Chemistry Reference
In-Depth Information
Fig. 8 Reaction mechanism for citral/acetone aldol condensation.
O
OH
OH
C=CH-C-CH
3
O
H
3
C
H
3
C
+ H
3
C-C-CH
3
C=CH-C-CH
3
+
H
3
C-CH-CH
3
H
3
C
H
3
C
4-methyl-3-penten-2-one
(MO)
2-propanol
4-methyl-3-penten-2 ol
(UOL)
Acetone
Fig. 9 Unsaturated alcohol (UOL) synthesis by hydrogen transfer reduction (HTR) of
mesityl oxide (MO) with 2-propanol.
coordination of the C
¼
O bond on the surface thereby decreasing
the chemoselectivity for the C
¼
O bond saturation [71, 72]. In addition,
the consecutive UOL isomerization to the corresponding saturated
ketone is usually an unavoidable side reaction on metallic catalysts [73].
Hydrogen transfer reduction (HTR) reactions is an alternative route
for the catalytic synthesis of UOL by asymmetric reduction of the
corresponding a ketone. In the HTR reaction, the carbonyl compound
(oxidant) is contacted with a hydrogen donor (reductant) at mild
conditions in liquid or gas phase without supply of molecular hydrogen.
Heterogeneously catalyzed HTR of unsaturated carbonyl compounds
would occur on metal oxides via a Meerwein-Ponndorf-Verley mech-
anism, which involves the selective reduction of the C
¼
O bond
preserving the C
¼
C bond [74, 75]. In particular, we have studied
the gas-phase HTR of 2-cyclohexenone and mesityl oxide (MO) with
2-propanol toward the corresponding unsaturated alcohol on base,
acid-base and metal/acid-base catalysts [18, 76, 77]. Here, we present
the results obtained for the gas-phase mesityl oxide/2-propanol reaction
(Fig. 9) on the MgO-x samples of Table 1.
When both reactants (MO and 2-propanol) are co-fed to the reactor, in
addition to the reaction of Fig. 9 several parallel or consecutive reactions
can take place, such as:
i) MO double bond isomerization
