Containing Axial Chiral Backbones

A selection of axial chiral bisphosphine ligands. The original BiNAP ligand is shown at the top left of the figure. 

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Concurrent with studies on cyclometalation, studies on the effects of the structure of phosphoramidite ligand had been conducted. Several groups studied the effect of the stmcmre of ligand on the rate and selectivity of these iridium-catalyzed allylic substitutions. LI contains three separate chiral components - the two phenethyl moieties on the amine as well as the axially chiral BINOL backbone. These portions of the catalyst structure can control reaction rates by affecting the rate of cyclometalation, by inhibiting catalyst decomposition, or by forming a complex that reacts faster in the mmover-limiting step(s) of the catalytic cycle. 

A novel intramolecular decarboxylative amination protocol of A -tosylcarbamates (10) for the synthesis of axially chiral allenes containing synthetically attractive functionalities (11) has been developed with excellent enantioselectivities (up to 92% ee) and good yields (Scheme 5). The biphenyl backbone in the ligands has been found responsible 0... 

When coordinated to the metal, the chiral ligand plays an important role in the control of enantioselectivity during the course of the catalysis reaction. These ligands can contain chirality in the backbone (e.g.. CHIRAPHOS, 1), at the phosphorus atom (e.g., DIP AMP, 2), or atropisomerically from C2 symmetric axial configurations (e.g., BINAP, 3). Typically, most chiral ligands possess di-phenylphosphine moieties (Figure 1). 

OA is a polyether compound containing a 38 carbon backbone. There are 17 chiral centers within the molecule and 3 spiroketal moieties. Of the many fnnctional groups, the C-1 carboxyl terminus and the C-7 hydroxyl gronp are both commonly modified by esterification. The two known natural analogs of OA are 35(R)-methyl OA (DTX-1) and 35(5)-methyl 31-desmethyl OA (DTX-2). The stereochemistry at C-35 has recently been confirmed as R for DTX-1 (Sasaki et al., 1998) and S for DTX-2 (Larsen et al., 2007). This variation in the stereochemistry may explain why DTX-1 and DTX-2 are each known to co-occur with OA but rarely with each other. It has been hypothesized (Larsen et al., 2007) that a specific methylation enzyme is responsible for adding a methyl group at C-35 and that it will add either an equatorial (DTX-1) or an axial methyl (DTX-2). Despite the complicated nature of the task, three full syntheses of OA have been completed (Ichikawa et al., 1987 Ley et al., 1998 Urbanek et al., 1998) and these are worthy of note. 

Bidentate Phosphorus Ligands Supported on Atropisomeric Biaryl Backbones. Most ligands of this group contain binaphthyl or biphenyl moieties-providing an axial element of chirality. The flagship of this class of compounds is the atropisomeric diphosphane BINAP (36), whose complexes (primarily Ru(II) complexes (37)) play a pivotal role in asymmetric scale-up hydrogenations and isomerizations (Fig. 5). 

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