Molecule-Induced Alkane Homolysis with Dioxiranes

Abstract

The mechanisms of C-H and C-C bond activations with dimethyldioxirane (DMD) were studied experimentally and computationally at the B3LYP/6-311+G**//B3LYP/6-31G* density functional theory level for the propellanes 3,6-dehydrohomoadamantane (2) and 1,3-dehydroadamantane (3). The sigma(C-C) activation of 3 with DMD (Delta G(*) = 23.9 kcal mol(-1) and Delta G(r) = -5.4 kcal mol(-1)) is the first example of a molecule-induced homolytic C-C bond cleavage. The C-H bond hydroxylation observed for 2 is highly exergonic (Delta G(r) = -74.4 kcal mol(-1)) and follows a concerted pathway (Delta G(*) = 34.8 kcal mol(-1)), in contrast to its endergonic molecule-induced homolysis (Delta G(*) = 28.8 kcal mol(-1) and Delta G(r) = +9.2 kcal mol(-1)). The reactivities of 2 and 3 with CrO(2)Cl(2), which follow a molecule-induced homolytic activation mechanism, parallel the DMD results only for highly reactive 3, but differ considerably for more stable propellanes such as 4-phenyl-3,6-dehydrohomoadamantane (1) and 2.

Metal‐Free, Selective Alkane Functionalizations

“…[35,38] Mechanistic studies of alkanes reacting with electrondeficient species reveal common mechanistic features arising from the formation of radical-cationic transition states or intermediates. [35] The recently suggested [38,39] H-coupled electron transfer mechanistic model agrees well with the experimental features of alkane halogenations with electrophiles and may be extended to a wide range of alkane transformations: from uncharged metaloxo species [40] and, possibly, dioxiranes [41,42] (™molecule induced homolytic∫ path, TS1, Scheme 1) to charged electrophiles like nitronium salts (™electrophilic∫ path Scheme 1. Alkane CÀH bond activation mechanisms with uncharged (E) and charged (E ) electrophiles as well as with radicals (E ) and electron-acceptors ( À e À ).…”

“…[74] The mechanistic dichotomy in the reactions of alkanes with peracids, i.e., concerted asc.wiley-vch.de insertion vs. molecule induced homolysis [75] is somewhat reminiscent of dioxirane chemistry, where clear evidence for concerted [76] paths was identified recently. [42] The selectivities for radical oxyfunctionalizations may also be substantially increased with the less reactive and bulky t-BuO radical. When generated from t-butyl chlorite (t-BuOCl) the participation of Cl radicals lowers the selectivities and even CH 3 groups are attacked (Table 3, Entry 1).…”

The Role of Radicals in Metal‐Assisted Oxygenation Reactions

“…[6] The occurrence of intermediate radical species in at least one of the pathways is supported by the results of experiments where alkynes and allene were used as substrates: Photolytically activated, matrix-isolated CrO 2 Cl 2 reacts with alkynes to yield complexes of the type [O = CrCl 2 (h 1 -O = C = CR 2 )] (R = H, Me) featuring ketene ligands in a h 1 -coordination mode which is unprecedented in structurally characterized compounds, [126] and the oxidation of allene yields the cyclopropanone complex 22 [Eq. (16)]. [126] The formation of 22 provides strong evidence for the hypothesis of a p-bond homolysis initiated by the attack of CrO 2 Cl 2 at the unsaturated bonds leading to radical intermediates in the matrices.…”

“…[4,14] The reactivity of CrO 2 Cl 2 is therefore reminiscent of that of oxygencentered radicals, even though it is not a radical, and its reactions with alkanes can consequently be viewed as examples of "molecule-assisted homolysis", organiclike reactions in which two radicals are formed from two closed-shell molecules. [15,16] As a result of these findings a progression has occurred from thinking about radicals and spin density to an approach based on the thermochemistry of the Hydrogen atom transfer step. [14,6] Based on this "bond-strength approach" the reinvestigation of the system MnO 4 À /alkyl aromatics considerably improved the understanding of the reactivity principles characteristic for permanganate oxidations: Mayer et al showed that in aprotic organic media permanganate can abstract hydrogen atoms similarly to CrO 2 Cl 2 .…”

Wenn C‐H‐Bindungen sprechen könnten – selektive Oxidationen von C‐H‐Bindungen

“…Bei der Untersuchung mehrerer anderer polycyclischer Systeme fand man, dass hier die Selektivitäten ebenfalls von den verwendeten Reagentien abhängen. [86] Die Oxidation von Ringverknüpfungen, bei denen es sich nicht um Brücken-kopfpositionen handelt, ist einfacher. Die Oxidation von cisoder trans-Decalin (68) erfolgt z.…”