2-Cyclopentadienylglycine, a new α-amino acid and its use for η5-complex formation

The reaction of α-bromohippuric acid methyl ester with nickelocene, cyclopentadienylthallium or cyclopentadienyltrimethylsilane afforded N-benzoyl-2-cyclopentadienylglycine methyl ester (1) as two double bond isomers 1a and 1b. Similarly, the cyclopentadienyl(Cp)-containing dipeptide Z-l-Phe-dl-(Cp)Gly-OMe (3) was obtained from Z-l-Phe-dl-Gly(Cl)-OMe and trimethylsilylcyclopentadiene. The amino acids 1a, 1b were susceptible to Diels–Alder cycloaddition and yielded dimer 2 as a mixture of regioisomers. Compound 1 and FeCl2 gave 1,1′-ferrocenylene-bis(glycine) (5), whereas with [(COD)RhCl]2 rhodium η5-cyclopentadienylglycine (6) and η5-cyclopentadienyldipeptide complexes like 7 were obtained from 1 or 3, respectively. Unexpectedly, the η5-cyclopentadienylglycine complexes proved to be very sensitive to light.