Hydroquinidine 1,4-phthalazinediyl diether

Of the quinuclidine-ligated transition metal catalyzed processes, the osmium-catalyzed asymmetric dihydroxylation of alkenes, developed by Sharpless, has had the greatest impact on synthetic chemistry (67). In a recent synthesis, 1,3-dienoates were dihydroxylated with high enantioselectivities in the presence of (DHQD)2PHAL (hydroquinidine 1,4-phthalazinediyl diether) (Fig. 19) (183). 

The best enantioselectivities (94-99% ee) were obtained with hydroquinidine 1,4-phthalazinediyl diether ((DHQD)2PHAL) as the ligand at 0-12 °C (Table 1.2, entries 1, 3, and 4). 

Backvall and coworkers have shown that other tertiary amines can assume the role of the N-methylmorpholine. They reported on the first example of an enantioselec-tive catalytic redox process where the chiral ligand has two different modes of operation (1) to provide stereocontrol in the addition of the substrate, and (2) to be responsible for the reoxidation of the metal through an oxidized form [17]. The results obtained with hydroquinidine 1,4-phthalazinediyl diether (DHQD)2PHAL both as an electron-transfer mediator and chiral ligand in the osmium-catalyzed dihydroxylation are comparable to those obtained employing NMM together with (DHQD)2PHAL. The proposed catalytic cycle for the reaction is depicted in Scheme 1.4. 

Ligand Hydroquinidine 1,4-phthalazinediyl diether. Hydroquinidinep-chlorobenzoate. 

Reaction conditions K2[0s02(0H)4], 50 °C, 2 mmol olefin, 25 mL buffer solution (pH 10.4), 10 mL BuOH. 10 mmol olefin, 50 mL buffer solution (pH 10.4), 20 mL BuOH. The autoclave was purged with air and then pressurized to the given value. l,4-Diazabicyclo[2.2.2.]octane. Hydroquinidine 1,4-phthalazinediyl diether. 

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