Addition to Carbon-Oxygen Double Bonds *
With esters, the initial reaction is a nucleophilic displacement, fol
lowed by reduction as above:
Oe OH
II f\ AIH.S || A1H.© | H.O |
R—CJ-OR' > R'O© + R—C—H > R—C—H > R—C—H
H^9 * H HA similar reduction takes place with amides (R-CO-NH^9 being
obtained by a preliminary removal of proton by,AlH 4 e, i.e. an
'active hydrogen' reaction) via an addition/eliniinatioh^fSge,
O jrO°
|| © AIH,© /%
R—C—NH > R—Ci-NH —> Ose + R—CH=NH
H AIH,^6
(VII)
R—CH 2 —NHa J^- R—CH 2 —NH
the Schiff base being obtained as it is easier to eliminate Ooe than
HNee from (VII). LiAlH 4 may obviously not be employed in hydro-
xylic solvents ('aatije hydrogen' reaction) or in those that are readily
rechjjjgd and ether oj tetrahydrofuran %(CH 2 ) 40 is, therefore, com
monly used. NaBH 4 may be used in water or alcohol but is, not sur
prisingly, a less reactive reagent and will not reduce amides.
(b) Meerwein-Ponndorf reduction: This is essentially the reduction
of ketones to secondary alcohols with aluminium isopropoxide in
isopropanol solution:
CMea CMe„
/
O H
(Me 2 CHO) 2 Al CR 2 (MejCHO)^ CR,
O (VIII); O (IX)
- ° CMe,
Me,CH-OH(Me 2 CHO) 2 Al CMe 2 +R 2 CHOH
V00