Theoretical and Mechanistic Studies

The effect of water on Diels-Alder reactions has been studied extensively by various theoretical and experimental methods. It was mentioned previously that Breslow studied the influence of the hydrophobic effect on the aqueous Diels-Alder reactions in detail while the volumes of activation for catalyzed Diels-Alder reactions were examined by Isaacs et al.  

Density functional theory study of aqueous-phase rate acceleration and endo/exo selectivity of the butadiene and acrolein Diels-Alder reaction shows that approximately 50% of the rate acceleration and endo/exo selectivity is attributed to hydrogen bonding and the remainder to bulk-phase effects, including enforced hydrophobic interactions and cosolvent effects. This appears to be supported by the experimental results of Engberts where a pseudothermodynamic analysis of the rate acceleration in water relative to 1-propanol and 1-propanol-water mixtures indicates that hydrogen-bond stabilization of the polarized activated complex and the decrease of the hydrophobic surface area of the reactants during the activation process are the two main causes of the rate enhancement in water. 

An ab initio MO calculation by Jorgensen revealed enhanced hydrogen bonding of a water molecule to the transition states for the Diels-Alder reactions of cyclopentadiene with methyl vinyl ketone and acrylonitrile, which indicates that the observed rate accelerations for Diels-Alder reactions in aqueous solution arise from the hydrogenbonding effect in addition to a relatively constant hydrophobic term. Ab initio calculation using a self-consistent reaction field continuum model shows that electronic and nuclear polarization effects in solution are crucial to explain the stereoselectivity of nonsymmetrical 

Diels-Alder reactionsJ Using a combined quantum-mechanical and molecular-mechanical (QM/MM) potential, Gao carried out Monte Carlo simulations to investigate the hydrophobic and hydrogenbonding effects on Diels-Alder reactions in aqueous solution. Enhanced hydrogen-bonding interaction and the hydrophobic effect were found to contribute to the transition-state stabilization. The number of hydrogen bonds was found to cause strong Coulomb interaction and discriminate heats of formation of transition states for exo/endo products. 

High stereospecificity is observed when the rotation of the diradical intermediate is slow in comparison with cyclization to cycloadduct or reversion to reactants. With the presence of external heavy atoms, it could facilitate the intersystem crossing (ISC) of the first-formed singlet diradical to the longer-lived triplet counterpart. The triplet diradical will have a chance to undergo rotation before it reverts back to singlet and cyclizes or cleaves to reactants. This then accounts for the reduced stereospecificity. The alternative possibility of a zwitterionic intermediate is considered unlikely because there is no interception of zwitteiions by water. 

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Cycloaddition involves the combination of two molecules in such a way that a new ring is formed. The principles of conservation of orbital symmetry also apply to concerted cycloaddition reactions and to the reverse, concerted fragmentation of one molecule into two or more smaller components (cycloreversion). The most important cycloaddition reaction from the point of view of synthesis is the Diels-Alder reaction. This reaction has been the object of extensive theoretical and mechanistic study, as well as synthetic application. The Diels-Alder reaction is the addition of an alkene to a diene to form a cyclohexene. It is called a [47t + 27c]-cycloaddition reaction because four tc electrons from the diene and the two n electrons from the alkene (which is called the dienophile) are directly involved in the bonding change. For most systems, the reactivity pattern, regioselectivity, and stereoselectivity are consistent with describing the reaction as a concerted process. In particular, the reaction is a stereospecific syn (suprafacial) addition with respect to both the alkene and the diene. This stereospecificity has been demonstrated with many substituted dienes and alkenes and also holds for the simplest possible example of the reaction, that of ethylene with butadiene ... 

This reaction has been the subject of a great deal of theoretical and mechanistic study, largely because of the commercial importance of the polymers to which it can give rise. Like the other radical reactions we have discussed, it can be said to involve three stages—(a) initiation, (i>) propagation, and (c) termination ... 

Other effective catalysts to synthesize formic acid are Rh complexes with formate114 or hexafluoroacetylacetonate115 ligands. New results of theoretical and mechanistic studies for these and other systems have been disclosed98 103 116-121... 

Remarkably, one study [19] reported that ODPM rearrangement is also possible from the singlet excited state, but this is a matter of debate. For the triplet excited state of these (3,y-unsaturated ketones, it was clearly pointed out that it is nn in origin based on low temperature phosphorescence measurements, theoretical and mechanistic studies [20-23]. Furtheron, these phosphorescene and mechanistic studies indicated that the 37i7i excited state is approx. 8-12kcal/mol lower to the corresponding 3nn excited state [25]. 

The most important cycloaddition reaction from the point of view of synthesis is the Diels-Alder reaction. The Diels-Alder reaction is the addition of an alkene to a diene to form a cyclohexene. It is called a [4 + 2]-cycloaddition reaction because four n electrons from the diene and two n electrons from the alkene are directly involved in the bonding change. The reaction has been the object of extensive theoretical and mechanistic studies, as well as synthetic application [52, 53]... 

Theoretical and mechanistic studies using QM/MM simulations have also looked at the solvent effects and on-water reactivity of the aromatic Claisen rearrangements of allyl p-R-phenyl ethers (R = CH3, Br, and OCHj) and allyl naphthyl ethers and showed that such aqueous systems can provide increased rate accelerations, yields, and specificity for several types of organic reaction classes compared to organic solvents [58]. Biologically relevant aromatic Claisen rearrangements have also been explored, which utilize the on-water effect [59]. 

The photochemical decarbonylation of ketones can be traced back to 1910 when acetone was photolysed in the gas phase to yield ethane and carbon monoxide. A radical process involving a-cleavage (Norrish type I reaction) and decarbonylation as two separate steps was proposed a few years later by Norrish and Appleyard (Scheme 1). Each of the two cleavage reactions has been the subject of numerous theoretical and mechanistic studies that have been covered in several reviews. 

Having considered how solvents can affect the reactivities of molecules in solution, let us consider some of the special features that arise in the gas phase, where solvation effects are totally eliminated. Although the majority of organic preparative reactions and mechanistic studies have been conducted in solution, some important reactions are carried out in the gas phase. Also, because most theoretical calculations do not treat solvent effects, experimental data from the gas phase are the most appropriate basis for comparison with theoretical results. Frequently, quite different trends in substituent effects are seen when systems in the gas phase are compared to similar systems in solution. 

The Brookhart laboratory has contributed much of the knowledge of the polymerization mechanism for the late transition metal a-diimine catalysts. The review by Ittel provides a concise summary of the mechanistic understanding as of the year 2000 [26]. Some of the early findings will be reviewed here and additional insights reported afterward will be presented. In addition to the experimental work, many theoretical and computational studies worthy of discussion have also been carried out. These efforts have been most important in providing insight into the mechanistic details of the highly reactive nickel system, which is often difficult to study experimentally. 

More mechanistic insight into the C-H functionalization process of arenes is provided by theoretical and experimental studies. Multifluorinated, electron poor, aromatic substrates readily undergo CH activation with coordina-tively unsaturated rhenium complexes, attributed to the stronger C-Re bond in the product, whereas with monofluorinated analogs, the if -complex predominates (Equation (60)).61... 

Metal ions play an important role as catalysts in many autoxidation reactions and have been considered instrumental in regulating natural as well as industrial processes. In these reactive systems, in particular when the reactions occur under environmental or in vivo biochemical conditions, the metal ions are involved in complicated interactions with the substrate(s) and dioxygen, and the properties of the actual matrix as well as the transport processes also have a pronounced impact on the overall reactions. In most cases, handling and analyzing such a complexity is beyond the capacity of currently available experimental, computational and theoretical methods, and researchers in this field are obliged to use simplified sub-systems to mimic the complex phenomena. When the simplified conditions are properly chosen, these studies provide surprisingly accurate predictions for the real systems. In this paper we review the results obtained in kinetic and mechanistic studies on the model systems, but we do not discuss their broad biological or environmental implications. 

From a theoretical and mechanistic point of view, small molecule rings are much easier to study than long macromolecular chains. Substitution reactions carried out on macromolecular... 

The term fast flow comes from the high flow speeds. In most of these systems, discharges are used to generate A or another species that is a precursor to A hence the term fast-flow discharge system (FFDS) is also commonly applied. Since fast-flow discharge systems have been applied in many kinetic and mechanistic studies relevant to tropospheric chemistry (e.g., see Howard, 1979 Kaufman, 1984), we concentrate on them. However, all fast-flow systems rely on the same experimental and theoretical principles. 

Asymmetric synthesis has evolved rapidly during recent years. Most of the progress is registered in synthetic methods less emphasis has been given to theoretical concepts and mechanistic studies. Methods have been devised for achieving optical yields exceeding 95%. A number of stochiometric reactions with respect to the chiral auxiliary moiety are now highly efficient. 

In this section we concentrate mainly on mechanistic aspects of reactions involving ft-silylcarbocations. The structure and properties of /J-silylcarbocations are discussed in detail in Chapter 12 on Silicon-substituted Carbocations in this volume. The strong stabilization of /J-silylcarbocations is of particular importance in relation to activating and directing effects in organic syntheses using silicon compounds, and is still an important area for mechanistic, theoretical and synthetic studies. 

It is fitting to begin the discussion with one of the most well-studied reactions in indoor air quality science. In 1999, Weschler and Shields (1999) showed that ozone will react with some terpenes at substantial rates in indoor environments. This prompted phenomenological, health, theoretical, kinetic and mechanistic studies to understand the potential and real impact of this chemistry on occupants. 

Many theoretical and experimental studies carried out in the past deal with the study of the solid motion inside a rotary cylinder, drum or kiln [1-8]. Saeman s model, developed in 1951, is undoubtedly the first attempt for modelling such motion [1] this mechanistic model is widely... 

In the literature of recent years, we indeed see more and more publications on the study and development of novel nonaqueous high energy density battery systems, nonaqueous electro-organic synthesis, and mechanistic studies. This parallels worldwide efforts to commercialize nonaqueous lithium and lithium-carbon batteries. Hence, it is important to gather from time to time the knowledge accumulated in these areas, to update the literature and provide the increasing number of people working in this field with a comprehensive compendium on the practical and theoretical aspects of nonaqueous electrochemistry. 

Theoretical and experimental studies revealed a mechanistic twist of the concerted [8+2] cycloaddition between dienylbenzo[c]furans and dimethyl acetylenedicarboxylate (DMAD). Thus, DFT calculation at the (U)-B3LYP/6-31+G(d) level had suggested a stepwise mechanism involving the formation of a zwitterionic intermediate for the [8+2] cycloaddition between DMAD and dienylbenzo[c]furans with electron-donating methoxy groups in the diene moiety. When no electron-donating substituents are present in the diene moiety of the dienylbenzo[c]furan, computational results indicated an alternative mechanism in which a [4+2] reaction occurs between the furan moiety and DMAD, and was followed by a [l,5]-vinyl shift <07JA10773>. 

The discovery of skeletal rearrangements produced a dramatic impact on the development of structural theory in the first half of this century. Numerous theoretical and experimental studies were carried out to formulate a consistent mechanistical description of the observed non-trivial transformations. Because of these efforts, the course of rearrangements in a diverse structural context can be now accurately predicted, or, at the very least, well accounted for. Skeletal rearrangements are therefore no longer a mere challenging curiosity of organic chemistry, but a new set of powerful and reliable synthetic tools. ° ... 

Theoretical, Structural and Mechanistic Studies of Phosphorus Ylides and the Wittig Reaction. - In contrast to previous years there are no significant new theoretical or mechanistic studies to report. However, investigations into the structural nature of ylides continue to provide much of interest. 

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