Structural Dualism in the Zwitterionic 7-RRNH-nido-7,8,9-C3B8H10 Tricarbollide Series: An Example of Absolute Tautomerism

摘要

Tautomerism, which is a dynamic equilibrium between two or more alternative structures of a single species, has been observed for many organic compounds. The commonest case of tautomerism, known as prototropy, is when the electrofuge (a leaving group that does not carry away the bonding electron pair) is a proton. This phenomenon is typical for compounds that contain a functional group that is able to donate a proton, and another functional group which is able to accept it, the functional groups must be in the same molecule and in close enough proximity to one another. The tautomerisation equilibrium is in all cases transmitted by a common anion. The most renowned textbook example of tautomerism is the equilibrium between the keto and enol forms of acetylacetone (Scheme 1).[1] The tautomerisation constant, defined as KT=[enol]/[keto], is in this case 3.6 for neat acetylacetone.[1]