Cycloalkenes cyclohexadiene

Cycloalkenes and cyclohexadienes. These unsaturated cyclic aliphatic compounds [49, p. 284] have one and two double bonds, respectively, in the ring. Eixamples are... 

Vanadium(V) fluoride converts polyfluoroaromatics into fluorinated cyclohexadienes and cyclohexenes, and reacts with polyfluorinated and polychlorinated alkenes,113 115 alkadienes, cycloalkenes and cyclodienes to give addition products of two fluorine atoms across C = C bonds.116117... 

The intermolecular enyne cross metathesis, and consecutive RCM, between a terminal alkyne and 1,5-hexadiene produces cyclohexadienes, by cascade CM-RCM reaction, and trienes, formed during the sole CM step. Studies of various parameters of the reaction conditions did not show any improvement of the ratio of desired cyclohexadiene product [25] (Scheme 12). The reaction with cyclopentene instead of hexadiene as the alkene leads to 2-substituted-l,3-cycloheptadienes [26]. After the first cyclopentene ROM, the enyne metathesis is favored rather than ROMP by an appropriate balance between cycloalkene ring strain and reactivity of the alkyne. 

Cyclic allyl residues enable the synthesis of cycloalkenes having a 1-hydroxyalkyl side chain. Cyclic Ti complexes, like (118) and (119) are readily obtained (Scheme 12) by treatment of (Ti -C5H5)2Tia (117) with i-butylmagnesium chloride and cyclohexadiene or cyclopentaidiene respectively, in TTIF at -40 C for 10 min. Such complexes are quite unstable and can be prepared only in situ. Propanal... 

The force-field model developed earlier for calculations of hydrocarbons has been applied to alkenes and cycloalkenes among the molecules considered were cyclohexene and cyclohexadiene. Calculations of the potential function for pseudorotation of cyclohexene have been carried out and contrary to earlier calculations of the same group these show that the boat form is the transition state in the pseudorotation from one chair form to another. This process is associated with a value of AH = 6.39 kcal mol" The value obtained bears comparison with two earlier cited studies. Thus Bucourt concluded that the boat form is ca. 7 kcal mol" less stable than the chair and that a monotonous energy profile connects the two, and Anet found from low-temperature n.m.r. studies that the barrier between chair forms is 5.3 kcal mol" ... 

But their cyclic analogs, the nonactivated cycloalkenes 2.174p,q forms tricyclic cyclohexadiene 2.177p,q with a low yield (Table 2.8, entry 4). 

This effect is attributed to the increased microenvironmental polarity around the sensitizer chro-mophore that stabilizes the exciplex or contact ion pair in nonpolar solvents. As a result of this effect, the stereochemical interaction between the sensitizer and the substrate is more intimate. Because significant enantioselectivities were only observed for dimer 44, an independent cyclodimerization pathway to 44 via an exciplex or contact ion pair of cyclohexadiene and the chiral sensitizer was suggested. Dimer 45 gave much lower ee values even at low temperatures, but the product chirality was inverted within the tested temperature range in favor of enantiomer ent-A5. Similar temperature switching of product chirality has been reported in the enantiodifferentiating photoisomerization of cycloalkenes and in the polar addition of alcohols to 1,1-diphenylalkenes. This effect has been rationalized by a non-zero differential activation entropy of the same sign as the differential activation enthalpy. 

---