A theoretical study of α‐substituted isopropyl and cyclopropyl anions

Abstract

Ab initio molecular orbital theory has been used to probe the effect of the substituent X on the structures, strain energies, stabilization energies, inversion barriers, and proton affinities of carbanions CH&X CH; and c~s -C~H~X -, where X = H, F, CN, and NC. All geometries have been optimized with a 3-21G basis set, and the parent anions (X = H) were also optimized with the same basis set with a diffuse function added (i.e. the 3-21 + G basis set). The anions, with the exception of the a-cyanoisopropyl anion, are pyramidal. The out-of-plane angle, a, for the pyramidal anions decreases in the order F > H = NC > CN, and the barriers to inversion follow the same order with the cyclopropyl anions consistently having higher barriers than the isopropyl anions. The substituents strongly stabilize the anions with the stabilization energy following the order CN > NC > F. The cyano group slightly reduces the strain energy of cyclopropane, but the isocyano and fluoro substituents are weakly and strongly destabilizing, respectively. The pyramidal cyclopropyl anions are less strained than the cyclopropanes except when the substituent is a cyano group where the strain energies are reversed but are very similar. The planar anions all have higher strain energies than the cyclopropanes.

A theoretical study of α‐substituted cyclopropyl and isopropyl radicals

“…Ringopening of cyclopropyl radicals to form ally1 radicals is exothermic, but except when the cyclopropyl radical has stabilizing phenyl groups in the p -p o s i t i~n s ,~~*~' the ring- -"Inversion barriers in corresponding anions at the 3-21G//3-21G level taken from ref. 4.…”

“…Similar basis set dependence was found for the much larger barriers in the a-X-cyclopropyl anions (X = F, CN, and NC). 4 Ring-opening to 2-X-ally1 radicals. Ringopening of cyclopropyl radicals to form ally1 radicals is exothermic, but except when the cyclopropyl radical has stabilizing phenyl groups in the p -p o s i t i~n s ,~~*~' the ring- -"Inversion barriers in corresponding anions at the 3-21G//3-21G level taken from ref.…”

Configurational stability of chiral lithiated cyclopropylnitriles: A density functional study

“…Cyclopropyl anion 8 has a significantly higher barrier to inversion than cyclopropylnitrile anion 6b, consistent with earlier calculations at the DZ//3-21G level. 10 On this basis, one might try to explain the successful retentive protonation, carboxylation, and halogenation of enantiomerically pure cyclopropyllithium 10 13 and the racemization observed in similar reactions of lithiated 1. The first step in such a racemization mechanism for lithiated 1 is ion pair separation, but to the best of our knowledge there are no published estimates of the barrier for this process in a nonpolar solvent.…”

“…The corresponding C S -symmetric free anions 6a 9 and 6b 10 and their C 2v -symmetric inversion transition states 7a and 7b were previously studied at up to the MP2(fc)/6-31+G*//HF/6-31+G* and DZ//3-21G levels, respectively.…”