![: modern physical organic chemistry, anslyn & doherty pdf : modern physical organic chemistry, anslyn & doherty pdf](https://images-na.ssl-images-amazon.com/images/I/51EDvaQ-nKL.jpg)
A second stabilizing interaction was also found and came from hyperconjugation of the CF 3 substituent, involving interactions between the empty 2pC orbital with the πCF 3 FO (−5.2 kcal⋅mol −1). The latter arose from a homoconjugation interaction (−5.3 kcal⋅mol −1) of one fluorine lone pair (πnF FO) with the empty 2pC orbital of the cationic carbon center ( Figure 1, top). Indeed, the quantitative PMO analysis at the 6-31G* level allowed, by calculating fragment orbitals (FO), the identification of the nature of this attractive interaction. The presence of this attractive interaction should, however, not be discarded.
![: modern physical organic chemistry, anslyn & doherty pdf : modern physical organic chemistry, anslyn & doherty pdf](https://demo.fdocuments.in/img/378x509/reader024/reader/2021012314/5ae601687f8b9a08778c7337/r-2.jpg)
Furthermore, the very small π-electron density calculated in the 2pC orbital of CF 3CH 2 + (0.04 electrons) led the authors to conclude that “there is no hyperconjugative stabilization by the CF 3 group”. However, exactly the same structural distortion was calculated for the ethyl cation. The optimization of the geometry for CF 3CH 2 + at the STO-3G level led to an energy minimum, in which one of the fluorine atoms is significantly closer to the positive carbon center ( Figure 1, top, θ = 101°). Thus, an increased π-electron transfer is present in the least-stabilized alkylcarbenium ions, in which a higher electronic contribution from neighboring substituents is required.ĭetailed ab initio studies have been focused on the stability of the CF 3CH 2 + cation and provide pieces of thoughts on the origins of the stabilizing interactions in α-(trifluoromethyl)carbenium ions. A trend exists in the magnitude of the parameter according to the nature of the carbenium ions, which is in line with the carbenium ion stability (alkyl < allylic < benzylic). It is noteworthy that amongst the several substituents studied, the CF 3 group exhibits the lowest π-electron-donation ability in each investigated carbenium series, reflecting, as one could expect, the very poor stabilizing power by π-electron donation. Ab initio calculations were performed to account for the π-electron-donating ability of several substituents conjugated with carbocations ( Table 2). Thus, despite the strong intrinsic electron-withdrawing character, the trifluoromethyl group was shown to modestly act as a π-electron donor when substituting a carbenium ion. , “the electronic effect of a substituent depends to a certain extent upon the electron demand in the system to which it is attached”. In this context, deciphering the impact that can be exerted by the trifluoromethyl group on a cation and the associated consequences when facing the challenge of developing innovative synthetic methods are the subjects of this review.Īσ values based on benzoic acid ionization.
![: modern physical organic chemistry, anslyn & doherty pdf : modern physical organic chemistry, anslyn & doherty pdf](https://ars.els-cdn.com/content/image/1-s2.0-S2666386421002630-fx1.jpg)
These fluorine effects are nowadays remarkably established in many domains, including medicinal, organic, and organometallic chemistry, catalysis, chemical biology, and material sciences. The increasing demand for fluorinated scaffolds, due to the striking beneficial effects generally resulting from the introduction of these fluorinated motifs, also participated in this development. Many efforts are currently devoted to develop methods allowing the efficient insertion of fluorine atoms or fluorinated groups into organic molecules. The so-called electron-deficient carbocations, i.e., carbocations substituted with electron-withdrawing groups, drive original reactions, and the most important one among these cations is probably the α-(trifluoromethyl) carbocation. Carbocations that are especially intriguing are the destabilized ones that have been elegantly reviewed over the past years by Gassman, Tidwell, and Creary. A countless number of carbocations have been generated and studied, and many famous organic chemists strongly participated in their development. Carbocations are pivotal intermediates in organic chemistry, and carbocation-based synthetic chemistry continues to be a vital part of industrial and academic chemistry.