Abstract
Oxidative dimerization of primary alcohols with 2-butanone in the presence of an amino alcohol-based Ir bifunctional catalyst was accomplished for the first time. The reaction proceeds with 1-2 mol% of the catalyst and 0.3 mol equivalents of K2CO3 in 2-butanone at room temperature to give the corresponding dimeric esters in 30-93% yield.
Key words
alcohols - esters - hydrogen transfer - iridium catalyst - oxidative dimerization
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General Procedure for the Oxidative Dimerization of 1.
A 10 mL test tube equipped with a magnetic stirring bar was charged with 42 mg (0.3 mmol) of K2CO3 and 1.0 mmol of alcohol under Ar. Then a solution of 11 mg (0.02 mmol, 2 mol%) of Ir complex 3 in butanone (0.24 mL, 2.7 mmol) was added to the above mixture and stirred at r.t. The mixture was passed through a short silica gel column (12 g, EtOAc) to remove the catalyst. The yields for the products of 2c and 2d were determined by gas chromatography using authentic samples and appropriate correction factors. The products of 2a, 2b, and 2d-m were purified by silica gel column chromatography (hexane-EtOAc).
22 Although TONs were not optimized at the moment, they were roughly calculated to be in range between 17 and 23 for most substrates [with single entry as high as 40 (entry 7)].
23 The mechanism of saturation is not clear at present. Partially saturated products were observed even without K2CO3 under a high-concentration condition (4.2 M). However, such saturated products were not observed with K2CO3 under a diluted condition (0.08 M). See also ref. 17c.
