Syn and Apparent Anti Addition of Hydrogen

An excellent example of stereospecific syn addition is seen in the hydrogenation of cis- and franv-dimethylstilbcnc with palladium catalyst (eq. 3.24).13 Under the same conditions, diethylstilbestrol (50, R = H) and its dimethyl ether (50, R = Me) were hydrogenated to the products containing, respectively, 90 and 97% of the corresponding racemic 3,4-diphenylhexane derivatives. Syn addition decreased to 86 and 70%, re- 

Nishimura et al. studied the hydrogenation of 19-21 over the six unsupported platinum group metals in f-butyl alcohol at 26°C and 1 atm H2.138 The hydrogenations of 19 over iridium and osmium have been found to be highly stereoselective, affording 

2 and 98.7% yields of Vl,2-dimethylcyclohexane, respectively (eq. 3.25). The order of the platinum metals in the formation of cis isomer for 19 (Ir Os Ru Rh Pt Pd) also holds approximately for the hydrogenation of 20 and 21 (Table 3.14). Iridium and osmium are always among the metals that give the highest yields of the cis isomer, and palladium always gives the trans isomer predominantly. In most cases ruthenium, rhodium, and platinum show the intermediate stereoselectivity between these extreme metals. In general, the tendency of the platinum metals for the formation of the cis isomer has been found to correlate inversely with their ability for isomeriz-ing 20 to 19 or 21 to 19 and 20 (see Table 3.14). 

TABLE 3.14 The Stereoselectivity and Isomerization Ability of the Platinum Metals in the Hydrogenation of 1,6-Dimethylcyclohexene and 2-Methyl met hvlenecyclohexanet 

Catalyst Proportion of cis Isomer in Saturate (%) Proportion of Isomerized Product (%)  

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