Die Synthese und der thermische Zerfall des 1,2-Diazabenzo[e]-semibullvalens und dessen Beziehung zur Thermolyse des Benzobenzvalens

The [4 + 2]-photoadduct of naphthalene and 4-phenyl-4H-1,2,4-triazol-3,5-dione (PTAD) was subjected to a triplet-sensitized di-π-methane rearrangement. Hydrolysis of the resulting urazol 15 gave a stable semicarbazide 16 which by nickel-peroxide oxidation gave 1,2-diazabenzo[e]semibullvalene (10). At 40 °C, the latter decays in CHCl₃ solution to produce benzofulvene (2) as the principal product, accompanied by benzobenzvalene (1) and small amounts of naphthalene. This decay reaction is rationalized in terms of a 1,3-dipolar cycloreversion which is followed by denitrogenation to give indenyl carbene (19). The thermolysis of 1 in solution is believed to proceed via the same carbene. At present, these findings provide only a partial answer to the benzobenzvalene enigma (i.e., in solution at 150°, 1 gives 2, but in the gas-phase at 300°, 1 yields naphthalene). MeOH is shown to add readily to 10 in a homologous Michael addition

Valence isomers of aromatic compounds: on the mechanism of the Katz reaction

Lithium salts of small and medium-sized aromatic anions react with methylene chloride and alkyllithium to afford bridged bicyclobutanes or related polycyclic compounds. We demonstrate that the three new carbon-carbon bonds formed in the course of these title reactions result from a tandem of carbenoid processes. Formation of exocyclic carbenoids precedes intramolecular cheletropic addition and ring enlargement. Neither H-migration nor CH-insertion is observed throughout these processes. Key intermediates have been generated by independent routes.</p

A Convenient Synthesis of Dibenzo[a,c]cyclooctene Based on Thioenol Ether Reduction

9,10,11,12-Tetrahydrodibenzo[a,b]cyclooctane-9,12-diol (6) is found to be reluctant to double bond forming elimination. It gives a bridge ether (7) instead. The corresponding diketone (5) reacts with benzyl mercaptan to provide the bis-thioenol ether (8-a. The latter, upon treatment with Raney nickle, gives the desired title diene 1. An alternative approach to 1, based on repeated carbane addition to phenanthrene, is also presented

The Photochemical Synthesis and Denitrogenation of 8,9-diazadibenzo[c, e]isobullvalene

Starting from dibenzo[a, c]cyclooctene (4) and 4-methyl-3H-1,2,4-triazol-3,5(4H)-dione (MTAD), the strained skeleton of the title azo compound 1 is assembled in a tandem photo-Diels-Alder addition/di-p-methane rearrangement sequence. The synthesis is completed by a stepwise hydrolytic oxidation of the ensuing triazolidine-dione 2 with nickel peroxide. Thermolysis of 1 in benzene solution is shown to be governed by an initial 1,3-dipolar cycloreversion which leads, via an intermediate diazo compound 11, to cyclobuta[1]phenanthrene 8 and two further carbene-derived C16H12 products. Photolysis of 1 at 350 nm leads in modest yield (12%), via a diazenyl diradical, to an unstable bridged bicyclobutane 10 (dibenzooctavalene). MNDO calculations suggest the latter to have a rapidly inverting, twisted structure of C2 symmetry