Deprotonative metalation substituted benzenes

As for the mechanism, initially, the reaction of exited photocatalyst (PC ) and benzene gave benzene radical cation and photocatalyst radical (PC ). The later species may undergo an electron transfer to the metal cocatalyst Co to produce Co and ground state photocatalyst to complete the photocatalysis cycle. The former species reacts with amine to give a dienyl radical. This adduct may be oxidized by Co° to furnish Co and dienyl cation, which afford the desired substituted benzene after deprotonation. The Co may reduce two protons produced during the reaction to a molecule of H2 (Scheme 2.4). 

Sn6garoff K, Komagawa S, Chevallier F et al (2010) Deprotonative metalation of substituted benzenes and heteroaromatics using amino/alkyl mixed lithium-zinc combinations. Chem Eur J 16 8191-8201. doi 10.1002/chem.201000543... 

A peculiar complex is formed by if coordination of Os(II) ammine complex to one of the double bonds of benzene rings, rather than rf coordination, and the coordinated benzene rings show interesting reactivity [82]. For example, Os(II) coordinates regioselectively to the 2,3-double bond of anisole to form the complex 333, and hence localization of the remaining 7r-electrons occurs. As a result, at 20 °C an electrophile attacks easily at C(4) due to electron-donation of the methoxy group. The 4H-cationic intermediate 334 is stabilized by backdonation from the metal, and the monosubstitution product 334 is formed without deprotonation. The / ara-substituted anisole 335 is... 

Metallation of 2,3,4,5-tetrahydrooxepin with Bu"Li or Bu Li leads to 7-lithio-2,3,4,5-tetra-hydrooxepin, i.e. vinylic deprotonation occurs <82JOC3094>. It is of interest that 2,3-dihydrooxepin, like 2,5-dihydrofuran, undergoes allylic deprotonation. The readily available trimethylsilyl ketene acetal of e-caprolactone is a convenient intermediate for the synthesis of 3-RO-substituted e-caprolactones (R is acyl or tosyl) using, as other reagents, lead(IV) carboxylates <83JOC4940> or [hydroxy(tosyloxy)iodo]benzene <89JOC1101>, respectively. 

The mechanism of direct arylation has been studied experimentally and computationally and possible pathways include electrophilic aromatic substitution, Heck-type coupling and concerted metalation-deprotonation (CMD). The reaction pathway is dependent on the substrate and the catalytic system employed,however, most electron-rich (hetero)arenes seem to follow a base-assisted CMD pathway. Two catalytic cycles for the coupling of bromo-benzene and thiophene are shown in Schemes 19.5 and 19.6. Scheme 19.5 depicts a carboxylate-mediated process where C-H activation occurs... 

The applicabihty of Turbo-Hauser A towards aromatic substrates is expertly proven by the susceptibility of the substituted aromatic ethyl 3-chlorobenzoate to undergo four consecutive direct magnesiations/electrophilic quenches to yield a hexasubstituted benzene derivative (Fig. 12) [30]. The second metallation requires careful choice of solvent in neat THF a competitive deprotonation occurs in a 90 10 ratio in favour of the desired product, which can be improved to a near quantitative level of 98.5 1.5 ratio by employing the less polar THF/Et20 (1 2) mixture. It should also be noted that the third and fourth metallations must be carried out at 50 °C, reflecting the greater sensitivity of the multi-functionalised benzenes. 

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