Intramolecular coupling reaction metal-catalyzed

Tandem Conjugate Addition/Metal-Catalyzed Intramolecular Coupling Reaction... 

The last method for the preparation of 2-quinolones described in this chapter relies on a intramolecular Heck cyclization starting from heteroaryl-amides (Table 2) [57]. These are synthesized either from commercially available pyrrole- and thiophene-2-carboxylic acids (a, Table 2) or thiophene-and furan-3-carboxylic acids (b, Table 2) in three steps. The Heck cyclization is conventionally performed with W,Ar-dimethylacetamide (DMA) as solvent, KOAc as base and Pd(PPh3)4 as catalyst for 24 h at 120 °C resulting in the coupled products in 56-89% yields. As discussed in Sect. 3.4, transition metal-catalyzed reactions often benefit from microwave irradiation [58-61], and so is the case also for this intramolecular reaction. In fact, derivatives with an aryl iodide were successfully coupled by conventional methods, whereas the heteroarylbromides 18 and 19, shown in Table 2, could only be coupled in satisfying yields by using MAOS (Table 2). 

Metal complexes of lanthanides beyond lanthanocenes were used to catalyze the reductive coupling reaction of dienes. La[N(TMS)2h was found to effect the cyclization of 1,5-hexadiene in the presence of PhSiH3 (Eq. 13) [50]. Cyclized products 88 and 89 were isolated in a combined yield of 95% (88 89 = 4 1). It was suggested that the silacycloheptane 89 resulted from competitive alkene hydrosilylation followed by intramolecular hydrosilylation. 

For the domino transition metal-catalyzed synthesis of macrocycles, conditions must be found for two distinct cross-coupling reactions, of which one is inter- and the other intramolecular. For this purpose, Zhu s group [115] has developed a process of a Miyura arylboronic ester formation followed by an intramolecular Suzuki reaction to give model compounds of the biphenomycin structure 6/1-232 containing an endo-aryl-aryl bond. 

Compared to the previously described transition metal-catalyzed transformations in this chapter, microwave-assisted Stille reactions [74] involving organotin reagents as coupling partners are comparatively rare. A few examples describing both inter- and intramolecular Stille reactions in heterocyclic systems are summarized in Scheme 6.38 [47, 75-77]. Additional examples involving fluorous Stille reactions are described in Section 7.3. 

Organic Electroreductive Coupling Reactions using Transition Metal Complexes as Catalysts Table 11. Co- or Ni-catalyzed intramolecular additions to Michael acceptors... 

Many of the limitations of C—C bond formation by C —H insertion outlined for intermolecular reactions (Section 1.2.1.) can be overcome by making the reaction intramolecular. Thus, hydrogen atom abstraction followed by intramolecular radical-radical coupling or radical addition to an alkene are increasingly popular processes. Two-electron carbene insertions, either thermal or transition metal catalyzed, have also been used extensively. In either case, ring construction involves net C—C bond formation at a previously unactivated C-H site. 

Intramolecular nucleophilic reaction of sp2-geminated organodimetal species gives cycloalkyl metal derivative, which can be converted into the corresponding alkenyl iodide. The iodide can be used for further transition-metal-catalyzed cross-coupling reaction (equations 61 and 62)86. 

Transition metal catalyzed insertion reactions offer a convenient route for the preparation of five membered heterocyclic rings. Besides intramolecular Heck-couplings and CO insertion, examples of the intramolecular insertion of an acetylene derivative constitute the majority of this chapter. Although some of these processes involve the formation of a carbon-heteroatom bond, they are discussed here. 

J. T. Link, L. E. Overman, Intramolecular Heck Reactions in Natural Product Chemistry in Metal-catalyzed Cross-coupling Reactions (F. Diederich, P. J. Stang, Eds.), Wiley-VCH, Weinheim, 1998, 231-266. 

Not many catalyzed processes involving free radicals are known with these metals. Some vanadium-catalyzed pinacol coupling reactions were developed (reviews [129, 171], [172, 173] and cited ref, [174]). Niobium and tantalum complexes were applied in pinacol coupling reactions [130]. Vanadium(IV) [175-179] and vanadium(V) ([129], reviews [180-186]) complexes are known to catalyze asymmetric oxidative dimerizations of phenols and naphthols in moderate to excellent ees applying oxygen as the terminal oxidant. Biaryls are accessible by intramolecular coupling of sodium tetraarylborates, catalyzed by EtOVOCl2 in the presence of air [187]. 

Link JT, Overman LE (1998) Intramolecular Heck reactions in natural product chemistry. In de Meijere A, Diederich F (eds) Metal-catalyzed cross-coupling reactions. Wiley, Weinheim, p 231... 

Metal mediated and catalyzed reactions have made significant contributions to organic synthesis over the past two decades [1]. One of the earliest and most useful of these is the Pauson-Khand carbon-carbon coupling reaction [2] first reported in 1971. In this reaction, a cyclopente-none is formed from an alkyne and an alkene in the presence of [Co2(CO) ] with insertion of carbon monoxide in a formal 12-h2+1 ]-cycloaddi-tion. The exceptional potential of this reaction has been demonstrated in many (mostly intramolecular) syntheses (Scheme 1) [3]. 

Much effort has been devoted to the extension of this chemistry by the use of new starting materials and by combining the carbonylation step with new modes of trapping reactions of the intermediate acylmetal complex. The combination of metal-catalyzed C-C coupling reactions with carbonylation chemistry to develop regio- and stereoselective atom-economic cascade reactions has been a major subject of interest. This resulted in a number of new inter- and intramolecular methods for the synthesis of complex organic intermediates, mainly heterocycles. 

Link, J. T., Overman, L. E. Intramolecular Heck reactions in natural product chemistry, in Metal-Catalyzed Cross-Coupling Reactions (eds. Diederich, F.,Stang, P. J.), 231-269 (Wiley-VCH, Weinheim, New York, 1998). 

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