Alkynes from halides

Vinyl halides from alkynes + hydrogen halides (3.20)... 

Hydrosilylation-halodesilylation sequence can be used for the stereoselective preparation of synthetically useful alkenyl halides from alkynes (180-183). The combination of the platinum-catalyzed hydrosilylation of terminal alkynes and the chloro- or bromodesilylation of the resulting ( )-alkenylsilanes gives substituted 1-haloalkenes (Scheme 26). 

Evidence from a variety of sources however indicates that alkenyl cations (also called vinylic cations) are much less stable than simple alkyl cations and their involve ment m these additions has been questioned Eor example although electrophilic addi tion of hydrogen halides to alkynes occurs more slowly than the corresponding additions... 

Problem 8.10 Show the terminal alkyne and alkyl halide from which the following products can be obtained. If two routes look feasible, list both. 

The result of the retrosynthetic analysis of rac-lO is 2-hydroxyphenazine (9) and the terpenoid unit rac-23, which may be linked by ether formation [29]. The rac-23 component can be dissected into the alkyl halide rac-24 and the (E)-vinyl halide 25. A Pd(0)-catalyzed sp -sp coupling reaction is meant to ensure both the reaction of rac-24 and 25 and the ( )-geometry of the C-6, C-7 double bond. Following Negishi, 25 is accessible via carboalumination from alkyne 27, which might be traced back to (E,E)-farnesyl acetone (28). The idea was to produce 9 in accordance with one of the methods reported in the literature, and to obtain rac-24 in a few steps from symmetrical 3-methyl-pentane-1,5-diol (26) by selective functionalization of either of the two hydroxyl groups. 

Tradition reserves the use of the name acid for substances that transfer protons measurably to water. Thus the carboxylic acids stand out from alkynes, halides, alcohols, and simple aldehydes and ketones in giving water solutions that are acidic to indicator papers or pH meters as the result of proton transfers from the carboxyl groups ... 

Before discussing examples of the reactions, it should be pointed out that most types of vinylic halides used in this reaction are easily available. The UV-catalyzed addition of HBr to terminal acetylenes forms the 1-bromo-l-alkenes. The cis isomer is formed almost exclusively if the addition is carried out at dry ice temperature. The 2-bromo or 2-iodo-l-alkenes are obtained from reacting aqueous hydrogen halides with alkynes. The 2-substituted-1-bromo-l-alkenes are available by the bromination-base dehydrobromination reactions. 

New reports of organozirconium-organic halide cross-coupling reactions have almost completely stopped. Nevertheless, the palladium-catalyzed cross-coupling of vinyl zir-conocenes (from alkyne hydrozirconation) with vinyl halides has been employed in the synthesis of the lipid isobutylamide natural product anacyclin (equation 98). ... 

The authors point out that, unlike vinyl halides, the vinyl chloronium ion 23a could hardly show double bond character in the C-Cl bond because of unfavourable charge repulsion in 23b. The unreactivity of vinyl halides may therefore primarily be a consequence of the strength of the a bond to the sp2, carbon. However if opening of 23a is assumed to be due to nucleophilic attack of the solvent (or any other nucleophile) to either C5 or C2, the greater amount of shifted products obtained from alkynes than from alkenes is a direct consequence of the relative facility of nucleophilic attack at the sp3 carbon and sp% carbon (Rappoport, 1969 Modena, 1971). 

Alkynes from halides also alkynes from alkynes Esters from alcohols also esters from carboxylic acids Alkyls from alkenes (Conjugate Reduction)... 

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