The dipeptide derivatives Boc-Gly-Val-OMe (2) and Boc-primary adduct 7-I (Ͼ 0 °C) resulted in methane elimination and formation of a mixture of the isomers 7-B and 7-A Ala-Val-OMe (3) selectively add the methylzirconocene cation 1 to the carbonyl oxygen atom of the N-terminal amino (isolated in a 3:4 ratio). According to the detailed NMR analysis 7-A shows the favored chelate coordination at the acid residue (κ-C 4 =O coordination) upon treatment with [Cp 2 ZrCH 3 (THF) + BPh 4 -] in dichloromethane at low N-terminus involving C 1 =O/N2/C 4 =O bonding, whereas the cation complex 7-B exhibits a similar chelate structure at the temperature (Ͻ 0 °C) to generate the complexes 2-I and 3-I, respectively. Above 0 °C methane is eliminated to give the internal Ala moiety, characterized by C 4 =O/N5/C 7 =O coordination to the zirconium center. Similarly, methane stable chelate peptide metallocene cation complexes 2-A and 3-A, respectively, both featuring a combined C 1 =O/N2/C 4 = liberation from 8-I and 9-I gave mixtures of the respective cationic chelate complex isomers 8-A/8-C (2:3) and 9-A/9-C O coordination to the zirconium center. Complex 3-A was characterized by X-ray diffraction. The analogous tripeptide
(1:1), respectively. The (8,9)-A isomers have the zirconium ion bonded at the N-terminus, whereas (8,9)-C exhibit the derivatives Boc-Gly-Val-Val-OMe ( 6), Boc-Ala-Ala-Val-OMe (7), Boc-Ala-Val-Val-OMe ( 8), and Boc-Val-Val-Gly-OMe (9) Cp 2 Zr + unit at the C-terminus of the peptide chain, involving the ester carbonyl group in forming the chelate framework all form analogous κ-C 4 =O Cp 2 ZrCH 3 + cation adducts (6-9)-I under kinetic control and after subsequent loss of CH 4 C 1 = (C 7 =O/N8/C 10 =O coordination). The (6-9)-A isomers are thermodynamically favored. The cation complexes 7-B, 8-C, O/N2/C 4 =O chelate complexes (6-9)-A under thermodynamic control, both involving selective bonding of the 9-C rearrange to the A-type isomers upon prolonged standing at temperatures Ͼ 0 °C in dichloromethane solution. organometallic cation to the terminal amino acid residue of the respective peptide derivatives.