Transition metal-catalyzed crosscoupling

Hydroxy acids have been protected as acetals which are l,3-dioxan-4-ones. Numerous examples of such dioxa-nones were reported, and they have been widely used in synthetic organic chemistry. In particular, dioxanone triflates prepared from 2,4,6-trihydroxybenzoic acid or analogs were used for several transition metal-catalyzed crosscouplings. A Suzuki coupling <2006EJ01678> and a Stille coupling <2005JOC3686> provide illustrations of this principle (Scheme 93). 

Much work has been directed toward the synthesis of thiophene oligomers and polymers. This is due to interest in conducting polymers and molecular electronics. Two main approaches have been used for making such polymers (1) chemical (e.g., FeCh) or electrochemical oxidation of monomeric thiophenes and (2) transition metal-catalyzed crosscoupling reactions. 

Scheme 1 Catalytic cycle of traditional transition metal-catalyzed crosscoupling reactions...

This chapter describes recent advances in transition metal-catalyzed crosscouplings involving the cleavage of C(sp )-0 bonds in non-sulfonylated phenol and enol derivatives. The chapter is divided into three main sections The first details the most intensively studied nickel-based catalyst system. The second deals with C-O bond transformations using metals other than nickel. Finally, the last section describes several examples that exemplify the potential utility of these classes of cross-coupling reactions. 

Functionalization at N-1. Transition metal-catalyzed crosscoupling of benzopyrazole with various halides and pseudohalides affords primarily the N- substituted benzopyrazoles. Such reactions have been carried out using noble metals as well as inexpensive transition metals, and include green reactions that use aqueous reaction solvents. 

Probably the most thoroughly studied transition metal catalyzed transformation of six membered systems is their participation in crosscoupling reactions. Due to the vastness of this field we present only some representative examples for the different reaction types. In this chapter... 

The transition-metal catalyzed cross-coupling reaction of (hetero)aryl hahdes and triflates with primary and secondary amines or (hetero)aryl amines is know as the Buchwald-Hartwig reaction [144]. Mechanistically, this reaction is related to the crosscoupling reactions outlined thus far (Fig. 4.6). The modification arises at the point of transmetalation. This step in the process is substituted with the coordination of the amine reactant. Deprotonation of the amine nitrogen now precedes the reductive elimination step to generate the aryl amine product. This reaction has foimd utility in the academic setting, for use in natural product total synthesis, and in industry, for the preparation of materials up to the multi-hundred kilogram scale. 

Over the past century, several methods have been developed for the synthesis of biaryl compounds [7]. Among these are the Ullmann-type coupling [8,9], the Scholl reaction [10], the Gomberg-Bachmann reaction [11], and recently transition-metal-catalyzed cross-coupling reactions [12], In particular, palladium-catalyzed crosscoupling reactions have been successfully applied to the synthesis of biaryls due to their generally high yields and excellent selectivities. 

Metal-catalyzed cross-coupling reactions have emerged as an important advancement in organic chemistry during the last few decades. Meanwhile, due to the importance of indoles in medicinal chemistry and many other fields, metal-catalyzed crosscoupling reactions have been extensively applied in the field of indole synthesis. While many books and reviews [1-3] have been published in the field, a book by the authors is solely dedicated to Palladium in Heterocyclic Chemistry [4]. In this chapter, we will cover applications of palladium- and other transition metal-catalyzed cross-coupling reactions in indole synthesis and reactimis. 

Several independent protocols using a combination of transition metal-catalyzed stereoselective hydrosilylation, such as palladium-catalyzed crosscoupling sequence leading to stereodefined r-conjugated alkene derivatives, have been successfully developed in the last decade (4). Alkenylsilanes or siloxanes, prepared via platinum or rhodium complex-catalyzed intermolecular hydrosilylation of terminal alkynes have been highly stereospecifically cross-coupled with aryl and alkenyl halides to give unsymmetrical stilbenes, alkenylbenzenes, and conjugated dienes (Scheme 24) (4). 

Transition metal-catalyzed C—P cross-coupling affords powaful tool for the preparation of aryl phosphorus compounds [198]. The scope of the phosphorus-based nucleophiles used in crosscoupling with aryl electrophiles in the presence of transition metal (Pd-, Cu-, and Ni-based) catalysts is very broad. Figure 20.5 summarizes the common phosphoms nucleophiles used for crosscoupling reactions (for mechanism, see Scheme 20.2). 

In biatyl synthesis, although transition metal catalyzed aryl-aryl crosscoupling reactions have been widely exemplified using Suzuki-type reactions, CDC reactions between sp C-H bonds leading to sp C-C bonds are scarce. In 2010, Katsuki reported an enantioselective CDC reaction between two naphthol moieties catalyzed by a chiral iron(salan) complex under aerobic oxidative conditions (Scheme 4.23). Using 4 mol% of the iron(salan) complex 23-A as the catalyst in toluene at 60 °C for 48 h, two different 2-naphthols were coupled and the cross-coupled derivatives were isolated with moderate yields (44-70%) and good ee (87-95%). It must be pointed out that the homocoupling derivatives are also obtained in low yields. 

Transition metal complex-catalyzed carbon-nitrogen bond formations have been developed as fundamentally important reactions. This chapter highlights the allylic amination and its asymmetric version as well as all other possible aminations such as crosscoupling reactions, oxidative addition-/3-elimination, and hydroamination, except for nitrene reactions. This chapter has been organized according to the different types of reactions and references to literature from 1993 to 2004 have been used. 

Shi el al. reported one of the first examples of bimetallic catalytic systems that allowed the insertion of C02 into the rather unreactive fin-carbon bond [45], The concept behind this system was to exploit, in the same system, the ability of a transition metal to catalyze crosscoupling reactions and C02 activation. For instance, tributyl(allyll)tin does not react with C02 in solution even under high-pressure. To run the same reaction in the presence of zero-valent palladium species (Pd(PPh3)4 or Pd(PBu3)4) will quantitatively afford carboxylates 2 (90%) and 3 (10%) (Scheme 5.10), although the reactivity of the system is limited to allylstannanes. 

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