Cyclopropenes isomerization

With unsymmetrically substituted cyclopropenes, isomeric cycloadducts (177) and (178) and pyridazines are formed (Scheme 99) (80LA590). 

In principle, the vinylcarbene-to-cyclopropene isomerization is reversible. While this has not been reported for 167 (where ring-opening could produce not only alkenyl(sulfonyl)carbene 166, but also the isomeric l-alkenyl(silyl)carbene), it was found that 1-trimethylsilylcyclopropenone acetal 168 (equation 49) by thermal ring-opening yields the (trimethylsilyl)vinylcarbene 169 besides traces of the isomeric vinylcarbene 170. Both carbenes are obviously nucleophilic since they are able to cyclopropanate the... 

Similarly, 4//-azepinc-2-carboxylates 16, obtained by cycloaddition of 1,2,4-triazines with cyclopropenes (see Section 3.1.1.1.2.), isomerize in practicable yields to 3//-azepines 17 in refluxing pyridine,84-85 or with sodium methoxide in refluxing methanol.28-83... 

The Diels-Alder reaction of cyclopropenes with 1,2,4,5-tetrazines (see Vol.E9c, p 904), a reaction with inverse electron demand, gives isolable 3,4-diazanorcaradienes 1, which are converted into 4H-1,2-diazepines 2 on heating. The transformation involves a symmetry allowed [1,5] sigmatropic shift of one of the bonds of the three-membered ring, a so-called walk rearrangement , followed by valence isomerization.106,107... 

Alkinyloxy)diazoacetic esters 11 give rise to product mixtures that could be separated only partially. The isolated products result from a tandem intramolecular cyclopropenation/cyclopropene —> vinylcarbene isomerization (12, 14) and from a twofold intermolecular (3+2)-cycloaddition of the intact diazo compound (13). 

Methylene cyclopropene (5), the simplest triafulvene, is predicted to be of very low stability. From different MO calculations5 it has been estimated to possess only minor resonance stabilization ranging to 1 j3. Its high index of free valency4 at the exocyclic carbon atom causes an extreme tendency to polymerize, a process favored additionally by release of strain. Thus it is not surprising that only one attempt to prepare this elusive C4H4-hydrocarbon can be found in the literature. Photolysis and flash vacuum pyrolysis of cis-l-methylene-cyclopropene-2,3-dicarboxylic anhydride (58), however, did not yield methylene cyclopropene, but only vinyl acetylene as its (formal) product of isomerization in addition to small amounts of acetylene and methyl acetylene65 ... 

Analogous reactivity was observed by Jones303 with methylene cyclopropene 554, which on treatment with diazomethane yielded the pyrazole 556, thought to arise from thermal isomerization of pyrazolenine 555 formed initially ... 

The ratio of isomeric ethers is strongly affected by polar substituents which induce an asymmetric distribution of charge in allylic cations. Photolysis of methyl 2-diazo-4-phenyl-3-butenoate (20) in methanol produced 24 in large excess over 25 as the positive charge of 22 resides mainly a to phenyl (Scheme 8).19 As would be expected, proton transfer to the electron-poor carbene 21 proceeds reluctantly intramolecular addition with formation of the cyclopropene... 

Welter, W., Hartmann, A., and Regitz, M., Isomerization reaction of phospho-ryl-vinyl-carbenes to phosphorylated cyclopropenes, acetylenes, indenes, and 1,3-butadienes, Chem. Ber., Ill, 3068, 1978. 

In another study the kinetics and mechanism of an unprecedented T/2-vinyl isomerization of a highly fluorinated tungsten(II) metalla-cyclopropene complex was studied (92). Photolysis of a tungsten(II) tetrafluoroaryl metallacycle 1 and perfluoro-2-butyne results in the formation of the kinetic rf -vinyl complex 2 in which the fluoride is trans to the inserted acetylene and cis to both carbonyl ligands. Upon heating 2 is converted to the thermodynamic rf -vinyl complex 3 in which the fluoride ligand is now cis to the inserted alkyne and trans to one CO and cis to the second CO ligand as shown in Scheme 1. 

The photolysis of the furan derivatives 78 yielded the butadienals 79 as the main products [123], Further isomerizations leading to allenic esters used the radiation of a cyclopropene-1 -carboxylic acid ester [124] or applied flash vacuum pyrolysis to 3 -ethoxy cyclobut- 2-en-l-one[125]. 

For the thermal isomerization of 2 to 1 [6] on the basis of kinetic data, it was suggested that at least 50% of 1 is formed via cyclopropene 3 as an intermediate [7] rather than a direct 1,3-H-shift as proposed before [8, 9]. Later theoretical calculations provided further support for that suggestion [10], A number of other publications, some of them with basic catalysts such as sodium hydroxide, deal with this reaction [11-17]. Furthermore, 2 can be isomerized to 1 by a stoichiometric reacion with [RuHCl(CO)(PPh3)3] and subsequent treatment with NaS2CNMe2 [18]. The 1,1-bisdeuterated 2 has also been investigated [19]. 

In 1987, Nitta reported the formation of an unexpected vinylketene complex from the reaction of an azido-substituted cyclopropene with diiron nonacarbonyl.104 They had previously investigated the chemical behavior of the complexed nitrene intermediates that result from the reaction of organic azides and iron carbonyls113 and were interested in replicating the thermal isomerization of 3-azido-l,2,3-triphenylcyclopropene (163) into 4,5,6-triphenyl-l,2,3-triazine using a metal carbonyl-promoted re-... 

Additional methods for preparing non-heteroatom-substituted carbene complexes include nucleophilic or electrophilic additions to carbyne complexes (Section 3.1.4), electrophilic additions to alkenyl or alkynyl complexes (Section 3.1.5), and the isomerization of alkyne or cyclopropene complexes (Section 3.1.6). 

Similar to alkynes, cyclopropenes also readily form transition metal 7i-complexes which can isomerize to carbene complexes thermally, photochemically, or che-... 

Fig. 3.29. Isomerization of cyclopropene complexes into vinylcarbene complexes [5831.

Interestingly, copper(I) salts also catalyze the cyclopropene-vinylcarbene isomerization [681]. In this case the transient carbene complexes again show electrophilic behavior, behavior similar to that of the complexes formed from copper(I) salts and diazoalkanes or sulfonium ylides. 

As discussed in Section 3.1.6, cyclopropenes can react with rhodium complexes [38,585,587-589,1061,1063] or other transition metal derivatives to yield vinylcarbene complexes (see Section 3.1.6). This reaction will proceed particularly smoothly with strained cyclopropenes, because these can already isomerize thermally to vinylcarbenes [1064]. Hence the formation of vinylcarbene complexes from alkynes can proceed by initial cyclopropanation, followed by reaction of the resulting cyclopropene with the complex L,M. 

The formation of the cyclopropene derivative Z-36 is very similar to the isomerization of p-phenylenebisnitrene (13) to 14 (Scheme 2) and both processes are likely to be photochemical, rather than thermal. Further irradiation of Z-36 causes its isomerization to another compound, which according to DFT simulations, is compatible with the structure of its E-isomer (E-36, Scheme 7). 

When subjected to strong bases, gem-dihalocyclopropanes undergo dehydro-halogenations, and cyclopropenes are formed. These are generally unstable under the reaction conditions and participate in further transformations. The most common of these processes is the isomerization of the newly formed double bond from the endo- to the exo-orientation, followed by a second dehydrohalogenation step. The methylenecyclopropenes thus generated are still not stable, and subsequently tend to rearrange to less strained systems. 

Section I1,D,1 The kinetics have been studied for competitive formation of 3//-pyrazoles and cyclopropenes thermally from the isomeric vinyldiazo compounds 2,3-dimethyl-l-phenyl-l-diazo-2-butene and 4-methyI-3-phenyl-2-diazo-3-pentene. The higher (12 kJ/mol) ground state energy of the latter accounts almost entirely for its larger (x68) rate of cyclization to a 3H-pyrazole, relative to its isomer.172... 

Isomerization of cyclopropenes. Irradiation of the cyclopropcne 1 results in the alkcncs 2 and 3, formed by cleavage of the h bond. Exposure of 1 to silver ion results in two different alkenes, 4 and 5, formed by cleavage of the a bond. 

Tetrazines (39) react with cyclopropenes (149) via the unisolated tricyclic intermediates (150) to form the diazanorcaradienes (151) by loss of nitrogen. Three reactions of the diazanorcaradienes have been observed (1) opening of the ring to give the 5H-diazepines (152) (2) isomerization by the walk mechanism to the diazanorcaradienes (153), which may be transformed into the 4//-diazepines (154) (3) reaction with another molecule... 

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