Ferrocene monodentate ligands

Photolysis of the arene complexes in the presence of monodentate ligands, e.g. carbon monoxide, leads to new complexes of the type CpFe(L) whereas in pure aprotic solvents, ferrocene and iron salts are formed Investigation of the photo-lytic reaction of an iron arene complex with excess ethylene oxide in methylene chloride solution (Meier and Rhis ) showed that a crystalline crown ether complex (structure shown in Fig. 9) was obtained in high yield. Only traces of dioxane could be detected. 

Introduction of amino and carboxyl groups to metal compounds has been tried and some compounds have been reported. For stability (against oxidation, heat, moisture, acid, base, etc.) and synthetic facility, the selection of the ligand is confined to chelates with N or O donor sets and sufficiently stable organometals like ferrocene. Among monodentate ligands, pyridine is very useful for chemical modifi-... 

An important point of these ligands may be the combination of the flexible ferrocene backbone (to increase the catalytic activity) with the rigid phosphetane moiety (to control stereochemistry). In this respect, it is noteworthy that simple monodentate phosphetanes such as 79 perform better than the corresponding phospholanes in various rhodium-catalyzed hydrogenations, as a result of their restricted flexibility. 

Fig. 2.10 The ligand universe A stunning breadth of structural variation with members representing various motifs, e.g. atropisomeric biaryls, ferrocene-based bisphosphanes, P-chiral bisphosphanes, bisphosphites, monodentate phosphoramidites, AJ,P-ligands.

Compared with asymmetric intramolecular hydroacylation reactions of alkenes, the corresponding intermolecular versions have been relatively less developed. In 2007, Stemmier and Bolm reported the first example of enanti-oselective intermolecular hydroacylation reactions between salicylaldehydes 40 and norbornadiene-type alkenes 41 (Scheme 8.20). It was found that the diastereoselectivity of the reaction could be modulated through changing the chiral ligands. With ferrocene-based bisphosphine ligand L5, the exo-product 42a was obtained exclusively with up to 82% ee. When monodentate phos-phoramidite ligand L6 was employed, the e do-isomer 42b was obtained as major product with moderate enantioselectivity (54% ee). 

A further diphosphine complex, [Ru(dppf)(S2CNEt2)2] (dppf= l,l -bis(di-phenylphosphino)ferrocene), has been prepared in high yields upon addition of the diphosphine to cw-[Ru(PPh3)2(S2CNEt2)2] (1364). The NMR spectra are temperature dependent being associated with a fluxional process that results from dechelation of one of the dithiocarbamates, and the authors suggest that at low temperatures in chloroform, the molecule exists completely in a form with one monodentate dithiocarbamate ligand (Fig. 165). 

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