Heterometallic alkylidyne clusters: Natural products as sources of chirality

Article abstract of DOI:10.1021/om00103a019

A series of mixed-metal clusters of the type MCo₂(CO)₆CCO₂(menthyl), where M = Co(CO)₃, (C₅H₅)-Mo(CO)₂, (i-PrC₅H₄)Mo(CO)₂, or (indenyl)Mo(CO)₂, have been synthesized and characterized by using FAB mass spectrometry and 2D NMR techniques. Treatment of the tricobalt cluster with Ph₂AsCH₂CH₂PPh₂ yields a pair of diastereomers which are interconverted via migration of the Ph₂As terminus of the arphos ligand from one cobalt vertex to another. This process has been followed by variable-temperature ³¹P NMR spectroscopy and has a barrier of ≈13 kcal mol^-1. When M ≠ Co(CO)₃, the two remaining cobalt vertices are diastereotopic and are, in principle, not equally susceptible to attack by an incoming ligand. To test for chiral discrimination, these molecules have been treated with phosphines of widely differing cone angles. For trimethyl phosphite, there is very little selectivity but with tricyclohexylphosphine the two diastereomers are formed in a 75:25 ratio.