Amidic hydrogen transfer

Substituted thiazolylindole 66 was formed via the Stille coupling of 6-iodoindole 64 and 5-tributylstannylthiazole 65 [42], Stannane 65 was not stable at temperatures higher than 60 °C and decomposition occurred presumably by amidic hydrogen transfer on the 5-position. Simply blocking NH with a methyl group greatly improved the yield. 

The third condition requires that no transfer reactions take place. For example, amide hydrogen transfer is to be expected in the copolymerization of vinyl compounds with lactams. 

Reductions of nitronitriles situated to favor interaction are apt to involve both functions (S4,93). Hydrogenation of o-nitrobenzonitrile over either palladium or platinum gave o-aminobenzamide (78), with the amide oxygen transferred from the nitro group (66). On the other hand, l-amino-2-cyanonaphthalene gave the amino amide on reduction over Pt02, but the amino nitrile over palladium (82). 

Considerable interest remains in catalyzed hydrogen-transfer reactions using as donor solvents alcohols, glycols, aldehydes, amides, acids, ethers, cyclic amines, and even aromatic hydrocarbons such as alkylben-... 

The synthesis of l,3-oxazin-4-ones of type 464 is the first example of the formation of a C-O bond in the course of the Norrish-Yang reaction. Upon treatment with 1-hydroxy-l-phenyl-A -iodanyl mesylate, /3-keto amide 460 was converted to the corresponding a-mesyloxy-/3-keto amide 461 in excellent yield. On ultraviolet (UV) irradiation (A>300nm) of 461, 5-hydrogen transfer to the excited carbonyl group occurred and the diradical 462 thus formed underwent MsOH elimination to enolate diradical 463, cyclization of which resulted in formation of 3-methyl-6-phenyl-3,4-dihydro-277-l,3-oxazin-4-one 464 (Scheme 89) <2001S1258>. 

Recently Liu and coworkers used (porphyrin)iron(III) chloride complex 96 to promote 1,5-hydrogen transfer/SHi reactions of aryl azides 95, which provided indolines or tetrahydroquinolines 97 in 72-82% yield (Fig. 24) [148]. The reaction starts probably with the formation of iron nitrenoids 95A from 95. These diradicaloids undergo a 1,5- or 1,6-hydrogen transfer from the benzylic position of the ortho-side chain. The resulting benzylic radicals 95B react subsequently with the iron(IV) amide unit in an Sni reaction, which liberates the products 97 and regenerates the catalyst. /V,/V-Dialkyl-w// o-azidobenzamides reacted similarly in 63-83% yield. For hydroxy- or methoxy-substituted indolines 97 (R2=OH or OMe) elimination of water or methanol occurred from the initial products 97 under the reaction conditions giving indoles 98 in 74—78% yield. 

While often considered undesirable pathways in the past, radical hydrogen transfers have also been shown to constitute intriguing possibilities for organic synthesis. Curran has for instance defined the concept of protection and radical translocation (PRT) groups, fii this context, 2-0-(2-bromo-aryl)dimethylsilyl ethers, aryl amides and 2-bromo-4-methoxyphenyl ethers... 

The reductive cyclization of the 2-nitrodihydrocinnamoyl group has found application for the protection of alcohols and amines using sodium phosphinate or cyclohexene as the hydrogen donor. - The hydrogen transfer reduction of both esters and amides regenerates the alcohols or amines under mild conditions (equation 12). 

The selective and rapid reduction of nitro compounds is an active area of research, particularly when other potentially reducible moieties are present in the molecules. Anhydrous ammonium formate has been developed as a catalytic hydrogen transfer agent for selectively reducing nitro groups in the presence of acid, ester, amide, halogen and nitrile groups (equation 13). ... 

Irradiation of the quinone (179) at X, > 410 nm in deuteriobenzene affords the dimer (180). The importance of hydrogen bonding between the amide hydrogen and the quinone carbonyl has been studied in the electron transfer reactivity of the ferrocenyl-quinone dyad (181)7 The quinones (182) on irradiation are converted into the quinone methide derivatives (183), which can be trapped by acetone. The methides (183) can also be trapped in a glass at 77 K. Ring contraction occurs on irradiation of the quinone (184) at X > 250 nm. The product (185) is obtained in 21 % yield. Steric factors control the ring contraction process. ... 

Figure 14.1. Hydrogen exchange mechanisms (a) Amide exchange at neutrai pH invoives base cataiyzed proton abstraction and acid cataiyzed transfer of deuterium from solvent. Measurable isotope effects on the amide hydrogen and a lack of a solvent isotope effect indicate that proton abstraction is rate limiting [1]. (b) Schematic diagram of the two-state model for hydrogen exchange in native proteins and kinetic prediction. The rate of...

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