E1CB elimination

A similar E1CB elimination of a nitro group in nitro sugars was involved in a recently reported synthesis of methyl 4,6-0-benzylidene-2,3-dideoxy-2-C-formyl-a-D-erH ro-hex-2-enopyranoside 163 (Scheme 49).107... 

ElcB eliminations often follow aldol reactions and lead to a,p-unsaturated products. In this case, though, DABCO is a much better leaving group than the hydroxyl group, so enollzatlon leads to loss of DABCO in an E1cB elimination, giving the product of the reaction. DABCO is recovered and is a catalyst. 

In this, as in many other cases in aqueous solution, OH" plays the role of the base. Note that for compounds such as 1,1,2,2-tetrachloroethane and pentachloroethane, the base catalyzed reaction is important at quite low pH values (/NB = 4.5, i.e., pH at which the neutral and base catalyzed reaction are equally important, see Table 13.7 and Section 13.3). In fact, for polyhalogenated alkanes a small7NB value (e.g., <7) is indicative of an E2 reaction, or, in special cases, of an E1CB reaction see below. Some other examples of compounds reacting by an E2-mechanism include 1,1,2-trichloro-ethane, 1,1,2-tribromoethane, and l,2-dibromo-3-chloroethane (see Table 13.7). A high /NB value (e.g., >10) does not, however, necessarily exclude ( elimination, because this reaction may also occur with water as base, or by an alternative to the SN1 mechanism (i.e., an El mechanism, see below). 

Examination of Equation 7.30 shows that the rate of an (E1cB)b reaction should be independent of the base concentration if the buffer ratio, B/BH+ is kept constant—that is, the reaction should exhibit specific base catalysis (see Section 7.1, p. 340 and Chapter 8, p. 405). An example of such a reaction is elimination of methanol from 33. Not only is specific base catalysis observed, but... 

The (EiCBJjp mechanism is a close cousin of the (E1cB)b mechanism. The difference is that in the former the free anion is not formed but exists as an ion pair with the protonated base as counter-ion. An example of a reaction that goes by this mechanism is the formation of bromoacetylene from cis-l,2-dibro-moethylene and triethylamine (Equation 7.31).77 If the rate of elimination from deuterated 1,2-dibromoethylene is compared to the rate from nondeuterated material, kHlkD x 1. Therefore proton abstraction is not involved in the ratedetermining step. Because added Et3 D X- does not affect the rate and be-... 

Fig. 11.7 (A) E1cB mechanism for the elimination reactions of 4-nitrophenylethylpyridinium ions catalysed by OH deprotonation is rate limiting at pKa s below the breakpoint data are from reference [16] and the line is calculated from Eqaution 11.11. (B) Effect of increasing 3-quinuclidinol buffer concentration on the rate constants for 2-methyl-3-thiosemicarbazone formation from 4-chlorobenzaldehyde at pH 11.10 results are consistent with a change in the rate-limiting step from proton transfer to formation of the intermediate data from reference [8],...

Explain why vinyl halides such as CH2=CHCI undergo elimination by an E1cB mechanism more readily than alkyl halides such as CH3CH2CI. 

Energy Surface Organizes Three Routes E1cB (Via Anionic Intermediate), E1 (Via Cationic Intermediate), E2 (Concerted) E1 in Detail E2 in Detail ElcB in Detail E1/E2/E1cB Spectrum Eliminations in Cyclic Systems Eliminations Yielding Carbonyls Approaches to Elimination Mechanisms Elimination Flowchart... 

The a-protons of the sulfonium cation are very acidic (highly stabilised conjugate base), and only weak base is required for deprotonation this induces an elimination (E1CB) reaction. 

Base catalysed elimination of water (E1cB mechanism)... 

The partial positive charge on carbon favors a E1cB-like transition state for elimination, in which the C — H bond breaks ahead of the C— L bond, because this kind of transition state places a 8 near the existing S ... 

Like E1 elimination, E1cB requires two steps. Unlike E1, though, the intermediate in E1 cB is a carbanion, not a carbocation. E1cB stands for Elimination, unimolecular, conjugate base. 

The reaction is an elimination process that has been interpreted through an ElcB mechanism, as described in Scheme 17.2. The only question left is to determine whether the first step (A i, removal of the proton) or the second step k2, elimination of the leaving group) is rate determining. In other words, to distinguish between an (ElcB)jir(ElcB irreversible) and an (E1cB)r (ElcB reversible) mechanism. 

The transition states of the E2 and ElcB mechanisms are represented in Fig. 37.3 together with the KIEs that should be observed in each case. Just by comparing the three transition states, it becomes clear that no primary D KIE should be observed in the (E1cB)r mechanism, as the proton has been already removed. As the experimental fact is a clear primary D KIE at C3, the EIcBr mechanism must be discarded. Additionally it is an experimental fact that no H/D exchange with the solvent has been observed in the elimination of substrates 1. Solvent H/D exchange is indicative for an EIcBr mechanism where the carbanion is reprotonated by the solvent in the fast initial step (see equation C in Scheme 37.2). 

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