A carbon-centered radical unreactive toward oxygen: Unusual radical stabilization by a lactone ring

[GRAPHICS] A lactone ring confers unusual stability to a diphenylmethyl-like radical t hat is virtually unreactive toward oxygen. Thus, the radical derived from H P-136 is about 10 000 times less reactive than typical carbon-centered radi cals, A reversible reaction with oxygen is proposed by analogy with triphen ylmethyl; however, the association constant is about 1000 times smaller for HP-136 than for triphenylmethyl. While the lactone ring greatly influences the reactivity, the spectroscopy of the HP-136-derived radical is in line with that expected for a substituted diphenylmethyl radical.