Non-benzenoid aromatic

Compounds of special interest whose preparation is described include 1,2,3-benzothiadiazole 1,1-dioxide (a benzyne precursor under exceptionally mild conditions), bis(l,3-diphenylimida-zolidinylidene-2) (whose chemistry is quite remarkable), 6- di-melhylamino)julvene (a useful intermediate for fused-ring non-benzenoid aromatic compounds), dipkenylcyclopropenone (the synthesis of which is a milestone in theoretical organic chemistry), ketene di(2-melhoxyethyl) acetal (the easiest ketene acetal to prepare), 2-methylcyclopenlane-l,3-dione (a useful intermediate in steroid synthesis), and 2-phenyl-5-oxazolone (an important intermediate in amino acid chemistry). 

X-ray crystallographic studies are available on the ethoxy compound 1858 and on N,N-dimethylcyclopent[c]azepin-3-amine(19).59 The former is a fully conjugated, planar, 14k, non-benzenoid aromatic system, while the latter is virtually planar and, on the basis of bond length measurements, is best represented as the dipolar mesomer 19B. 

Non-benzenoid aromatic compounds containing a hydroxy group also react with arenediazonium ions and form arylazo derivatives. The first case of such an azo coupling process was found by Nozoe (1949) in his classic work on the natural product hinokitiol (12.15, R=CH3 Nozoe, 1959, 1991). Shortly afterwards Nozoe et al. 

As another example, the tropylium ion [3 ], which is stabilized by virtue of the 67t electrons spread over a heptagonal sp hybridized carbon framework [Hiickel s (4n 4- 2)v rule with = 1], is also unstable in the gas phase. Its formation from toluene or the benzyl cation has been a long-standing problem in organic mass spectrometry, and the reaction mechanism and energetics have recently been exhaustively discussed (Lif-shitz, 1994). It was, however, isolated as the bromide salt by Doering and Knox (1954, 1957), and was the first non-benzenoid aromatic carbocation. 

On the other hand, the anodic oxidation of 1,3,5-cycloheptatrienes is one of the most powerful key tools for the preparation of a variety of non-benzenoid aromatic compounds such as tropylium salts, tropones, tropolones, 2H -cyclohcpta h furan-2-oncs and azulenes14. 

Untch, K. First International Symposium on the Chemistry of Non-benzenoid Aromatic Compounds, Sendai, Japan, August, 1970. See also Jones, W.M. Angew. Chem. Int. Ed. Engl. 1972, 11, 325. 

Non-benzenoid aromatic compounds, flash vacuum pyrolysis for, 21 148 Non-benzenoid quinones, 21 238 Nonbenzidine-based dyes, 9 448 Nonblack fillers, 21 781... 

Non-Benzenoid Aromatic Compounds.—The synthesis of aromatic molecules containing small, medium, and large rings, using the reaction of dicarbonyl compounds and bis-ylides, has been thoroughly reviewed.95... 

R. A. Raphael, in Non-Benzenoid Aromatic Compounds (Ed. D. Ginsburg), Chapter VIII, Interscience Publishers, New York, 1958. 

T. Nozoe, in Non-Benzenoid Aromatic Compounds (D. Gins-burg, ed.) p. 339. Wiley (Interscience), New York and London, 1959. 

Fig. 3.2 reflects the dependence of benzenoid aromatic ions correlate linearly with the it electron density relative to benzene (Qn = 1)... 

J. P. Collman, L. S. Hegedus, J. R. Norton and R. G. Finke, Principles and Applications of Organotransition Metal Chemistry, University Science Books, Mill Valley, California, 1987 P. L. Pauson, in Non-Benzenoid Aromatic Compounds (Ed. D. Ginsburg), Interscience Publishers, New York, 1959. 

Were we to relax our restriction to consider solely hydrocarbyl substituents and accept both benzenoid and non-benzenoid aromatic imines, we would find other relevant compounds. For example, there is ALf-butyl-p-nitrobenzaldi mi ne with a gas-phase enthalpy of formation of 49.4 3.6 kJmol-1 from W. E. Acree, Jr., J. J. Kirchner, S. A. Tucker, G. Pilcher and M. D. M. C. R. Ribeiro da Silva, J. Chem. Thermodyn., 21, 443 (1989) and A-methyl-7-(methylamino)-troponimine (misnamed in our principal archive, Reference 16) with a gas-phase enthalpy of formation of 211.2 4.2 kJmol-1. Another relevant species is ammonium murexide with its 100-year-old enthalpy of formation of — 1212 kJ mol-1 as chronicled by Domalski. These three compounds are interesting, but it is precisely the non-hydrocarbyl part of these species that confounds simple comparison with other interesting species in this chapter. 

Jongsma, C Bickelhaupt, F. Topics in Non-Benzenoid Aromatic Chemistry, vol. II, Hirokowa Publ., Tokyo, 1977, p. 139... 

D. Lloyd, Carbocyclic Non-Benzenoid Aromatic Compounds, p. 98. Elsevier, Amsterdam, 1966. 

Nozoe T (1959) In Ginsburg D (ed) Non-benzenoid Aromatic Compounds. Intere-science Publishers Inc., New York, pp 339-463... 

Many aromatic compounds have considerable resonance stabilization but do not possess a benzene nucleus, or in the case of a fused polycyclic system, the molecular skeleton contains at least one ring that is not a benzene ring. The cyclopentadienyl anion C5HJ, the cycloheptatrienyl cation C7H+, the aromatic annulenes (except for [6]annulene, which is benzene), azulene, biphenylene and acenaphthylene (see Fig. 14.2.2(b)) are common examples of non-benzenoid aromatic hydrocarbons. The cyclic oxocarbon dianions C Of (n = 3,4,5,6) constitute a class of non-benzenoid aromatic compounds stabilized by two delocalized n electrons. Further details are given in Section 20.4.4. 

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