Aromatic atropisomeric ligands

The introduction of chirality into the NHC framework follows diffoent principles than that of traditional phosphane ligands. Whaeas phosphanes are conically shaped [1] ligands with a tet-rahedrally coordinated central phosphorus atom, NHC are planar disc-like ligands based on the aromatic imidazole system (see Chapter 1). It follows that phosphanes are asymmetric owing to a stericaUy active and stable lone pair, whereas the chirality in the traditionaT NHC defines itself through a peripheral chiral element This element can be an asymmetric atom (central chirality), an axial chiral element (an atropisomeric binaphthyl substiment) or planar chirality (double substitution on an aromatic ring system, planar chiral substituent). Combinations of these three types of chirality are possible and can be realised in a more or less facile protocol. 

This stereospecificity is caused by the structural features of the BINAP ligand C2 chirahty, full aromatic substitution, the flexible atropisomeric skeletal backbone, and phenyl rings attached to the phosphorous atoms that align the transition states. 

The synthesis and reactivity of the six membered ring, potentially aromatic, phosphinine system has received further attention. The first examples of a new class of axially chiral phosphorus ligands, the atropisomeric phosphinines (204), have been prepared and isolated in an enantiopure state." Also reported are syntheses of the wide-bite-angle diphosphinine (205)" and the diphosphinine analogue of terpyridine (206), a new class of neutral, 7i-acceptor PNP-pincer ligand." Studies of the reactivity of phosphinines include the first observations of protonation, alkylation and silylation in systems usually viewed as extremely weak bases," " the proto-desilylation of 2,6-disubstituted silylphosphinines," a theoretical study of... 

Using a related cycloaddition protocol, Schreiber found that the chiral P,N-ligand QUINAP (121) exhibited comparable high enantioselectivity in silver-catalyzed dipolar cycloadditions (Equation 6) [98], The highest asymmetric induction was observed for tert-hutyl acrylate (119) with aromatic imino esters such as 118. The process shows a wide substrate scope, as the transformation was tolerant both of a-substitution in 118 and of /3-substitution of the acrylate dipolarophile 119. Carreira has crafted and studied the use of a family of atropisomeric P,N-ligands, as exemplified by 122 [99]. 

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