EPR STUDIES OF THE ADDITION OF 1,1-BIS(ALKOXYCARBONYL)ALKYL RADICALS AND TRIS(ETHOXYCARBONYL)METHYL RADICALS TO ALKENES AND TO ALKYL ISOCYANIDES

EPR spectroscopy has been used to monitor the addition of RC(CO2Et)2 (R = H or Me) and of .C(CO2Et)3 to terminal alkenes, furan, MeNC and Bu(t)NC in cyclopropane solution at low temperatures. The radical addenda were generated by UV photolysis of di-tert-butyl peroxide in the presence of the corresponding malonate or triethyl methanetricarboxylate and trimethylamine-butylborane complex, which acts as a polarity reversal catalyst for hydrogen-atom abstraction from the electron-deficient alpha-C-H groups in the esters. For all acceptors, addition of each of the electrophilic alpha-alkoxycarbonyl(alkyl) radicals is more rapid than the corresponding addition of simple (nucleophilic) alkyl radicals, a result which is attributed to the importance of charge-transfer interactions in the transition states (polar effects). Relative rates of addition of HC(CO2Et)2, MeC(CO2Et)2 and .C(CO2Et)3 to H2C=CH2, MeCH=CH2, Me2C=CH2 and Me3SiCH2CH=CH2 are also governed mainly by polar effects. Approximate absolute rate constants for addition of HC(CO2Et)2 and .C(CO2Et)3 to ethene at 221 K have been determined to be 7.3 x 10(3) and 1.4 x 10(3) dm3 mol-1 s-1, respectively. The radical H2C=CH(CH2)3C(CO2Et)2 undergoes rapid 5-exo-cyclisation with a lower activation energy than that for corresponding cyclisation of the unsubstituted hex-5-enyl radical; this is attributed to the electrophilic nature of the radical centre in the former species. Addition of alpha-alkoxycarbonyl(alkyl) radicals to alkyl isocyanides occurs at the terminal carbon atom to give imidoyl radicals which are strongly bent at C(alpha). Addition of HC(CO2Et)2 to Bu(t)NC takes place 16 times faster than its addition to ethene at 220 K.