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Concerted step

Ptn(5 -GMP-Ar7)2(NH3)(py)]2+ (7rG2) (Fig. 9). These results again argue for stepwise (photo)dissociation of the two azides, rather than reductive elimination of both azides in a concerted step. [Pg.17]

Skell s hypothesis proved to be extremely useful in carbene chemistry even though it was frequently opposed. The principal significance of these rules is represented in the scheme below. The singlet reaction occurs in a concerted step, the cis-addition product being formed in a stereospeciiic manner. In the triplet addition, which is a two-step reaction, rotation is thought to be faster than intersystem crossing (spin inversion) and ring closure, i.e., which would... [Pg.112]

Ab initio and density functional calculations indicate that the first step of the abnormal Reimer-Tiemann reaction involves barrierless formation of an intermediate by nucleophilic attack on CCl2 of the /3-carbon of pyrrole anion. " This is followed by a single, concerted step to give the product, 3-chloropyridine. [Pg.269]

The discussion by Braterman and Cross of reductive elimination from square planar or octahedral complexes is a special case of frontier orbital theory. A transition metal L MR, R2 is taken to lose grcwps R, R2 in a concerted step to give L M + Rj - R2. By the usual book-keeping convention, the electrons in the initial M—R a bonds are assigned to the R groups but this is of course a mere convention of naming and does not affect the argument. The in-phase combination of metal-carbon a bonds correlates... [Pg.150]

It is well known that base-induced elimination reactions can proceed either by a single, concerted step (E2), or by two steps, proton transfer and leaving group expulsion, with a carbanion intermediate (ElcB) to yield an alkene. " The... [Pg.97]

Thermal extrusion of a sulfur atom is the most common thermal reaction of a thiepin. The mechanism of this thermal process involves two orbital symmetry controlled reactions (69CC1167). The initial concerted step involving a reversible disrotatory electrocyclic rearrangement is followed by a concerted cheleotropic elimination of sulfur (Scheme 29). Similar aromatization reactions occur with thiepin 1-oxides and thiepin 1,1-dioxides, accompanied by the extrusion of sulfur monoxide and sulfur dioxide respectively. Since only a summary of the major factors influencing the thermal stability of thiepins was given in Section... [Pg.575]

The second and final example of a computational study of a reaction mechanism that will be considered here is drawn from work carried out by the author s group and serves to illustrate some of the points discussed in the previous section. The reaction in question is the catalytic hydrogenation of ketones by ruthenium(bisphosphine) (diamine) complexes. This reaction was developed by the group of Professor Ryoji Noyori20 and was also studied by the group of Professor Robert Morris. The initial computational work discussed here was a collaboration with Professor Morris. It was motivated by the desire to test the feasibility of a proposed mechanism, involving a key ruthenium dihydride complex, that would transfer a hydride (from Ru) and a proton (from N) in a concerted step to the ketone (Figure 10.9). [Pg.481]

In path A, the electrons on the negative oxygen reform the double bond as hydride leaves. Hydride is too basic to leave by itself, so it is transferred to the electrophilic carbonyl carbon of another aldehyde molecule in a concerted step. This step is relatively slow and occurs only when no other reaction pathways, such as an aldol condensation, are available. [Pg.888]

Numerous concerted electron transfer/bond breaking or formation mechanisms have been suggested. We have already touched upon one (p. 84). the concerted electron transfer/coupling mechanism for EHD. If this mechanism is correct, it should have stereochemical consequences in a cyclic EHD process involving a conjugated system, such as that shown in eqn (108). A concerted step would be... [Pg.105]

The Sn2 reaction takes place in a single (concerted) step. A strong nucleophile attacks the electrophilic carbon, forcing the leaving group to leave. [Pg.233]

Osmium tetroxide (0s04, sometimes called osmic acid) reacts with alkenes in a concerted step to form a cyclic osmate ester. Oxidizing agents such as hydrogen peroxide (H202) or tertiary amine oxides (R3N+—O-) are used to hydrolyze the osmate ester and reoxidize osmium to osmium tetroxide. The regenerated osmium tetroxide catalyst continues to hydroxylate more molecules of the alkene. [Pg.364]

The MOs of the reactants can flow into the MOs of the products in one concerted step according to the rules of conservation of orbital symmetry. In a symmetry-allowed cycloaddition, there is constructive overlap (+ phase with + phase, — phase with — phase) between the HOMO of one molecule and the LUMO of the other, (p. 693)... [Pg.708]

The MOs of the reactants are of incorrect symmetries to flow into those of the products in one concerted step. (p. 694)... [Pg.708]

The product of the stereospecific cycloaddition is an osmate ester . This isn t the required product, and the reaction is usually done in the presence of water (the usual solvent is a f-BuOH-water mixture), which hydrolyses the osmate ester to the diol. Because both oxygen atoms were added in one concerted step during the cycloaddition, their relative stereochemistry must remain syn. [Pg.937]

The adsorbed butane is activated (by hydrogen abstraction) to give butadiene before the concerted step to form MA (Figure 9). The formation of MA creates seven oxygen vacancies on the surface. The reoxidation of the surface is proposed to be the rate-determining step. [Pg.198]

The oxidation mechanism, based on an early one proposed for a tungsten catalyst, entails a concerted step in which a C—H bond undergoes electrophiUc attack by the distal peroxy oxygen of Ti—OOH (Scheme 18.12) [110, 111, 113]. [Pg.729]

In an important report, Janowicz and Bergman have described the photochemical incorporation of an alkyl group from an alkane into an iridium complex. The process does not appear to involve alkyl radicals, but is thought to occur via two successive concerted steps. This report may point the way to new catalytic processes for the functionalization of alkanes. The photoaddition of alkyl halides to alkenes catalysed by Cu complexes represents a novel process for formation of C-C bonds, and may have useful synthetic applications (Mitani et ai). The possibility of analogous intramolecular reactions comes readily to mind. [Pg.620]

State (IV) involving a single concerted step is preferable to the ion pair mechanism. Its attributes are simplicity and predictability. When experimental. 4-factors differ appreciably from those estimated on the basis of this transition state, the experimental results are probably not reliable. [Pg.397]


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See also in sourсe #XX -- [ Pg.1273 ]




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