Pseudoaromaticity

The fused hetero rings of aromatic or pseudoaromatic character on the 4,5 bond as, for example, benzothiazole, naphthothiazole, thieno[2,3ci]-thiazole, benzthieno[2,3d]thiazole, and so forth (Scheme 8), do not appear in the tables. 

Except in the case of formaldehyde, the electrophilic character of carbon atom of an aliphatic aldehyde is not strong enough to allow its condensation on a CH3 reactive group. However, such a condensation can occur with an aromatic or pseudoaromatic substance such as benzal-dehyde or pyrroloaldehyde, and the of the resulting dimethine dyes have been used in this last case to obtain the basicity scale of various rings (16). 

The squarato ligand, which is a resonance-stabilized pseudoaromatic, forms an unusual complex in the solid state, [Be(C404)(H20)3], whose structure is shown in Fig. 24 (42). Here the ligand is mono-... 

SSJ Bergman, E. D., Pullman, B., (ed.) Aromaticity, pseudoaromaticity, and antiaromaticity. Jerusalem The Israel Academy of Sciences and Humanities 1971... 

The term homoconjugation (a through-space interaction of orbitals) was introduced by Winstein and Adams (1948) in connection with the cholesteryl cation [1]. Applequist and Roberts (1956) were the first to progress to the concept of homoaromaticity in considering the homocyclopropenium cation [2], Soon after this Doering et al. (1956) invoked pseudoaromatic ... 

Prin2bach, H. Kndfel, H. Woischnik, E. In Aromaticity, Pseudoaromaticity, Anti-aromaticity, The Jerusalem Symposia on Quantum Chemistry and Biochemistry HI Bergmann, E. D. Pullman, B., Eds. Jerusalem, 1971, p. 269. 

Through the presence of aromatic or pseudoaromatic groups in the molecule (tetraphenylborate and related derivatives, the tetraphenyl-arsonium ion, cationic triphenylmethane dyes, complexes of o-phenanthroline with divalent nickel or ion, where the hydrophobicity is increased by further substitution by a hydrophobic group on the pseudoaromatic nucleus). 

Besides being of considerable commercial interest, Diels-Alder reactions are clean, well-characterized reactions that generally proceed in a single step through a pseudoaromatic transition state. There have been studies on the pressure effect on ionic reactions in SCFs by Zhang et al. (1996) who measured the rates of aryhnethyl cation ion-neutral reactivity in SCF. 

Azo coupling of diazonium salts with aromatic (or pseudoaromatic) hydrocarbons is possible if the coupling agent is highly substituted. For example, azo compounds have been produced from pentamethylbenzene [12], benzpyrene [13], and azulene [1-4]. 

B-71MI30101 A. Julg in Aromaticity, Pseudoaromaticity, Anti-aromaticity , ed. E. D. 

E. D. Bergmann and B. Pullman (Eds.), Aromaticity, Pseudoaromaticity, Antiaromaticity, Jerusalem Symposium on Qunatum Chemistry and Biochemistry, Vol. 3, Israel Academy of Science and Humanities, 1971. 

Another VB type application to antiaromatics is the Craig Rule. His rule, based on symmetry properties of the electronic state, distinguished for the first time between the aromatic (47V + 2) species and the (47 ones which Craig called pseudoaromatic . See Craig, D. P. J. Chem. Soc. 1951, 3175. 

Jerusalem Symposia on Quantum Chemistry and Biochemistry. Aromaticity, Pseudoaromaticity, Antiaromaticity, Israel Academy of Sciences and Humanities 1971 Vol. 3 E. Heilbronner On Mr Binsch s slide the following definition is proposed A conjugated electron system is called aromatic if it shows neither strong first-order nor second-order double bond fixation . Now, could you point out a molecule, except benzene, which classifies as aromatic G. Binsch Benzene is a perfect example E. Heilbronner Name a second one. 

Polypyrazoles of the type 62 have been prepared.65 These polymers are formed from the low-temperature solution condensation of 1,1 -bis(diketo)ferrocenes with aromatic or pseudoaromatic dihydrazines to give polyhydrazones which are transformed thermally to the polypyrazoles. Polybenzimadazole (63) has been prepared68 from l,l -ferrocenedicarboxylic acid and 3,3 -diaminobenzidine. 

D. P. Craig, J. Chem. Soc. 3175 (1951). Cyclobutadiene and Some Other Pseudoaromatic Compounds. 

Pyrrole-ZV-carbodithioate bisalkyne complexes display two distinct flux-ional processes. Rotation around the C—N bond of the S2C—NC4H4 ligand equilibrates both halves of the pseudoaromatic NC4H4 ring (AG = 10.7 kcal/mol) (88). Alkyne rotation exchanges both ends of the alkyne ligands at somewhat higher temperatures (AG =13.7 kcal/mol for MeC CMe, 13.8 kcal/mol for HC=CH). 

The expansion of the concept to encompass cyclic electron delocalization or homoaromaticity occurred in the late 1950s. In 1956 Applequist and Roberts pointed out that the cyclobutenyl cation resembles the cyclopropenium cation . Doering and colleagues suggested that the cycloheptatriene carboxylic acids could be regarded as planar pseudoaromatic type structures with a homoconjugative interaction between C(l) and C(6) . Based on the results of solvolytic studies on the bicyclo[3.1. OJhexyl system, Winstein set out the general concept of homoaromaticity in 1959 ... 

The reaction of 247 with DMAD in methanol results in the exclusive formation of thiazinone 24. The reaction can be catalyzed by sodium methoxide. In the absence of added base, thiazolidinone 23 is formed in addition to compound 24. This is in contrast to the flndings in the reactions of thioureas with DMAD in which thiazolidinones are the only isolable products. The difference may be due to.the enhanced stability of the 6,5,6-ring system because of pseudoaromaticity and to the inherent strain present in the 6,5,5-ring system, or it may be due to the difference in basicity between a benzimidazolyl-2-thione and thioureas. 

Fourth, metallocycles where the metal fragment forms part of a pseudoaromatic ring which is 7r-bonded to a second transition metal fragment are omitted on the grounds of a metal-metal bond. This group includes metallometallocenes, metalloarenes, ferroles, etc. (44,45). 

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