Oxidative addition of the formyl C-H bond

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. 

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. 

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. 

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. 

Two distinct pathways for this reaction have been suggested oxidative addition of the C-H bond followed by reductive elimination of HCl, and direct electrophilic attack on the formyl group by platinum(II) with displacement of a proton. 

This mechanism is quite general for this substitution reaction in transition metal hydride-carbonyl complexes [52]. It is also known for intramolecular oxidative addition of a C-H bond [53], heterobimetallic elimination of methane [54], insertion of olefins [55], silylenes [56], and CO [57] into M-H bonds, extmsion of CO from metal-formyl complexes [11] and coenzyme B12- dependent rearrangements [58]. Likewise, the reduction of alkyl halides by metal hydrides often proceeds according to the ATC mechanism with both H-atom and halogen-atom transfer in the propagation steps [4, 53]. 

The chemistry is clearly complex most pertinently, a methyl radical can be oxidized by flame species to formaldehyde either directly or via a methoxy radical intermediate (Eqns (7)—(9)). Subsequent hydrogen atom abstraction yields formyl radical (Eqn (10)) once collisionaUy stabilized, formyl radical can form CO (Eqn (11)) and, presuming complete combustion, it ultimately reacts to form CO2 (Eqn (13)). The methyl radical intermediate, alternatively, can combine with another methyl radical to form ethane (Eqn (12)), which can then yield ethyl radicals by chemically activated C—H bond fission or via H atom abstraction, opening up a wide variety of side reactions. (In unsaturated systems common to HC fuel combustion, another common initiation step involves addition of a reactive flame species to a double bond to yield a radical center.)... 

Nucleophilic addition of LiR or H , from Li[BEt3H], to (14) occurs at the cis-CO to give acyl or formyl anions [Re C(0)R X(C0)4] , which may be protonated or alkylated to give the corresponding Fischer carbenes [Re C(OR0R X(CO)4]. In contrast to the dinuclear formyls [Re2(CO)9(CHO)], these are not stabilized by SnBu3pI because rapid X loss produces Re(CO)4(CHO), which decomposes to ReH(CO)5 or oxidatively adds the Sn-H bond. Remarkably, double nucleophilic alkylation is possible without X replacement, and upon protonation bis(hydroxy)carbene complexes, for example, fac-[ReX C(OH)Me 2(CO)3] are obtained. ... 

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