Chloronium ion, bridged

Initial addition of the electrophile CF (from CI2) occurs from either side of the planar double bond to form the bridged chloronium ion. In this example, both modes of addition (from above and below) generate the same achiral intermediate, so either representation can be used to draw the second step. 

In the second step, nucleophilic attack of CF must occur from the back side—that is. from the side of the five-membered ring opposite to the side having the bridged chloronium ion. Because the nucleophile attacks from below in this example and the leaving group departs from above, the two Cl atoms in the product are oriented trans to each other. Backside attack occurs with equal probability at either carbon of the three-membered ring to yield an equal amount of two enantiomers—a racemic mixture. 

The dependence of AGci on has an even greater slope agreeing vith the tendency of p-chloro-subhituted carbocations to form bridged chloronium ions. 

Both proton loss and rearrangement reflect the greater positive charge at carbon in a chloronium ion than in a bromonium ion because of the weaker bridging by chlorine. 

Twenty years later, by studying the chlorination of cis- and trans-di-t-butylethylenes, Fahey (1966) showed that halogen bridging is a general rule. It is expected that, owing to steric repulsions between the two branched groups, the cis-alkene would prefer to react via an open / -chlorocarbocation [14] where free rotation can occur rather than via a chloronium ion [15]. 

The appearance of a new double bond yields a number of mechanistic possibilities. Most important is the fact that this C=C bond gives access to the introduction of Cl and OH groupings via the chloronium ion pathway with the two different configurations that are needed to account for those precursors of type IX and X, namely, XV and XVI, since the stereochemical restrictions imposed by the gem-dimethyl bridge are no longer present. 

Crystal structures have also been obtained for the corresponding chloronium and iodonium ions and for the bromonium ion with a triflate counterion. Each of these structures is somewhat unsymmetrical, as shown by the dimensions below. The significance of this asymmetry is not entirely clear. It has been suggested that the bromonium ion geometry is affected by the counterion and it can be noted that the triflate salt is more symmetrical than the tribromide. On the other hand, the dimensions of the unsymmetrical chloronium ion, where the difference is considerably larger, has been taken as evidence that the bridging is inherently unsymmetrical. Note that the C- C bond lengthens considerably from the double-bond distance of 1.35 A. 

Similar results were obtained for chlorination of several of alkenes in methanol. Whereas styrene gave only the Markovnikov product, propene, hexene, and similar alkenes gave more of the anti Markovnikov product. This result is indicative of strong bridging in the chloronium ion. 

Theoretical calculations also support the intermediacy of chloronium ions in addition reactions. Rodriquez, C. F. Bohme, D. K. Hopkinson, A. C. /. Am. Chem. Soc. 1993,115,3263. Also see Reynolds, C. H. /. Am. Chem. Soc. 1992,114, 8676. There was also experimental evidence for the existence of chloronium ions from ion cyclotron resonance spectrometry experiments. Berman, D. W. Anicich, V. Beauchamp, J. L. J. Am. Chem. Soc. 1979,101,1239 reported that the stability of cyclic ions varies with the atom or group that is bonded to two carbon atoms in a three-membered ring. For the following species as the bridging group, relative stability is OH < Cl < Br < SH, PH2 < NH2. 

The structure formed in this step is called a chloronium ion. Notice that the chlorine atom bridges two carbon atoms and is the atom carrying the positive charge, (b) In the second step, a Cl attacks one of the carbon atoms in the bridge from the backside. 

The stereochemical behavior observed in the addition of BrCl has been compared with that related to the Br2 and CI2 additions to the same diene and discussed in terms of steric hindrance of the nucleophile approach (chloride ion with respect to bromide or tribromide ion) and different bridging in the bromonium or chloronium intermediates77. 

A stereo specific addition of BufOI to /1-methylstyrene was observed in the presence of BF3, yielding Markovnikov products. This result contrasts with the non-stereospecific addition of Bu OCl and Bu OBr. It has been suggested that the bridging in the intermediate chloronium and bromonium ion derived from PhCH=CHMe is not as symmetrical as in the iodonium ion. Consequently, charge develops on the benzylic carbon in the first two cases, and rotation occurs about the C—C bond190. By contrast, a radical mechanism is assumed in the absence of BF3 as anti-Markovnikov products are formed (both in the dark and upon UV irradiation)190. 

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