Concerted processes

Sigmatropic rearrangements are normally classified as concerted processes with relatively nonpolar transition states. However, the Fischer cyclization involves rearrangement of a charged intermediate and ring substituents have a significant effect on the rate of the rearrangement. The overall cyclization rate... 

The E2 mechanism is a concerted process m which the carbon-hydrogen and carbon-halogen bonds both break m the same elementary step What if these bonds break m separate steps s... 

Section 5 15 Dehydrohalogenation of alkyl halides by alkoxide bases is not compli cated by rearrangements because carbocations are not intermediates The mechanism is E2 It is a concerted process m which the base abstracts a proton from the p carbon while the bond between the halogen and the a carbon undergoes heterolytic cleavage... 

The mechanism of alkene epoxidation is believed to be a concerted process mvolv mg a single bimolecular elementary step as shown m Figure 6 14... 

Figure 10 12 shows the interaction between the HOMO of one ethylene molecule and the LUMO of another In particular notice that two of the carbons that are to become ct bonded to each other m the product experience an antibondmg interaction during the cycloaddition process This raises the activation energy for cycloaddition and leads the reaction to be classified as a symmetry forbidden reaction Reaction were it to occur would take place slowly and by a mechanism m which the two new ct bonds are formed m separate steps rather than by way of a concerted process involving a sm gle transition state... 

Only one exception to the clean production of two monomer molecules from the pyrolysis of dimer has been noted. When a-hydroxydi-Zvxyljlene (9) is subjected to the Gorham process, no polymer is formed, and the 16-carbon aldehyde (10) is the principal product in its stead, isolated in greater than 90% yield. This transformation indicates that, at least in this case, the cleavage of dimer proceeds in stepwise fashion rather than by a concerted process in which both methylene—methylene bonds are broken at the same time. This is consistent with the predictions of Woodward and Hoffmann from orbital symmetry considerations for such [6 + 6] cycloreversion reactions in the ground state (5). 

MO (STO-3G) calculations on the gas-phase hydration reaction of formaldehyde suggest a concerted process involving two water molecules as a low-energy mechanism for hydration. 

There is an intermediate mechanism between these extremes. This is a general acid catalysis in which the proton transfer and the C—O bond rupture occur as a concerted process. The concerted process need not be perfectly synchronous that is, proton transfer might be more complete at the transition state than C—O rupture, or vice versa. These ideas are represented in a three-dimensional energy diagram in Fig. 8.1. 

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 ... 

Diels-Alder reactions are almost always stereospecific, which implies that if an intermediate exists, it cannot have a lifetime sufficient to permit rotation or inversion. The prevailing view is that the majority of Diels-Alder reactions are concerted processes, and most current theoretical analyses agree with this view." ... 

Classify the following reactions as electrocyclizations, sigmatropic rearrangements, cycloadditions, etc., and give the correct symbolism for the electrons involved in each concerted process. Some of the reactions proceed by two sequential processes. 

For the discussion of the rearrangements in section III only stepwise reaction sequences will be considered. We must note, however, that many of these transformations may just as well result from concerted processes in accordance with the rules for orbital symmetry conservation,... 

Intermediate 134 could then collapse to 136, which upon loss of the trialkyl- or triarylphosphine oxide would give the enamine (137). The conversion of 134 to 137 is probably best viewed as a concerted process. 

The mechanism of the cycloaddition of sulfenes to enamines does not involve a concerted process in many if not all cases, but rather a two-step process in which a zwitterion is the initially formed intermediate (158,159). 

A particularly elegant route to metallacarbor-anes is the direct oxidative insertion of a metal centre into a c/oso-carborane cluster the reaction uses zero-valent derivatives of Ni, Pd and Pt in a concerted process which involves a nett transfer of electrons from the nucleophilic metal centre to the cage ... 

In this solvent the reaction is catalyzed by small amounts of trimethyl-amine and especially pyridine (cf. 9). The same effect occurs in the reaction of iV -methylaniline with 2-iV -methylanilino-4,6-dichloro-s-triazine. In benzene solution, the amine hydrochloride is so insoluble that the reaction could be followed by recovery. of the salt. However, this precluded study mider Bitter and Zollinger s conditions of catalysis by strong mineral acids in the sense of Banks (acid-base pre-equilibrium in solution). Instead, a new catalytic effect was revealed when the influence of organic acids was tested. This was assumed to depend on the bifunctional character of these catalysts, which act as both a proton donor and an acceptor in the transition state. In striking agreement with this conclusion, a-pyridone is very reactive and o-nitrophenol is not. Furthermore, since neither y-pyridone nor -nitrophenol are active, the structure of the catalyst must meet the conformational requirements for a cyclic transition state. Probably a concerted process involving structure 10 in the rate-determining step... 

Loss of Nt and migration of the group R is likely to be a concerted process, since evidence for a free acyl nitrene RCON in the thermal reaction has not been found " ... 

Concerted (Section 30.1) A reaction that Lakes place in a single step without intermediates. For example, the Diels-Alder cycloaddition reaction is a concerted process. 

The authors formulate the mechanism in two steps, first an electron transfer from phenoxide ion to diazonium ion forming a radical pair, followed by attack of the diazenyl radical at the 4-position of the phenoxy radical and a concerted proton release, i. e., without involving the o-complex. Admittedly, there is no experimental evidence against such a concerted process, but also none for it It seems that those authors wanted only to demonstrate the occurrence of radical intermediates, but did not consider the question of the mechanism of the proton release. 

It is clear from the results that there is no kinetic isotope effect when deuterium is substituted for hydrogen in various positions in hydrazobenzene and 1,1 -hydrazonaphthalene. This means that the final removal of hydrogen ions from the aromatic rings (which is assisted either by the solvent or anionic base) in a positively charged intermediate or in a concerted process, is not rate-determining (cf. most electrophilic aromatic substitution reactions47). The product distribution... 

The higher strain energy in thiirene dioxides (19) compared to thiirane dioxides (17) is obvious. Yet, the elimination of sulfur dioxide from the latter is significantly faster than one would expect for a thermally allowed concerted process. Consequently, either aromatic-type conjugative stabilization effects are operative in thiirene dioxides2,12 or the relative ease of S02 elimination reflects the relative thermodynamic stability of the (diradical )99 intermediates involved in the nonconcerted stepwise elimination process. 

The racemization mechanism of sec-alcohols has been widely studied [16,17]. Metal complexes of the main groups of the periodic table react through a direct transfer of hydrogen (concerted process), such as aluminum complexes in Meerwein-Ponn-dorf-Verley-Oppenauer reaction. However, racemization catalyzed by transition metal complexes occurs via hydrogen transfer processes through metal hydrides or metal dihydrides intermediates (Figure 4.5) [18]. 

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