Biphenyls: steric interactions

Rotation about the 1,1 -bond is resisted by van der Waals interactions between the hydrogens shown in the structures. These hydrogens crowd each other when the two naphthyl groups are coplanar, and the racemization process requires tjie hydrogens to move past each other. The existence of enantiomeric substituted biphenyls also depends on steric interactions between substituents. The relationship between the rate of racemization and... 

On page 132, atropisomerism was possible when ortho substituents on biphenyl derivatives and certain other aromatic compounds prevented rotation about the bond. The presence of ortho-substituents can also influence the conformation of certain groups. In 88, R= alkyl, the carbonyl unit is planar with the trans C=0 - F conformer more stable when X=F. When X=CF3, the cis and trans are planar and the trans predominates. When R = alkyl there is one orthogonal conformation but there are two interconverting nonplanar conformations when R=0-alkyl. In 1,2-diacylbenzenes, the carbonyl units tend to adopt a twisted conformation to minimize steric interactions. " ... 

Figure 5.3 Steric interaction between the ortho chlorine atoms in biphenyl prevents the two rings from adopting a coplanar orientation.

Attack on the electrophilic C-2 may occur as in the 2-aminothiazoles series, which probably explains the rearrangements observed in acidic medium (121, 711, 712, 723, 724), in aqueous medium with NaOAc (725), or with aqueous NaHC03 (725) (Scheme 232). That the initial attack probably involves the C-2 atom is substantiated by the fact that this rearrangement occurs under extremely mild conditions for 2-imino-3-substituted-5-nitro-4-thiazolines (725). As the whole mechanism proposed (see p. 92) is reversible, when imino derivatives are submitted to such rearrangement conditions the rearrangement is expected to occur faster if steric interaction between 3- and 4-substituents exists in the 2-imino isomer. Another reaction may occur in acidic medium phenylimino-2-biphenyl-3,4-4-thiazoline hydrolyzed with hydrochloric acid gives the corresponding 4 thiazoline-2-one and aniline (717). 

The intramolecular potential for biphenyl consists of two parts. One is the steric interaction of the atoms at the ortho position modelled in terms of the non-bonded interactions between hydrogens and carbons of the two rings at the ortho position. The second is the variation of the conjugation energy [14]. Several models have been proposed for the former. Of the available potentials, the one proposed by Bartell [21] is the oldest. Most potentials include only the H-H non-bonded interaction, but as... 

C. Substitution in the phenyl ring at the a-position raises Jg to 115 °C. Jg in the case of a naphthalene substitution has a value of 135 °C. Jg for biphenyl is 145 °C. Tg of the more sterically hindered poly(a-methylstyrene) is 175 °C and that of polyacenaphthalene is 265 °C. A further illustration of the influence of subtle changes on T g can be seen in the case of poly(butyl methacrylate)s. The normal butyl methacrylate has a T g value of 56 °C, that of secondary butyl methacrylate is 22 °C and that of isobutyl methyacrylate is 43 °C. Once more, the greater the steric interaction the higher the value of T g. 

A classic demonstration of this effect is in the racemization of the chiral biphenyl compound characterized by Mislow and shown in Eq. 8.6. Rotation about the central bond race-mizes the material and forces a severe steric clash between the two methyl groups in the transition state. Indeed, it is found that the deuterio compound racemizes faster than the protio, consistent with the notion that D is effectively smaller than H. The effect is certainly not a large one, as only a 15% difference is seen in a system with a really severe steric interaction and with multiple H/D substitutions. Nevertheless, steric isotope effects can be comparable to other secondary isotope effects, and so should be considered when evaluating experimental data. 

There are three isomeric binaphthyls 1, T, the 1,2, and 2,2. There is enthalpy of formation data only for the first one as the solid,ii° 170 kj/mol. No doubt it is nonplanar in the gas phase, thus minimizing any steric interaction between the 8,8 peri hydrogens. The enthalpy of reaction 32 where Ar, Ar = 1-naph-thyl is +14 kJ/mol in the solid phase. Evidently, the more aromatic benzene undergoes a more endothermic reaction to form biphenyl. 

When equation (9) is applied to the transition state of the biphenyl system, it gives directly the isotopic difference in the activation enthalpy per interacting pair of atoms, provided we make the reasonable assumption that initial-state steric effects are independent of isotopic substitution in the 6- and 6 -positions. Since there are two pairs of interacting atoms in the coplanar transition state, the final expression is... 

At very long separations, for example, transfer to the biphenyl cation radical over 34 A in 1C)2 s, electronic interactions seemed to be propagated by ion states of the solvent (25), although quantum mechanical tunnelling may be important when diffusion is blocked by steric factors or by immobilization of the reagents (26). Perhaps most convincing are Miller and Closs demonstration of intramolecular electron transfer between donor-acceptor pairs separated by a rigid steroid spacer (27, 28). In 1, for example,... 

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