Group 4 organometallic reagents. Part 6. The organotin-mediated monofunctionalization of diols: an insight into the selective monoesterification with acyl chlorides

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Abstract
The monoesterification of ethylene glycol with acyl chlorides through its dibutylstannylene derivative (1) has been selected as a model reaction to investigate, by n.m.r. spectroscopy, the origin of the organotin-mediated selective monofunctionalization of diols. The reaction has been found to take place in two separate steps, namely, an initial fast formation of a stannylated diol monoester followed by a slower intermolecular transesterification. The latter affords diester products and regenerates the starting dioxastannolane (1). Thus, the success of monoesterification depends on the timing of quenching the intermediate and is ensured by the large rate difference between the two steps. The above transesterification is an equilibrium reaction, shifted towards (1), that eventually leads to complete formation of diesters. Dynamic phenomena exhibited by n.m.r. spectra reveal that such a transesterification equilibrium also takes place intramolecularly at a much faster rate, showing intramolecularity factors of the order of 107 with respect to its intermolecular counterpart. Dibutyltin dichloride (3), which forms in the reaction, exerts a catalytic effect enhancing the reactivity of dioxastannolane toward the ester functionality. Such a catalytic effect may be accounted for by the very fast and strongly biased equilibrium reaction that occurs between (1) and (3), leading to ring-opening of the highly stable dioxastannolane. The generality of the stannylation method is further confirmed by monosilylation, carried out with silyl chlorides.

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