Oxidation of Isopropylcyclopropane by Chromyl Chloride: Ring-Opened Products Support a Hydrogen Atom Abstraction Mechanism.

Abstract

Chromyl chloride (CrO(2)Cl(2)) reacts with neat isopropylcyclopropane at 65 degrees C to give a dark precipitate along with at least 20 organic products. Both cyclopropyl products and ring-opened products are observed: 2-cyclopropyl-2-chloropropane (1, 0.4% yield based on chromium), 2-cyclopropyl-2-propanol (2, 0.2%), 5-chloro-2-methyl-2-pentene (3, 0.3%), and 4-methyl-3-penten-1-ol (4, 0.5%) as well as other ring-opened products. Authentic samples of 1-4 were synthesized, and their GC and GC/MS data were compared with the reaction mixture. Other organic products (5-10) were tentatively assigned by GC/MS on the basis of their m/z and fragmentation patterns. The ratio of (1 + 2) vs (3 + 4) increases by a factor of 2 when the initial concentration of CrO(2)Cl(2) increases from 0.3 to 1.12 M. The reaction was also carried out in the gas phase, and essentially all the products from the liquid phase reaction were observed. The products are explained by a mechanism involving initial hydrogen atom abstraction from the substrate. The resulting dimethylcyclopropylcarbinyl radical can either be trapped by CrO(2)Cl(2) (to form 1 and 2) or ring-open to give 4-methyl-3-pentenyl radical, which reacts with CrO(2)Cl(2) to form 3 and 4 as well as further oxidized products. The oxidation of isopropylcyclopropane by MnO(4)(-) in pyridine was also examined. Acetone, an expected ring-opened product, was the only product observed by our analytical techniques. Me(2)C(18)O is produced from (18)O-labeled MnO(4)(-). These results suggest that the reactions of CrO(2)Cl(2) and MnO(4)(-) with isopropylcyclopropane proceed by hydrogen atom transfer to form organic radical intermediates.