Ferrocenes as ligands

Analogues of [RhH(CO)(PPh3)3] with other phosphorus ligands also act as hydroformylation catalysts. Using complex (34) in the presence of l,l -bis(diphenylphosphino)ferrocene as ligand,... 

The development of novel chiral metal complexes and chiral ligands is crucial for both progress and development of asymmetric catalytic synthesis [1-3]. Within this area, the appearance of planar-chiral ferrocenes as ligands in asymmetric catalysis has been an important advancement [4-7]. While most of these complexes bear side chains or atom groups with stereogenic centres, it is often the 1,2-disubstitution pattern of the n-complexed ring that creates an inherent planar chirality [8] and exercises efficient stereochemical control. 

Chiral ferrocenes have received niucli attenlion as ligands in metal-calalyzed reactions [39], bul tiieir use in copper cliemislry has been very limited [40, 41]. Hie ferrocene moiety offers die possibility of utilizing botli central and planar cliirality in die ligand. By analogy witli tlie copper arenetiiiolales described above, ferrocenyl copper complex 33 iSclieme 8.20) is extremely inleresling. 

Hie necessity of an anionic tbiolale ligand was established by perforniing reactions with ferrocene tliioetliets 37 as ligands. Here, essentially racemic products... 

Phosphine ligands based on the ferrocene backbone are very efficient in many palladium-catalyzed reactions, e.g., cross-coupling reactions,248 Heck reaction,249 amination reaction,250 and enantioselective synthesis.251 A particularly interesting example of an unusual coordination mode of the l,l -bis(diphenylphosphino)ferrocene (dppf) ligand has been reported. Dicationic palladium(II) complexes, such as [(dppf)Pd(PPh3)]2+[BF4 ]2, were shown to contain a palladium-iron bond.252,253 Palladium-iron bonds occur also in monocationic methyl and acylpalladium(II) complexes.254 A palladium-iron interaction is favored by bulky alkyl substituents on phosphorus and a lower electron density at palladium. 

The rhodium complexes of the ferrocene derivatives 39 have shown useful characteristics for the reduction of itaconates as well as dehydroamino acid derivatives [15, 167-170]. These compounds are hybrids between ferrocene-based ligands and the various other types. The P-chiral compounds, which in some ways are DIPAMP hybrids, showed tolerance for the reduction of N-methyl en-amides to produce N-methyl-a-amino acid derivatives [169-171]. 

The linkers may be nitrogen and carbon, as in BoPhoz - that is, a ferrocene-type ligand, as has already been mentioned. An example of a phosphine-phos-phoramidite is provided by QuinaPhos (48) [292]. 

As a general mle, most phospholanes are relatively air-sensitive and usually sold as rhodium-diene complexes. The ferrocene-based ligands, as well as phos-phoramidites and phosphites, are relatively easy to handle, and the corresponding Rh, Ir and Ru complexes are usually prepared in situ. Even though most biaryl type ligands are quite insensitive, most of them are applied as preformed Ru complexes. 

As mentioned in Chapter 4, Section 1, to date, several hundred ferrocene derivatives have been synthesized. A further increase in their number has occurred in recent years due to the use of ferrocene derivatives as ligands in metal complexes.2 Since some examples of metallic centres branched by several ferrocene units have already been presented in Chapter 4, Section 1.4.2 (when dealing with dendrimers), we will limit our discussion... 

Despite the resolution which was required to produce the enantiomerically pure starting materials (which fortunately is highly efficient—recrystalhzation of the mother liquors allows isolation of both enantiomers) , Ugi s lithiation provided the basis for a rapid growth in the use of enantiomerically pure, planar chiral ferrocenes which has continued since. Several reviews have covered applications of enantiomerically pure ferrocenes as chiral ligands, which until the 1990 s were all made using Ugi s method... 

Although oxazolines can be used as auxiliaries and later removed they have also been retained in target molecules which have then been used as ligands for a variety of asymmetric transformations. Ferrocenes carrying oxazoline and phosphine coordination sites ° ° " , oxazoline and amine coordination sites, and ferrocene bis-oxazoli-nes have been synthesized by the method of Scheme 141. 

Dilithiation of a C2-symmetric bis(oxazoline)-substituted ferrocenes as well as biaryls provides a versatile method for preparation of C2-symmetric tetradentate ligands. This reaction was originally described in 1995 by Park, Ahn, and co-workers. The same group and others have further expanded this reaction to... 

---