Phosphaferrocene

The nucleophilic properties of phosphorus in phosphaferrocene were demonstrated by reaction with n-butyllithium occurring at the phosphorus atom (81IC3252 820M312). 

The redox chemistry of several phosphaferrocenes,31,50 l,l -diarsaferro-cene (7),13 the complete series of 2,2, 5,5 -tetramethyl-l,r-diheteroferro-cenes (89, 26, 29, 32),22 and octamethyl-1,1 -diheteroferrocenes (90, 44, 48, 49)22 has been investigated by cyclic voltammetry. These compounds undergo quasi-reversible one-electron oxidations (0/+) to their radical cations and irreversible one-electron reductions (0/-) to their radical anions. The data are summarized in Table VI. 

As shown in Scheme 12, acetylation of 1-phosphaferrocene (12) gives a mixture of 85% 2-acetyl-1-phosphaferrocene (97) and 15% 3-acetyl-l-phosphaferrocene (98).54 In contrast, the acetylation of 1,1 -diarsaferrocene (8) gives 99 as the exclusive monoacetylation product.13 Further acetylation of 99 affords a mixture of the two diastereomeric products 100 and 101.55 Apparently the a position is more reactive in both the P and As series, but the relative rates for a- vs. -substitution are greater in the As series. If the more reactive a position is blocked, substitution can take place at the /3 position in both series. Thus the acetylation of 2,2, 5,5 -tetraphenyl-l, l -diphosphaferrocene (102) gives the 3-substituted product 103 as well as substantial amounts of phenyl-substituted product... 

Aromatic electrophilic acylations were described only in the coordination sphere of the phospholide anion involving phosphacymantrenes [43, 44] or phosphaferrocenes [45],... 

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... 

Metal-catalyzed C-H bond formation through isomerization, especially asymmetric variant of that, is highly useful in organic synthesis. The most successful example is no doubt the enantioselective isomerization of allylamines catalyzed by Rh(i)/TolBINAP complex, which was applied to the industrial synthesis of (—)-menthol. A highly enantioselective isomerization of allylic alcohols was also developed using Rh(l)/phosphaferrocene complex. Despite these successful examples, an enantioselective isomerization of unfunctionalized alkenes and metal-catalyzed isomerization of acetylenic triple bonds has not been extensively studied. Future developments of new catalysts and ligands for these reactions will enhance the synthetic utility of the metal-catalyzed isomerization reaction. 

The phosphorus atom in a r-coordinated phospholyl ligand still exhibits a lone-pair of electrons that is capable of binding a second metal atom, so that a phospholyl complex such as phosphaferrocene is—in contrast to ferrocene—itself a ligand. This special property which has no direct correspondence in the chemistry of cyclopentadienyl complexes has recently been fruitfully exploited as the underlying principle which allowed the introduction of phosphaferrocenes as effective ligands in catalytic applications [10, 13, 14]. 

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). 

An X-ray crystal structure of [Rh(PF-PPh2)(COD)]PF6 reveals a distorted square-planar geometry around rhodium (Fig. 4.1). The Rh-phosphaferrocene bond length (2.25 A) is shorter than the Rh-(tertiary phosphine) bond length (2.30 A). We postulated that the strong -accepting capacity of the phosphaferrocene [9] may lead to effective Rh P back-bonding that decreases the conformational flexibihty of the metal-... 

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.

All the reactions described above rely on the electrophilic reactivity of the carbonyl C atom of the aldehyde 2. Much effort was devoted to the development of phosphaferrocenes with nucleophilic reactivity at this position. For example, transformation of the formyl group into a halomethyl function would pave the way for the preparation of Grignard or lithium derivatives by halogen-metal exchange. However, all attempts to do this were unsuccessful. 

This is also true for the donor/acceptor properties, because the phosphaferrocene is a reasonable Jt-acceptor due to the large p orbital contribution of the P to the LUMO of the molecule [15]. 

Reaction of the anion 21 with Cp or Cp metal fragments provides further metallocene-type complexes with a pendant phosphaferrocene side-chain. For example, the reaction of the thallium derivative T1 21 with [Cp RhCl2]2 yields the cationic pentamethylrhodocenium 24 as its chloride (Scheme 1.5.10). This is an interesting species because it is a chiral water-soluble P ligand. The chloride anion can be exchanged by PF,s to make the compound more soluble in organic solvents. 

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