Deprotonation arenes

Further examples of a cyclic heterometallic potassium amide, obtained from KBu, MgBu2 and tmpH in CgH, or PhMe, are the isostructural complexes [(tmp)i2K6-Mg6(C6H5)6] (Figure 2.8) and [(tmp)i2K6Mg6(C6H4CH3)6] comprised of 24-membered N22K6Mg6 macrocycles that play host to six mono-deprotonated arene molecules... 

A remarkable aspect of the heterobimetal lie magnesium salts is the discovery that compounds, derived from the pairing of two different metal atoms usually Li, Na, or K and Mg or Zn that are bound by amido ligands, can deprotonate arene rings [(tmeda)Na-... 

Truong T, Alvarado J, Tran LD, Daugulis O (2010) Nickel, manganese, cobalt, and iron-catalyzed deprotonative arene dimerization. Org Lett 12(6) 1200-1203... 

To avoid die difliculties in bandling die bigbly air-sensitive copper arenediiolates, a metliod for dieir preparation and utilization in situ bas been developed, die aren-ediiol 29 being deprotonated witii n-BuLi and mixed widi a coppetfl) salt to yield die active catalyst [34]. 

CpFe(arene)+ Mono-cations Benzylic Deprotonation.63... 

The acidity of benzylic protons of aromatics complexed to transition-metal groups was first disclosed by Trakanosky and Card with (indane)Cr(CO)3 [61]. Other cases are known with Cr(CO)3 [62], Mn(CO)3 [63], FeCp+ [64, 65], and Fe(arene)2+ [31, 66] but none reported the isolation of deprotonated methyl-substituted complexes. We found that deprotonation of the toluene complex gives an unstable red complex which could be characterized by 13C NMR ( Ch2 = 4.86 ppm vs TMS in CD5CD3) and alkylated by CH3I [58] Eq. (13) ... 

Fe+ + has been deprotonated, but the reaction is complicated by further nucleophilic attack of the methylene unit with the starting material [17]. Enhanced acidity of the ring hydrogens in arene-metal complexes is shown [21] by the formation of complexes of alkyllithium by proton abstraction. 

A subsidiary approach involves nuclear modification of the arylsilanes so obtained. The requisite organometallics can be prepared from aryl halides, or by deprotonation of a suitably activated (c.g. methoxy-substituted) arene. A more specialized route involves cycloaddition between alkynylsilanes and diynes. 

Although deprotonation at the benzylic position of arenes coordinated to ruthenium and chromium was reported,27 in the case of the coordinated oxo-ri5-dienyl unit, nucleophilic attack at one terminus of the complexed r 5-dienyl ligand, rather than deprotonation, was expected.28 The reason for the successful deprotonation (even at relatively hindered isopropyl sites) is, according to the authors, the cationic nature of the Cp M fragment. In addition, the transition state for the deprotonation might involve stabilization by the metal (Scheme 3.15). 

The formation of the Wheland intermediate from the ion-radical pair as the critical reactive intermediate is common in both nitration and nitrosation processes. However, the contrasting reactivity trend in various nitrosation reactions with NO + (as well as the observation of substantial kinetic deuterium isotope effects) is ascribed to a rate-limiting deprotonation of the reversibly formed Wheland intermediate. In the case of aromatic nitration with NO, deprotonation is fast and occurs with no kinetic (deuterium) isotope effect. However, the nitrosoarenes (unlike their nitro counterparts) are excellent electron donors as judged by their low oxidation potentials as compared to parent arene.246 As a result, nitrosoarenes are also much better Bronsted bases249 than the corresponding nitro derivatives, and this marked distinction readily accounts for the large differentiation in the deprotonation rates of their respective conjugate acids (i.e., Wheland intermediates). 

Electron-rich olefins with substituents Y = phenyl, vinyl, amino, or alkoxy can be coupled by anodic oxidation to tail-tail dimers being either deprotonated to dienes and/or substituted a to Y, depending on Y and the reaction conditions (Eq. 6). Alkyl substituted arenes can be dehydrodimer-ized to diphenyls or diphenylmethanes depending on the kind of substitution (Eq. 7). 

One of the first notions of EGA-catalyzed reactions was the rationalization [8, 14] of the unexpected outcome of anodic oxidation of methyl arenes, (1), in MeGN containing various amounts of water. Preferentially A-benzyl acetamides, (3), rather than the benzyl alcohols, (2), were formed [15, 16] (with increasing amounts of water, increasing amounts of aldehyde was formed as a side product [16]). Since water is a more powerful nucleophile than MeCN, it is reasonable to believe that the carbocation formed by overall two-electron oxidation and deprotonation is initially trapped by water. However, the process is reversible in the presence of a strong EGA (protons liberated from the oxidized substrate), and the carbocation is eventually trapped by the excess MeCN, Scheme 1. 

To avoid the difficulties in handling the highly air-sensitive copper arenethiolates, a method for their preparation and utilization in situ has been developed, the aren-ethiol 29 being deprotonated with n-BuLi and mixed with a copper(l) salt to yield the active catalyst [34]. 

Since different reactivity is observed for both the stoichiometric and the catalytic version of the arene-promoted lithiation, different species should be involved in the electron-transfer process from the metal to the organic substrate. It has been well-established that in the case of the stoichiometric version an arene-radical anion [lithium naph-thalenide (LiCioHg) or lithium di-ferf-butylbiphenylide (LiDTBB) for using naphthalene or 4,4 -di-ferf-butylbiphenyl (DTBB) as arenes, respectively] is responsible for the reduction of the substrate, for instance for the transformation of an alkyl halide into an alkyllithium . For the catalytic process, using naphthalene as the arene, an arene-dianion 2 has been proposed which is formed by overreduction of the corresponding radical-anion 1 (Scheme 1). Actually, the dianionic species 2 has been prepared by a completely different approach, namely by double deprotonation of 1,4-dihydronaphthalene, and its X-ray structure determined as its complex with two molecules of N,N,N N tetramethylethylenediamine (TMEDA). ... 

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