Asymmetric dihydroxylations with 2 PHAL ligands

Deng et al. later found that dimeric cinchona alkaloids such as (DHQ AQN (8, Scheme 6.6) and (DHQD PHAL (9, Scheme 6.7) - both well known as ligands in the Sharpless asymmetric dihydroxylation and commercially available - also catalyze the highly enantioselective cyanosilylation of acetal ketones with TMSCN... 

More recently, Petri et a/.[136] have copolymerized the chiral ligand (QHN)2-PHAL (Scheme 9.3) directly with ethylglycol dimethacrylate using AIBN as radical initiator. This material revealed high activities (68-80 % yield) and enantioselectivities (ee > 98 %) for asymmetric dihydroxylation of frans-stilbene using K3Fe(CN)6 as secondary oxidant. However, the authors noted that the catalytic material still contained unbound bis-alkaloid. 

A soluble polymer-bound version of the asymmetric dihydroxylation ligand is provided by DHQD-PHAL-OPEG-OMe, a polyethylene glycol derivative [152, 153]. The use of a PEG system with a ligand at each end allowed the dihydroxylation reaction to be run in a continuous flow system with a membrane to retain the ligand. Under these conditions, metal leaching was observed [154]. 

These ligands were superseded with the development of the phthalazine (PHAL) ligands in which two cinchona alkaloid units are connected together. The two most widely used ligands are the phthalazine ligands (DHQD)2-PHAL 93 and (DHQ)2-PHAL and these are used in the two commercially available asymmetric dihydroxylation mixes - AD-mix-a and AD-mix-p. 

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