Boronic acids Synthesis from alkynes

One distinguishes palladium(0)- and palladium(ll)-catalysed reactions. The most common palladium(O) transformations are the Mizoroki-Heck and the cross-coupling transformations such as the Suzuki-Miyaura, the Stille and the Sonogashira reactions, which allow the arylation or alkenylation of C=C double bonds, boronic acid derivates, stan-nanes and alkynes respectively [2]. Another important palladium(O) transformation is the nucleophilic substitution of usually allylic acetates or carbonates known as the Tsuji-Trost reaction [3]. The most versatile palladium(ll)-catalysed transformation is the Wacker oxidation, which is industrially used for the synthesis of acetaldehyde from ethylene [4]. It should be noted that many of these palladium-catalysed transformations can also be performed in an enantioselective way [5]. 

This transformation was applied in the sequential one-pot synthesis of disubsti-tuted benzo[6]furan from dichlorophenol, terminal alkyne, and boronic acid [42]. In this case, water was used as the cosolvent to conduct successfully the SuzuM-Miyaura coupling reaction after chlorobenzo[(i]furan formation (Scheme 23). The use of two ligands, Cy-DHTP and XPhos, was necessary to promote the formation... 

A synthesis of tylophorine 6.207 used an electron-rich arene as the carbon nucleophile (Scheme 6.92). The substrate was constructed by a sequence of two Suzuki couplings (Section 2.6), first an aryl-aryl coupling between iodide 6.201 and boronic acid 6.202, then an aryl-alkyne coupling, using an alkyne boronate 6.204 derived from proline. Platinum-catalysed cyclization generated the pyrrolidine 6.206, and the synthesis was completed by a one-pot deprotection-Pictet-Spengler reaction. 

The in situ preparation of aryl and heteroaryl azides from the corresponding aryl halides via L-proline-promoted Cnl-catalyzed coupling reactions in the presence of alkynes allows the one-pot synthesis of 1,4-disubstituted 1,2,3-triazoles (e.g. 226). Liang et al. also reported the one-pot synthesis of 1,4-disubstituted 1,2,3-triazoles (e.g. 227) from aryl bromides or iodides and terminal alkynes in the presence of sodium azide using diamine-promoted Cnl-catalyzed reactions. It has also been shown that this type of synthesis can be carried out in a mixture of the ionic liquid [bmim][BF4] and water (Scheme 3.32). Starting with boronic acids, the catalytic approach to aryl azides and l-aryl-1,2,3-triazoles can be carried out under milder reaction conditions and improved substrate tolerance (Scheme 3.33). In fact, it was demonstrated that both electron-rich and electron-poor aryl boronic acids 228 could be efficiently converted into the corresponding aryl azides (229) in the presence of sodium azide and CUSO4. A one-pot protocol... 

Interestingly, a similar method can be used for the synthesis of 3-borylated pyrroles 24 from enamides and internal alkyne AT-methyliminodiacetic acid (MIDA) boronates (Eq, (5.23)) [17]. Product 24a undergoes Suzuki-Miyaura coupling with bromobenzene to give 24c in good yield, with a concomitant removal of the acetyl group. 

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