Condensation alkyl halide elimination

Depending on the reaction system, that is, precursor and solvent, several aprotic condensation reactions have been established (Scheme 2.3). Alkyl halide elimination involves the condensation between a metal alkoxide and a metal halide (Scheme 2.3a), ester elimination occurs between metal carboxylates and metal alkoxides, for example, by transesterification (Scheme 2.3b), and ether... 

Scheme 2.3 Condensation mechanisms (a) alkyl halide elimination (b) ester elimination and (c) ether elimination.

Table III also shows that hydrogen and the chlorinated butanes are reduced substantially when ethyl chloride is irradiated in the presence of benzene. The other products are essentially unaffected by this additive. In the radiolysis of certain alkanes (4), benzene, added in small amounts, does not interfere with the fast ion-molecule reactions of primary ionic fragments or with free radical processes, but it will efficiently condense unreactive or long-lived ions in the system. It is reasonable to assume that this is also true for alkyl halide systems and that the reduction in product yields compared with the pure compound upon adding benzene may be attributed to the interception of unreactive ions. Since the rate constants for reactions of the expected primary ions with ethyl chloride are very large (see Table II), the concentration of benzene used in our experiments should not interfere with the initial fast ion-molecule reactions. For ethyl chloride ion-molecule reactions, C4Hi0C1+ is the only unreactive ion of appreciable abundance which is expected in this system at the elevated pressures used in the radiolysis experiments. Thus, the reduced product yields in the presence of benzene additive can be identified tentatively with the removal of this stable ion and the elimination of its resultant neutralization products.

The enolate ion is nucleophilic at the alpha carbon. Enolates prepared from aldehydes are difficult to control, since aldehydes are also very good electrophiles and a dimerization reaction often occurs (self-aldol condensation). However, the enolate of a ketone is a versatile synthetic tool since it can react with a wide variety of electrophiles. For example, when treated with an unhindered alkyl halide (RX), an enolate will act as a nucleophile in an Sn2 mechanism that adds an alkyl group to the alpha carbon. This two-step a-alkylation process begins by deprotonation of a ketone with a strong base, such as lithium diisopropylamide (LDA) at -78°C, followed by the addition of an alkyl halide. Since the enolate nucleophile is also strongly basic, the alkyl halide must be unhindered to avoid the competing E2 elimination (ideal RX for Sn2 = 1°, ally lie, benzylic). 

In 2006, Lam and Gao reported a regioselective procedure for the synthesis of 1,2,3-triazoles via a [3 -b 2] cycloaddition of resin-bound vinyl sulfone and sodium azide in the presence of an alkyl halide. The resin-bound vinyl sulfone 71 was effectively generated by the Knoevenagel condensation of resin 44 (Scheme 12.18). Resin 71 then acted as the vinyl sulfone dipolarophile and the electronegative sulfone that was eliminated in the reaction directed the regiochemistry of the cycloaddition. 

Non-hydrolytic Aprotic Reactions. Actually, rigorously non-hydrolytic conditions are more surely preserved by the use of aprotic condensation reactions (excluding hydroxyl groups), which involve the elimination of ester, alkyl halide or ether. 

Elimination of Alkyl Halide. The non-hydrolytic condensation between metal chlorides and metal alkoxides with elimination of alkyl chloride provides an attractive method to form the oxide network. This reaction (Eq. 26.6) involves the nucleophilic cleavage of 0-R bonds and is therefore influenced by the electronic effects on the carbon center. 

The reaction of trimethyl phosphate, (MeO)3PO, with NiBr2 results in the neutral bis-dimethoxyphosphato complex by elimination of one methyl group from the neutral phosphoric esters (equation 185).1708 But when nickel halides are reacted at higher temperatures with the neutral diisopropylmethylphosphonate, an alkyl elimination and a phosphato condensation occur (equation 186).1709... 

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