Camphor, hydrosilylation

Although there are now several catalysts useful for hydrogenation of saturated carbonyl compounds to alcohols (see Section XII), an alternative approach has involved initial hydrosilylation (Chapter 9 in this volume) followed by acid hydrolysis [Eq. (41)]. The area first developed using principally the RhCl(PPh3)3 catalyst (207-210), and has since proved particularly useful in asymmetric syntheses (see Section III,A,4). Besides simple aliphatic and aromatic aldehydes and ketones, the ter-pene-ketones camphor and menthone were stereoselectively reduced to mainly the less stable alcohols e.g., camphor gave 9 (209). 

Menthone and camphor undergo asymmetric hydrosilylation to give alkoxysilanes with up to 82% optical purity using neutral rhodium(I) catalysts containing DIOP or neomenthyl- or menthyl-diphenylphos-phine even triphenylphosphine gave about 65% ee (300). Hydrolysis to alcohols was not reported. The ferrocenyl ligands (28, 29) are similarly effective for asymmetric hydrosilylation (255), and could be used for production of the optically active alcohols. 

In the reaction between an activated diene and a prochiral aldehyde, which is catalyzed by chiral Lewis acids, 5,6-dihydropyranoncs such as 6 are formed enantiomerically enriched. By attachment of a chiral auxiliary (3-heptafluorobutanoyl camphor derivatives such as 7 to a soluble poly-siloxane by hydrosilylation the catalyst should be recyled easily by precipitation or ultrafiltration while the cycloaddition reaction can be performed in homogeneous solution [lOj. 

The hydrosilylation of chiral (-)-menthone and (+)-camphor has also been investigated. Results are presented in Tables 5 and 6. 

Reaction of (-)-menthone or (+)-camphor resulted in the formation of four diastereomeric alkoxysilanes, depending on the configuration, R or 5, at the silicon atom and mode of attack at one face or the other of the ketone. The hydrosilylation of (-)-menthone gave two menthoxy- and two neomenthoxysilanes, that of (+)-camphor the bornyloxy- and isobornyloxysilanes. The relative pro-... 

Use of the oxime of D-camphor in the catalytic hydrosilylation leads to the formation of only 5 % of endo- and 13 % of e.YO-bornylamine. A further product is obtained in 79 % yield, the formation of which requires the hydrogenolytic cleavage of the five-membered ring in the camphor skeleton between the bridgehead carbon atom and the oxime carbon atom. The product, (+ )-(l/ ,3A)-l -(2-aminoethyl)-2,2,3-trimethylcyclopentane, was shown to be enantiomerically pure88. 

Stereoselective hydrosilylation of terpene ketones such as camphor and menthone catalyzed by RhCI(PPh3)3 followed by hydrolysis exhibits significant differences in stereochemistry from other reductions using metal hydrides123,115. The bulkiness of hydrosilanes exerts a remarkable influence on the stereochemical course of the reaction, i.e., a bulky hydrosilane favors the production of the more stable alcohols (equation 61). 

Asymmetric hydrosilylation can be accomplished (i) by [RhO(PPh3)3] if the organic substrate is optically active, e.g. (-)-menthane or ( + )-camphor, or (ii) if chiral phosphine-rhodium catalysts are used. In the particular case where the catalyst is a (diop)rhodium(i) derivative molecular models of intermediates based on oxidative addition of the silane to Rh, e.g. (61), can be used to predict the chirality of products... 

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