Selective Alkane Transformations via Radicals and Radical Cations:  Insights into the Activation Step from Experiment and Theory

The heterogeneous reaction of hydroxyl radicals with sub-micron squalane particles: a model system for understanding the oxidative aging of ambient aerosols

“…Reaction rates and mechanisms of hydrocarbon free radicals in the condensed-phase have been similarly well-studied (Avila et al, 1993;Bennett and Summers, 1974;Ferenac et al, 2003;Fokin and Schreiner, 2002;Ingold, 1969;Russell, 1957;Vonsonntag and Schuchmann, 1991). However, it remains unclear how well these gas and condensed-phase chemical mechanisms apply to the more complex heterogeneous reactions of OH radicals with organic aerosol particles.…”

Metal‐Free, Selective Alkane Functionalizations

“…The selectivities of these reactions are controlled by the charge transfer from the hydrocarbon to the electrophile through the C À H À E bonding arrangement in the transition structures. The selectivities often are similar [35,38] to those observed for alkane oxidations via SET (™oxidative∫ path, Scheme 1). [43] It may be concluded that radical chemistry still is the preparatively most useful approach for alkane functiuonalizations.…”

“…Thermodynamically driven products form from the most stable cationic intermediates. As we pointed out recently, [35] the overall functionalizations in such systems are selective (giving one predictable and well-defined product), while the activations are not (various starting materials give the same thermodynamic product through a series of rearrangements). Functionalizations in the presence of oxidizing electrophiles [12,34,36] are quite selective, but they are limited to a small number of reagents and are only applicable to the more reactive hydrocarbons.…”

“…Selective single-electron transfer alkane oxidations are only possible with strong electron acceptors [37] and when structurally distinct alkane radical cations form. [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.…”

“…In other words, alkane activations with different electron-deficient reagents fit into the same mechanistic regime. [35] Exceptions may be C À H bond cleavages promoted by electron-deficient metal complexes where the occupied d-orbitals of the metal may be involved in agostic C À H bonding; [45] this usually inverts the 18 vs. 28 selectivities. [7] The activations of alkanes with transition metal complexes were the subject of a number of excellent recent reviews [6±8,17,46] and we will not cover these here.…”