Synthesis of oxochromium(VI) alkoxides via epoxide cleavage. Structure, reactivity, and mechanism

In an NMR spectroscopic study the cleavage of epoxides [ethylene, propylene , and cis/trans-butylene oxide] by chromyl chloride giving access to oxochr omium(VI) alkoxides was shown to proceed via a bimolecular rate-determining step where two molecules of a complex CrO2Cl2... epoxide collide. Subseque ntly one Cl ligand at the first Cr center attacks the backside of an epoxid e molecule complexed at the Cr center of a second CrO2Cl2... epoxide molecu le and vice versa. The traits-opening of the epoxides was proved by determi ning the configuration of the chlorohydrins resulting from hydrolysis of th e corresponding alkoxide products in the cases of cis- and trans-butylene o xide. The NMR data provide evidence that each oxochromium(VI) alkoxide adop ts one preferred conformation in solution although DFT calculations did not indicate any special stabilizing effects. The product formation was ration alized by DFT calculations concerning the thermodynamics of the reactions.