Metallacyclopentene/metallaoxanorbornadiene chemistry. Synthesis, characterization, and reactivity of 1-(cyclopentadienyl)-1-cobalta-2-oxanorbornadiene complexes

Article abstract of DOI:10.1021/om00147a015

Reaction of (η⁵-C₅H₅)Co[C(R)C(CO₂CH ₃)CH(CO₂CH₃)CH(CO₂CH ₃)][P(C₆H₅)₃] (R = C₆H₅, 1a; R = CO₂CH₃, 1b) with silver salts or sulfur resulted in loss of triphenylphosphine to yield new metallabicyclic complexes 2a and 2b, respectively. The structure of 1-(cyclopentadienyl)-1-cobalta-5,7-dicarbomethoxy-3-methoxy-2-oxa-6- phenylnorbornadiene (2a), C₂₁H₂₁O₆Co, was determined by X-ray diffraction. The complex crystallized in the orthorhombic space group Pbca with a = 27.445 (5) A?, b = 8.826 (2) A?, c = 16.261 (3) A?, and Z = 8. The structure was refined by full-matrix least squares to R = 0.054. The [2.2.1] ring system of 2a is formed from the cobaltacyclopentene 1a by coordination to cobalt of the carbonyl oxygen atom from the ester on the β-sp³ carbon. Oxygen coordinates to cobalt solely on the opposite face of the cobaltacyclopentene from the phosphine ligand, resulting in inversion of the configuration of the chiral cobalt atom. Angles within the metallabicyclic complex are comparable to those in norbornadiene and imply the existence of considerable ring strain. Phosphines, phosphites, isocyanides, nitriles, and thiocyanate react cleanly with 2a and 2b to afford ligand-substituted cobaltacyclopentene complexes, many of which are not readily accessible from 1a or 1b. Ligands other than acetonitrile and thiocyanate add to 2 only from the face opposite the coordinated oxygen. ¹H and ¹³C NMR characteristics of cobalta-cyclopentenes and cobaltaoxanorbornadienes are presented.