Abstract:Bei der Bestrahlung in Eisessig, Methanol, Aceton, Dioxan, Benzol und Cyclohexan sowie als kristallines Pulver geht 1,3,5-Tri-rert-butyl-2-nitrobeml (1) in 5,7-Di-tert-butyl-3.3dimethyl-3H-indol-l-oxid (5) uber, welches je nach den Reaktionsbedingungen als solches isoliert werden kann oder zu verschiedenen Produkten photoisomerisiert bzw. zu 5,7-Di-tertbutyl-I -hydroxy-3,3-dimethyI-2-indolinon (44 oxidiert wird. In Di-bzw. Triathylamin findet glatte Photoreduktion der Nitrogruppe. ohne Beteiligung der ortho-te… Show more
“…Solvents were of the highest grade commercially available and were used as received. The hydroxylamines 1a , 5a , 6a , and 7a and the stable nitroxides 2b , 3b , and 4b were prepared according to literature procedures, while 8a and NHPI were commercial products. The hydroxylamines 2a (mp = 159−161 °C from benzene/petroleum ether; IR ν = 3475 and 1600 cm -1 ), 3a (mp = 176−177 °C from ligroin; IR ν = 3400 and 1600 cm -1 ), and 4a (mp = 184−185 °C from benzene/petroleum ether; IR ν = 3415 a 3300 cm -1 ), were obtained from 2b , 3b , and 4b , respectively, by reduction with phenylhydrazine according to the method described in ref .…”
Bond dissociation enthalpies (BDE) of hydroxylamines containing alkyl, aryl, vinyl, and carbonyl substituents at the nitrogen atom have been determined by using the EPR radical equilibration technique in order to study the effect of the substituents on the O-H bond strength of these compounds. It has been found that substitution of an alkyl group directly bonded to the nitrogen atom with vinyl or aryl groups has a small effect, while substitution with acyl groups induces a large increase of the O-H BDE value. Thus, dialkyl hydroxylamines have O-H bond strengths of only ca. 70 kcal/mol, while acylhydroxylamines and N-hydroxyphthalimide (NHPI), containing two acyl substituents at nitrogen, are characterized by BDE values of ca. 80 and 88 kcal/mol, respectively. Since the phthalimide N-oxyl radical (PINO) has been recently proposed as an efficient oxidation catalyst of hydrocarbons or other substrates, the large BDE value found for the parent hydroxylamine (NHPI) justifies this proposal. Kinetic studies, carried out in order to better understand the mechanism of the NHPI-catalyzed aerobic oxidation of cumene, are consistent with a simple kinetic model where the rate-determining step is the hydrogen atom abstraction from the hydroxylamine by cumylperoxyl radicals.
“…Solvents were of the highest grade commercially available and were used as received. The hydroxylamines 1a , 5a , 6a , and 7a and the stable nitroxides 2b , 3b , and 4b were prepared according to literature procedures, while 8a and NHPI were commercial products. The hydroxylamines 2a (mp = 159−161 °C from benzene/petroleum ether; IR ν = 3475 and 1600 cm -1 ), 3a (mp = 176−177 °C from ligroin; IR ν = 3400 and 1600 cm -1 ), and 4a (mp = 184−185 °C from benzene/petroleum ether; IR ν = 3415 a 3300 cm -1 ), were obtained from 2b , 3b , and 4b , respectively, by reduction with phenylhydrazine according to the method described in ref .…”
Bond dissociation enthalpies (BDE) of hydroxylamines containing alkyl, aryl, vinyl, and carbonyl substituents at the nitrogen atom have been determined by using the EPR radical equilibration technique in order to study the effect of the substituents on the O-H bond strength of these compounds. It has been found that substitution of an alkyl group directly bonded to the nitrogen atom with vinyl or aryl groups has a small effect, while substitution with acyl groups induces a large increase of the O-H BDE value. Thus, dialkyl hydroxylamines have O-H bond strengths of only ca. 70 kcal/mol, while acylhydroxylamines and N-hydroxyphthalimide (NHPI), containing two acyl substituents at nitrogen, are characterized by BDE values of ca. 80 and 88 kcal/mol, respectively. Since the phthalimide N-oxyl radical (PINO) has been recently proposed as an efficient oxidation catalyst of hydrocarbons or other substrates, the large BDE value found for the parent hydroxylamine (NHPI) justifies this proposal. Kinetic studies, carried out in order to better understand the mechanism of the NHPI-catalyzed aerobic oxidation of cumene, are consistent with a simple kinetic model where the rate-determining step is the hydrogen atom abstraction from the hydroxylamine by cumylperoxyl radicals.
“…More impurity is formed from protiated than from deuterated substrate (Withers, 1977). These observations have a rational, if not unique, explanation in the known [see Dopp & Sailer (1975)] abstraction of adjacent alkyl hydrogen atoms by the nitro groups of ortho-nitroaryl compound; such a process on the anomeric hydrogen of 2-nitrophenyl galactoside will be subject to a primary isotope effect. The known (Angyal, 1974) complex-formation of Mg2+ by sugars then presumably occurs in such a way as to fix the glycoside in a favourable conformation for hydrogen transfer; alternatively traces of transition-metal cations could be removed by the EDTA necessary for work with apoenzyme and these traces could catalyse the hydrogen transfer.…”
1. Removal of Mg2+ from Escherichia coli (lacZ) beta-galactosidase slightly increases the rate of hydrolysis of galactosyl pyridinium salts, but decreases the rate of hydrolysis of arylgalactosides. 2. Fair correlation of logkcat. and log (Km) with the pKa of aglycone is now observed for arglygalactosides, as well as for glycosyl pyridinium salts. 3. Degalactosylation of Mg2+-free enzyme is the rate-limiting step in the hydrolysis of 2,4-dinitrophenyl galactoside. 4. alpha-Deuterium kinetic isotope effects for both sets of substrates are consistent with the rate-determining generation of a glycosyl cation. 5. The pH-independent, SNl hydrolysis of 3,4-dinitrophenyl galactoside has been measured: it is as fast as that of the galactosyl 3-chloropyridinium ion. 6. Hydrolysis of these two substrates by Mg2+-free enzyme proceeds at very similar rates. 7. It is concluded that loss of both types of aglycone takes place, without acid catalysis, from the first ES complex of substrate and apoenzyme. 8. Data for galactosyl azide and thiopicrate confirm that neither charge nor change of atom is the cause of the differences in behavior between aryl galactosides and galactosylpyridinium salts.
“…Irradiation inside the EPR cavity of solutions containing the boranes 2a,b and 1f , in the presence of peroxide, led at room temperature to the formation of two paramagnetic species. One was the corresponding boroxy nitroxide, and the other was the cyclic nitroxide radical with structure 4 , whose formation takes place via an initial intramolecular hydrogen abstraction from an ortho tert -butyl substituent by the photoexcited nitro group. , …”
The radical-initiated reaction of amine-boranes and phosphine-boranes, LBH(3) (L = R(3)N, R(3)P) with aliphatic nitro compounds has been investigated in order to explore the possibility of reducing tertiary nitroalkanes to the corresponding hydrocarbons. In all the examined cases boroxy nitroxides, RN(O(*))OBLH(2), resulting from the addition of ligated boryl radicals, LBH(2)(*), to an oxygen atom of the nitro group were detected and characterized by EPR spectroscopy. This reaction occurs at room temperature with a rate constant of 1.5 x 10(7) M(-)(1) s(-)(1) for LBH(2)(*) = Me(3)NBH(2)(*) and RNO(2) = Me(3)CNO(2). The boroxy nitroxides from tertiary nitroalkanes decay by a fragmentation reaction occurring with cleavage of the nitrogen-oxygen bond, rather than of the carbon-nitrogen bond as would be required for the reduction to the corresponding alkane to take place. The Arrhenius parameters for this fragmentation have been determined in few cases.
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