TETRAHEDRON LETTERS Tetrahedron Letters 42 (2001) 2739–2742 Pergamon Transition structures for the stepwise insertion of oxygen into alkane tertiary CH bonds by dimethyldioxirane Mauro Freccero, a Remo Gandolfi, a Mirko Sarzi-Amade ` a, * and Augusto Rastelli b a Dipartimento di Chimica Organica, Universita ` di Pavia, V.le Taramelli 10, 27100 Pavia, Italy b Dipartimento di Chimica, Universita ` di Modena, Via Campi 183, 41100 Modena, Italy Received 8 February 2001; revised 13 February 2001; accepted 19 February 2001 Abstract—Oxygen insertion into the isobutane CH bond by dimethyldioxirane 1 was computationally studied at the R(U)B3LYP level to address the mechanistic concerted-stepwise problem. We located genuine TSs, diradicaloid in nature, that can lead to final products via radical pair intermediates. These TSs have lower energies than their concerted counterpart. Thus, calculations support the viability of radical pair formation in the reaction of dioxiranes with alkanes. © 2001 Elsevier Science Ltd. All rights reserved. The synthetically interesting oxygen insertion into unactivated CH bonds of alkanes by dioxiranes is one of the most recent additions to the armory of oxidation reactions (Scheme 1). 1 The hypothesis of a concerted process advanced by Curci and Adam 2 was challenged by Minisci 3 who suggested a stepwise mechanism (Scheme 2). Recently, the feasibility of a concerted process was supported by the RB3LYP/6-31G* transition structures (e.g. conc-3 in Scheme 3) located in three computational studies. 4–6 Actually, these TSs exhibit considerable diradical character and are highly polarized. At the TS the oxygen being transferred not only interacts strongly with the H atom (the O 1 H is almost completely formed) but also, even if much more weakly, with the C 4 atom. Moreover, IRC calculations starting from conc-3 produce on one side a dioxirane+alkane and on the other side lead directly to an alcohol+carbonyl compound. 4–6 However, TSs of type conc-3 exhibit an RHFUHF instability 7 (for example, 5.3 kcal/mol for conc-3) that casts some doubt on their nature. In the context of our systematic theoretical search on oxidation reactions with dioxiranes 8 and peroxy acids, 9 we report here that the stepwise process is also a viable route, as demonstrated by the ‘genuine’ diradicaloid TSs (rad-4 and rad-5, Scheme 3 and Fig. 1) located by Scheme 1. Scheme 2. Keywords : dioxiranes; hydroxylation; transition states. * Corresponding author. 0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII:S0040-4039(01)00276-3