Chromium Arene Synthesis
This application note demonstrates the preparation of arene chromium tricarbonyl complexes using continuous-flow processing: (η⁶ – C₆H₅CH₃)Cr(CO)₃ as an example.
Arene chromium tricarbonyl complexes, (arene)Cr(CO)₃, have found extensive use in organic synthesis. The coordination of an aromatic ring to the chromium tricarbonyl unit has a profound effect on its reactivity. Due to the electron withdrawing nature of the Cr(CO)₃ moiety, nucleophilic substitution reactions can be performed on the aromatic ring. The electron-deficient ring is able to stabilize negative charge, allowing both ring and benzylic deprotonation. In addition, the chromium unit effectively blocks one side of the ring and this directs incoming reagents to the face opposite the metal. The preparation of chromium arene complexes generally involves heating chromium hexacarbonyl under reflux either in the arene as the solvent or in a high boiling solvent containing the arene (Scheme 1). Reactions can take in the order of 1-4 days to reach completion and need to be performed under an inert atmosphere. The procedure is complicated by the sublimation of Cr(CO)₆ as well as the competitive decomposition of the arene chromium product over time. This can be somewhat overcome by heating a 1 : 1 mixture of arene and Cr(CO)₆ in a mixture of dibutyl ether and heptane in the presence of little tetrahydrofuran.
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