From an Amine via C-H Activation by Zr

Zirconocene methyl amide complexes 1 are readily prepared by addition of lithiated secondary or N-silyl amines to zirconocene methyl chloride [17] or zirconocene methyl triflate [18] (Eq. 1). Loss of methane from 1 yields zirconaaziridines which, in the presence of THF or PMe3, can be isolated as the adducts 2 in high yield and purity. This synthetic method is ideal when the isolation and characterization of the resulting zirconaaziridine is desired, as the C-H activation and concomitant methane evolution occur with the formation of little side product. 

This method is most practical for complexes where methane elimination is facile and occurs below room temperature. Zirconaaziridines such as those in Fig. 1 (with N-aryl and N-silyl [19] substituents) have been readily prepared in this manner [20-22]. Buchwald noted that methane elimination from 1 is facile when the availability of the lone pairs on nitrogen is reduced [20] Whitby suggests that such reduced availability... [is] due to conjugation with the aromatic n system, or overlap with the Si d orbitals (or Si-C o orbitals) [23]. The rate of zirconaaziridine formation from le, for example, is faster by a factor of 1,000 than the rate of the formation from If (Fig. 2). 

Substituents in the R2 position that increase the acidity of the methylene protons on the amide ligand also increase the rate of methane elimination significantly. For example, methane elimination is facile to yield both the phenyl-substituted zirconaaziridine 2b and the a-silyl-substituted zirconaaziridine 2c [24], whereas the rate can decrease by a factor of 100 for alkyl-substituted zirconaaziridines [23]. The amide complex lb eliminates methane readily below -30 °C, whereas lg requires over 1 h at 60 °C (Fig. 3) [24]. 

These observations, along with kinetic isotope effect studies and Hammett correlation studies, support a concerted elimination by a-bond metathesis involving a four-membered transition state (Eq. 2) [23]. A large kinetic isotope effect is observed for the loss of methane from methyl amide complexes lb and lh (Eqs. 3 and 4), comparable to those observed by Buchwald and coworkers for formation of zirconium rj2-thioaldehyde complexes [25] and by Bercaw and coworkers for formation of tantalum rf- imine complexes [5a] through similar transition states. 

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