The benzylic anomeric effect. Internal rotational potentials of ArCH₂X compounds (X = F, Cl, SH, SCH₃, S(O)CH₃, SO₂CH₃)

Abstract

. Can. J . Chem. 65, 1845Chem. 65, (1987. The apparent twofold barriers to rotation about the exocyclic C-C bonds in ArCH2X (X = SH, SMe, S(O)Me, S02Me) compounds have been determined in CS, and in benzene-d6, from the magnitudes of long-range proton-proton coupling constants. The work extends earlier studies of the systems X = F, C1. Except for X = F, all of the title compounds have a low energy conformation in which the C-X bond is perpendicular to the plane of the benzene ring. The introduction of two meta-chlorine substituents leads to a 2 kJ/mol relative destabilization of this structure for X = F, C1, SH. The trend in the rotational barriers associated with the sulfur substituents is SH, SMe < < S(O)Me, S02Me. Ab initio calculations, with geometry optimization at the STO-3G level for X = F, C1, SH, reproduce the experimental results, and also the magnitudes of the rotational barriers. Quantitative PMO analyses of these a b initio wavefunctions have been employed to distinguish between "steric" and "other" interpretations, and reveal that the preferred conformation of X = F is determined by a first order perturbational effect, manifested in an electrostatic attraction between the heteroatom and a syn-periplanar ortho-hydrogen. For X = C1, the PMO analysis suggests that a T T~, -u & interaction, analogous to the n -~:~ interaction postulated as the origin of the anomeric effect in heterocyclohexanes, plays a dominant role. For X = SH, both T T~~-~:~ and ucx-n:, interactions are found; the latter is not possible in the anomeric effect itself. It is concluded that a benzylic anomeric effect exists, and that its magnitude, as a function Chem. 65, 1845Chem. 65, (1987.Utilisant les amplitudes des constantes de couplage proton-proton i longue distance, on a dCterminC les barrikres binaires apparentes 21 la rotation autour des liaisons exocycliques C-C des composCs ArCH2X (X = SH, SMe, S(0)Me et S0,Me) dans le CS2 et dans le benzene-d6. Ces Ctudes sont une extension d'etudes anttrieures sur des systkmes dans lesquels X = F et C1. Except6 pour le cas oB X = F, tous les composCs mentionnCs dans le titre prtsentent une conformation de basse Cnergie dans laquelle la liaison C-X est perpendiculaire au plan du noyau benzCnique. L'introduction de deux chlores en positions me'ta conduit a une distabilisation relative de 2 kJ/mol de cette structure pour X = F, C1 et SH. Cette tendance dans les barrikres rotationnelles associCes avec les substituants contenant du soufre est la suivante : SH. SMe << S(O)Me, S02Me. Des calculs a b initio impliquant des optimisations de la gComCtrie au niveau STO-3G ont CtC rCalisCs sur les composCs contenant X = F, C1 et SH et ils permettent de reproduire les resultats expkrimentaux ainsi que les amplitudes des barrikres a la rotation. On a utilisC des analyses quantitatives PMO de ces fonctions d'onde a b initio pour distinguer entre les interpretations ste'riques et antres; ces analyses rCvklent que la conformation privilCgiCe pour X = F est dCterminCe par un effet de perturbation du pre...

The barrier to internal rotation in benzyl fluoride investigated using the N.M.R. spectra of samples dissolved in liquid crystalline solvents

“…Measurements of 5JcF coupling for samples dissolved in a range of samples led Schaefer et al [1][2][3][4][5][6][7][8][9][10][11][12] to conclude that V(~b) is of two-fold symmetry, having a minimum at 0 ~ in CS2 which changes to 90 ~ as the dielectric constant of the solvent increases. The barrier height is never more than 2kJmol -~, which agrees with an ab initio MO calculation, which gives a barrier of 1-7kJmol -~ and places (~min at 0 ~ [13]. In view of the evidence that V(~b) is solvent dependent we have studied samples of benzyl fluoride dissolved in two liquid crystal solvents: 152, which has the structure F and which has TcN= 297K and TN1 : 381 K, and ZLI 1132, which is a mixture of 4-cyano-(4'-trans-n-alkylcyclohexyl)benzenes, and for which TcN= 267 K and TN~ = 344 K.…”

The anomeric effect revisited. A possible role of the CH/n hydrogen bond

“…In Figure 7 are illustrated several examples: alkyl 1-phenylethylketones 4 (R = CH 3 , C 2 H 5 , i-C 3 H 7 , t-C 4 H 9 ), 30 dimethoxymethane 5, 14,31 and benzyl fluoride 6. 32 We suggest that the stabilizing CH/n hydrogen bond, which may occur between these hydrogens and Z, plays an important role in making these conformations favorable. Table 2.…”

Theoretical and experimental approaches to the barrier to rotation about the Csp2—Csp3 bond in benzyl silane. Hyperconjugative stabilization of the perpendicular conformation

“…In solution (2-4), 1 is more stable than the in-plane conformer ($ = 0") by 5-6 kJ/mol, with an outlier at 9.6 kJ/mol ( 3 , attributable to a-IT conjugation (hyperconjugation) of the C-C bond with the IT electron system and also, more simply, partially to steric repulsion between the methyl group and the ortho C-H bond when $ = 0" (6). In terms of perturbational molecular orbital theory, the relative stability of 1 arises from a two-orbital four-electron destabilizing interaction when $ = 0" (7).…”