Phosphaferrocene ligands, planar chirality

Fu has reported a planar-chiral bisphosphorus ligand 45 with a phosphaferrocene backbone. The ligand has provided enantioselectivity up to 96% ee in the hydrogenation of a-dehydroamino acid derivatives.99 Another planar-chiral ferrocene-based bisphosphorus ligand 46 has been reported by Kagan recently and enantioselectivity up to 95% ee has been obtained in the reduction of dimethyl itaconate.100... 

In 2000, we demonstrated that the planar-chiral phosphaferrocene PF-PPhj is a useful ligand for rhodium-catalyzed asymmetric isomerizations of several allylic alcohols, providing the first catalyst system that furnishes the target aldehyde in >60% ee (Eq. 6) [7]. It appears that, in order to obtain high enantiomeric excess (>0% ee), the olefin should bear a relatively bulky substituent (for example, Pr Eq. 6). 

In recent years we have developed a new type of planar-chiral ligand system which is based on a phosphaferrocene skeleton equipped with an additional donor function Y [8]. This structure is closely related to the well-known ferrocene-type ligands in that a CH unit of the latter has been replaced by a P atom. The phosphaferrocene moiety serves as both a chiral metallocene-type backbone and as a donor group via the phosphorus atom lone pair. These new ligands are unique in their topological architecture and show interesting ligand properties. 

Fig. 1.5.1 The planar-chiral donor-substituted phosphaferrocene chelate ligand.

The inherent planar chirality of substituted phosphaferrocenes can be exploited for the diastereoselective deriva-tization of the heterometallocenes themselves. This is nicely illustrated by the preparation of the alcohol derivative 323 as a single diastereomer (cf. LiAlH4 reduction of 307, which affords a 1 1 mixture of diastereomers Scheme 107) <1998CEJ2148>. Alcohol 323 was then used to prepare the enantiomerically pure P,P-diphosphoms ligand 324. 

Planar chiral phosphaferrocene-oxazolines (379) constitute another family of complexes that are usefiil as ligands in asymmetric catalysis. Preparation of these takes advantage of a modified Friedel-Crafts acylation of (373) and an unusual conversion of the resulting trifluoromethyl ketone into an amide that is then cyclized to an oxazoline. The diastereomeric complexes thus formed are chromatographically separable and are used in a palladium-catalyzed asymmetric allylic substitution. Modification of this complex by using the anion derived from 3,4-dimethyl-2-phenylphosphole gives more... 

Subsequently, better ligands than 44 have been designed for the asymmetric palladium-catalyzed reaction of 1,3-diphenylallyl acetate with sodium mal-onate. Both the bis-(phosphaferrocene) 48 synthesized as shown in Eq. (32) [77] and the phosphaferrocene-oxazolines 49 [78] perform this condensation with much higher ees (79% in the first case and up to 82% in the second case). In the second case, the stereochemical outcome of the reaction is determined by the planar chirality at the phosphaferrocene, rather than the chirality of the oxazo-line [78] ... 

C) Planar Chiral Phosphaferrocene-Based Ligands. Phosphaferrocene-based ligands are related to ferro-cenes iron-7r-complexes where a phosphorous atom provides a lone pair for coordination. Compared with numerous ligands with central, chiral, or planar chirality that have been synthesized to date, the... 

Ganter C. Planar chiral phosphaferrocene-based ligands. In Bomer A, editor. Phosphorus ligands in asymmetric catalysis. Synthesis and applications. Weinheim Wiley-VCH 2008. p 393 07. 

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