Cyclopropenes methyl substituents

Cyclopropanes are now readily available and have become useful, through hydrogenolysis, for synthesis of compounds containing quaternary carbons, em-dialkyl, r-butyl, and angular-methyl substituents (779), compounds often available only with difficulty otherwise (.77,5i,55,750,756), Cyclopropanes can be formed in good yields by hydrogenation of cyclopropenes (26). 

A wide range of 3,3-disubstituted cyclopropenes, e.g. the dimethyl and diphenyl derivatives, as well as spiro[2.4]hept-l-ene or 6,6-dimethyl-4,8-dioxaspiro[2.5]oct-l-ene, can thus be reacted with a variety of mono- and disubstituted alkynes. Usually, the chemoselectivity of this cy-clocotrimerization reaction is remarkably high. The norcaradiene derivatives initially obtained are in equilibrium with the valence tautomeric cycloheptatrienes. The equilibrium ratio of the valence tautomers is strongly dependent on the position and kind of substituents, especially those in the 7-position of the newly formed norcaradienes and cycloheptatrienes. When one or two phenyl groups are present in this position, only norcaradiene products can be detected by and NMR spectroscopy at room temperature, whereas in the case of methyl substituents, both valence tautomers are formed in almost equal amounts. When cyclic alkynes, such as cyclooctyne, are employed in the reaction, only norcaradienes are formed regardless of the substituents present in the cyclopropene cosubstrate. ... 

The AMI estimated activation barrier for nitrogen elimination was 17.5 kcal/mol. This means that isolation of the cycloadduct between cyclopropene and 2,5-bis(trifluoromethyl)-l,3,4-oxadiazole might be very hard. If we explore other cycloadducts between cyclopropene and 2 and 5 disubstituted 1,3,4-oxadiazoles and their activation barriers for nitrogen elimination, we can see that transition state structures are very similar with differences in C-N distance for the carbomethoxy substituent of 0.536 A, and the methyl substituent of 0.605 A instead of trifluoromethyl substitution. These results proved that all the transition state structures were very similar. That was also true for computed activation barriers. With carbomethoxy as a substituent, the activation barrier was 14.7 kcal/mol and with methyl as substituent, 16.7 kcal/mol. All of these values suggested that isolation of the cycloadduct might not be possible and that a substituted pyran should be expected as the final cycloaddition product. 

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... 

The position of the alkyl substituent in the product indicates that cyclisation occurs with rearrangement of the double bond, ie., by 1,1-elimination and formal formation and cyclisation of a vinylcarbene. Although the overall yields are not always good, the reagents are readily available and large quantities of the simple alkylcyclopropenes can be produced. 1,2-Dimethylcyclopropene has been prepared in a similar process by treatment of methallyl chloride with two equivalents of phenyl lithium, followed by quenching with methyl iodide presumably, the initial reaction leads to 1-methylcyclopropene which is converted in situ to the 2-lithio-species 10). The elimination of HBr from brominated alkylidenemalonates also leads to cyclopropenes, though in low yield U) ... 

The addition of phenyllithium to cyclopropene occurred with 99% stereoselectivity to give cw-l-lithio-2-phenylcyclopropane, albeit in 3 /o yield. With 3-methylcyclopropene, the reaction was more efficient and, after protonation of the lithiocyclopropane, 2-methyl-1-phenyl-cyclopropane was obtained in 44.5% yield. The product is about 94% in the trans form thus attack of the organolithium occurs on the opposite face to that bearing the larger substituent at C3. When the cyclopropene is 3,3-disubstituted, e.g. 17, both faces are sufficiently hindered to slow the addition process and metalation of a vinylic hydrogen predominates. ... 

The addition of bromine to methyl 2,3-diphenylcycloprop-2-ene-l-carboxylate, in the dark or diffuse light, in chloroform or acetic acid solution, gave the tran.v-addition product like cyclopropene itself, whereas in carbon tetrachloride or chloroform and under irradiation, cis addition from the face opposite to the ester substituent predominated. ... 

In the case of the cyclopropene 5, bearing different substituents at Cl and C2, a single regioisomer of the ene product 6 was observed, in which hydrogen transfer has occurred to the carbon bearing the phenyl group, and C-C bond formation to that bearing the methyl. ... 

When phosphane-free nickel complexes, such as bis(cycloocta-l,5-diene)nickel(0) or te-tracarbonylnickel, are employed in the codimerization reaction of acrylic esters, the codimer arising from [2-1-1] addition to the electron-deficient double bond is the main product. The exo-isomer is the only product in these cyclopropanation reactions. This is opposite to the carbene and carbenoid addition reactions to alkenes catalyzed by copper complexes (see previous section) where the thermodynamically less favored e Jo-isomers are formed. This finding indicates that the reaction proceeds via organonickel intermediates rather than carbenoids or carbenes. The introduction of alkyl substituents in the /I-position of the electron-deficient alkenes favors isomerization and/or homo-cyclodimerization of the cyclopropenes. Thus, with methyl crotonate and 3,3-diphenylcyclopropene only 16% of the corresponding ethenylcyc-lopropane was obtained. Methyl 3,3-dimethylacrylate does not react at all with 3,3-dimethyl-cyclopropene, so that the methylester of tra 5-chrysanthemic acid cannot be prepared in this way. This reactivity pattern can be rationalized in terms of a different tendency of the alkenes to coordinate to nickel(O). This tendency decreases in the order un-, mono- < di-< tri- < tet-... 

The elimination of hydrogen chloride from an allylic chloride provides one of the most effective routes to simple alkylcyclopropenes, and to cyclopropene itself The choice of base can be critical, particularly when the product cyclopropene has a 2-alkyl substituent and may undergo a base-induced rearrangement to a methylenecyclopropane, e.g. compare formation of methyl-enecyclopropane and 3-methylcyclopropene. ... 

The biradicals formed upon thermolysis or photolysis of cydoproparenes bearing alkyl substituents at Cl rearrange via hydrogen shift to styrenes. Thus, 1-methoxy-l-methylbenzo-cyclopropene gave (l-methoxyvinyl)benzene (la) quantitatively upon standing at room temperature, and methyl 1-methylbenzocyclopropene-l-carboxylate gave a mixture of methyl 2-... 

Dehydrochlorination of a 2,2-disubstituted t-butyl l-chloro-3-phenylcyclo-propanecarboxylate results in the corresponding t-butyl cyclopropene-1-catboxylate when the substituents are methyl or phenyl, and this represents only the second synthesis of cyclopropene-1-carboxylic acid derivatives. Dehydrobromination of the analogous 1-bromocyclopropyl esters is unsuccessful. With t-butoxide in THF,... 

A number of approaches to tetrakis(trifluoromethyl)-pyrroles was developed using tetrakis(trifluoro-methyl)-Dewar-thiophene (183) [71]. The 1,3-dipolar cycloaddition with azides led to the tricyclic thiiranes 184. Subsequent desulfurization by treatment with PPha afforded the cyclobutenes 185 in good to quantitative yields. The result of thermolysis of 185 was strongly depended on the substituent on the amine nitrogen. Pyrroles 188 were formed in high yields (cases a and d), while only cyclopropene 189, or a mixture of 188 and 189 (cases b and c) were isolated. Pyrrole 188a was also synthesized by the reaction of 183 with aniline in 19 % yield [72]. 

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