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A natural bond orbital analysis of aryl‐substituted polyfluorinated carbanions: negative hyperconjugation
Author(s) -
Suenaga Masahiko,
Nakata Kazuhide,
Abboud JoséLuis M.,
Mishima Masaaki
Publication year - 2018
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.3721
Subject(s) - hyperconjugation , chemistry , natural bond orbital , carbanion , substituent , lone pair , stereochemistry , crystallography , computational chemistry , medicinal chemistry , density functional theory , molecule , organic chemistry
Aryl‐substituted polyfluorinated carbanions, ArCHR f − where R f = CF 3 ( 1 ), C 2 F 5 ( 2 ), i ‐C 3 F 7 ( 3 ), and t ‐C 4 F 9 ( 4 ), were analyzed by means of the natural bond orbital (NBO) theory at the B3LYP/6‐311+G(d,p) computational level. A lone pair NBO at the formal anionic center carbon (Cα) was not found in the Lewis structure. Instead, significant donor/acceptor NBO interactions between π(Cα‐C1) and σ*(Cβ‐F) or σ*(Cβ‐Cγ) were observed for 1 , 2 , 3a (strong electron‐withdrawing substituent, from p ‐CF 3 to p ‐NO 2 ), and 4 . Their second‐order donor/acceptor perturbation interaction energy, E (2), values decreased with the increase of the stability of carbanions. A larger E (2) value corresponds to longer Cβ‐F and Cβ‐Cγ bonds and a shorter Cα‐Cβ bond, indicating that the E (2) values can be associated with the negative hyperconjugation of the Cβ‐F and Cβ‐Cγ bonds. In accordance with this, the E (2) values for π(Cα‐C1) → σ*(Cβ‐F) were linearly correlated with the Δ G o β‐F values (an empirical measure of β‐fluorine negative hyperconjugation obtained from an increased acidity). In 3b (weak electron‐withdrawing substituents, from H to m ‐NO 2 ) very large E (2) values for LP(Fβ) → π*(Cα‐Cβ) were obtained. This was attributed to the Cβ‐F bond cleavage and the Cα‐Cβ double bond formation in the Lewis structure that is caused by the extremely strong negative hyperconjugation of the Cβ‐F bond.