Internal barriers to rotation in 2,6-difluoroisopropyl and 2,6-difluoroethylbenzenes. Overlap of dynamic nuclear magnetic resonance (DNMR) and the J method

Abstract

The temperature-dependent I9F nrnr spectra of 2,6-difluoroisopropylbenzene yield AGh5, AH*, and AS* as 6.93 (5) kcal/mol, 6.1(1) kcal/mol, and -5.0(8)cal/mol K, respectively, for the internalrotation ofthe isopropylgroupaboutthespz-sp3 carbon-carbon bond. The long-range spin-spin coupling constant over six bonds, 6JpH-CH, combined with the J method gives a twofold internal potential barrier of 5.0 f 1.6 kcal/mol at 305 K. Although in this barrier range the J method suffers from large errors, the two methods yield comparable values for the banier height. The lineshape method is inapplicable to 2,6-difluoroethylbenzene. The J method finds the preferred conformation and a twofold barrier of 6.0 + 2 kcal/mol, again in a barrier region where this method is inaccurate.Relative to hydrogen, the fluorine substituents cause substantial increases in the barriers to internal rotation. Signs of the stereospecific couplings, 4J0F,CH, are determined. La mesure de spectres de rmn du I9F en fonction de la temperature du difluoro-2.6 isopropylbenzene perrnet d'obtenir les valeurs des ACTs5, AH* et AS* qui sont respectivement de 6,93 (5) kcal/mol, 6,1(1) kcal/mol et -5,08(8) cal/mol K pour la rotation interne du groupe isopropyle autour de la liaison carbone-carbonesp2-sp3. Laconstantede couplage alongue distance a travers six liaisons 6JpH,CH, combinCe ? I la mkthode J donne une baniere de potentiel interne binaire de 5,O + 1,6 kcal/rnol 305 K. Bien que dans cet ordre de grandeur, pour la baniere, la mCthode J cornporte degrandes erreurs, les deux methodes donnent des valeurs comparables pour la hauteur de la baniere. La methode de la forrne des lignes ne peut s'appliquer dans le cas du difluoro-2,6 ethylbenzene. La mCthode J perrnet d'etablir la conformation prkintkgrke et une barriere binaire de 6,O f 2 kcal/rnol; il s'agit encore d'une rkgion de la baniere ou cette methode n'est pas precise. Par rapport a I'hydrogtne, les substituants fluor provoquent une augmentation substantielle des banieres internes la rotation. On a dktermine des signes de couplages stkrCospkcifiques 4J0F,CH.[Traduit par le journal]

Long-range 13C,1H and 13C,19F coupling constants as indicators of the conformational properties of 4-fluoroanisole, 2,3,5,6-tetrafluoroanisole, and pentafluoroanisole

“…That two ortho C-F bonds can raise a barrier by steric forces is clear from 2,6-difluoroethylbenzene (38). In this molecule the twofold barrier is most likely mainly steric and is 4 to 5 kcal/mol higher than in ethylbenzene itself.…”

Some unique properties of in 2-fluorotoluene derivatives. Conformational dependence

“…The results derivative appeared on reduction of 2,6-difluorobenzyl bromide are examined for evidence concerning the relative importance with tri-n-butyl tin hydride. The 2,6-difluoroisopropylbenzene and of the three coupling mechanisms, the qualitative veracity of 2-chloro-6-fluoroisopropylbenzene were synthesized from thc corresponding carboxylic acids by methods recently described (18,19), as the INDo prediction of the conformations' dependence of was 2,6-difluorocthylbenzenc from the acctophenone precursor (I 8).…”

The proximate spin–spin coupling, 5J(F,CH3), as a quantitative conformational indicator in alkylfluorobenzenes and related compounds

“…The preparation and analyses of the 'H and IyF nmr spectra of 1 have been described (ll), as have the synthesis and analysis of the 'H nmr spectrum of 2 (11,30). The analysis of the 'H nmr spectrum of 3, the compound being an intermediate in the synthesis of 2 (30), yielded 'J(F,cH~) as , 2.032(1) Hz for a 3.1 mol% solution in CS2 at 305 K. The 'H nmr I spectrum was calibrated on a HA100 spectrometer by procedures 1 previously described in detail (31).…”

“…For example, it is very likely that OH this potential in 2,6-difluoroethylbenzene, 1, is predominantly 1 twofold (11)(12)(13)(14) and that, as in ethylbenzene itself (12,14), the…”

“…Hence There is considerable freedom of motion about the C,2-CV3 bond, as shown by the 6~(~,~~) of -0.233(5) Hz. This coupling vanishes when the a C-H bond sits in the ring plane (11,(21)(22)(23). The simplest potential describing the motion of the isopropyl group contains onefold and twofold terms.…”