Hydrogen atom abstraction

About: Hydrogen atom abstraction is a research topic. Over the lifetime, 7059 publications have been published within this topic receiving 151781 citations.

An isolable radical anion based on the borole framework.

Abstract: The element boron is known to have a variety of ways to relieve its inherent electron deficiency. The acceptance of an electron pair (Lewis acidity) has applications in catalysis and activation of element–element bonds (frustrated Lewis pairs). The combination of boron with p-donating substituents (e.g. BF3) and its incorporation into organic p-conjugated systems allows the empty pz orbital of boron to participate in p bonding and p conjugation, respectively, and the latter enables the use of boron in optoelectronic materials with unique properties. The absence of p-donating substituents at the boron center may result in multiple-center bonding to form nonclassical frameworks (e.g. B2H6 or clusters). In addition, organoboranes and -diboranes(4) are prone to accept a single electron by chemical reduction. Likewise, hydrogen atom abstraction from N-heterocyclic carbene(NHC)-stabilized boranes (NHC-BH3) can lead to neutral, persistent boryl radicals of the type NHC-BH2C, [5] which have been studied by means of cyclic voltammetry, EPR, and UV/Vis spectroscopy as well as trapping reactions. However, examples of isolated boron radicals are rare owing to the reactive nature of the species, and only little is known about their structural properties. Steric protection of the boron center combined with spin delocalization over the organic substituents, both achieved by substitution with mesityl groups (Mes= 2,4,6-trimethylphenyl), has occasionally enabled isolation and structural characterization of radical anions such as [Li([12]crown-4)2][BMes3] (1) or [K([18]crown-6)(thf)2][Mes2BB(Ph)Mes] (2). [7] Our group has recently studied a persistent radical anion as an intermediate in the stepwise reduction of 1-ferrocenyl2,3,4,5-tetraphenylborole (3). Boroles are a class of antiaromatic compounds with interesting chemical and photophysical properties that are well-known for their ability to accept two electrons with formation of an aromatic borole dianion. Encouraged by these recent results on the radical anion [3]C , which indicated the presence of a highly unusual C4B p system bearing five electrons, [8] we set out to isolate and characterize a stable borol radical anion. As we report here, this was possible by choice of steric protection and an appropriate reducing agent. The synthesis of MesBC4Ph4 (1-mesityl-2,3,4,5-tetraphenylborole, 4) by means of the commonly employed tin–boron exchange reaction was unsuccessful because of the low reactivity of dihalo(mesityl)boranes (MesBX2; X=Cl, Br). However, 4 was obtained in 41% yield by functionalization of the boron center in 1-chloro-2,3,4,5-tetraphenylborole (5) through nucleophilic displacement of the chlorine ligand with LiMes (Scheme 1). A more efficient alternative was found to be the salt-elimination reaction of MesBCl2 with 1,4-

Size-dependent catalytic activity of supported vanadium oxide species: oxidative dehydrogenation of propane

Abstract: Possible reaction pathways for the oxidative dehydrogenation of propane by vanadium oxide catalysts supported on silica are examined by density functional theory. Monomeric and dimeric vanadium oxide species are both considered and modeled by vanadyl-substituted silsesquioxanes. The reaction proceeds in two subsequent steps. In a first step, hydrogen abstraction from propane by a vanadyl (O═V) group yields a propyl radical bound to a HOVIV surface site. Propene is formed by a second hydrogen abstraction, either at the same vanadia site or at a different one. VV/VIV redox cycles are preferred over VV/VIII cycles. Under the assumption of fast reoxidation, microkinetic simulations show that the first step is rate-determining and yields Arrhenius barriers that are lower for dimers (114 kJ/mol at 750 K) than for monomers (124 kJ/mol). The rate constants predicted for a mixture of monomers and dimers are 14% larger (750 K) than for monomers only, although the increase remains within experimental uncertainty lim...

Computational models for cytochrome P450: a predictive electronic model for aromatic oxidation and hydrogen atom abstraction.

Abstract: Experimental observations suggest that electronic characteristics play a role in the rates of substrate oxidation for cytochrome P450 enzymes. For example, the tendency for oxidation of a certain functional group generally follows the relative stability of the radicals that are formed (e.g., N-dealkylation > O-dealkylation > 2 degrees carbon oxidation > 1 degree carbon oxidation). In addition, results show that useful correlations between the rates of product formation can be developed using electronic models. In this article, we attempt to determine whether a combined computational model for aromatic and aliphatic hydroxylation can be developed. Toward this goal, we used a combination of experimental data and semiempirical molecular orbital calculations to predicted activation energies for aromatic and aliphatic hydroxylation. The resulting model extends the predictive capacity of our previous aliphatic hydroxylation model to include the second most important group of oxidations, aromatic hydroxylation. The combined model can account for about 83% of the variance in the data for the 20 compounds in the training set and has an error of about 0.7 kcal/mol.

Mesoporous Graphitic Carbon Nitride as a Heterogeneous Visible Light Photoinitiator for Radical Polymerization

Abstract: The use mesoporous graphitic carbon nitride (mpg-C3N4) in conjunction with tertiary amines as initiators in visible-light-induced free radical polymerization is described. The initiation mechanism involves photoinduced free radical generation by scavenging holes with amines and subsequent hydrogen abstraction. The efficiency of the photoinitiation is controlled by the nature of the amines and specific surface area of the carbon nitride powder. Apparently, amines with higher basicity and available hydrogens provide more favorable conditions for the photoinitiation process. Due to its heterogeneous nature, the photoinitiator preserves its photoinitiation activity after the polymerization and can easily be separated and used for further polymerizations.