Abstract

The chemistry of aromatic compounds was originally confined to benzenoid hydrocarbons and their derivatives but has been enriched in the last decade by the discovery of numerous non‐benzenoid, cyclically conjugated systems that are more or less like benzene in their properties. This has necessitated a new conception of “aromatic character”. Theoretical and experimental studies have been applied to this problem. In this connection, several selected representatives of this class of compound are described, mostly from studies by the author's own group, and relationships between structure and “aromatic character” are indicated.

6‐(Dimethylamino)fulvene solvent: 1.5 l. of petroleum ether product: 6‐(dimethylamino)fulvene

Abstract

Photolysis or thermolysis of alkyl azidoformates ( I ) causes elimination of nitrogen leading to alkoxycarbonylazenes (2), which insert themselves into C-H bonds or dehydrogenate the solvent to form the corresponding alkylurethanes 121. In order to avoid these secondary reactions, we investigated the pyrolysis of alkyl azidoformates in the gas phase in the presence of nitrogen as inert diluent. At 300 OC/20 mm, the ethyl, isopropyl, and t-butyl esters ( I a)-(I c) release nitrogen to form oxazolidones (3) in yields of 45-75 "/,.Polycyclic conjugated non-benzenoid hydrocarbons are of great theoretical interest in connection with the question of the relationships between structure and aromaticity 111. Starting from 4,6-dimethyl-l,8-cyclopentenoazulene ( I ) 121, we have prepared a non-benzenoid isomer of methylpyrene, viz. the tetracyclic hydrocarbon ( 4 ) . which can formally be viewed as consisting either of a pentalene and a heptalene system or of two azulene systems. Moreover the structural elements of sesquifulvalene and heptafulvene are recognizable. Hydrocarbon ( I ) reacts with N-methylanilinoacrolein in the presence of phosphorus oxide chloride at 2OoC to form the immonium salt (2) in 9 0 % yield; the perchlorate of (2) forms dark violet crystals which decompose above 26OoC. When (2) is heated with sodium methoxide in boiling methanol, it loses a molecule of N-methylaniline and undergoes intramolecular condensation to give (3), m.p. 62-63 "C, in 35 "/, yield; the trinitrobenzene x-complex of (3) melts at 196OC. Dehydrogenation of (3) with chloranil in benzene at 20°C produces a 65 yield of (4) as thermostable black needles, m.p. 109"C, which ( 3 i When ethyl azidoformate was thermolysed in carbon tetrachloride or 1,1,2-trichlorotrifluoroethane, it is not the oxazolidone (3a) that is formed [2a]; instead, two molecules of the azido ester interact with loss of nitrogen to form diethyl azodicarboxylate [2a, 31. This compound could not so far be detected during pyrolysis in the gas phase. We assume that the concentration of ( I ) and (2) in the gas phase is so small as a result of the dilution with nitrogen that the dimerization observed in solution cannot occur [4].In contrast, t-butyl azidoformate ( I c) reacts exclusively intramolecularly to form the oxazolidone f3c) in solution as well. It seems that the t-butyl group shields the azene nitrogen and that the close proximity of the methyl group promotes the cyclization I2a, 2 b].According to Smolinsky and Feuer [5], thermal decomposition of (S)-2-methylbutyl azidoformate in the gas phase yields 4-ethyl-4-methyl-2-oxazolidone. The configuration of the asymmetric carbon atom remains unchanged during this reaction.