Barrier, rotation, biphenyls

A rationally designed system for the study of the steric effects of flanking substituents on the rotational barriers in biphenyl was introduced by Bott et al. (80JA5618). The barriers were determined by DNMR. These barriers allowed the definition of interference values that can be used to predict the rotational barriers in 2,2 -biphenyls. These parameters were used to estimate the barrier in a 2-iodo-3-(2-methylphenyl)thiophene derivative 222. 

Biphenyl, terphenyl, and quaterphenyl all have room-temperature structures that behave similarly, and also all have low-temperature phases in which only one conformer is present. The torsion angle about the central C-C bond in biphenyl is estimated to be about 10° in the low-temperature form, which is appreciably less than that in the gas phase. In the case of terphenyl, it has been established (41) that the room-temperature structure is disordered each molecule librates in a double-well potential, with the barrier height being about 0.6 kcal/ mol. In the low-temperature form the molecule is stabilized in one of the two minima of the well, and has its terminal rings rotated in the same sense, so that the molecule conserves its center of symmetry. This alternation of rotations between adjacent rings is found (42) also in the low-temperature form of quaterphenyl and results in the molecule being noncentric (even though the crystal structure has a center of symmetry). 

Buttressing effects are known to raise the barrier to rotation in the biphenyl series by preventing bond angle deformations of a substituent involved in direct interaction in the transition state. Similar effects were found in the 9-arylfluorene series (108). The barrier to rotation of 9-(3-bromo-6-methoxy-2,4-dimethyl-phenyl)fluorene (67, X = H) in chloroform-d at 56.3°C is 25.7 kcal/mol for... 

F. Leroux, Atropisomerism, biphenyls, and fluorine A comparison of rotational barriers and twist angles, ChemBioChem 5(5) (2004) 644-649. 

The barrier to internal rotation in biphenyl is smaller than crystalline forces, thus the considerable nonplanarity of the molecule disappears in the solid state. G. Bastiansen, Acta Chem. Scand., 1952, 6. 205 C. P. Brock, K. L. Haller, J. Phys. Chem., 1984, 88, 3570 G. P. Charbonneau, Y. Delugeard, Acta Crystallogr. Sect. B, 1976, 32, 1420. 

In spite of the uncertainty in the equilibrium conformation of biphenyl in the liquid phase, it is reasonable to anticipate an energy barrier to free rotation and coplanarity. Adoption of the highly successful Westheimer-Mayer model (Westheimer, 1956) for the racemization of optically active biphenyls leads to a calculated value of 3.9 kcal mole-1 for the barrier to free rotation (Howlett, 1960). The calculation was made on the basis of a large stabilization energy, 7 kcal mole-1, for the coplanar molecule (Guy, 1949). 

There are perhaps only two research areas in which the quantitative estimation of steric factors is routinely done, namely for hindered biphenyls (Cooke and Harris, 1967) and rotational or pseudorotation barriers in ethanes and cyclic compounds (Allinger et al., 1967). There are indications, however, that this interest is widening. Simonetta and Favini (1966) performed conformational calculations on the Cope rearrangement via chair or boat transition states and indicated that the orbital preference emphasized in a previous section should be supplemented with a steric factor. 

Substitution in the ortho positions of the heteroaromatic ring progressively increases the interplanar angle between the heterocycle and the carbonyl plane and thus gives rise, with sufficient substitution, to a steric barrier around the heteroaromatic amide bond similar to that encountered in biphenyls. The transition state to rotation around this bond may be even higher than that for rotation around the C—N bond (vide infra Section III,C,2) (Scheme 77). 

Barrier of rotation (racemization) for biphenyls (diagram XXXIII) (from [42,43])... 

Schurig, V Reich, S., Determination of the rotational barriers of atropisomeric polychlorinated biphenyls (PCBs) by a novel stopped-flow multidimentional gas chromatographic technique Chirality 1998, 10, 316-320. 

Harju, M.T. Haglund, R, Determination of the rotational energy barriers of atropisomeric polychlorinated biphenyls Fresenius J. Anal. Chem. 1999, 364, 219-223. 

Biphenyls containing four large groups in the ortho positions cannot freely rotate about the central bond because of steric hindrance. For example, the activation energy (rotational barrier) for the enantiomerization process was determined, AG = 21.8 0.1 kcal mol , for the chiral 2-carboxy-2 -methoxy-6-nitrobiphenyl. In such compounds, the two rings are in perpendicular planes. If either ring is symmetrically substituted, the molecule has a plane of symmetry. For example, consider the biaryls ... 

Almenningen, A., Bastiansen, O., Fernholdt, L., Cyvin, B. N., Cyvin, S. J., and Samdal, S. Structure and barrier of internal rotation of biphenyl derivatives in the gaseous state. Part I. The molecular structure and normal coordinate analysis of normal biphenyl and perdeuterated biphenyl. J. Molec. Struct. 128, 59-76 (1985). 

Steric congestion can raise the barrier about single bonds enough to bring it into the NMR range. Rotation about the single bond in the biphenyl 5-6 is raised to a measurable... 

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