Systems of Pseudoazulenes

Replacement of one C=C bond in the seven-membered ring by a heteroatom generates compounds of the [b]- and [c]-series (2 and 3) 

Replacement of two C=C bonds in the seven-membered ring (5 or 6 here system 6 must be a zwitterion) 

Replacement of one C=C bond in the seven- and five-membered rings 

These C=C bonds can be replaced by an oxygen, sulfur, selenium, or tellurium atom or by an NR group. The number of possible systems is considerably increased by multiple substitutions. 

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Table I, containing compounds 26-87, summarizes all pseudoazulenes known through July 1979, as well as the corresponding literature. Not included are hydroderivatives of known and hitherto unknown pseudo-azulene systems containing many oxo and thio compounds. Such pseudoazulenes are predominantly of the type produced by introduction of an NR group. Only one system (78) of pseudoazulenes containing a selenium group is known, whereas tellurium compounds are unknown because of difficulties...

The condensation reaction gives very high yields, but the preparation of the reactants (106) is often complicated. For the preparation of pseudoazulene systems 5 and 6 formed by exchange of two C—C bonds of the seven-membered ring in azulene (Scheme 1), five-membered dicarbonyl compounds of type 107 are used. Their reaction with alkyl or aryl hydrazines gives heterocyclic bases (108), which can be transformed to the pseudoazulene system (49) by alkylation and deprotonation (see Section III,A,l).136... 

The different electron density distribution causes a considerable dipole moment in the ground state of pseudoazulenes (Section IV,D). Corresponding calculations have been carried out for various systems.54,60-62-63,1 17,170-171... 

Quantum chemical calculations also allow statements to be made about the reactivity of pseudoazulenes. The results have been generalized for simple systems in Scheme 15. 

Detailed studies of the mass spectra of pseudoazulenes have not been reported data are only described in the experimental sections of papers, usually without any discussion. More detailed data have been reported for pseudoazulene systems 35 and 36,105 39,113,114 42,123 65,163 80,195 81,197 82,198 and 83.198 Mass ion data have been reported for 2655,56 and 56.143... 

Several authors have reported that they did not succeed in isolating a number of pseudoazulenes when solutions in dilute mineral or glacial acetic acids were diluted with water or other bases.51-71,77-86 It was only possible to recover polymer-like material. In the subsequent decomposition the nucleophile attacks the conjugated acid of the pseudoazulenes at a position adjacent to the heteroatom, probably resulting in a fission. The hydrogen-deuterium exchange catalyzed by acids has been reported for systems 29,86 31,86 and 49.135... 

Closer investigations of pseudoazulenes 26, 27, 39, and 4()50,214 revealed that irradiation of these systems in the presence of 302 produced singlet oxygen. It was possible to demonstrate the formation of 02 by quenching experiments with DABCO and 2,5-dimethylfuran. In a thermal reaction 02 adds to the pseudoazulenes, whereby dioxetane derivatives (126) are... 

Some pseudoazulene systems show weak fluorescence (measured for 26, 27, 33, 39, 40, 44, 56, and 68)." 2 With the exception of pseudoazulenes 68 and 56, the emission by the other systems is due to the transition S2- So. Thus, with respect to fluorescence spectra, many pseudoazulenes show essentially the same properties as azulenes. The quantum yields of fluorescence are very low (between 10 and 10 ), They are markedly dependent upon the energy gap (S1-S2) and the flexibility of the pseudoazulene systems. The fluorescence quantum yields increase when additional benzene rings are introduced onto the uncondensed ring skeletons of the pseudoazulene systems, A slight increase in quantum yields is also observed if halides or phenyl groups are introduced. To explain this anomalous fluorescence the energy-gap model and a quantum chemical model have been invoked however, neither could explain all effects, Hitherto, no other luminescence spectra (particularly Sj Sq fluorescence and phosphorescence) have been observed for pseudoazulenes... 

Detailed studies of the mass spectra of pseudoazulenes have not been reported data are only described in the experimental sections of papers, usually without any discussion. More detailed data have been reported for pseudoazulene systems 35 and 36, ° 39, "" 42, 65, 80, 81, 82, and 83. Mass ion data have been reported for 26 and 56. The mass spectra of benzofused pseudoazulenes 65 and 80 -83 show the mass ion as the base peak, and intense peaks are observed that correspond to the doubly charged molecular ion. The authors have attributed this fact to the aromaticity of these systems. This conclusion, however, cannot be drawn for all pseudoazulenes since the relative intensity of the mass ion for 26b is only 5.5%, for 4-methyl-4//-cyclopenta[b]quinoline 64%, for... 

Just as there is a family of heterocycles isoelectronic with the indene monoanion (13)27 (indole, isoindole, indolizine, pseudoazulenes, etc.), there are several hundred possible systems derived from the pentalene dianion (14).2 Scheme 3 shows some examples (15-20) derived (formally) from each canonical form of 14. 

Another peculiarity appears in the case of replacement by an NR group it can be introduced into the carbon skeleton in such a way that it links both ring systems. Structures of this type, e.g., indolizine (23),43 quinolizine (24),44 and some azapentalenes3 (such as 25),45 are pseudoazulenes, also,... 

Many of the pseudoazulenes summarized in Table I were prepared from the corresponding quaternary salts (90 and 92) by deprotonation reactions (Scheme 3). This type of synthesis is especially advantageous for azalenes and oxalenes, although thialenes are seldom prepared in this way (e.g., systems 37,107 51,137 and 67l76) because of the difficult synthesis of the corresponding thiopyrylium salts.204 However, with the exception of condensed compounds (e.g., systems 35,104,105 38,94 61,92 72,24-29,32-36 and 73,35,185,186 tjjjs approach also has disadvantages for the preparation of azalenes (93 X = NR) of the [c]-series. 

Yields of the deprotonation reactions are about 60 to 80% and are essentially dependent on the stability of the resulting pseudoazulene systems (see Section IV,A). 

Both methods explained above start from a molecular lattice already having the structure of the corresponding pseudoazulene system. The essential last step of a pseudoazulene synthesis, however, can also be to create... 

In some condensation reactions the five-membered fragment is functionalized so that the condensation reaction with a suitable partner results directly in a pseudoazulene system. For this purpose, cyclopentendiones (104) are especially useful. They condense with o-substituted anilines (103) on heating in pyridine the primary condensation product (105) immediately gives the pseudoazulene product by elimination of H20 (Eq. 4).1 S8 162 The... 

For pseudoazulene systems containing one X—Y bond in the molecule, the condensation of hydrazine1 2,1 38,1 39,144-1 50 1 53,199 or hydroxyl-amine138,145 with five-membered dicarbonyl compounds or their heterocyclic products199 (106) is the most effective method of preparation (Scheme... 

The essential requirement for an exact determination of the physicochemical properties of a compound is its stability under the conditions of measurement. Thus, pseudoazulenes are difficult to measure because they are rather unstable in comparison to most azulenes. The simplest unsubstituted representatives of some systems are so thermally unstable that it is nearly impossible to isolate them. Phenyl substitutents have a stabilizing effect, so polyaryl-substituted compounds can be kept at room temperature for some months without decomposing noticeably. An especially strong stabilizing effect is given by the picryl group.106... 

Pseudoazulenes possess a heteroaromaticity produced essentially by the [4n + 2k electron system extending over the ring periphery. The trans-annular bond between the five- and six-membered ring enforces coplanarity thus, the pseudoazulenes can also be considered as bicyclic analogs of the heteronins.2 Dipolar structures (Eq. 7) contribute to the resonance stabili-... 

The aromaticity obtained by means of the DE values and their differences in the single pseudoazulene systems are also the result of calculations of the bond orders (Fig. 1). The calculated bond alternations46 57-77-79-82-117118-129 are in line with the heteroaromaticity of the systems. Similar relationships were calculated for azulene.219 The extent of the bond alternation, however, differs from one system to the other (see Fig. 1). In indeno[2.1-f>]-l-benzo-pyran (44a) the bond lengths of the central cyclo[fo]pyran system indicate pronounced bond alternation, and in the opinion of the authors, the systems have little aromatic character.129 Calculations with 1 //-pyrindines (26) and 2//-pyrindines (29) show that 26 is more stable than 29, but both are less aromatic than indole.63 A comparison of SCF calculations for pseudoazulenes 26 and 29 with their aza analogs 68 and 69-73, which contain a pyrrole-type nitrogen, shows only minor perturbations of carbon-carbon bond lengths on replacing a =CH— moiety by =N—.66... 

X-ray crystallographic studies have been carried out on four compounds belonging to three different pseudoazulene systems.9-61129 All studied compounds crystallized in monoclinic or orthorhombic space groups, a situation requiring Cs symmetry of each molecule. The size of the unit cells of the crystal lattice strongly depends upon the pseudoazulene system. The atoms of the pseudoazulene skeleton are all coplanar to within <5 ... 

All these frequencies are in the region of other heteroaromatic compounds and of azulenes. Infrared absorption spectra for several derivatives of the following pseudoazulene systems have been reported 26,56 28,77 2985 86 33 96.uio 35,165 39,"3114 42, 23 49,135 136 and 56.143144-146 The key frequencies for substituents at positions 1 or 3 in the five-membered ring are shifted to lower wavelengths in a typical manner. This is especially pronounced in the case of carbonyl groups. 

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