Dewar benzenes synthesis

Synthesis The FGI is easily done and the product was used by Van Tamelen (J. Amer. Chem. Soc.. 1963, 3297) in one of the early syntheses of a Dewar benzene ... 

This procedure is illustrative of the synthetic use of cyelobuta-dieneiron tricarbonyl2 as a source of highly reactive cyclobutadiene. Cyclobutadiene has been employed, for example, in the synthesis of oubane, Dewar benzenes, and a variety of other systems.3,1... 

A key step in one route to the synthesis of hexamethyl Dewar benzene is the cycloaddition of 2-butyne to tetramethylcyclobutadiene (stabilized by A1 cation). Using the parent compounds (no methyls), develop a Woodward-Hoffmann orbital correlation diagram for the reaction and determine whether the reaction is thermally allowed. 

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. 

Anodic and cathodic elimination is simply the reverse of cathodic [eqn (16)] and anodic [eqn (15)] addition, respectively. Important cases are anodic bisdecarboxylation, either in the 1,2- (Corey and Casanova, 1963 Radlick et al., 1968 Westberg and Dauben, 1968) or 1,3-fashion (Vellturo and Griffin, 1966), with the preparation of Dewar benzene and dimethyl bicyclobutane-2,4-dicarboxylate as the more prominent cases [eqns (25) and (26)], and cathodic dehalo-genation of dihalides with the halogens in the 1,2- (Zavada et al., 1963), 1,3- (Casanova, 1974 Gerdil, 1970 Rifi, 1967, 1969), 1,4-(Casanova and Rogers, 1974 Wiberg et al., 1974) and 1,6- (Covitz, 1967) positions. The synthesis of bicyclobutanes (27) and [2,2,2]propellane (28) bear witness to the usefulness of this reaction type. 

T. J. Katz, N. Acton, Synthesis of Prismane. J. Am. Chem. Soc. 1973, 95, 2738-2739 V. Ramamurthy, T. J. Katz, Energy-Storage and Release -Direct and Sensitized Photoreactions of Dewar Benzene and Prismane. Nouv. J. Chim. 1977, 1, 363-365. 

These observations indicate the existence of high energy barriers that literally lock the Dewar benzene inside the energy well and prevent its immediate conversion to benzene. Kinetic stability makes the synthesis of Dewar benzene feasible. The methods employed for its preparation are sufficiently mild so the opportunity for the concurrent reaction (the formation of the more stable isomer benzene 3) was safely excluded. 

Intramolecular [2 + 2] cycloaddition also occupies a pivotal position among the methods available for the synthesis of highly strained compounds. Owing to the proximity effect, this reaction occurs easily even in sterically encumbered cases. The synthesis of one of the first representatives of exotic hydrocarbons, Dewar benzene 388 by van Tamelen, was achieved by a surprisingly short route (Scheme 2.130). The readily available Diels-Alder adduct 389 was first converted into diene 389a. The latter underwent intramolecular [2 -i- 2] cycloaddition which led to the formation of the [2.2.0] bicyclohexene framework of the key intermediate 390. 

Osmium tetraoxide has also been used in the oxidation of bicyclic and polycyclic dienes. Thus, oxidation of norbomadiene (26) in a stoichiometric reaction was found to yield the exo-cis diol exclusively. On the other hand, in the NMO catalytic system a mixture of the exo-cis and endo-cis products was reported. However, by use of the NMO catalytic procedure for the substituted norbomadiene 27, the exc-diol was formed exclusively at the sterically crowded unsubstituted double bond and this product was utilized in the synthesis of pentalenolactone. Somewhat surprisingly, oxidation of hexamethyl Dewar benzene (28) exclusively gave the endo-cis diol as sole product. The tricyclic compound 29 gave the usual c/s-diol oxidation product of one of the double bonds. ... 

In their synthesis of Dewar benzene (10) van Tamelen and Pappas effected oxidative decarboxylation of the anhydride (9) with lead tetraacetate in pyridine at 43-45°. 

The synthesis of unsubstituted Dewar benzene was accomplished by van Tamelen (3) 4). Thus, dihydrophthalic anhydride was photochemically isomerized to a [2.2.0]-ring system which was decarboxylated oxidatively to Dewar benzene. This compound has a higher stability than expected from the high strain of its ring system (t1/2 = 37.2 h at 24.3°). This stability was later explained by the rule of the conservation of the orbital symmetry by Woodward and Hoffmann. 

The synthesis of Dewar benzenes from hydrocarbons has been briefly surveyed. These examples show that photoreactions of strained benzenes give Dewar isomers, whereas other benzenes do not give Dewar isomers in preparative quantities so that the latter must be synthesized by cyclization reactions. In contrast to these hydrocarbons, the photoreaction of perfluorinated aromatic compounds gives the corresponding valence-bond isomers in good yields. Three of the most typical examples are described by Eqs. (11) to (13). 

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