Terminal alkynes abstract

Acetylene and terminal alkynes are CH-acidic compounds the proton at the carbon-carbon triple bond can be abstracted by a suitable base. Such a deprotonation is the initial step of the Glaser reaction as well as the Eglinton... 

Allylzirconation of alkynes with allylzirconocene chloride reagents (obtained by hydrozir-conation of allenes) takes place in the presence of a catalytic amount of methylaluminox-ane (MAO) [67,68]. MAO presumably abstracts chloride to form an allylzirconocene cation, which coordinates to the alkyne triple bond. The subsequent migratory insertion is regioselective, as it is found that the new bond is mainly formed between the a-carbon of the allylzirconium species and the internal carbon of a terminal alkyne (Scheme 8.33). 

Allenylcobaloximes, e.g. 26, react with bromotrichloromethane, carbon tetrachloride, trichloroacetonitrile, methyl trichloroacetate and bromoform to afford functionalized terminal alkynes in synthetically useful yields (Scheme 11.10). The nature of the products formed in this transformation points to a y-specific attack of polyhaloethyl radicals to the allenyl group, with either a concerted or a stepwise formation of coba-loxime(II) 27 and the substituted alkyne [62, 63]. Cobalt(II) radical 27 abstracts a bromine atom (from BrCCl3) or a chlorine atom (e.g. from C13CCN), which leads to a regeneration of the chain-carrying radical. It is worth mentioning that the reverse reaction, i.e. the addition of alkyl radicals to stannylmethyl-substituted alkynes, has been applied in the synthesis of, e.g., allenyl-substituted thymidine derivatives [64],... 

Trust s group has shown that another selective reaction involving C—O bond formation followed by rearrangement and C—C bond formation occurred when Cp-containing ruthenium complexes were used as catalytic precursors. With RuCl(Cp)(PPh3)2 in the presence of NH4PF6, an additive known to facilitate chloride abstraction from the metal center, the addition of allylic alcohols to terminal alkynes afforded unsaturated ketones [46, 47]. It has been shown that the key steps are the... 

The organozinc prepared from propargyl bromide did not react with terminal alkynes because, under the reaction conditions, abstraction of the acidic terminal acetylenic hydrogen from the propargylic metabotropic form presumably occurred (equation 154)185. 

A recent theoretical study by Takeuchi et al. [140] has examined the mechanism for the reaction of both alkenes and alkynes with the H-terminated silicon surface using periodic DFT calculations, and the results are in good agreement with the proposed radical-based mechanism [137]. In particular, the calculations show that the reaction occurs through a carbon-based radical intermediate which must be sufficiently stabilized to proceed by abstraction of a surface hydrogen (as in the case of styrene) if the intermediate is not stable enough, it will preferentially desorb (as in the case of ethylene). The calculations also show that reaction with terminal alkynes should proceed faster and lead to more stable products than with terminal alkenes [140]. 

Acetylenic Grignard reagents of the type RC CMgBr are prepared, not from an acetylenic halide, but by an acid-base reaction in which a Grignard reagent abstracts a proton from a terminal alkyne. 

Aldehydes and ketones have been alkynylated using indium(III) and Hunig s base (PrjNEt) as catalysts.213 IR and NMR evidence support a dual-activation role for indium it is a Lewis acid for the hard electrophile (carbonyl compound), and has sufficient n -coordination ability for a soft nucleophile such as a terminal alkyne. For the latter substrate, the amine then assists proton abstraction. 

A regioselective iodoperfluoroalkylation of terminal alkynes (R—C = CH) has been reported, and is based on photolysis ofthe C—I bond in perfluoroalkyl iodides (Rp-I). Addition of the thus-formed RF" radical onto the alkyne afforded a vinyl radical that in turn abstracts an iodine atom from the starting Rp—I to form the end olefin R-C(I)= CH-Rf. A xenon lamp through Pyrex (hv > 300 nm) was used for the reaction, where aliphatic alkynes gave a better alkylation yield with respect to phenylacetylene [81],... 

Terminal alkyne complexes of zirconium, while challenging to isolate, have been implicated in a number of organometallic transformations. Mixing an alkenyl zirconocene 218 with a transient dialkyl zirconocene 111 furnishes a yu-acetylide complex 219 (Scheme 33).112 This reaction is believed to proceed by initial transmetallation to form a zirconocene alkenyl alkyl 220, which undergoes subsequent /3-hydrogen abstraction to generate the terminal alkyne complex 221. This proposed intermediate can be trapped with PMe3 222 from the alkylation of the zirconocene alkenyl bromide with butyllithium. Comparison of the spectroscopic features of this product to the... 

However, terminal alkynes gave only the corresponding indenes derived by further rearrangement, and alkynes with larger substituents gave the products of hydrogen abstractions. 

Terminal alkynes have weakly acidic hydrogens that can be abstracted by strong bases such as sodium amide. 

The mechanism of the reaction is the same as that for the addition of HBr to an alkene in the presence of a peroxide. That is, the peroxide is a radical initiator and creates a bromine radical (an electrophile). If the alkyne is a terminal alkyne, the bromine radical adds to the sp carbon bonded to the hydrogen if it is an internal alkyne, the bromide radical can add with equal ease to either of the sp carbons. The resulting vinylic radical abstracts a hydrogen atom from HBr and regenerates the bromine radical. Any two radicals can combine in a termination step. 

Abstract Palladium readily catalyzes the cross-coupling of functionally substituted aryl or vinylic halides and alkynes to afford a wide variety of heterocycles and carbocycles in one efficient step. Terminal alkynes presumably initially generate aryl (vinylic) alkynes, which under the reaction conditions are rapidly cyclized by the palladium or copper salts employed in the first step to produce the final product. Internal alkynes apparently react by carbo-palladation of the alkyne and subsequent intramolecular nucleophilic substitution of the palladium moiety to generate the observed products. A variety of other closely related processes have also been reported, including the simultaneous annulation of alkynes and car-... 

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