Systems amine catalysis

The contrasting behavior in the DNBF-C6H5NH2 system, where tertiary amine catalysis is not required, reflects the greater stability of a complexes formed by this highly electron deficient substrate. These results and Scheme IV indicate that the more reactive DNBF electrophile can differenti-... 

The use of secondary amine catalysis in combination with radical chemistry was first introduced by MacMillan in 2007 in a process he termed as organo-SOMO catalysis [32]. hi this system, the enamine that is generated in the condensation of a chiral secondary amine and a carbonyl, is oxidized via a single electron process. This generates a three-7i-electron radical cation with a singly occupied molecular orbital (SOMO) which can react asymmetrically in a variety of different processes (Scheme 1.25). 

The success of simple Au(I) PPhj systems for catalysis inspired the development of less strongly donating phosphine hgands in order to enhance it-acidity to improve reactivity with protected amines. Using triphenyl phosphite as a hgand, intermolecular hydroamination of alkenes with sulfonamides can be accomphshed with low catalyst loadings (Scheme 15.63) [263]. 

The 1-azirine ring also undergoes a number of reactions in which the heterocycle plays the role of the nucleophile. Although the basicity of the nitrogen atom in the azirine ring is much lower than in simple aliphatic amines, this system can still function as a nucleophilic reagent. One example of this involves the acid-catalyzed hydrolysis of 1-azirines to a-aminoketones (200) which represents a well-established reaction. In fact, in many reactions of 1-azirines where acid catalysis is used, formation of a-aminoketones is difficult to avoid (67JA44S6). 

The catalytic effect of protons has been noted on many occasions (cf. Section II,D,2,c) and autocatalysis frequently occurs when the nucleophile is not a strong base. Acid catalysis of reactions with water, alcohols, mercaptans, amines, or halide ions has been observed for halogeno derivatives of pyridine, pyrimidine (92), s-triazine (93), quinoline, and phthalazine as well as for many other ring systems and leaving groups. An interesting displacement is that of a 4-oxo group in the reaction of quinolines with thiophenols, which is made possible by the acid catalysis. 

As a result the research emphasis in this field focused on efforts to design experiments in which it might be possible to determine to which one of the foregoing three rate equations the observed second-order rate coefficient actually corresponded. More specifically, the objective was to observe one and the same system first under conditions in which complex decomposition (fcp) was rate-determining and then under conditions in which complex formation (kF) was ratedetermining. A system in which either formation or decomposition was subject to some form of catalysis was thus indicated. In displacements with primary and secondary amines the transformation of reactants to products necessarily involves the transfer of a proton at some stage of the reaction. Such reactions are potential-... 

This possibility warrants more detailed consideration. For the reaction of 2,4-dinitrochlorobenzene with an amine it is highly probable that the rate-determining step is intermediate formation. One possible mode of base catalysis in this system would involve transfer of the proton in the step in which the intermediate is formed. 

Bacteria have been Implicated in the formation of N-nitroso compounds under a wide variety of conditions representing both vitro and vivo situations Mechanisms of participation and/or catalysis Include a) decrease of the pH of the system, b) reduction of nitrate to nitrite, c) adsorption of amine onto the cell surface or cytoplasmic membrane, d) actual enzymatic formation. The literature of the field will be reviewed and experimental evidence which tests the above mechanisms will be presented ... 

Poly(2,6-dimethyl-l,4-oxyphenylene) (poly(phenylene oxide), PPG) is a material widely used as high-performance engineering plastics, thanks to its excellent chemical and physical properties, e.g., a high 7 (ca. 210°C) and mechanically tough property. PPO was first prepared from 2,6-dimethylphenol monomer using a copper/amine catalyst system. 2,6-Dimethylphenol was also polymerized via HRP catalysis to give a polymer exclusively consisting of 1,4-oxyphenylene unit, while small amounts of Mannich-base and 3,5,3, 5 -tetramethyl-4,4 -diphenoquinone units are always contained in the chemically prepared PPO. 

This reaction type has been intensely studied °. The application of highly polar solvents, catalysis with tertiary amines" or with acids mesomeric stabilization of intermediate carbenium ions " (allylic and benzylic systems propargylic systems" ) as well as derivatives of sulfinic acids with increasing acidity - usually indicate an ionic pathway (intra- and/or inter-molecular) ... 

Phase-transfer catalysis is a special type of catalysis. It is based on the addition of an ionic (sometimes non-ionic like PEG400) catalyst to a two-phase system consisting of a combination of aqueous and organic phases. The ionic species bind with the reactant in one phase, forcing transfer of this reactant to the second (reactive) phase in which the reactant is only sparingly soluble without the phase-transfer catalyst (PTC). Its concentration increases because of the transfer, which results in an increased reaction rate. Quaternary amines are effective PTCs. Specialists involved in process development should pay special attention to the problem of removal of phase-transfer catalysts from effluents and the recovery of the catalysts. Solid PTCs could diminish environmental problems. The problem of using solid supported PTCs seems not to have been successfully solved so far, due to relatively small activity and/or due to poor stability. 

In addition, there are a few examples of heterogeneous nonaqueous sonochemistry, in both liquid-liquid and liquid-solid systems. Two recent reports have utilized ultrasonic agitation in place of or along with phase transfer catalysis for the preparation of dichlorocarbene from aqueous NaOH/CHCl3 (166), and for N-alkylation of amines with alkyl halides (167). Along the same lines, several papers have appeared in which... 

A report in 1977 (3) of an active system prepared from [Rh(CO)2CI]2, CH3CO2H, cone. HC1 and Nal in water demonstrated that a basic medium is not a necessary condition for WGSR catalysis. This result stimulated us to examine the potential activity of several simple metal carbonyls in acidic solution as well. Attempts with Fe(CO)5 and Iri (CO)12 (17), both active in alkaline and amine solutions, proved unfruitful. However Ru3(CO)12 in acidic (0.5 N H2SOtf) aqueous ethoxyethanol gave WGSR activity substantilly larger than found in basic solutions under otherwise analogous conditions (Pco=0.9 atm, T=100°C, [Ru]Total=0 036 mol/L) (15). This solution proved unstable and... 

Some of the most important evidence for the two-step mechanism comes from studies of base catalysis, in this regard, reactions involving primary and secondary amines have played a central role1-5. The initially formed cx-adduct, 1, is zwitterionic and contains an acidic proton, which can be removed by a base which may be the nucleophile itself. Conversion of 1 to products can then occur via the uncatalysed k2 pathway or via the base-catalysed hl pathway. The influence of Brpnsted base catalysis, the experimental observation of 1,1- and 1,3-cr-adducts, the sensitivity of the system to medium effects, are some experimental evidence of the mechanism depicted in equation 1. 

Furthermore, in many cases, changes in the mechanism have also been observed and they will be discussed in a later section. Nevertheless, by selecting a system that exhibited the same rate-determining step in a variety of solvents it would be possible to assess how the rate of a given process may be affected by a solvent transfer. Such is the case of the reaction of l-chloro-2,4-dinitrobenzene with piperidine, where the rate dependence with amine concentration has been studied in 12 aprotic solvents483 as well as in 10 protic solvents4815. It was found that the reaction does not exhibit base catalysis in any of the solvents studied that is, addition of piperidine is rate-limiting in all the... 

The nucleophilic attack by alkoxides, amines, and water is of great interest to homogeneous catalysis. A dominant reaction in syn-gas systems is the conversion of carbonyls with water to metal hydrides and carbon dioxide ("Shift Reaction"), see Figure 2.27. 

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