EPR studies of the additon of 1,1-bis(alkoxycarbonyl)alkyl radicals and tris(ethoxycarbonyl)methyl radicals to alkenes and to alkyl isocyanides

Abstract

EPR spectroscopy has been used to monitor the addition of RĊ(CO2Et)2(R = H or Me) and of ˙C(CO2Et)3 to terminal alkenes, furan, MeNC and ButNC 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 α–C–H groups in the esters. For all acceptors, addition of each of the electrophilic α-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 HĊ(CO2Et)2, MeĊ(CO2Et)2 and ˙C(CO2Et)3 to H2CCH2, MeCHCH2, Me2CHCH2 and Me3SiCH2CHCH2 are also governed mainly by polar effects. Approximate absolute rate constants for addition of HĊ(CO2Et)2 and ˙C(CO2Et)3 to ethene at 221 K have been determined to be 7.3 × 103 and 1.4 × 103 dm3 mol–1 s–1, respectively. The radical H2CCH(CH2)3Ċ(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 α-alkoxycarbonyl(alkyl) radicals to alkyl isocyanides occurs at the terminal carbon atom to give imidoyl radicals which are strongly bent at Cα. Addition of HĊ(CO2Et)2 to ButNC takes place 16 times faster than its addition to ethene at 220 K.