Triple aromaticity

An enhancement of the simple substructure approach is the Fragment Reduced to an Environment that is Limited (FREL) method introduced by Dubois et al. [7] With the FREL method several centers of the molecule are described, including their chemical environment. By taking the elements H, C, N, O, and halogens into account and combining all bond types (single, double, triple, aromatic), the authors found descriptors for 43 different FREL centers that can be used to characterize a molecule. 

If one correlates the calculated electron density at the bond critical point p and the CC bond distance R for a variety of hydrocarbons, a linear relationship will be obtained which holds for both single, double, triple, aromatic and homoaromatic CC bonds. On the basis of this relationship, a bond order n(CC) has been defined according to equation 780,82 ... 

According to this analysis, Hfj in the singlet electronic state has three 2c-2e Hf-Hf o-bonds formed of hybrid 6s-, 5d-AOs and three completely delocalized bonds formed of pure d-AOs one completely bonding 3c-2e d-radial-based o-bond responsible for the presence of o-aromaticity, one completely bonding 3c-2e d-radial-based tr-bond responsible for the presence of ir-aromatidty, and one completely bonding 3c-2e d-AO-based 8-bond responsible for the presence of 8-aromaticity in this cluster (4n -F 2 rule for every kind of aromaticity for n = 0). The 3c-2e d-AO-based 5-bond is formed by the overlap of the d 2 AO on each Hf atom. The triple aromaticity is consistent with the bond equalization and high symmetry of Hfj. Tsipis et al. [119] obtained NlCS -scan curve for D3, A/ state of Hf3, Ti3 and Zr3 clusters, and concluded that they all three are indeed triply (a-, tr- and 5-) aromatic in this state on the basis of magnetic criterion. 

Iminoyl chloride 17 reacted with tetrazoles 18 analogously to afford the 1,2,4-triazoles 19 in excellent yield. The electrocyclizaton of A -imino nitrile imines of the type 20 provided the key step for this general synthesis of 1,2,4-triazoles. Cyanuric chloride, as a triple aromatic imidoyl chloride, was treated with three equivalents of 5-phenyltetrazole to give 21 in 79% yield. ... 

Atom,- Atomy Single Double Triple Aromatic... 

Bonds Single, double, triple, and aromatic (or conjugated) bonds are indicated by the symbols and " respectively single and aromatic bonds should be omitted. 

The correction term in Eq. (9) shows that the basic assumption of additivity of the fragmental constants obviously does not hold true here. Correction has to be appHed, e.g., for structural features such as resonance interactions, condensation in aromatics or even hydrogen atoms bound to electronegative groups. Astonishingly, the correction applied for each feature is always a multiple of the constant Cu, which is therefore often called the magic constant . For example, the correction for a resonance interaction is +2 Cj, or per triple bond it is -1 A detailed treatment of the Ef system approach is given by Mannhold and Rekker [5]. 

The benzene derivative 409 is synthesized by the Pd-catalyzed reaction of the haloenyne 407 with alkynes. The intramolecular insertion of the internal alkyne, followed by the intermolecular coupling of the terminal alkyne using Pd(OAc)2, Ph3P, and Cul, affords the dienyne system 408, which cyclizes to the aromatic ring 409[281]. A similar cyclization of 410 with the terminal alkyne 411 to form benzene derivatives 412 and 413 without using Cul is explained by the successive intermolecular and intramolecuar insertions of the two triple bonds and the double bond[282]. The angularly bisannulated benzene derivative 415 is formed in one step by a totally intramolecular version of polycycli-zation of bromoenediyne 414[283,284],... 

The cyclization of the enediynes 110 in AcOH gives the cyclohexadiene derivative 114. The reaction starts by the insertion of the triple bond into Pd—H to give 111, followed by tandem insertion of the triple bond and two double bonds to yield the triene system 113, which is cyclized to give the cyclohexadiene system 114. Another possibility is the direct formation of 114 from 112 by endo-rype. insertion of an exo-methylene double bond[53]. The appropriately structured triyne 115 undergoes Pd-catalyzed cyclization to form an aromatic ring 116 in boiling MeCN, by repeating the intramolecular insertion three times. In this cyclization too, addition of AcOH (5 mol%) is essential to start the reaction[54]. 

Aryne (Section 23 8) A species that contains a triple bond within an aromatic nng (see benzyne)... 

In presence of one carbon-nitrogen triple bond —C—C=N In compounds with tendency to dipole formation, e.g., C=C—C=0 In aromatic compounds... 

In order for dipole—dipole and dipole-iaduced dipole iateractioas to be effective, the molecule must coataia polar groups and/or be highly polarizable. Ease of electronic distortion is favored by the presence of aromatic groups and double or triple bonds. These groups frequently are found ia the molecular stmcture of Hquid crystal compouads. The most common nematogenic and smectogenic molecules are of the type shown ia Table 2. 

In general, peroxomonosulfates have fewer uses in organic chemistry than peroxodisulfates. However, the triple salt is used for oxidizing ketones (qv) to dioxiranes (7) (71,72), which in turn are useful oxidants in organic chemistry. Acetone in water is oxidized by triple salt to dimethyldioxirane, which in turn oxidizes alkenes to epoxides, polycycHc aromatic hydrocarbons to oxides and diones, amines to nitro compounds, sulfides to sulfoxides, phosphines to phosphine oxides, and alkanes to alcohols or carbonyl compounds. 

Arynes are intermediates in certain reactions of aromatic compounds, especially in some nucleophilic substitution reactions. They are generated by abstraction of atoms or atomic groups from adjacent positions in the nucleus and react as strong electrophiles and as dienophiles in fast addition reactions. An example of a reaction occurring via an aryne is the amination of o-chlorotoluene (1) with potassium amide in liquid ammonia. According to the mechanism given, the intermediate 3-methylbenzyne (2) is first formed and subsequent addition of ammonia to the triple bond yields o-amino-toluene (3) and m-aminotoluene (4). It was found that partial rearrangement of the ortho to the meta isomer actually occurs. 

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