Amination amido complex formation

Noyori and coworkers reported well-defined ruthenium(II) catalyst systems of the type RuH( 76-arene)(NH2CHPhCHPhNTs) for the asymmetric transfer hydrogenation of ketones and imines [94]. These also act via an outer-sphere hydride transfer mechanism shown in Scheme 3.12. The hydride transfer from ruthenium and proton transfer from the amino group to the C=0 bond of a ketone or C=N bond of an imine produces the alcohol or amine product, respectively. The amido complex that is produced is unreactive to H2 (except at high pressures), but readily reacts with iPrOH or formate to regenerate the hydride catalyst. 

A number of amide derivatives of 114 have also been synthesised. The reactions of the methoxy and hydroxo complexes 162 and 165 with a range of primary and secondary aliphatic and aromatic amines resulted in formation of a number of air-and solution-stable amido derivatives 171-176.74,75 The related monoamido complex containing a cycloaurated benzylpyridine ligand, 177 was also prepared.75 The stability of the toluidine 172 and xylidene 173 complexes in water or a physiological-like buffer solution has been investigated by UV-visible and NMR spectroscopies, and rapid hydrolysis occurred, probably giving the hydroxo complex 165.79... 

A direct consequence of the enhanced acidity of co-ordinated amines is seen in the reactions with chlorine in aqueous solution of some platinum(iv) complexes. In these reactions the nucleophilic attack of an intermediate amido complex upon chlorine leads to the formation of dichloroamido complexes (Fig. 5-29). 

The protonation and complex formation of a number of poly(amido-amine)s in aqueous solution have been studied by potentiometric, calorimetric, viscosimetric, spectrophotometric, esr, 13C nmr, and quantum chemical techniques. 

For the case of tri(o-tolyl)phosphine-ligated catalysts, the upper pathway appears to predominate. Oxidative addition occurs first via loss of a ligand from the bisphosphine precursor to form the oxidative adduct, which exists as a dimer bridged through the halogen atoms (equation 33). This dimer is broken up by amine, the coordination of which to palladium renders its proton acidic. Subsequent deprotonation by base leads to the amido complex, which can then reductively eliminate to form the product. When tert-butoxide is used as the base, the rate is limited by formation of and reductive elimination from the amido complex, while for the stronger hexamethyldisilazide, the rate-determining step appears to be oxidative addition. ... 

Some fundamental inorganic chenustry that is important for understanding which complexes will undergo the aromatic C—and C—O bond-forming processes will be presented before the catalytic transformations. First, the three reaction types involved in the catalytic cycle to form arylanunes are similar to those found in the catalytic cycle for C—C bond formation oxidative addition of aryl halide to Pd(0) complexes, transmetallation that converts an arylpalladium halide complex to an arylpaUadium amido complex, and reductive elimination to form a C—or C—O bond. The oxidative addition step is identical to the addition that initiates C—C bond-fomting cross-couplings,f f but the steps that form the arylpalladium amido complexes and that produce the arylamine product are different. The mechanism for these steps is discussed after presentation of the scope of the amination process. 

The final pathway, which involves alkoxide intermediates, has been shown to occur when reactions employ NaO-t-Bu as base and DPPF as ligand (Eq. 52). DPPF-ligated arylpalladium t-butoxide complexes have been isolated. Reactions of these complexes with arylamines led to rapid formation of the amido complex and subsequent reductive elimination of diarylamine. Reactions with secondary alkylamines led to formation of di-alkylanilines, presumably through the intermediacy of an arylpalladium dialkylamido intermediate. Reaction of the DPPF-ligated, arylpalladium halide complexes with alkoxide and dialkylamine together first formed the alkoxide complex. This complex subsequently reacted with amine and reductively eliminated the dialkylaniline. The observation of the alkoxide complex in this reaction strongly suggests the intermediacy of the alkoxide complex in the catalytic reactions. ... 

Insertion and a-elimination reactions of early-metal-amido complexes are presented in Chapters 9 and 10 of this book. In addition to these processes, amido complexes undergo several important reactions that cause elimination of amine or formation of imido complexes. 

The mechanism of this process has been studied in detail. The identity of the palladium(O) species that lies on the catalytic cycle/ the effect of anions on the oxidative addition step/ " the effect of amines in the dissociation of chelating ligands from the palladium(O) complex during the oxidative addition/ the mechanism of formation of the amido complex/ and the mechanism of reductive elimination of amine - - have all been studied. The oxidative addition of aryl chlorides and bromides is generally the turnover-limiting step of the catalytic cycle. 

The insertion of CO into Pd-carbon bonds has also been employed in several tandem/cascade reactions that afford five-membered nitrogen heterocycles [97]. A representative example of this approach to the construction of heterocydes involves synthesis of isoindolinones via the Pd-catalyzed coupling of 2-bromobenzaldehyde with two equivalents of a primary amine under an atmosphere of CO [97bj. As shown below (Eq. (1.57)), this method was used for the preparation of 144 in 64% yield. The mechanism of this reaction is likely via initial, reversible condensation of 2-bromobenzaldehyde with 2 equiv of the amine to form an aminal 145. Oxidative addition of the aryl bromide to Pd° followed by CO insertion provides the acylpalladium spedes 146, which is then captured by the pendant aminal to afford the observed product. An alternative mechanism involving intramolecular imine insertion into the Pd—C bond of a related acylpalladium species, followed by formation of a paUadium-amido complex and C—N bond-forming reductive elimination has also been proposed [97b],... 

In careful stoichiometric reactivity with unactivated aminoalkene substrates, Wolfe prepared a bis(diphenylphosphino)ferrocene (dppf)-Hgated Pd(II)-mixed aryl amido complex, which could be observed by NMR spectroscopy to undergo alkene insertion and subsequent product formation via reductive eHmination (Scheme 15.90) [146]. Labeling experiments confirmed this as the first example of the syn addition of the N- and aryl-substituent to the face of the alkene, as would be expected for this often postulated step in various Pd-catalyzed reactions including alkene carboamination, diaminations, and oxidative amination [146]. 

D.v.a. Formation of C—N Bonds. Though Pd-catalyzed amination— the Hartwig-Buchwald reaction—is normally performed in anhydrous media in the presence of strong bases, no steps of the mechanism of this reaction strictly require the absence of water. Moreover, it has been shown that amido complexes of Pd, the key intermediates of this reaction, can easily form by ligand exchange of water or hydroxyl (Scheme 56). ... 

Three general mechanisms can be envisioned for the formation of amido complexes from arylpalladium halides direct substitution of halide by alkali amide generated by simple deprotonation of free amine by free base, coordination of amine to the metal center followed by deprotonation of the more acidic coordinated amine, or formation of a palladium alkoxide... 

NMR spectroscopy has been used to determine the thermodynamics of borate ester formation by three readily grafted carbohydrates. Boron specia-tion in borate-poly(amido amine) dendrimer has been performed using B NMR spectroscopy. The role of inositol in the synthesis of FAU- and LTA-type zeolites has been investigated using H, C, Na, and Al NMR spectroscopy. Ulvan-boron complex formation has been studied using B and NMR spectroscopy. The complexation of borate ions and humic acid fractions has been analysed with B and H NMR spectroscopy. ... 

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