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 hig h yield. This complex reacts further with additional imine to give N-t ert-butylphenylimine (5) and diazazirconacyclobutane 3. The latter rea ction, an imine metathesis, was subjected to a detailed kinetic study. The observed saturation kinetic behavior established that the first s tep in the reaction involves reversible apparent [2 + 2] retrocycloadd ition of diazametallacycle 2 to give imine 5 and the transient interme diate Cp(2)ZrN=Ph(6). The transient imido species is then trapped by a n analogous cycloaddition reaction with imine 4 to give 3. Supporting evidence for this mechanistic picture was provided by a parallel kinet ic study of the reaction of diazametallacycle 2 with diphenylacetylene and 2-butyne. These reactions give azazirconacyclobutenes 7a and 7b b y a reaction that is first order in 2 and zero order in alkyne. The fa ct 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 t hree reactions. Competition and equilibration studies provided data su fficient to construct a complete free energy diagram for the interconv ersion of azametallacycles 2 and 3 with one another and with azametall acyclobutene 7b. Imido complex 1 also undergoes [3 + 2] cycloaddition reactions with tert-butyl, phenyl, and p-tolyl azides, leading to meta llatetrazenes 8, 9a, and 9b. The structure of 8 was determined X-ray d iffraction (crystal data: space group: P2(1)2(1)2(1); a = 8.5618(13) A ngstrom, 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 te rt-butyl azide at 45 degrees C, followed by reaction with additional a zide to give tetrazene complexes 10a and 10b. Trapping studies with im ines and bicyclo-[2.2.1]hept-2-ene provide evidence that the first pha se of the azide metathesis involves retro[3 + 2]cycloaddition, extrudi ng t-BuN(3) to give Cp(2)Zr=NPh, followed by reaction of the latter sp ecies 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 structur e 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 mon itoring and control reactions demonstrated that these materials arise by initial formation of azametallacyclobutenes, followed by reaction o f the latter materials with organic azide.