Metal aryls protonation

When arylhydrazones of aldehydes or ketones are treated with a catalyst, elimination of ammonia takes place and an indole is formed, in the Fischer indole synthesis.515 Zinc chloride is the catalyst most frequently employed, but dozens of others, including other metal halides, proton and Lewis acids, and certain transition-metals have also been used. Arylhydrazones are easily prepared by the treatment of aldehydes or ketones with phenylhydrazine (6-2) or by aliphatic diazonium coupling (2-7). However, it is not necessary to isolate the arylhy-drazone. The aldehyde or ketone can be treated with a mixture of phenylhydrazine and the catalyst this is now common practice. In order to obtain an indole, the aldehyde or ketone must be of the form RCOCH2R (R = alkyl, aryl, or hydrogen). 

Work by K Travis Holman and Jerry Atwood at the University of Missouri, USA has resulted in a tricationic host 4.66 based on the macrocycle cyclotriveratrylele (CTV, Section 7.7), which exhibits a deep anion binding pocket surrounded by three metal centres. A guest PF6 anion fits neatly into the cavity, stabilised by C—H F interactions, which may be differentiated from symmetry-related, noninteracting protons on the other side of the metallated aryl ring by H NMR spectroscopy. The X-ray crystal structure of this material is shown in Figure 4.26. 

Oxidation of the porphyrin 7r-system of iron(IH) (45) and iron(IV) tet-raarylporphyrins (8) leads to large chemical shifts for the meso-aryl protons. This is a result of spin density on the raeso-carbon, which is delocalized into the aryl rings via a 7r-spin delocalization mechanism (50). Accordingly, the chemical shifts of ortho-, meta-, and para-protons have alternating signs. Coupling of the porphyrin spin to the metal spin in both a ferro- and antiferromagnetic manner has been demonstrated (45). 

The large paramagnetic contribution to the chemical shifts for the meso-aryl protons is consistent with the presence of a porphyrin 7r-radical species (45). The shift pattern for these signals (o-H upfield, m-H and p-CH3 downfield) is similar to that observed for (TPP )Fein(Cl04)2 (o-H, p-H upfield, m-H downfield), which exhibits ferromagnetic coupling between the porphyrin 7r-radical spin and the metal spin (45). The small... 

With few exceptions arynes used in the nucleophilic additions discussed so far are generated either by removing with base an aryl proton adjacent to a leaving group (Type A2, Section III.A.2), or by decomposing an anthranilic acid derivative (Type Bl, Section III.A.3). Reactions described in the remainder of this section are initiated instead by metal-halogen exchange (Type Al, Section IILA.l). 

Thus the proton abstraction mechanism seems to be the favored pathway in the metalation step of transition-metal-catalyzed arylations. This may seem surprising because a relatively weak base as carbonate is taking away an arylic proton, which has very low acidity. The presence of the metal is in this case key to stabilize the development of a negative charge in the carbon center, as proved by the structure of the transition states, where the metal-carbon bond is practically already formed. It is worth mentioning that external bases are key in both direct arylation and the Suzuki-Miyaura cross-coupling, but their roles are completely different. 

Most electrophilic carbene complexes with hydrogen at Cjj will undergo fast 1,2-proton migration with subsequent elimination of the metal and formation of an alkene. For this reason, transition metal-catalyzed cyclopropanations with non-acceptor-substituted diazoalkanes have mainly been limited to the use of diazomethane, aryl-, and diaryldiazomethanes (Tables 3.4 and 3.5). 

The Birch reduction has been used by several generations of synthetic organic chemists for the conversion of readily available aromatic compounds to alicyclic synthetic intermediates. Birch reductions are carried out with an alkali metal in liquid NH3 solution usually with a co-solvent such as THF and always with an alcohol or related acid to protonate intermediate radical anions or related species. One of the most important applications of the Birch reduction is the conversion of aryl alkyl ethers to l-alkoxycyclohexa-l,4-dienes. These extremely valuable dienol ethers provide cyclohex-3-en-l-ones by mild acid hydrolysis or cyclohex-2-en-l-ones when stronger acids are used (Scheme 1). 

In this section the reactions of selenophenes and tellurophenes with metal alkoxides, alkyls, aryls and amides will be considered. These reactions result primarily in proton abstraction and, in the monocyclic and benzo[6] fused systems, protons are removed preferentially from positions a to the heteroatom. In conformity with the general format of the reactivity sections in this work, the reactivity of the metal derivatives formed is dealt with in Section 3.16.8.8 on metalloid substituents. 

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