Cross-coupling reactions Stille reaction

There are many other examples in the literature where sealed-vessel microwave conditions have been employed to heat water as a reaction solvent well above its boiling point. Examples include transition metal catalyzed transformations such as Suzuki [43], Heck [44], Sonogashira [45], and Stille [46] cross-coupling reactions, in addition to cyanation reactions [47], phenylations [48], heterocycle formation [49], and even solid-phase organic syntheses [50] (see Chapters 6 and 7 for details). In many of these studies, reaction temperatures lower than those normally considered near-critical (Table 4.2) have been employed (100-150 °C). This is due in part to the fact that with single-mode microwave reactors (see Section 3.5) 200-220 °C is the current limit to which water can be safely heated under pressure since these instruments generally have a 20 bar pressure limit. For generating truly near-critical conditions around 280 °C, special microwave reactors able to withstand pressures of up to 80 bar have to be utilized (see Section 3.4.4). 

Stille reaction (cross-coupling) for the synthesis of alkenes [120]... 

Hartwig-Buchwald, Suzuki, and Stille type cross-coupling reactions with key intermediate 46 led to a wide range of substituted sulfoximines such as 47-49 [37]. In order to demonstrate the synthetic utility of the resulting products, pseudo tripeptide 50 was prepared from a related intermediate. 

Palladium Catalysts Yu s group has carried out systematic studies on Pd-catalyzed alkylations of aryl C—H bonds. Stille-type cross-coupling reactions have been developed by directed C—H activation (Equation 11.30) [68]. The reaction rate is enhanced by benzoquinone and microwave irradiation. Significantly, carboxylic acid functionality can be used as an efficient directing group for aryl C— H bond activation (Equation 11.31) [69]. The reaction conditions can be applied to the carboxylation of vinyl C— H bonds. The possible intermediacy of a palladacycle has been confirmed by NMR spectra and X-ray crystallography. 

Bi- and terthiophenes 2.178 and 2.179 (n = 1) in which 15-crown-5 units were directly attached to one of the thiophenes (Chart 1.37) were synthesized by Stille-type cross-coupling reactions [273]. The redox behavior was largely influenced by the recognition process of metal cations. Thus, the oxidation potential of 2.178 was more affected by the presence of Na+ ions (AEp = 48 mV) than by Li+ (23 mV) or K+ (22 mV) ions, whereas 2.179 (n = 1) showed a negligible effect upon addition of metal cations. 

Trihexyl-2,2 5, 2"-terthiophene 196 was similarly prepared, but by Stille-type cross-coupling reactions [177], In this reaction sequence, the less effective step is the Pd(0)-catalyzed coupling of the organotin thiophene and 2-bromo-3-hex-ylthiophene to yield 3,4 -dihexyl-2,2 -bithiophene 163 in only 23%. Bromination of the latter and reaction with the stannylated thiophene under Stille-conditions results in 69% yield of the desired regioregular terthiophene 196 [Eq. (57)]. 

The use of the Stille reaction (cross coupling and carbonylative variation) has been the subject of numerous excellent reviews and books over the past few years and we will mention in this part only a few examples to illustrate the power and broad applicability of the method. 

Despite being activated by the nitrogen atom, 2-chloroquinoline (25) is still a poor substrate for the Stille cross-coupling reactions, though yields are usually improved under Negishi conditions. For instance, the coupling of... 

AT-acetyltryptamines could be obtained via microwave-assisted transition-metal-catalyzed reactions on resin bound 3-[2-(acetylamino)ethyl]-2-iodo-lH-indole-5-carboxamide. While acceptable reaction conditions for the application of microwave irradiation have been identified for Stille heteroaryla-tion reactions, the related Suzuki protocol on the same substrate gave poor results, since at a constant power of 60 W, no full conversion (50-60%) of resin-bound 3-[2-(acetylamino)ethyl]-2-iodo-lH-indole-5-carboxamide could be obtained even when two consecutive cross-coupling reaction cycles (involving complete removal of reagents and by-products by washing off the resin) were used (Scheme 36). Also under conventional heating at 110 °C, and otherwise identical conditions, the Suzuki reactions proved to be difficult since two cross-coupling reaction cycles of 24 h had to be used to achieve full conversion. 

Stille cross coupling reactions usually proceed under mild neutral conditions. 2-Substituted thiazoUnes can be obtained by the cross coupling reaction of 2-bromothiazolines with various tributylstannyl compounds. Previous attempts at a palladium mediated coupling of 2-trimethylstannylthiazoUne led to only decomposition of the substrate <96TL4857>. 

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. 

Transition metal-catalyzed transformations are of major importance in synthetic organic chemistry [1], This reflects also the increasing number of domino processes starting with such a reaction. In particular, Pd-catalyzed domino transformations have seen an astounding development over the past years with the Heck reaction [2] - the Pd-catalyzed transformation of aryl halides or triflates as well as of alkenyl halides or triflates with alkenes or alkynes - being used most often. This has been combined with another Heck reaction or a cross-coupling reaction [3] such as Suzuki, Stille, and Sonogashira reactions. Moreover, several examples have been published with a Tsuji-Trost reaction [lb, 4], a carbonylation, a pericyclic or an aldol reaction as the second step. 

Domino transition metal-catalyzed processes can also start with a cross-coupling reaction most often, Suzuki, Stille and Sonogashira reactions are used in this context They can be combined with another Pd-catalyzed transformation, and a number of examples have also been reported where a pericydic reaction, usually a Diels-Alder reaction, follows. An interesting combination is also a Pd-catalyzed borina-tion followed by a Suzuki reaction. 

The Pd-catalyzed cross coupling reaction of 3-stannylated pyrroles is also known. Muchowski has thus prepared and utilized 96 to effect Stille couplings leading to 97 [59]. 

Doi and Mori made excellent use of dihydroindole triflate 189 in Pd-catalyzed cross-coupling reactions. This compound was discussed earlier in the Suzuki section, and it also undergoes Stille couplings as illustrated below [140]. A final dehydrogenation completes the sequence to indoles. 

The Pd-mediated cross-coupling reactions on the pyrazine ring of quinoxaline with organometallic reagents are more cumbersome. In one example, the Stille reaction between 2-chloroquinoxaline 87 and benzylstannane 88, attainable from the SN2 displacement of... 

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