Conformational analysis of alkylated biuret and triuret: evidence for helicity and helical inversion in oligoisocyanates
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 (DeltaG(SA)(double dagger) = 9.3 +/- 0.4 kcal/mol) but also a reversal of helicity (DeltaG(PM)(double dagger) = 9.0 +/- 0.4 kcal/mol). The coalescence pattern for the latter process provides direct evidence for a concerted, conrotatory helical inversion.
Roth
J
Department of Chemistry, Pomona College, Claremont, California 91711, IBM Almaden Research Center, San Jose, California 95120, and Department of Chemistry, Santa Clara University, Santa Clara, California 95053, USA.
O'Leary
DJ
Wade
CG
Miller
D
Armstrong
KB
Thoburn
JD