The photochemical addition of N-haloamides to olefins: the influence of various factors on the competition between 1,2-addition and hydrogen abstraction

Abstract

In the photodecomposition of N-haloamides (ZCONRX) in the presence of olefins, the 1,2-addition chain competes with the hydrogen abstraction chain(s) leading to the parent amide (the quantum yields for these processes are greater than unity). The following factors were shown to have an influence on this competition as measured by the yield of 1,2-addition and the yield of parent amide in methylene chloride solutions: (i) the N-halogen (higher yields of addition with X = Cl than with X = Br); (ii) the electronegativity of Z (increase of the yield of addition as the electronegativity of Z increases); (iii) the temperature (higher yields of addition at lower temperatures, and at −70 °C, better yields of addition (>90%, R = H) for X = Br than for X = Cl); and (iv) the size of R (dramatic decrease of the yield of 1,2-addition in going from R = H to R = CH3). Surprisingly, the presence of a scavenger for HX had no influence on the yield of 1,2-addition. Both the size and electronegativity of Z had an effect on the stereochemistry of 1,2-addition to cyclohexene. High yields of addition to a variety of olefins were obtained with N-chloroamides such as ClCH2CONHCl, C2H5OCONHCl, CF3CONHCl. Their addition to enol ethers at −70 °C led to the synthesis of α-amido acetals or ketals (aldehydes or ketones) and to an α-amido glycoside in good yields.