Novel Regiospecific Aldol Addition of Nitroalkane Dianion

Abstract

The aldol addition reactions of dianions derived fro m nonbranched nitroalkanes with electrophiles are known to give products in which carbon-carbon bonds are formed exclusively at the a-carbon of the nitro group via a, a-dianions. A change in the regioselectivity, from a, a- to a, p-dianion, was observed when the sequence of addition of reagents was changed, Addition of n-BuLi to a THF solution of nitroalkane and HMPA (procedure A)gave an 'a-addition product whereas a-addition was predominant when nitroalkane was added to the THF solution of n-BuLi and HMPA (procedure B). The electrophile was added to the solution of the dianion finally in both procedures. In a typical example, the reaction of benzaldehyde with nitroethane dianion (procedure A)gave 14% of the 9-adduct. On the contrary, procedure B (molar ratio of HMPA: n-BuLi=2.6: 1; nitroethane was added at 90°C) afforded the j9-adduct with 95% selectivity and in high yield. Exte nsive studies of this reaction with other electrophiles showed that /3-selectivity depends on the molar ratio of HMPA/n-BuLi and the reaction temperature at the stage of the deprotonation of nitroalkane.