Chiral amidinate ligands

Chiral amidine ligands bearing sulfenyl groups have been developed.ui In these cases, more electron-rich imino groups (amidines) improve the enantioselectivity and the catalytic activity in Pd-catalyzed allylic substitution in comparison with oxazolines. 

In addition to the substituents listed in Scheme 1, chiral groups may be introduced and unsymmetrically substituted amidinate anions are also possible. The amidinate anions may also contain additional functional groups, or two such anions can be linked with or without a suitable spacer unit. Yet another variety comprises the amidinate ligands being part of an organic ring system. All these aspects will be covered in the present review. 

A new class of chiral amidine-phosphine and -sulfide hydrid ligands with a variety of modifications is used for the palladium mediated allylic substitutions of both acyclic and cyclic compounds [74] (Figure 3.5). High levels of asymmetric induction were achieved for both substrates. 

Some ferrocenylphosphine-amidine ligands (Figure 3.6) with central and planer chirality were prepared and their efficiency and diastereomeric impact in the palladium catalysed asymmetric allylic substitution were examined [75]. Up to 96% ee with 98% yield was achieved by the use of a ligand with a methyl-substituted ligand. 

Hu, X., Chen, H., Hu, X. er a/. (2002) Synthesis of novel ferrocenylphosphine-antidine ligands and their application in Pd-catalysed asyrmnetric allylic alkylation. Tetrahedron Letters, 43, 9179-9182 Hu, X., Chen, H., Dai, H. ef a/. (2003) S3mthesis of novel ferrocenylphosphine-amidine ligands with central and planar chirality and their diastereomeric effect in Pd-catalysed asymmetric allylic alkylation. Tetrahedron Asymmetry, 14, 2073-2080. 

The catalytic activation of allylic carbonates for the alkylation of soft car-bonucleophiles was first carried out with ruthenium hydride catalysts such as RuH2(PPh3)4 [108] and Ru(COD)(COT) [109]. The efficiency of the cyclopen-tadienyl ruthenium complexes CpRu(COD)Cl [110] and Cp Ru(amidinate) [111] was recently shown. An important catalyst, [Ru(MeCN)3Cp ]PF6, was revealed to favor the nucleophilic substitution of optically active allycarbonates at the most substituted allyl carbon atom and the reaction took place with retention of configuration [112] (Eq. 85). The introduction of an optically pure chelating cyclopentadienylphosphine ligand with planar chirality leads to the creation of the new C-C bond with very high enantioselectivity from symmetrical carbonates and sodiomalonates [113]. 

Among chiral ligands known to contain sulfenyl groups, high enantioselectivity was obtained with chiral oxazoline, amidine, and imine ligands. 

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