Homogeneous, Heterogeneous and Nanocatalysis - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 17

MICHAEL ADDITION

OXIDATION

Ph
OSiMe 3
Ph

E E

CuCl (10 mol %)
TEMPO (10 mol %)

O

OH

H

(10mol %)

d n 4 r 4 n g | 1

H Ph H Ph

O 2 , DMF, r.t., 1h CH 2 (CO 2 Et) 2 (3 equiv.)

E = CO 2 Et

additive, air, DMF, r.t., 18h

Scheme 1.15 Tandem oxidation-Michael addition.

alcohols were converted into b-substituted aldehydes and a,b-substituted
aldehydes in good yields and with excellent levels of stereoselectivity, ex-
tending the substrate scope of a,b-unsaturated aldehydes to low molecular
weight aliphatic alcohols in addition to aromatic alcohols (Scheme 1.15).
Furthermore, merging iminium and enamine catalysis provides a highly
stereoselective protocol for the formation of a,b-substituted aldehydes as a
single diastereomer with remarkably high enantioselectivity. Large amounts
of solvents and silica gel to purify the resulting aldehydes are not required.

1.2.3.7 Esterification of Alcohols
Traditionally, alcohols could be converted into esters by multiple steps.
However, the direct conversion of alcohols to esters in the presence of
catalysts may represent a step forward towards green, economic and sus-
tainable processes. Despite the fact that great attention has been paid to the
palladium-catalyzed selective oxidation of alcohols to aldehydes in recent
decades, reports on the oxidative esterification of alcohols are scanty. The
first palladium-catalyzed direct aerobic oxidative esterification of benzylic
alcohols with methanol and various long-chain aliphatic alcohols was re-
ported by Lei et al. 63 Benzylic alcohol and methanol were used in the model
reaction and molecular oxygen was used as oxidant. The applicability of this
method, where [PdCl 2 (CH 3 CN) 2 ] with NaOtBu gave the best results in MeOH
as solvent, was shown for a range of different substrates to give their cor-
responding esters in moderate to high yields. The challenging esterification
reactions of long-chain aliphatic alcohols were accomplished by using a
P-olefin ligand to control the selectivity. The direct nature of this route and
the use of O 2 as oxidant represent a step towards an environmentally benign
and sustainable process.

.

1.3 Heterogeneous Catalysis for the Aerobic
Oxidation of Alcohols
Even through heterogeneous catalysts are preferable to homogeneous ones
owing to their easier recovery and reuse, there are still some concerns and
practical problems in the use of heterogeneous catalysts in the aerobic oxi-
dation of alcohols owing to lower catalytic activity and deactivation. 64,65