Dehydrogenation amine complexes

Complexes with other Bidentate 1,4-Diimines. The thermodynamic stability of the five-membered iron(II) 1,4-diimine chelate ring in low-spin complexes is illustrated by the facile autoxidation of saturated amine complexes to the corresponding colored 1,4-diimine complexes. The first example of this oxidative dehydrogenation is shown in equation (14), and the more recently studied " reaction (15) is known to involve iron(III) species and radical intermediates. Interestingly, the complex [Fe(CN)4(HN=CHCH=NH)] can be obtained by oxidizing [Fe(CN)4(en)] , whereas the tris-ligand complex of this smallest diimine ligand has yet to be prepared. 

The oxidative dehydrogenation polymerization of 2,6-dialkylphenoles has been known for many years to be catalyzed by copper amine complexes [1] see figure for 2,6-dimethylphenol oxidation. The mechanism of action of this industrially very important reaction has been studied for some time by several groups. 

As first shown in the laboratory here, (raHjr-Ru(TMP)(0)2 can oxidatively dehydrogenate primary and secondary amines imder stoichiometric and catalytic conditions in benzene at 50°C (with O2 as oxidant) . For the former, the reaction stoichiometry depends on the number of H-atoms in the a-position of the amine, and for primary amines can be presented primarily by eqs. 38 and 39 Ru = Ru(TMP)) imines or nitriles are formed with generation of isolable Ru(II)-bis(amine) complexes, and an X-ray stmcture of Ru(TMP)(NH2CH2Ph)2 was determined . 

Like mthenium, amines coordinated to osmium in higher oxidation states such as Os(IV) ate readily deprotonated, as in [Os(en) (NHCH2CH2NH2)] [111614-75-6], This complex is subject to oxidative dehydrogenation to form an imine complex (105). An unusual Os(IV) hydride, [OsH2(en)2] [57345-94-5] has been isolated and characterized. The complexes of aromatic heterocycHc amines such as pyridine, bipytidine, phenanthroline, and terpyridine ate similar to those of mthenium. Examples include [Os(bipy )3 [23648-06-8], [Os(bipy)2acac] [47691-08-7],... 

Oxidative dehydrogenations of many macrocyclic ligand complexes have now been documented. Typically, these reactions involve conversion of coordinated secondary amines to imine groups. 

Remarkably, the same Shvo complex can be used for the catalytic transfer dehydrogenation of aromatic amines to give imines (Scheme 7.14) [80]. This reaction produces high yields when carried out for 2-6 h in refluxing toluene with 2 mol.% catalyst. A quinone is used as the hydrogen acceptor, giving the corresponding hydroquinone. 

Consequently, by choosing proper conditions, especially the ratios of the carbonyl compound to the amino compound, very good yields of the desired amines can be obtained [322, 953]. In catalytic hydrogenations alkylation of amines was also achieved by alcohols under the conditions when they may be dehydrogenated to the carbonyl compounds [803]. The reaction of aldehydes and ketones with ammonia and amines in the presence of hydrogen is carried out on catalysts platinum oxide [957], nickel [803, 958] or Raney nickel [956, 959,960]. Yields range from low (23-35%) to very high (93%). An alternative route is the use of complex borohydrides sodium borohydride [954], lithium cyanoborohydride [955] and sodium cyanoborohydride [103] in aqueous-alcoholic solutions of pH 5-8. 

Primary amines at a primary carbon can be dehydrogenated to nitriles. The reaction has been carried out with a variety of reagents, among others, IF5,"9 lead tetraacetate, 20 nickel peroxide,121 NaOCl in micelles,122 S g-NiSO, 2-1 and CuCl-02-pyridine.124 Several methods have been reported for the dehydrogenation of secondary amines to imines.125 Among them126 are treatment with(l) iodosylbenzene PhIO alone or in the presence of a ruthenium complex, 27 (2) Me2SO and oxalyl chloride, 2" and (3) f-BuOOH and a rhenium catalyst. 29... 

We have already seen that imines may be formed by the oxidative dehydrogenation of co-ordinated amines and that this is a commonly observed process, particularly in macrocyclic systems. Likely mechanisms for these dehydrogenations were suggested in Chapter 5, which emphasised the role of the variable oxidation state metal ions in the process. These reactions are quite general and many examples involving iron or ruthenium complexes have been studied in detail. 

Although oxidative dehydrogenation reactions are particularly well characterised with macrocyclic complexes, even very simple amine ligands such as 1,2-diaminoethane may be oxidised to the corresponding imines (Fig. 9-23). 

Such an example has been demonstrated by Johnson and Sames, who chose a platinum-mediated dehydrogenation as a key step in the synthesis of the antimitotic rhazinilam 33 (Scheme 6) [20], The key intermediate 27 was converted into the imine 28, which was allowed to react with Me Pt(//-SMe2)]2 to afford the platinum complex 29. Subsequent treatment with triflic acid resulted in elimination of methane and furnished the cationic complex 30. Upon thermolysis in trifluoroethanol, the complex lost a second methane molecule, which resulted in the activation of the ethyl group. A subsequent /1-hydride elimination gave the hydrido-Pt(n) complex 31. Treatment with aqueous KCN followed by hydrox-ylamine removed the platinum and yielded the liberated amine 32. Johnson and Sames added a homologization and a macrolactamization and completed the total synthesis of rhazinilam (33) by removal of the carboxyl group. 

A primary alcohol and amines can be used as an aldehyde precursor, because it can be oxidized by transfer hydrogenation. For example, the reaction of benzyl alcohol with excess olefin afforded the corresponding ketone in good yield in the presence of Rh complex and 2-amino-4-picoline [18]. Similarly, primary amines, which were transformed into imines by dehydrogenation, were also employed as a substrate instead of aldehydes [19]. Although various terminal olefins, alkynes [20], and even dienes [21] have been commonly used as a reaction partner in hydroiminoacylation reactions, internal olefins were ineffective. Recently, methyl sulfide-substituted aldehydes were successfully applied to the intermolecu-lar hydroacylation reaction [22], Also in the intramolecular hydroacylation, extension of substrates such as cyclopropane-substituted 4-enal [23], 4-alkynal [24], and 4,6-dienal [25] has been developed (Table 1). 

Due to its enhanced jr-deficiency 3,5-dinitropyridine has a lower site selectivity and it can be expected that oxidative amino-dehydrogenations will take place at all three positions 2, 4, and 6. This has indeed been found. 3,5-Dinitropyridine gives a complex mixture of 2-amino-, 2,6-diamino-, 2,4-diamino-, and 2,4,6-triamino-3,5-dinitropyridines (Scheme 5) (85JOC484, 93ACS95). Mixtures of diamino- and triamino-3,5-dinitropyridines have also been found in the oxidative amination of 2-R-3,5-dinitropyridines (R = NH2, OH, Cl, OMe) and 4-R-3,5-dinitropyridines (R = NH2, Cl). 

Schwoegler and Adkins (93) reacted alcohols with primary and secondary amines over Raney nickel to form secondary and tertiary amines, respectively. Since tertiary alcohols do not undergo the reaction, it is assumed that the catalyst dehydrogenates the alcohol to a carbonyl compound which reacts with an amine to give a product that can be readily hydrogenated to a more complex amine. Piperidine was reacted with ethyl, n-butyl, and n-dodecyl alcohols to give 82, 70, and 69% yields, respectively, of the corresponding alkylpiperidines. 

---