Five 1-alkyl-3-methylindazoles were prepared by treatment of l-(2-bromophenyl)-l-methoxy-l-(2-alkylazo)ethanes with tri-n-butyl stannane and AIBN at 80°C in benzene. Yields in the radical cyclization step ranged from 39 to 92%. 1-Phenyl-3-methylindazole was prepared by an analogous route but in very poor yield (<5%). Rate constants for the 5-endo radical closures ( k: O: : ) , estimated by the radical clock method, were 5.2 X 10's-' and 9.2 X 10's-' for two of the alkyl systems. Rate constants for analogous 5-endo cyclizations of aryl radicals onto C-C double bonds are much smaller than those for cyclizations to the azo functional group.
210ChemInform Abstract (V). -For R: -Ph, however, the respective (V) is obtained in only < 5% yield. Some concurrent reactions are studied as well to gain insight into the reaction mechanism, especially the same reaction sequence starting from acetophenone instead of the bromo derivative (I). Rate constants are estimated for the 5-endo radical (from (IVa) + Bu3Sn•) ring closure to be 5.2•109 s-1and 9.2•108 s-1for R: cyclohexyl and -tBu, respectively.
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