β-Alkylation of alcohols
I noted with interest the paper by Allen and Crabtree (Green Chem, 2010, 12, 1362) wherein they describe the formation of a C-C bond between a secondary and a primary alcohol without transition metal catalysis. The reaction is outlined in the scheme below.
The authors are mindful of the fact that similar transition metal catalysed chemistry is known, operating through ‘borrowing hydrogen’ type mechanisms so were careful to report the use of electronics grade KOH and NaOH (99.99% pure based on trace metal analysis), so it would seem the reaction is not as a result of trace amounts of catalyst. The reaction is reported for a number of substrates and all gave mixtures of the desired alcohol and corresponding ketone, the latter potentially the result of oxidation owing to the aerobic reaction conditions. The authors point out that the reaction only reached about 18% conversion when operated under nitrogen, suggesting aerobic oxidation is a key part in the mechanism.
In short, to me this represents an interesting paper for two reasons:
- it does represent the basis of what could be a scalable, low cost and environmentally friendly C-C bond forming process
- it serves as a possible cautionary tale to all process chemists when using benzylic alcohols under basic conditions in the presence of air – various oxidative and possible coupled by-product may be forming and contributing to the impurities profile