Phthalimide tricarbonylchromium complexes: Synthesis, characterization, nucleophilic addition, and unanticipated syn adduct formation upon addition of propynyllithium

The complexation of phthalimide derivatives with hexacarbonylchromium in dibutyl ether/THF (10:1) afforded the corresponding tricarbonylchromium complexes, which were characterized spectroscopically. Subsequent nucleophilic addition with Grignard or organolithium reagents as well as with sodium tetrahydridoborate gave high yields of the corresponding anti adducts. However, in certain cases, addition of 1-lithio-1-methoxyallene, 1-propynyllithium, or ethenylmagnesium bromide, the unanticipated formation of the corresponding syn adducts was observed with the relative configuration of the propynyllithium adduct rac-20 confirmed by a crystal structure analysis. The unusual stereochemistry is explained by the intermediacy of a highly resonance-stabilized acyliminium ion formed from the original exo adduct. Finally, dehydration of the methyl adduct made available the exo-methylene derivative rac-37, bearing another fully sp²-hybridized ligand system. This compound was also characterized by a crystal structure analysis. The synthesis of the first phthalimide tricarbonylchromium complexes is reported. Nucleophilic addition to these complexes takes place at one of the imide carbonyl groups from the face opposite the Cr(CO)₃ group to give endo-hydroxy products. In some cases, however, the unanticipated formation of the exo-hydroxy derivatives is observed as a result of isomerization following the original reaction.