Oxidation of cyclopropenes

Oxidation of cyclopropenes by peroxy acids (67HCA1669, 69JA6534, 70JOC450) or photochemically 80JOC2334, 81H(15)1643> gives enones. The products and kinetics are... 

Although definitive evidence for the intermediacy of oxabicyclobutanes in the peracid oxidation of cyclopropenes is lacking, the production of conjugated carbonyl compounds from such reactions is most readily rationalized from the formation and spontaneous rearrangement of such species (equation Calculations suggest... 

Whilst 1-methylcyclopropene is inert towards singlet oxygen, the photosensitized oxidation of cyclopropenes 288 rapidly produces numerous products which include enones. This latter free radical oxidation is thought to provide 289 and 290 which afford products by secondary rearrangement singlet oxygen is not involved in these reactions. 

Oxidation of cyclopropenes by osmium(VIII) oxide and sodium periodate, by potassium permanganate or by ozone normally gives 1,3-diketones, although in some cases further reactions occur. Examples are the reaction of 7 with osmium(VIII)oxide " 8, " and with... 

Oxidation of cyclopropenes 12 by P-oxygen atoms gave a mixture of productsd" ... 

Enones are also the normal products of oxidation of cyclopropenes with 3-chloroperoxybenzoic acid. 

Second-order rate constants for the oxidation of cyclopropenes with 3-chloroperoxybenzoic acid in carbon tetrachloride at 0 "C were in the order 1,2-diphenyl- (3.4), 3-methyl-l, 2-diphenyl-(1.24), 3,3-dimethyl-l,2-diphenyl- (0.4), 1,2,3-triphenyleyelopropene (0.122). The rates are somewhat lower than those of the corresponding 3-substituted cyclopentenes. On this scale, the relative rates of reaction of 1,2-diphenylcyclobutene and cyclopentene are 3.3 and 38.8. " ... 

In an attempt to determine whether peracid oxidation of cyclopropenes to enones proceeds by ct- or ic-bond oxidation, Friedrich and Fiato compared the second-order rate constant for allylically substituted cyclopropenes and cyclopentenes. The results (Table 1) indicate rather small allylic substitution effects in the cyclopentenes, but larger rate reductions in the cyclopropenes. The magnitude of the effect is not in line with an epoxidation-type transition state and may be more consistent with o attack, but insufficient background data are available to allow firm conclusions from these results. A comparison of oxidation rates of 1,2-diphenyl-cyclopropene, -cyclobutene, and -cyclopentene yielded data which were much more consistent with oxabicyclobutane formation, as did an examination of substituent effects at the vinylic positions in cyclopropene. ... 

As presented in the example of ethylene oxide above, it is often beneficial to obtain the IR spectra of isotopomers of the system under study. The isotopomers also were useful in the interpretation of the IR spectra of cyclopropene. In Table 2 the observed and calculated (MP2/6-31G ) isotopic shifts for three of the isotopomers of cyclopropene are given. Comparison of the calculated shifts with those observed indicates that theory reproduces well experimental results. Such calculated shifts can be extremely useful in assigning the origins (symmetries) of the fundamental vibrational frequencies of the parent molecule. 

The stability of the metallocene complexes is strongly dependent on the nature of the cyclopropene substituents, and the reaction conditions. Thus, when equimolar amounts of 3,3-dimethylcyclopropene and Cp2Ti(PMe3)2 react at 0 °C, a 2 1 mixture of alkylidene and cyclopropene complexes is formed. However, when excess of cyclopropene is used, a dicyclopropyl titanacycle is exclusively formed by oxidative coupling reaction of the intermediate cyclopropene complex (equation 215)77. The analogous zirconium oxidative-coupling product is obtained upon reaction of 3,3-dimethylcyclopropene with Cp2(PMe3)Zr( j2-CH2=CHEt) (Section IV.B.2). 

Oxidative activation of cyclopropenes is much less frequently encountered. The reactions of various platinum(O) complexes with the electron-deficient methylenecyclopropene 170 affords platinacyclobutene complexes, as reported nearly 30 years ago <1978ICA19>. More recent investigation has established that in the presence of two or more equivalents of the metal, bicyclic diplatinum complexes can be generated (Scheme 40) <1996JBS75>. 

A number of Ni(0) complexes containing a cyclopropene ligand have been prepared and shown to catalyze the oxidative coupling of cyclopropene derivatives to compounds with a cyclobutane core (Scheme 21). A recent study of the kinetics of these reactions has led to the isolation... 

This type of fatty acid is usually found at low levels (around 1%) in plant oils that contain cyclopropene acids. Cyclopropene acids are characteristic for oils from the Malvalaceae, Stercu-liaceae, Bombaceae, Tiliaceae, and Sapicidaceae families with sterculic acid (9,10-methyleneoctadec-9-enoic acid) and mal-valic acid (8,9-methyleneheptadec-8-enoic acid) as two major compounds. Sterculic acid is more abundant (about 50% of the total fatty acids in Sterculiafoetida oil). 2-Hydroxysterculic acid may also be present in this oil as a possible intermediate in the biosynthesis of malvalic acid (through a-oxidation of sterculic acid). 

Pyrazolines can also be prepared by the oxidation of pyrazolidines. 2-Pyrazolines, which are readily obtained in the reaction of a,j -unsaturated ketones with hydrazine, also undergo nitrogen extrusion at elevated temperature usually in the presence of a basic catalyst. The reaction is believed to proceed via 1-pyrazolines. Treatment of 3,3,5-trialkyl-2-pyrazolines with lead tetraacetate followed by thermolysis affords cyclopropyl acetates ". Oxidation of certain 2-pyrazolines with manganese dioxide gives 3H-pyrazoles, which in turn produce cyclopropenes in the photolysis (equation 7). ... 

Cyclobutanols substituted by a leaving group at the 2-position are transformed into cyclopropanecarbaldehydes or cyclopropyl ketones upon treatment with base . The reaction of a 3,4-dichlorocyclobutene derivative with aqueous base to give a 2-cyclopropen-l-yl ketone probably proceeds via a 4-chlorocyclobut-2-en-l-ol (equation 104) 67 jjj oxidation of 1-methylcyclobutene with metallic salts in aqueous medium to... 

Dipolar cycloadducts 176 are available from the reactions of cyclopropenes with azomethine ylids, pyridinium ylids aryl nitrile oxides " ", alkyl nitrile... 

The cycloaddition of phosphorus-substituted carbonitrile A-oxides to a range of cyclopropenes was also successful, e.g. formation of 6, although for 1-halo- or 2-halocyclopropenes more complex reactions occurred and bicyclic products were not isolated. ... 

The carbocupration of cyclopropenes has been especially investigated with cyclopropenone ketals as reactants. The cuprio cyclopropanes formed can serve as a synthon for the cyclo-propanone enolate. " Achiral ketals with an unsubstituted cyclopropene double bond 15 undergo instantaneous reaction with lithium dimethylcuprate at — 78 C to give the methyl derivative 17 (R = Me) in 96% yield after quenching with methanol. Similar reactions, with a deuterium oxide or iodomethane quench, indicate that the carbocupration takes place in a cis fashion. 

The corresponding analog of cyclopropene 17 with methyl in place of hydrogen at C3 and 1,2-di- cr/-butyl-3-methyl-3-phenylcyclopropene do not react under these conditions, and a mechanism involving the formation and trapping of a cyclopropenylium ion by direct oxidation of the hydrocarbon, followed by fragmentation has been proposed. ... 

Although the dipolar addition of nitrile oxides to cyclopropenes normally gives 2-oxa-3-aza-bicyclo[3.1.0]hex-3-enes (Section 1.A.1.1.6.1.5.3.2.), the dichlorocyclopropene 1 (X = H) reacted at 0-20 C to give oxazines 2. ... 

The hydrogen at the tertiary position of cyclopropenes trisubstituted with bulky substituents, such as the /cri-butyl group, was abstracted as the hydride ion when the cyclopropene was treated with one equivalent of 3-chloroperoxybenzoic acid in dichloromethane or with chro-mium(VI) oxide in 80-90% aqueous acetic acid. °... 

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