Bridged intermediate, formation

The rate-determining expulsion of bromide ion through a bridged intermediate requires an anti orientation of the two bromides. The nucleophilic attack of iodide at one bromide enhances its nucleophilicity and permits formation of the bridged ion. The stereochemical preference in noncyclic systems is also anti, as indicated by the fact that /neso-stilbene... 

In the oxidative Eschenmoser sulfide contraction (Scheme 11), thioamide 59 is oxidized by benzoyl peroxide to give either a symmetrical disulfide or the O-benzoate of the thiolactam-S-oxide. In any event, the once-nucleophilic thioamide sulfur atom is now forced to adopt the role of electrophile a reactivity umpolung has, in effect, been achieved.13 The nucleophilic enamide 65 attacks the sulfur atom leading to the formation of sulfur-bridged intermediate 66. The action of a phosphine or a phosphite thiophile on the putative episulfide then gives vinylogous amidine 67. 

Chen YX, Heinen M, Jusys Z, Behm RJ. 2006a. Bridge-bonded formate Active intermediate or spectator species in formic acid oxidation on a Pt film electrode Langmuir 22 10399-10408. 

The electrochemistry of cobalt-salen complexes in the presence of alkyl halides has been studied thoroughly.252,263-266 The reaction mechanism is similar to that for the nickel complexes, with the intermediate formation of an alkylcobalt(III) complex. Co -salen reacts with 1,8-diiodo-octane to afford an alkyl-bridged bis[Co" (salen)] complex.267 Electrosynthetic applications of the cobalt-salen catalyst are homo- and heterocoupling reactions with mixtures of alkylchlorides and bromides,268 conversion of benzal chloride to stilbene with the intermediate formation of l,2-dichloro-l,2-diphenylethane,269 reductive coupling of bromoalkanes with an activated alkenes,270 or carboxylation of benzylic and allylic chlorides by C02.271,272 Efficient electroreduc-tive dimerization of benzyl bromide to bibenzyl is catalyzed by the dicobalt complex (15).273 The proposed mechanism involves an intermediate bis[alkylcobalt(III)] complex. 

To summarize, when the kinetic data predict that only bromonium ions or only bromocarbocations are formed, the bromination products are obtained stereospecifically and regiospecifically, respectively, whatever the solvent. Olefin brominations involving open intermediates lead to more solvent-incorporated products in methanol or acetic acid than those involving bridged ions. This chemoselectivity can be interpreted in terms of the hard and soft acid and base theory (Dubois and Chretien, 1978). Methanol assistance to intermediate formation also plays a role in determining product-selectivity (Ruasse et al, 1991). 

The formation of the bridged intermediate has been represented as an SN2-like displacement of the leaving group from the sulfenyl sulphur of 85116, or alternatively, as reported in equation 90 in agreement with the addition of other electrophiles to alkenes, it has been proposed that the reaction involves the initial formation of jr-complex 86 in a rapid equilibrium with the reagents76. 

Although it was not possible to verify whether this product is formed under kinetic or thermodynamic control, the authors suggest119 that if 90 arises from a kinetically controlled reaction, its formation could be rationalized on the basis of the stability of the involved intermediate. The bridged intermediate i is expected to be more stable than ii (equation 100) owing to the effect of the dimethoxymethyl substituent. 

N=C=S—OlOHsh]2 are produced, (t has been postulated that both remote and adjacent atiacks are involved in the formation of these products. Draw bridging intermediates consistent with this view for the formation of both products. ... 

The kinetics for the acid hydrolysis of A,A/A,A-(phen)2Cr(OH)2-Cr(phen)24+ revealed that the formation of cis diaqua mononuclear species proceeds in one step without significant buildup of mono-hydroxo-bridged intermediates (336). The rate law observed is kobs = a + 6[H+] at low acidities ([H+] = 0.01-0.05 M) and kobs = 6[H+] at... 

H2P04, and CH3COOH in acidic solution give complexes of type X. As shown in Eq. (64), the conversions are reversible and involve intermediate formation of the corresponding singly bridged complexes IX (376-386). 

However, special experiments showed that diketones of type 220 are not oxidized with chromic anhydride under the described conditions. Such an oxidation occurs via formation of bridged intermediates 221 (89UP1). 

In the oxidation of Cr2+(aq) by IrCl -, there is evidence for the formation of a chloro-bridged intermediate, but no overall chlorine transfer takes place the products are Cr3+(aq) and IrCl -. Bridging does appear to assist the process of electron transfer, but ultimate atom transfer is not absolutely essential. 

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