Abstract
Cinchona alkaloids in combination with zinc triflate catalyze the addition of acetylenes
to aldehydes at room temperature with ee up to 89% for aliphatic aldehydes. Cinchonidine
has proven to be the ligand of choice for the addition reaction whose outcome is demonstrated
to be very sensitive to the alkaloid structure. Aromatic aldehydes are less reactive
and give adducts of low optical purity. The more sterically hindered 2-bromo- and
2-nitrobenzaldehydes are considerably more reactive in the reaction as compared to
their 4-substituted congeners.
Key words
addition reactions - aldehydes - alkaloids - alkynes - asymmetric catalysis
References and Notes
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In a typical experiment the reactions were run under conditions employed by Carreira
and coworkers for catalytic addition of acetylenes to aldehydes in toluene.
[16 2 (0.036 g, 0.10 mmol, 20 mol%) was heated in a standard Radley reaction carousel test
tube at 125 °C in vacuo (<1 mbar) for 5 h. After cooling the tube to r.t. and releasing
vacuum the cinchona alkaloid was added (0.11 mmol, 22 mol%) and the mixture was evacuated
for 0.5 h. To it was added dry toluene (0.5 mL) and Et3 N (0.025 g, 0.25 mmol), and the tube content was stirred for 2 h at r.t. Subsequent
addition of acetylene (0.60 mmol) and stirring for 0.25 h at r.t. was followed by
addition of aldehyde (0.50 mmol). The reaction was performed at r.t. or 60 °C (temperature
and time are presented in Table
[1 2
