SolutionsAsymmetric metal catalysis: Sharpless and Noyori
Exercise 1
What product is obtained in the Sharpless epoxidation of (E)-hex-2-en-1-ol with D-(−)-diethyl tartrate? Why is the C2=C3 double bond of geraniol epoxidized and not the more electron-rich C6=C7? Would geranyl methyl ether react?
[(2R,3R)-3-Propyloxiran-2-yl]methanol, the same sense of induction as for geraniol with D-(−)-DET. Oxygen is transferred from a peroxide bound to the titanium that also holds the alkoxide of the substrate, so it can reach only the double bond adjacent to the carbon bearing the OH; C6=C7, although more nucleophilic, is too far away (with m-CPBA, by contrast, C6=C7 is epoxidized preferentially). The methyl ether cannot bind to titanium as an alkoxide and hardly reacts.
Exercise 2
Which reagent would you use to obtain (S,S)-1,2-diphenylethane-1,2-diol from trans-stilbene? What roles do the potassium osmate, the ligand and the potassium ferricyanide of AD-mix play? What would be obtained from cis-stilbene with OsO4 and NMO, without a ligand?
AD-mix-α, with the (DHQ)2PHAL ligand. The osmate is the source of the OsO4, used in catalytic amounts; the chiral ligand coordinates to osmium, accelerates the reaction and selects the face of the alkene; the ferricyanide reoxidizes osmium(VI) to osmium(VIII) to close the cycle. cis-Stilbene would give, by syn addition, the meso diol, (1R,2S), which is achiral: there are no enantiomers to distinguish.