A natural bond orbital analysis of aryl-substituted polyfluorinated carbanions: negative hyperconjugation

作者：Suenaga Masahiko; Nakata Kazuhide; Abboud Jose Luis M^*; Mishima Masaaki^*

来源：Journal of Physical Organic Chemistry, 2018, 31(1): e3721.

DOI：10.1002/poc.3721

• donor
• acceptor NBO interaction energy
• natural bond orbital analysis
• negative hyperconjugation
• polyflourinated carbanions

Aryl-substituted polyfluorinated carbanions, ArCHRf- where R-f=CF3 (1), C2F5 (2), i-C3F7 (3), and t-C4F9 (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 (C) was not found in the Lewis structure. Instead, significant donor/acceptor NBO interactions between (C-C1) and sigma*(C-F) or sigma*(C-C) were observed for 1, 2, 3a (strong electron-withdrawing substituent, from p-CF3 to p-NO2), 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 C-F and C-C bonds and a shorter C-C bond, indicating that the E(2) values can be associated with the negative hyperconjugation of the C-F and C-C bonds. In accordance with this, the E(2) values for (C-C1)sigma*(C-F) were linearly correlated with the G(-F)(o) values (an empirical measure of -fluorine negative hyperconjugation obtained from an increased acidity). In 3b (weak electron-withdrawing substituents, from H to m-NO2) very large E(2) values for LP(F)*(C-C) were obtained. This was attributed to the C-F bond cleavage and the C-C double bond formation in the Lewis structure that is caused by the extremely strong negative hyperconjugation of the C-F bond.

• 出版日期2018-1