Antiaromatic dimerization

Ab initio molecular orbital calculations for the model systems RCN3S2 (R = H, NH2) show that these dithiatriazines are predicted to be ground state singlets with low-lying triplet excited states (Section 4.4). The singlet state is stabilized by a Jahn-Teller distortion from C2v to Cj symmetry. In this context the observed dimerization of these antiaromatic (eight r-electron) systems is readily understood. 

The oxazolo[3,4-a]azepinones 4, in which 5 7 ring fusion imparts considerable planarity and hence antiaromatic character on the ring system, undergo spontaneous dimerization.153 The mode of dimerization appears to depend on the nature and position of substituents. The unsubstituted system and the 9-chloro derivative 4 (R1 = Cl R2 = H) produce the exo.anti-dimers, e.g. 5, upon spray-vacuum pyrolysis at 300 C, whereas the 7-/ert-butyl, 7-bromo, 7-methyl, and 7,9-dichloro (4, R1 = R2 = Cl) compounds yield the exo,syn-dimcrs, e.g. 6. 

Of enormous interest to our present discussion would be XRD data on monomeric boroles in order to determine the structural consequences of 7r-electron delocalization in this four-electron, formally antiaromatic system.30 However, many attempts to grow suitable crystals of 94 have failed, and prolonged storage of solutions has produced only yellow dimers, presumably similar in structure to 100. Hence, in estimating the relative importance of structures 94a-c, spectral data must be our guide. 

The oxazolo[3,4-a]azepin-2-one (75) is unusual in that, because of 5,6-ring fusion, it has considerable planarity unlike most other monocyclic JV-acylazepines. This enhanced antiaromatic character is reflected in its spontaneous, highly peri-, site and regio-specific [6 + 4] dimerization to the exo-anti cycloadduct (76) (80CC1161). 

The question of aromaticity arises. Neither thiophenium salts nor thiophene sulfoxides are especially stable, making the classical reactivity test of electrophilic aromatic substitution difficult. The former dealkylate readily and the latter, at least for the case of thiophene sulfoxide, readily undergo self-dimerization (65CCC1158) (the bulky substituents of (57) impede this reaction). Aromaticity requires that the lone pair on sulfur participate in the aromatic sextet. If the lone pair, because of sp3 hybridization and improper symmetry, is not delocalized into the butadiene segment, the system will be antiaromatic. 

Bicyclo[3.3.0]octa-l,3,5,7-tetraene (2), trivially called pentalene [26, 27], is the second member in the series of fully unsaturated oligoquinanes. Huckel MO theory predicts that this planar hydrocarbon with its 8 r-electron system should be an antiaromatic species [25]. 2-Methylpentalene (37) has been generated by a retro-Diels-Alder reaction and deposited as a film at —196 °C on a NaCl or quartz plate for its spectroscopic characterization. It rapidly dimerized upon warming the cold plates to temperatures above —140 °C [26]. Only two stable derivatives of pentalene not complexed to a metal [29], the hexaphenyl- (38) [30] and 2,4,7-tri-tert-butylpentalene (39) [31], have hitherto been reported (Figure 4). 

Quadrelli, P Romano, S. Toma, L. Caramella, P. A bispericyclic transition structure allows for efficient relief of antiaromaticity enhancing reactivity and endo stereoselectivity in the dimerization of the fleeting cyclopentadienone, 7. Org. Chem. 2003, 68, 6035-6038. 

The energy of the double bond may be raised by other means, e.g. by strain or by antiaromaticity [224]. Miller, a pioneer in the field of organofluorine chemistry, generated tetrakis(trifluoromethyl)cyclobutadiene and showed that it reacts to form a tricyclic dimer [242] which can be converted to the corresponding cubane and cuneane derivatives by ultraviolet radiation [243] (Figure 7.72). 

The structural similarities between S-alkylthiophenium salts and thiophene sulfoxides have prompted the suggestion that both classes are antiaromatic (70JA7610, 70JCS(C)1764). Thiophene sulfoxides are exceptionally reactive, undergoing spontaneous Diels-Alder dimerization unless stabilized by bulky substituents at the 2- and 5-positions. This type of reactivity is reminiscent of cyclobutadiene. In stark contrast to the sulfoxides, S-alkylthiophenium salts show no tendency to take part in Diels-Alder reactions either in an intra- or intermolecular sense. Pentamethylthiophenium hexafluorophosphate (11, X = PFg ) was shown to be completely unreactive toward both electron-rich and electron-deficient dienophiles under conditions where thiophene sulfoxides undergo efficient cycloaddition reactions. 

There are some simple cyclobutadienes that are stable at room temperature for varying periods of time. These either have bulky substituents or carry certain other stabilizing substituents such as seen in tri-tert-butylcyclobutadiene (83). ° Such compounds are relatively stable because dimerization is sterically hindered. Examination of the NMR spectrum of 83 showed that the ring proton (8 = 5.38) was shifted upfield, compared with the position expected for a nonaromatic proton, for example, cyclopentadiene. As we will see (pp. 89-90), this indicates that the compound is antiaromatic. 

Figure 3.16 shows the correlation diagram for the peiicyclic four-center reaction via a Hiickel-type transition complex as mentioned above. It contains two crossing correlation lines (72 - und 72-), which are indicative of a thermaUy forbidden transition (2). Examples for such a reaction via an antiaromatic four-membered Htickel ring are the dimerization of ethylene and the disrotatory ring opening of cyclobutene (41), both of which occur only photo-chemically but not thermaUy. 

The 1,3,2,4,6-dithiatriazines (7) are also formally 8ti electron systems, and are thus antiaromatic. The formation of the dimer on reducing the dichloro compounds is attributed to the coupling of the triplet states of (7) to achieve a singlet ground state <85JA7710,92IC1417>. 

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