4,5-Dimethylphenanthren

The molecules most profitably studied in connection with purely steric isotope effects have been isotopically substituted biphenyl derivatives. Mislow et al. (1964) reported the first more or less clearcut example of this kind in the isotope effect in the configurational inversion of optically active 9,10-dihydro-4,5-dimethylphenanthrene (7), for which an isotopic rate ratio ( d/ h) of 1-17 at 295-2°K in benzene solution was determined. The detailed conformation of the transition state is not certain in this case, as it involves the mutual passage of two methyl groups, and thus it is difficult to compare the experimental results with... 

The unsigned average deviation between calculated and experimental energies is 0.36 kcal/mol for a collection of 35 benzenoid molecules. This result does not include 7,12-dimethylbenz[a]anthracene (36) the discrepancy of 16 kcal/mol between theory and experiment is in all likelyhood due in part to an error in the latter. Although certainly real, steric interactions involving the methyl group in position 12 are probably not so severe as to cause a destabilization exceeding that found in 1,8-dimethylnaphthalene and 4,5-dimethylphenanthrene—molecules that are discussed further below. 

Example 14.4 4,5-Dimethylphenanthrene. Using the C NMR spectrum measured by Stothers et al. [275], we deduce A/// = 36.8 kcal/mol assuming planarity. Closely neighboring methyl groups that are separated by five bonds in the molecular skeleton, however, result in chiral nonplanar conformations [276]. Modeling, where appropriate, the CC bonds on those described for biphenyl and m-stHbene, one predicts A/// = 47.8 for the nonplanar form, in acceptable accord with the reported value [248], 46.26 + 1.46 kcal/mol, a result that is self-explanatory. 

Phenanthrene (7), the smallest member of the helicene series, is planar in its neutral state and shows aromatic character. Reducing the compound with lithium affords a highly paratropic dianion34 (72 ) that, according to calculations, is not planar35. It is possible to twist the sp2 framework of phenanthrene by its alkylation at the 4- and 5-positions36, i.e. in 4,5-dimethylphenanthrene (8), 2,4,5,7-tetramethylphenanthrene (9) and 2,4-di-ferf-butyl-5,7-dimethylphenanthrene (10). When these systems are reduced with lithium metal, three... 

Fig. 1 Stracture and ORTEP drawing of 4,5-dimethylphenanthrene [5]. Reprinted with permission from [5]. Copyright (1987) American Chemical Society...

The twisted structure of 4,5-dimethylphenanthrene (1) contributes to the chirality of the molecule. It was recognized early on that the ability to resolve the two enantiomers could provide supporting evidence for the nonplanarity of the aromatic system [1]. Determinations of the rates of racemization and the activation barriers of twisted chiral polyarenes have been actively pursued. Resolutions of the enantiomers to allow these investigations to proceed were achieved in several cases. In other cases, variable-temperature nuclear magnetic resonance (NMR) experiments were employed to provide insights into the configurational stabilities of the molecules. For practical applications, such as using these twisted compounds... 

The strain energies in the twisted polyarenes have been determined by comparison of heats of combustion with those of the corresponding nonstrained isomers. In the case of 4,5-dimethylphenanthrene, the comparison was with that of 2,7-dimethylphenanthrene [15]. The bathochromic and hyperchromic shifts of the UV spectra of twisted arenes have been used to detect the presence of twisted distortion [16]. 

The history of the chemistry of l,12-dimethylbenzo[c]phenanthrene (2) resembles that of 4,5-dimethylphenanthrene (1). The first symthesis of 2 was reported by Newman and Wolf in 1952 [9]. The synthetic sequence included preparation of diacid 67 for the Friedel-Crafts acylation reactions to form diketone 68 (Scheme 15). Reduction followed by dehydration and dehydrogenation then produced 2. 

The D(g) for 4,5-dimethylphenanthrene is only 9.5 6.4 kj/mol. Since 4-methylphenanthrene is the only monomethylated species available for comparison, it appears that introduction of another methyl in the bay region is not significantly more destabilizing than the first, inasmuch as both compounds are likely nonplanar. 

The 50.5 6.3 kj/mol difference between the 2,7- and 4,5-dimethylphenanthrene enthalpies of formation more clearly illustrates the large destabilizing steric effect of two methyl groups in the bay region of phenanthrene. 

The ammonium salt obtained from 6,6 -dimethylbiphenyl-2,2 -diylbis(methyl bromide) and dimethylamine produces an ylide that rearranges under ring contraction. Upon Hofmann elimination, the strained 4,5-dimethylphenanthrene (335) is obtained harboring two substituents in the congested "bay region" (Scheme 1-261). ... 

Diphenyl- 1-methyIcyclopropane, A35.12 9.10-Dihydro-4,5-dimethylphenanthrene, X5.1 1 -Methyl-1 -phenylindane, A 40.5... 

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