Mechanistic aspects

Fatty acid compounds usually contain single or multiple double bonds with varying E/Z geometries. Under hydroformylation conditions, they can be hydrogenated before hydroformylation takes place [21]. A special problem is Z/ isomerization or the migration of double bonds, since most hydroformylation catalysts are also active isomerization catalysts. Isomerization may take place before, during, or after (multiple double bonds ) the reaction with syngas. 

The analysis of reaction products can be rather tedious, especially when mixtures of vegetable oils are subjected to hydroformylation. Usually, NMR spectroscopy, GC/MS, and ICP-MS are utilized in an integrated approach. 

Highest turnover numbers of 400 - 500 h were observed with a ratio of sub-strate/rhodium of 910. Mainly 9- and 10-formyl octadecanoic acid methyl esters were produced. Detailed investigations showed that the cis double bond isomer-izes rapidly to a trans-configured olefin. The latter undergoes hydroformylation only slowly. Below 50 C, isomerization did not take place. Microwave irradiation could accelerate the hydroformylation of these substrates [24]. 

Mainly 9- and 10-formylstearates were formed. Interestingly, ligands with five-and six-membered lactams induced preferentially Z/ isomerization (more than 50%) and performed as poor hydroformylation catalysts. 

Hydroformylation of Esters of Linoleic Acid, Linolenic Acid, and Ridnoleic Add 

Diels-Alder reactions can be divided into normal electron demand and inverse electron demand additions. This distinction is based on the way the rate of the reaction responds to the introduction of electron withdrawing and electron donating substituents. Normal electron demand Diels-Alder reactions are promoted by electron donating substituents on the diene and electron withdrawii substituents on the dienophile. In contrast, inverse electron demand reactions are accelerated by electron withdrawing substituents on the diene and electron donating ones on the dienophile. There also exists an intermediate class, the neutral Diels-Alder reaction, that is accelerated by both electron withdrawing and donating substituents. 

The way the substituents affect the rate of the reaction can be rationalised with the aid of the Frontier Molecular Orbital (FMO) theory. This theory was developed during a study of the role of orbital symmetry in pericyclic reactions by Woodward and Hoffinann and, independently, by Fukui Later, Houk contributed significantly to the understanding of the reactivity and selectivity of these processes.  

In a recent experimental study of the femtosecond dynamics of a Diels-Alder reaction in the gas phase it has been suggested that both concerted and stepwise trajectories are present simultaneously It is interesting to read the heated debates between Houk and Dewar on the 

Under the usual conditions their ratio is kinetically controlled. Alder and Stein already discerned that there usually exists a preference for formation of the endo isomer (formulated as a tendency of maximum accumulation of unsaturation, the Alder-Stein rule). Indeed, there are only very few examples of Diels-Alder reactions where the exo isomer is the major product. The interactions underlying this behaviour have been subject of intensive research. Since the reactions leadirig to endo and exo product share the same initial state, the differences between the respective transition-state energies fully account for the observed selectivity. These differences are typically in the range of 10-15 kJ per mole.  

In summary, it seems that for most Diels-Alder reactions secondary orbital interactions afford a satisfactory rationalisation of the endo-exo selectivity. However, since the endo-exo ratio is determined by small differences in transition state energies, the influence of other interactions, most often steric in origin and different for each particular reaction, is likely to be felt. The compact character of the Diels-Alder activated complex (the activation volume of the retro Diels-Alder reaction is negative) will attenuate these eflfects. The ideas of Sustmann and Mattay ° provide an attractive alternative explanation, but, at the moment, lack the proper experimental foundation. 

This topic has been thoroughly reviewed (72MI50500, 64HC(19-1)1, 59CRV737). 

The main features of the effect of structure on the site of attack are summarized in Table 3, and can be understood in.terms of a borderline 5n2 (59CRV737) transition state (48) which somewhat resembles an 5n1 transition state in charge distribution because C—O bond breaking runs ahead of Nu—C bond making. 

Substituent group Favored position of attack Nonacidic conditions Acidic conditions  

Electronegative, nonconjugating groups (which interact with an incipient carbocation only by an inductive or field effect) discourage attack at C. This is due to destabilization of transition state (49) by the juxtaposition of positive charge. 

TT-Conjugating groups tend to favor attack at C, but the ratio of Ca. C attack depends strongly on a balance of steric and electronic factors arising from both substituent and nucleophile (Table 4). The results can be rationalized, to a first approximation, by assuming that with good vr-donors stabilization of the incipient carbocation in (50) offsets steric hindrance. 

In a generahzed and simpHfied mechanism, the reaction usually follows the standard catalytic cycle of metal-catalyzed cross-coupling reactions oxidative addition of the C(sp ) -X bond to paUadium(O), followed by coordination of the amine to the resulting palladium complex, occurring with extrusion of HX that is captured by the base. Finally, reductive elimination yields the couphng product, regenerating the catalyticaUy active paUadium(O) species. 

In order to elucidate the mechanism involved, a variety of spectroscopic techniques as well as reaction calorimetry have been used for kinetic investigations, along with crystallographic and computational methods [30, 50, 62, 67, 95, 204, 205). 

In a recent study, when the mechanism of reactions catalyzed by B INAP-ligated palladium complexes was revisited [39], it was shown that the bromoarene undergoes oxidative addition prior to a reaction of the palladium complex with the 

Subsequently, Green investigated the oxidahve addition of aryl chlorides to palladium complexes derived from NHCs and their role in catalytic aminations [206], by using density functional theory (DFT) methods. Here, the most likely transition-state model for oxidative addition is the monocarbene-palladium complex (NHC)Pd(ArCl). For the amination of chlorobenzene, coordination of the T-shaped oxidative addition product by the amine occurs with a rearrangement, placing the amine in a ds-position to the aryl group, a prerequisite for reductive elimination [2]. 

Claisen rearrangements are formally intramolecular processes that involve concomitant cr bonds breaking and the formation as well as migration of n bonds. On the other hand, numerous investigations support a more or less concerted mechanism for this type of reaction depending on the nature of substituents on the 1,5 diene unit [Id]. 

In open chain series the Claisen-Johnson rearrangement is known to give rise to E-configuration for the resulting double bond [20]. An unprecedented stereochemical reversal from alkyl to aryl substituents has been described by Basavaiah 

Although the mechanism of the Povarov reaction is still debated, a stepwise mechanism via ionic intermediates is the 

SCHEME 13.106 Use of Povarov reaction in total synthesis of Inotonin A 502. 

Various systems were examined and proven as applicable for the achvahon of the Schiff base HI. Among them are 

The stereochemical result of the Povarov reaction is closely linked to the applied alkene species. In the case of an acyclic alkene, the 2,4-cis product is highly favored because of a bis-equatorial arrangement of both residues in the transition state. Cyclic alkenes generate predominantly the exo product. However, 2,3-dihydrofuranes and dihydropyrroles give poor diastereomeric ratios (endolexo 1 1-1 2), while 3,4-dihydro-27/-pyranes give excellent ra- 

In organic solvents, it was demonstrated that the Barbier reaction did not necessarily involve the formation of an organometallic species. In some cases there is a radical pathway in which the anion radical resulting from single electron transfer from the metal to the halogenated compound is trapped by the ketone or the ketyl radical on the surface of the metal (Molle and Baner, 1982). 

In aqueous zinc-promoted allylation, wherein the allylzinc species is considered unlikely, no evidence of the formation of a radical arising from allylic bromide could be obtained, but the radical intermediate formed could be associated with the metal surface (Wilson and Guazzaroni, 1989). The formation of a ketyl radical cannot be excluded either, as some ketones dimerized to the pinacol in the absence of the halide. But the process is slow and limited (Einhorn and Luche, 1987). The formation of a ketyl radical 

The initiation of the zinc-mediated reaction could then be attributed to the formation of an allyhc radical anion of the type [CH2=CH-CH2-X] on the metal surface. This radical surface could then react with the carbonyl group to give an alkoxide radical, which could add an electron and form the alcohols (Einhom and Luche, 1987). This process takes place with an increase of the pH value of the reaction mixture (Sjohohn et al, 1994). Likewise, Wurtz coupling products were detected with cinnamyl chloride, which was supposed to stem from an allylic radical anion intermediate. Moreover, in one experiment without aldehyde, Sjdholm et al. (1994) observed that all cinnamyl chloride was transformed into dicinnamyl. 

Likewise, Li and Loh (1996) observed that premixing indium trichloride, tin and allylic bromide increased the rate and the diastereoselectivity of allylations, which could be another indication of the existence of transient organoindium species. 

At this stage of the discussion, let us remember the stereochemical aspects of tin- and indium-mediated allylation. Though the indium-mediated 

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Again with platinized Ti02, ultraviolet irradiation can lead to oxidation of aqueous CN [323] and to the water-gas shift reaction, CO + H2O = H2 + CO2 [324]. Some mechanistic aspects of the photooxidation of water (to O2) at the Ti02-aqueous interface are discussed by Bocarsly et al. [325]. 

Halpern J 1978 Mechanistic aspects of homogeneous catalysis Trans. Am. Crystallogr. Assoc. 14 59-70... 

This chapter introduces the experimental work described in the following chapters. Some mechanistic aspects of the Diels-Alder reaction and Lewis-acid catalysis thereof are discussed. This chapter presents a critical survey of the literature on solvent ejfects on Diels-Alder reactions, with particular emphasis on the intriguing properties of water in connection with their effect on rate and selectivity. Similarly, the ejfects of water on Lewis acid - Lewis base interactions are discussed. Finally the aims of this thesis are outlined. 

In Chapter 1 mechanistic aspects of Are Diels-Alder reaction are discussed. The literature on the effects of solvents and Lewis-acid catalysts on this reaction is surveyed. The special properties of water are reviewed and the effects of water on the Diels-Alder reaction is discussed. Finally, the effect of water on Lewis acid - Lewis base interactions is described. 

No mechanistic aspects of organic chemistry (or, for this reason, any reaction intermediates) were ever mentioned by Zemplen in his lectures or writings, nor did he consider or accept their existence. I never heard him mention the names of Meerwein, Ingold, Robinson, or any other pioneers of the mechanistic electronic theory of organic chemistry. The possible role of organic ions was similarly never mentioned. He was. 

These studies at the same time aroused my interest in the mechanistic aspects of the reaetions, including the complexes of RCOF and RF with BF3 (and eventually with other Lewis acid fluorides) as well as the complexes they formed with aromatics. 1 isolated for the first time at low temperatures arenium tetrafluoroborates (the elusive (T-complexes of aromatic substitutions), although I had no means to pursue their structural study. Thus my long fascination with the chemistry of car-bocationic complexes began. 

The hydrocarbon research program of the Toker Institute was able in many ways to build on and utilize results of our fundamental work on superacid-catalyzed reactions and their mechanistic aspects (includ-... 

In the first chapter, devoted to thiazole itself, specific emphasis has been given to the structure and mechanistic aspects of the reactivity of the molecule most of the theoretical methods and physical techniques available to date have been applied in the study of thiazole and its derivatives, and the results are discussed in detail The chapter devoted to methods of synthesis is especially detailed and traces the way for the preparation of any monocyclic thiazole derivative. Three chapters concern the non-tautomeric functional derivatives, and two are devoted to amino-, hydroxy- and mercaptothiazoles these chapters constitute the core of the book. All discussion of chemical properties is complemented by tables in which all the known derivatives are inventoried and characterized by their usual physical properties. This information should be of particular value to organic chemists in identifying natural or Synthetic thiazoles. Two brief chapters concern mesoionic thiazoles and selenazoles. Finally, an important chapter is devoted to cyanine dyes derived from thiazolium salts, completing some classical reviews on the subject and discussing recent developments in the studies of the reaction mechanisms involved in their synthesis. 

The mechanistic aspect of these reactions is summarized by saying that the reactions occur in steps. Thus the formation of an ester linkage is not essentially different between two small molecules or in a polyester. 

K AIE 20, has emerged as a highly efficient, noncorrosive, and nonhazardous flux for brazing aluminum parts of heat exchangers. Nocolok 100 Flux (Alcan Aluminum Corp.) developed by Alcan (Aluminum Co. of Canada) has been the first commercial product. Its use and mechanistic aspects of the associated brazing process have been weU documented (33—37). 

Synthetic and mechanistic aspects of iatramolecular cycli2ation in the tricycHc diterpenoid area have been studied in detail. In general, the presence of electron withdrawing groups such as carbonyl in the side chain retard the rates of cycli2ation (61). 

Thickness of the laminar layer is deterrnined both by the need to reproduce fine detail in the object and by the penetration depth of the actinic laser light into the monomer bath (21,76). There is thus a trade-off between precision of detail in the model and time required for stereohthography, ie, the number of layers that have to be written, and an optimum Light-absorbing initiator concentration in the monomer bath corresponding to the chosen layer thickness. Titanocene-based initiators, eg, bis-perfluorophenyltitanocene has been recommended for this apphcation (77). Mechanistic aspects of the photochemistry of titanocenes and mechanisms of photoinitiation have been reviewed (76). 

Hydrolysis. Esters are cleaved (hydroly2ed) into an acid and an alcohol through the action of water. This hydrolysis is cataly2ed by acids or bases. The mechanistic aspects of ester hydrolysis have received considerable attention and have been reviewed (16). For most esters only two reaction pathways are important. Both mechanisms involve a tetrahedral intermediate and addition-elimination reactions i7i7... 

Mechanistic aspects of the action of folate-requiring enzymes involve one-carbon unit transfer at the oxidation level of formaldehyde, formate and methyl (78ACR314, 8OMI2I6OO) and are exemplified in pyrimidine and purine biosynthesis. A more complex mechanism has to be suggested for the methyl transfer from 5-methyl-THF (322) to homocysteine, since this transmethylation reaction is cobalamine-dependent to form methionine in E. coli. 

The addition of carbon nucleophile, including organometallic compounds, enolates, or enols, and ylides to carbonyl gro is an important method of formation of carbon-carbon bonds. Such reactions are- ctremely important in synthesis and will be discussed extensively in Part B. Here, we will examine some of the fundamental mechanistic aspects of addition of carbon nucleophiles to carbonyl groups. 

Enolates of aldehydes, ketones, and esters and the carbanions of nitriles and nitro compounds, as well as phosphorus- and sulfur-stabilized carbanions and ylides, undergo the reaction. The synthetic applications of this group of reactions will be discussed in detail in Chapter 2 of Part B. In this section, we will discuss the fundamental mechanistic aspects of the reaction of ketone enolates with aldehydes md ketones. 

H. Cerfontain, Mechanistic Aspects in Aromatic Sulfonation and Desulfonation, Interscience, New York, 1968, pp. 68-69. 

Extensive work in this area by the Syntex group provides additional important information with respect to the mechanistic aspects of such cycloadditions. At the same time, the method proved of considerable utility for the preparation of cyclobutane compounds bearing a wide variety of substituents. 

The most important contnbution of this technology to the science of fluonne chemistry has been its ability to probe mechanistic aspects of elemental fluorine attack on organic molecules As has been long accepted fluorine attack on nonaromatic organic molecules is free radical in nature The great oxidizing power of fluonne and... 

The Nenitzescu process is presumed to involve an internal oxidation-reduction sequence. Since electron transfer processes, characterized by deep burgundy colored reaction mixtures, may be an important mechanistic aspect, the outcome should be sensitive to the reaction medium. Many solvents have been employed in the Nenitzescu reaction including acetone, methanol, ethanol, benzene, methylene chloride, chloroform, and ethylene chloride however, acetic acid and nitromethane are the most effective solvents for the process. The utility of acetic acid is likely the result of its ability to isomerize the olefinic intermediate (9) to the isomeric (10) capable of providing 5-hydroxyindole derivatives. The reaction of benzoquinone 4 with ethyl 3-aminocinnamate 35 illustrates this effect. ... 

There has been relatively little work on the mechanism of arylation of heterocyclic compounds, but the arylation of homocyclic compounds has been studied in great detail. Those mechanistic aspects of the reactions with heterocycles which have been examined parallel closely the corresponding aspects of the reactions of homocycles, and there is... 

The mechanism of hydroxylation of heterocyclic compound has not been studied, but the mechanistic aspects of hydroxylation of homocyclic compounds may again serve as a basis for discussion. 

There are only a few examples for a preparative use of this reaction of more importance have so far been mechanistic aspects. ... 

The major part of each chapter deals with mechanistic aspects however, for didactic reasons, in most cases not with too much detail. Side-reactions, variants and modified procedures with respect to product distribution and yields are described. Recent, as well as older examples for the application of a particular reaction or method are given, together with references to the original literature. These examples are not aimed at a complete treatment of every aspect of a particular reaction, but are rather drawn from a didactic point of view. 

Despite the major technical importance of hydrogen embrittlement, and the wealth of research work on the subject, the mechanism (or perhaps mechanisms) of hydrogen embrittlement remains uncertain. Much of the book edited by Oriani, Hirth and Smialowski is concerned with mechanistic aspects of hydrogen embrittlement, and the reader is referred in particular to the summary by Thomson and Lin . 

In contrast with the above situation, the polymerization of 2-furfurylidene methyl ketone, di-2-fiirfurylidene ketone and their homologues has been the subject of a large volume of (mainly technical) publications because of the useful applications of the final cross-linked products. As pointed out in the introduction, this review does not deal with the technological aspects of furan resins and in this section only the mechanistic aspects of the first phase of these polymerizations will be discussed. 

Structural and Mechanistic Aspects of Additions of Nucleophiles to Diazonium Ions... 

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