Ring strain reactions

Angle strain is the main source of strain in epoxides but torsional strain that re suits from the eclipsing of bonds on adjacent carbons is also present Both kinds of strain are relieved when a ring opening reaction occurs... 

Ethylene oxide is a highly reactive compound, and so is used iudustriaHy as an iatermediate for many chemical products. The three-membered ring is opened iu most of its reactions. These reactions are very exothermic because of the tremendous ring strain iu ethylene oxide, which has been calculated (39). Reviews of ethylene oxide reactions are given iu References 40 and 41. 

Azabicyclo[2.2.0]hexa-2,5-diene, pentakis-(pentafluoroethyl)-synthesis, 2, 304 2-Azabicyclop.2.0]hexadiene reactivity, 7, 360 thermal isomerization, 7, 360 2-Azabicyclo[2.2.0]hexa-2,5-diene synthesis, 2, 304 1 -Azabicyclo[3.2.0]hexadiene synthesis, 7, 361 1 - Azabicyclo[2.2.0]hexane reactions, 7, 344 ring strain... 

Selenophene, 2-methylmercapto-conformation, 4, 944 Selenophene, 2-nitro-mercuration, 4, 946 Selenophene, 2-phenyl-irradiation, 4, 42, 946 mass spectra, 4, 942 Selenophene, 3-phenyl-mass spectra, 4, 942 Selenophene, tetrachloro-applications, 4, 971 reactions, 4, 955 synthesis, 4, 963 Selenophene, tetrahydro-conformation, 4, 34, 944 IR spectra, 4, 942 mass spectra, 4, 24, 943 molecular structure, 4, 938 NMR, 4, 10, 13 reactions, 4, 88, 958 ring strain, 4, 28 synthesis, 4, 118, 962, 968 Selenophene, tetraphenyl-synthesis, 4, 118, 962, 964 Selenophene, 2-thienyl-... 

Intramolecular cycloadditions of substrates with a cleavable tether have also been realized. Thus esters (37a-37d) provided the structurally interesting tricyclic lactones (38-43). It is interesting to note that the cyclododecenyl system (w = 7) proceeded at room temperature whereas all others required refluxing dioxane. In each case, the stereoselectivity with respect to the tether was excellent. As expected, the cyclohexenyl (n=l) and cycloheptenyl (n = 2) gave the syn adducts (38) and (39) almost exclusively. On the other hand, the cyclooctenyl (n = 3) and cyclododecenyl (n = 7) systems favored the anti adducts (41) and (42) instead. The formation of the endocyclic isomer (39, n=l) in the cyclohexenyl case can be explained by the isomerization of the initial adduct (44), which can not cyclize due to ring-strain, to the other 7t-allyl-Pd intermediate (45) which then ring-closes to (39) (Scheme 2.13) [20]. While the yields may not be spectacular, it is still remarkable that these reactions proceeded as well as they did since the substrates do contain another allylic ester moiety which is known to undergo ionization in the presence of the same palladium catalyst. 

The retro-ene reaction also is of synthetic importance. While the application of high pressure facilitates the ene reaction, the retro-ene reaction is favored at higher temperatures. Furthermore small-ring strain can shift the equilibrium towards the side of the dienes. The vinylcyclopropane 11 rearranges by a synchronous process to the open-chain diene 12. Formally this process is the reverse of an intramolecular ene reaction ... 

The unreactivity of cyclohexene (Section II.A) may be explained by the fact that in this case the ring strain of the dimer is much higher than that of the monomer. The observation that cyclohexene can be a reaction product [Eq. (8)] supports the assumption that thermodynamic rather than kinetic limitations prevent cyclohexene from polymerizing. Calderon and Ofstead (24, 100) have observed that bicyc o-[2.2.2]2-octene can be polymerized via ring opening ... 

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