Atom-Transfer Cyclization with CuSO4/KBH4: A Formal “Activators Generated by Electron Transfer” Process Also Applicable to Atom-Transfer Polymerization

The 4-exo and 5-exo-trig atom-transfer cyclizations of 1, 8a–e, 9, 12, and 13 can be mediated with as little as 0.05 mol % of Cu(TPMA)SO4·5H2O in the presence of 2.5 mol % of borohydride salts in 10 min at room temperature in air. This formal “activators generated by electron transfer” (AGET) procedure utilizes a cheap and oxidatively stable copper source (CuSO4·5H2O) and can be carried out in environmentally benign solvents (EtOH). It is possible to alter the product distribution in the 5-endo radical–polar crossover reactions of 10a,b and 11 by tailoring the amount of borohydride. Cyclization onto alkynes 14 and 15 is also possible in only 20 min. Controlled radical polymerization of styrene, with increased rates over conventional atom-transfer radical polymerization (ATRP), can be carried out in a controlled fashion (Mn, PDI) using either CuBr or CuSO4·5H2O and Bu4NBH4.