Nitrogenation-transmetallation

Scheme 2.176 Palladium-catalyzed nitrogenation-transmetallation process.

In 1991, a new C-N-C bond formation reaction using a nitrogenation-transmetalation process was described. Ketones and aryl or vinyl halides couple to give divinyl or arylvinyl amines in the presence of the titanium isocyanate complex [3THF MgaClaO TiNCO] and a palladium catalyst, via transmetallation of the titano imine complex with aryl or vinyl palladium bromide. Moderate to good yields of the desired products were observed (Scheme 2.176). 

Silver(I) complexes with macrocyclic nitrogen ligands are also very numerous. Mono- or homodi-nuclear silver-containing molecular clefts can be synthesized from the cyclocondensation of functionalized alkanediamines or triamines with 2,6-diacetylpyridine, pyridine-2,6-dicarbalde-hyde, thiophene-2,5-dicarbaldehyde, furan-2,5-dicarbaldehyde, or pyrrole-2,5-dicarbaldehyde in the presence of silver(I).486 97 The clefts are derived from bibracchial tetraimine Schiff base macrocycles and have been used, via transmetallation reactions, to complex other metal centers. The incorporation of a range of functionalized triamines has provided the conformational flexibility to vary the homodinuclear intermetallic separation from ca. 3 A to an excess of 6 A, and also to incorporate anions as intermetallic spacers. Some examples of the silver(I) complexes obtained are shown in Figure 5. 

Low chemical yields and a-selectivity on the allyl copper reagent are observed with the phenylthio analogues. These observations suggest that the pyridine nitrogen facilitates transmetalation and or cuprate reactivity and plays a role in the regio-selectivity of the reaction. 

In 2000, Chamboumier and Gawley reported the failure of a conformationaUy locked 2-(tributylstannyl)piperidine to transmetalate when the 2-(tributylstannyl) moiety of a 4-fert-butylpiperidine is equatorial, Sn/Li exchange is facile, but when the 2-tributylstannyl group is axial, transmetalation fails (compare Figure 3d,i). In other words, in these conformationaUy rigid piperidines, there appears to be a configurational requirement for transmetalation the nitrogen atom lone pair has to be synclinal to the adjacent carbon-tin bond for transmetalation to succeed. 

Examples exist of rearrangements in which a halogen atom migrates from annular nitrogen to carbon, or from one ring carbon to another ( halogen-dance or transmetalation). Such processes occasionally have preparative importance, and they will be covered in the appropriate sections. 

The chemistry outlined in Scheme 24 was then put into effect catalytic hydrogenation of the tris-isoxazole (302) and recyclization with triethylamine gave a tricyclic ligand which was chelated with nickel ions to give (303). Introduction of the fourth nitrogen atom was accomplished by treatment of (303) with ammonium acetate, giving (304). Treatment with cyanide removed the nickel ion which was then replaced with zinc(II) to give (305). The reasons for this transmetallation step were two-fold firstly, zinc(II) corrins, as shown by Eschenmoser, can be readily demetallated, and this fact opens up many options later in the synthesis, but secondly, and more importantly, Eschenmoser s photochemical cyclization of seco-corrins (see Section 3.07.3.4.2.3) does not proceed with nickel complexes of seco-corrins, whereas zinc(II) seco-corrins can be cyclized in almost quantitative yield... 

---