Enaminoesters (also called vinylogous carbamates), such as

Chapter 23, Problem 23.95

(choose chapter or problem)

Enaminoesters (also called vinylogous carbamates), such as compound 4 below, can serve as building blocks in the synthesis of nitrogen heterocycles. Compound 4 was prepared from compound 1 in a one-pot method (the entire transformation took place in one reaction vessel). Compound 1 was first converted to compound 2, but in the presence of methyl acetoacetate, compound 2 was further converted into imine 3, even without the introduction of an acid catalyst. Upon its formation, compound 3 then tautomerized to give enamine 4 (Tetrahedron Lett. 2005, 46, 979982). O OMe O H N N3 t-BuO O NH OMe O H t-BuO N O Pd H2 1 2 3 4 (a) Draw the structures of 2 and 3. (b) Imine- enamine tautomerization is very similar to keto-enol tautomerization. Draw an acid-catalyzed mechanism (using H3O+) for the conversion of 3 to 4. (c) The equilibrium for imine-enamine tautomerization generally favors the imine, but in this case, the enamine is favored. Give at least two reasons why the equilibrium favors enamine 4 over imine 3.