Subject Uncatalyzed reactions

A few polymerizations can be reasonably employed either in a catalyzed or an uncatalyzed process. Polyurethane formation is an example of this type of behavior. The reaction between diols and diisocyanates is subject to base catalysis. However, the polymerization is often carried out as an uncatalyzed reaction to avoid various undesirable side reactions. 

Where (H20) is the first order rate constant for the uncatalyzed reaction at a given water concentration, k3 = A2/[H20], A2 = Ac/[C1 ], kc = ki (catalyzed) + kx (uncatalyzed), and [Cl-] is the concentration of chloride ion from neutral salts. They again oppose the Swain mechanism on the basis a) that the unimolecular dissociation of the pentacovalent complex should be subject to electrophylic catalysis, b) that the steric effects are too great, c) that symmetrical chloride exchange is... 

Another subject that captured the attention of researchers in the 1970s was the identification of reaction conditions under which catalyzed and uncatalyzed reactions exhibit multiple steady states and/or oscillatory behavior. Theoretical investigations demonstrated that such behavior could arise from the nonlinear character of the reaction kinetics or from an interplay between the kinetics of a reaction and mass transport processes. A rich body of literature has now emerged detailing the space of reaction conditions and parameters within which multiple steady states and oscillations can be expected [15]. 

Since their introduction in the late 1970 s (see G G), uncatalyzed reactions have become the subject of numerous studies. Several systems with various substrates have been experimented with. These reactions are summarized in Table III.K., and contributions are briefly discussed in this section. 

This hypothesis is well in line with the finding that the formation of 9 by condensation of 6 and 8 can proceed in the absence of a catalyst under mild conditions. Specifically, the reaction occurs with appreciable velocity at room temperature in aqueous solution of neutral pH with reactant concentrations in the low mmolP range. The regiospecificity of the uncatalyzed reaction is significantly lower as compared to the strictly regiospecific enzyme-catalyzed reaction. Most likely, the spontaneous reaction is subject to partitioning... 

Fig. 3.28. Schematic components of pH rate profiles (a) acid catalyzed with uncatalyzed component, (b) base catalyzed with uncatalyzed component, (c) acid catalyzed with saturation, (d) base catalyzed with saturation, (e) uncatalyzed reaction, (f) reaction subjected to both acid and base catalysis. Adapted from J. Chem. Soc., Perkin Trans., 2, 365 (2000).

Chorismate mutase (CM) catalyzes the Claisen rearrangement of chorismate to prephenate in the shikimic acid pathway used in the biosynthesis of aromatic amino acids. It represents a reference enzyme to explore the fundamentals of catalysis and has been the subject of extensive experimental and computational research. These have shown both that catalysis proceeds without covalent binding of the substrate to the enzyme, and that the uncatalyzed reaction in water proceeds by the same mechanism. This makes CM a particularly convenient target for QM/MM studies. 

Imines of a-jd-unsaturated carbonyl compounds (1-azadienes) 239 can be subjected to [4 + 2] cycloadditions with ketenes to give rise to dihydropyr-idones 240. When the imine moiety is part of a conjugated system, both 1,2- and 1,4-cycloadducts can be obtained. Although challenging [2 + 2] cycloaddition to the imine moiety of azadienes infrequendy leads to product mixtures containing jd-lactams 241 (Scheme 75), the [4 + 2] cycloaddition commonly proceeds smoothly, giving the products in satisfactory yield at room temperature or sl hdy elevated temperatures via an uncatalyzed reaction (1970TL245). 

Reduction of complexes [(NHsjsCoCIIIjL], where L is a reducible ligand by Eu " and is subject to catalysis by the L/L° couple when the ligand is released from the cobalt(II) product.These catalysts have a variety of fates. Addition of excess Eu " or suppresses catalysis " by oxidation of the active species L and allows examination of the uncatalyzed reaction. For the 4-carboxamide derivative (5) a positive hydrogen dependence in the rate law is... 

The first term of the rate law requires acid-catalyzed decomposition of the conjugated acid of the ester. This term predominates only under strongly acidic conditions. It has not been investigated in detail, but the major product of the acid catalyzed reaction is the corresponding hydroxylamine. The second term predominates under neutral to mildly acidic conditions. This term is consistent with uncatalyzed heterolysis of the N—O bond of the neutral ester to generate a heteroaryinitrenium ion. " The rate law is more complicated than that for reactive esters of carbocyclic hydroxylamines or hydroxamic acids that show pH-independent decomposition over a wide pH range. The kinetic behavior of the heterocyclic esters is caused by protonation of a pyridyl or imidazolyl N under mildly acidic conditions. The protonated substrates are not subject to spontaneous uncatalyzed decomposition, so decreases under acidic conditions until acid-catalyzed... 

Reactions of amines with substituted phenyl acetates are subject to a change in pathway as a function of basicity of the leaving phenoxide group. In the ammonolysis, hydrazinolysis, and imidazole-catalyzed hydrolysis of substituted phenyl acetates, both first- and second-order terms in amine are observed (Bruice and Mayahi, 1960 Bruice and Benkovic, 1964). Out of five phenyl acetate derivatives (p-NOg, m-N02, p-H, p-Cl, and p-CHs) the general-base term kz is observed with only the least acidic leaving groups, that is p-CHs, p-H, and p-Cl in the case of ammonolysis and hydrazinolysis, and p-CHj in the case of the imidazole reaction. Uncatalyzed k terms are observed with all... 

The second and third terms in rate expression Eq. 11.1 ( rLHsO" ] and Aoh[OH"], respectively) clearly indicate that the hydrolysis of styrene oxides is subject to both and OH" catalysis. Acid-base catalyzed reactions have characteristic profiles when plotting log Aobs versus pH (a typical profile is represented in Fig. 11.1). The plots would reflect the contribution of the acid catalysis (a), the spontaneous (uncatalyzed) mode (b) and the basic catalyzed reaction (c). ... 

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