Heterocyclic analogues of biphenyl

Compared to the extraordinary potential of these heterocyclic analogues of biphenyl in the study of steric effects, relatively few barrier determinations... 

The heterocyclic analogues of biphenyl, that this review survey, have also been studied but in a much lesser degree. The evolution of computational chemistry allows predicting that only HF, DFT, and higher level methods will survive in the near future. For this reason we will not report semiempirical (mostly PM3, MNDO, and AMI) nor molecular mechanic calculation results save in exceptional cases. 

An appreciable amount of information concerning the conformational preferences of substituted heterocycles has accrued, largely through dipole moment and NMR studies. However, the earliest appreciation of this topic apparently arose out of the extension of studies of restricted rotation in biphenyls to heterocyclic analogues. 

As it can be seen, lUPAC definition covers more situations than the present review that is restricted to biphenyl heterocyclic analogues. Natta and Farina (72M11) offer an interesting discussion on this subject (they used the term atropisomerism, also found in the old references). In the authoritative book by Eliel et al. (94MI1), Chapter 14-5 is devoted to biphenyls atropisomerism. They report that this type of enantiomerism was discovered by Christie and Kenner in 1922 (22JCS614) in the case of 6,6 -dinitro-2,2 -diphenic acid (1) that they were able to resolve. It was later called (33MI1) atropisomerism. An important aspect of all the concepts related to a barrier is (we quote) It is immediately obvious that the term suffers from all the problems discussed previously How slow must be the interconversion of the enantiomers (i.e., how long is their half-life) before one speaks of atropisomerism At what temperature is the measurement to be made Does atropisomerism still exists when isolation of stereoisomers becomes difficult or impossible but their existence can be revealed by NMR (or other spectral) study and so on. ... 

Benzene (1) is the simplest aromatic hydrocarbon upon which our knowledge of aromatic chemistry is based. This hydrocarbon, the alkylbenzenes (2), the arylmethanes [e.g. diphenylmethane (3)], the biphenyls [e.g. biphenyl (4)] and the condensed polycyclic systems [e.g. naphthalene (5) and anthracene (6)] all exhibit chemical reactivity and spectroscopic features which are markedly different from their aliphatic and alicyclic hydrocarbon counterparts. Indeed the term aromatic character was introduced to specify the chemistry of this group of hydrocarbons and their substituted functional derivatives, and it was soon used to summarise the properties of certain groups of heterocyclic compounds having five- and six-membered ring systems and the associated condensed polycyclic analogues (Chapter 8). 

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