Organic Letters

Conformational analysis of alkylated biuret and triuret: Evidence for helicity and helical inversion in oligoisocyanates

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(matrix presented) The conformations of several oligoisocyanates have been investigated by NMR in order to study the onset and dynamics of helicity in polyisocyanates. Pentaethylbiuret and hexaethyltriuret were found to adopt turns and helices in solution. For hexaethyltriuret, symmetric and asymmetric helices are present. Not only is an interconversion of these forms observed (ΔGSA‡ = 9.3 ± 0.4 kcal/mol) but also a reversal of helicity (ΔGPM‡ = 9.0 ± 0.4 kcal/mol). The coalescence pattern for the latter process provides direct evidence for a concerted, conrotatory helical inversion.