Cyclization reactions metal-mediated/catalyzed

A unique method to generate the pyridine ring employed a transition metal-mediated 6-endo-dig cyclization of A-propargylamine derivative 120. The reaction proceeds in 5-12 h with yields of 22-74%. Gold (HI) salts are required to catalyze the reaction, but copper salts are sufficient with reactive ketones. A proposed reaction mechanism involves activation of the alkyne by transition metal complexation. This lowers the activation energy for the enamine addition to the alkyne that generates 121. The transition metal also behaves as a Lewis acid and facilitates formation of 120 from 118 and 119. Subsequent aromatization of 121 affords pyridine 122. 

Superheated and supercritical water are used in several applications. Supercritical water is most often used in the destruction of organic wastes, including some chemical warfare agents, as an alternative to incineration (Katritzky et al., 1996 Sherman et al., 1998). Recent reports describe the use of both forms as a solvent and as a reactant in synthetic chemistry (Katritzky et al., 1996 An et al., 1997). Some of the reactions investigated include metal-mediated alkyne cyclizations, Pd-catalyzed al-kene arylations, aldol reactions, the Fischer indole synthesis, and hydrolysis reactions. Waterborne coatings and the destruction of wastes in supercritical water are fully... 

The use of hydrogen as terminal reductant has been accomplished by its activation with transition metal complexes. The resulting weak M-H bonds can be used in both radical generation and reduction through HAT. In this manner, conceptually novel radical chain reactions, such as hydrogen mediated cyclizations, or metal catalyzed processes with coupled catalytic cycles for radical generation and reduction, have been realized. The latter transformations are especially attractive for enantioselective synthesis. 

Abstract This review gives an insight into the growing field of transition metal-catalyzed cascades. More particularly, we have focused on the construction of complex molecules from acyclic precursors. Several approaches have been devised. We have not covered palladium-mediated cyclizations, multiple Heck reactions, or ruthenium-catalyzed metathesis reactions because they are discussed in others chapters of this book. This manuscript is composed of two main parts. In the first part, we emphasize cascade sequences involving cycloaddition, cycloisomerization, or ene-type reactions. Most of these reaction sequences involve a transition metal-catalyzed step that is either followed by another reaction promoted by the same catalyst or by a purely thermal reaction. A simple change in the temperature of the reaction mixture is often the only technical requirement to go from one step to another. The second part covers the cascades relying on transition metalo carbenoid intermediates, which have recently undergone tremendous... 

Metal-mediated approaches to the synthesis of imidazoles have been reported. PaUadium(ll)-catalyzed intermolecular 1,2-diamination of conjugated dienes with ureas led to 4-alk enyl-2-imidazolones in good yields rmder mild conditions <05JA7308>. Palladium-catalyzed cyclization of O-pentafluorobenzoylamidoximes 74 furnished l-benzyl-2-substituted-4-methylimidazoles 75 <050L609>. Direct copper(I)-chloride mediated reaction of nitriles 76 with a-amino acetals 77 followed by acidic reaction led to a variety of 2-substituted imidazoles 78 <05TL8369>. 

Metal-mediated electrophilic cyclizations of o-ethynylanilines leading to indoles have also been reported. For example, the gold(III)-catalyzed heteroannulation of bis-aniline 119 produced 2,2 -biindole 120 <04S610>. A similar heteroannulation reaction was reported utilizing copper triflate as the catalyst <04JOC1126>. 

Abstract An overview of recent transition metal-catalyzed syntheses of pyrroles and carbazoles is presented. The focus is on methods which have been applied to the preparation of biologically active namrally occurring pyrrole and carbazole alkaloids. For pyrroles, special attention is paid to silver(I)-catalyzed cyclization reactions. For carbazoles, iron(0)-mediated and palladium(0/ll)-catalyzed cychza-tion reactions are highlighted and their broad range of applications is discussed. 

Metal-mediated cyclizations that rely on the initial complexation of an alkene or alkyne around a low oxidation state metal center are often sensitive to the presence of additional substituents (particularly electron-donating substituents), and relatively more stringent reaction conditions are often required for successful cychzation. This effect was noted in the Ni-catalyzed formal [4 -I- 4] cycloaddition reactions developed by Wender and Tebbe and is apparent when one compares the reported facility of Pd-catalyzed linear dimerization of 1,3-butadiene versus that of substituted 1,3-dienes. Similarly, the initial attempts at Pd-catalyzed cyclization of bisdiene 70a (Scheme 22) were rather disappointing. Using 0.05 equiv of [Pd(OAc)2/3 PhjP] (THF, 65 °C, 12 h), only a small... 

Transition-metal-catalyzed functionalization of propargyfic alcohols and their derivatives (including cyclization reactions) 12S1131. Transition-metal mediated carbon—sulfur bond activation and transformations (including those in thiophene, benzothiophene, and dibenzo-thiophene derivatives) 13CSR599. 

Transition metal-mediated cycloaddition and cyclization reactions have played a vital role in the advancement and applications of modem synthetic organic chemistry. Rhodium-catalyzed cycloadditions/cyclizations have attracted significant attention because of their versatility in the transformations of activated and unactivated acetylenes, olefins, allenes, etc. These reactions are particularly valuable because of their ability to increase molecular complexity through a convergent and highly selective combination of acyclic components. In addition, these reactions allow for the preparation of molecules with chemical, biological, and medicinal importance with greater atom economy. Recent developments in rhodium-catalyzed cycloaddition and cyclization reactions are described in this section. 

Transition-metal-mediated cyclization reactions play an increasing role in the synthesis of heterocycles, as shown by furan syntheses catalyzed by Pd, Cu, and Au. 

Beyond the numerous applications of palladium in transition metal-catalyzed domino reactions there are a lot of other metals inducing domino processes. Ihara et al. found a strategy for the enan-tioselective synthesis of (+)-equilenin catalyzed by mangan and palladium complexes,1841 Whitby et al.1851 initiated domino cyclizations on a zirconocene template and furthermore Scherf et al. generated phenantrones by a nickel-mediated one-pot domino reaction.1861... 

TL627>. Other metal-catalyzed reactions include the CdC -mediated cyclization of enamines in the presence of a cyano substituent (Scheme 98) <1995JOC5243>. Zinc and copper (i) salts can be used in place of the cadmium however, organocadmium promoters allow room temperature cyclization. A two-step process involving tungsten alkynols, aldehydes, and nitriles in the presence of a Lewis acid yields pyridines (Scheme 99) <1998JA4520>. 

The transition metal catalyzed or mediated cyclizations of alkynes are also classified into this category. The Ni complex catalyzed reaction of alkynes with... 

Highly enantioselective Rh-catalyzed intramolecular Alder-ene reactions for the synthesis of chiral 3-alkylidene-4-vinyltetrahydrofurans were reported by Zhang, as illustrated below <02AG(E)3457>. Metallic indium was also shown to mediate the intramolecular cyclization of tethered allyl bromides onto terminal alkynes to afford 3-methylene-4-vinyltetrahydrofurans in 50-62% yield <02SL2068>. 

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