Olefin stereospecific hydration

Stereospecific hydration of olefins is another reaction involving a coordinated hydroxide. This reaction with maleate monoester (Figure 6.12, top) involves essentially alkene hydration by the coordinated nucleophile, hydroxide. The reaction is stereospecific in terms of the site of reaction, as it selects five-membered ring formation exclusively over six-membered... 

The addition of OH at the olefinic center creates a chiral center at carbon in conjunction with the chiral center at cobalt. Two diastereoisomers are thereby produced. Addition of the bound OH is very rapid for the maleato system and essentially pH independent between pH 8-10. In this range the diastereoisomer ratio is 2 1 as shown below. However, at higher pH values the rate becomes first order in OH- and in 0.1 M NaOH at 25 the half-life for the production of malate is 3 sec (at least 106-fold faster than the uncoordinated hydration). Under these conditions the reaction becomes much more stereospecific with a 9 1 ratio of diastereoisomers. While it is clear that in the pH independent region, H+ addition at the 8 carbon atom is the rate-determining step, it looks as if in the high base region deprotonation of the CoOH entity and addition of Co-0 could be rate-determining. 

Reduction of Unsaturated Steroids.—Reduction with di-imide [provided by hydrazine hydrate and copper(ii) acetate in methanol] provides a novel and stereospecific conversion of steroidal 4-en-3)5-ols into 5a-dihydro-compounds. By contrast, catalytic hydrogenation is non-stereospecific and is often accompanied by partial hydrogenolysis, so the new method offers considerable promise. Catalytic reduction of 3jS-hydroxyandrost-4-en-17-one with tritium and a Pt catalyst gave 3)5-hydroxy-5a-androstan-17-one with the tritium distribution 4a, 37% 5a, 43% and 6a, 20%. The 5jS-isomer was also formed, with tritium at 4, 29 % 5, 54 % 6, 4.7 %, and 6a, 13 %. Tritium analysis was achieved by a combination of equilibration with base, bromination-dehydrobromination, and dehydrogenation. The appearance of tritium at C-6 indicates olefinic bond migration in contact with the catalyst. ... 

Oxytelluration of olefins with PhTeBrs or Ph2Te2-Br2 in aqueous THF generates (P-hydroxyalkyl)aryltelluiium dihalides. The reaction is tran -stereospecific in the case of cw-2-butene and cis- and trans-4-octenes and regiospecific in the cases of all terminal olefins. Their reactions with reducing agents in aqueous sodium hydroxide generate the Markovnikov hydration products. 

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