Tantalum complexes with imines

Low-valent niobium and tantalum react with imines to give the corresponding imine-niobium or -tantalum complexes. Some of these are isolated, and structures of the complexes are confirmed by X-ray analyses. These imine complexes react with aldehydes to give /3-hydroxyamines in good to excellent yields (Equation... 

Although monodentate isocyanide-tantalum complexes are available by alternative routes,204 the majority of studies are concerned with the resultant 2-imines. Conversely, similar reactions with nitriles give monodentate imine moieties, which can undergo reactions while remaining attached to the metal.195,203-209... 

Acetylenes 191 (R1 = C5H11, Me3Si or MeS R2 = C5H11 or C10H21) react with tantalum ) chloride and zinc to form tantalum complexes. Addition of the imine... 

Treatment of tantalum-alkyne complexes, prepared in situ from TaCls, Zn and RC=CR (R = n-C5Hn), with the lithio-imine Li N=CR Me (R = n-CgHn), followed by aqueous NaOH, gives primary ( )-allylic amines E-RCH=CR-CR Me(NH2). Treatment of these complexes with the terminal alkyne R"C=CH (R" = n-CeHis), followed by aqueous NaOH, yields tetrasubstituted benzene derivatives (99a-d). ... 

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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