with in situ generated sulfur ylides with moderate to excellent
selectivities to give the corresponding cyclopropyl carboxylic
esters. Electronic effects exerted by the enal component and
also by the ylide strongly influenced stereoselectivity. The
proper choice of the alcohol in this three component reaction
is crucial for successful transformation. Reactions occur under
mild conditions and are easy to conduct.
Notes and references
1 (a) J. Salaun, Chem. Rev., 1989, 89, 1247; (b) A.-H. Li, L.-X. Dai
and V. K. Aggarwal, Chem. Rev., 1997, 97, 2341; (c) H.-U. Reissig
and R. Zimmer, Chem. Rev., 2003, 103, 1151.
2 (a) T. Tsuji and S. Nishida, The Chemistry of the Cyclopropyl
Group, Wiley and Sons, New York, NY, 1987; (b) M. P. Doyle and
M. N. Protopopova, Tetrahedron, 1998, 54, 7919; (c) A. Pfaltz, in
Comprehensive Asymmetric Catalysis, ed. E. N. Jacobsen, A. Pfaltz
and H. Yamamoto, Springer, Heidelberg, 1999, p. 513.
3 (a) M. J. Gaunt and C. C. C. Johansson, Chem. Rev., 2007,
107, 5596; (b) X. L. Sun and Y. Tang, Acc. Chem. Res., 2008,
41, 937.
Scheme 2 Suggested catalytic cycle.
4 Selected examples on organocatalytic cyclopropanations:
(a) R. K. Kunz and D. W. C. MacMillan, J. Am. Chem. Soc.,
2005, 127, 3240; (b) A. N. Alba, X. Companyo, M. Viciano and
R. Rios, Curr. Org. Chem., 2009, 13, 1432; (c) A. E. Raveendran,
R. R. Paul, E. Suresh and V. Nair, Org. Biomol. Chem., 2010,
8, 901; (d) M. Rueping, H. Sunde
Chem. Commun., 2012, 48, 2201.
5 Reviews: (a) D. Enders, O. Niemeier and A. Henseler, Chem. Rev.,
2007, 107, 5606; (b) N. Marion, S. Dıez-Gonzalez and S. P. Nolan,
´
n, L. Hubener and E. Sugiono,
´
´
Angew. Chem., Int. Ed., 2007, 46, 2988; (c) V. Nair, S. Vellalath
and B. P. Babu, Chem. Soc. Rev., 2008, 37, 2691; (d) A. T. Biju,
N. Kuhl and F. Glorius, Acc. Chem. Res., 2011, 44, 1182;
(e) A. Grossmann and D. Enders, Angew. Chem., Int. Ed., 2012,
51, 314.
Scheme 3 Preparation of bicyclic lactones 13a–c.
6 Selected examples of NHC catalyzed internal redox reactions:
(a) K. Y.-K. Chow and J. W. Bode, J. Am. Chem. Soc., 2004,
126, 8126; (b) N. T. Reynolds, J. R. de Alaniz and T. Rovis, J. Am.
Chem. Soc., 2004, 126, 9518; (c) S. S. Sohn and J. W. Bode, Angew.
Chem., Int. Ed., 2006, 45, 6021; (d) K. Zeitler and C. A. Rose,
J. Org. Chem., 2009, 74, 1759; (e) Y. Yamaki, A. Shigenaga, J. Li,
Y. Shimohigashi and A. Otaka, J. Org. Chem., 2009, 74, 3278.
7 (a) J. Guin, S. De Sarkar, S. Grimme and A. Studer, Angew.
Chem., Int. Ed., 2008, 47, 8727; (b) S. De Sarkar, S. Grimme and
A. Studer, J. Am. Chem. Soc., 2010, 132, 1190; (c) S. De Sarkar and
A. Studer, Org. Lett., 2010, 12, 1992; (d) S. De Sarkar, A. Biswas,
C. H. Song and A. Studer, Synthesis, 2011, 12, 1974.
8 Oxidative esterification (a) B. E. Maki, A. Chan, E. M. Phillips and
K. A. Scheidt, Tetrahedron, 2009, 65, 3102; (b) C. Noonan,
L. Baragwanath and S. J. Connon, Tetrahedron Lett., 2008,
49, 4003; (c) C. A. Rose and K. Zeitler, Org. Lett., 2010, 12, 4552.
9 C–C bond formation via unsaturated acylazolium: (a) S. De Sarkar
and A. Studer, Angew. Chem., Int. Ed., 2010, 49, 9266;
of cyclopropanation of hexafluoroisopropyl cinnamate, which
is formed by the addition of hexafluoroisopropanol to the
unsaturated acyl azolium, leading to racemic 7e and hence a
reduced selectivity. In contrast, the cinnamic acid esters
formed with the non-fluorinated alcohols as side products
are not as reactive as Michael acceptors.
The suggested catalytic cycle is depicted in Scheme 2.
Reaction between carbene 2b0 and enal 1 in the presence of
4 provides acyl azolium ion 8,9a,b which reacts with ylide 3
via 1,4-addition to give enolate 9. Cyclization to 10 and
alcoholysis eventually afford 11. As a side reaction, the alcohol
can directly react with acylazolium 8 to give the corresponding
alkyl cinnamate.
Finally, we tested salt 3e bearing a keto functionality in the
reaction with enals. An external alcohol is not necessary for
catalyst turnover since the intermediate acyl azolium 12 can
after deprotonation be intramolecularly acylated to give 13
(Scheme 3). Catalyst 2b, which delivered best results in the
three component reactions, provided 13a in 43% yield with
good diastereo but moderate enantioselectivity (ee 73%).
A better ee (76%) was obtained with triazolium 2d and 13a
was isolated in 43% yield.11 Good enantioselectivities but
diminished yields were achieved with p-nitro cinnamaldehyde
(13b) and p-tolyl acrolein (13c).
(b) A. Biswas, S. De Sarkar, R. Frohlich and A. Studer, Org.
¨
Lett., 2011, 13, 4966; (c) S. J. Ryan, L. Candish and D. W. Lupton,
J. Am. Chem. Soc., 2009, 131, 14176; (d) S. J. Ryan, L. Candish
and D. W. Lupton, J. Am. Chem. Soc., 2011, 133, 4694;
(e) J. Kaeobamrung, J. Mahatthananchai, P. Zheng and
J. W. Bode, J. Am. Chem. Soc., 2010, 132, 8810; (f) Z.-Q. Rong,
M.-Q. Jia and S.-L. You, Org. Lett., 2011, 13, 4080; (g) Z.-Q. Zhu,
X.-L. Zheng, N.-F. Jiang, X. Wan and J.-C. Xiao, Chem. Commun.,
2011, 47, 8670; (h) B. Wanner, J. Mahatthananchai and J. W. Bode,
Org. Lett., 2011, 13, 5378; (i) F.-G. Sun, L.-H. Sun and S. Ye,
Adv. Synth. Catal., 2011, 353, 3134.
10 For 5d, f and j a third isomer was formed (see ESIw).
11 Relative configuration was assigned by NMR spectroscopy (see ESIw)
and assignment of the absolute configuration was based on the model
depicted in Scheme 3. Note that catalyst 2d delivers the opposite
enantiomer to 2b.
We showed that a,b-unsaturated acyl azoliums generated
from the corresponding enals by oxidative NHC catalysis react
c
5192 Chem. Commun., 2012, 48, 5190–5192
This journal is The Royal Society of Chemistry 2012