Efficient Synthesis of β-Unsaturated α-Amino Esters
General Procedure for YbIII-Catalyzed Addition Reaction of Alk-
ynyltrifluoroborate Salts to α-Imino Esters: Ytterbium triflate
(10 mol-%, 0.030 g) was added to a stirred solution of α-imino ester
(0.5 mmol, 0.103 g) in CH2Cl2 (5 mL). Potassium trifluoroborate
(0.6 mmol) was then added, and the reaction mixture was stirred at
room temperature until there was total consumption of the starting
material. Afterwards, the reaction mixture was extracted with
NaOH (0.5 n). The organic phase was dried using MgSO4, and the
solvent was removed under reduced pressure.
Ethyl
2-(4-Methoxyphenylamino)-4-phenylbut-3-ynoate
(3a):
1
(0.125 g, 81% yield). H NMR (300 MHz, CDCl3): δ = 7.41–7.38
(m, 2 H), 7.30–7.24 (m, 3 H), 6.77 (dd, J = 9.0 Hz, 4 H), 4.94 (s, 1
H), 4.30 (q, J = 7.1 Hz, 2 H), 3.74 (s, 3 H), 1.32 (t, J = 7.1 Hz, 3
H) ppm. 13C NMR (75 MHz, CDCl3): δ = 168.9, 153.3, 139.4 (2
C), 131.8, 128.5 (2 C), 128.1, 122.1, 116.0 (2 C), 114.7 (2 C), 84.3,
84.2, 62.3, 55.6, 50.6, 14.0 ppm. HRMS: calcd. for C19H19NO3 [M
+ H]+ 310.1437; found: 310.1435.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental details and analytical data for all new com-
1
pounds as well as the H and 13C NMR spectra.
Scheme 2. Proposed mechanism.
Acknowledgments
An interesting aspect of using rare earth metal triflates[15]
is that the catalyst can be easily recovered from the reaction The authors gratefully acknowledge the São Paulo Research Foun-
dation (FAPESP; fellowship to F. M., 2010/15677-58, grant number
2012/00424-2) for financial support and The National Council for
Scientific and Technological Development (CNPq) for a fellowship
(308.320/2010-7, to H. A. S.).
medium and recycled. This prompted us to evaluate the
possibility of reusing the Yb(OTf)3 in our reactions. In this
regard, the catalyst was separated from the reaction me-
dium by simple filtration, washed with dichloromethane,
and dried at 70 °C for 2 h. This process was repeated two
times to afford the desired product in 81% (first run), 75%
(second run), and 50% (third run).
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Conclusions
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through the alkynylation of imino esters with ytterbium
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Experimental Section
General Methods: All reactions were carried out under nitrogen.
All compounds were characterized by using 1H and 13C NMR
spectroscopy as well as ESI-MS. Copies of the H and 13C NMR
1
1
spectra can be found in the Supporting Information. The H and
13C NMR spectroscopic data were recorded with a 300 MHz in-
1
strument. The chemical shifts (δ) for the H NMR experiments are
reported in parts per million (ppm) and measured relative to the
signals for TMS (δ = 0.00 ppm). The chemical shifts for the 13C
NMR spectra are reported in ppm relative to deuterated chloro-
form (δ =77.23 ppm), unless otherwise stated, and all data were
1
recorded using H decoupling. Solvents and reagents were analyti-
cal grade or the highest grade commercially available and were used
without further purification. The reactions were monitored by in
situ Fourier transform infrared (FTIR) spectroscopy using a
Mettler Toledo ReactIR 15 that was fitted with an articulated arm
and a diamond probe.
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