Thermolysis of α-hydroperoxyalkyl diazenes. Spin trapping of radical intermediates and spin trapping kinetics

1991 ◽  
Vol 69 (9) ◽  
pp. 1398-1402 ◽  
Author(s):  
Lukose Mathew ◽  
Emmanuel Y. Osei-Twum ◽  
John Warkentin
Keyword(s):  
Free Radical ◽  
Rate Constant ◽  
Ring Opening ◽  
Spin Trapping ◽  
Ethyl Vinyl Ether ◽  
Trapping Rate ◽  
Ethyl Vinyl ◽  
Enol Ethers ◽  
Radical Intermediates ◽  
Radical Trapping

α-Hydroperoxyalkyl diazenes (Me2C(OOH)N=NR, 1, R = CH2CF3, CH2CH2OMe, CH(Me)2, CMe3, CH2Ph, Ph, CH2CH2OPh, and c-C3H5CD2) decompose in benzene, at 50 °C or less, by a mechanism involving free radical (R•) intermediates. The radicals were trapped with 1-methyl-4-nitroso-3,5-diphenylpyrazole, 2, to afford spin adducts (nitroxyls) that were observed by ESR spectroscopy. When the solvent was ethyl vinyl ether, radicals from 1 (R = CH2CH2OPh) were trapped by the solvent and the adduct radicals so formed were spin trapped by 2. These observations support free radical mechanisms for thermolysis of 1 and for the hydroxyalkylations that occur when 1 are decomposed in solutions containing enol ethers or other unsaturated substrates. The ring-opening of cyclopropylmethyl radicals (cpm) to 3-butenyl radicals was used to estimate the rate constant for radical trapping by 2. For cpm the rate constant is given by log kcpm = (10.7 ± 0.4) − (3.9 ± 0.5)/θ where θ = 2.3 RT kcal mol−1. At 25 °C, the spin trapping rate constant has the value 6.9 × 107 M−1 s−1. Key words: hydroperoxyalkyl diazenes; radicals, spin trapping; spin trapping, rate constant.

scholarly journals Dihydrofurans from α-diazoketones due to facile ring opening – cyclization of donor–acceptor cyclopropane intermediates

1996 ◽  
Vol 74 (12) ◽  
pp. 2401-2412 ◽  
Author(s):  
Elizabeth A. Lund ◽  
Isaac A. Kennedy ◽  
Alex G. Fallis
Keyword(s):  
Vinyl Ether ◽  
Ring Opening ◽  
Ethyl Vinyl Ether ◽  
Reaction Conditions ◽  
Substitution Pattern ◽  
Initial Product ◽  
Ethyl Vinyl ◽  
Enol Ethers ◽  
Donor Acceptor ◽  
Rhodium Acetate

A series of α-diazoketones, 8, 25, 28, 31, and 34, have been synthesized and their reaction with ethyl vinyl ether examined under various reaction conditions. In the presence of metal salts (Rh2(OAc)4, Pd(OAc)2, CuCl) the ethoxy-dihydrofurans 12, 37, 39, 41, and 43 are produced. Sensitized irradiation of the α-diazoketone 8 afforded the dihydrofuran 12 plus cyclobutanone 7, while direct photolysis of α-diazoketones 8, 25, 28, 31, and 34 gave the cyclobutanones 7, 38,40,42, and 44, respectively. A sample of the cyclopropylketone 45 was isolated from the rhodium(II) acetate mediated reaction of 34 and its facile rearrangement to dihydrofuran 43 demonstrated. Collectively, these results indicate that the initial product from the reaction of an α-diazoketone with an electron-rich alkene such as ethyl vinyl ether is a cyclopropylketone. The donnor–acceptor substitution pattern of this intermediate results in spontaneous rearrangement to a dihydrofuran. Thus a direct dipolar cycloaddition mechanism is not involved when α-diazoketones react with enol ethers under metal-mediated conditions. Instead, these reactions follow a cyclopropanation rearrangement or, more accurately, cyclopropanation – ring opening – cyciization pathway. Key words: diazoketone, rhodium acetate, dihydrofuran, cyclopropylketone, vinyl ether.

Initiation Processes in Copolymerization Studied by the Nitroxide Radical-Trapping Technique: Ethyl Vinyl Ether and Acrylonitrile

1997 ◽  
Vol 50 (1) ◽  
pp. 1 ◽  
Author(s):  
W. Ken Busfield ◽  
W. Ken Busfield ◽  
Ian D. Jenkins ◽  
Ian D. Jenkins ◽  
Michael J. Monteiro ◽  
...  
Keyword(s):  
Hydrogen Bonding ◽  
Vinyl Ether ◽  
Nitroxide Radical ◽  
Ethyl Vinyl Ether ◽  
Initiation Mechanism ◽  
Ethyl Vinyl ◽  
Radical Trapping ◽  
Selective Interaction ◽  
End Groups ◽  
Nitroxide Free Radical

The nitroxide free-radical trapping technique has been applied to an investigation of the initiation mechanism of the copolymerization of ethyl vinyl ether and acrylonitrile initiated by t-butoxyl radicals. In addition to a range of products normally produced from reactions with individual monomers, four new trapped products each involving both monomers have been observed. These arise because the strongly electron-accepting acrylonitrile reacts so fast with the strongly nucleophilic ethyl vinyl ether radical end groups that the reaction competes successfully with radical trapping. t-Butoxyl radicals react 3-6 times faster with ethyl vinyl ether than with acrylonitrile depending on solvent, illustrating the strong electrophilic nature of the t-butoxyl radicals. Reactions carried out in non-olefinic solvents show that polarity is not a major factor in the solvent e®ect. It is more likely to be due to selective interaction of one monomer with the radical end enhancing its electrophilic nature. A similar e®ect is caused by a hydrogen-bonding solvent.

Cyclopropanation of benzylidenemalononitrile with dialkoxycarbenes and free radical rearrangement of the cyclopropanes

2001 ◽  
Vol 79 (3) ◽  
pp. 312-318
Author(s):  
Nadine Merkley ◽  
Paul C Venneri ◽  
John Warkentin
Keyword(s):  
Free Radical ◽  
Double Bond ◽  
Rate Constant ◽  
Ring Opening ◽  
Radical Addition ◽  
Radical Pairs ◽  
Radical Chemistry ◽  
Radical Rearrangement ◽  
Benzyl Radicals

Thermolysis of 2-cinnamyloxy-2-methoxy-5,5-dimethyl-Δ3-1,3,4-oxadiazoline (1a) and the analogous 2-benzyloxy-2-methoxy compound (1b) at 110°C, in benzene containing benzylidenemalononitrile, afforded products of apparent regiospecific addition of methoxycarbonyl and cinnamyl (or benzyl) radicals to the double bond. When the thermolysis of 1a was run with added TEMPO, methoxycarbonyl and cinnamyl radicals were captured. Thermolysis of the 2,2-dibenzyloxy analogue (1c) in the presence of benzylidenemalononitrile gave an adduct that is formally the product of addition of benzyloxycarbonyl and benzyl radicals to the double bond. In this case, a radical addition mechanism could be ruled out, because the rate constant for decarboxylation of benzyloxycarbonyl radicals is very large. A mechanism that fits all of the results is predominant cyclopropanation of benzylidenemalononitrile by the dialkoxycarbenes derived from the oxadiazolines, in competition with fragmentation of the carbenes to radical pairs. The cyclopropanes so formed then undergo homolytic ring-opening to the appropriate diradicals. Subsequent β-scission of the diradicals to afford radical pairs, and coupling of those pairs, gives the final products. Thus, both carbene and radical chemistry are involved in the overall processes.Key words: cyclopropane, dialkoxycarbene, β-scission, oxadiazoline, radical.

Peroxyl radical trapping activity of anthocyanins and generation of free radical intermediates

2007 ◽  
Vol 41 (7) ◽  
pp. 854-859 ◽  
Author(s):  
Monica Rossetto ◽  
Paola Vanzani ◽  
Michele Lunelli ◽  
Marina Scarpa ◽  
Fulvio Mattivi ◽  
...  
Keyword(s):  
Free Radical ◽  
Peroxyl Radical ◽  
Radical Intermediates ◽  
Radical Trapping
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