Stereoelectronic Effects and Stereoselectivity

According to the reaction scheme of the Schenck reaction one should expect four more products in the case of (+)-carvomenthene (19) and at least four more products in the case of (+)-limonene (16). The fact that with limonene no products were obtained which originate from an oxygen attack on the A8 double bond, is in agreement with the general rule that type II photosensitized oxygenation reactions occur much faster with tri- and tetrasubstituted ethylenes than with di- or monosubstituted ones (see p. 71). 

Compounds 16 and 19 each deliver the expected six alcohols after reduction of the primarily formed hydroperoxide mixtures as a result of an oxygen attack on the trisubstituted A1 double bonds of these molecules. The ratio of tertiary/secondary hydroperoxides (or alcohols) is about 44 56, as has also been found with 1-methylcyclohexene (30)13S while open-chain olefins such as trimethylethylene (S3), 1,1-dimethyl-2-ethylethylene (id), 2,6-dimethyl-2-octene (39), myrcene (42), / -citronellol (45), linalool (48), and l,l-dimethyl-2-benzylethylene (51) give ratios of tertiary/secondary hydroperoxides between 54 46 and 60 40.104-1 7 7 1 79 The slight deviations from 1 1 ratios in all these cases are probably due to stereochemical rather than electronic effects exerted by the olefins on the reaction with oxygen. 

As the quasi-axial hydrogens are in more favorable positions than are the corresponding quasi-equatorial (e ) hydrogens at C3 and C6, the formation of hydroperoxides corresponding to the alcohols 22,26 and 18,21 should be favored over those corresponding to 23,27 and 17,20, respectively. This is the case, as the product distribution 

The results so far discussed and those to be discussed are well in agreement with a concerted mechanism involving a six-membered transition state for the termination reaction of photosensitized hydroper- 

Structures 55a and 56a can be excluded on the basis that oxygen attack on a nonsterically hindered, unsymmetrical double bond should occur almost exclusively on the less-substituted carbon atom, thus giving rise to the formation of the most-stable alkyl radical or carbonium 

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