PNNP ligands

Table 3.12 surveys current industrial applications of enantioselective homogeneous catalysis in fine chemicals production. Most chiral catalyst in Table 3.12 have chiral phosphine ligands (see Fig. 3.54). The DIP AMP ligand, which is used in the production of L-Dopa, one of the first chiral syntheses, possesses phosphorus chirality, (see also Section 4.5.8.1) A number of commercial processes use the BINAP ligand, which has axial chirality. The PNNP ligand, on the other hand, has its chirality centred on the a-phenethyl groups two atoms removed from the phosphorus atoms, which bind to the rhodium ion. Nevertheless, good enantio.selectivity is obtained with this catalyst in the synthesis of L-phenylalanine. 

Homobimetallic iridium(I) complexes containing the binucleating PNNP ligand undergo oxidative addition and reductive elimination reactions with acetyl chloride and methyl iodide. Thus,... 

Two distinct classes of planar bimetallic complexes containing the PNNP ligand were isolated and characterized the neutral [M2(PNNP)(/i-X)L2] and the cationic [M2(PNNP)(L)4]+ (208). 

Recently, iron(II) complexes with chiral N2P2 and N2P4 macrocychc hgands were introduced by Mazzetti and Gao for apphcations in ATH catalysis. In 2004, Gao and co-workers reported the ATH of ketones with the catalysts in situ generated from iron(ll) complexes and chiral PNNP ligands [154], ligands that were later incorporated into the well-defined Fe-PNNP complexes by Morris et al., vide supra. The 22-membered macrocycles 174 and 175 (Fig. 55) synthesised by Gao et al. were used to in situ generate precatalysts from different iron sources (e.g. 

In addition to oxygen, the phosphorus can be tied to the backbone through nitrogen. Indeed, this was one of the earliest variations of the DIOP family (see Section 23.3) with PNNP (45) [22, 280, 287]. Care must be taken to avoid hydrolysis of the labile P-N bonds [22, 288]. Other examples of nitrogen-linked compounds are BDPAB (46 and 47) and its derivatives (Fig. 23.4). These ligands are clearly variations of the BINAP series [289-291]. 

RuCl(PNNP)KPF ) and [RuCl(H,0)(PNNP)](PF ) (PNNP=tetradentate chiral ligands, e.g.iV,A -[bis(o-diphenylphosphino)-benzylidene]-(15, 25)-di-iminocyclohexane (Fig. 1.42) [928,929] andiV,iV -[bis(o-diphenylphosphino)-benzylidene]-2,2-di-imino-l, F-(S)-binaphthylene) [928], These complexes were made from the ligands and RuCljCPPhj) in CH Cl P NMR spectra were measured [929]. 

Ruthenium complexes [RuCl2(PNNP)] containing tetradentate hybrid ligands 7.53 with P-and N-donors (PNNP) catalyze the asymmetric epoxidation of unfunctionalized alkenes... 

The PNNHP molecule, when deprotonated, behaves as a planar tetradentate ligand consisting of a central pyrazolate unit with symmetrically disposed meth-ylenediphenylphosphine arms (PNNP). Its geometry provides for two metals to reside within a cooperative distance but does not allow for metal-metal bond formation (208). 

Scheme 12. Oxidative-addition reactions on complexes containing the functionalized pyrazolate ligand PNNP. Depending on the electron richness of the metals, reversible or irreversible addition takes place. (Adapted from (2).]...

Introduction Catalytic hydrogenation with soluble catalysts Hydrogenation with C2-Symmetrical ftis-Phosphine Rhodium Complexes C2 symmetric ligands (DIPAMP, DIOP, PNNP)... 

The groups of Gao (326) and Morris (327-330) reported that iron complexes containing PNNP tetradentate ligands (Fig. 75) are efficient catalysts for the ATH of aryl/alkyl ketones obtained from 2-propanol. Although some systems give low ee values (range = 12-99% ee), the use of iron has the advantages of its low cost and toxicity. 

Fig. 75. PNNP tetradentate ligands used in iron catalysts for ATH reactions.

There are many other enantioselective reactions catalyzed by chiral metal complexes. These include complexes of various chiral phosphines, tartrate ion, and others as ligands. Chiral cyclopentadienyl metal complexes (32) also are used. Chiral ligands are not limited to those containing an asjonmetric carbon. [RuCbinap)] " is a catalyst for enantioselective hydrogenation. In some cases such as [Rh(dipamp)] and [Rh(pnnp)] well defined complexes are used. In many cases the reaction is carried out in the presence of a chiral ligand and a metal compound. The reaction involving... 

Several iron complexes for imine ATH have been developed by Morris et al., with IPA used in the reduction. Early examples of the PNNP-complexes (containing imine ligands) of iron gave 100 % reduction of PhCH=NPh but only 5 % of PhCMe=NPh [124]. Beller reported the use of an in situ-generated catalyst for ATH of imines with high yield and enantioselectivity (Rg. 39) [125]. 

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