Heteroarene complexes

Heteroarene complexes, (C6R3H2E)2Ti (E = N, R=Buc E = P, R = Buc E = As, R = H, 20), can be prepared by metal-ligand vapor co-condensation of titanium with the corresponding arene (Scheme 4).15,16 Distinct 111 NMR resonances are observed for the aromatic protons at ambient temperature, suggesting restricted arene rotation. Variable-temperature NMR experiments provided barriers of 16 and 17kcalruol respectively, for the ring rotation. Reduction of either compound with potassium metal furnished the titanium(l) salts, KhC BuffTE )2Ti] (E = N, P 21). 

Ais-Heteroarene complexes, like [Mo( -C5H3N-2,6-Me2)2], can also be prepared using this methodology and their reactivity mirrors that of the 6/s-arene molybdenum complexes. ... 

The reactions of the phosphabenzene system [124] confirm these conclusions. Phosphabenzenes have low basicity towards hard acids. They are not protonated by CF3CO2H nor alkylated by trialkyloxonium salts. However, soft acids attack at phosphorus. For instance, 2,4,6-triphenyl-phosphabenzene forms compounds 4 with the hexacarbonyl derivatives of Cr, W and Mo in which the phosphorus coordinates to the metal, possibly with metal-P back-donation. The complexes 4 rearrange photochemically or thermally affording the 67i-heteroarene complexes 5. Although 2,4,6-triphenyl-pyridine is protonated on nitrogen, it undergoes complex formation with chromium hexacarbonyl exclusively on the phenyl moieties yielding the ri -arene complexes 6 [125]. 

T -Arene complexes containing late metals have been known for many years, but the scope and utility of these complexes have increased in recent years. Copper(I) and silver form labile arene complexes of various stoichiometries that are apparently T -arene complexes. A few of these complexes have been structurally characterized. More recently, a large number of V-arene and heteroarene complexes of osmium, rhenium, molybdenum, and tungsten have been prepared for the purpose of dearomatization of the arene or heteroarenes. Two examples are shown in Figure 2.33. This dearomatization creates a diene or vinyl unit that imdergoes the organic chemistry of ttiese isolated units, instead of the chemistry of an arene. n -Arene complexes of rhodium and platinum have been characterized structurally and studied in the context of their likely intermediacy in the oxidative addition of arene C-H bonds. ... 

V-Arene complexes have been developed as reagents for organic synthesis. A vast majority of this work has been reported by Harmann. - V Arene and heteroarene complexes have been generated on (NHjjjOs " and more recently on TpM(CO)(L) in which M = Re, Mo, and W, and L = CO or NO. The latter chiral-at-metal system has been resolved. This t coordination leads to de-aromatization of the arene, in effect isolating the diene unit for reactions, as depicted in Figure 11.6. The chemistry of T -arene complexes is therefore more akin to the chemistry of dienes, and the chemistry of T -furans is more akin of that of vinyl ethers than it is to the chemistry of electron-poor T -arenes. Harmann has developed Diels-Alder reactions of the diene unit and additions of electrophiles to the vinyl ether unit. - In one case, coordination of furan also led to the ability to conduct nucleophilic substitution on the coordinated olefin. ... 

The direct arylation of heteroaryls is particularly attractive due to the fact that these moieties are present in many biologically active compounds [58], Recently, etinkaya and co-workers reported the direct arylation of benzoxazoles and ben-zothiazoles with aryl bromides catalysed by a bis-NHC-palladium complex [59], Also, Sames and co-workers have described the C-H arylation of different SEM-protected heteroarenes, catalysed by NHC-Pd complex 28 (Scheme 7.12, pathway a) [60],... 

Remarkable carbon-boron bond-forming reactions are catalyzed by iridium complexes and proceed at room temperature with excellent regioselectivity, governed by steric factors. Heteroarenes are borylated in the 2-position and this reaction is generally tolerant of halide substituents on the arene (Equations (87) and (88)). 

Desulfuration.1 This complex as such or in combination with 2,2 -bipyridyl (bpy) or triphenylphosphine (a NiCRAL) can effect desulfuration of heteroarenes. aryl thioethers, dithioketals, sulfoxides, or sulfones in DME or THF at 63° in 1.5-30 hours. NiCRA is sufficient for aryl thioethers, dithioketals, but NiCRALs are more efficient for desulfuration of heteroarenes. Yields can be comparable with those obtained with Raney nickel. 

The formation of six-membered or larger rings by intramolecular C-H bond insertion normally requires the attacked position to be especially activated towards electrophilic attack [1157,1158]. Electron-rich arenes or heteroarenes [1159-1162] and donor-substituted methylene groups can react intramolecularly with electrophilic carbene complexes to yield six- or seven-membered rings. Representative examples are given in Table 4.8. 

The SULPHOS-containing rhodium and ruthenium complexes retained their catalytic activity in heteroarene hydrogenation when supported on styrene-divinylbenzene polymer [180] or on silica [181], and showed even higher activity than in homogeneous solution. This effect is attributed to the diminished possibility of dimerization of the active catalytic species to an inactive dimer on the surface of the support relative to the solution phase. The strong hydrogen bonds between the surface OH-groups on silica and the -SO3 substituent in 31 withheld the catalyst in the solid phase despite the rather drastic conditions (100 °C, 30 bar H2). 

The direct borylation of arenes was catalyzed by iridium complexes [61-63]. Iridium complex generated from [lrCl(cod)]2 and 2,2 -bipyridine (bpy) showed the high catalytic activity of the reaction of bis (pinaco la to) diboron (B2Pin2) 138 with benzene 139 to afford phenylborane 140 (Equation 10.36) [61]. Various arenes and heteroarenes are allowed to react with B2Pin2 and pinacolborane (HBpin) in the presence of [lrCl(cod)]2/bipyridne or [lr(OMe)(cod)]2/bipyridine to produce corresponding aryl- and heteroarylboron compounds [62]. The reaction is considered to proceed via the formation of a tris(boryl)iridium(lll) species and its oxidative addition to an aromahc C—H bond. 

Some work has been performed with bis (arene)vanadium and bis(heteroarene) vanadium complexes [7-10]. As indicated for the selected complexes shown in Table 1, replacement of benzene by phos-phabenzene and arsabenzene lowers the reduction potential. This counterintuitive result has been explained in terms of a greater positive charge on the metal... 

Bis(guanidinato)bis(benzyl) complexes, with Zr(IV), 4, 776 Bis(heteroarene) vanadium complexes, preparation, 5, 48 Bis(heteroatom) polysilanes, synthesis, 3, 584 Bis(iV-heterocyclic carbene) ligands, in silver(I) complexes,... 

Heteroalkenes, with iron, 6, 132 Heteroannulation, allylic benzylamines, 10, 156 Heteroarene chromium carbonyls, preparation and characteristics, 5, 260 Heteroarenes borylation, 10, 242 C—H functionalizations, 10, 127 as metal vapor synthesis milestone, 1, 237 with titanium, 4, 246 vanadium complexes, 5, 48 7]6-Heteroarenes, with platinum, 8, 664 Heteroaromatic compounds... 

Arenes and heteroarenes which are particularly easy to metalate are tricarbo-nyl( 76-arene)chromium complexes [380, 381], ferrocenes [13, 382, 383], thiophenes [157, 158, 181, 370, 384], furans [370, 385], and most azoles [386-389]. Meta-lated oxazoles, indoles, or furans can, however, be unstable and undergo ring-opening reactions [179, 181, 388]. Pyridines and other six-membered, nitrogen-containing heterocycles can also be lithiated [59, 370, 390-398] or magnesiated [399], but because nucleophilic organometallic compounds readily add to electron-deficient heteroarenes, dimerization can occur, and alkylations of such metalated heteroarenes often require careful optimization of the reaction conditions [368, 400, 401] (Schemes 5.42 and 5.69). 

In addition to the ligands above, considerable attention is given to more complex ligand systems [4,5] aromatic and heteroaromatic compounds (heteroarenes) (i.e., five- or six-member cyclic structures with delocalized 7i-bonds in the ring containing, besides carbon atoms, either N, P, As, O, S, Se, or Te compounds [6-8]), various chelate-forming compounds, such as macrocyclic crown-ethers, cryptands, porphyrins, and phthalocyanines. 

It is generally admitted that skeletal transformations of hydrocarbons are catalyzed by protonic sites only. Indeed good correlations were obtained between the concentration of Bronsted acid sites and the rate of various reactions, e g. cumene dealkylation, xylene isomerization, toluene and ethylbenzene disproportionation and n-hexane cracking10 12 On the other hand, it was never demonstrated that isolated Lewis acid sites could be active for these reactions. However, it is well known that Lewis acid sites located in the vicinity of protonic sites can increase the strength (hence the activity) of these latter sites, this effect being comparable to the one observed in the formation of superacid solutions. Protonic sites are also active for non skeletal transformations of hydrocarbons e g. cis trans and double bond shift isomerization of alkenes and for many transformations of functional compounds e.g. rearrangement of functionalized saturated systems, of arenes, electrophilic substitution of arenes and heteroarenes (alkylation, acylation, nitration, etc ), hydration and dehydration etc. However, many of these transformations are more complex with simultaneously reactions on the acid and on the base sites of the solid... 

The Au-catalysed [3,3] sigmatropic rearrangement of propargyl propynoates 29 leads to 5-vinylpyran-2-ones probably by way of a cationic intermediate 30 (Scheme 23). This oxocarbenium ion can be intercepted by electron-rich arenes and heteroarenes with attack occurring at the vinylic double bond leading to more complex pyranones <07AG(E)8250>. 

An interdisciplinary approach should lead to their future prospects as building blocks of a variety of chemical structures. Thus, betaines 1 can be incorporated as a subunit(s) in host molecules and could confer unusual properties to the supramolecules, either cavitates or clathrates. Their capacity for specific physical behavior should also be considered together with their use as neutral ligands (azolate ligands without counterion) in forming metal complexes. Advances in the chemistry of betaines 1, to be of any real significance, must result from coordinate efforts directed toward supramolecular chemistry, advanced organic materials, and heteroarene coordination chemistry. 

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