Piperidine carbonylation, ruthenium

The ruthenium carbonyl complexes [Ru(CO)2(OCOCH3)] n, Ru3(CO)12, and a new one, tentatively formulated [HRu-(CO)s ] n, homogeneously catalyze the carbonylation of cyclic secondary amines under mild conditions (1 atm, 75°C) to give exclusively the N-formyl products. The acetate polymer dissolves in amines to give [Ru(CO)2(OCOCH3)(amine)]2 dimers. Kinetic studies on piperidine carbonylation catalyzed by the acetate polymer (in neat amine) and the iiydride polymer (in toluene-amine solutions) indicate that a monomeric tricarbonyl species is involved in the mechanism in each case. 

The stoichiometric carbonylation observed using [HRu(CO)3] and the proposed catalytic schemes all involve tricarbonyl species as the active catalyst the relatively high activity of Ru3(CO)i2 is consistent with this. The relative activity of the complexes for piperidine carbonylation is [HRu(CO)3L Ru3(CO)12 > [Ru(CO)2(OCOMe)]n. The major cause of the decrease in carbonylation rates is the accumulation of formyl product although the decrease in amine concentration is also a contributing factor. This catalyst poisoning is likely attributable to com-plexation to the ruthenium, presumably via the carbonyl grouping as commonly found for formamide ligands (26). The product could compete with either amine or CO for a metal coordination site. 

In the ruthenium catalysed carbonylation of piperidine (60 °C, 10 bar CO) the catalyst precursor, [Ru3(CO)i2] was found to be converted mainly to [Ru(CO)5], although IR absorptions due to other minor species were also observed [94]. A catalytic mechanism was tentatively proposed, which involved [RuCO)4] as the active... 

This paper summarizes our efforts to date and is concerned primarily with kinetic and mechanistic studies on the catalytic carbonylation of piperidine to N-formyl product using a ruthenium (I) -bridged acetate dicarbonyl polymer [Ru(CO)2(OCOCH3)] (6,7) and a less well-characterized polymeric hydridocarbonyl [HRu(CO)3]n (7). Kealy and Benson (8) noted the formation of N-cyclohexylformamide when car-bonylating cyclohexylamine in the presence of allene using Ru2(CO)9 [now known (9) to be Ru3(CO)i2] at high temperature and pressure. 

Adding N-formyl product (and other amides such as N,N-dimethyl-acetamide) decreases the carbonylation rate, and thus accumulation of product slowly poisons the catalyst. The rate for the piperidine system (Table I) after 70 hrs decreased to 0.4 X 10"5M sec1, and at this stage about 100 moles of amine were carbonylated per mole of ruthenium. Amine solutions of the dimers are quite stable at 75 °C for long periods in vacuo or under argon, and there is no trace of N-formylamine. 

Other Ruthenium Catalysts. Ru3(CO)i2 readily dissolved in piperidine to give a solution effective for catalytic carbonylation of the amine. The uptake plots resemble those shown in Figure 1 (curves B-E), and the maximum rate given in Table I refers to the initial rate. Attempts to characterize the ruthenium complexes formed from reaction of the dodecacarbonyl with amines have been unsuccessful. 

The results show that a number of ruthenium carbonyl complexes are effective for the catalytic carbonylation of secondary cyclic amines at mild conditions. Exclusive formation of N-formylamines occurs, and no isocyanates or coupling products such as ureas or oxamides have been detected. Noncyclic secondary and primary amines and pyridine (a tertiary amine) are not effectively carbonylated. There appears to be a general increase in the reactivity of the amines with increasing basicity (20) pyrrolidine (pKa at 25°C = 11.27 > piperidine (11.12) > hexa-methyleneimine (11.07) > morpholine (8.39). Brackman (13) has stressed the importance of high basicity and the stereochemistry of the amines showing high reactivity in copper-catalyzed systems. The latter factor manifests itself in the reluctance of the amines to occupy more than two coordination sites on the cupric ion. In some of the hydridocar-bonyl systems, low activity must also result in part from the low catalyst solubility (Table I). 

Reaction 4 shows that the ruthenium center with three coordinated carbonyls can transfer one such ligand to the piperidine (presumably coordinated). The mechanism suggested for the acetate complex includes exactly analogous steps (Reactions 6 and 7). The kinetics for the hydride-catalyzed system, however, are quite different and show a first-order dependence in Ru and a more complex dependence on CO (Figure 4). Further, no autocatalysis is evident. 

Recent work by Ford et al. demonstrates that a variety of metal carbonyl clusters are active catalysts for the water-gas shift under the same reaction conditions used with the ruthenium cluster (104a). In particular, the mixed metal compound H2FeRu3(CO)13 forms a catalyst system much more active than would be expected from the activities of the iron or ruthenium systems alone. The source of the synergetic behavior of the iron/ruthenium mixtures is under investigation. The ruthenium and ruthenium/iron systems are also active when piperidine is used as the base, and in solutions made acidic with H2S04 as well. Whether there are strong mechanistic similarities between the acidic and basic systems remains to be determined. 

When the neutral cluster Ru3(CO),2 (1) was employed as the catalyst for the carbonylation of cyclic amines, the carbamoylato clusters of type 2 were isolated from the reaction mixture and characterized. In the case of piperidine, the corresponding carbamoylato cluster HRu3(CO)10 [OCN(CH2)s] (2a) was shown to have the same catalytic properties as 1. On the basis of these findings, a catalytic cycle involving exclusively trinu-clear ruthenium species has been proposed (Scheme 2) The amine is believed to attack a carbonyl ligand in 1, which provokes the formation of... 

The results of catalysis with primary and secondary amines indicate that ruthenium carbonyl forms active catalysts in aqueous ethylenediamines, diethanolamine, pyrrolidine and piperidine solutions. No detectable amounts of hydrogen are formed with aromatic and unsaturated primary and secondary amines under the present WGS conditions. Tertiary amines were found not to initiate catalyst systems as active as those produced by the best primary and secondary amines. Aliphatic tertiary amines exhibited only weak activity. 

Ruthenium. Cyclic secondary amines, including morpholine and piperidine, can be carbonylated to give iV-formyl derivatives in the presence of ruthenium carbonyl compounds [Ru(OaCMe)(CO)2ln or Ru3(CO)ig. With the former catalyst it is possible to isolate an intermediate [RuCOgCMe)-(amine)(CO)2]OT, where m is probably 2. ... 

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