Elimination reactions alkynes

Double dehydrohalogenation of gemmal dihalides (Section 9 7) An E2 elimination reaction of a gemmal dihalide yields an alkenyl halide If a strong enough base IS used sodium amide for example a second elimination step follows the first and the alkenyl halide IS converted to an alkyne... 

The primary and secondary products of photolysis of common diazirines are collected in Table 4. According to the table secondary reactions include not only isomerization of alkenes and hydrogen elimination to alkynes, but also a retro-Diels-Alder reaction of vibrationally excited cyclohexene, as well as obvious radical reactions in the case of excited propene. 

In a reaction similar to the (>-alkoxide elimination reactions seen with zir-conocenes, catalytic Rh(OH)(cod)2 and 2 eq. of arylboronic acids gave cyclic products 165 from enynes 166 (Scheme 35) [100]. In this reaction, transmet-allation of Rh - OR with B - Ph gave Rh - Ph species 167, which inserted into the alkyne, cyclized to 168, and finally underwent [>-alkoxidc elimination to provide Rh-OCH3. This reaction is limited to the formation of five-membered rings, but it can also undergo cascade type reactions of enediynes to give multicyclic products [100]. 

Some interesting modifications with respect to the base-induced isomerization have recently been developed. For example, conversion of 4-hydroxy-l-thiophenyl-2-alkynes 299 into the corresponding 4-hydroxy-substituted thiophenylallenes 300 was achieved by treatment with potassium hexamethyldisilazide at low temperature (Scheme 8.79) [165], If the hydroxyl group is protected as the THP ether an elimination reaction occurred, resulting in the formation of an enyne instead of allene 300. 

For a number of other pharmacologically active unsaturated compounds, it is assumed that a reactive allene is formed in situ by an alkyne isomerization [160] or an elimination reaction [161]. The prime example of the formation of such a highly reactive allene through chemical activation of an unsaturated precursor is the ene-diyne antibiotic neocarzinostatin (Scheme 18.57) [162],... 

Table 9.1). The rate of dehydrobromination from the intermediate bromoalkenes follows the pattern 2-bromoalkenes > Z-l-bromoalkenes > E- -bromoalkenes the corresponding chloro derivatives react more slowly. For optimum yield, the reaction temperature should be <100°C to reduce decomposition of the catalyst, and the concentration of base should be kept low to prevent isomerization of the resulting alkynes. [3-Elimination of HBr from 1,2-dibromo-1 -phenylethane can be controlled to yield 1-bromo-l-phenylethene in 83% yield [15]. The addition of alcohols and diols have a co-catalytic effect on the elimination reaction, as the alkoxide anions are transferred more effectively than the hydroxide ions into the organic phase [13]. 

Similar facile cycloaddition-elimination reactions are observed when the nitrile group is replaced by an alkyne or alkene group. Thus, thermolysis of thiatriazolines (56) in refluxing benzene for 3 days provided the intramolecular adducts (57) (Equation (5)) <9314439 >. 

The proposed mechanism involves the formation of ruthenium vinylidene 97 from an active ruthenium complex and alkyne, which upon nucleophilic attack of acetic acid at the ruthenium vinylidene carbon affords the vinylruthenium species 98. A subsequent intramolecular aldol condensation gives acylruthenium hydride 99, which is expected to give the observed cyclopentene products through a sequential decarbonylation and reductive elimination reactions. 

The oxoniastannaboratole derivatives 357 are synthesized by the reaction of zwitterionic r/2-(alkynylborate)alkenyl stannane derivatives 356 with tetrabutylammonium fluoride (TBAF) followed by water. The stannanes 356 react readily with TBAF even at — 65 °C yielding the Sn-F compounds, which then react with water with concomitant elimination of alkyne to yield the stannaboratole 357 in high yield (Scheme 57) <1999JOM93>. 

We seem not to be getting closer to 58, but 58 would actually result by dehydration of 62. In real life, the alcohol 62 is turned into the chloride and a double elimination with NaNth gives 58 after acidification.14 Though we saw elimination reactions used to make alkenes in chapter 15, this is the first we have seen to make alkynes. 

The exceptional nucleofugality of the phenyliodonio group has been determined in an alkenyl salt and it is about 106 times greater than that of triflate [30]. This remarkable property makes alkenyl iodonium salts excellent vinyl cation equivalents in nucleophilic substitutions. The chemistry of alkenyl iodonium salts is dominated by the transfer of their aliphatic moiety to a variety of nucleophiles other important reactions involve Michael-type addition and alkylidenecarbene generation, along with elimination to alkynes which is actually an undesirable side-reaction. 

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