Cyclo Propenes

Double bonds are accommodated by rings of all sizes The smallest cycloalkene cyclo propene was first synthesized m 1922 A cyclopropene nng is present m sterculic acid a substance derived from one of the components of the oil present m the seeds of a tree (Sterculia foelida) that grows m the Philippines and Indonesia... 

The irradiation of the thiophene in gas phase yields ethylene, allene, methyl-acetylene, carbon disulfide, and vinylacetylene. No Dewar thiophene or cyclo-propene derivatives were isolated (69CJC2965). The irradiation in liquid phase gave the Dewar thiophene which can be trapped as a Diels-Alder adduct with furan (85JA723). The Dewar thiophene and cyclopropene-3-thiocarbaldehyde can be obtained by irradiation in argon matrices at 10 K (86JA1691). 

Diazomalonic esters serve as intermediates for the synthesis of a wide variety of compounds including cyclopropanes, cyclo-propenes, cycloheptatrienes, sulfur ylides, lactones, and substituted malonates. ... 

Accompanying this trend is a monotonic decrease in the internal angle at the adjacent position (P), from 119.6° in tetralin to 113.0° in cyclopropabenzene (Figure 16). The bond lengths at the ring fusion are demonstrably shorter in cyclopropabenzenes. This has caused some discussion of bond fixation in cyclopropabenzene however, parallel effects are seen for cyclopropane and cyclo-propene among cycloalkanes and cycloalkenes, respectively. Thus, the shorter bonds in cyclopropabenzene are expected or rationalized by simple rehybridization independent of any bond fixation. 

The carbocupration of (5./ ,77 )-l-cthyl-5,7-dimcthyl-4,8-dioxaspiro[2.5]oct-l-ene, a cyclo-propene carrying an auxiliary group, gave a 96 4-mixture of two diastereomeric addition products for which the relative configuration at the newly formed stereogenic centers had to be determined (see p 479)123. 

The addition to a carbon-carbon triple bond results in the formation of cyclo-propene products, and with diazoacetates the catalyst of choice for intermolecular addition is the dirhodium(II) carboxamidate 13 (e.g., Eq. 26). The reactions are general, except for phenylacetylene whose cyclopropene product undergoes [2 + 2]-cycloaddition, and selectivities are high. However, high selectivities have not been reported for reactions with allenes. 

Acetoxy-(trifluoro-1 -cyclopro-penyl)-methylene]-3-methoxy-E10b2, 104 ( —HF - Cyclo-propene)... 

Sections VILA and VII.B cover cyclic derivatives which contain the cyclopropane or cyclo-propene unit in a fused ring system according to scheme 9 or 10. Structures of many simple molecules of this class were determined long ago. Some of them will be mentioned here further structural data and references are given in previous reviews. Bridged derivatives of bicyclo[ 1.1.0]butane are discussed in Sections VII. C. 1 and VII. C.2. 

The application of such complexes in the cyclopropanation of alkenes was investigated, mostly in benchmark reactions between styrene and diazo esters or aromatic diazomethane derivatives (Scheme 9.19) to form the cis- and trans-cyclo-propene derivatives 23 and 24. 

As might be expected from their inherent strain, many cyclopropenes undergo rearrangement, dimerisation or even polymerisation under relatively mild conditions. The conditions required for reaction are, however, very variable and some cyclo-propenes, such as 3,3-dimethylcyclopropene, are stable at relatively high temperature (150 °C in this case). Three main reactions are described below — the ene-reaction, [2+ 2]-dimerisation, and rearrangement to vinylcarbenes. 

The reaction of the dichlorides (213, n = 3-7) with potassium t-butoxide in DMSO leads to the elimination of two molecules of HC1 and the formation of (214, n = 3-7) respectively. In each case the reaction can be explained by a sequence of elimination to a chlorocyclopropene, prototropic shifts, then a second elimination to a cyclo-propene followed by prototropic shifts. Thermolysis of (214, n = 3 or 4) leads to (215, n = 3 or 4), although the mechanism of this reaction is rather uncertain in contrast the larger ring species (214, n = 6 or 7) rearrange to a methylenecyclo-pentene, (2 1 6) 148,149). In the case of (213, n = 2), the analogous elimination product (214, n = 2) is not observed, presumably rearranging under the reaction conditions to the observed product (217)149). 

Acetylenes and cyclopropenes (37) are related to each other in the same formal way as olefins and cyclopropanes are. Recall that cyclopropanation of ethylene is almost slightly endothermic and the endothermicity was asserted to increase by some (3 2) kJ moT per alkyl group. Cyclopropanation of acetylene to form cyclopropene (37, X = X = H) is endothermic by (48.9 2.6) kJ mol. Cyclopropanation of propyne (monomethy-lacetylene) to form 1-methylcyclopropene (37, X = Me, X = H) has an increased endothermicity of (58.7 1.4) kJ mol. By contrast, the cyclopropanation of 2-butyne (dimethy-lacetylene) using a derived value for the enthalpy of formation of 1,2-dimethylcyclopropene (37, X = X = Me) has an accompanying endothermicity of only 41 kJ moT. We suspect that the last value is in error and so suggest remeasurement of the enthalpy of formation of dimethylcyclopropene as well as measuring the enthalpy of formation of other cyclo-propenes. ... 

Another mechanism for the 2n -f 27t ) photocycloaddition of alkenes via electron transfer is the reaction that proceeds via a triplet state which is produced by a back-electron transfer from a radical anion of the electron acceptor to a radical cation of the electron donor. The triplet state alkenes generated by this way can undergo the cyclodimerization (Scheme 22). Farid showed that the DCA-sensitized (2n 3- 2n) photocyclodimerization of 1,2-diphenylcyclo-propene-3-carboxylate occurs via the triplet state of the cyclo-propene in acetonitrile [84]. In this photoreaction, two types of the (An -(- 27t) photocycloaddition reactions take place between DCA and the cyclopropene depending upon solvents. One type of the cycloadduct is produced in benzene via exciplex and the other type of the photocycloadduct is produced in... 

Cyclopropan l-Chlor-l-(trichior-ethenyl)-2-(trimethylsilyl-me-thyl)- E17a, 587 f. (Cl4 —cyclo-propen/A + En) E19b, 742 (Cl4-cyclopropen/A + En)... 

Cyclopropyne has a dipole moment in the same direction as cyclo-propene but the magnitude of the moment is substantially larger... 

Triola G, Fabrias G, Casas J, Llebaria A. Synthesis of cyclo-propene analogues of ceramide and their effect on dihydroce- 161. ramide desaturase. J. Org. Chem. 2003 68 9924-9932. 

Klett and Johnson have demonstrated the interrelationship between the photoreactions of allenes (93) and cyclopropenes. Their results indicate that irradiation of (93) leads to the carbenes (94) and (95) such carbenes have been proposed as the key intermediates in the photoreactions of cyclopropenes. Thus the irradiation of (93a) yields the indene (96) as the primary photo-pnoduct via the carbene (94) produced by phenyl migration. With aUene (93b) irradiation affords the carbene (95), by hydrogen migration, which then reacts to form photoproducts (97)—(99). For comparison, the irradiation of cyclo-propene (100) was carried out it yielded the allene (93b) and the two indenes (97) and (99). 

In this 3-phcnylalkyne system the triple bond becomes incorporated into a cyclo-propene ring, whereas from the enynes 10 and 11 there is no sign of a vinylcyclo-propene product. The preference for reaction to take place at the alkene unit rather than at the alkyne unit in the photochemistry of enynes is seen again in the photochemistry of enepoly-yne chlorides, where only cyclopropyl chlorides are produced (equation 18). This preference is probably a reflection of the lower strain energy in a cyclopropane than in a cyclopropene ring. 

These compounds upon irradiation in the (n, n ) region give rise to cyclo-propenes via carbenoid decomposition of a vinyl diazoalkane intermediate , viz. 

Allen, F. H. The geometry of small rings — IV Molecular geometry of cyclo-propene and its derivatives. Tetrahedron 38, 645-655 (1982). 

When the dichlorocarbene adduct 231 of 9-methoxyphenanthrene is reacted with 2 equivalents of (-BuOK in tetrahydrofuran in the presence of a crown ether, the phenanthro[9,10-b]furan 232 is obtained in 80% yield. The intermediate methoxy-substituted chlorocarbene is derived from cyclo-propene. The key step consists in an intramolecular carbene insertion into the C—H bond of the methoxy group (84C79). 

Franck-Neumann et al. 61) have succeeded in preparing m-chrysanthemic acid methyl ester (8) by selective catalytic hydrogenation of the corresponding cyclo-propene derivative 7 (Eq. 7). 

Other examples of C-C bond cleavage of cyclopropane derivatives like cyclo-propene, benzocyclopropane, bicyclo [1.1.0] butane, bicyclo [2.1.0] pentane, meth-ylenecyclopropane, and vinylcyclopropane have been reviewed comprehensively [19-23]. 

Diazoalkanes undergo facile cycloaddition to a range of cyclopropenecarboxylates and car-bonitriles. In the case of cyclopropene-l-carboxylates, the 2,3-diazabicyclo[3.1.0]hex-2-ene produced normally has the ester substituent at Cl, although a bulky 2-substituent on the cyclo-propene can lead to mixtures of regioisomers being produced (Table 18). 

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