Indoles transition metal complexes

The (5)-tryptophan-derived oxazaborolidenes utilized in this aldol study have been previously examined by Corey as effective catalysts for enantioselective Diels-Alder cycloaddition reactions [6]. Corey has documented unique physical properties of the complex and has proposed that the electron-rich indole participates in stabilizing a donor-acceptor interaction with the metal-bound polarized aldehyde. More recently, Corey has formulated a model exemplified by 7 in which binding by the aldehyde to the metal is rigidified through the formation of a hydrogen-bond between the polarized formyl C-H and an oxyanionic ligand [7], The model illustrates the sophisticated design elements that can be incorporated into the preparation of transition-metal complexes that lead to exquisite control in aldehyde enantiofacial differentiation. 

The potential for transition metal complexes to provide new reactivity patterns continues to be explored by the preparation of complexes and the study of their reactivity patterns. The aminoalkyl substituents of gramine, tryptamine and methyl tryptophanate promoted metalation at C2 of the indole ring by Pt(DMSO)2Cl2. The crystal structure of the gramine product was determined. 

Arylamines are commonplace. They are part of molecules with medicinally important properties, of molecules with structurally interesting properties, of materials with important electronic properties, and of transition metal complexes with catalytic activity. An aryl-nitrogen linkage is present in nitrogen heterocydes such as indoles [1, 2] and benzopyr-azoles, conjugated polymers such as polyanilines [3-9], and readily oxidizable triarylamines used in electronic applications [10-13]. The ability of aryl halides and triflates to form arylamines allows a single group to be used as a synthetic intermediate in aromatic carbon-... 

Among the first hydroaminations of alkynes to be published were cyclizations catalyzed by palladium complexes to form indoles. Hydroaminations of alkynes catalyzed by low-valent, late transition metal complexes occur with a narrower scope than the reactions catalyzed by lanathanide complexes. More recently, examples of the hydroaminations of alkynes catalyzed by rhodium complexes have been reported. 

Westemacher, S. Kisch, H. (1996) Transition-metal complexes of diazenes. 36. Formation of indoles from azobenzene and diphenylacetylene through supported aqueous-phase catalysis by Rh(l) complexes, Monatsh. Chem., 127,469-73. 

As part of our investigation of the hydroarylation of alkynes (or alkenylation of arenes) catalyzed by electrophilic transition metal complexes, our group reported the intra- and intermolecular reaction of indoles with alkynes catalyzed by gold (see Ref. [118, 133] in Chap. 1). Thus, alkynyUndole III-l cycUzes readily in the presence of a cationic gold(I) complex to give azepino[4,5-h]indole derivative III-2, whereas the use of AuCls leads to indoloazocine III-3 by a S-endo-dig process, this cyclization mode has not been observed in other hydroarylation of alkynes (Scheme 4.7). Under certain forcing conditions, aUenes and tetracyclic compounds were also obtained (see Refs. [118, 133] in Chap. 1). 

The cationic imidazolium rhodium complex (56) has been found to catalyze the intramolecular hydroamination of alkynes in refluxing THF. In the case of 2-ethynylaniline, indole is formed in 100% yield over 9h at 55 °C (Scheme 38).173 One of the earliest examples of late transition metal-catalyzed hydroamination involved the use of the iridium(I) complex [Ir(PEt3)2(C2H4)Cl] as... 

Supercritical carbon dioxide with a minute co-solvent addition is an effective medium for the 1,3-dipolar cycloaddition of azomethine ylides with DMAD to produce substituted pyrroles.67 The 1,3-dipolar cycloaddition of nitrile ylides [e.g. benzonitrile (4-nitrobenzylide) and 4-nitrobenzonitrile(benzylide)] with acrylamides provided a synthesis of 3,4-dihydro-2//-pyrroles with moderate to good yields.68 The Pt(II)-or Au(III)-catalysed 3 + 2-cycloaddition of the transition metal-containing azomethine ylide (63) with electron-rich alkenes provided a carbene complex (64), which yields tricyclic indoles (65) having a substituent at 3-position (Scheme 17).69 The 1,3-dipolar cycloadditions of azomethine ylides with aryl vinyl sulfones are catalysed by Cu(MeCN)4C104-Taniaphos with nearly complete exo- selectivity and enantioselec-tivities up to 85% ee.10 The 3 + 2-cycloaddition of benzol/>]thiophene 1,1-dioxide... 

Diels-Alder reaction of 2-bromoacrolein and cyclopentadiene using 10 mol% of titanium catalyst 74 gave the synthetically versatile (R)-bromoaldehyde adduct 75 in 94% yield, 67 1 exo. endo diastereoselectivity, and 93% ee. The absolute stereochemical outcome of the reaction is consistent with the proposed transition state assembly 76 in which the dienophile coordinates at the axial site of the metal, proximal to the indane moiety through Ji-attractive interactions. In this complex, the 7t-basic indole and the Ji-acidic dienophile can assume a parallel orientation facilitated by the octahedral geometry of the transition metal. The aldehyde would then react through a preferential s-cis conformation (Scheme 17.27).54... 

The transition metals and their salts and coordination complexes such as RhCla, Pd(PPh3)4, Pd(OAc)2, Cu(0), CuCl and CuX2 can catalyze the aryl-aryl coupling, carbonylation and phenylation reactions of organobismuth compounds with indoles, acyl chlorides, acetylenes and olefins, as illustrated in Table 5.12. Most of these reactions proceed under mild conditions. 

The following section looks at how palladium-catalyzed C-H functionalization has been successfully applied in synthetic strategies enabling rapid and elegant routes to complex natural products containing the indole and pyrrole nucleus. There are a number of metal-mediated examples where stoichiometric quantities of transition metals are employed to affect the desired transformation however, there are very few cases of catalytic functionalization with in the context of complex molecule synthesis. 

Although occasional examples had been described earlier, the design and development of new highly active ligands for palladium gave new impetus to transition-metal-catalysed aminations. A number of relatively complex ligands were used in earlier work, but simpler versions, such as JohnPhos, have now become prominent. " These methods work well with heterocycles, for example A-arylation of indoles, using triflates, bromides and chlorides. ... 

The spirit is to show some of the results, but also to guide users of the approach by pointing to the problems and limitations of the method. The review covers some of the newer applications in the spectroscopy of organic molecules acetone, methylenecyclopropene, biphenyl, bithiophene, the protein chromophores indole and imidazole, and a series of radical cations of conjugated polyenes and polyaromatic hydrocarbons. The applications in transition metal chemistry include carbonyl, nitrosyl, and cyanide complexes, some dihalogens, and the chromium dimer. 

Transition metal catalysed prenylation. There is a new one-step N-tert-prenylation of indole developed by Baran and co-workers [42] which still outcom-petes the chemoenzymatic approach (Scheme 5). Isobutene (21) as prenyl source is reacted with side-chain Fmoc-protected tryptophan methyl ester 20 in the presence of catalytic amounts of Pd(OAc)2 and superstoichiometric amounts of Ag(I) trifluoroacetate and Cu(II) acetate. The protocol also requires the presence of oxygen. After about 1 day at 35°C, the N-tert-prenylated indole is obtained in a yield of about 60%. The mechanism has not been elucidated, but may involve a 7i-allyl-Pd(II) complex which is coordinated by the indole nitrogen or by C3. In the latter case, a Pd-Claisen rearrangement of a 3-palladated indole would follow. Ag (I) functions as reoxidant of Pd(0). 

Noncovalent binding of luminescent transition metal polypyridine complexes to avidin, indole-binding proteins and estrogen receptors 07CCR2292. 

In addition, a number of reports on the indole synthesis utilizing a similar concept of the in sUu generation of the key reactive arylhydrazones, similar to 284, with the aid of transition metal catalysts recently appeared in the literature. Among them, the domino Rh-catalyzed hydroformylation [263] of alkenes leading to aldehydes followed by a subsequent hydrazone formation and Fischer indolization process, which was introduced by Eilbracht [264—267] and later investigated by Beller [268], represents an attractive and efficient 3 + 1 + 1 approach for a one-pot construction of complex indoles. 

Following our interest on the redox properties of transition metal isocyanide and carbene complexes [1], we report the investigation of the electrochemical behaviour of new phosphonium-fiinctionalized isocyanide (A), and derived carbene (B), indole (C) and protonated indole (D) complexes of Cr, Mo and W pentacarbonyls. These studies appear to have been undertaken for the Erst time for complexes with such types of ligands. It was also our object to correlate the redox properties of these compounds with the electron donor/acceptor ability of these ligands. Moreover, this study would also extend to novel carbene complexes the rather limited electrochemical investigation reported [2] for compounds with multiple metal-carbon bonds. 

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