Chem 313: Pyridoxal Phosphate (PLP)
Imines and Enamines
Imine formation and hydrolysis come up many times in this course, so it is worthwhile looking at their
reactions in some detail.
A primary amine can react with an aldehyde or ketone to give an imine. The reaction usually requires
removal of water, as it is reversible and the amine/carbonyl are thermodynamically favored over
water/imine. There are two main steps, addition and collapse of the tetrahedral intermediate (TI).
Collapse of the TI is the rate-determining step (RDS) under neutral and basic conditions because
hydroxide is a poor leaving group and the protonated alcohol (needed for water to leave) is in low
concentration. Since amines are pretty good nucleophiles, addition if relatively fast. Under acidic
conditions, the concentration of amine is low, as it is mostly ammonium, so addition is the RDS.
Imine hydrolysis: Generally, protonated aldehydes are more electrophilic than iminiums, but imines
are more basic aldehydes, so the concentration of iminiums is usually higher than the protonated
alcohol. Iminiums are more electrophilic than neutral aldehydes, which are more electrophilic than
imines. Under basic conditions, the RDS is addition of hydroxide to the iminium. As basic pH goes up,
the concentration of hydroxide goes up while the concentration of iminium goes down, the result being
a fairly constant rate at high pH’s. Collapse of the TI is fast as formation of the protonated amine in the
TI is facile. A neutral pH, the nucleophile becomes water and the rate slows down, but the RDS is still
addition. At acidic pH, the RDS is collapse of the zwitterionic TI, which is in low concentration; the
hydroxylamine form of the TI is apparently slow to collapse.
Enamines: With secondary amines, imines cannot form. Iminiums can, but they revert to starting
amines or give enamines. Enamines from primary amines are generally not stable, but judicious choice
of substituents at the alpha and beta carbons can stabilize primary enamines. As with imines, water
removal is needed to form enamines in appreciable quantities. Enamines are both nucleophilic and
basic, with reaction occurring at the beta carbon. Hydrolysis under basic conditions proceeds with the
RDS being protonation at the beta-carbon. Under neutral or weakly basic conditions, the RDS is
collapse of the TI, as with imine hydrolysis.
Enamines are often used synthetically to alkylate at the beta carbon, followed by hydrolysis, to yield
selectivity overall in alkylating alpha to carbonyls. We will not study such reactions. But we will use
imine formation, hydrolysis, tautomerization, conversion to enamines, enamine formation, hydrolysis,
and isomerization in our studies of ninhydrin reactions, PLP reactions, and thiamin reactions.
Pyridoxal phosphate is a vitamin B de6ivative. It is a coenzyme that is used by a great many and great
variety of enzymes. OH
O H O
HO OH P OH
Pyridoxine, Vitamin B6 Pyridoxal Phosphate, PLP
Many enzymes function largely as organizers of substrates, and rely on coenzymes such as PLP to
carry out catalysis of the particular reaction. In PLP, the phosphate and methyl group serve largely as
binding/recognition sites for the enzymes. The aldehyde is the reactive center. The pyridyl serves an
electron sink; that is, it stabilizes carbanions that are remote from, but conjugated to the ring. The
hydroxyl serves as a proto