Cyclohexadiene isomers

Electrocyclic reactions of 1,3,5-trienes lead to 1,3-cyclohexadienes. These ring closures also exhibit a high degree of stereospecificity. The ring closure is normally the favored reaction in this case, because the cyclic compound, which has six a bonds and two IT bonds, is thermodynamically more stable than the triene, which has five a and three ir bonds. The stereospecificity is illustrated with octatrienes 3 and 4. ,Z, -2,4,6-Octatriene (3) cyclizes only to cw-5,6-dimethyl-l,3-cyclohexadiene, whereas the , Z,Z-2,4,6-octa-triene (4) leads exclusively to the trans cyclohexadiene isomer. A point of particular importance regarding the stereochemistry of this reaction is that the groups at the termini of the triene system rotate in the opposite sense during the cyclization process. This mode... 

Figure 30.3 Electrocyclic interconversions of 2,4,6-octa-triene isomers and 5,6-dimethyi-1,3-cyclohexadiene isomers.

We have also observed competition between products resulting from C-C and C-H bond activation in reactions of Y with propene,138 propyne,143 2-butyric,143 four butene isomers,138 acetaldehyde,128 acetone,128 ketene,144 and two cyclohexadiene isomers,145 as well as for Zr, Nb, Mo, and Mo with 2-butyne.143 In this chapter, we use the term C-C activation to describe any reaction leading to C-C bond fission in which the hydrocarbon reactant is broken into two smaller hydrocarbon products, with one hydrocarbon bound to the metal. It is important to note, however, that C-C activation does not necessarily require true C-C insertion. As will be shown in this chapter, the reaction of Y, the simplest second-row transition metal atom, with propene leads to formation of YCH2 +C2H4. The mechanism involves addition to the C=C bond followed by H atom migration and C-C bond fission, rather than by true C-C insertion. 

Diene)tricarbonyliron complexes have found use as synthons for the preparation of functionalized dienes. Substituted 4-vinylcyclohexene derivatives are isomerized by pen-tacarbonyliron into a mixture of conjugated cyclohexadiene tricarbonyl iron complexes . When the 4-vinyl cyclohexene 90 was refluxed with 1.2 equivalents of fclCOjs in di-n-butyl ether, a 3 1 mixture of cyclohexadiene isomers 91 and 92 was acquired in 75% overall yield (equation 48). 

By contrast, the cyclohexadiene isomers in reaction (7.11) undergo ring opening with rotations in opposite directions as shown. These reactions are termed disrotatory. 

In contrast, ring opening of the cyclohexadiene isomers 16 and 18 to give 17 and 19 are both disrotatory. For this reaction, with a Htickel transition state involved, 4 + 2 electrons is the favoured number, and for this number of electrons the conrotatory mode which would give a Mobius transition state is not favoured. 

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