Ferrocenyl catalysts

In addition to the planar chiral ferrocenyl catalysts 15-18, 24 developed by Fu, a number of other chiral derivatives of 4-DMAP and 4-PPY [4, 47, 48] have been explored by other groups as organocatalysts for KR of ec-alcohols. Contributions have been made by the groups of Vedejs [104, 105, 110, 111], Fuji and Kawabata... 

Given that the Birman catalysts are readily prepared in just two steps from commercially available enantiomerically pure phenylalaninol, these catalysts constitute attractive alternatives to Fn s planar chiral ferrocenyl catalysts 15-18. 

Ferrocenyl groups are useful tools for stereoselective syntheses. Hayashi and coworkers have discovered a novel class of ferrocenyl catalysts allowing the kinetic resolution of benzylic zinc derivatives, such as 448 . The racemic mixture of the benzylic zinc reagent... 

A ferrocenyl catalyst system able to perform highly enantioselective Cu-catalysed allylic alkylations with Grignard reagents has been described.89 The alkylated products have been obtained in high yields with good to excellent enantioselectivities (ee up to 98%). 

Using a chiral 4-dimethylaminopyridine-ferrocenyl catalyst, acyclic silyl ketene acetals react with anhydrides to furnish 1,3-dicarbonyl compounds containing allcarbon quaternary stereocentres in good yield and ee.144 Evidence for dual activation (anhydride -> acylpyridinium, and acetal -> enolate) is presented. 

The most serious limitation associated with the use of Fu s planar chiral ferrocenyl catalysts (4a-d) is that their preparation is via a multi-step synthesis followed by chiral-HPLC resolution however, catalysts 4b and 4c are commercially... 

Thomas and Johnson described a direct asymmetric hydrogenation of ethyl nicotinate using Pd ferrocenyl catalyst anchored within MCM 41 [48]. The confined catalyst (5%) displayed a good activity (TON = 291), but no increase in enantioselec tivity (17% ee) was observed compared to the corresponding homogeneous catalyst (Scheme 10.35). 

Martha CT, van Zeist W-J, Bickelhaupt FM, Irth H, Niessen WMA. Tandem mass spectrometry of silver-adducted ferrocenyl catalyst complexes in continuous-flow reaction detection systems. J Mass Spectrom. 2010 45 1332 3. 

Upon completion of the polymerisation of polyolefins by the Ziegler-Natta, low pressure routine, the organo-aluminium-titanium halide catalyst is decomposed and neutralised by the addition of low molecule weight alcohols and possibly aqueous wetting agents and soaps whilst pH control may be effected by the addition of aqueous alkalies such as sodium carbonate. This stage of the process can therefore introduce alcohols and alkali metal salts into the polymer. Similar comments apply in the case of the ferrocenyl catalysts now being used in polyolefin synthesis. 

An iinportanl advance in this synthesis of allylsilanes involved the use of optically active palladium ferrocenyl complexes as catalysts to provide optically active allylsilanes with good enantiomeric excesses being obtained for (/. (-allylsilanes and less good enantiomeric excesses for (Z)-allylsilanes26,27. 

The gold complex, generated in situ from bis(4-isocyanocyclohexyl)gold(I) tetrafluoroborate and (A)-A-methyl-,V-[2-(dialkylamino)ethyl]-l-[(5)-r,2-bis(diphenylphosphino)ferrocenyl]eth-ylamine, is an effective catalyst for the aldol reaction of various aldehydes with methyl iso-cyanoacetate to give the trans- and cw-4,5-dihydro-l,3-oxazoles. Depending on the aldehyde, the transjeis product ratio ranges from 84 16 to 100 0, and the ee of the main diastereomer is between 72 and 97%26. 

To a mixture of vinyl bromide (40 mmol) and the catalyst dichloro-[(R)-Af,N-dimethyl-l-[(.S)-2-(diphenylphosphino)ferrocenyl]ethylamine]-palladium(n) (0.2 mmol) was added an ethereal solution of [a-(trimethyl-silyl)benzyl]magnesium bromide (0.6-1 m, 80 mmol) at —78 °C. The mixture was stirred at 30 °C for 4 days, and then cooled to 0 °C and hydrolysed with dilute aqueous HC1 (3 m). The organic layer was separated, and the aqueous layer was re-extracted with ether. The combined organic extracts were washed with saturated sodium hydrogen carbonate solution and water, and dried. Concentration and distillation gave the chiral allylsilane (79%, 66% ee), b.p. 55°C/0.4mmHg. 

Palladacycles are defined as compounds with a Pd-C CT-bond with the Pd being stabilized by one or two neutral donor atoms, typically forming 5- or 6-membered rings [51]. Ferrocenyl palladacycles constitute a particularly attractive catalyst class partly due to the element of planar chirality. The first diastereoselective cyclopallada-tion of a chiral ferrocene derivative was reported in 1979 by Sokolov [52, 53]. 

In 1997 the first asymmetric aza-Claisen rearrangement was reported by Overman et al. [55], which made use of diamines as bidentate ligands for Pd(II), allowing for moderate enantioselectivities. In the same year, Hollis and Overman described the application of the planar chiral ferrocenyl palladacycle 38 as a catalyst for the enantioselective aza-Claisen rearrangement of benzimidates 39 (Fig. 19) [56]. A related ferrocenyl imine palladacycle provided slightly inferior results, while a benzylamine palladacycle lacking the element of planar chirality was not able to provide any enantioselectivity [57]. 

Hollis TK, Overman LE (1997) Cyclopalladated ferrocenyl amines as enantioselective catalysts for the rearrangement of allylic imidates to allylic amides. Tetrahedron Lett 38 8837-8840... 

Cohen F, Overman LE (1998) Planar-chiral cyclopalladated ferrocenyl amines and imines as enantioselective catalysts for allylic imidate rearrangements. Tetrahedron Asymmetry 9 3213-3222... 

Donde Y, Overman LE (1999) High enantioselection in the rearrangement of allylic imidates with ferrocenyl oxazoline catalysts. J Am Chem Soc 121 2933-2934... 

Weiss ME, Fischer DF, Xin ZQ, Jautze S, Schweizer WB, Peters R (2006) Practical, highly active, and enantioselective ferrocenyl-imidazoline palladacycle catalysts (FIPs) for the Aza-Claisen rearrangement of A-para-methoxyphenyl trifluoroacetimidates. Angew Chem Int Ed 45 5694-5698... 

In situ derived systems, in general, performed similarly to preformed complexes, in telomerisation of butadiene with MeOH, Tables 4.1 and 4.2 [68,70,71,77,78], In situ systans may be generated from free NHC or from imidazolium salt in combination with an appropriate Pd(0) or Pd(ll) source. Typically, 2-4 equivalents of imidazolium salt relative to Pd have been nsed [68,70,77], In situ catalysts derived from mono- and bis-Fc-snbstituted (Fc = ferrocenyl) imidazohnm and benzimidazolium salts (64-68) (Table 4.2) showed interesting telomerisation activities ascribed to the steric bulk of the Fc substituents [70]. Unsymmetrical salts 65 and 66 bearing A -Fc and A -Me... 

Chiral Pd catalysts can achieve enantioselectivity. The best catalysts developed to date are dimeric ferrocenyl derivatives.264... 

Platinum complexes [PtCl2(diphosphine)] and [PtCl(SnCl3)(diphosphine)] of the ferrocenyl diphosphine ligands (35a), (35b), and (36) have been synthesized. Complexes [PtCl2(35)] and [PtCl2(36)] have been structurally characterized by XRD. Both the preformed and the in situ catalysts have been used in the hydroformylation of styrene.112... 

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