Übergangsmetall-methylen-komplexe. XLVIII. Addition und ringschluss bei reaktionen von α-ketodiazoalkanen mit metall-metall-doppelbindungen der metallcarbonyl-reihe: Aufbau metallacyclischer strukturen

The α-diazoketones 2a-h,k,l react with the dinuclear organorhodium complex 1 with elimination of dinitrogen and concomitant addition of their carbonyl-flanked carbene moieties upon the metal-metal double bond. The μ-alkylidene complexes 3f-h thus accessible, represent the final products; the trimethylsilyl derivative 3i was synthesized for comparison. By contrast, the derivatives 3a-e,l behave rather as IR-spectroscopically identified intermediates that, at room temperature, undergo rapid consecutive intramolecular cycloaddition reactions yielding the novel compounds 4a-e and 5l, respectively. Formation of 4a-e results from nucleophilic attack of the keto groups of the bridging ligands upon one of the metal carbonyl units. This type of cyclization process comprises the construction of a new oxygen-carbon bond. The metallacyclic structures thus obtained have been elucidated in the cases of 4c and 4e by means of single crystal X-ray diffraction techniques and no longer obey the structural criteria of type-3 μ-alkylidene complexes. An internal experiment concurrently carried out by use of the diazoalkane 2e furnished proof that the cyclization process occurred regiospecifically: only the more nucleophilic keto-oxygen atom is subject to ring closure, whereas the oxygen atom of the ester carbonyl group does not participate in this reaction. The bis(acyl)methylene derivatives 3f-h resist cyclization and retain their structures even at elevated temperatures (cf. boiling tetrahydrofuran). The carbocyclic diazo precursor 2k (diazodimedone) requires more drastic conditions (boiling benzene) for the reaction with compound 1 to occur; here, the cycloadduct 5k is formed following nitrogen elimination. According to a single crystal X-ray diffraction study, the ketocarbene fragment derived from 2k has been added across the dinuclear metalcarbonyl core, finally acting as a bridging structural unit. In contrast to the cycloadducts 4a-e, a new carbon-carbon bond is formed during the synthesis of 5k. The dimetallacycle 5l results from a two-step cyclo-elimination between the bisdiazoalkane 2l and the organorhodium compound 1.