Bisalkaloid ligands

Although the bisligand systems provide faster reactions and better selectivities in the majority of cases, more hindered substrates, such as tetrasubstituted alkenes or alkenes with a bulky substituent can benefit from the use of a monoalkaloid ligand system [90]. For a discussion on relative rates of reaction for different substrate classes and selectivities see Section 3.2.5.1. 

With the bisalkaloid ligands, potassium ferricyanide can be used as the stoichiometric oxidant [84, 91]. As with the parent achiral osmium oxidation, NMMO can also be used as the oxidant (see above) [92]. However, rather than using NMMO in stoichiometric amounts, this morpholine component can be used in catalytic amounts by the addition of the biomimetic flavin 4 to set up a triple catalytic system where hydrogen peroxide is the oxidant [93-95], Methyltrioxorhenium can be used in place of the flavin mimic [96], as can tungsten(VI) [97] and carbon dioxide [98]. 

Other alternatives for the oxidant for stoichiometric oxidations include the use of a selenoxide [99], including a photochemical oxidation of catalytic selenium [100], iodine [101], sodium chlorite [102], hypochlorite [103], and electrochemical methods [101,104]. Even air can be used as the oxidant [99,100], but care has to be taken with regard to the choice of solvent as cleavage of the product 1,2-diol can occur, especially when the alkene has an aryl substituent [53, 105, 106]. 

The use of a constant pH (12) for the reaction with internal alkenes provides faster reaction rates, and the need for the hydrolysis aid, methanesulfonamide, is alleviated. For terminal alkenes, the use of a constant pH 10 affords higher enantioselectivities compared to reactions where the pH is not controlled [107]. 

K20s02(0H)4 can be used as the nonvolatile form of osmium and this can be mixed with the oxidant, potassium ferricyanide, and the ligand to allow ease of use. 

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The bisalkaloid ligands are simple to make from the dichloride of the heterocyclic spacer with KOH and K2C03 as solid bases in toluene (Scheme 3.5) [2]. 

For many AD reactions, the bisalkaloid systems (cf. Figure 3.1), in particular the (DHQ)2PHAL and (DHQD)2PHAL systems, provide good asymmetric induction. Studies with mixed ligand systems showed that the (DHQD)2PHAL complex was... 

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