Alkynes, cross-coupling

A palladium-catalysed carbometallation-alkyne cross coupling cascade process has been reported for the stereo- and regio-controlled synthesis of dibenzoxepines with substituted exocyclic alkene functionality <06OL1685>. 

Alkyne cross-coupling reactions over the last 25 years have become one of the most valuable assets in the synthetic chemist s toolbox. The now famous Sonogashira coupling (50, 114) of terminal alkynes with aryl or vinyl halides is readily achieved with a palladium catalyst, a copper(l) cocatalyst, and amine base. In the catalytic cycle (Scheme 14a), copper-and palladium-alkyne complexes are the key intermediates that lead to coupling of R and R units via the alkyne. Analogously, the Stille coupling... 

Silylpalladium complexes have attracted attention due to their relevance in the mechanism of Pd complex catalyzed reactions such as hydrosilylation of alkenes and dienes, bis-silylation of dienes and alkynes, carbosilylation of alkynes, cross-coupling of organic halides with disilanes, and ring-opening oligomerization and polymerization of cyclic disilanes and polysilanes. 

Pd-catalyzed alkyne cross-coupling has become one of the most satisfactory methods for the synthesis of alkynes. As discussed in Sect. HI.2.8, there are two related but discrete protocols, that is, Sonogashira coupling, which does not involve formation of alkynylmetals as preformed and discrete reagents (Sect. HI.2.8.1), and the general... 

In this section, Pd-catalyzed homocoupling of tenninal alkynes, cross-coupling of terminal alkynes with internal alkynes, and cross-coupling of terminal alkynes with allenes will be discussed. All three types of reactions involve (i) activation of the C—bond of a terminal alkyne, (ii) alkynylpalladation of another molecule of alkyne or allene, and (iii) reductive elimination or protonation to produce a conjugated enyne. For alkynylpallada-tions of allenes followed by trapping with nucleophiles, see Sect. IV.7. 

Further development of this reaction mode by Trost and co-workers led to the discovery of the alkyne-alkyne cross-coupling protocol. It was shown that addition of a terminal... 

An interesting sequential reaction, consisting of an intermolecular alkene carbometallation and subsequent intermolecular alkyne cross-coupling, has been reported (Scheme 8.87) [601]. Starting from an immobihzed tropane framework 459, stoichiometric carbopalladation yields a stable organopalladium intermediate, which in the presence of copper(I) iodide undergoes coupling with an added terminal acetylene. 

Alkyne cross-coupling is also an important tool in the assembly of unique nanoarchitectures to investigate fundamental stmcture-property relationships. One pertinent example by Haley et al. [169] is the synthesis of DBA trefoils 61 and 62, which were prepared by a sequential Sonogashira cross-coupling/Pd-mediated... 

An interesting sequential reaction consisting of an intermolecular alkene carbometallation and subsequent intermolecular alkyne cross-coupling has been reported by... 

Synthetic strategy Copper-free Sonogashira — aryl bromide - alkyne cross-coupling... 

Finke, A. D., Elleby, E. C., Boyd, M. J., Weissman, H., and Moore, J. S. (2009). Zinc chloride-promoted aryl bromide-alkyne cross-coupling reactions at room temperature. J. Org. Chem., 74, 8897-8900. 

In 2006, Trost and co-workers reported a one-pot synthesis of enantiopure N- and O-heterocyclic compounds using the combination of an achiral ruthenium catalyst and a chiral palladium complex. After the completion of the ruthenium-catalysed alkene-alkyne cross-coupling reaction between the... 

Scheme 3.10 Tandem alkene-alkyne cross-coupling reaction-intramolecular het-erocyclisation reaction catalysed hy a combination of ruthenium catalysis and chiral palladium catalysis.

Xi Z, Kara R, Takahashi T (1995) Highly selective and practical alkyne-alkyne cross-coupling using Cp2ZrBu2 and ethylene. J Org Chem 60 4444—4448... 

Recently, [CpRu(CH3CN)3]PF6-catalyzed alkyne-alkyne cross coupling between a,co-cyanoalkynes and tertiary propargyl alcohols was developed in the presence of water and 20 mol% of tartaric acid and produced predominately cyanodienylketones via a nonsymmetric mthenacyclopentadiene followed by successive dehydration and addition of water [10] [Eq. (7)]. 

In 2006, Trost et al. discovered a tandem alkene/alkyne cross-coupling/hetero-cyclization reaction using a combination of Ru and Pd catalysis [7]. Impressively, this strategy allowed for the synthesis of all diastereomers of the functionalized chiral... 

The requisite dihydroxyketones are commonly assembled via iterative aldol coupling reactions [1], but other methods including Nef reactions [17,18], acetylide additions [19, 20], 1,3-dipolar nitrile oxide cycloadditions [21], iterative alkylation of dithianes [22-28], hydrazones [29], oximes [30], nitriles [31], or dihalomethylene species [32-34], cross-metathesis/hydroboration/oxidation [35], iterative substitution of a xanthate [36], dihydroxylation/desymmetrization of alkenes [37], Homer-Wadsworth-Emmons olehnations [38, 39], allylmetallations [40], and alkyne-alkyne cross-coupling [41] have also been reported. 

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