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 Bu^tNC 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 C_α. Addition of HĊ(CO₂Et)₂ to Bu^tNC takes place 16 times faster than its addition to ethene at 220 K.