Negative hyperconjugation in organic fluorine chemistry; myth or reality?

Abstract

B15 2 T l ) Suintnary New evidence from exchanges in neutral D,0-CU3COCD3, involving (CF,),CH and bridgehead compounds together, reopens a n old controversy.

Substituent effects on acidity of aryl-substituted fluoroalkanes: a computational study

“…For example, the contribution of resonance effect of p ‐nitro group involved in the δΔG o β‐F value (7.9 kcal mol −1 ) for R f = i ‐C 3 F 7 is evaluated to be 3.6 kcal mol −1 , indicating the importance of resonance effect for the reduction of the β‐fluorine negative hyperconjugation by an electron‐withdrawing substituent X. In terms of molecular orbital theory, the β‐fluorine negative hyperconjugation was attributed to orbital interaction of the formal lone pair at the anionic center with the σ* orbital of the C β F bond . This interaction stabilizes the negative charge of the conjugate anion, resulting in the increase of the acidity of polyfluorinated alkanes.…”

“…In particular, polyfluorinated alkanes have received attention in a wide field of fundamental and applied sciences . Strong acidity characteristic of these compounds was considered to result from the stabilization of the conjugate anion by β‐fluorine negative hyperconjugation in addition to strong inductive effect of the polyfluorinated alkyl group . In this connection, we found recently that the magnitude of β‐fluorine negative hyperconjugation involved in the gas‐phase acidity ( GA ) of PhCH 2 R f increases in the order of R f = CF 3 < CHF 2 < CH 2 F by 5.8 kcal mol −1 and CF 3 < C 2 F 5 < CF(R f ) 2 by 9.5 kcal mol −1 .…”

Gas‐phase Acidities of 2‐Aryl‐2‐chloro‐1,1,1‐trifluoroethanes and Related Compounds. Experimental and Computational Studies

“…In 1a , 1b , 2a , and 2b having β‐fluorines to a carbanion site, there would be another mechanism to stabilize their conjugate anions because β‐fluorine can stabilize the negative charge through negative hyperconjugation of the C β F bond . According to molecular orbital theory, β‐fluorine negative hyperconjugation is caused by the interaction between a lone pair orbital of the anionic center carbon and the energetically low‐lying antibonding orbital (σ*) of the C β F bond .…”

“…Recently, we found that the acidity ( GA ) of polyfluorinated alkanes is governed by accumulated inductive effect of fluorine atoms contained in the fluoroalkyl group and by β‐fluorine negative hyperconjugation . The negative hyperconjugation is one of the important concepts in organic chemistry and has been extensively studied so far . An excellent linear correlation between the GA values and the corrected number of fluorine atoms contained in the fluorinated alkyl group (R f ) was observed for the polyfluorinated alkanes having no fluorine atom at β‐position of the conjugate acid's anion center.…”

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

“…Negative hyperconjugation is one of the fundamental concepts in organic chemistry [1][2][3][4][5][6][7][8][9][10][11][12][13] and has been utilized for interpretation of many phenomena such as stabilities of conformers, [14,15] anomeric effect, [16] and strong electronwithdrawing ability of the polyfluorinated alkyl group. [1,13] However, it is difficult to determine experimentally the strength of negative hyperconjugation.…”