Chem. Commun., 1990, 1467; (c) Y. H. Lin,
I. D. Williams and P. Li, Appl. Catal., A, 1997, 150, 221;
(d) M. Hamamoto, K. Nakayama, Y. Nishiyama and Y. Ishii,
J. Org. Chem., 1993, 58, 6421; (e) J. Cossy, D. Bellotti,
V. V. Bellosta and D. Brocca, Tetrahedron Lett., 1994, 35, 6089;
(f) C. Bolm, G. Schlingloff and K. Weikhardt, Tetrahedron Lett.,
1993, 34, 3405.
4 Gold catalysts below showed good activities in this catalytic
O2–MeOH–CH2Cl2 system at 25 1C: 2 mol% PPh3AuCl–AgSbF6
(12 h; 2a, 75%; 3a, 10%), 4 mol% AuCl–AgSbF6 (26 h; 2a, 68%;
3a, 9%), 4 mol% AuCl/AgOTf (26 h, 2a, 73%; 3a, 15%). See the
ESI, Table S1w for catalysts screenings.
5 For formation mechanism of enone 3a, see selected examples:
(a) D. A. Engel and G. B. Dudly, Org. Lett., 2006, 8, 4027;
(b) M. Edens, D. Doerner, C. R. Chase, D. Nass and
M. D. Schiavelli, J. Org. Chem., 1977, 42, 1977; (c) J. Andres,
R. Cardenas, E. Silla and O. Tapia, J. Am. Chem. Soc., 1988, 110,
666; (d) K. H. Meyer and K. Schuster, Ber. Dtsch. Chem. Ges.,
1922, 55, 819.
6 Analysis of CO, CO2 and H2 were performed with MS 4 A-column
at room temperature. The solubility of these gases in CH2Cl2
causes a deficiency of the carbon balance.
7 We observed no NMR peak pertaining formic acid or methyl
formate in the oxidative cleavage of 1a with an in situ NMR study
in CD2Cl2. Formic acid and aldehyde C0 underwent no cleavage in
the PPh3AuNTf2–O2–MeOH system (25 1C, 12 h), and portions of
these two species were converted to methyl formate and acetal
species in 55% and 87%, respectively.
8 In eqn (3), the 18O-enriched samples 2a did not show additional
18
band due to n(CQ O) band at 1600–1560 cmꢁ1 region. The mass
spectra of 18O-enriched 2a is provided in the ESIw.
9 For electrophilic activation of vinylgold(I) complex, see: (a) Y. Shi,
S. D. Ramgren and S. A. Blum, Organometallics, 2009, 28, 1275;
(b) A. S. K. Hashmi, T. D. Ramamurthi and F. Rominger,
J. Organomet. Chem., 2009, 694, 592; (c) L.-P. Liu, B. Xu,
M. S. Mashuta and G. B. Hammond, J. Am. Chem. Soc., 2008,
130, 17642; (d) G. Zhang, Y. Peng, L. Cui and L. Zhang, Angew.
Chem., Int. Ed., 2009, 48, 3112.
Notes and references
1 (a) Activation of Unreactive Bonds and Organic Synthesis, in
Topics in Organometallic Chemistry, ed. S. Murai, Springer, Berlin,
1999, vol. 3; (b) M. Tobisu and N. Chatani, Chem. Soc. Rev., 2008,
37, 300; (c) C.-H. Jun, Chem. Soc. Rev., 2004, 33, 610;
(d) F. E. Kuhn, R. W. Fischer, W. A. Hermann and
T. Weskamp, in Transition Metals for Organic Synthesis, ed.
M. Beller and C. Bolm, Wiley-VCH, Weinheim, Germany, 2004,
´
vol. 2, p. 427; (e) E. J. Jimenez-Nu´ rnez and A. M. Echavarren,
Chem. Commun., 2007, 333; (f) S. I. Lee and N. Chatani, Chem.
Commun., 2009, 371.
10 A radical process was proposed for the activation of O2 by
PPh3AuOTf in Liu’s work; see ref. 2d.
11 D. H. Gibson, K. Owens and T.-S. Ong, J. Am. Chem. Soc., 1984,
106, 1125.
12 This air-oxidative cleavage is inapplicable to tertiary alkynyl ethers
which gave enones through a Meyer–Schuster rearrangement.5
ꢀc
This journal is The Royal Society of Chemistry 2009
4048 | Chem. Commun., 2009, 4046–4048