Proton transfer enamines undergo

The mechanism of this cyclization involves a conjugate addition of the enamine (100) to the nitroallyl ester (101) to give 102, which on elimination produced 103. The immonium salt 103 undergoes proton transfer to give enamino nitro olefin 104, which cyclizes to an enamine (107) via 105 and 106. Hydrolysis of 107 produces the ketone (108). Depending on the reaction conditions and the structure of the enamine and nitroolefin components employed, intermediates can be isolated (equation 19). 

The possibility that the stereoselectivity arises from either a thermodynamic preference or a subsequent process (cyclization) is less likely with imines as conjugate addition leads to an N-protonated immonium ion, which should rapidly undergo proton transfer. The resulting neutral product should be substantially less likely to undergo reversal to starting material than the dipolar intermediate involved in enamine Michael additions. 

Prior to 2001, when the first serious computational approaches to the problem appeared in print, four mechanistic proposals had been offered for understanding the Hajos-Parrish-Wiechert-Eder-Sauer reaction (Scheme 6.8). Hajos and Parrish proposed the first two mechanisms Mechanisms A and B. Mechanism A is a nucleophilic substitution reaction where the terminal enol attacks the carbinolamine center, displacing proUne. The other three mechanisms start from an enamine intermediate. Mechanism B invokes an enaminium intermediate, which undergoes C-C formation with proton transfer from the aminium group. Mechanism C, proposed by Agamii to account for the nonlinear proline result, has the proton transfer assisted by the second proline molecule. Lastly, Mechanism D, proffered by Jung, proposed that the proton transfer that accompanies C-C bond formation is facilitated by the carboxylic acid group of proline. 

The first step in the Combes reaction is the acid-catalyzed condensation of the diketone with the aromatic amine to form a Schiff base (imine), which then isomerizes to the corresponding enamine. In the second step, the carbonyl oxygen atom of the enamine is protonated to give a carbocation that undergoes an electrophilic aromatic substitution. Subsequent proton transfer, elimination of water and deprotonation of the ring nitrogen atom gives rise to the neutral substituted quinoline system. 

Donor-acceptor monosubstituted hydrazones (328) have been reported to add in the Michael manner to the iminium species (329), generated from the corresponding enal and the prolinol-type catalyst (R)-(325). The initially formed enamine (330) undergoes a proton transfer followed by hydrolysis to produce adducts (332), which were converted into stable hydrazones (333) by oxidation involving a [1,3] proton shift. ... 

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