We demonstrate the gold(III)-catalyzed direct substitution of benzylic alcohols in water. These atom economic and environmentally benign protocols afford S-benzylated products in moderate to excellent yields. In contrast, common Lewis or Brønsted acids as catalyst, and organic solvents such as dichloromethane or toluene were ineffective for the S-benzylation of mercaptobenzoic acids. Water can be an attractive tool for new transition metal-catalyzed reactions. A Hammett study for the rate constants with various substituted alcohols shows a good correlation (R2=0.97) between the log(kX/kH) and the σ+ value of the respective substituents. From the slope negative ρ values of 2.35 are obtained, suggesting that there is a build-up of positive charge in the transition state. Our catalytic system can be performed with the use of only 2 mol% of gold(III) catalyst without any other additives in water, and scaled up to 10 mmol scale (85% isolated yield). Notably, the present method can accomplish the S-benzylation of unprotected mercaptobenzoic acids, which is chemoselective and leaves the carboxyl group intact. Furthermore, the direct substitution of allylic and propargylic alcohols also proceeded smoothly in good yields.
CITATION STYLE
Hikawa, H., Tada, A., Kikkawa, S., & Azumaya, I. (2016). Direct Use of Benzylic Alcohols for Gold(III)-Catalyzed S-Benzylation of Mercaptobenzoic Acids in Water. Advanced Synthesis and Catalysis, 358(3), 395–402. https://doi.org/10.1002/adsc.201500839
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