Arenes deprotonation reactions

Not included in the former sections are methodologies involving arene-catalyzed lithiation that are based on deprotonation reactions, carbon-carbon bond cleavage, addition processes as well as the preparation of lithium telluride. 

Fomarini and colleagues gave also experimental evidence that in the reaction of trime-thylsilicenium ion with arenes, cr-complexes such as 54 are produced29-31. The kinetic isotope effect of the gas phase deprotonation reaction and specific deuterium labeling... 

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. 

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. 

Several chapters deal with the synthesis of and the synthetic applications of organolithium compounds such as orthometallation, arene catalysed lithiation, addition to carbon-carbon double bonds, their reaction with oxiranes, and asymmetric deprotonation with lithium (-)-sparteine. We gratefully acknowledge the contributions of ah the authors of these chapters. 

When, under the reaction conditions outlined above for the preparation of 33, ferrocene was added as the arene to be deprotonated, it appeared that 1,1-di-deprotonation occurs resulting in an aggregated species [((C5H4)2Fe 3Mg3Na2(TMP)2(TMPH)2] (36) . Unfortunately, its X-ray structure determination showed disordered moieties especially with respect to the coordinated TMPH molecules. Its lithium analog [[(C5H4)2Fe 3Mg3Li2 (TMP)2(TMPH)2] (37) was prepared in a similar way and, after treatment with pyridine (to... 

The magnesium bis(amide) Mg(TMP)2 (TMP = 2,2,6,6-tetramethylpiperidide) has been shown to be a useful base in the selective deprotonation of arenes to produce aryhnagne-sium amide intermediates . For example, reaction of Mg(TMP)2 with methyl benzoate... 

A special class of linked phenols are the so-called calixarenes. Reaction of p-f-Bu-calix[4]arene, abbreviated (FljL), with excess Et2Zn in the presence of TMEDA affords the fully deprotonated calixarene complex Et2Zu5(L)2(TMEDA)2 (168), the structure of which was determined in the sohd state (Figure 84) °. The two cahxarenes are fused by three zinc atoms. One of these is coordinated by two oxygen atoms from each calixarene unit. The other two zinc atoms, connecting the calixarenes, are five-coordinate, bound by... 

More recent investigations have shown that the reaction of a Na(tmp)/Bu Mg(tmp)/ tmeda mixture with 0.5 equivalent of benzene leads to dideprotonation of the arene to afford [ (tmeda)Na(p-tmp)Mg(tmp)2 2C6H4]. 5 The reaction of the magnesium amide Mg(tmp)2 with LiN(SiMe5)2 resulted in a deprotonation/amine elimination process as shown in Scheme 3.7. The product, a dimeric lithium/magnesium mixed amide has a conformationaUy locked structure with two stereogenic nitrogen centres. ... 

The following example shows that the results of some reactions may be unexpected by allowing P(NMe2)3 to react with p-tert-butylcalix[4]arene, Khasnis et al214 obtained the hexacoordinated compound 236 instead of the expected phosphorane with a PH bond, 238 (Scheme 61). This result is, however, consistent with the particular reactivity of the pyrocatechols in basic media mentioned above (Schemes 54 and 55). Conversely, submitting 238 to heat or to the action of CF3COOH yields nearly quantitatively the P(III) derivative 237 rather than 238. The equilibrium P(III) (237) P(V) (238), if it exists, must be totally shifted to the tricoordinated form. The action of LiBu on 236 does not affect the PH bond but rather results in the deprotonation of the NH bond and the formation of a lithium salt whose structure, as determined by XRD275, places the P atom nearly in the plane of the four O atoms, the H atom of the PH bond within the aromatic cone and the N atom at the apex of the structure. 

As with arene-amine radical ion pairs, the ion pairs formed between ketones and amines can also suffer a-deprotona-tion. When triplet benzophenone is intercepted by amino acids, the aminium cation radical can be detected at acidic pH, but only the radical formed by aminium deprotonation is detectable in base (178). In the interaction of thioxanthone with trialky lamines, the triplet quenching rate constant correlates with amine oxidation potential, implicating rate determining radical ion pair formation which can also be observed spectroscopically. That the efficiency of electron exchange controls the overall reaction efficiency is consistent with the absence of an appreciable isotope effect when t-butylamine is used as an electron donor (179). 

The Knoevenagel reaction is a base-catalyzed condensation between a dialdehyde and an arene possessing two sites with relatively acidic protons. In this polymerization, shown schematically in Scheme 34, deprotonation affords a difunctional nucleophile that subsequently attacks the carbonyl functionalities present in the other monomers. Elimination is the final step in the Knoevenagel sequence, and the use of monomers with highly acidic protons drives the reaction to completion. A number of research groups have employed this method to obtain PPV and its substituted analogs 29 [126,140-146]. 

A new approach for the generation of NHCs has been reported using a cyclopentadi-ene(arene)iron complex (17).22 The method converts imidazolium to the free carbene in the presence of oxygen (peroxyradical anion is the base deprotonating the imidazolium salt). A colour change and precipitation of the oxidized iron complex are evidence for the reaction outcome. 

---