C-N bond formation via hydrogen transfer

Figure 12.1 General mechanisms of C-N bond formation via hydrogen transfer.

C-N Bond Formation via Hydrogen Transfer Alcohol Preactivation... 

The 9-(aminoalkyl)anthracenes 67 and 68 also undergo intramolecular addition upon irradiation in benzene solution to yield the 1.4-adducts 69 and 70, respectively. The formation of these adducts is proposed to occur via photoinduced electron transfer followed by N-H proton transfer to yield 10-anthryI-aniIino biradical intermediates (Scheme 9). In the case of the biradical from 68, C-N bond formation affords adduct 70. The biradical from 67 undergoes C-C bond formation at the ortho position of the anilino radical, followed by hydrogen transfer to yield 69, rather than C-N bond formation to form the highly strained lower homolog of 70. The formation of 62 from the intramolecular reaction of 58 and of 5 from the iniermolecular reaction of anthracene and dimethylaniline may also occur via C-C bond formation in the biradical or radical pair intermediates. 

Noyori and coworkers reported well-defined ruthenium(II) catalyst systems of the type RuH( 76-arene)(NH2CHPhCHPhNTs) for the asymmetric transfer hydrogenation of ketones and imines [94]. These also act via an outer-sphere hydride transfer mechanism shown in Scheme 3.12. The hydride transfer from ruthenium and proton transfer from the amino group to the C=0 bond of a ketone or C=N bond of an imine produces the alcohol or amine product, respectively. The amido complex that is produced is unreactive to H2 (except at high pressures), but readily reacts with iPrOH or formate to regenerate the hydride catalyst. 

The pyrolysis of pyrrole produces a variety of products hydrogen cyanide, propyne, allene, acetylene, c/ -crotonitrile, and allyl cyanide, among them. Lifshitz et al. hypothesized that pyrrole undergoes 1,2-bond (N—C) cleavage, then an internal H-atom transfer, to yield a radical intermediate that can isomerize to either c/ -crotonitrile or allyl cyanide, or dissociate to HCN and propyne.Bacskay et al. completed quantum chemical comparisons of the isoelectronic pyrrolyl and cyclopentadienyl radicals they hypothesized that pyrrolyl radical is formed via C—H bond scission in the intermediate pyrrolenine (2/f-pyrrole) rather than directly via N—H bond cleavage (Fig. 14). Mackie et al. explained a similar finding, postulating that it was the formation of pyrrolenine that dictated the rate at which pyrrole pyrolysis occurred. 

In the past, this field has been dominated by ruthenium, rhodium and iridium catalysts with extraordinary activities and furthermore superior enantioselectivities however, some investigations were carried out with iron catalysts. Early efforts were reported on the successful use of hydridocarbonyliron complexes HFcm(CO) as reducing reagent for a, P-unsaturated carbonyl compounds, dienes and C=N double bonds, albeit complexes were used in stoichiometric amounts [7]. The first catalytic approach was presented by Marko et al. on the reduction of acetone in the presence of Fe3(CO)12 or Fe(CO)5 [8]. In this reaction, the hydrogen is delivered by water under more drastic reaction conditions (100 bar, 100 °C). Addition of NEt3 as co-catalyst was necessary to obtain reasonable yields. The authors assumed a reaction of Fe(CO)5 with hydroxide ions to yield H Fe(CO)4 with liberation of carbon dioxide since basic conditions are present and exclude the formation of molecular hydrogen via the water gas shift reaction. H Fe(CO)4 is believed to be the active catalyst, which transfers the hydride to the acceptor. The catalyst presented displayed activity in the reduction of several ketones and aldehydes (Scheme 4.1) [9]. 

The photolytic cleavage of alkyl aryl sulfoxides has been shown to occur via initial C—S bond homolysis, in accordance with the common mechanistic assumption. Secondary and tertiary alkyl groups show high chemoselectivity for alkyl C—S cleavage. Uniquely, alkene products have been isolated, formed by disproportionation of the initial alkyl radical, with the formation of benzaldehyde and racemization of primary alkyl compounds. An investigation into the photochemical conversion of N-propylsulfobenzoic imides into amides in various solvents revealed a solvent dependence of the observed mechanism. In ethanol, sulfur dioxide extension forms a biradical which abstracts a hydrogen atom from the solvent, whereas in aromatic solvents biradical formation by a single electron transfer is implicated. The photolysis and thermolysis of l,9-bis(alkylthio)dibenzothiophenes and /7-aminophenyl disulfide have been studied. 

In contrast, hydrogen atom transfer to carbon of the C=S bond occurs in excited (both n,jt and jt,jt ) 2,4,6-tri-tcrt-butylthiobenzaldehyde (441), resulting in the six-membered thiolane derivative 442 formation via a radical mechanism (Scheme 6.211) in nearly quantitative chemical yield.1273... 

In contrast, there are some examples in which parent pyridylidenes have been implicated in the functionalization of pyridines. In this context Bergman and coworkers found that the formation of the Rh—NHC complex 18 via C-H bond activation of the heterocycle 17 (see Scheme 28) was the key step in the Rh-catalyzed coupling of N-heterocycles and olefins.The 1,2-hydrogen shift involved in this process was studied in detail, using experimental and computational methods, and an intramolecular hydrogen transfer pathway through Rh—H intermediates was found to be the more plausible mechanism. 

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