Dewar forms

T orbital for benzene obtained from spin-coupled valence bond theory. (Figure redrawn from Gerratt ], D L oer, P B Karadakov and M Raimondi 1997. Modem valence bond theory. Chemical Society Reviews 87 100.) figure also shows the two Kekule and three Dewar benzene forms which contribute to the overall wavefunction Kekuleform contributes approximately 40.5% and each Dewar form approximately 6.4%. 

I should like to see a point of union between yours and Bamberger s views and the ideas I have developed that benzene has a Dewar form tautomeric with the two Kekule individuals. 29... 

Figure S.6. Schematic representation of So and Si energy profiles for DEWAR formation in TB9A and TB9ACN. 2 The excited state funnel F is very close to the ground stale surface and therefore leads to fluorescence quenching (identifiable with rate constant k). Most of the molecules return to the anthracene form via pathway a, while only a few proceed to the Dewar form (pathway b), because F is placed to the left of the ground state barrier. The steric effect of the tert-butyl substituent is indicated by the broken line. Without this prefolding" of the anthracence form. Dewar formation is not observed. The top part of the figure contains a schematic description of the butterfly-type folding process, while the bottom part contains examples of actual molecules.

Proof for the existence of benzene isomers in irradiated benzene has been obtained in several ways. These will not be discussed in detail, but they may be classified broadly as physical and chemical. Nuclear magnetic resonance has been used by Wilzbach and Kaplan to identify benzvalene.39 Prismane has also been identified by NMR and by vapor-phase chromatography. The Dewar form has been synthesized in several steps which start with ris-1,2-dihydrophthalic anhydride. Photochemically this compound yields bicyclo(2,2,0)hexa-5-ene-2,3-dicarboxylic aqid anhydride. This was followed by catalytic reduction and oxidative decarboxylation to give the Dewar form of benzene.39 The method of synthesis alone provides some basis for structure assignment but several other bits of supporting evidence were also adduced. Dewar benzene has a half-life of about 48 hr at room temperature in pyridine solution and its stability decreases rapidly as the temperature is raised. 

A substituted Dewar form of benzene has also been prepared by the reaction of cyclobutadiene with an acetylenic compound.57... 

Irradiation of 1,3,5-trideuterobenzene in the vapor phase has been shown recently to yield small amounts of a scrambled form.43 87 In view of the substantiation and isolation of nonaromatic intermediates in previous cases, it seems likely that the scrambling observed proceeds through similar intermediates. One such isomer has been isolated in the case of hexofluorobenzene, where a small amount of the Dewar form was obtained in an irradiation in the vapor phase.59... 

For benzene we have considered so far only the Kekule structures in which the bonds are formed between neighbouring atoms such forms represent the greatest possible interaction of the electron clouds, since the distance between neighbouring carbon atoms is only i 4 A, and will thus contribute more than other possible structures to the resultant state of the molecule. But it is necessary to consider all possible valency structures among which there is resonance and thus for benzene the three Dewar forms must be considered ... 

The five structures are the two Kekul and the three Dewar forms. It is possible, however, to construct other structures of benzene e.g. by joining atoms in the meta position with a tt bond or by making all three it bonds of the Dewar type ... 

Irradiation of tetrakis(trifluoromethyl)thiophene gave a valence-bond isomer which Heicklen first proposed to have a Dewar structure,9 then a cyclopropenyl thioketone,10 and still later11 a Dewar form. The structure was shown to be Dewar form 1 by 19F and 13C NMR and by the study of its reactions (Eq. 8).12 14 This is the first isolated example of a Dewar viner of a five-membered heterocycle. 

The photoreaction of a substituted pyrylium salt yields an intermediate which is postulated to have a benzvalene and not a Dewar form. A walk... 

Peculiarly, the 3,6-bonded Dewar pyridazine appears to be more stable than pyridazine. This conclusion could originate from the large approximations and the difference in the structure between the aromatic and the Dewar forms. Thus, the order in the Dewar isomers of one diazine might be correct. The photochemical and thermal isomerization of perfluoroalkylated diazines have been investigated mainly by Chamber s group, and Dewar intermediates were proposed for photoisomerizations. The photoisomerization of tetrafluoropyridazine to the pyrazine was given a mechanism shown in Eq. (34).101... 

Fits. 1-9 The three, equivalent. "singly-excited-Kekuie" (or Dewar ) forms of benzene. 

It is very likely that the thermal interconversion between silaaromatics and their Dewar isomers is facile. The ultimate transformation products from pyrolytically produced hexamethyl-l,4-disilabenzene have been accounted for by a scheme in which isomerization to the Dewar form represents the first step319. 

Irradiation of matrix-isolated silabenzene at 320 nm causes a transformation to the Dewar isomer318,324. This conversion is readily followed by the shift of the Si-H stretching frequency from that characteristic of an sp2 to that characteristic of an sp3 hybridized silicon. Irradiation of the Dewar isomer with 240 nm light318 causes partial reversion to silabenzene. A similar photostationary state can be reached in the case of 1-phenyl-1-silabenzene315. Since there are no wavelengths at which the Dewar isomer absorbs while the silaaromatic itself does not, it is not possible to achieve complete conversion of the Dewar form into the silaaromatic form. 

All three of the isomers of disilabenzene have been studied by ab initio methods. Using the 3-21G/STO-2G level of theory all of the isomers show some delocalization with the 1,3-isomer being the most stable <84JOM(27i)369>. Further ab initio calculations on the 1,4-isomer show that the disilabenzene, its Dewar form, and one of its silylene isomers all have similar thermodynamic stabilities <85JOM(289)5i>. Calculations on aromaticity in disilabenzenes and other group 14 containing heterocycles are also discussed in a review <93AHC(56)303>. 

Okoyuma et al. have reported that the strained Dewar paracyclophanes (98) can be converted into the benzenoid parent compounds (99) by irradiation of the compounds in a diethylether-isopentane glass at 77 K using 365 nm light. Interestingly when (99, R = CN, = H) is allowed to thaw to room temperature there is a thermal reaction that converts it back into the Dewar form which provides the first observation of such a thermal cyclization. Irradiation at 254 nm of the tetraene (100), a hk-Dewar benzene, at 77 K in a matrix results in its conversion into the paracyclophane (101). Continued irradiation transforms (101) into the (4+4)-adduct (102). The products from this low temperature... 

When a compound is so constituted that more than one normal valency formula may be used to represent its molecule, as with benzene where the two Kekule forms and the three Dewar forms are possible, the energy of formation usually proves to be greater than that calculated for any of the possible individual formulae. The molecule thus appears to exist in a state which is more stable than that corresponding to any of the conventional valency-bond representations. Detailed evidence from many somces, indeed, suggests that these formulae are quite often inadequate. Benzene, for example, is best regarded not as possessing three single and three double carbon-carbon bonds, but six equal bonds of order approximately 1 -S. 

Scheme 1 Synthesis of a [6]annulene via its Dewar benzene isomer. The Dewar form is prepared via a [47H-27r] cycloaddition with [4]annulene and a substituted acetylene.

In the Dewar benzene structure, the bulky tert-butyl groups are farther apart and interact less with each other than they do in the aromatic structure and, in conjunction with other factors, render the Dewar form stable enough to be isolable. Reversion to the aromatic form occurs only on heating. The parent compound was reported by van Tamelen and Pappas in 1963 ... 

Figure 3.21. Resonance forms (connected by resonance arrows) for methylbenzene (toluene) (a) Kekule forms vide infra and (b) Dewar forms (suggested as early as 1867 by James Dewar as another potential C6He isomer).

WORKED PROBLEM 13.3 There are three other resonance forms for benzene, which can be included to get a sUghtly better electronic description of the molecule than is provided by the pair of Kekule forms alone. In these other resonance forms, overlap between two p orbitals on across the ring carbons is taken into account. Draw these three resonance forms, called Dewar forms, showing the orbitals involved in the cross-ring bond. Be careful This problem uses bonding that you probably haven t seen before. 

In the Dewar forms, 1,4-overlap is explicitly emphasized. There are three Dewar forms that differ in which 1,4-overlap (1,4 3,6 or 2,5) is counted. 

PROBLEM 13.4 Do you think the Dewar forms will contribute strongly to the benzene structure Why or why not ... 

Dewar forms (Section 13.3) Resonance forms for benzene in which overlap between 2p orbitals on two para carbons is emphasized. These forms superficially resemble Dewar benzene (bicyclo[2.2.0]hexa-2,5-diene). 

---