Formyl C-H bonds

The mechanism for this process is presumed to involve activation of the formyl C-H bond, following pyridine coordination, by the Ru catalyst, which may be mononuclear or in a cluster form. Such chelation suppresses a decarbonylation route (Scheme 26).119... 

With formamide hydrogen abstraction is quite selective using hydroxy and alkoxy radicals. With A-substituted formamides the site of the hydrogen abstraction depends on the nature of the abstracting species and both the formyl C-H bond and the substituent can be involved. 

Research on intermolecular hydroacylation has also attracted considerable attention. The transition-metal-catalyzed addition of a formyl C-H bond to C-C multiple bonds gives the corresponding unsymmetrically substituted ketones. For the intermolecular hydroacylation of C-C multiple bonds, ruthenium complexes, as well as rhodium complexes, are effective [76-84]. In this section, intermolecular hydroacylation reactions of alkenes and alkynes using ruthenium catalysts are described. 

Later, they also reported an intermolecular hydroacylations of 1,3-dienes with aromatic aldehydes yielding the corresponding j8,y-unsaturated ketones (Eq. 51) [79]. This reaction does not require a CO atmosphere. The addition of formyl C-H bond in formic acid esters and amides to olefins and conjugate... 

The unique transformation of formamides to ureas was reported by Watanabe and coworkers [85]. In place of carbon monoxide, formamide derivatives are used as a carbonyl source. The reaction of formanilide with aniline was conducted in the presence of a catalytic amount of RuCl2(PPh3)3 in refluxing mesitylene, leading to N,AT-diphenylurea in 92% yield (Eq. 56) [85]. They proposed that the catalysis starts with the oxidative addition of the formyl C-H bond to the active ruthenium center. In the case of the reaction of formamide, HCONH2, with amines, two molecules of the amine react with the amide to afford the symmetrically substituted ureas in good yields. This reaction evolves one molecule of NH3 and one molecule of H2. 

Arylative Coupling via Cleavage of Aromatic and Formyl C-H Bonds... 

Addition of aliphatic aldehydes to perfluoroalkenes and cycloalkenes under y-ray irradiation or with heating in the presence of dibenzoyl peroxide gives partially fluorinated ketones by homolysis of the formyl C H bond for example, formation of 1,1,1,2,3.3-hexafliiorohep-tan-4-one. ... 

Chelation-assisted additions of formyl C-H bonds to olefins and dienes have been reported by Jun et al. [120]. In the case of the reaction of 8-quinolinecar-boxaldehyde, they proposed that the formation of the stable 5-membered met-allacyclic complex [121] suppressed the undesired decarbonylation reaction (Eq. 53) [120]. The intermolecular hydro acylation of 1-alkene with 2-(diphenyl-phosphino)benzaldehyde by rhodium(I) catalyst has been conducted on the basis of this working hypothesis [122]. 

The catalytic addition of a formyl C-H bond across an olefin to generate a ketone (Equation 18.83) is called "hydroacylahon." Intramolecular hydroacylations were reported before intermolecular hydroacylations. The intramolecular reaction can be a valuable route to carbocycles, and it has been conducted enantioselectively. The intermolecular reaction could be a valuable route to acyclic ketones. However, both reactions are under... 

A simple catalytic cycle for hydroacylation is shown in part A of Scheme 18.19. Hydroacylation occurs by oxidative addition of the formyl C-H bond to generate an acyl hydride complex. Insertion of olefin into the metal hydride then generates an alkyl acyl intermediate. These complexes undergo reductive elimination, as described in Chapter 8. Although these basic steps constitute the catalytic cycle, many other processes occur outside of this cycle in the catalytic system. Some of these steps lying off the cycle lead to poisoning of the catalyst and others are unproductive reversible processes that have been revealed by H/D exchange experiments. Part B of Scheme 18.19 shows a catalytic cycle that includes these side processes. 

Scheme 7.3 Proposed mechanism for ruthenium-catalyzed intramolecular olefin hydrocar-bamoylation through direct activation of the formyl C-H bond.

Two mechanistic pathways, which differed in the way of ruthenium-mediated initial cleavage of formyl C-H or amido N-H bond, were proposed for the catalytic cycle. As shown in Scheme 7.3, an irreversibly cleavage of formyl C-H bond by the active ruthenium complex was followed by reversible insertion of the olefin into the Ru-H bond, which afforded either six-membered or seven-membered ruthenacycle. After reductive elimination, indolin-2-ones or 3,4-dihydroquinolin-2-one was formed. According to isotopic studies, pathway leading to six-membered lactams is postulated to be less favored. Another cyclization process initiated by Ru-catalyzed oxidative addition of formyl N-H bond (Scheme 7.4) was similar to Carreira s proposal for their hydrocarbamoyla-tion reaction of allylic formamides under similar ruthenium catalysis conditions [7]. The 6-endo cyclization process is proposed to be favored under the catalytic system B. 

Exchange of formyl hydrogens for tritium is observed to occur in both aryl aldehydes (with concurrent ortho labeling in appropriate cases) and aliphatic aldehydes using [(cod)Ir(PCy3)(py)]PF6. Partial reduction of aldehydes to alcohols may occur in some cases. Labeling by other iridium phosphine catalysts has not been reported but is likely to occur. This type of catalytic activity, which likely involves reversible oxidative addition of the iridium center into the formyl C—H bond, is different in outcome from that of organorhodium complexes, whose insertion into formyl C—H bonds proceeds instead to decarbonylation. 

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