Ketene aminals tautomerism

Very recently, an enamine-imine tautomerism of ketene aminals has been reported by Huang and coworkers64-67. Due to competition by increased aromaticity, very stable enaminone isomers 63-65 equilibrate to their imine isomers 63-65 (equation 10). It is... 

Amination of ketene has been studied by ab initio methods.Reactions of ammonia, its dimer, and its (mono)hydrate with ketene have been calculated and compared with earlier smdies of ammonia (at lower levels of theory), of water, and of water dimer. In general, the results favour initial addition of ammonia to the C=0 bond (giving the enol amide), as against addition to the C=C bond (which gives the amide directly). Amide formation is compared with the corresponding hydration reaction where enol acid and acid are the alternative immediate products. Most of the reactions, i.e. both additions and tautomerizations, are suggested to involve cyclic six-membered transition states. 

Simple enols stabilized by bulky aryl groups have been reviewed.131 Amide enols, tip2C=C(OH)NR1R2 (tip = 2,4,6-triisopropylphenyl), can be generated by reaction of amines with ditipyl ketene, are observable by NMR, and slowly tautomerize. Vinyl alcohols with two or three bulky aryls have propeller conformations and are chiral, but are not easily resolved. 

Cyclic ketene 7V,S-aminals (187) with different ring sizes were used by Marcelis and van der Plas to form (188) (Equation (5)) <87JHC545>. Cyclic lactim ethers (189) and donor substituted 2,5-dihydropyrazines (191) react in their tautomeric ketene (9,A -aminal forms (190) and (192) (194) and (193) are accessible as has been shown in a number of examples (Scheme 34) <77AP(310)936, 90AP(323)89>. The sulfur analogue reacts in the same way. Finally, lactam acetals (195) can be used as an in situ source of cyclic ketene 0,A -aminals (196) and form pyridazines (197) on reaction with... 

The amination of ketenes to produce amides (see Scheme 1) has been subjected to a variety of computational methods, including several treatments of the solvent, with explicit roles for actively participating amine and water molecules. All the results favour a two-step process with initial addition to the C=0 bond, rather than a concerted reaction involving the C=C bond. The former involves a 1-amino-1-hydroxy ene intermediate (6), formally the enol of the amide. Inclusion of a second amine molecule lowers the barrier to the two-step reaction. Replacing the second amine with a water molecule lowers it even further, an effect which should be even greater when water is the bulk solvent. Some experimental evidence is presented for the highly hindered substrates, bis(mesityl)ketene and bis(pentamethylphenyl)ketene. Addition of primary or secondary amines clearly shows, from IR and UV spectra, the build-up and subsequent tautomerization of the intermediate enols. The kinetics of these more hindered substrates are first order in amine this is not inconsistent with the theoretical results, as such hindered ketenes may only react rather slowly with amine dimer, which is also in low concentration under the conditions used. 

Synthon 7 A small modification of the previous pattern carried out by a nucleophilic addition of (N-substituted) amine to ketene with subsequent enol-oxo-tautomerization. 

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