Cyclohexenes allylic strain

Novel results were reported for allylic bromination. In radical bromination of cyclohexene in CCI4 under light the selectivity of substitution over addition was shown to be controlled by bromine concentration.304 Substitution via the corresponding allyl radical, while relatively slow, is irreversible and fast enough to maintain the concentration of bromine at a sufficiently low level to prevent significant addition. The reaction of two strained alkenes, fZ)-1,2-dimethyl-1,2-di-ferf-butylethylene and the -isomer (14), leads to the corresponding bromosubstituted product, instead of addition 305... 

Step a The predominant cyclohexene conformer places (i) the /-Bu equatorial and (ii) the allylic-Me pseudoaxial to minimize A1,3 strain, syn-Addition of the H-B bond at the least, after oxidation, to B. 

Step a. 5y -Addition of the H-B bond at the sterically less hindered face. To minimize A strain, the favored cyclohexene conformer has the r-Bu group equatorial and the allylic-Me group pseudoaxial. 

Palladium-based catalysts also bring about cyclopropanations in high-yield. With palladium acetate/CHjNj, styrene , unactivated terminal olefins strained olefins , 1,3-dienesan enamine , as well as a,3-unsaturated carbonyl compounds have been cyclopropanated (Table 1). Contrary to an earlier report, the reaction also works well with cyclohexene if the conditions are chosen appropriately it seems that the notniyst is rapidly deactivated in the presence of this olefin >. Trisubstituted a,p-unsaturated carbonyl compounds were found to be unreactive, and the same is true for the double bonds in diethyl fumarate, maleic anhydride, coumarin and 1,3-dimethyluracil. Whereas the latter two were totally unreactive, [3-1-2] cycloaddition of diazomethane gave pyrazolines in the former two cases. The last entry of Table 1 shows that an allyl alcohol function can still be cyclopropanated, but methylene insertion into the O—H bond is a competing process. 

Dramatically differing effects of phase-transfer catalysts on the cyclopropanation of cw,trans,trans-cyclododecatriene (61) and a series of dienes have been reported. Addition of dichlorocarbene to (61) results in tris-cyclopropanation when cetyltri-methylammonium bromide (i) is employed, whereas with benzyl-P-hydroxyethyl-dimethylammonium ion (ii) as catalyst only monocyclopropanation (of the more strained bond) is observed (Scheme 7). From the extensive study it may be concluded that, for dichlorocarbene addition, the P-hydroxyethyl catalyst restricts potential polycyclopropanation to monocyclopropanation at the most highly substituted (or strained) double bond. With dibromocarbene a different situation results. Catalyst (i) does not effect the addition of dibromocarbene to styrene, cyclohexene, or allyl bromide while catalyst (ii), with the P-hydroxyethyl function, effects dibromocyclo-propanation, in yields of up to 80 %. 

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