Metal-catalysts piperidines

Pipendine. the hydrogenation product of pyridine, is used as an intermediate for drugs and for making rubber-vulcanization accelerators, e.g., piperidinium pentamechylenedithiocarbamate (also known as Accelerator 552 ). On a commercial scale, piperidine (hexahydropyridine) is prepared by the catalytic hydrogenation of pyridine, e.g., with nickel catalysts at from 68 to 136 atmospheres pressure and at l50-20C,c,C. or under milder conditions with noble-metal catalysts. Pyridine derivatives can be similarly reduced to substitute piperidines. See formulas below. 

During the past several years the use of secondary phosphine oxides, chloro-phosphines, and related species as ligands for late-transition metal catalysts has been explored by several groups (reviews [81, 82]). Catalysts supported by these unusual ligands have shown some utihty in Pd-catalyzed A-arylation reactions. For example, treatment of 4-chlorobenzotrifluoride with piperidine and NaOfBu in the presence... 

Using the above preformed catalysts, ethylene can be hydroaminated by primary and secondary amines under much lower pressures (3-55 atm) than those required for the reactions catalyzed by alkali metals (800-1200 atm). The example of N-ethyl-ation of piperidine has been described in full details in Organic Syntheses (Eq. 4.14) [120]. 

Other poisons (modifiers) used to create such selective Pd catalysts may be metals 23 Zn, Cd, Zr, Ru, Au, Cu, Fe, Hg, Ag, Pb, Sb, and Sn or solvents (organic modifiers) 24 pyridine, quinoline, piperidine, aniline, diethylamine, other amines, chlorobenzene, and sulfur compounds. Hydroxides have also been used to increase the half-hydrogenation selectivity of Pd. 

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. 

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. 

The aza-Michael reaction yields, complementary to the Mannich reaction, P-amino carbonyl compounds. If acrylates are applied as Michael acceptors, P-alanine derivatives such as 64 and 65 are obtained. The aza-Michael reaction can be catalyzed by Bronsted acids or different metal ions. Good results are also obtained with FeCl3, as shown in Scheme 8.29. The addition of HNEt2 to ethyl acrylate (41f), for example, requires 10mol% of the catalyst and a reaction time of almost 2 days [94], The addition of piperidine to a-amino acrylate 41g is much faster and yields a,P-diaminocarboxylic acid derivative 65 [95]. 

A significant improvement in the selectivity of the addition of perhalomethane to olefins has been attained by researchers in Dow who have utilized CuCl or Cu combined with amines (e.g. piperidine)652 and by workers at Tokuyama Soda Co. who have used iron powder combined with triphenylphosphine653. Metal oxides, particularly Cu20, CuO, T1203 and Ag20, are also active catalysts in the presence of amines or phosphines654. 

The structure of the catalyst as determined by 13C NMR involves the bonding of binaphthol to the metal through oxygen atoms along with two hydrogen bonds to the nitrogen of the trimethyl piperidine as shown below [164],... 

However, PEG supported metal-free catalysts have also been shown to perform well in water. For example the synthesis of a PEG-supported TEMPO (2,2,6,6-tetramethyl-piperidine-l-oxyl), and its use as a highly efficient, recoverable and recyclable catalyst in oxidation reactions was described (Pozzi et al. 2004). 

RhCl(C2H4)pip2] is formed as the actual catalyst. The complex, whose structure is established by X-ray crystal structure analysis [18], can be easily prepared by piperidine addition to the well-known [Rh(C2H4)2Cl]2, which shows identical catalytic properties. The catalytic efficiency is limited by the thermal instability of the bis(piperidine) ethylene complex at higher temperatures. The thermal decomposition proceeds with formation of metallic rhodium, presumably according to eq. (5). 

Anionic bridged bis(amidinate) lithium lanthanide complexes have been found to be efficient catalysts for the amidahon of aldehydes with amines under mild conditions (Scheme 56). The achvity was found to follow the order of yttrium < neodymium < europium ytterbium. The catalysts are available for the formahon of benzamides derived from pyrrolidine, piperidine, and morpholine with good to excellent yields. In comparison with the corresponding neutral complexes, the anionic complexes showed higher achvity and a wider range of scope for the amines. A cooperation of the lanthanide and lithium metals in this process was proposed to contribute to the high activity of these catalysts [66,67]. 

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