Cyclopropenes oxidation

Analysis of the second-order rate constants for various methyl and phenyl substituted cyclopropenes shows effects similar to those observed with alkenes undergoing epoxidation, again in agreement with the intermediacy of the oxabicyclo[1.1.0]butane in cyclopropene oxidation 263 ... 

NMR, 3, 542 oxidation, 3, 546 phosphorescence, 3, 543 photoelectron spectra, 3, 542 photolysis, 3, 549 reactions, 3, 543-555 with alkenes, 3, 50 with alkynes, 3, 50 with IH-azepines, 3, 552 with azirines, 3, 554 with cyclobutadiene, 3, 551 with cyclopropenes, 3, 550 with dimethylbicyclopropenyl, 3, 551 with heterocyclic transition metal complexes, 7, 28 29... 

Cycloalkene Derivatives Cyclopropenes readily interact with nitrile oxides. Reactions of a broad series of 3,3-disubstituted cyclopropenes with 4-substituted benzonitrile, methoxycarbonyl- and cyanoformonitrile oxides (229) as well as with di(isopropoxy)phosphorylformonitrile oxide (230) give 2-oxa-3-azabicyclo[3.1.0]hexene derivatives 62. Stereoselectivity of the cycloaddition is governed by both steric and polar factors. In particular, steric factors are supposed to prevail for 3-methyl-3-phenylcyclopropene affording 62 [R1 =... 

In the addition of Me2CuLi reagents to electron-deficient acetylenes [85-88], DCD-type complexes have been identified by NMR [84, 89]. As shown below, an ynoate affords a vinylcopper intermediate, while an ynone instead affords an allenolate (Eq. 10.9). The origin of this diversity remains unclear. A related carbocupration mechanism has also been proposed for the reaction with allenylphosphme oxide [53]. Olefin carbocupration of dienes [90] and cyclopropenes [34, 36] is known, but these mechanisms also remain unclear. 

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. 

Another method used to prepare dialkyl-substituted diazomethanes involves the photolysis of 2-alkoxy-2,5-dihydro-1,3,4-oxadiazoles (209), which can be prepared by the oxidative cyclization of A(-acetyUiydrazones. The diazoalkanes are trapped in situ by cycloaddition with dimethyl acetylenedicarboxylate (54) (Scheme 8.49). The resulting pyrazoles 210 are converted into cyclopropenes 211 by continued irradiation. 

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

Furans (133) are obtained from acyltriafulvenes [(acylmethylene)cyclopropenes] (131) and azomethine-imines, -ylides or oxides (132) (75TL3919). The reaction involves the transfer of the group X from the azomethine dipole to the triafulvene and may proceed as shown in Scheme 28. I... 

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). 

A remarkably diastereoselective synthesis of a palladacycloheptane has been achieved via the oxidative cyclisation of the cyclopropene 113 by Pd2(dba)3 followed by treatment with 2,2-bipyridyl to provide the crystallographically characterised derivative 114 [Eq. (45)]. 

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>. 

The iridium cyclopropene complex 171 reacts with an additional equivalent of Ir(l) to open the strained ring, affording bimetallic iridacyclobutene complex 172 (Equation 89) <1994JA10032>. Labeling experiments were used to establish that the external Ir(l) reagent mediates the oxidative activation of the intact coordinated cyclopropene, with the original Ir(l)-cyclopropene coordination retained throughout the reaction. 

Cazes et al. reported the Pd-catalyzed intermolecular hydroamination of substituted allenes using aliphatic amines in the presence of triethylammonium iodide leading to allylic amines [19]. In a way similar to the Pd-catalyzed hydrocarbona-tion reactions we reported that the hydroamination of allenes [20], enynes [21], methylenecyclopropanes [22], and cyclopropene [10] proceeds most probably via oxidative addition of an N-H bond under neutral or acidic conditions to give allylic amines. The presence of benzoic acid as an additive promotes the Pd-medi-ated inter- and intramolecular hydroamination of internal alkynes [23]. Intramolecular hydroamination has attracted more attention in recent years, because of its importance in the synthesis of a variety of nitrogen-containing heterocycles found in many biologically important compounds. The metal-catalyzed intramolecular hydroamination/cyclization of aminoalkenes, aminodienes, aminoallenes, and aminoalkynes has been abundantly documented [23]. 

In the second, the use of diazopropane is avoided the alcohol (250) is converted to the pyrazole (251) by a two step sequence of reaction with hydrazine in acetic acid and then oxidation with manganese dioxide. Photolysis then leads cleanly to the cyclopropene, without any interference by cyclisation of an intermediate carbene to the alcohol group the product is hydrogenated directly, the conversion of (251) to (252) occurring in 94% yield. Elimination of the elements of water leads to m-chrysanthemic acid in good overall yield 175) ... 

Cyclopropene itself and the 3,3-dimethyl-derivative add to nitrile oxides or nitrile imines in good yield 251) ... 

In the case of the dimethyl-substituted cyclopropene (330), the initial product is (331) but further oxidation occurs more easily. The presence of large substituents at C-2 reduces the regioselectivity of the initial oxidation, (332) producing a 4 1 mixture of isomeric enones 264). 

---