Cobalt metallacycles. Part 15. Regioselective formation and rearrangement of 4-iminocobaltacyclobutene complexes

Abstract

The reaction of [Co(η⁵-C₅H₅)(PPh₃)(R¹CCR²)](1)(R¹= H, Me, or Ph; R²= Ph, CO₂Me, or CN) with isocyanides R³NC (R³= Ph, p-MeC₆H₄, or 2,6-Me₂C₆H₃) gives the 1-(η⁵-cyclopentadienyl)-4 imino-1-(triphenylphosphine)cobaltacyclobutene complexes [[graphic omitted](NR³)}(η⁵-C₅H₅)(PPh₃)](3). When (1) containing an unsymmetrical acetylene is used, the reaction is highly regioselective as only one of the two possible regioisomers has been detected. It has been concluded that the regioselectivity originates not from a steric effect but from an electronic requirement such that the acetylenic carbon with an electron-withdrawing substituent becomes the a carbon of the metallacycle. That this requirement leads to kinetically-controlled products has been demonstrated by thermal skeletal rearrangement of (3bi)(R¹= Ph, R²= CO₂Me) and (3ci)(R¹= Ph, R²= CN) to [[graphic omitted](NC₆H₄Me-p)}(η⁵-C₅H₅)(PPh₃)](8a)(R¹= CO₂Me, R²= Ph) and (8b)(R¹= CN, R²= Ph), respectively. These regioselectivity observations have been rationalized by ab initio molecular orbital calculations. The imino configuration in these complexes is syn with respect to the metal and the substituent on the nitrogen. Methyl iodide reacts with (3bi) to give a PF₆^– salt of [[graphic omitted](NMeC₆H₄Me-p)}(η⁵-C₅H₅)(PPh₃)]⁺(9a), where the methyl group is bonded to the nitrogen anti to the metal. In solution, (9a) isomerizes to (9b) at room temperature by rotation of the CN bond. Reactions of (3ai) with PhC₂Ph, CO, CS₂, and MeNCS have been studied.