Reactive intermediates in olefin oxidations with chromyl chloride. IR-spectroscopic proof for O=CrCl2-epoxide complexes

After photolytic induction (411 nm) of the reaction between chromyl chlorid e and olefins in low-temperature matrixes, for the first time primary produ cts of this system have been isolated and identified: the reaction of propy lene yields O = CrCl2 ... O = CHCH2CH3 and O = CrCl2 ... OCH2CHCH3, while i n the case of trans- and cis-2-butylene, OCrCl2 ... O = C(CH3)(CH2CH3) as w ell as OCrCl2 (trans-2,3-butylene oxide) and OCrCl2...(cis-2,3-butylene oxi de) are obtained, respectively. Epoxide formation proceeds under complete r etention of stereochemistry, and the regioselective reaction route leading to the carbonyl products can be suppressed if all hydrogen atoms at the ole finic bond are substituted: tetramethylethylene is selectively oxidized to tetramethylethylene oxide, which forms a complex with OCrCl2. All products were characterized by IR spectroscopy, which was further supported by DFT c alculations in certain cases. The mechanism leading to the formation of the se products, their electronic states, and the implications of the matrix re sults on the thermal reactions of chromyl chloride are discussed.