N H oxidative addition

Known examples of N—H oxidative addition prior to 1980 usually involved the addition of relatively acidic N—H bonds, for example that in phthalimide, to Pd(0) or Pt(0) centres. Work by Milstein in the 1980s showed that the parent amine NH3 can oxidatively add to Ir(I) centres (see Section 6.2.4) under mild conditions to afford parent amido hydrides in good... 

Functionalized tertiary aryl phosphines play an important role in transition metal coordination chemistry. These compounds have been used as ligands in synthesis, catalysis, mechanistic studies, and in the study of coordination compounds as structural models. In this contribution the syntheses of two new types of these ligands, tertiary aryl phosphines functionalized by an amide group, are detailed. The published coordination chemistry of these compounds includes the study of intramolecular N—H oxidative addition, the synthesis of chelates stabilized amido complexes, and the preparation of complexes with both ftve- and six-membered chelate rings. ... 

Scheme 2. Catalytic cycle for the hydroamination of olefins with activation of the amine by N-H oxidative addition to the transition metal.

As shown in Scheme 20, the insertion step from 21 to 22a in route 1 is more favored both kinetically and thermodynamically than the addition step from 21 to 23a in route 2 (9.6 and —37.2 vs. 24.7 and —18.8 kJ/mol, respectively). In addition, route 2 (23b to 23c) has much higher N-H oxidative addition barrier than that of route 1 (22c to 22d) (237.7 vs. 205.9 kJ/mol, respectively), and such high barriers are responsible for the dramatic reaction conditions. Direct comparison indicates that route 1 is more favored and less complicated than route 2. Therefore, the insertion-addition route 1 should be the most likely catalytic mechanism. 

Jones and coworkers described the formation of 7-methyl indoles from ruthenium-catalyzed C(sp )-H bond activation of 2,6-xylylisocyanide in 1986 (Eq. (7.1)) [6]. The catalytic reaction proceeded by using ruthenium(II) complex Ru(dmpe)2H2 or Ru(dmpe)2(2-naphthyl)H as catalyst (20mol%) in CgDg at high temperature (140°C). Further studies indicated that the substrate scope is restricted to 2,6-disubstituted (2,6-dimethyl, 2-ethyl-6-methyl, 2,6-diethyl) isocyanides, since less substituted isocyanides only produced stoichiometric indole N-H oxidative addition adducts with [Ru(dmpe)2]. 

Pyridine-functionalized N-heterocyclic carbene Rh and Ir complexes have also been described as active precatalysts for C=0 bond TH. For example, Peris and coworkers observed the formation of metal hydrides by C—H oxidative addition of a pyridine-N-substituted imidazolium salt such as N-"Bu-N -(2-pyridylmethyl-imidazolium) hexafluorophosphate in the reaction leading to M-pyNHC complexes, that is [lr(cod)H(pyNHC)Cl] (58) [54]. Transmetallation from silver carbene... 

The first successful catalytic animation of an olefin by transition-metal-catalysed N—H activation was reported for an Ir(I) catalyst and the substrates aniline and norbornene 365498. The reaction involves initial N—FI oxidative addition and olefin insertion 365 - 366, followed by C—FI reductive elimination, yielding the animation product 367. Labelling studies indicated an overall. vyw-addition of N—FI across the exo-face of the norbornene double bond498. In a related study, the animation of non-activated olefins was catalysed by lithium amides and rhodium complexes499. The results suggest different mechanisms, probably with /5-arninoethyl-metal species as intermediates. 

This shows that an excited state of the system is not required for C-H activation and suggests that the role of the light is merely to liberate an open site at the metal. The C-H oxidative addition itself is a dark reaction. Another interesting feature of this system is that initial insertion takes place into 1° and 2° C-H bonds of n-pentane, but that the system isomerizes at 110° to give the n-pentyl species. In a thermal equilibration experiment, it was estimated that the M-(n-pentyl) bond is 5.5 kcal/mol more stable than the 2° alkyl M-C6H bond. Kinetically, the relative reactivity of 1° and 2° C-H bonds is ca. 1.5 1 as expected for a very reactive and therefore unselective Cp IrL fragment. Thermal reaction was also used to form the... 

Although the oxidative addition of the N-H bond of NH3 and amines to transition metal complexes had been known for some time [140], it was only in the late 1980s that Milstein et al. succeeded in designing a homogeneously catalyzed hydroamina-tion reaction involving such an activation process (Eq. 4.27) [141]. 

The reaction mechanism clearly involves the oxidative addition of aniline to an unsaturated Ir(I) complex (Scheme 4-4). Interestingly, the azametallacyclobutane intermediate could be characterized by single-crystal X-ray diffraction [141]. This result confirms that insertion of an olefin into the M-H bond is less favorable than insertion into the M-N bond [142]. 

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