α-Amino Acids with Metallocenyl Side Chains

A straightforward method for the synthesis of enantiomerically pure bis(valine)metallocenes is presented. Derivatives of lithium cyclopentadienylvaline 1 a, b were obtained by addition of the (R)- or (S)-Schöllkopf reagents to 6,6-dimethylfulvene as single enantiomers and gave with FeCl2 or [RuCl2(dmso)4] the chiral metallocenes [Fe{C5H4-CMe2-[C4H2N2(OMe)2iPr]}2] (2 a, b) and [Ru{C5H4-CMe2-[C4H2N2(OMe)2iPr]}2] (3 a, b). Complex 2 b was hydrolyzed to the ferrocenylene-bis(valine-methylester) [{Fe[C5H4-CMe2-CH(NH3+)COOMe]2}2+(Cl−)2] (7) without racemization. Complex 7 could be used as ligand and was treated with [{Cp*IrCl2}2] to afford [Fe{C5H4-CMe2-CH(COOMe)(NH2-IrCp*Cl2)}2] (10). The reactions of 1 with CoCl2, [Re(CO)5Br], [{(cod)RhCl2}2] (cod=1,5-cyclooctadiene) or [Cp*MCl3] (M=Ti, Zr) gave the cyclopentadienyl complexes [{Co{C5H4-CMe2-[C4H2N2(OMe)2iPr]}2}+I−] (11) and [Re{C5H4-CMe2-[C4H2N2(OMe)2iPr]}(CO)3] (13), [(C8H12)Rh{C5H4-CMe2-[C4H2N2(OMe)2(iPr)]}] (14), [{Rh{C5H4-CMe2-[C4H2N2(OMe)2(iPr)]}I}2(μ-I)2] (15), [Cp*Cl2Ti{C5H4-CMe2-[C4H2N2(OMe)2(iPr)]}] (16), and [Cp*Cl2Zr{C5H4-CMe2-[C4H2N2(OMe)2(iPr)]}] (17), with chiral valine derivatives as substituents on the cyclopentadienyl ring and with excellent diastereoselectivities. Also the Seebach reagent (Boc-BMI) or O′Donnell reagent could be added to 6,6-dimethylfulvene to give the lithium cyclopentadienides Li[C5H4-CMe2-{C3H2(tBu)(N-Boc)(NMe)O}] (18) and Li[C5H4-CMe2-CH(NCPh2)(COOEt)] (21), which formed the ferrocene derivatives [Fe{C5H4-CMe2-[C3H2(tBu)(N-Boc)(NMe)O]}2] (19) and [Fe{C5H4-CMe2-CH(NCPh2)(COOEt)}2] (22). The stable cobaltocinium cation in 11 and the complex 19 could be hydrolyzed to the metallocenes 12 and [Fe{C5H4-CMe2-CH(NH3+)(COO−)}2] (20) with two valines in the 1,1′-position. The structures of 2 a, b, 11, 15, and 16 were determined by X-ray diffraction and confirm the diastereomeric purity of the compounds.