A Synthetic and Mechanistic Study of Oxycarbene-like Coupling Reaction Patterns of Actinide η²-Acyl Complexes with Carbon Monoxide and Isocyanides

This contribution reports the synthesis and characterization of the ylide complexes Cp’₂Th[OC(CH₂-t-Bu)C-(PR₃)O](C1) (Cp‘ = η⁵-C₅Me₅; R = Me, Ph) formed by the carbonylation of Cp’₂Th(Cl)(η²-COCH₂-t-Bu) (1) in the presence of phosphines. Isotopic tracer studies with ¹³CO indicate that the labeled carbon atom is incorporated regiospecifically at the ylide a-carbon atom position. The carbonylation of 1 to yield the enedionediolate Cp’₂Th(Cl)(η²-COCH₂-t-Bu) (2) or the ylide complexes was found to obey a second-order rate law where rate = KP_CO[1]; k = 1.50 (5) X 10^-5 min^-1 torr^-1for both carbonylations at 30.8 °C. The rate of formation of the ylide complexes was also found to be independent of solvent, phosphine concentration, and type of phosphine used. The carbonylation of 1 is therefore inferred to involve a rate-determining coupling of CO with the acyl to yield an intermediate ketene or ketene-like complex. The reaction of acyl 1 or Cp’₂Th-(Cl)(η²-COCH₂Ph) with a variety of isocyanides (RNC; R = t-Bu, C₆H₁₁, 2,6-Me₂C₆H₃) results in the formation of ketenimine complexes of the type Cp’₂Th[OC(CH₂-t-Bu)CNR](Cl) and Cp^/₂Th[OC(CH₂Ph)CNR](Cl). Addition of a second equivalent of isocyanide yields simple adducts of the type Cp^/₂Th[OC(CH₂-t-Bu)CNR](CNR)(Cl) and Cp’₂Th[OC(CH₂Ph)CNR]-(CNR)(C1). These new complexes have been fully characterized by standard techniques and the ketenimine Cp’₂Th(Cl)-[OC(CH₂-t-Bu)(CN-2,6-Me2C₆H₃)] by single-crystal X-ray diffraction. Under the appropriate conditions, the N-aryl complexes suffer secondary reactions in which the η²-acyl ultimately undergoes coupling with 2 equiv of isocyanide to yield acetylindolate and acetylindoleninate derivatives. The η²-acyl was also found to undergo coupling with 1 equiv of diphenylketene to yield the complex Cp^/₂Th[OC(CH₂-t-Bu)C(CPh₂)O](Cl). These coupling products have been fully characterized by conventional analytical and structural methodology. The mechanistic and reactivity results of this study are discussed in terms of the anchored Fischer carbene-like character of actinide η²-acyls and the mechanism by which the carbonylation of 1 yields the enedionediolate 2.