Possible Mechanisms for Elimination

As mentioned at the beginning of the chapter, alkenes can be prepared via elimination reactions, in which a proton and a leaving group are removed to form a it bond  

The four fundamental electron transfer steps for ionic processes. 

All elimination reactions exhibit at least two of the four patterns (1) proton transfer and (2) loss of a leaving group  

Every elimination reaction features a proton transfer as well as loss of a leaving group. But consider the order of events of these two steps. In the figure above, they occur simultaneously (in a concerted fashion). Alternatively, we can imagine them occurring separately, in a stepwise fashion  

In this stepwise mechanism the leaving group leaves, generating an intermediate carbocation (much like the S l reaction), which is then deprotonated by a base to produce an alkene. 

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Figure 3.43 Proposed reaction route with two possible mechanisms for -elimination.

We will now use these general ideas to discuss specific structural effects that favor the various possible mechanisms for elimination reactions. We have a back-... 

As we begin to explore the possible mechanisms for elimination reactions, recall from Chapter 6 that ionic mechanisms are comprised of only four types of fundamental arrow-pushing patterns (Figure 8.8). All four of these steps will appear in this chapter, so it might be wise to review SldUBuilders 6.3 and 6.4. 

The possible mechanism for the reactions involving stoichiometric amount of preformed Ni(0) complexes is shown in Fig. 9.8. The first step of the mechanism involves the oxidative addition of aryl halides to Ni(0) to form aryl Ni(II) halides. Disproportion of two aryl Ni(II) species leads to a diaryl Ni(II) species and a Ni(II) halide. This diaryl Ni(II) species undergoes rapid reductive elimination to form the biaryl product. The generated Ni(0) species can reenter the catalytic cycle. 

Fig. 5. Possible mechanisms for the MMO hydroxylation step. Pathway A insertion of the oxygen atom of Q into the C-H bond B concerted addition of the C-H bond to Q followed by reductive elimination C, D homolytic attack of Q on the C-H bond E reaction of the peroxo species with substrate.

Cascade silylcarbocyclization reactions tiave been developed based on the fact that it is possible to realize successive intramolecular carbocyclizations, as long as the competing reductive elimination is slower than the carbometallation. For example, the reaction of dodec-6-ene-l,ll-diyne 67 with PhMe2SiH catalyzed by Rh(acac)(CO)2 at 50°C under 1 atm CO gives bis(exo-methylenecyclopentyl) 68 in 55% yield [44]. The reaction is stereo-specific that is, (6 )- and (6Z)-dodec-6-ene-l,ll-diynes, ( )-67 and (Z)-67, afford R, R )-68 and (S, R -68 respectively. A possible mechanism for this reaction is outlined in Scheme 7.20. It should be noted that none of the tricyclic product is formed even though a third carbocyclization in the intermediate III.2c is conceptually possible. 

The products of alkylation of iV-sulfonyl-protected aziridines (32) by alkyllithiums are unstable and undergo subsequent rapid elimination a possible mechanism for the reaction is shown in Scheme 16.49... 

In the non-3-hydroxylation pathway, the initial step from GA] 2 alenzyme-catalyzed oxidation of carbon-7 which yields the acid, GAqg. This is followed by the loss of carbon-20 to give GAq. The mechanism for the loss of carbon-20 is still unresolved. There are several lines of evidence which eliminate a number of possible mechanisms. For instance the two oxygens in the lactone of GAq have been shown to have their origin from the 19-oic acid group of the Cgo precursor (25). 

The possible mechanism for the synthesis of alkenyl bromides involves the usual trans-addition followed by case induced nms-elimination 1741 as illustrated in Eq. 110. 

Although the experimental evidence suggests that the observed photogeneration of hydrogen may not involve homogeneous metal dithiolene photocatalysts, a theoretical study by Alvarez and Hoffmann addressed possible mechanisms for hydrogen elimination from ds square-planar bis(dithiolene) complexes (77). Concerted elimination of H2 from protonated sulfur atoms in the complex was proposed to be a thermally forbidden but photochemically allowed pathway, and protonation of a metal hydrido complex was also considered, as shown in Scheme 3. 

Two possible mechanisms for methanol oxidation by MDH enzymes have been proposed in the literature, the Addition-Elimination (A-E) and the Hydride Transfer (H-T) mechan-isms. " The A-E is a three-step mechanism (Fig. 2a) that involves a proton transfer from methanol to an active site base, which is proposed to be ASP303. It is believed that the presence of this catalytic base at the MDH active site initiates the oxidation reactions by subtracting a proton (H16) from methanol (Fig. 2a). This proton addition to ASP303 leads to the formation of a covalent hemiketal intermediate, since the resulting oxyanion (016 ) in the methanol molecule is then attracted to the C5 of PQQ. The second step consists of the proton (H16) elimination from ASP303 and transfer to 05 of PQQ, and the final step is characterized by a... 

A possibility not suggested as a possible mechanism for the exclusive formation of the cis reduction product of a ir-system is the reductive elimination of an organometallic hydride intermediate as illustrated in equation (40). 

There are two possible mechanisms for cyclometallation oxidative addition or electrophilic substitution followed by reductive elimination of a small molecule. The balance of electronic and steric effects determines the course of the reaction. The metallation of 8-methylquinoline is the prototype of most cyclometallation reactions. It can be viewed in two ways, as shown in reactions (ar) and (as). 

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