Selective dealkylations of aryl alkyl ethers and thioethers by sodium in HMPA

The reaction of sodium with bis- and tris(alkoxy)benzenes in HMPA gives selectively the products of monodealkylation. The reaction proceeds through a dianion which fragments into an alkyl and an aryloxy anion. The positional selectivity of this fragmentation is governed by the structure of both the alkyl and aryloxy groups. With bis- and tris(alkoxy)benzenes which for symmetry reasons can afford aryloxy anions having the same basicity, the dealkylation involves exclusively the less substituted alkyl group. On the contrary, in the asymmetric terms, the positional selectivity of the dealkylation process is governed by the basicity of the aryloxy anion. On the basis of these concepts several efficient and synthetically useful reactions have been developed. In most cases the selectivity obtained in the present reactions is different from that observed with other previously developed methods which use sodium methoxide or sodium alkanethiolates in HMPA. It is shown that the appropriate chioce of the reagent allows selective dealkylation of the desired alkoxy group of a poly(alkoxy)benzene.

The reaction of sodium with bis(alkylthio)benzenes in HMPA gives the bis(mercapto)benzenes. If the reduction is caried out with a solution of sodium in HMPA, the reaction gives instead the products of monodealkylation. This however is not selective. It is suggested that in the case of thioethers the dealkylation products originate from the fragmentation of the radical anions.