Steric Interactions in Organic Chemistry: Spatial Requirements of Substituents

The synthesis and measurement of the barrier to rotation [Formula: see text] in 2,2″-dimethyl-o-terphenyl (62 kJ/mol) and the following 3,3″-disubstituted derivatives is described (Cl, Cl, 74 kJ/mol), (Cl, CN, 70 kJ/mol), (CM, CN 70 kJ/mol), (CHO, 75 kJ/mol). For 3,3″-bis-CH2Br, Δv, the separation of the —CH3 peaks in the 1Hmr spectrum, was too small to obtain [Formula: see text]

Section: Proton Magnetic Resonance Resultssupporting

confidence: 54%

The barrier to rotation in 2,2″-dimethyl-o-terphenyl and several 3,3″-derivatives (rotational isomers of 1,2-di-o-tolylbenzene)

Mitchell¹,

Yan²

1980

“…In the 2,6-dichloro derivative 'J(C,C) drops to 0.77 Hz. The chloro and methyl substituents are often (19) methyl groups cause (8) to be nearer 90" than do the two ortho chlorine atoms. There are some experiments, for example, on ortho disubstituted isopropyl benzene derivatives (19,20), that agree that a methyl has larger steric requirements than a chlorine substituent.…”

Section: Resultsmentioning

confidence: 99%

Mechanisms of long-range ¹³C, ¹³C spin–spin coupling in thioanisole and its derivatives. Conformational applications

Schaefer¹,

Penner²

1988

. Can. J. Chem. 66, 1229Chem. 66, (1988.The I3c nuclear magnetic resonance chemical shifts and the long-range I3C,l3c spin-spin coupling constants are reported for 23 thioanisole derivatives enriched in I3C at the methyl position. Forpara and metn substituted thioanisole derivatives, "J(C,C) (n being the formal number of bonds intervening between the coupled nuclei) can be related to functions of the angle by which the thiomethyl group twists out of the aromatic plane. For n = 3 , 4 , 5 , the ensuing relationships yield estimates of the twofold barriers to rotation about the C( l ) -S bond. The barrier is lower in ethyl phenyl sulfide than in thioanisole derivatives. Complications arise for ortho substituted thioanisole derivatives but estimates of the torsional motion about the C( 1) -S bond can be obtained from the observed "J(C,C). Among the complications is the expected fact that 3~( C , C ) , which is shown to be larger in the cis than the trans orientation of the intervening bonds ("anti-Karplus" behaviour), is perturbed by the substituent attached to the coupled nucleus. It is confirmed that in 2-hydroxythioanisole the thiomethyl group is oriented effectively perpendicular to the benzene plane, attributable to a stereospecific hydrogen bond between the hydroxyl group and the 3p lone-pair on the sulfur atom. In acetone-d6 solution, an equilibrium exists between this conformation and one in which an intermolecular hydrogen bond exists with solvent molecules. In the latter, the thiomethyl group prefers a coplanar orientation. In 2-aminothioanisole, the thiomethyl group twists out of the plane by about 60" so as to optimize the N-H. . .3p interaction. This twist angle is changed very little in acetone-d6 solution because the second N-H bond can hydrogen bond to the solvent molecules without disrupting the intramolecular N-H. . .3p interaction. It is also shown that the chemical shift of the I3C nucleus in the methyl group is a good conformational indicator in rneta and para substituted thioanisoles. Therefore it can be used as such for molecules in which "J(C,C) is difficult to find, in 1,4-dithiomethylbenzene, for example.On a mesurk les dkplacements chimiques en rLsonance magnktique nuclCaire du I3C ainsi que les constantes de couplage spin-spin A longue distance I3c,l3C pour 23 dkrivCs du thioanisole enrichis en I3C dans la position du mCthyle. Pour les dkrivks du thioanisole substitutts en positionspara ainsi que me'ta, les constantes 'J(C,C), pour lesquelles n correspond au nombre de liaisons formelles intervenant entre les noyaux qui sont couplks, peuvent Ctre relikes aux fonctions de l'angle de torsion du groupement thiomkthyle par rapport au plan aromatique. Pour n = 3 , 4 ou 5, les relations qui en dkrivent permettent d'kvaluer les banikres binaires A la rotation autour de la liaison C( 1) -S.La barrikre est plus faible dans le cas du sulfure d'ithyle et de phknyle que dans les cas des dkrivks du thioanisole. On fait face i des complications avec les dCrivks du thioanisole substituks en ortho; tout...

“…However, the internal barriers in benzyl silane and benzyl trimethylsilane (10) are also equal to within experimental error. There are conflicting estimates in the literature concerning the steric sizes of chlorine atoms and methyl groups (32,33), so that this possibility is moot. Second, it is not at all clear that electron transfer to, or, at least, polarization of, the n electron system of the phenyl group by u-n conjugation should be a measure of the size of the internal barrier4 to rotation about the ~s~~-~s~~ bond.…”

Section: Remarks On Benzyl Trichlorosilanementioning

confidence: 99%

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

Schaefer¹,

Sebastian²,

Penner³

1991

. Can. J. Chem. 69,496 (1991). The 'H nuclear magnetic resonance spectra of benzyl silane and benzyl trichlorosilane, obtained in CS2 and benzene-d6 solutions, are analyzed. The long-range coupling constants between the methylene and para ring protons are used to derive apparent twofold barriers about the Csp2-Csp3 bonds of 7.4 2 1.6 and 8.1 + 1.1 kJ/mol for the silane and the trichlorosilane, respectively. These are higher than that for ethylbenzene and are attributed mainly to the stabilization of the perpendicular conformer, that with the C-Si bond in a plane perpendicular to the phenyl plane, by u-IT conjugation (hyperconjugation) of the C-Si bond and the IT electron system. Molecular orbital computations confirm the predominantly twofold nature of the internal barrier in benzyl silane and also for benzyl germane and stannane. The calculated barriers for the silane derivatives are rather higher than the experimental values. The computed barriers have magnitudes that appear to change with X in much the same order as do the hyperconjugative interactions deduced in other ways for CH2X(CH3)3 groups (X = Sn, Ge, Si, C). The angles CCX in benzyl-X (X = CH3, SiH3, SiC13, GeH3, SnH3) are all computed to decrease smoothly as sin2$, where $ is the angle by which the C-X bond twists out of the phenyl plane.Key words: conformations, benzyl silane and trichlorosilane; NMR, benzyl silane and trichlorosilane; MO calculations, benzyl silane and trichlorosilane. Des calculs d'orbitales molCculaires confirment la nature principalement binaire de la barrikre interne dans le benzyl silane ainsi que dans les benzyl germane et stannane. Les barrikres calculCs pour les dCrivCs silanes sont sensiblement plus hautes que les valeurs expCrimentales. La variation des amplitudes des barrikres calculCes pour les dCrivCs silanes avec X semble suivre l'ordre des interactions d'hyperconjugaison dCduites sur d'autres bases pour les groupements CH2X(CH3), (X = Sn, Ge, Si, C). Les calculs indiquent que les angles CCX, des benzyl-X (X = CH3, SiH3, SiC13, GeH3 et SnH,), doivent diminuer doucement comme le sin2$, oh IJJ est l'angle que fait la liaison C-X avec le plan du noyau phCnyle.