Coupling reactions, metal catalysed Stille

IV. METAL-CATALYSED COUPLING REACTIONS A. The Stille Coupling Reaction... 

Transition metal-catalysed reactions have emerged as powerful tools for carbon-carbon (C-C) bond formation [1], Cross-coupling reactions (Suzuki-Miyaura, Mizoroki-Heck, Stille, etc.) are recognised to be extremely reliable, robust and versatile. However, some other catalysed arylation reactions have been studied and have been reported to be very efficient [2]. In recent years, A -heterocyclic carbenes (NHC) have been extensively studied and their use as ligands for transition-metal catalysis has allowed for the significant improvement of many reactions [3]. This chapter highlights the use of NHC-bearing complexes in those arylation reactions. 

For reviews see (a) Mitchell TN. In Diedrich F, Stang PG, eds. Metal Catalysed Cross-Coupling Reactions. Weinheim Verlag GmbH, Wiley VCH, 1988 197-202 (b) Farina V, Krishnamurthy V, Scott WJ. In Paquette LA, ed. The Stille Reaction. Organic Reactions 1997, Vol. 50. New York John Wiley Sons, Inc., 1997 1-676. 

What the true catalyst is in Pd-catalysed cross coupling reactions is still an open problem and this topic has been the subject of recent review articles. Metal nanoparticles (Met NPs) prepared in and stabilized by ILs are emerging as catalysts for reactions under multiphase conditions. 

Several years ago transition metal mediated reactions in the area of C-glycoside synthesis were primarily limited to palladium and to a lesser extent nickel and manganese. Over the last few years several other metals, including chromium, molybdenum, tungsten, cobalt, and rhodium, have been utilized in C-glycoside synthesis. This section discusses the chemistry of palladium, which is divided into Stille-type couplings and 7T-allyl complexes. This is followed by considerations of the chemistry of chromium and the above-listed metals. A review by Frappa and Sinou entitled Transition Metal Catalysed Fimctionalization at the Anomeric Center of Carbohydrates appeared in early 1997 [55]. 

In domino transition-metal-catalysed processes, cross-coupling reactions can also be nsed as the starting transformation. Most often, Suzuki, Stille and Sonogashira reactions are employed in this context. They can be combined with a Mizoroki-Heck reaction and other palladium-catalysed transformations. 

An area that has been subject to more investigation is the non-metal or organocatalytic coupling of esters and amines. Numerous catalysts such as l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and l,5,7-triazabicyclo[4.4.0]-dec-5-ene (TBD) have shown significant potential in this area. However, the drawback of these methods is that slightly higher catalytic loadings of 20 and 30 mol% were required respectively. DBU has been employed as a catalyst for several transformations prior to its use in the aminolysis of esters." This method is only applicable in the case of methyl esters and has only been tested with benzylamine, aniline and pyrrolidine. Additionally, these catalysed reactions still required reaction times of up to 48 hours to reach a maximum of 90% conversion (in most cases, the conversions were lower). 

---