E2C elimination

Figure 7.5 (a) Transition state for f 2H-syn elimination, (b) Transition state for fi H-anti elimination, (c) Transition state for E2C elimination. 

Reactions of 5-(alkoxy)thianthrenium perchlorates with weakly basic nucleophiles Br, P, and PhS in MeCN or DMSO led to 5n2 substitution, E2C elimination, and... 

A study of reaction of weakly basic nucleophiles (X ) with 5-(alkoxy)thianthrenium ions (18a-e) in MeCN and DMSO has revealed that E2C elimination competes effectively with 5n2 reaction and reaction at sulfonium sulfur when X = r is used. The proportion of E2C product (21, 53, 52, 35 and 6.6% cycloalkene, respectively) is much higher for (18a-e) than found previously for reaction at primary and acyclic secondary alkyloxy groups (RO). 

Certain alkyl halides and tosylates undergo E2 eliminations faster when treated with such weak bases as Cl in polar aprotic solvents or PhS than with the usual E2 strong bases such as RO in ROH. In order to explain these results Parker et al. proposed that there is a spectrum of E2 transition states in which the base can interact in the transition state with the a carbon as well as with the p hydrogen. At one end of this spectrum is a mechanism (called E2C) in which, in the transition... 

However, the E2C mechanism has been criticized, and it has been contended that all the experimental results can be explained by the normal E2 mechanism. McLennan suggested that the transition state is that shown as 18. An ion-pair mechanism has also been proposed. Although the actual mechanisms involved may be a matter of controversy, there is no doubt that a class of elimination reactions exists that is characterized by second-order attack by weak bases. " These reactions also have the following general characteristics (1) they are favored by good leaving groups (2) they are favored by polar aprotic solvents (3) the reactivity order is tertiary > secondary > primary, the opposite of the normal E2 order (p. 1319) (4) the elimination is always anti (syn elimination is not found), but in cyclohexyl systems, a diequatorial anti elimination is about as favorable as a diaxial anti elimination (unlike the normal E2 reaction, p. 1302) (5) they follow Zaitsev s rule (see below), where this does not conflict with the requirement for anti elimination. 

Fig. 26 Energy diagrams showing the mechanistic spectrum of elimination reactions (a) concerted E2, (b) concerted E2 with El character, (c) stepwise El, (d) concerted E2 with ElcB character, (e) stepwise ElcB. (f) concerted E2C and (g) stepwise E2C-I...

Extension of this line of reasoning suggests that there should be a stepwise mechanism based on the SN2 configuration. Thus, just as a concerted E2/E1 pathway finally turns into a stepwise El pathway, the E2H/E2C pathway is predicted, in the limit, to lead to a radical anion intermediate which then undergoes elimination. We have termed this, as yet undiscovered pathway, the E2C-I (I = intermediate) pathway (Pross and Shaik, 1982a). The E2C-I pathway constitutes the missing link in the chain of potential elimination processes based on three intermediate configurations (Fig. 26g). 

However, the E2C mechanism has been criticized, and it has been contended that all the experimental results can be explained by the normal E2 mechanism.66 McLennan has suggested that the transition state is that shown as 18.w An ion-pair mechanism has also been proposed.70 Although the actual mechanisms involved may be a matter of controversy, there is no doubt that a class of elimination reactions exists that is characterized by second-order attack by weak bases.71 These reactions also have the following general characteris-... 

Effect of solvent on El vs. E2 vs. ElcB. With any reaction a more polar environment enhances the rate of mechanisms that involve ionic intermediates. For neutral leaving groups, it is expected that El and ElcB mechanisms will be aided by increasing polarity of solvent and by increasing ionic strength. With certain substrates, polar aprotic solvents promote elimination with weak bases (the E2C reaction). 

From the effect of changing solvents on rates, it is apparent that an E2C transition state is loose—that is, both base and leaving group are solvated ions. For example, elimination of toluenesulfonic acid from cyclohexyl tosylate by Cl- proceeds only approximately 50 times faster in acetone than in methanol. Compare this with the rate enhancement of about 106 when the Sw2 reaction of CH3OTs is transferred from protic to aprotic solvent.93 (See also Section 4.3.) This indicates that the double bond must be highly developed in the transition state,... 

E2C reactions give entirely anti elimination. This fact seems to be universal, and the need for anti elimination is even more important than formation of the most stable product.109, 110 Thus, for example, 50 with N(Bu)4C1 gives >99.9 percent 51, whereas the other diastereomer, 52, gives >99.9 percent 53.111 Of course, 51 is the more stable olefin. 

The leaving group The relative reactivity of a leaving group in an E2 elimination depends on where, in the spectrum of transition states, the transition state of the particular reaction lies. If the reaction is very E2C-like, the reactivities... 

Effect on El versus E2 versus ElcB. In the El mechanism, an external base is generally not required The solvent acts as the base. Hence, when external bases are added, the mechanism is shifted toward E2. Stronger bases and higher base concentrations cause the mechanism to move toward the ElcB end of the El-E2-ElcB spectmm."° However, weak bases in polar aprotic solvents can also be effective in elimination reactions with certain substrates (the E2C reaction). Normal E2 elimination has been accomplished with the following bases " HjO, NR3, OH, OAc, OR, OAr, NHj, COa ,... 

The stereochemistry of dehydrohalogenation with DBU has been studied by several authors. Wolkoff (82JOC1944) studied in detail the stereochemical consequences of dehydrohalogenation of secondary and tertiary alkyl and cycloalkyl halides with DBU. A comparison of the product distributions obtained in the elimination reactions of alkyl halides with DBU, with weak bases, and with anionic bases indicated that the elimination reactions with DBU very probably follow an E2C-like mechanism. 

E2C Bimolecular elimination with the base interacting primarily with the carbon. 

The observed absence of a correlation of the rates of XI with the OTs solvent ionizing power parameter (3, 6) or with the new Y0Tf parameters for triflates (29, 30) is also indicative of a significant kinetic effect of the solvent acting as a base in assisting the elimination according to equation 4. This process is an example of the E2C+ mechanism and has also been invoked to explain the results of solvolysis of some a-carbonyl- and a-phosphoryl-substituted mesylates (31-34). 

All of the 1,2-elimination mechanisms discussed here have assumed that, at some point, a base abstracts a proton jS to the leaving group by directly attacking that proton. Some authors have distinguished between the E2H ("normal E2") pathway and an E2C pathway, in which the base interacts with the a-carbon atom attached to the leaving group prior to removal of the -hydrogen... 

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