Cyclization reactions transition metal catalysts

The mechanistic similarity between Ziegler-Natta polymerization of olefins and the alkene cyclization reactions described above suggested that early transition metal catalysts would be effective catalysts for the coupling of... 

The very first example of the catalytic reductive cyclization of an acetylenic aldehyde involves the use of a late transition metal catalyst. Exposure of alkynal 78a to a catalytic amount of Rh2Co2(CO)12 in the presence of Et3SiH induces highly stereoselective hydrosilylation-cyclization to provide the allylic alcohol 78b.1 8 This rhodium-based catalytic system is applicable to the cyclization of terminal alkynes to form five-membered rings, thus complementing the scope of the titanocene-catalyzed reaction (Scheme 54). 

In contrast to the palladium-catalyzed reactions, little attention has been paid to other transition-metal catalysts. Recently some efficient reactions using copper(i or 11), gold(m), platinum(n), and tungsten(O) have been developed for the synthesis of nitrogen-heterocycles. The copper-catalyzed cyclizations of 2-alkynylaniline derivatives into... 

Inter-ring metal migrations, dynamic NMR studies, 1, 412 Intracyclic germanium-carbon bond formation large rings, 3, 706 small rings, 3, 703 Intramolecular Alder-ene reactions with metals, 10, 576 with palladium, 10, 568 with rhodium, 10, 575 with ruthenium, 10, 572 with transition metal catalysts, 10, 568 Intramolecular allylations, in cyclizations, with indium compounds, 9, 679... 

A wide variety of synthetic processes have been rendered asymmetric through the use of a chiral catalyst. In addition to the types of reaction described above, chiral transition metal catalysts have been used to influence the stereochemical course of isomerization, cyclization, and coupling reactions. As an example, an approach towards the natural product (-)-epibatidine (158) was recently reported by Namyslo and Kaufmann (166). Epibatidine is a potent analgesic and a nicotinic receptor agonist. The synthesis involves an asymmetric Heck-type hydroarylation between the bicyclic alkene (155) and pyridyl iodide (156). A number of bidentate chiral li-... 

Enantioselective C-H insertion is clearly the domain of chiral dinuclear rhodium catalysts (see Chap. 16.2). Only very few examples of enantioselective copper-catalyzed reactions of this type have been reported. As a possible approach to the mitomycine ring system, Sulikowski has studied the cyclization of diazo esters 28 quite extensively using various chiral transition metal catalysts... 

It has been proven that the chiral Pd(II) complexes as transition metal catalysts vs Lewis acid catalysts bring a breakthrough in the frontier of catalytic asymmetric organic synthesis. Here we discussed the key issues based on asymmetric carbon-carbon bond formations anomalous six-membered ring formation, ene-type cyclization leading to five-membered rings, spiro cycliza-tion, alkaloid and quinoline synthesis, Suzuki-Miyaura coupling, and C-H bond activation/C-C bond formation by transition metallic Pd(II) catalysts. On the other hand, the carbonyl-ene reaction, hetero Diels-Alder reaction, and... 

There were a number of examples of the use of transition metal catalysts in cyclization reactions to form azepine derivatives. A paUadium(II) catalyst was employed for the intramolecular cyclization of sulfonamide-tethered propargylic carbonates (14AGE1915) and for the cyclization of N-tosyl... 

As mentioned above, although some examples of intramolecular MBH reactions have been reported in the literature, this aspect is still in its infancy. Most known reports are based on the cyclizations of combinations of enone-enone, enone-acrylate, enone-aldehyde, unsaturated thioester-aldehyde, enone-allylic carbonate frameworks, etc. More recently, Krafft et al. have developed a novel, entirely organo-mediated intramolecular MBH reaction by using allyl chloride 277 as an alternative electrophile to afford densely functionalized cyclic enones 278. This reaction tolerates modification of the enone and the use of primary and secondary allylic chlorides and generates both five-and six-membered rings in excellent yields. Both mono- and disubstituted alkenes are formed with excellent selectivity in the absence of a transition metal catalyst (Scheme 1.100). ... 

In this chapter, the applications of douhle-functionalized arenes in the synthesis of five-memhered heterocycles will he discussed. The eontents are divided according to the different types of substrates applied. The subchapters are organized based on the types of nucleophiles o/t/jo-substituted to aryl halides. For the heterocycle synthesis based on C-X bond activation, in general, transition metal catalyst promoted activation of the C-X bond initiated the reaction sequence and was followed by intramolecular or intermolecular cyclization. 

Phenanthridinone derivatives have been reported to be found in a number of natural alkaloids and exhibit a wide range of biological activities. In the case of starting from bi-functionalized arenes with transition metal catalysts, the intramolecular cyclization of 2-bromo-iV-arylbenzamides via C-H activation is the most direct pathway, which has been applied in the synthesis of anti-hepatitis C virus agents and materials. Remarkably, Yao, Xu and their co-workers developed a one-pot procedure for the synthesis of a pyrrolophenanthridone skeleton via an intramolecular Heck reaction and oxidation of N-(2-bromobenzyl) substituted indoles. Moderate to good yields of the desired products were isolated in one step (Scheme 3.67). From the point view of academic interest, the... 

Akiya et al. [55] developed a photoirradiated method for the synthesis of o-alkynylaiyl isocyanides in the presence of iodine via the intramolecular cyclization of o-alkynylaryl isocyanides affording the corresponding 2,4-diiodoquinolines 28 (Scheme 10.21). 2,4-Diiodoquinolines can be used in different cross-coupling reactions catalyzed by transition metal catalysts. 

In terms of synthetic planning, it must be noted that a transition-metal catalyst is not required for this cyclization. o-Alkynylamines can be cyclized to indoles under basic conditions, especially if the nitrogen atom has an electron-withdrawing substituent to lower the pKa. Just such a base-catalysed cyclization was employed in a synthesis of goniomitine (Scheme 6.20), after a Sonogashira reaction was employed to synthesize the substrate. Similarly, benzofurans are easily prepared by base catalysed cyclization as in a synthesis of Ailanthoidol (Scheme 2.115). The transition-metal catalysed cyclization is of particular importance when... 

In the addition reaction of Grignard reagents to a carbon-carbon unsaturated bond, the yields and selectivities become higher by using transition metal catalysts, and so these reactions came to public notice. Metal compounds of Ti, Ni, Cu and Zr are used for the catalysts [55,56,99]. For example, the adduct is obtained in a high yield by a carbometalation reaction with zirconium catalysts as shown in eq. (4.36) [100]. On the other hand, cyclization proceeds in a high yield by using a diene as shown in eq. (4.37) [101]. 

The intramolecular cyclization of phenoxyethynyl diols into unsaturated lactones has been achieved using a silver salt as the catalyst for the reaction (Scheme 2.99) [144]. The authors screened a variety of transition metal catalysts for activity toward the cyclization reaction including gold/silver combinations and determined that silver triflate alone was a successful catalyst. Furthermore, the catalyst loading was quite low (0.5%). The reaction conditions were quite mild, and moderate to excellent yields of the target compounds... 

As described in this chapter, transition-metal catalysts promote various types of cyclization reactions between C=N, C=0, N—H, O—H, and S—H bonds and alkynes in 5/6-endo/exo-dig manners. These reaction modes provide facile and atom-economical pathways to aromatic compounds such as pyrroles, indoles, isoquinolines, quinolines, furans, thiophenes, oxazoles, pyrones, and isoquinolones and their aza analogs and fused-ring congeners. Particularly notable is their utility in cascade reactions, which are step-economical approaches to target molecules, which increase rapidly in structural complexity. Therefore, these reactions can help provide solutions to meet the increasing demands of environmentally benign synthesis in modern organic chemistry. 

Other transition-metal catalysts can be used for the alkylation of allylmagnesium halides with alkyl bromides such as cobalt 6 -(l,3-diphenylphosphino)propane complex Co(dppp)Cl2 (Scheme 2-34, eq. (a)) and cobalt chloride in combination with a diamine (Scheme 2-34, eq. (b)). Cyclization reactions are possible under these conditions. ... 

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