Nonbenzenoid aromatic

Fulvene-type enamines, which possess some nonbenzenoid aromatic character, have been synthesized by treating cyclopentadienylsodium with an amide-dimethyl sulfate eomplex (117aJ17b) or quaternary pyridinium salts (117c). One of the simplest ones produced is 6-(dimethylamino)fulvene (117a,117d). 

This procedure illustrates formylation by N,N-dimethylamino-methoxymethylium methyl sulfate, a compound which can be produced readily by reaction of easily available materials. 6-(Dimethylamino)fulvene is a useful intermediate for the synthesis of various f used-ring nonbenzenoid aromatic compounds. 

The structural features and the spectroscopic characteristics of the thiirene dioxide system (22) are of special theoretical interest since, on the basis of analogy with cyclopropenone (23), it is a possible nonbenzenoid aromatic system with all the physical and chemical implications involved. Aromatic and/or conjugative effects, if any, require transmission through the d-orbitals of the sulfur atom. 

The thiirene oxide system is of particular interest due to it being simultaneously both a potentially nonbenzenoid aromatic (4n + 2)n and antiaromatic 4nn Hiickel system. 

Eor reviews of 76 (X = N) and other nine-membered rings containing four double bonds and a hetero atom (heteronins), see Anastassiou, A.G. Acc. Chem. Res., 1972,5, 281, Top. Nonbenzenoid Aromat. Chem., 1973,1,, Pure Appl. Chem., J975, 44, 691. For a review of heteroannulenes in general, Anastassiou, A. G. Kasmai, H.S. Adv. Heterocycl. Chem., 1978, 23, 55. 

Heilbronner, E. Nonbenzenoid aromatic compounds (ed. D. Ginsburg). New York Interscience Publishers 1959. 

A79. J. P. Snyder, ed. Nonbenzenoid Aromatics. Academic Press, New York. Volume 2 (1971) contains ... 

Of course, in reality new chemical substances are not synthesized at random with no purpose in mind—the numbers that have still not been created are too staggering for a random approach. By one estimate,1 as many as 10200 molecules could exist that have the general size and chemical character of typical medicines. Instead, chemists create new substances with the aim that their properties will be scientifically important or useful for practical purposes. As part of basic science, chemists have created new substances to test theories. For example, the molecule benzene has the special property of aromaticity, which in this context refers to special stability related to the electronic structure of a molecule. Significant effort has gone into creating new nonbenzenoid aromatic compounds to test the generality of theories about aromaticity. These experiments helped stimulate the application of quantum mechanical theory to the prediction of molecular energies. 

Of the condensed nonbenzenoid aromatics, azulene (10) has gained greatest popularity [80]. Although it is an isomer of naphthalene, its electrochemical behavior... 

A further group of nonbenzenoid aromatics is the series of odd-membered cations and anions such as cycloprope-nium (14) and tropylium cations (15) as well as cyclopentadienyl (16) and cyclononatetracenyl anions (17). Regarding the arguments for the properties of Hiickel-like 4 + 2 jr-systems, all these molecules should be energetically stabilized. Obviously, this is not fulfilled in all cases. The tropylium cation (15) can be reduced in a one-electron step to the tropyl radical even at A = +0.06 V vs. SCE [85, 86]. The radical is unstable and rapidly dimerizes to bitropyl. The hep-taphenyl tropylium radical is stable on the voltammetric timescale, but decays... 

Tropones, tropolones, and tropylium salts have been known since the early 1950s. Today, they are counted among the most important classes of nonbenzenoid aromatic compounds. 

Azulene. The absorption spectrum of azulene, a nonbenzenoid aromatic hydrocarbon with odd-membered rings, can be considered as two distinct spectra, the visible absorption due to the 1Lb band (0-0 band near 700 nm) and the ultraviolet absorption of the 1L0 band.29 This latter band is very similar to the long wavelength bands of benzene and naphthalene CLb) and shows the same 130 cm-1 blue shift when adsorbed on silica gel from cyclohexane.7 As in the case of benzene and naphthalene, this blue shift is due to the fact that the red shift, relative to the vapor spectra, is smaller (305 cm"1) for the adsorbed molecule than in cyclohexane solution (435 cm"1). Thus it would appear that the red shifts of the 1La band are solely due to dispersive forces interacting with the aromatic molecule, in agreement with Weigang s prediction,29 and dipole-dipole interaction is negligible. 

B-69MI51600 L. A. Paquette in Nonbenzenoid Aromatics , ed. J. P. Snyder Academic Press,... 

B-73MI52000 A. G. Anastassiou in Topics in Nonbenzenoid Aromatic Chemistry , ed. T. 

James P. Snyder (Editor). Nonbenzenoid Aromatics, Volume I, 1969 ... 

For a review of cyclopeniadienyl cations, see Breslow Top. Nonbenzenoid Aromat. Chem. 1973,1, 81-94. Clark Chem. Commun. 1969, 637 Ref. 136. 

A linear correlation between 13C chemical shifts and local n electron densities has been reported for monocyclic (4n + 2) n electron systems such as benzene and nonbenzenoid aromatic ions [76] (Section 3.1.3, Fig. 3.2). In contrast to theoretical predictions (86.7 ppm per n electron [75]), the experimental slope is 160 ppm per it electron (Fig. 3.2), so that additional parameters such as o electron density and bond order have to be taken into account [381]. Another semiempirical approach based on perturbational MO theory predicts alkyl-induced 13C chemical shifts in aromatic hydrocarbons by means of a two-parameter equation parameters are the atom-atom polarizability nijt obtained from HMO calculations, and an empirically determined substituent constant [382]. 

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