E2C mechanism

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. 

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-... 

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). 

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... 

For convenience, we will refer to this class of reactions as E2C reactions, though the actual mechanism is in dispute. 

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). 

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). 

When a hard base is used as catalyst, the reaction will be more E2H-like, whereas a soft base will cause it to be E2C-like. Weakly acidic substrates and good leaving groups also shift the reaction path to a more E2C-like mechanism. 

The value A Ef = —35.6 mV has the particular interest of corresponding to a 50 % of character E e-E c and E2c At the average formal potential Ef, the intermediate species reaches half of its maximum value and, hence, at this A Ef species 02 may or may not gain a second electron (and as a direct consequence, for higher AEf it will be considered that the intermediate species is no more stable at the average formal potential). So, this AEf could be considered as the boundary between anti-cooperative and cooperative behavior of both electron transfer reactions [35, 43]. Indeed, it is well known that the voltamogram of an EE mechanism under these conditions is identical to that of an E mechanism multiplied by a factor... 

A mechanistic rationale for the Pd-catalyzed addition of a C-H bond at nitriles to allenes is outlined in Scheme 3. The oxidative insertion of Pd(0) into the C-H bond of nitrile 1 produces the Pd(II) hydride species 16 (or alternatively a tautomeric structure E E2C=C=N PdH Ln may be more suitable, where E = H, alkyl, aryl and/or EWG). Carbopalladation of the allene 2 would afford the alkenylpalladium complex 17 (carbopalladation mechanism), which would undergo reductive coupling to give the addition product 3 and regenerates Pd(0) species. As an alternative mechanism, it may be considered that the hydropalladation of allenes with the Pd(II) intermediate 16 gives the jr-allylpalladium complex 18 which undergoes reductive coupling to afford the adduct 3 and a Pd(0) species (hydropalladation mechanism). 

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. 

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... 

---