Triazole-Gold-Promoted, Effective Synthesis of Enones from Propargylic Esters and Alcohols
Experimental Section
Typical Experimental Procedure
To a solution of 1a (288 mg, 1.25 mmol) in wet MeOH
(5 mL, 0.25M, MeOH:H2O=100:1) was added Au(I) cata-
lyst (1.8 mg, 0.0025 mol, 0.2 mol%). The reaction mixture
was stirred at 608C. After the reaction was completed (4 h)
according to TLC, the solvent was removed under reduced
pressure and the residue was purified by flash chromatogra-
phy on silica gel (ethyl acetate/hexane=1:20, v/v) to give 4a
as a colorless oil; yield: 92%.
Acknowledgements
We thank the NSF (CHE-0844602), WVU-PSCoR for finan-
cial support. YFC thanks NSFC (No: 21002076) for support.
Scheme 7. Comparison with the best results obtained in the
literature.
References
Heating the [(IPr)AuCl]/AgBF4 in wet MeOH at
608C produced a black solution/participate within
30 min, indicating the rapid decomposition of the cat-
alyst. TA-Au, on the other hand, showed much im-
proved stability, with no decomposition after more
than 6 h under the same conditions. Considering the
good thermal stability of TA-Au, we wondered wheth-
er this catalyst could also be used to promote the
challenging Meyer–Schuster arrangement of propar-
gylic alcohols 5 at higher temperature.[11] Impressively,
the desired enone products were formed with excel-
lent yields. The reaction tolerated a large group of
substrates (Scheme 6). Surprisingly, the bulky tert-
butyl-substituted alkyne 5b was also suitable for this
reaction, which suggested the effective water addition
to the sterically hindered alkyne at higher tempera-
ture. The terminal alkyne[12] propargylic alcohol gave
enals in modest yields,[13] which were likely caused by
the longer reaction time required for the unfavored
anti-Markovnikov addition. Continuous addition of
catalysts could improve the overall yields. Neverthe-
less, the feasibility of this challenging substrate high-
lighted the strength of the TA-Au catalyst over the
[NHC-Au]+ catalysts (Scheme 7) by tolerating the
much harsher conditions.[14]
In conclusion, we have reported herein the applica-
tion of triazole-coordinated Au(I) complex (TA-Au)
as effective catalyst in promoting propargylic ester re-
arrangement and hydration for the synthesis of substi-
tuted enones. The key for the success of the TA-Au
catalyst was the combination of the unique chemose-
lectivity and acidity. Extension of this transformation
to the Meyer–Schuster rearrangement by taking ad-
vantage of the thermal stability of the TA-Au further
improved the atom economy. Compared with the
more expensive NHC-Au catalysts, TA-Au promoted
the reaction through a different mechanism and
achieved better performance with lower overall costs.
[1] For selected recent reviews on homogeneous Au cataly-
sis, see: a) A. S. K. Hashmi, M. Rudolph, Chem. Soc.
Rev. 2008, 37, 1766–1775; b) D. J. Gorin, B. D. Sherry,
F. D. Toste, Chem. Rev. 2008, 108, 3351–3378; c) A.
Arcadi, Chem. Rev. 2008, 108, 3266–3325; d) E. Jime-
nez-Nunez, A. M. Echavarren, Chem. Rev. 2008, 108,
3326–3350; e) A. S. K. Hashmi, Chem. Rev. 2007, 107,
3180–3211; f) A. Furstner, P. W. Davies, Angew. Chem.
2007, 119, 3478–3519; Angew. Chem. Int. Ed. 2007, 46,
3410–3449; g) S. Diez-Gonzalez, N. Marion, S. P. Nolan,
Chem. Rev. 2009, 109, 3612–3676; h) E. Jimenez-
Nunez, A. M. Echavarren, Chem. Commun. 2007, 333–
346; i) A. S. K. Hashmi, G. J. Hutchings, Angew. Chem.
2006, 118, 8064–8105; Angew. Chem. Int. Ed. 2006, 45,
7896–7936; j) L. Zhang, J. Sun, S. A. Kozmin, Adv.
Synth. Catal. 2006, 348, 2271–2296.
[2] a) B. Bovio, A. Burini, B. R. Pietroni, J. Organomet.
Chem. 1993, 452, 287–291; b) J. H. Teles, S. Brode, M.
Chabanas, Angew. Chem. 1998, 110, 1475–1478; Angew.
Chem. Int. Ed. 1998, 37, 1415–1418; c) J. H. Teles, S.
Brode, M. Chabanas, Angew. Chem. Int. Ed. 1998, 37,
1475–1478; d) X. Hu, I. Castro-Rodriguez, K. Olsen, K.
Meyer, Organometallics 2004, 23, 755–764; e) C. Nieto-
Oberhuber, S. Lopez, A. Echavarren, J. Am. Chem.
Soc. 2007, 129, 7772–7773; f) P. Frꢂmont, N. M. Scott,
E. D. Stevens, S. P. Nolan, Organometallics 2005, 24,
2411–2418; g) A. S. K. Hashmi, C. Lothschꢃtz, C.
BÅhling, T. Hengst, C. Hubbert, F. Rominger Adv.
Synth. Catal. 2010, 352, 3001–3012.
[3] a) S. Dꢄez-Gonzꢅlez, N. Marion, S. P. Nolan, Chem.
Rev. 2009, 109, 3612–3676; b) P. Frꢂmont, N. Marion,
S. P. Nolan, Coord. Chem. Rev. 2009, 253, 862–892;
c) N. Marion; S. P. Nolan, Acc. Chem. Res. 2008, 41,
1440–1449; d) N. Marion, S. P. Nolan, Chem. Soc. Rev.
2008, 37, 1776–1782; e) S. Dꢄez-Gonzꢅlez, S. P. Nolan,
Acc. Chem. Res. 2008, 41, 349–358.
[4] a) H. Duan, S. Sengupta, J. L. Petersen, N. G. Akhme-
dov, X. Shi, J. Am. Chem. Soc. 2009, 131, 12100–12102;
b) Y. Chen, W. Yan, N. G. Akhmedov, X. Shi, Org.
Lett. 2010, 12, 344–347; c) D. Wang, X. Ye, X. Shi, Org.
Adv. Synth. Catal. 2011, 353, 2584 – 2588
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2587