Rh(II)-Catalyzed Reaction Of Some α‘,α‘- and β‘-Branched-O-Alkyl α-(Alkoxycarbonyl)-α-diazoacetates

alpha',alpha'- and beta'- Branched-O-alkyl alpha-(alkoxycarbonyl)-alpha-diazoacetates 1a,b and 2a,b were prepared. The regio- and chemoselectivities in the Rh(II)-catalyzed reaction of these compounds were studied. The model systems 1a,b showed that the reactivity of the ester-substituted Rh(II) carbenoid is responsive to the nature of the Rh(II) catalyst; rhodium(II) acetamide favored insertion into the tertiary C-H bond and rhodium(II) perfluorobutyrate favored insertion into the secondary C-H bond. We also found that the nature of the O-alkyl group in the ester moiety had no influence on the reactivity of the Rh(II) carbenoid. In more complex systems such as compounds 2, it was found that rhodium(II) acetate promoted insertion into tertiary C-H bonds. Intramolecular cyclopropanation, to give 3-oxabicyclo[5.1.0]octane and 3-oxabicyclo[6.1.0]nonane products, was found to be competitive with tertiary C-H insertion; it is favored when rhodium(II) perfluorobutyrate was used as the catalyst. The Rh(II)-catalyzed reaction of 3 ("carbon" analog of 2a) resulted mainly in the cyclopentanone derivative, and no cyclopropanation product was detected. This result indicates that the ester oxygen in compounds of type 2 plays an important role in influencing the regio- and chemoselectivities of their reaction. Overall the results indicate that for compounds of a particular type of structure the regio- and chemoselectivities can be controlled via the judicious choice of the Rh(II) catalyst.