Elimination reactions of vinyl halides

Elimination Reactions of Vinyl Halides Formation of Alkynes... 

Rappoport has presented a detailed outline of the mechanisms of the reactions of vinyl halides with nucleophilic reagents. Modena et /. " have provided further evidence in support of a spectrum of transition states for elimination from activated vinyl halides induced by alkoxide bases. Cristol and Whittemore have shown that the stereoselectivity of elimination from vinyl halides is largely determined by the choice of basic reagent alkoxide bases encourage am/-elimination, whereas syn-elimination and alpha-elimination become dominant with lithium alkyls. 

Nucleophilic substitution by cyanide ion (Sections 8.1, 8.13) Cyanide ion is a good nucleophile and reacts with alkyl halides to give nitriles. The reaction is of the S m2 type and is limited to primary and secondary alkyl halides. Tertiary alkyl halides undergo elimination aryl and vinyl halides do not react. 

AJkynes can be made by dehydrohalogenation of vinylic halides in a reaction that is essentially an E2 process. In studying the stereochemistry of this elimination, it was found that (Z)-2-chloro-2-butenedioic acid reacts 50 times as fast as the corresponding isomer. What conclusion can you draw about the stereochemistry of eliminations in vinylic halides How does this result compare with eliminations of alkyl halides ... 

Alkyl halides possessing / -hydrogens are usually poor substrates for carbonylative cross-coupling due to competitive / -hydride elimination/ Allyl chlorides can be used in carbonylative cross-coupling with allylstannanes/ phenyl-, 3-furyl, or vinylstannanes " to afford allylketones in modest to good yields. Divinylketones can be accessed through the reaction of vinylstannanes with vinyl iodides or vinyl triflates, with the latter requiring the addition of LiCl. Synthetic potential of this method has been proved in the formation of macrocyclic ketone jatrophone. In the reaction of vinyl triflates with tetramethyltin or aryltrimethylstannanes the additional activation by ZnCle is required. 

The choice as to which fragment of the molecule to be synthesized should be the vinylic halide and which should be the olefin will depend on several factors. In the cases where elimination to form conjugated dienes is the favored reaction, either possible combination of vinylic halide and olefin may produce the same diene however, different intermediates are involved and in some instances different products may be formed. The situation is more complex when allylic amines are produced since these products always will be different from the two different combinations of reactants. For example, Z-... 

Castle and Widdowson were first to disclose alkyl-alkyl Kumada coupling reactions catalyzed by Pd(dppf)Cl2 [195]. This report was later questioned and corrected by Scott [196]. Matsubara and colleagues established formal Stille-type coupling reactions of perfluoroalkyl halides with allyl, alkynyl, or vinyl stannanes catalyzed by 10 mol% of Pd(PPh3)4, which have to be considered, however, better as radical addition/elimination reactions rather than as coupling reactions (see Sect. 3.1) [184],... 

Ruthenium(O) complexes such as Ru(COD)(COT) catalyze the dehydrohalo-genative coupling of vinyl halides with olefins to give substituted conjugated dienes in a Heck-type reaction [11]. Thus, alkenyl halides readily react with activated olefins to produce dienes 16 (Eq. 7). Oxidative addition of vinyl halide, followed by regioselective insertion of an electron-deficient olefin and by -hydrogen elimination leads to the diene. 

Vinyl bromides can themselves be made by elimination reactions of 1,2-dibromoalkanes. Watch what happens when 1,2-dibromopropane is treated with three equivalents of K NLi first, elimination to the vinyl halide then, elimination of the vinyl halide to the alkyne. The terminal alkyne is amply acidic enough to be deprotonated by R2NU, and this is the role of the third equivalent. Overall, the reaction makes a lithiated alkyne (ready for further reactions) from a fully saturated starting material. This may well be the first reaction you have met that makes an alkyne from a starling material that doesn t already contain a triple bond, making an alkyne from 1,2-dibromopropane... 

The kinetics of dehydrohalogenation from the configurational isomers of vinyl halides have been determined for numerous reactions. Thus cw-/7-nitro-P-bromo-styrene in the presence of ethanolic NaOH is converted quantitatively by trans elimination to /7-nitrophenylacetylene within a few minutes (equation 21), whereas the trans isomer hardly reacts at all in that short time. However, the latter affords l,l-diethoxy-2-p-nitrophenylethane in high yield when kept under the above conditions for 20 days (equation 22). The mechanism of the latter reaction could not... 

Bromine and chlorine react with vinylsilanes to afford vinyl bromide and chloride with net inversion of configuration. Addition of these halogens proceeds with anti stereochemistry. Elimination of halo-silane in the presence of a nucleophile like F or RO also is assumed to take place in an anti manner (Scheme 16). The same transformation using iodine is applicable only to 1,2-dialkylvinylsilanes. The process is a reliable method for the preparation of vinyl halides of defined configuration. The reactions are not, overall, reductions, and they are included here only to emphasize the usefulness of the products of hydrosilylation. 

The transformation that has come to be known as the Heck reaction is broadly defined as the palladium(O)-mediated coupling of an aryl or vinyl halide or triflate with an alkene. The basic mechanism for the Heck reaction of aryl halides or trifiates (as outlined in more detail in the Key Chemistry), involves initial oxidative addition of the chiral palladium(O) catalyst to afford a a-arylpalladium(II) complex. Coordination of an alkene and subsequent carbon-carbon bond formation by syn insertion provide a a-alkylpalladium(II) intermediate, which readily undergoes P-hydride elimination to release the alkene product. Finally, the hydridopalladium(II) complex has to be converted into the active palladium(O) catalyst to complete the catalytic cycle. 

The base-induced dehydrohalogenation of vinyl halides and allyl halides often gives low yields of allenes because of the competing reaction to alkynes alkynes can either be formed by direct elimination from vinyl halides or by isomerization of the allene first formed to the isomeric alkyne. Since it has been established that anti elimination of hydrogen halide from vinyl halides to yield alkynes is much faster than syn elimination, the proper choice of the starting material is often important for a successful allene synthesis. When ( )-4-bromo-4-octene was treated with NaOMe, the sole product was 3,4-octadiene, whereas the conesponding Z-educt yielded 4-octyne (Scheme 66). ... 

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