Cyclohexenes, bicyclic, alkenes

Similar isomerization occurs in the presence of silica-alumina-thoria.104 As it might be expected, this reaction is similar to the isomerization of cyclohexane to methylcyclopentane. Both processes involve the same intermediate cyclohexyl car-bocation, which is formed, however, in different reactions. It may be formed from cyclohexane by hydride ion transfer, or by protonation of cyclohexene. Bicyclic alkenes undergo complex interconversions via carbocations over acidic catalysts.105... 

Although it is generally accepted that the exo approach to norbomene is favored over the endo, the magnitude of the AAE is rarely measurable experimentally unless some fraction of the endo product can be detected. The exo approach to norbomene is favored over the endo orientation by nearly 3 kcalmol for both PFA and DMDO, while the exo TS for benzonorbomadiene is favored by nearly 5 kcalmol (Figure 27). The preferred exo approach does not appear to result from steric interactions. It is of particular interest that the AE for norbomene is only 0.9 kcalmol greater than that for cyclohexene despite the SE = 19.2 kcalmol" for the strained bicyclic alkene. Thus, in the absence of twist-strain we observe little rate enhancement due to strain energy. 

This subject has recently been reviewed.647 Several additional papers have appeared on the catalytic oxidation of alkenes by 02 in the presence of PdCl(MeCN)2N02(148).64S Terminal alkenes and trans- cyclooctene yield the corresponding ketones, cyclopentene and cyclohexene the corresponding allyl alcohol, and bicyclic alkenes the corresponding epoxide. Heterometallacy-clopentanes such as (152) have been isolated from the reaction of (148) with norbornene (dicy-clopentadiene), and characterized by X-ray crystallography.6486 Glycol monoacetates were obtained from the reaction of (148) with terminal alkenes in acetic acid.649... 

Unfortunately, these reactions tend to afford mixtures. Similar mixtures are observed when using 2-cy-clohexen-l-ol,315 3-cyclohexen-l-ol,31s,32s or the corresponding ethers or esters.326-327 The regio- and stereo-selectivity of the hydration of bicyclic alkenes has also been carefully examined (equation 2 1 3).328-330... 

Reactions of the hafnium analog of (6), [Cp2HfHCl], differ little from those of the zirconium com-plex. It was suggested that the hafnium reagent is somewhat more sensitive to steric inhibition, as cyclohexene and related cyclic alkenes failed to react but this might well be due to the specific preparation of the solid (see above), rather than to any inherent molecular differences. There are some changes in the stereochemical outcome of hydrometallation of bicyclic alkenes (see Section 3.9.3.3.S), but it is far from clear that hafnium will provide any features sufficient to overcome the availability and cost advantages of zirconium. 

A major difficulty with the Diels-Alder reaction is its sensitivity to sterical hindrance. Tri- and tetrasubstituted olefins or dienes with bulky substituents at the terminal carbons react only very slowly. Therefore bicyclic compounds with polar reactions are more suitable for such target molecules, e.g. steroids. There exist, however, several exceptions, e. g. a reaction of a tetrasubstituted alkene with a 1,1-disubstituted diene to produce a cyclohexene intermediate containing three contiguous quaternary carbon atoms (S. Danishefsky, 1979). This reaction was assisted by large polarity differences between the electron rich diene and the electron deficient ene component. 

The formation of cyclopropane derivatives by photolysis of diazoalkanes in the presence of alkenes is believed to occur by photolytic decomposition of the diazoalkane to yield the carbene, followed by addition of this carbene to the alkene. Cycloaddition of this type has been reported in furan, dihydrofuran, and thiophene.198 Thus, photolysis of ethyl diazoacetate in thiophene yields the bicyclic sulfur heterocycle (215). Alternatively, photolysis of 3-diazo-l-methyl-oxindole (216) in cyclohexene leads to the formation of two isomers which are thought to have the spirocyclopropyl structure (217) photolysis in ethanol yields 3-ethoxy-1-methyloxindole.194... 

The other approach to 2-thioxo-l,2-thiaphospholanes 108 and 110 is based on the direct reaction of an unsaturated hydrocarbon, phosphorus pentasulfide and a thiophosphoric acid halide under drastic conditions (autoclave, 150-200 °C, 10-18 h) [142, 143], When cyclic alkenes (e.g. cyclohexene) are used as the starting substrates, bicyclic compounds are formed with one of the rings representing 1,2-thiaphos-pholane (Scheme 64). These reaction products are used as antioxidant additives for motor oils, polymers and agrochemicals. 

A variety of cyclopropane derivatives have been prepared by generating dichlorocarbene in the presence of alkenes. Cyclohexene, for example, reacts with dichlorocarbene generated by treating chloroform with potassium fert-butoxide to give a bicyclic product ... 

The reaction of bromine with cyclohexene involves anti addition, which generates, initially, the diaxial conformation of the addition product that then undergoes a ring flip to the diequatorial conformation of rn r-l,2-dibromo-cyclohexane. However, when the unsaturated bicyclic compound I is the alkene, instead of cyclohexene, the addition product is exclusively in a stable diaxial conformation. Account for this. (You may find it helpful to build handheld molecular models.)... 

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