Three-center concerted process

Such three-center concerted process is also proposed for the oxidative addition of alkenyl halide (Scheme 3.14) [20,21]. Reaction of Pt(PPh3)3 with ( )-yd-bromostyrene gives tran5-[PtBr ( )- iS-styryl (PPh3)2], where C-Br bond oxidative addition proceeds via prior jr-coordination of alkenyl group resulting in retention of configuration at carbon. 

Molecular hydrogen interacts with a metal center using its a- and a -orbitals as donor- and acceptor orbitals, as explained in Section 2-15. The approach of H2 leads to the polarization, elongation, and finally cleavage of the H—H bond in a concerted, three-center addition process (21-VI). 

In insertion, as the name suggests, the carbene inserts itself between two atoms. Insertions have been observed into C—H, C—C, C—X, N—H, O—H, S—S, S—H, and M—C bonds, among others. The mechanism of the process is often concerted. A three-centered transition state is usually written for the concerted mechanism ... 

Boron has only three electrons in the valence shell, and therefore its compounds are electron deficient and there is a vacant p-orbital on the boron atom. Borane (BH3) exists as a mixture of BaHe/BHs, as dimerization partially alleviates the electron deficiency of the boron. This equilibrium is fast, and most reactions occur with BH3. The addition of borane to a double bond is a concerted process going through a four-centered transition state. The formation of the C-B bond precedes the formation of the C-H bond so that the boron and the carbon atoms are partially charged in the four-centered transition state. 

The Simmons-Smith cyclopropanation is a concerted process, and it proceeds via a three-centered "butterfly-type" transition state. This is in agreement with the result of theoretical studies as well as the stereochemical outcome of a large number of reactions. 

Most synthetically-useful reductive eliminations occur by a concerted three-centered process (the reverse of three-center OA) outlined in equation 7.49. 

It may be assumed, that the reaction of the chloroaquoplatinum(ll) complex with atkanes begins as oxidative addition, proceeds through a three-center transition state, and terminates by the synchronous formation of a platinum-carbon bond with elimination of a proton (which can be transferred to a molecule of water). A similar mechanism has been proposed [35a] for the cyclometalation of 8-alkylquinolines by palladium(II). It has also been suggested that the reverse process (the protolysis of the platinum-carbon bond in alkyl complexes) involves a three-center transition state [35b], and the concerted oxidative addition to Ir(I) complex has been proposed [35c]. 

If a concerted three-center oxidative addition mechanism is realized (path b), first a lone electron pair of an atom X (X = O, N) is used to form the M-X bond resulting in a complex 5 (step bi). Formation of the M-C bond and cleavage of the C-X bond constitute a concerted process resulting in a five coordinate intermediate, which can be stabilized by coordination of a solvent molecule (Sol) as in structure 6 (step b-2) similar to 4. Finally, by coordination of ligand L, the intermediate 6 can produce an octahedral reaction product (step b ) shown in Fig. 3, b. The configuration of the carbon atom present in R-Z is retained during oxidative addition to the metal. 

Various mechanisms for the 1,3-acyl shift have been proposed, the following three pathways being the most widely discussed a) a Norrish type I cleavage to an acyl and an allyl radical followed by recombination either to starting ketone or to the rearranged ketone [Eq. (19)]. i i>,21,25,33, 37,59,96,96) For some cases it has been pointed out that the radical centers do not become independent of each other but exist as a radical pkir in a solvent cage. 21,37,59) b) a concerted process as depicted in Eq. (20) and c) a concerted n2 -f <,2 cydoaddition of the C—1,C—2 bond with the C—3,C—4 double bond [Eq. (21)]. > Mechanisms (b) and (c) have not always been properly distinguished. Circumstantial evidence has been reported which supports the nonconcerted and/or... 

The mechanism of the reactions of aryl halides cannot occur by the common S 2 patii for the oxidative addition of methyl halides, and most aryl halides lack substituents that would make them sufficiently electrophilic to react by nucleophilic aromatic substitution pathways. As presented in the section on radical pathways for oxidative addition, aryl halides react with metal complexes that readUy imdergo one-electron oxidation by radical mechanisms. However, metal complexes that do not readily undergo one-electron processes tend to react by two-electron mechanisms. Thus, aryl halides typically react with tP" palladium(O) complexes by concerted pathways through three-centered transition states. No strong data for a radical pathway has been gained during the many studies on the oxidative addition of aryl halides to Pd(0). In contrast, evidence that oxidative addition of aryl halides to P, iridium, Vaska-t)q)e complexes occurs by a radical pathway has been published. ... 

The formation of the less favored product XII proceeds in a manner similar to the dimerization of two molecules of ethylene. There are no intermediates on the reaction PES, a type III three-center interaction arises also in the transition state. First a bond is formed between the unsubstituted carbon atoms. Even though the reaction is concerted, the process of the formation of two new carbon-carbon bonds is asynchronous. The calculated activation barrier is 59.7 kcal/mol. 

B-alkyl-9-borabicyclo[3.3.1]nonanes undergo olefin-alkyl group exchange when refluxed with an olefin in THF. Kinetic and competition studies support a dehydroboration-hydroboration process rather than a concerted mechanism. Ab initio M.O. calculations show that the reaction between C2H4 and BH3 proceeds through a two-step process. A loose three-center (C—B—C) tt complex is formed which is then transformed into the product via a four-center transition state in a rate-determining step. 

As expected the calculated energy barrier for the oxidative addition reaction was rather low (17.0 kcal mol ) and involves the concerted formation of the Pd-I and Pd-C bonds, and the cleavage of the C-I bond through a three-centered transition state (OA-TS). This transition state results in the oxidative addition product OA-P, which evolves to the more stable trans isomer through a cis-to-trans isomerization. This isomerization is known that may take place following different pathways, [54] but in any case it is an easy process [64]. Thus, we focused our further analysis on the proposed mechanisms starting from the trans-[Pd(Ph)(I)(PH3)2] (1) complex. 

In the classical Passerini reaction [11], an isocyanide is condensed with a carbonyl compound and a carboxylic acid to afford a-acyloxyamides 7 (Scheme 1.2). When the carbonyl compound is prochiral, a new stereogenic center is generated. It is generally accepted that the reaction proceeds through intermediate 6, which rearranges to the product. The way this intermediate is formed is more debated. A possibility is a concerted non-ionic mechanism involving transition state 5. Since the simultaneous union of three molecules is not a very likely process, another possibility is a stepwise mechanism, with the intermediacy of a loosely bonded adduct 4 between the carbonyl compound and the carboxylic acid [2], Since all three... 

Kinetically, the fluxional process is first order in tetramer. Thomas and coworkers discussed three mechanisms 1) a dissociation-recombination of dimers, 2) unfolding of the tetramer into an eight-membered ring with alternating a-carbons and lithiums and 3) a concerted center to edge rotation of three of the alkyl groups. For different reasons, many authors have preferred the dissociation-recombination mechanism in which the transition structure is a very loose tetramer. 

In the case of ene reactions between cycloalkene or alkenes and diethyl azodi-carboxylate (DEAD) (e.g. Table 2.9, entry (3)), the ratio (AV AV) was foimd to be smaller than unity (0 < 1). This result can be considered as an indication of a stepwise process in which a pericyclic transition state is not involved. Stephenson and Mattern, however, observed that the ratios S/R and kH/kp in the ene reaction with DEAD as enophile shown in Table 2.9 entry (4), were roughly equal, which was explained in terms of a concerted ene reaction. To explain this discrepancy fenner and coworkers proposed a mechanism comparable to the formal ene reactions of alkenes with singlet oxygen or triazolindiones as enophiles forming in the first step three-membered rings between the alkenes and one center of the enophile prior to hydrogen transfer. To clarify the mechanism of the ene reaction with DEAD, it... 

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