Metal-free synthesis of nitrile based partially reduced thia-and oxa-thia[5]helicenes: conformation and dynamics

Abstract

An expeditious, metal free, simple and convenient synthesis of partially reduced thia[5]helicenes and oxa-thia[5]helicenes, appended with nitrile and amino functionalities such as 3-sec.amino-5,6-dihydro-2H-1-thia-dibenzo[c,g]phenanthrene-4-carbonitriles 7 and (Z)-2-(5,6-dihydrobenzo[f]thiochromeno[3,4-c]-3-(2H)-ylidene)acetonitriles 6 has been delineated through base-catalyzed ring transformation of 4-sec.amino-2-oxo-2,5-dihydrothiochromeno[4,3-b]pyran-3-carbonitriles 4 by 2-tetralones 5. The molecular structure of one of the partially reduced thia[5]helicenes has been determined by X-ray crystallographic analysis. In order to calculate the inversion barrier of the helimeric enantiomers P-7a and M-7a, the molecular geometries of both, the ground states and the transition states have been optimized by the density functional theory (DFT) using B3LYP/6-311G.** Based on these structures, helimerization barrier of 44.36 K cal mol⁻¹ has been predicted using MP2/6-311G** single-point energy calculations.