Rearrangements involving alkynes

The first rearrangement involving an acetylenic compound was reported by Favorskii133 as early as 1887. He found that when 1-butyne and higher-molecular-weight straight-chain 1-alkynes are heated with an alcoholic solution of potassium hydroxide, they are isomerized to 2-alkynes ... 

Most reactions of this category involve the base-induced generation of alkylidene-carbenes (R2C = C ) which undergo an intramolecular 1,5-carbon-hydrogen insertion providing a useful route for the construction of substituted cyclopentenes a competing intramolecular pathway is rearrangement to alkynes. 

E. Rearrangements Involving Reactions of Alkynes with Electrophiles. . . 500... 

II. REARRANGEMENTS INVOLVING ACETYLENES A. Alkyne-Allene Interconversions... 

In contrast, kinetic studies of the reaction between [Cr(CO)5 C(Ph)OMe ] and various arylalkynes to give [Cr(CO)3(Tj -substituted-l-naphthol)] complexes indicate initial CO ligand loss to form a steady state concentration of [Cr(CO)4 C(Ph)OMe ], followed by addition of the alkynes to the metal. The subsequent rearrangements, involving attack at the carbene, to give the naphthol products are rapid. 

In the present review, preparation of 1,2,3-triazoles with substituents at positions 1, 4, and 5 has been divided into three parts condensation reactions and rearrangements, selective preparation of trisubstimted triazoles from disubstituted precursors, and formal [3+2] cycloaddition reactions involving alkynes (Fig. 2). Fused polycyclic 1,2,3-triazoles are out of the scope of this review. For simplicity, all the triazoles described in this review will be 1,2,3-triazoles and will be called triazoles. ... 

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 skeletal rearrangements are cycloisomerization processes which involve carbon-carbon bond cleavage. These reactions have witnessed a tremendous development in the last decade, and this chemistry has been recently reviewed.283 This section will be devoted to 7T-Lewis acid-catalyzed processes and will not deal, for instance, with genuine enyne metathesis processes involving carbene complex-catalyzed processes pioneered by Katz284 and intensely used nowadays with Ru-based catalysts.285 By the catalysis of 7r-Lewis acids, all these reactions generally start with a metal-promoted electrophilic activation of the alkyne moiety, a process well known for organoplatinum... 

Alkylidenecyclopentene-l,4-diones (cf. 12, 130).2 The cobaltacyclopente-nedione (2) formed on reaction of a cyclobutenedione with 1, when complexed with dimethylglyoxime, reacts with 1-alkynes to form 5-alkylidenecyclopentene-1,4-diones (3). The reaction involves an alkyne-vinylidene rearrangement. Ben-zoquinones are usually formed in minor amounts. 

O Connor proposed a mechanism involving deinsertion of carbon monoxide from the vinylketene complex 106 to form the new cobaltacyclobutene 109. The cobalt may then undergo a 1,3-shift to the carbonyl of the ester group to create the oxycobaltacycle 110, before deinsertion of the cobalt moiety forms the furan 108. Alternatively, 109 may rearrange to the vinyl-carbene 111, which then undergoes ester-carbonyl attack on the carbene carbon to form the zwitterionic species 112, which finally aromatizes to yield the furan 108. Notice that this latter postulate is identical to the final steps of the mechanism formulated by Wulff (see Section V,B) for the reaction between a cobalt carbene and an alkyne, in which a cobaltacyclobutene is a key intermediate.51... 

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 ... 

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