1,4-Halogen participation

Tetramethylenehalonium Ions. 1,4-Halogen participation was first postulated to occur in the acetolysis of 4-iodo- and 4-bromo-l-butyl tosylates.402 In subsequent studies, Peterson and coworkers found anomalous rates in the addition of trifluoroacetic acid to 5-halo-l-hexenes403,404 and 5-halo-1-pentynes.405 406 Such observations were recognized as due to 1,4-halogen participation.341 Also, a study of the solvolysis of 8-chloroalkyl tosylates indicated rate-accelerating 1,4-chlorine participation effects up to 99-fold.407... 

Direct experimental evidence for 1,4-halogen participation comes from the direct observation (by NMR spectroscopy) of five-membered-ring tetramethylenehalonium ions by Olah and Peterson408 and by Olah et al.400... 

A number of Intermediates of halogen participation (halonium ions, e.g., 21 and... 

These results have been rationalized74 by Heasley and coworkers by assuming that the primary function of the complexes is to limit the concentration of free halogen. In the reaction of free bromine where the reaction is second order in bromine, two or more molecules of halogen participate in the transition state while the halogen complexes with pyridine or amines impose a first-order mechanism by limiting the availability of free halogen (equation 40). 

A number of intermediates of halogen participation (halonium ions),88 e g., 19 and 20, have been prepared as stable salts in SbF5-S02 or SbFs-SC ClF solutions.89 Some have even... 

Neighbouring halogen participation through halonium ions (p. 91) has been postulated to account for features of some substitution reactions. 5a-Bromo-6jS hydroxy steroids (17) are converted by thionyl chloride into the 5a-bromO"6 -chloro derivatives (19) with over-all retention of configuration [124], An intermediate 5a,6a bromonium ion (18) controls the stereochemistry of subsequent introduction of chloride ion. 

Neighbouring halogen participation effects have been investigated for pm-chloro- and pcn-bromo-substituted 0-tosyl oximes under acid-catalysed Beckmann rearrangement conditions. Evidence for stabilization of a nitrogen cation by nearby halogen is presented, including diversion of expected pathways. 

A French group have described the low-temperature dealkoxycarbonyla-tion of jS-ketoesters by boric anhydride, B2O3. A further example of thermal demethoxycarbonylation of substituted malonate esters has been reported halogen participation in charge dispersal has been invoked to account for the production of methyl bromoacetate from dimethyl bromomalonate (Scheme 26). 

In the Heck reaction the educt participates in the oxidative addition. The reductive elimination occurs without the target molecule. In the Ni catalyzed polyphenylene synthesis the target molecule participates also in the reductive elimination and halogen is removed in another reaction step. Fundamentally different classes of products can be expected if oxidative addition and reductive elimination involves fragments of the product and no halogen participate in the reaction. This can be expected with ArH activated reactions. The regioselectivity of activation of... 

It has been shown that halogen-substituted alkenes can participate in the metathesis reaction, e.g. 5-bromo-l-pentene reacts with 2-pentene 11). A very interesting reaction is the conversion of methyl-9-octa-decenoate into 9-octadecene and dimethyl-9-octadecenedioate 12) ... 

The reaction was also found to be inhibited by addition of dioxan and tetra-hydropyran, the rate decrease being proportional to the ether concentration. The results were rationalised by the assumption that 2 1 and 1 1 phenol ether complexes were formed, respectively. The inhibition was attributed to participation of the hydroxyl group in solvation of the halogen atom of the alkyl halide, though this seems much less likely than a straightforward modification of the electron-supplying effect of the substituent3 54. 

Scheme 5.7 illustrates these and other applications of the hydride donors. Entries 1 and 2 are examples of reduction of alkyl halides, whereas Entry 3 shows removal of an aromatic halogen. Entries 4 to 6 are sulfonate displacements, with the last example using a copper hydride reagent. Entry 7 is an epoxide ring opening. Entries 8 and 9 illustrate the difference in ease of reduction of alkynes with and without hydroxy participation. 

When X—Y is R3Sn—H the net reaction is decarboxylation and reduction of the original acyloxy group. Halogen atom donors can also participate in such reactions. 

Coulometry. Even in water, controlled potential or potentiostatic coulometry is a difficult and often time-consuming technique, as the analyte must participate in a direct electrode reaction. Therefore, in non-aqueous media there are only a few examples of its application, e.g., the potentiostatic coulometry of nitro and halogen compounds in methanol (99%) with graphical end-point prediction, as described by Ehlers and Sease153. 

Despite a recent computational article suggesting that organofluorine compounds could participate in halogen bonding [190], to date no F2 complexes have been observed in the solid state. 

It is important to recognize that the intermolecular long-distance bonding with the participation of halogen derivatives represents a specific example of the broad general area of donor/acceptor interactions. Moreover, the complexes of molecular iodine, bromine and chlorine with aromatic donors represent classic examples of charge-transfer compounds [26-28] that are vital for the development of Mulliken theory of intermolecular association [29-31]. The latter thus provides the convenient framework for the... 

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