A MECHANISTIC STUDY OF THE CYCLOADDITION-CYCLOREVERSION REACTIONS OF THE ZIRCONIUM IMIDO COMPLEX CP(2)ZR(N-T-BU)(THF) WITH ORGANIC IMINES AND AZIDES

Treatment of Cp(2)Zr(=N-t-Bu)(THF) (1) with benzaldehyde phenylimine (4) at room temperature gives 2,4-diaza-1-zirconacyclobutane (2) in high yield. This complex reacts further with additional imine to give N-tert-butylphenylimine (5) and diazazirconacyclobutane 3. The latter reaction, an imine metathesis, was subjected to a detailed kinetic study. The observed saturation kinetic behavior established that the first step in the reaction involves reversible apparent [2 + 2] retrocycloaddition of diazametallacycle 2 to give imine 5 and the transient intermediate Cp(2)ZrN=Ph(6). The transient imido species is then trapped by an analogous cycloaddition reaction with imine 4 to give 3. Supporting evidence for this mechanistic picture was provided by a parallel kinetic study of the reaction of diazametallacycle 2 with diphenylacetylene and 2-butyne. These reactions give azazirconacyclobutenes 7a and 7b by a reaction that is first order in 2 and zero order in alkyne. The fact that the saturation rate constants measured in these reactions are identical to the one observed in the imine metathesis provides strong evidence that the same intermediate, Cp(2)Zr=NPh, is involved in all three reactions. Competition and equilibration studies provided data sufficient to construct a complete free energy diagram for the interconversion of azametallacycles 2 and 3 with one another and with azametallacyclobutene 7b. Imido complex 1 also undergoes [3 + 2] cycloaddition reactions with tert-butyl, phenyl, and p-tolyl azides, leading to metallatetrazenes 8, 9a, and 9b. The structure of 8 was determined X-ray diffraction (crystal data: space group: P2(1)2(1)2(1); a = 8.5618(13) Angstrom, b = 14.3507(18) Angstrom, c = 15.1037(17) Angstrom; Z = 4; R = 5.1; R = 4.8%; R(w) = 6.6%). These materials undergo extrusion of tert-butyl azide at 45 degrees C, followed by reaction with additional azide to give tetrazene complexes 10a and 10b. Trapping studies with imines and bicyclo-[2.2.1]hept-2-ene provide evidence that the first phase of the azide metathesis involves retro[3 + 2]cycloaddition, extruding t-BuN(3) to give Cp(2)Zr=NPh, followed by reaction of the latter species with additional azide. Attempts to trap Cp(2)Zr=NPh with alkynes in the presence of azide led to new complexes 13 and 14. The structure of 14 was determined by X-ray diffraction (crystal data: space group: P2(1)/n; a = 9.927(2) Angstrom, b = 13.377(2) Angstrom, c = 18.068(3) Angstrom; alpha = 90.0 degrees, beta = 97.605(14)degrees, gamma = 90.0 degrees; Z = 4; R = 13.6; R = 7.2%, R(w) = 8.3%). Spectroscopic monitoring and control reactions demonstrated that these materials arise by initial formation of azametallacyclobutenes, followed by reaction of the latter materials with organic azide.