Mechanism concerted

Concerted mechanism of the oxidative addition is regarded as a reverse reaction of reductive elimination where effective overlapping between the filled 

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Reactions of this type are rather rare and seem to proceed in a stepwise fashion rather than by way of a concerted mechanism involving a single transition state... 

Substitution nucleophilic bimolecular (Sn2) mechanism (Sec tions 4 12 and 8 3) Concerted mechanism for nucleophilic substitution in which the nucleophile attacks carbon from the side opposite the bond to the leaving group and assists the departure of the leaving group... 

The stereoregulating capability of Ziegler-Natta catalysts is believed to depend on a coordination mechanism in which both the growing polymer chain and the monomer coordinate with the catalyst. The addition then occurs by insertion of the monomer between the growing chain and the catalyst by a concerted mechanism [XIX] ... 

A versatile method for the synthesis of a variety of five-membered heterocycles and their ring-fused analogs involves the reaction of a neutral 47r-electron-3-atom system with a 27T-electron system, the dipolarophile, which is usually electron deficient in nature. Available evidence, e.g. retention of dipolarophile stereochemistry in the product and solvent polarity exerting only a moderate influence on the reaction, indicates that the cycloaddition proceeds via a concerted mechanism 63AG(E)565, 63AG(E)633, 68JOC2291) and may be represented in general terms by the expression in Scheme 8. 

This ring closure is the final step of the reaction of hydrazines with 1,3-difunctional compounds (Section 4.04.3.1.2(ii)), and numerous examples in the literature of pyrazoles have been described. In some cases the N—C ring closure occurs by a concerted mechanism, classified by Huisgen (80AG(E)947) as 1,5-electrocyclizations. 

Figure 13.12 A concerted mechanism for GTP hydrolysis by Ga in which transfer of a proton to the y phosphate is coupled to deprotonation of the attacking water by Gin 200. (Adapted from J. Sondek et al., Nature 372 276-279, 1994.)...

Racemization, however, does not alwiys accompany isotopic scrambling. In the case of 5ec-butyl 4-bromobenzenesulfonate, isotopic scrambling occurs in trifluoroethanol solution witiiout any racemization. Two mechanisms are possible. Scrambling may involve an intimate ion pair in which the sulfonate can rotate with respect to the caibocation without allowing migration to die other face of the caibocation. The alternative is a concerted mechanism, which avoids a caibocation intermediate but violates the prohibition of front-side displacement. ... 

For many secondary sulfonates, nucleophilic substitution seems to be best explained by a concerted mechanism with a high degree of carbocation character at the transition state. This has been described as an exploded transition state. Both the breaking and forming bonds are relatively weak so that the carbon has a substantial positive charge. However, the carbocation per se has no lifetime because bond breaking and fonnadon occur concurrently."... 

Table S.16 presents data on some representative nucleophilic substitution processes. The first entry illustrates the use of 1-butyl-l-r/p-bromobenzenesulfonate to dononstrate at primary systems react with inversion, even under solvolysis conditkms in formic acid. The observation of inversion indicates a concerted mechanism in fids weakly nucleophilic solvent.

It is not difficult to incorporate this result into the general mechanism for hydrogen halide additions. These products are formed as the result of solvent competing with halide ion as the nucleophilic component in the addition. Solvent addition can occur via a concerted mechanism or by capture of a carbocation intermediate. Addition of a halide salt increases the likelihood of capture of a carbocation intermediate by halide ion. The effect of added halide salt can be detected kinetically. For example, the presence of tetramethylammonium... 

Fig. 8.1. Representation of transition states for the first stage of acetal hydrolysis, (a) Initial C—O bond breaking (b) concerted mechanism with C—O bond breaking leading O—H bond formation (c) concerted mechanism with proton transfer leading C—O bond breaking (d) initial proton transfer.

Fig. 8.2. Contour plot showing a favOTed concerted mechanism for the first step in acetal hydrolysis, in which proton transfer is more complete in the transition state than C—O bond breaking.

Is the reaction concerted As was emphasized in Chapter 11, orbital symmetry considerations apply only to concerted reactions. The possible involvement of triplet excited states and, as a result, a nonconcerted process is much more common in photochemical reactions than in the thermal processes. A concerted mechanism must be established before the orbital symmetry rules can be applied. 

Note that the endo product is predicted by the concerted mechanism. It is the major product, even though it is sterically more congested than the exo isomer. This stereospecificity is characteristic of the reaction. 

To make this more specific. Table 5-3 gives examples of several reaction types that fit the RIP pattern. Consider nucleophilic substitution on saturated carbon. The concerted mechanism is the one-step bimolecular 5, 2 process ... 

The E2 P-elimination reaction is a base-catalyzed concerted mechanism ... 

A similar concerted mechanism accounts for the water-catalyzed reaction, which becomes predominant at pH values lower than 4. The transition in rate-determining step has not been observed for the other two enamines. This point will meet attention in Section III. 

Finally, it should be noted that at alkaline and neutral pH a concerted mechanism, involving /S-carbon protonation of the enamine and a simultaneous addition of a water molecule, leading to the amino alcohol, can be rejeeted, since the immonium ion appeared to be a real and deteetable intermediate. 

The mechanism of the cycloaddition of phenyl azide to norbornene has been shown to involve a concerted mechanism with a charge imbalance in the transition state (199). In a similar manner the cycloaddition of phenyl azide to enamines apparently proceeds by a concerted mechanism (194, 194a). This is shown by a rather large negative entropy of activation (—36 entropy units for l-(N-morpholino)cyclopentene in benzene solvent at 25°C), indicative of a highly ordered transition state. Varying solvents from those of small dielectric constants to those of large dielectric constants has... 

The stereospecificity of the methylene transfer provides compelling support for a concerted mechanism and this conclusion has rarely been disputed. It is instructive, however, to review the experimental evidence that allowed for the elimination of the alternative mechanistic proposals, namely, a radical addition and a carbome-tallation (Scheme 3.4). 

The major developments of catalytic enantioselective cycloaddition reactions of carbonyl compounds with conjugated dienes have been presented. A variety of chiral catalysts is available for the different types of carbonyl compound. For unactivated aldehydes chiral catalysts such as BINOL-aluminum(III), BINOL-tita-nium(IV), acyloxylborane(III), and tridentate Schiff base chromium(III) complexes can catalyze highly diastereo- and enantioselective cycloaddition reactions. The mechanism of these reactions can be a stepwise pathway via a Mukaiyama aldol intermediate or a concerted mechanism. For a-dicarbonyl compounds, which can coordinate to the chiral catalyst in a bidentate fashion, the chiral BOX-copper(II)... 

Honk et al. concluded that this FMO model imply increased asynchronicity in the bond-making processes, and if first-order effects (electrostatic interactions) were also considered, a two-step mechanisms, with cationic intermediates become possible in some cases. It was stated that the model proposed here shows that the phenomena generally observed on catalysis can be explained by the concerted mechanism, and allows predictions of the effect of Lewis acid on the rates, regioselectivity, and stereoselectivity of all concerted cycloadditions, including those of ketenes, 1,3-dipoles, and Diels-Alder reactions with inverse electron-demand [2],... 

For most Diels-Alder reactions a concerted mechanism as described above, is generally accepted. In some cases, the kinetic data may suggest the intermediacy of a diradical intermediate 18 ... 

The overall reaction includes allylic transposition of a double bond, migration of the allylic hydrogen and formation of a bond between ene and enophile. Experimental findings suggest a concerted mechanism. Alternatively a diradical species 4 might be formed as intermediate however such a species should also give rise to formation of a cyclobutane derivative 5 as a side-product. If such a by-product is not observed, one might exclude the diradical pathway ... 

In certain cases the reaction may proceed by a concerted mechanism. With allyl ethers a concerted [2,3]-sigmatropic rearrangement via a five-membered six-electron transition state is possible " ... 

Metathesis is a catalyzed reaction that converts two olefin molecules into two different olefins. It is an important reaction for which many mechanistic approaches have been proposed by scientists working in the fields of homogenous catalysis and polymerization. One approach is the formation of a fluxional five-membered metallocycle. The intermediate can give back the starting material or the metathetic products via a concerted mechanism ... 

Treatment of the S.yV-ylide 3 with dimethyl acetylenedicarboxylate yields (amongst other products) the stable 1,2-thiazocine derivative 4 as a colorless crystalline solid, via a [6 + 2]-cycload-dition reaction. The formation of 2-ethyl, 3,4-dimethyl 5,6,7,8-tetrachloro-2//-l,2-thiazocine-2,3,4-tricarboxylate (4) may not necessarily involve a concerted mechanism.2... 

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