Mechanisms of alkynes

The mechanism of alkyne additions is similar but not identical to that of alkene additions. When an electrophile such as HBr adds to an alkene (Sections 6.7 and 6.8), the reaction takes place in two steps and involves an alkyl carbocation intermediate. If HBr were to add by the same mechanism to an alkyne, an analogous vinylic carbocation would be formed as the intermediate. 

This postulate has several implications regarding the mechanism of alkyne hydrogenation these will be discussed in Sect. 4.3. It should be noted, however, that there is as yet little or no direct evidence for structure L, although analogous structures are known to exist with organometallic complexes [161], Such a structure is also consistent with the positive surface potentials observed for acetylene adsorption on evaporated nickel films [88]. 

Alternatively, with known metathesis catalysts such as (PCy3)2Cl2Ru=CHPh, a metal carbene mechanism of alkyne trimerization has been proposed.141... 

Reaction of acetone with DsO" yields hexadeuterioacetone. That is, all the hydrogens in acetone arc exchanged for deuterium. Review the mechanism of alkyne hydration, and then propose a mechanism for this deuterium incorporation. (See Section 22.2.)... 

The mechanism of alkyne metathesis also consists of a series of [2 + 2] and [2 + 2] retro-cycloadditions. The key intermediate, a metallacyclobutadiene, appears to be antiaromatic, but apparently the metal uses a d orbital to form the W=C 77 bond so that the ring is aromatic. 

We first encountered alkyne metathesis in Chapter 10 in connection with reactions of metal-carbyne complexes. The mechanism of alkyne metathesis, first proposed by Katz,64 is analogous to that for alkenes, and it is shown in Scheme 11.9. 

Abstract Bimetallic catalysts are capable of activating alkynes to undergo a diverse array of reactions. The unique electronic structure of alkynes enables them to coordinate to two metals in a variety of different arrangements. A number of well-characterised bimetallic complexes have been discovered that utilise the versatile coordination modes of alkynes to enhance the rate of a bimetallic catalysed process. Yet, for many other bimetallic catalyst systems, which have achieved incredible improvements to a reactions rate and selectivity, the mechanism of alkyne activation remains unknown. This chapter summarises the many different approaches that bimetallic catalysts may be utilised to achieve cooperative activation of the alkyne triple bond. 

The mechanism of alkyne oligomerization induced by PdCl2 is therefore seen to involve a series of stepwise c -insertion of coordinated alkynes as shown in Scheme Similarly, phenylacetylene can react with PdCLj " to give the tetramer product as shown in Scheme... 

This reaction is called the Birch reduction, named after the Australian chemist Arthur Birch, who systematically explored the details of this reaction. The mechanism, which is believed to be very similar to the mechanism of alkyne reduction via a dissolving metal reduction, is comprised of four steps (Mechanism 18.1). 

The insertion of alkynes into Pd-C or Pd-ER bonds (ER = SiR3, SnRs, BR2, etc.) of palladium(ll) derivatives is an important reaction involved in the mechanism of alkyne polymerization and in the numerous Pd-catalyzed additions to alkynes. It must occur by previous coordination of the alkyne to the metal, but the intermediate Pd( 7 -alkyne) complex is usually not detected either in catalytic studies or in examples of stoichiometric reactions reported in recent years,and those studies will not be collected here. Double insertion of alkynes into Pd-C bonds leads sometimes to <7-77 -enyl derivatives and some examples will be given in Section 8.06.6.5. Reviews are available dealing with palladium-catalyzed processes of interest where addition of two different groups to alkynes with Pd(ll) complexes takes place. 

Recent theoretical studies on the cyclotrimerization of alkynes catalyzed by Cp RuCl have proposed a new mechanism from ruthenacyclopentadiene to mthenacycloheptatriene (Scheme 5.27). This mechanism is not the simple insertion of alkyne into a Ru—C bond. Ruthenecyclopentadiene 66 undergoes [2 + 2] cycloaddition with alkyne to give l-ruthenabicyclo[3.2.0]hepta-l,3,6-triene 67, which then isomerizes to ruthenacycloheptatriene 68 [10a,41]. Isolation of 64 from the reaction of iridacyclopentadiene 63 with 2-butyne and the observation of isomerization from 65 to 64 constitute direct evidence of the proposed mechanism of alkyne cyclotrimerization based on the theoretical study. 

Ananikov VP, Gayduk KA, Orlov NV, Beletskaya IP, Khrus-talev VN, Antipin MY. Two distinct mechanisms of alkyne insertion into the metal-sulfur bond combined experimental and theoretical study and application in catalysis. Chem. Eur. J. 2010 16 2063-2071. 

Isolation of the alkyne complexes with the ir) -cyclopentadienyl dicar-bonylcobalt(I) catalyst [CpCo(CO)2] 2.370 and 2.371 was possible (Scheme 2.116). These are the essential intermediates that were being considered at the beginning of this chapter during the analysis of the reaction mechanism of alkynes cyclotrimerization [172] (structures 5 and 6 in Scheme 2.1). 

Fig. 4.20 Mechanism of alkynes hydration. (From O.N. Temidn, Kinetic models of multi-route reactions in homogeneous catalysis with metal complexes (a review), Kinet. Catal. 53 (2012) 313—343. Copyright 2012...

Further elucidations on the mechanism of alkynes hydration arise from the isolation of both enol and keto tautomers of organometallic intermediates, starting from an alkyne-carboxylic acid ester as tetrolic acid ethyl ester [43]. 

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