Simple Benzenoid Compounds

To the organic chemists of the early 1900s and before, an aromatic compound was one that underwent substitution reactions, as opposed to the addition reactions of ordinary alkenes and polyenes. They also noted that if one had fused benzene rings in different arrangements, such compounds were by and large aromatic. Smaller members of this group included naphthalene, anthracene, phenanthrene, and coronene (Structure 4)  

Benzene, and most aromatic compounds, usually undergo aromatic substitution reactions and not the addition reactions of ordinary alkenes. Phenanthiene (Structure 5), however, is different When treated with bromine, it simply adds the bromine to the 9,10 double bond to yield 9,10-dibromophenanthrene. This unusual reactivity of phen-anthrene can easily be understood from the valence bond viewpoint. In terms of valence bond pictures, we can desaibe the structure of the skeleton of this molecule to a first approximation with the complete set of five Kekule forms (resonance forms), as shown in Structure 5  

The MM4 program calculates the structures of these and many other simple ben-zenoid compounds to within experimental error, and of equal importance it calculates correctly their heats of formation (Chapter 11). This is not, of course, any great surprise since similar calculations have been made repeatedly in the past and, subsequently, with increasingly better approximations. The bond order-bond length relationship idea proposed by Dewar works quite well when incorporated into more sophisticated molecular mechanics procedures. 

It is reassuring that barriers such as these for corannulene and its cyclopentene derivative can be well calculated because it shows that the o-jr separation approximations employed in MM4 work well. 

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Simple benzenoid compounds typically undergo substitution reactions in their electronic ground state, most commonly electrophilic substitution. The relative rates of reaction and the preferred positions... 

The enantioselective ctT-dihydroxylation of benzothiophenes and benzofnrans in the heterocyclic ring, by Pseudomonas putida, is analogous to well-known conversions of simple benzenoid compounds, bnt in the heterocyclic context, hydroxyl groups introduced at an a-carbon easily epimerise. Indole gives indoxyl probably via dehydration of an intermediate 2,3-diol. In contrast, cw-dihydroxylation of quinolines, or of 2-phenylpyridines, takes place selectively in the benzene ring. ... 

In the simple benzenoid compound 3,4-dimethoxybenzoylcyanide in ether, benzene, toluene or dichloromethane, reaction with phenylhydroxylamine occurred at ambient temperature to give O-(3,4-dimethoxybenzoy0-N-phenylhydroxylamine in 90% yield accompanied by HCN evolution (ref.87). 

The second possibility requires an alternative to the elimination-addition mechanism to account for the formation of the cine-substitution product. Several such reactions are known, the von Richter reaction [equation (16)] being the oldest and most thoroughly studied. Since the accepted mechanism of this reaction specifically requires the presence of a nitro group, it has been suggested that if nitro groups are absent cine-substitution may be taken as good evidence of an aryne mechanism. While this statement may be valid for simple benzenoid compounds it does not necessarily hold for highly substituted or heterocyclic aromatics. 

In many cases, however, the ortho isomer is the predominant product, and it is the meta para ratio which is close to the statistical value, in reactions both on benzenoid compounds and on pyri-dine. " There has been no satisfactory explanation of this feature of the reaction. One theory, which lacks verification, is that the radical first forms a complex with the aromatic compound at the position of greatest electron density that this is invariably cither the substituent or the position ortho to the substituent, depending on whether the substituent is electron-attracting or -releasing and that when the preliminary complex collapses to the tr-complex, the new bond is most likely to be formed at the ortho position.For heterocyclic compounds such as pyridine it is possible that the phenyl radical complexes with the nitrogen atom and that a simple electronic reorganization forms the tj-complex at the 2-position. 

The protonation studies are of interest in another connection. If protonation of metallocenes can be considered to be a simple form of electrophilic attack, it is possible that other types of electrophilic substitution reactions may proceed through initial coordination of the electrophile with the central metal atom (14, 93). The mechanism of acylation of metallocenes may therefore be more complex than might be expected by analogy to similar reactions of benzenoid compounds. Clearly more studies are needed along these lines, better to define specific metal effects on the properties and reactions of these remarkable compounds. 

The photochemistry of substituted furans can be very complex indeed and may differ considerably from that found for benzenoid compounds. However, the photocyclization of 2,2 -ethylenedifuran to a benzodifuran is a simple reaction that is an exact counterpart to the photocyclization of stilbene to phenanthrenes.19 Some side chains allow ready dissociation to radicals typical is the nitro group and Scheme 4 displays what happens to irradiated... 

Cycloaddition of benzyne and of tetrahalogenobenzynes also occurs with simple monocyclic benzenoid compounds, and at higher temperatures (e.g., if benzyne is generated from phthalic anhydride) adducts such as 24 may rearomatize by elimination of acetylene. 1,4-Cycloadducts of benzyne and heterocyclic substrates are sometimes isolable, but frequently they too react further by a variety of pathways to give secondary products (Sections V, VI, and VII). 

The simple unsubstituted heterocyclic systems show a wide range of electronic absorption, from the simple 200 nm band of furan, for example, to the 340 nm maximum shown by pyridazine. As is true for benzenoid compounds, the presence of substituents that can conjugate causes profound changes in electronic absorption, but the many variations possible are outside the scope of this section. 

Benzenoid compounds have been treated in this way by Boyd, who has looked at such molecules without considering the quantum mechanics of the electronic system (Boyd, 1968 Shieh et al., 1969 Boyd et al., 1971 Chang et al., 1970). He has treated a variety of paracyclophanes and related compounds with generally good results. Thus it seems clear that simple benzenoid hydrocarbons can be treated in the usual way, if a special set of parameters is assigned to the benzene ring. 

In the many recorded examples of the reaction, and because of its very nature, reports have tended to concentrate on compounds derived from a range of simple carbonyl compounds and a single (or at most two) dialkydiaryP or diheteroaryP hydrogenphosphonate or, alternatively, on combinations of a selection of hydrogenphosphonates with a relatively few carbonyl compounds, including propanal, benzenoid aldehydes " furan and thiophene aldehydes 3-formylindole ° 2- and 3-formyl-chromones " , diethyl oxomalonate and others . It is worthy of comment that... 

The greater intensity of the band of the metabolite at 220 mis probably due to the presence of a second, superimposed chromophore which could also account for the shift of the minimum. On the other hand, the band near 300 m/u. has the expected intensity. Its broadness and displacement towards longer wavelength are probably due to the presence of a substituent on the double bond or benzenoid ring. That the assignment to a coumaroyl chromophore is essentially correct is evidenced by the fact that both M and the model compound underwent the same type of reaction on irradiation in the near-ultraviolet (Figure 4). The observed isosbestic points imply that the photoreaction is a simple one, such as A -> B or A = B, and is obviously the well-known light-induced trans- to c/r-isomerization (7) of cinnamic acid derivatives. 

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. 

Early in 1963 Culbertson and Pettit6 reported the base strengths of ten simple polycyclic benzenoid aldehydes and ketones and attempted to relate the pKa values to molecular structure. They were able to show the existence of a linear correlation between -pKa and the protonation it energy, that is, the gain in n energy of the protonated over the neutral compound. In their treatment... 

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