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45648-AC3
Lewis Acid Chemistry at the Edge of Ferrocene

Frieder Jäkle, Rutgers, the State University of New Jersey (Newark)

Bidentate Lewis acids have attracted much recent interest due to their superior performance in diverse areas ranging from Lewis acid catalysis of organic and organometallic reactions, activation of catalysts in olefin polymerization, selective and efficient molecular recognition of anions and neutral nucleophiles, to new electronically interesting materials. The purpose of the research under this grant was to explore the chemistry of ferrocenes with two adjacent Lewis acid centers attached to one of the cyclopentadienyl rings. While the more easily accessible 1,1Õ-dimetalated ferrocenes have been widely studied in the past, the chemistry of the related 1,2-difunctionalized ferrocenes is virtually unexplored. The latter are of particular interest since they represent three-dimensional, redox-active analogs of the well-established and highly useful bidentate Lewis acids with phenylene and naphthalene backbones.

During the current grant period we have

(A)   Succeeded in the chiral resolution of heteronuclear bidentate Lewis acids that feature Lewis acidic organotin and organoboron moieties adjacent to one another at the edge of ferrocene and pursued their application in stereoselective organic transformations,

(B)   Studied the electronic structure and binding behavior of diboradiferrocenes, in which two adjacent Lewis acidic organoboron moieties lead to a doubly-bridged diferrocene system; a new route to these bifunctional Lewis acids that allows us to optimize the electronic structure through variation of the exocyclic boron substituents was developed.

(A) Chiral Resolution of Heteronuclear Bidentate Lewis Acids.

We first converted the bidentate Lewis acid 1-Cl to the methoxy derivative 1,2-Fc(BMeOMe)(SnMe₂Cl) (1-OMe) and then treated the latter with 0.5 equiv of (1R,2R)-(–)-N-methyl pseudoephedrine ((–)-MPE) and (1S,2S)-(+)-N-methyl pseudoephedrine ((+)-MPE), respectively (Scheme 1). The chelate complexes R_p-2-(–)-MPE and S_p-2-(+)-MPE were isolated as yellow solids with >97% de. The remaining unreacted isomers R_p-1-OMe and S_p-1-OMe, respectively, were subsequently converted back to the now enantiomerically pure bidentate Lewis acid R_p-1-Cl and S_p-1-Cl.

Scheme 1. Chiral resolution of the bidentate Lewis acid 1-Cl by complexation with N-methyl pseudoephedrine (MPE).

The new planar chiral organoboranes were then converted to the allyl derivatives, which in turn were successfully employed in the stereoselective allylation of ketones (upto 80% ee).

(B) Binding studies on diboradiferrocenes and tuning of the electronic structure.

We have developed a new and more versatile route to diboradiferrocenes starting from 1,2-bis(chloromercurio)ferrocenes. Conversion to the chloro-substituted diboracycle (fc₂B₂Cl₂) was achieved by treatment with BCl₃ at elevated temperature. The latter serves as a universal precursor to a variety of new diboradiferrocenes. Of particular interest to us was the possibility to tune the electronic structure of the bridge by variation of the exocyclic substituent on boron. The crystal structures of two new diboraferrocenes with mesityl (2-Mes) and electron-withdrawing pentafluorophenyl (2-Pf) groups are shown in Figure 1.

Figure 1. Molecular structures of 2-Mes and 2-Pf.

Importantly, studies on the electronic structure of 2-Mes and 2-Pf by UV-visible spectroscopy and cyclic voltammetry suggest a strong impact of the exocyclic substituent on the degree of electronic communication between the ferrocene moieties. Specifically, CV studies on 2-Pf revealed an exceptionally large redox coupling between the first and second oxidation (Figure 2), to our knowledge the largest reported to date for a heteroatom-bridged diferrocene species.

Figure 2. Cyclic voltammogram of 2-Pf reported vs. FcH/FcH⁺; * denotes a trace of ferrocene.

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