Ar-X bonds

Figure lb points out that log(rate) is better correlated with an homo-lytic than an heterolytic cleavage of Ar-X bond. Therefore, we can postulate a low positive charge on the migrating species. 

Figure 1. Isomerisation of para-substituted bromobenzenes (a) and fluoroben-zenes (b) over HBETA (Si/A1 =8) i reaction temperature, 613 K reactant pressure = 130 Pa.(a) withdrawing effect of the substituent X. (b) effect of the homolytic, or heterolytic, splitting of Ar-X bond.

During the past ten years, the SN1 photoarylation reaction and some of its synthetic implications have been extensively explored and recently reviewed [6-8]. This reaction involves the mediation of aryl cations (Ar+) formed by photoheterolysis of the Ar— X bond which allows the use of a variety of precursors, such as readily available aryl chlorides and aryl esters (i.e., mesylates and phosphates). Interestingly, the reaction applies also to aryl fluorides, although this is difficult by other methods with nonactivated aromatic derivatives. 

The aryl SN1 reaction results from the unimolecular heterolytic photofragmentation of an Ar X bond, with the generation of an aryl cation (Scheme 10.4, reaction 5). Subsequent addition of this cation to the nucleophile affords the substitution product (Scheme 10.4, reaction 6). 

Once the Ar-X bond oxidatively adds to the Pd(0), the resulting complex, [L2Pd(Ar)X], is expected to react with CO to give [L2Pd(COAr)X]. As far as the kinetics of the carbonylation is concerned, the rate constant was found to de-... 

Oxidative addition of Pd(0) into Ar-X bond 2) Nortoomene mediated Pd(ll)-Pd(IV) cycle generates atylpalladium(ll) intermedaite VI i) Heck reaction between arylpalladium(ll) complex I and norbomene generates II ii) Directed ortho-palladation on arene forms III... 

Mechanistic interpretations of the copper-catalyzed aromatic nucleophilic substitution reactions remain unsettled even after half-a-century of debate [19, 20]. Possible pathways involve an S Ar reaction mediated by copper complexation to the pi-system (Scheme 4a), an electron transfer reaction followed by halide dissociation (Scheme 4b), four-centered c-bond metathesis reaction (Scheme 4c) and Cu(l) oxidative addition to the Ar-X bond, followed by the nucleophile exchange and reductive elimination in the resulting Cu(lll) system (Scheme 4d). There is presently a considerable body of experimental and theoretical data for and against each of the proposed mechanisms [21]. While the mechanistic studies were mostly related to the formation of C-C, C-O and C-N bonds, it is likely that the copper-catalyzed halogen exchange reactions follow a similar trend. 

Oxidative addition of substrates possessing C-X or H-X bonds of medium polarity and of substrates possessing Ar-X bonds that cannot undergo S 2 pathways often occur by concerted pathways involving three-centered transition states more like those of the oxidative additions of nonpolar substrates. The clearest cases in which reactions occiu by concerted pathways are the oxidative additions of aryl halides and sulfonates to paUadium(0) complexes. These reactions have been studied extensively because they are the first step of transition-metal-catalyzed nucleophilic aromatic substitution reactions called cross couplings. The oxidative additions of the O-H and N-H bonds in water, alcohols, and amines also appear to occur by concerted three-centered transition states in many cases. 

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