Radical scavengers TEMPO

The competitive formation of the silyloxytelluride detected during the progress of the reaction, and of the silylated TEMPO formed by adding the radical scavenger TEMPO, suggests the following reaction pathway ... 

Scheme 4.4 Capture of singlet oxygen radicals by the radical scavenger TEMPO [49].

Fig. 19 Photolysis in the presence of radical scavenger TEMPO suppressed formation of ethane, methane, and methyl chloride...

The copper-catalyzed synthesis of imidazo[l,2-a]pyridines via C—H activation using oxygen as the sole oxidant has been reported by Adimurthy et al. ° The reaction proceeds in the presence of the radical-scavenger TEMPO, thns confirming that it does not occur via a radical reaction pathway. 

Davies NM, Jamali F Pharmacological protection of NSAID-induced intestinal permeability in the rat Effect of tempo and metronidazole as potential free radical scavengers. Hum Exp Toxicol 1997 16 345-349. 

The reaction rate is half-order in palladium and dimeric hydroxides of the type shown are very common for palladium. The reaction is first order in alcohol and a kinetic isotope effect was found for CH2 versus CD2 containing alcohols at 100 °C (1.4-2.1) showing that probably the (3-hydride elimination is rate-determining. Thus, fast pre-equilibria are involved with the dimer as the resting state. When terminal alkenes are present, Wacker oxidation of the alkene is the fastest reaction. Aldehydes are prone to autoxidation and it was found that radical scavengers such as TEMPO suppressed the side reactions and led to an increase of the selectivity [18],... 

Another improvement is the use of a Ru/TEMPO catalyst combination for the selective aerobic oxidations of primary and secondary alcohols to the corresponding aldehydes and ketones, respectively (Fig. 1.22) [72]. The method is effective (>99% selectivity) with a broad range of primary and secondary aliphatic, allylic and benzylic alcohols. The overoxidation of aldehydes to the corresponding carboxylic acids is suppressed by the TEMPO which acts as a radical scavenger in preventing autoxidation. 

HALS was based on the discovery that the 2,2,6,6-tetramethyl-l-piperidinyloxy, free radical (TEMPO) (1)), which already was known as an effective radical scavenger [46,47], was a very effective UV stabilizer too [48,49]. However, due to its physical and chemical properties TEMPO itself did not led to practical use. TEMPO is colored and will impart color to the to be stabilized polymer, it is thermally unstable and volatile [49]. Furthermore, it reacts with phenolic antioxidants present in many polymers leading to a reduction of processing and/or long-term heat stability. The discovery that compounds in which the /V-oxyl functionality was replaced by a N—H functionality also showed good UV stabilization activity was the key finding that led to the development of HALS stabilizers [49]. 

It was speculated that the second step of over-oxidation to acid might take place via a free radical pathway, arising from the catalytic decomposition of the t-BuOOH. It was further thought that the use of a free radical inhibitor might reduce the extent of the acid formation and inqjrove the overall aldehyde selectivity. The use of free radical scavengers such as 2,6-di-ier/-butyl-4-methylphenol (Table 4, Run 24), the stable free-radical, TEMPO, (Run 25) or the amine type inhibitor, N-Phenyl -2-Naphthylamine (Run 26), did not show any improvement in the reaction selectivity towards the formation of the aldehyde. The lack of any significant reduction in the amoimts of ester formed when using these modifiers showed that both steps of aldehyde and acid formation most likely do not include the involvement of free radical intermediates. 

As mentioned at the beginning of this section, Kirmse and coworkers (Bunse et al., 1992) found the first clear case of a homolytic aliphatic dediazoniation As shown in Scheme 7>24, the aqueous diazotization of 2>amino>2>methylpropane" nitrile (7.59) with two equivalents of NaN02 (or N2O4) yields products that are likely to be formed from the carbocation 7.60, namely 2>hydroxy-2-methylpropane-nitrile (7.62) and 2-methylprop-2-enenitrile (7.61), but also 2-methyl-2-nitropropane-nitrile (7.64), and the 7V,A-disubstituted 2>amino-2>methylpropanenitrile (7.66). The two last-mentioned products are probably formed via the radical 7.63. Direct evidence for this radical was found by experiments conducted in the presence of the radical scavenger 2,2,6,6-tetramethylpiperidin-l-oxyl (TEMPO) to give 7.65 in good yield. In other experiments, dimerization and oxidation products of the radical 7.63 were identified. 

Carroll and coworkers have published a series of papers on the use of aryliodonium salts in the synthesis of fiuorine-containing aromatic and heterocyclic products [63, 74, 75]. It has been found that the addition of radical scavengers such as TEMPO (2,2,6,6-tetramethylpiperidine-l-oxyl) during the fluoridation of diaryliodonium salts leads to a significant improvement of both the reproducibility of the process and the... 

Fronts of concentrations of 0.03, 0.06, and 0.15% (AIBN to MMA) were run at the temperatures ranges of 42 to 47,47 to 52 and 66 to 68 C. The ranges were a result of temperature fluctuations due to the type of thermostat used. Three samples at each set of conditions were run to determine an average and standard deviation. To ensure that the fronts were initiated by the polymeric seed, controls were run that did not contain seeds. To ensure that the gradient movement detected was not solely seed dissolution, controls using seeds and solutions of 4.0% TEMPO, a free-radical scavenger used as an inhibitor, in... 

Experiments of disappearance of DPPH (2,2 diphenyl-l-picrylhydrazyl radical), which is widely used to test the ability of compounds to act as free radical scavengers or as hydrogen donors at different temperatures, are shown in Figure 1. Fresh distilled styrene or styrene and recently sublimated MA and a solution 10 M of DPPH were put in a vial in the presence OH-TEMPO or without it, and heated at 80°, 100° or 120 °C after oxygen had been... 

The addition of radical scavengers, such as TEMPO, suppressed the formation of ethane, methane, and methyl chloride, instead leading rally to the observation of TEMPO-Me (35) and Pd(II) complex 34 (Fig. 19). The observed reaction with radical scavengers is consistent with photo-induced homolytic Pd-C bond cleavage... 

The hydrogen-bonded intermediate complex of a hindered phenolic antioxidant with a stable aminoxyl radical (TEMPO), used as a model for a hydrocarbon oxidant, has been isolated and its structure determined, as confirmation of the radical scavenging mechanism (79). 

Thus, in stable free radical polymerization (SFRP), also called nitroxide-mediated polymerization or NMP (which was discovered while using TEMPO as a radical scavenger in investigating the rate of initiation during free radical polymerization), it is believed that reversible combination of a polymer radical, P, with a stable niUoxyl radical, N, takes place forming an adduct, P-N, that exists as a dormant species ... 

The facts that the reactions require a large excess of acceptor, the appearance of the reduced haloarene with an H/D isotope effects, and inhibition by TEMPO or other radical scavengers are consistent with radicals being involved. 

More recently, a novel catalytic system based on the iron complex (Cp-NHC)Fe(CO) (Fig. 10.15a) combined with AgBF4/PhSiH3 resulted to be an efficient and selective method for the reduction of sulfoxides to sulfides [34]. The catalytic reaction is suitable for a variety of sulfoxides including aromatic and aliphatic. Radical scavenger experiments indicate the presence of free radicals in the catalytic reaction, suggesting a radical-base mechanism. The addition of both carbon- and oxygen-centered spin traps such as TEMPO (2,2,6,6-tetramethyl-piperidinyloxy) and BHT (2,6-di-rcrr-butyl-4-methylphenol) has a clear effect on the efficiency of the catalytic reaction. So far, the nature of the radical species is unknown. 

Charette has demonstrated an intra-molecular alkylation-cyclization, promoted by Ni(PPh3)4 and NaHMDS. The mechanism was shown to be radical-based by addition of TEMPO or other radical scavengers (Scheme 5.33). ... 

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