A Guidebook to Mechanism in Organic Chemistry

* Stereochemistry of Addition to Carbonyl Compounds

The reactions to form the so-called Mannich bases (XLVII) are be­
lieved to proceed as follows, e.g. with Ph-CO'Me:

C

O eO HO

i\£\ I © H* • I ®

H 2 CT :NR 2 > H 2 C—NR 2 > H 2 C—NR 2

I I I

H H H

Ph CO CH 2 CH 2 NR 2 PhC°^H' |H 2 C—NR 2 ** H 2 C==^R 2 ]

(XLVII) (XLVI)

Initial attack by the unshared electron pair of nitrogen on the car­
bonyl carbon atom is followed by protonation and elimination of
water to yield the ion (XLVI). Attack by the carbanion derived from
acetophenone on the positive carbon atom of (XLVI) then yields the
Mannich base (XLVII). If ammonia or primary amines are used, the
first formed Mannich base, still carrying hydrogen on the nitrogen
atom, can itself participate further in the reaction leading to more
complex products, hence the preference for secojjdary amines. m
The Mannich base formed will readily eliminate R 2 NH, hence the
synthetic usefulness of the reaction:

PhCOCH=CH 2

heat/'*

/Ni/H,

Ph-CO-CH.-CHj-NR, > PhCOCH,Me

Ac,o\^

PhCOCHjCH.OAc

-H.O

STEREOCHEMISTRY OF ADDITION TO CARBONYL COMPOUNDS

Whether the mechanism of addition to a carbon-oxygen double

bond is cis or tfans clearly has no meaning for, unlike a carbon-

carbon double bond, (p. 139) different products will not be obtained

by the two mechanisms because, of free rotation about the C—O

single bond that results: