Alkyl-substituted acetylene

The regioselectivity of the reaction with unsymmetrical alkynes is poor. Mixtures of isomers are obtained with alkyl substituted acetylenes, if the alkyl groups do not differ much in size. A solution to this problem has been reported by Semmelhack et al. The reactants are connected by a -OCHaCHaO-tether, which can later be removed the coupling step thus becomes intramolecular ... 

The addition of any one of several dialkyl chlorophosphates to an arylalkyne-derived vinyl zirconocene in the presence of catalytic amounts of CuBr in THF leads to the corresponding vinyl phosphonate in high yields (78—92% see, for example, Scheme 4.38) [25]. Here, alkyl-substituted acetylenic starting materials do not react beyond the initial hydrozirconation stage. Vinyl phosphonates may be readily converted to acyloins by oxidation to the diol followed by base-induced cleavage. 

Alkynes react with bromine via an electrophilic addition mechanism. A bridged bromonium ion intermediate has been postulated for alkyl-substituted acetylenes, while vinyl cations are suggested for aryl-substituted examples.119 1-Phenylpropyne gives mainly the anti addition product in acetic acid, but some of the syn isomer is formed.120 The proportion of dibromide formed and stereoselectivity are enhanced when lithium bromide is added to the reaction mixture. 

The literature of the vibrational spectra of adsorbed alkynes (acetylene and alkyl-substituted acetylenes) is very much in favor of single-crystal studies, with fewer reported investigations of adsorption on oxide-supported metal catalysts. Fewer studies still have been made of the particulate metals under the more advantageous experimental conditions for spectral interpretation, namely, at low temperatures and on alumina as the support. (The latter has a wide transmittance range down to ca. 1100 cm-1.) A similar number of different single-crystal metal surfaces have been studied for ethyne as for ethene adsorption. We shall review in more detail the low-temperature work which usually leads to HCCH nondissociatively adsorbed surface structures. Only salient features will be discussed for higher temperature ethyne adsorption that often leads to dissociative chemisorption. Many of the latter species are those already identified in Part I from the decomposition of adsorbed ethene. 

Similarly, irradiation of a mixture of alkyl iodide (69) and trifluoromethanesulfonyl-acetylenic derivative in the presence of Bu3SnSnBu3 with a mercury lamp generates an alkyl-substituted acetylenic derivative (70) through the same addition-elimination reaction at the sp carbon (eq. 4.28) [78]. 

The bromochlorination of phenyl- and alkyl-substituted acetylenes with tetrabutylammonium di-chlorobromate gives (T)-olelins stercoselectively. Unsymmetrical disubstituted acetylenes give the preferred Markovnikov adducts, but the formation of zznZz-Markovnikov adducts is more pronounced with this reagent than with molecular bromine chloride, which is less stereoselective139. 

Alkyl-substituted acetylenes can react with HCI by either the Ad S or the Ad 2 mechanism. The Ad S mechanism leads to anti addition. The preference for one or the other mechanism depends on the reactant structure and the reaction conditions. Added halide ion promotes the Ad S mechanism and increases the overall rate of reaction. " Reaction of 4-octyne with TFA in CH2CI2 containing O.l-l.OMBr leads mainly to Z-4-bromo-4-octene by an anti addition. The presence of Br greatly accelerates the reaction as compared to reaction with TFA alone, indicating the involvement of the Br in the rate-determining protonation step. " ... 

For alkyl-substituted acetylenes, there is a difference in stereochemistry between mono- and disubstituted derivatives. The former give syn addition while the latter react by anti addition. The disubstituted (internal) compounds are considerably ( 100 times) more reactive than the monosubstituted (terminal) ones. This result suggests that the transition state of the rate-determining step is stabilized by both alkyl substituents and points to a structure with bridged character. This would be consistent with the overall stereochemistry of the reaction. 

The Au(III)-catalyzed double hydroamination cascade reaction of ortho-alkynyla-nilines 167 with terminal alkynes 168 affording N-vinylindoles 169 was reported by li (Scheme 9.63) [221]. In the case of alkyl-substituted acetylenes, this protocol provided mixtures of isomeric N-vinylindoles with both terminal and internal double bonds. This transformation is believed to occur via the Au(III)-catalyzed cydoisomerization of transient key alkynyl imines, similar to 162 utilized by Yamamoto, which were generated via the initial Au(111)-catalyzed hydroamination of the corresponding anilines 167 with alkynes 168. 

Recently, Fagnou reported a very interesting, atom-economical route to the 1,2,3-trisubstituted indole derivatives 273 via the Rh(II)-catalyzed oxidative coupling-indolization reaction (Scheme 9.95) [251]. Accordingly, simple acetanilides 271, upon a directed C-H activation with the Rh(II)-catalyst [252] followed by a subsequent carborhodation-indolization sequence of alkyne 272, gave N-acylated indoles 273. Both electron-rich and electron-deficient acetanilides 271, possessing different functionalities were perfectly tolerated under these reaction conditions. In the case of unsymmetrical alkyl-alkyl-substituted acetylenes, a mixture of indole products... 

Alkyl-substituted acetylenes are believed to be susceptible to reaction by either the Ade2 or Ade3 process with the precise balance determined by individual structural features and reaction conditions. 

Tertiary alkyl-acetylenes are not usually available by the reaction of metal acetylides with tertiary alkyl h ides, but the Lewis-acid-catalysed alkylation of trimethylsilylacetylenes (279) can be applied to the synthesis of tertiary-alkyl-substituted acetylenes (280). ... 

Bromination of alkynes occurs via an electrophilic mechanism. A bridged intermediate is postulated for alkyl substituted acetylenes, while vinyl cations are... 

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