Rhodium catalysis oxidation

Close inspection of 2 reveals that it can be thought of as a metal-aryl complex in a low oxidation state. Thus it meets the conditions for another type of C-H transformation - cross-coupling. Various N-heterocycles bearing the N-CH-X motif will react with aryl iodides, under the action of rhodium catalysis, to give arylated products in moderate yield [11]. 

Rhodium catalysis for effecting allylic oxidation has been developed and has led to considerable controversy over the operative mechanistic pathway. ... 

The fust example of rhodium catalysis for this purpose utilized chlorotiis(triphenylphosphine)rho-diumG) to catalyze the allylic oxidation of a range of alkenes. 47oxidize cyclic allylsilanes " to afford -silyl-2-cycloalkoiones in very good yields and with exclusive fearrangemrat (equation 43). 

Scheme 4.11 Ketene formation by alkyne oxidation with rhodium catalysis.

Tandem hydrogenation-Baeyer-Villiger oxidation reaction catalysed by rhodium catalysis and biocatalysis. 

Under rhodium catalysis, pyridazine iV-oxide is able to undergo hydrometalation reactions (eq 5). In this case, CsOAc was determined to be essential for reactivity and has been postulated to facilitate the insertion of Rh into the heteroaryl C-H bond. 

Other functionalizations described include the direct alkylation of the title compound. For example, it was demonstrated that applying similar alkenes describe vide supra under rhodium catalysis leads to a direct hydroalkenylation, furnishing the corresponding 2-alkyl pyridine -oxides. Conversely, benzyl chlorides were effectively coupled to the title compound under similar conditions described for direct sp arylation (eq 30). ... 

A hydroxymethyl group acts as a removable directing group, in a manner similar to that of the carboxyl group in Schemes 25.8 to 25.12, to induce homologation. Under rhodium catalysis, triphenylmethanol (20) couples with 2c in the presence of [RhCl(cod)]2, l,2,3,4-tetraphenyl-l,3-cyclopentadiene (21), and Cu(0Ac)2-H20 as catalyst, ligand, and oxidant, respectively, accompanied by liberation of benzophe-none to produce naphthalene 5b quantitatively (Scheme 25.13) [16]. 

In 2007, Ueura et al. reported the first effective example for oxidative coupling of aromatic substrates with alkynes through C—H bond cleavage under rhodium catalysis [22]. Thus, benzoic acids 9 and 50 react with alkynes 2 efficiently in the presence of 0.5 mol % of [Cp RhCl2]2 and 2 equiv of Cu(OAc)2 H2O in o-xylene at 100 °C to afford the corresponding isocoumarins 51 as 1 1 coupling products (Scheme 25.26) [5,22]. Five years later, the same reaction was conducted using ruthenium catalysts in place ofRh [23]. 

Too et al. demonstrated that isoquinoUne synthesis can be conducted by coupling of aryl ketone O-acyloxime 110 with 2c under rhodium catalysis without addition of any external oxidant (Scheme 25.54) [51]. Similar annulation of aryl ketone O-benzyloximes under nickel catalysis was also disclosed by Yoshida et al. [52]. 

In 2010, Stuart and Fagnou reported an oxidative Larock-type heterocycle synthesis using rhodium catalysis and applied it to the synthesis of paullone (Scheme 16.8) [17]. In the synthesis of paullone, acetanilide (41) was coupled with alkyne 42 in the presence of a rhodium catalyst and Cu(0Ac)2-H20 as an oxidant under air to form disubstituted indole 44 via intermediate 43 in 71% yield. Removal of acetyl and Boc groups, followed by treatment with DBU (l,8-diazabicyclo[5.4.0]undec-7-ene), resulted in paullone in a total of eight steps. 

Expansion of the substrate scope on asymmetric cycloisomerization came from the advancement of rhodium catalysis. Following the same catalytic cycle, rhodium(l) complexes coordinate enynes followed by oxidative... 

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