Alkynes, from alkenes alkyne anions

The two basic methods used to prepare alkynes are double elimination from 1,2-dihaloalkanes and alkylation of alkynyl anions. This section deals with the first method, which provides a synthetic route to alkynes from alkenes Section 13-5 addresses the second, which converts terminal alkynes into more complex, internal ones. 

The desired synthetic route becomes apparent when it is recognized that the Z alkene stereoisomer may be obtained from an alkyne, which, in turn, is available by carboxylation of the anion derived from the starting material. 

After addition of the alkyne anion to the alkylborane, iodination facilitates alkyl group migration from boron to carbon in a transfer that resembles the one seen in the synthesis of (Z)-alkenes described in Section B4.1. Elimination to give the product alkyne occurs under the iodination reaction conditions (Figure B4.4). 

It was reported by Rozhkov and Chaplina130 that under mild conditions perfluorinated r-alkyl bromides (r-RfBr) in nonpolar solvents can be added across the n bond of terminal alkenes, alkynes and butadiene. Slow addition to alkenes at 20 °C is accelerated in proton-donating solvents and is catalyzed by readily oxidizable nucleophiles. Bromination of the it bond and formation of reduction products of t-RfBr, according to Rozhkov and Chaplina, suggest a radical-chain mechanism initiated by electron transfer to the t-RfBr molecule. Based on their results they proposed a scheme invoking nucleophilic catalysis for the addition of r-RfBr across the n bond. The first step of the reaction consists of electron transfer from the nucleophilic anion of the catalyst (Bu4N+Br , Na+N02, K+SCN , Na+N3 ) to r-RfBr with formation of an anion-radical (f-RfBr) Dissociation of this anion radical produces a perfluorocarbanion and Br, and the latter adds to the n bond thereby initiating a radical-chain process (equation 91). 

Reactions that form carbon-carbon bonds are extremely important in synthesis because they enable larger compounds, containing more carbons, to be constructed from smaller compounds. This requires the reaction of a carbon nucleophile with a carbon electrophile. The most important carbon nucleophiles that we have encountered so far are cyanide ion and acetylide anions (see Section 10.8). If we remember that acetylide anions can be reduced to c/.v-alkenes (see Section 11.12), then all of the addition products of this chapter are accessible from simple alkynes. 

From a synthetic point of view, bond forming steps are the most important reactions of radical ions [202]. Several principle possibilities have been described in Section 8.1 and are summarized in Scheme 52. Many carbo- and heterocyclic ring systems can be constructed by (inter- and intramolecular) radical addition to alkenes, alkynes, or arenes. Coupling of carbonyl radical anions leads to pinacols either intra-or inter-molecular which can be further modified to give 1,2-diols, acyloins or alkenes. Radical combination reactions with alkyl radicals afford the opportunity to synthesize macrocyclic rings. These radical ion-radical pairs can be generated most efficiently by inter- or intramolecular photoinduced electron transfer. 

Isomerization of Alkenes and Alkynes. Isomerization of alkenes proceeds through anion intermediates by abstraction of an allylic proton from alkene molecules by solid bases. In the case of 1-butene isomerization, high cisitrans ratio of 2-butene is characteristic of the base-catalyzed isomerization. On the... 

The new functional group exchange reactions presented in this chapter can be combined with reactions from previous chapters to expand the ability to synthesize molecules. Alkene 85 is synthesized from aldehyde 86, for example. The first task is to identify the four carbons of 86 in 85. It appears that the carbons marked in blue are the best candidates. Rather than disconnect the C-C=C unit marked in blue, first disconnect the ethyl group of 85 to give 87 and 88. This choice is made because no reaction has been presented that will allow direct incorporation of EtCHCH to X-C-CMea. Disconnection of the ethyl group takes advantage of the fact that an alkyne anion reacts with an alkyl halide. However, before this reaction can be used, the alkene unit in 87 needs to be changed to an alkyne unit in 89. 

Elimination of halogen from a,P-dihalides in a variety of circumstances has been observed and can be used to prepare alkenes and alkynes from vicinyl dihalides. The process is usually of limited utility since the dihalides themselves are often prepared from the unsaturated compounds. As shown in Scheme 7.42, both erythro (meso) and threo ( )-2,3-dibromobutane diastereomers undergo stereospecific elimination of bromine to their respective (E)- (or trans) and (Z)- (or cis) 2-butenes on treatment with iodide anion (T) in ethanol (ethyl alcohol, CH3CH2OH). 

The enamine (250) was formed when l,2-0-isopropylidene-cx-D-A y/o-pento-dialdo-1,4-furanose (251) reacted with diethylamine. Dehydrochlorination of the terminal alkene (252) [prepared by a Wittig reaction on (251)] using the anion derived from A-methylaniline gave the alkyne (253). Other alkynic derivatives of sugars are noted in Chapter 3. 

The transition states of 5-endo-dig and 5-endo-trig anionic ring closures have been reported as examples of nonpericyclic reactions with transition states stabilized by aromaticity resulting from delocalization of the lone pair at the nucleophilic centre, a a CC-bond, and an in-plane alkyne (or alkene) r-bond (Scheme 78). ... 

The facile photosensitized oxidation of tetraalkylstannanes (R4 n) and related group-14 compounds has been widely exploited by Mella and co-workers to form carbon-centered radicals. Photoinduced electron transfer from tetraalkylstannanes to a sensitizer, such as aromatic nitriles and esters (including tetramethyl pyromellitate, TMPM), affords a radical cation [R4 n] that can fragment to form an alkyl radical (R) together with the R3Sn+ cation. The alkyl radicals can then react with electron-poor alkenes, alkynes, aromatics, or the radical anion formed from the photosensitizer to form new carbon-carbon bonds. Careful choice of the photosensitizer can ensure that the radical anion selectively reduces the radical adduct (derived from addition to a double bond) rather than the first-formed alkyl radical (Scheme 5). 

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