Catalysed reactions kinetic control

Representatives of the bridged sulfone system 70 have been subjected to ruthenium catalysed ring-closing metathesis reactions (Grubbs catalyst) and shown to afford, in low yields, a few selected cyclic dimers and trimers, of all the possibilities available. The diastereoselectivities observed were rationalised in terms of kinetic control involved with internal ruthenium/sulfonyl oxygen coordination . 

In reactions where several different outcomes are possible, the final product distribution reflects the relative free energies of each transition state when the reaction is under kinetic control (Schultz and Lemer, 1993). Baldwin s rules predict that for acid-catalysed ring closure of the hydroxyepoxide [65] the tetrahydrofuran product [66] arising from 5-exo-tet attack will be preferred... 

Tetraphenylcyclopent-3-enone and dimethyl phosphonate are the major products from the base-catalysed reaction of methyl phosphonate with tetra-cyclone.75 A mechanism involving initial hydride transfer from dimethyl phosphinate anion to the ketone followed by kinetically controlled protonation to give (98) is suggested. 

Thermal cracking of wax. From thermal cracking a thermodynamic mixture might have been expected, but the wax-cracker product contains a high proportion of 1-alkenes, the kinetically controlled product. Still, the mixture contains some internal alkenes as well. For several applications this mixture is not suitable. In polymerisation reactions only the 1-alkenes react and in most cases the internal alkenes are inert and remain unreacted. For the cobalt catalysed hydroformylation the nature of the alkene mixture is not relevant, but for other derivatisations the isomer composition is pivotal to the quality of the product. 

Figure 14.13 The kinetic sequence of reactions that control the cyclic AMP concentration, and its binding to the effector system, and the kinetic sequence that controls the concentration of a neurotransmitter and its binding to the receptor on the postsyn-aptic membrane. Processes (1) are reactions catalysed by adenyl cyclase, and exocytosis. Reactions (2) are catalysed by phosphodiesterase and, for example, acetylcholinesterase. Reactions (3) are the interactions between the messenger and the effector system both the latter are equilibrium binding processes. (See Chapter 12 (p. 266) for discussions of equilibrium binding.)...

The initial products of organic reactions are formed under conditions of kinetic control - the products are formed in proportions governed by the relative rates of the parallel (forward) reactions leading to their formation. Subsequently, product composition may become thermodynamically controlled (equilibrium controlled), i.e. when products are in proportions governed by the equilibrium constants for their interconversion under the reaction conditions. The reaction conditions for equilibrium control could involve longer reaction times than those for kinetic control, or addition of a catalyst. The mechanism of equilibrium control could simply involve reversal of the initial product-forming reactions (as in Scheme 2.4, see below), or the products could interconvert by another process (e.g. hydrolysis of an alkyl chloride could produce a mixture of an alcohol and an alkene, and the HsO"1" by-product could catalyse their interconversion). 

How can the Z selectivity in Wittig reactions of unstabilized ylids be explained We have a more complex situation in this reaction than we had for the other eliminations we considered, because we have two separate processes to consider formation of the oxaphosphetane and decomposition of the oxaphosphetane to the alkene. The elimination step is the easier one to explain—it is stereospecific, with the oxygen and phosphorus departing in a syn-periplanar transition state (as in the base-catalysed Peterson reaction). Addition of the ylid to the aldehyde can, in principle, produce two diastere-omers of the intermediate oxaphosphetane. Provided that this step is irreversible, then the stereospecificity of the elimination step means that the ratio of the final alkene geometrical isomers will reflect the stereoselectivity of this addition step. This is almost certainly the case when R is not conjugating or anion-stabilizing the syn diastereoisomer of the oxaphosphetane is formed preferentially, and the predominantly Z-alkene that results reflects this. The Z selective Wittig reaction therefore consists of a kinetically controlled stereoselective first step followed by a stereospecific elimination from this intermediate. 

Further work on the acid-catalysed rearrangement of 4,5-dihydro-l,3-dioxepins (157) to tetrahydrofuran-3-carbaldehydes (158) and (159) has shown that the reaction is stereoselective and favours the (Z)-isomer (158) under kinetically controlled conditions (cfl 8 1 at — 78°C, for the example shown). At higher temperatures, these isomerize to the more stable (E)-... 

The complex of Co(II) with trisoxazoline 76 catalysed the 1,3-DC between a variety of nitrones and alkylidene malonates to give the corresponding isoxazolidines with both high enantio- and diastereoselectivity. The cycloaddition was reversibile and the endo/exo selectivity could be effectively controlled by reaction temperature. For example, 66 and 73 reacted in the presence of catalytic amounts of Co(C104)2 6H20 (5 mol%) and 76 (3.3 mol%) at -40 °C under kinetic control affording mainly the cis isoxazolidine 74, but at 0 °C the thermodinamically more stable trans isomer 75 was the major product <04OL1677>. 

Although the kinetics of binding may control the reaction if enzyme complementarity is too good and the mobility of the enzyme is too restricted, this is unlikely to be a general situation. Even if, because of weak binding, the rate of dissociation of an initially formed enzyme-substrate complex was fast, say 10 /sec, there is still plenty of time for conformational changes which may take place in less than 10 ° sec. This is compatible with those enzyme-catalysed reactions where the rate-limiting step is diffusion-controlled encounter of the enzyme and substrate. 

Under conditions of kinetic control, the dehydration of 2-butanol follows the Saytzeff rule, but a greater yield of m-2-butene than trans-l-batenc is obtained. These observations have no parallel in acid- or base-catalysed or pyrolytic eliminations. However, the dehydration of 2,3-dimethyl-2-butanol gives 2,3-dimethyl-l-butene (88.4%) and 2,3-dimethyl-2-butene (9.9%) and is thus oriented towards the Hofmann rule despite being more probably a carbonium ion process. Under similar reaction conditions the quite distinctly different products arising from the secondary alcohol, 3,3-dimethyl-2-butanol [3,3-dimethyl-1-butene (70%), 2,3-dimethyl-l-butene (23.5%), 2,3-dimethyl-2-butene (3.9%), l,l-dimethyI-2-methylcyclopropane (2.1%)] are accommodated in terms of concerted rather than a carbonium-ion mechanism. 

The catalytic performance of the MIP and appropriate controls were investigated with the hydrolysis of diphenylcarbonate. Reaction kinetics, followed by HPLC analysis of aUquots, were calculated as pseudo first-order rate constants. Rather encouragingly, the imprinted polymer showed typical Michaehs-Menten kinetics, in line with natural enzymes. The catalytic activity of the MIP (expressed as fccat/kuncat. the ratio of turnover munber of the catalysed reaction to turnover number in the absence of catalyst) was calculated to be 6900. This is markedly higher than has been reported for catalytic antibodies for carbonate hydrolysis (fccat/ uncat = 810), clearly demonstrating the great potential of MIPs for use as artificial enzymes. 

Acid (and base) catalysis, however, is not just controlled by the pH of the solution, which quantifies variation in [H ] (or [HO ]), but can also be influenced by the concentration of other weaker acids in solution in addition to the conjugate acid of solvent. To kinetically distinguish the two options, the terms specific and general acid catalysis have been defined according to eqn (3.6) and (3.7) where is the pseudo first-order rate constant (s ) for the acid-catalysed reaction of substrate. The former refers to a reaction showing a kinetic dependence only on the concentration of the conjugate acid of solvent (eqn (3.6)), whereas the latter (eqn (3.7)) also involves a... 

The kinetics of curing reaction of PU elastomers is analysed by in situ quantitative FTIR. The reactions, uncatalysed and catalysed by dibutyltin dilaurate (DBTDL), follow second-order kinetics at low conversion. At high conversion, however, the second-order rate constants decrease with the increase of reaction extent, due to the influence of diffusion control. The reaction kinetics parameters are given and the possible reaction mechanism is further offered. DBTDL is an effective positive catalyst for the reaction of -NCO with -OH. 8 refs. 

Alkylation of organic compounds with olefins is very important industrially (30-34). It appears that most acid-catalysed alkylations are very slow, hence kinetically controlled. However, Lee and Harriot (33) found that H2S04-catalysed alkylation of isobutane with butenes to be rather fast. Dixon and Saunders (34) studied the H2S04-catalysed alkylation of o-xylene with acetaldehyde and found that the reaction may be too fast to be kinetically controlled. 

The triethylamine-catalysed Michael addition of PhSH to cyclopent-2-en-l-one and its 2- and 3-methyl derivatives (151) proved to be appreciably reversible in chloroform solution (Scheme 22). Rates and equilibria were measured at 25 °C in order to assess the negative influence on addition exerted by methyl groups attached to the carbon-carbon double bond. 2-Methylcyclopent-2-en-l-one (151b) has been found to react with PhSH under kinetic control to give the m-adduct (153) as the sole detectable product. However, on prolonged reaction times, the system slowly evolved toward a new equilibrium, in which the more stable tran -adduct (154) was the predominant isomer. ... 

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