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. Enantiomerically pure α‐amino acids with metallocenyl side chains, such as 1 a,b and 2 a,b, were obtained by addition of the Schöllkopf and Seebach reagents to 6,6′‐dimethylfulvene and reactions of the lithium cyclopentadienides with metal salts.