Alkynes propargylic rearrangement

When X=OH, this conversion of acetylenic alcohols to unsaturated aldehydes or ketones is called the Meyer-Schuster rearrangement The propargyl rearrangement can also go the other way that is, 1-haloalkenes, treated with organocopper compounds, give alkynes. ... 

The addition of sulfur nucleophiles to alkynes is a less developed transformation. However, Yamamoto described the attack of the sulfur atom of aryl thioethers to afford benzothiophenes (equation 31). More recently, it has been showed that propargylic thioethers or thioacetals undergo migration to give carbenes that cyclize in hydroarylation processes and thiocarbamates that evolve by propargylic rearrangement. Thiosilanes can perform as both sulfur nucleophiles and silicon electrophiles in intramolecular reactions to afford benzothiophenes (equation 32). ... 

Cleavage of propargyl ethers by Grignard reagents 12-2 Rearrangement of alkynes... 

The ester/alkyne rearrangement system can be extended by conjugation to involve a more distant site. The reaction of diethyl phosphorochloridate with 3-methylhex-4-yne-2-ene-l-ol leads to the vinylogous propargylic ester rearrangement as shown in Equation 4.39.176... 

The isomerization of the smallest alkynes 80 with halogens in a propargylic position has been described for chlorine [151, 152], bromine [153] and iodine [154] (Scheme 1.35), but often might proceed by an SN2 -type substitution rather than a prototropic rearrangement [155-159]. On the other hand, transformations such as 82 —> 83 [160] or 84 —> 85 [161] are clearly prototropic (Scheme 1.36). This is also true for propargylic halides such as 86 with its additional ester group assisting the prototropic isomerization [162,163] (Scheme 1.37). 

If the alkene is an alkyne instead, we are dealing with a propargylic alcohol and now the thermodynamics are more favourable and the product is an enone. Commercial application is found in the production of citral from dehydrolinalool via vanadium-catalysed isomerisation (Figure 5.9). Note that the last step involves a transfer of hydrogen as well when the enol rearranges to the aldehyde ... 

A transition state model proposed by Marshall for the rearrangement is shown in Scheme 1. The chiral amide 63 coordinates either one of the two enantiotopic propargylic protons in the chair-like conformation (Scheme 1). Complex I should be of lower energy as it lacks the unfavorable 1,4-phenyl/alkyne interactions present in complex n, thus the pro-S proton was removed predominantly. This rationale suggests that the enantioselec-tivity is predominantly determined at the Uthiation step. 

It is noteworthy that the indenyl complex RuCl(ri -C9H7)(PPh3)2l4 provides an efficient catalyst precursor for the anti-Markovnikov hydration of terminal alkynes in aqueous media, especially in micellar solutions with either anionic (sodium dode-cylsulfate (SDS)) or cationic (hexadecyltrimethylammonium bromide (CTAB)) surfactants [38]. This system can be applied to the hydration of propargylic alcohols to selectively produce P-hydroxyaldehydes, whereas RuCl(Cp)(PMe3)2 gives a,P-unsat-urated aldehydes (the Meyer Schuster rearrangement products)(Scheme 10.8) [39]. 

Electron-withdrawing m-substituents will decrease the electron density at the aromatic terminus of the [3,3]-rearrangement, thereby retarding the reaction. This allows polymerization of the terminal alkyne to compete successfully with the cyclization. However, the presence of a m -substituent has a much more significant effect the cyclization of m-substituted aryl propargyl ethers can lead to two isomeric products (46a and b). 

Further mechanistic work with optically active and labeled compounds confirmed that the rate-determining step in such rearrangement was the silver coordination to the alkyne moiety and revealed that the silver-catalyzed allene epimerization was 2-40 times faster than the propargyl ester rearrangement.53... 

The reaction is highly dependent on the alkyne used. When structurally simple 3-hydroxy alkynes such as propargyl alcohol are used, acceptable yields of the addition products are obtained, whereas reactions with alkynes prone to carbocation formation and rearrangement, such as 1-ethynyl-cyclohexanol, give a complex mixture of products.68,73... 

Propargyl naphthyl ethers can be rearranged to to naphthofurans in shorter reaction times when exposed to microwave irradiation (Scheme 89) <1996JCM338>. These alkynes first undergo Claisen rearrangement to form an allene intermediate which in turn spontaneously cycloisomerizes to the benzofuran product. 

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