DFT study of the stereo-selectivity of oxygenated heterocycles from 10 to 12 links
Résumé
Intra-molecular Diels-Alder (IMDA) reactions of tethered trienes can furnish two distinct diastereoisomeric products, the cis (i.e., endo) stereoisomer and the trans (i.e., exo) stereoisomer. Experimental evidence shows a quite high cis stereoselectivity for 10-link compounds (cis/trans = 70: 30), while 11- and 12-links compounds exhibit no particular selectivity. DFT (B3LYP/6-31G*) computations provide useful insights into the origins of this amazing stereo-selectivity. The cyclization path towards trans stereo-isomer is always thermodynamically favored, whatever the size of the system. The high cis stereo-selectivity displayed by the 10-link system is kinetically controlled by a tug-of-war between ring strain and electronic effects in the transition structure. The dual descriptor of chemical reactivity, a conceptual DFT based descriptor designed to delineate electronic effects, has been used to unravel the stabilizing processes that take place at the TSs.