Oxidative addition of N-H bond

These three emerging synthetic approaches appear to be of growing importance in the current literature. The first route concerns the deceptively simple oxidative addition of N H bonds to metal centres which is considered to be an essential step in the development of catalytic cycles for the addition of ammonia or amines to hydrocarbon substrates. A generalized cycle for the catalytic addition is illustrated in Scheme 6.1. 

Treatment of (silox)3Ta with arylamines H2NC6H4X leads to the oxidative addition of N-H bonds to give (silox)3-Ta(H)[NH(C6H4X)] and/or C-N activation to yield (silox)3-Ta(NH2)(CgH4X) (X = 4-NMe2, 4-OMe, 4-Me, 4-F, 3-F, 3-CF3, 3,5-CF3, 4-CF3, 4-Ph). (silox)3Ta activates the C-0 bonds in 1,2-dihydrofuran and 3,3-dimethyloxetane, forming the cyclic complexes (98a-b). These reactions mimic key steps in HDN and HDO processes. 

Oxidative addition of N-H bond is also known as shown in Eq. 3.60 [108]. 

Late-transition-metal-amido complexes have been prepared by metathetical substitution reactions, or-bonded ligand exchange, deprotonation of amine complexes, and oxidative addition of N-H bonds. Metathetical substitution is the most common route to late-metal-alkylamido complexes, whereas metathetical substitution and a-bonded ligand exchange have both been used commonly to prepare arylamido compounds. 

Reaction of Cp Re(CO)2(Bpin)2 (16), prepared from Cp Re(CO)j (15) and puiaBa, led to the regiospecific formation of 1-borylpentane in quantitative yield under irradiation of light in pentane. Thus, the catalytic cycle involves oxidative addition of pin2B2 to Cp Re(CO)j with photochemical dissociation of CO, oxidative addition of C-H bond to Cp Re(CO)2(Bpin)2 (16) giving a rhenium(V) intermediate (17), and finally reductive elimination of an alkylboronate with association of CO (Scheme 2.4) [51]. The interaction required for C-H activation of alkane with 16 is not known but higher reactivity of primary over secondary C-H bonds has been reported in both oxidative addition (17) and bond metathesis (18) processes [52]. Isomerization of a sec-alkyl group in Cp Re(H)(R)(CO)(Bpin)2 (17) to an n-alkyl isomer before reductive elimination of pinB-R is another probable process that has been reported in metal-catalyzed hydroboration of internal alkenes [15c]. 

The agostic -H-M bond sometimes plays a keyrole in polymerization according to this mechanism. Indeed, in the complexes that are responsible for the polymerization, the metal coordination sphere is unsaturated most often N E 1 and then traps the a -H bond of the a carbon i.e. the agostic bond. If the complex is d, the oxidative addition of this -H bond cannot occur, and the agostic bond that is involved all along the insertion mechanism can influence the stereochemistry of this insertion... 

The fact that complex 38 does not react further - that is, it does not oxidatively add the N—H bond - is due to the comparatively low electron density present on the Ir center. However, in the presence of more electron-rich phosphines an adduct similar to 38 may be observed in situ by NMR (see Section 6.5.3 see also below), but then readily activates N—H or C—H bonds. Amine coordination to an electron-rich Ir(I) center further augments its electron density and thus its propensity to oxidative addition reactions. Not only accessible N—H bonds are therefore readily activated but also C—H bonds [32] (cf. cyclo-metallations in Equation 6.14 and Scheme 6.10 below). This latter activation is a possible side reaction and mode of catalyst deactivation in OHA reactions that follow the CMM mechanism. Phosphine-free cationic Ir(I)-amine complexes were also shown to be quite reactive towards C—H bonds [30aj. The stable Ir-ammonia complex 39, which was isolated and structurally characterized by Hartwig and coworkers (Figure 6.7) [33], is accessible either by thermally induced reductive elimination of the corresponding Ir(III)-amido-hydrido precursor or by an acid-base reaction between the 14-electron Ir(I) intermediate 53 and ammonia (see Scheme 6.9). 

The surface complex [=Si-NH2] can be obtained on siUca by reaction of ammonia on silicajiooo) (Scheme 2.7) [9]. The addition of ammonia on siUca, normally unable to induce N-H cleavage at room temperature, becomes possible in this case because of the drastic dehydroxylation treatment, which produces a few highly strained [=Si-0-Si=] bridges (0.15 nm ) that undergo cleavage by bimolecular oxidative addition of otherwise unreactive bonds [10-13]. 

This review is written to cover the needs of synthetic chemists with interests in oxidizing alkenes by addition of nitrogenous substituents. Whilst some aspects have been covered in previous reviews (noted in the text), most notably in the Tetrahedron Report No. 144, Amination of Alkenes and prior reviews on aziridines and nitrenes, the present review is the fust conq>ilation of references to the whole range of these particular bond-forming processes. A review by Whitham provides a useful general introduction to reaction mechanisms of additions to alkenes in greater detail than can be covered here. The oxidation requirement excludes from the scope the additions of N H and most additions of N + Metal or N + C. Hence, unmodified Michael and Ritter reactions are excluded. These topics are mostly covered in Volume 4 of the present series. 

Campian MV, Harris JL, Jasim N, Perutz RN, Marder TB, Whitwood AC (2006) Comparisons of photoinduced oxidative addition of B - H, B - B, and Si - H bonds at Rhodium 5-cyclopentadienyl)phosphine centers. Organometallics 25 5093... 

Oxidative addition of the P-H and S-H bonds in phosphines and thiols are less well studied but are more favorable than oxidative aditions of N-H and 0-H bonds for thermodynamic and kinetic reasons. S-H and P-H bonds in thiols and phosphines are weaker and more acidic " than the 0-H and N-H bonds in alcohols and amines. Moreover, the resulting products from additions to late transition metal complexes possess proportionately stronger metal-sulfur and metal-phosphorus bonds. " ... 

This same palladium jt-addity can be employed to activate alkenes towards intramolecular cydization with heteroatom-hydrogen bonds. In contrast to alkynes or allenes, the addition of N H, O—H and other heteroatom-hydrogen bonds across the alkene would formally create a saturated carbon-carbon bond, rather than the unsaturation necessary for aromatic heterocydes. As such, subsequent P-hydride elimination is often required to dired these readions towards aromatic produds, in a Wacker-type alkene oxidation (Scheme 6.28). 

The C-H bond addition reactions across alkynes using a Rh catalyst such as RhC PPhjjj described earlier appear to proceed through oxidative addition of C-H toward a Rh species, alkyne insertion into the resulting Rh-H bond, and reductive elimination (path A in Scheme 18.82). An alternative pathway may be a sequence involving C-H metallation by an electrophilic Rh species, alkyne insertion into the resulting Rh-C bond, and protodemetallation (path B). The latter mechanism was proposed by Schipper ei al. [82] for their reaction of N-carbamoylindoles with alkynes in the presence of a cationic Rh catalyst (Scheme 18.83). 

Iridium The intermolecular hydroamination of unactivated C=C bonds in ct-olefins (RCH=CH2) and bicycloalkenes (norbornene and norbornadiene) with arylamides (ArCONH2) and sulfonamides has been attained upon catalysis by chiral iridium complexes (PP)IrHCl(NHCOAr)(NH2COAr) [PP = chiral bidentate diphosphine]. Mechanistic studies identified the product of N-H bond oxidative addition and coordination of the amide as the resting state of the catalyst. Rapid, reversible dissociation of the amide precedes reaction with the alkene, but an intramolecular, kinetically significant rearrangement of the species occurs before the reaction with alkene. ... 

Scheme 2 shows an alternative inner-sphere mechanism, first involving the initial oxidative addition of NuH to the metal followed by olefin insertion into the M-Nu bond. The resulting M-C bond is cleaved by a C-H reductive elimination or by protonolysis (Scheme 2).While this mechanism is generally preferred for more electron-rich metals such as rhodium and iridium, several studies suggest that platinum and palladium-catalyzed additions of N-H or O-H nucleophiles more likely run by the outer-sphere electrophilic activation mechanism shown in Scheme 1.

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