Phosphaferrocenes reactions

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

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. 

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. 

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. 

Selected derivatives of the ligands and complexes described above have been tested in catalytic applications. Early tests with bidentate P,P or P,N ligands such as 4, 6, and 8 in Rh-catalyzed asymmetric hydrogenation were disappointing, with ee values below 20%. However, as was demonstrated mainly by the Fu group, phosphaferrocene derivatives do have the potential for successful applications in asymmetric catalytic reactions, provided the phosphaferrocene is endowed with sufficient steric bulk. Examples are depicted in Eig. 1.5.8 the Cp derivative 32,... 

Similar to the reaction of allylamines, allyl alcohols also undergo enantioselective isomerization in the presence of [Rh(BINAP)(COD)]+ [10]. Yields and enantio-selectivity are usually moderate, however. Considerable improvement was recently achieved by application of Rh(I) catalysts bearing phosphaferrocenes, 27, as chiral ligands (Scheme 7) [11]. These air-stable complexes, which can be recovered after the reaction, afford chiral aldehydes with up to 93 % ee. 

On account of their very important biological activity, /9-lactams are important synthetic targets [4-9]. Fused polycyclic -lactam subunits appear in many natural products such as penicillins [4-6] and trinems/tribac-tams [10-13]. Fu et al. reported that such frameworks can be prepared with high levels of enantioselectivity via the intramolecular Kinugasa reaction [ 14, 15] of alkyne-nitrone in the presence of a planar chiral Cu/phosphaferrocene-oxazoline catalyst [16]. For instance, compound 1 was transformed into tricyclic /9-lactam 3 in good stereoselectivity and yielded (88% ee and 74% yield) using 5 mol % of CuBr and 5.5 mol % of complex 2 (Scheme 1). 

Reaction of chlorobis(methylene)phosphorane 55 with K[(i73-C3H5)Fe(CO)3] in a mixture of THF and toluene gives the metallobis(meth-ylene)phosphorane 56 and the bis(methylene)propenylphosphorane 57 as major products. The dihydrophosphole complex 58 is obtained as a minor product (<10% yield) (Scheme 13).40 In contrast, treatment of 55 with K[Fe(CO)2Cp] gives rise to the formation of the metallobis(methylene) phosphorane 59 (39% yield) and the phosphaferrocene 60 (34-72% yield, dependent upon the reaction time). The phosphole ligand of 60 is con-... 

Similarly, the reaction of dicyclopentadienyl-tetracarbonyl-diiron with 1-phenylphos-pholes yields phosphaferrocenes, although in lower yields ... 

Phosphole radicals are known to be more readily made and more stable than analogous phosphine radicals . This is probably due to the delocalization of the unpaired electron over the ring. On the other hand, complexes such as XLVIII are isolated from the reaction mixture and their mass spectra contain a medium-intensity peak corresponding to the final phosphaferrocene however, their thermolysis in the pure state does not produce phosphaferrocenes in detectable amounts. Phenyl-substituted phosphaferrocenes are always among the by-products of the synthesis of phosphaferrocenes. No satisfactory explanation of this fact (the formation of phenyl substituted phosphaferrocene by radical arylation as initially proposed is not very likely since ferrocene cannot be arylated in this way could be found until the very recent discovery that, at around... 

This reaction provides a mild way to decomplex the phosphole ring. The much reduced nucleophilicity of phosphorus consequently does not interfere with the reactions of electrophiles at carbon. It is thus possible to acylate phosphacymantrenes and phosphaferrocenes. Some examples are given below ... 

In the presence of aluminum chloride, chloroformates are known to decompose readily to give carbon dioxide and alkyl chlorides. The success of this carboxylation reaction means that the electrophilic attack on phosphaferrocene takes place faster than the decarboxylation of ethyl chloroformate. 

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] ... 

A theoretical study of the Diels-Alder reactions between 1,3-butadiene and, respectively, cyclopentadiene and 2H-phosphole, has revealed a remarkable similarity between the two reactions. Further studies of photocycloaddition reactions of phosphole moieties have also been reported. Transition metal complexes of phospholide anions continue to attract attention, and in particular the chemistry of phosphaferrocene systems remains a major interest . The past year has also seen significant activity in the chemistry of di- and tri-phospholes, related polyphospholide anions, and also heterodiphosphole systems. Routes have been developed to the diphosphonio-l,2-diphospholes 356,... 

An interesting new development in aromatic phosphole chemistry has been the preparation and subsequent study of the electrophilic substitution reactions of transition-metal complexes of phospholyl anions, e.g. the phosphaferrocenes (165), which are obtained by the reaction of the appropriate P-phenylphosphole with transition-metal cyclopentadienyls or carbonyls, - and which undergo acetylation at... 

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