Eliminations Not Involving C-H Bonds

These reagents are not effective for primary alcohols, as chlorides are formed instead. Alcohols can be dehydrated in a two-step sequence by conversion to a sulfonate ester (methanesulfonyl chloride or p-toluenesulfonyl chloride are used most frequently), followed by base-catalyzed E2 elimination. 

The jS-elimination processes discussed thus far have focused on those that involve abstraction of a proton bound to carbon. It is the electrons in the C-H or-bond, however, that are primarily involved in the elimination process. Compounds bearing substituents, other than protons, that are either easily abstracted by Lewis bases or bonded to the carbon framework by a o--bond polarized so that carbon is electron-rich should undergo similar eliminations. A great many such processes are known, and in those instances that lend themselves to mechanistic study, it has been determined that the reactions are usually stereospecific. 

Vicinal dibromides may be debrominated by treating them with certain reducing agents, including iodide ion and zinc. The stereochemical features of these reactions 

The iodide-induced reaction may proceed through a bridged intermediate as shown below  

This mechanism is closely related to the reverse of the halogenation of alkenes. The rate-determining transition state leading to the formation of the bridged intermediate requires an anti orientation of the two bromines, and is lowered in energy by nucleophilic attack by iodide ion at the bridging bromine. The stereochemical requirements in noncyclic systems are probably similar, as indicated by the fact that meso-stilbene dibromide yields /rans-stilbene, while d,/-stilbene dibromide gives mainly c/s-stilbene under these conditions.  

Vicinal dibromides may be debrominated by treating them with certain reducing agents, including iodide ion and zinc. The stereochemical course in the case of 1,1,2-tribromocyclohexane was determined using a Br-labeled sample prepared by anti addition of Br2 to bromocyclohexene. Exclusive anti elimination would give unlabeled bromocyclohexene whereas Br-labeled product would result from syn elimination. Debromination with sodium iodide was found to be cleanly an anti elimination, whereas debromination with zinc gave mainly, but not entirely, anti elimination.  

The iodide-induced reduction is essentially the reverse of a halogenation. Application of the principle of microscopic reversibility would suggest that the reaction would proceed through a bridged intermediate as shown below. ° 

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 noncychc systems is also anti, as indicated by the fact that me o-stilbene 

One example of elimination reactions of this type is acid-catalyzed deoxymercura- 

One of the pieces of evidence in favor of this mechanism is the fact that the for deoxymercuration of h an5-2-meflioxycyclohexylmercuric iodide is about 8 kcal/mol less than for the cis isomer. Only the trans isomer can undergo elimination by an anti process 

There are a number of synthetically important /3-elimination processes involving 

Attack on the intermediate mercurinium ion by some nucleophilic species yields the olefin to complete the elimination. This reaction mechanism is essentially the reverse of oxymercuration. 

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Oxidative additions involving C-H bond breaking have recently been the topic of an extensive study, usually referred to as C-H activation the idea is that the M-H and M-hydrocarbyl bonds formed will be much more prone to functionalization than the unreactive C-H bond. Intramolecular oxidative additions of C-H bonds have been known for quite some time see Figure 2.15. This process is named orthometallation or cyclometallation. It occurs frequently in metal complexes, and is not restricted to "ortho" protons. It is referred to as cyclometallation and is often followed by elimination of HX, while the metal returns to its initial (lower) oxidation state. 

A final common arylation mechanism also involves C-H bond palladation with a Pd(II) catalyst, but then a transmetallation with an organometallic such as a boronic acid. Reductive elimination to form the desired product also releases Pd(0) and this species must be oxidized back to the active Pd(II) catalyst. A key aspect of this process is developing an oxidative system that does not result in homo-coupling of the aryl boronic acid (Scheme 9). 

It should be mentioned here that if no other leaving group is present, sulfonyl can act as its own leaving group in hydroxide- or alkoxide-catalyzed elimination from sulfones. Carbanion formation is not involved in this but the promotion of the ionization of a C—H bond by the sulfonyl group is seen at the /1-carbon rather than the a-carbon, e.g. equation 21. 

For the C-C bond-forming step coordination of an electrophilic aryl palladium halide to a cyclopentadienyl anion is assumed, followed by reductive elimination. Presumably the Pd catalyst is not involved in the C-H bond-breaking step, which is interpreted as an apparently simple deprotonation with cesium carbonate as base. The overall process is similar to the arylation of other soft nucleophiles [9]. 

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