Lewis studies

Nil Ratan Dhar, Coefficient de temperature de reactions catalytiques (Paris These d Universite, 1916) "Catalysis, Pt. IV, Temperature Coefficients of Catalysed Reactions," JCS.Trans. Ill (1917) 707762 and The Chemical Action of Light (London Blackie and Son, 1931). And Alfred Lamble and William C. McCullagh Lewis, "Studies in Catalysis, Pt. I, Hydrolysis of Methyl Acetate, with a Theory of Homogeneous Catalysis," JCS.Trans. 105 (1914) 23302342 and W. C. McCullagh Lewis, "Studies in Catalysis, Pt. VII, Heat of Reaction, Equilibrium Constant, and Allied Quantities, from the Point of View of the Radiation Hypothesis," JCS.Trans. Ill (1917) 457469. 

Table 4. Relationship between caries incidence and calculus prevalence at the end of the Isle of Lewis study... 

The most common flow arrangement (Fig. 1) has liquid alternating direction as it passes down the tower. Lewis studied three different flow arrangements and found that the most efficient method is to have the liquid flowing in the same direction in plug flow on each tray. The reversing arrangement is, however, mechanically expedient. 

Pearl and Lewis studied the reduction of nitrobenzene with sulfite-pulp waste liquor with respect to the variables of time, temperature, pressure, order of addition, concentration, and composition of the liquor. At atmos-1 /nd. Eng, Ckem., 36,664 (1944). 

K. Butler, B. Slater and D. W. Lewis, Studies in Surface Science and Catalysis, 2008, 174A, 725-728. 

As Chapter 58 explains, excited states display three types of conformational dynamics. The Norrish type II reaction can be subject to any of these different dynamic boundary conditions. The first dramatic example of ground-state control of triplet reactivity, in which conformational interconversion is slower than reaction of the initial excited-state conformation, was provided by Lewis study of the Norrish type II reaction of benzoylcyclohexanes. ° Photoenolization of ortfco-alkyl ketones was the first reaction recognized to show rotational control of triplet reactivity in which an irreversible bond rotation produces a reactive conformer of the excited state. Conformational equilibrium, in which bond rotations are faster than excited-state decay, is much more common an early example is the type II reaction of benzoylcy-clobutanes. ... 

Lewis J W, Warner J, Einterz C M and Kliger D S 1987 Noise reduction in laser photolysis studies of photolabile samples using an optical multichannel analyzer Rev. Sol. Instrum. 58 945-9... 

Unfortunately, the number of mechanistic studies in this field stands in no proportion to its versatility" . Thermodynamic analysis revealed that the beneficial effect of Lewis-acids on the rate of the Diels-Alder reaction can be primarily ascribed to a reduction of the enthalpy of activation ( AAH = 30-50 kJ/mole) leaving the activation entropy essentially unchanged (TAAS = 0-10 kJ/mol)" . Solvent effects on Lewis-acid catalysed Diels-Alder reactions have received very little attention. A change in solvent affects mainly the coordination step rather than the actual Diels-Alder reaction. Donating solvents severely impede catalysis . This observation justifies the widespread use of inert solvents such as dichloromethane and chloroform for synthetic applications of Lewis-acid catalysed Diels-Alder reactions. 

Studies on solvent effects on the endo-exo selectivity of Diels-Alder reactions have revealed the importance of hydrogen bonding interactions besides the already mentioned solvophobic interactions and polarity effects. Further evidence of the significance of the former interactions comes from computer simulations" and the analogy with Lewis-acid catalysis which is known to enhance dramatically the endo-exo selectivity (Section 1.2.4). 

Perhaps the most extensively studied catalytic reaction in acpreous solutions is the metal-ion catalysed hydrolysis of carboxylate esters, phosphate esters , phosphate diesters, amides and nittiles". Inspired by hydrolytic metalloenzymes, a multitude of different metal-ion complexes have been prepared and analysed with respect to their hydrolytic activity. Unfortunately, the exact mechanism by which these complexes operate is not completely clarified. The most important role of the catalyst is coordination of a hydroxide ion that is acting as a nucleophile. The extent of activation of tire substrate througji coordination to the Lewis-acidic metal centre is still unclear and probably varies from one substrate to another. For monodentate substrates this interaction is not very efficient. Only a few quantitative studies have been published. Chan et al. reported an equilibrium constant for coordination of the amide carbonyl group of... 

Bcamples of metal-ion catalysed organic reactions in water where the catalyst acts exclusively as Lewis acid are the hromination of diketones" " and the decarboxylation of oxaloacetate. The latter reaction has been studied in detail. In 1941 it was demonstrated that magnesium(II) ions catalyse this reaction" Later also catalysis by other multivalent metal ions, such as Zn(II), Mn(II), Cu(II), Cd(ir), Fe(II), Pb(II), Fe(III)... 

Lewis-acid catalysed decarboxylatiom. Later studies have focnsed on the effects of ligands on the efficiency of the catalysed reaction" This topic will be discussed extensively in Chapter 3. 

Searching for a suitable system for studying Lewis-acid catalysis of Diels-Alder reactions in water, several points have to be considered. 

Furthermore, the number of diene - dienoplrile combinations that can be expected to undergo a Lewis-acid catalysed Diels-Alder reaction is limited. Studies by Wijnen leave little doubt that the rate of typical Diels-Alder reactions, where the dienophile is activated by one or more carbonyl functionalities, does not respond to the presence of Lewis acids in aqueous solution , at least not beyond the extent that is expected for non-specific interactions (salt effects). No coordination of the Lewis acid to the dienophile was observed in these cases, which is perhaps not surprising. Water is... 

Fortunately, azachalcone derivatives (2.4a-g, Scheme 2.4) turned out to be extremely suitable dienophiles for Lewis-add catalysed Diels-Alder reactions with cyclopentadiene (2.5). This reaction is outlined in Scheme 2.4 and a large part of this thesis will be devoted to the mechanistic details of this process. The presence of a chromophore in 2.4 allows kinetic studies as well as complexation studies by means of UV-vis spectroscopy. Furthermore, the reactivity of 2.4 is such that also the... 

In the kinetic runs always a large excess of catalyst was used. Under these conditions IQ does not influence the apparent rate of the Diels-Alder reaction. Kinetic studies by UV-vis spectroscopy require a low concentration of the dienophile( 10" M). The use of only a catalytic amount of Lewis-acid will seriously hamper complexation of the dienophile because of the very low concentrations of both reaction partners under these conditions. The contributions of and to the observed apparent rate constant have been determined by measuring k pp and Ka separately. ... 

The effect of substituents on the rate of the reaction catalysed by different metal ions has also been studied Correlation with resulted in perfectly linear Hammett plots. Now the p-values for the four Lewis-acids are of comparable magnitude and do not follow the Irving-Williams order. Note tlrat the substituents have opposing effects on complexation, which is favoured by electron donating substituents, and reactivity, which is increased by electron withdrawirg substituents. The effect on the reactivity is clearly more pronounced than the effect on the complexation equilibrium. 

In summary, the effects of a number of important parameters on the catalysed reaction between 2.4 and 2.5 have been examined, representing the first detailed study of Lewis-acid catalysis of a Diels-Alder reaction in water. Crucial for the success of Lewis-acid catalysis of this reaction is the bidentate character of 2.4. In Chapter 4 attempts to extend the scope of Lewis-acid catalysis of Diels-Alder reactions in water beyond the restriction to bidentate substrates will be presented. 

A similar approach is followed in a recent study of the Lewis-acid catalysis of a Michael addition in acetonitrile. See Fukuzumi, S. Okamoto, T. Yasui, K Suenobu, T. Itoh, S. Otera, J. Chem. Lett. 1997, 667. 

When exclusively considering Lewis-add catalysis, the literature on ligand effects can be divided into studies describing quantitatively the effect of ligands on rates and equilibria of the individual steps in the catalytic cycle on one hand, and studies focused on the enantioselectivity of the reaction on the other. Interestingly, in the majority of the former investigations, aqueous media are employed. 

Studies of ligand effects on Lewis-acid catalysed reactions in water... 

In Chapter 2 the Diels-Alder reaction between substituted 3-phenyl-l-(2-pyridyl)-2-propene-l-ones (3.8a-g) and cyclopentadiene (3.9) was described. It was demonstrated that Lewis-acid catalysis of this reaction can lead to impressive accelerations, particularly in aqueous media. In this chapter the effects of ligands attached to the catalyst are described. Ligand effects on the kinetics of the Diels-Alder reaction can be separated into influences on the equilibrium constant for binding of the dienoplule to the catalyst (K ) as well as influences on the rate constant for reaction of the complex with cyclopentadiene (kc-ad (Scheme 3.5). Also the influence of ligands on the endo-exo selectivity are examined. Finally, and perhaps most interestingly, studies aimed at enantioselective catalysis are presented, resulting in the first example of enantioselective Lewis-acid catalysis of an organic transformation in water. 

Clearly, complete understanding of solvent effects on the enantioselectivity of Lewis-acid catalysed Diels-Alder reactions has to await future studies. For a more detailed mechanistic understanding of the origins of enantioselectivity, extension of the set of solvents as well as quantitative assessment of the strength of arene - arene interactions in these solvent will be of great help. 

---