Cp2Mo2 with alkynes, reaction

Dicobalt-hexacarbonyl-alkyne complexes are another class of organometallic compounds with good stability imder physiological conditions. Complexation of the alkyne proceeds smoothly under mild conditions by reaction with Co2(CO)g imder loss of two molecules of CO [79]. The applicability of this reaction to peptides was shown by Jaouen and coworkers by the reaction of Co2(CO)g with protected 2-amino-4-hexynoic acid (Aha) and dipeptides thereof (Boc-Phe-Aha-OMe and Ac-Aha-Phe-OMe) [80]. Similarly, Cp2Mo2(CO)4 complexes of these alkynes were obtained. It has been shown that the C-terminal Met" in SP can be replaced by isostere analogs without appreciable loss of physiological activity. The same is true for the C-terminal Met in neurokinin A (NKA), another tachykinin peptide hormone (Scheme 5.16). Alkyne analogs of SP and NKA were obtained by replacement of these methionines with norleucine acetylene residues. Alternatively, Lys in NKA may be replaced by an alkyne derivative which can also be complexed to Co2(CO)g as shown in Scheme 5.16. Complexation with Co2(CO)g proceeds smoothly in about 50% yield for all derivatives [81]. After HPLC purification, these cobalt alkyne peptides were comprehensively characterized spectroscopically. Most notably, they exhibit typical IR absorptions for the metal carbonyl moieties between 2000-2100 cm [3]. Recently, there is renewed interest in Co2(CO)5(alkyne) complexes because of their cytotoxicity [82-84]. 

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