Cyclic triene chromium complexes

The metal-mediated and metal-catalyzed [6 + 2]- and [6 + 4]-cycloaddition reactions, pioneered by Pettit and co-workers105 106 and Kreiter and co-workers,107 respectively, involve the cycloaddition of metal-complexed cyclic trienes with 7r-systems such as alkenes, alkynes, and dienes. The [6 + 2]-reactions produce bicyclo[4.2.1]nonadiene derivatives and the [6 + 4]-reactions produce bicyclo[4.4.1]undecatrienes (Scheme 32). Trienes complexed to chromium, which can be prepared on large scale (40 g) as reported by Rigby and co-workers,108 react with 7r-systems upon thermolysis or irradiation.109-111 Chromium and iron-catalyzed [6 + 2]-reactions of cycloheptatrienes and disubstituted alkynes... 

In dramatic contrast to the normal course of higher-order cycloadditions with most cyclic triene substrates, the photochemically induced [6 h- 4] cycloaddition reactions exhibited by tricarbonyl(T -l,3,5-cy-cloheptatriene)chromium(O) complexes appear to be quite suitable for preparative applications. Yields of the resultant diene cycloadducts are usu ly good and the metal can be removed in high yield on treatment of the cycloadduct complex with triethylphosphine. Scheme 14 shows the results of several cycloadditions between the complex and substituted diene addends. While not specifically delineated in the original work, these reactions provide the corresponding endo adducts exclusively. ... 

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