Thermal reactions of acyloxy and alkoxy carbene complexes with imines: Metathesis, acetate rearrangements, and a new route to imino carbene complexes via Peterson type eliminations

Article abstract of DOI:10.1021/om00162a028

The first examples of the reactions of imines (R³N=CHR²; R³ = CH₃, R² = Ph, p-MePh) with acetoxy Fischer carbene complexes ((CO)₅M=C(OAc)R¹; M = Cr, W, R¹ = Ph) are described. The reaction of the chromium complex gives exclusively the 0-acyl imidate 14 (MeN=C(OAc)Ph), which is the first example of a metathesis of an imine and a carbene complex. The tungsten complex gives a product that results from an unprecedented insertion of the imine into the carbene carbon-heteroatom bond along with the O-acyl imidate 14 as a minor product. This new insertion product was characterized by spectroscopic methods and by X-ray diffraction and determined to be the animal carbene complex 13b. Crystal data for 13b: space group P1, Z = 2, a = 8.269 (3) ?, b = 10.766 (5) ?, c = 13.851 (7) ?, α = 68.83 (5)°, β = 75.97 (6)°, γ = 81.39 (4)°, R = 0.046, and R_w = 0.050 for the 2267 reflections with F_o ≥ 5σ(F_o). It was determined that this insertion product is the result of an acetate transfer rather than a hydride transfer and that a likely intermediate in the mechanism is the zwitterion that results from initial addition of the imine nitrogen to the carbene carbon. The reaction of the acetoxy carbene complexes (CO)₅M=C(OAc)R¹ (M = Cr, W, R¹ = Ph, Me, tBu) with N-trimethylsilyl imines Me₃SiN=CHR² provides a new method for the synthesis of imino carbene complexes (CO)₅M=C(R¹)N=CHR² in a process that involves a Peterson type elimination. In a related reaction, it was found that imino complexes could also be accessed from the reaction of alkoxy carbene complexes (CO)₅M=C(OMe)R¹ (M = Cr, W, R¹ = Ph, Me, tBu) with N-trimethylsilyl imines.