Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC
Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC
Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC
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2.3 Selective liquid‐phase oxidation reactions<br />
O OH<br />
O<br />
Cu/Al2O3 Cu/Al2O3 N 2<br />
Scheme 2‐4: Anaerobic oxidation/ isomerization of carveol over Cu/Al2O3 [101].<br />
“external” hydrogen acceptor, the selectivity could be tuned to the dehydrogenated product.<br />
Primary benzylic alcohols reacted slowly (benzyl alcohol 51.5% conversion (20 h); cyclohexanol 99%<br />
conversion (3h)) [100] in contrast to aerobic alcohol oxidation over Au catalysts where similar or<br />
higher reaction rates were often observed for benzylic alcohols (Figure 2‐8) [98, 109, 110]. While in<br />
this report primary alcohols exhibited low reactivity, Cu supported on basic supports, i.e. MgO (entry<br />
4) [51] and La2O3 (entry 5) [53] were suitable for primary aliphatic alcohols like 1‐octanol at high<br />
reaction temperatures (150 °C), also with styrene as hydrogen acceptor. Both Cu catalysts were<br />
stable against leaching. Cu was fully reduced in these studies.<br />
Figure 2‐8: Different reactivities of alcohols on gold [98] and copper [100] catalysts.<br />
The hydrogen transfer principle offers interesting reaction routes. Copper‐aluminum<br />
hydrotalcite was used as a catalyst for the amination of primary alcohols (Scheme 2‐5) which<br />
proceeds via the imine (Table 2‐1, entry 6) [102]. K2CO3 as a base was necessary under the harsh<br />
reaction conditions (160 °C).<br />
Ph<br />
Scheme 2‐5: Reaction scheme for the amination of primary alcohols [102]. Note that the proposed imine was<br />
not observed as an intermediate.<br />
21<br />
2 ,N 2<br />
Cu/Al O<br />
2 3 + styrene Au/Cu Mg Al O + O<br />
5 1 2 x 2<br />
4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0<br />
OH + Ph NH2 TOF (h -1 )<br />
CuAl-HT/ K 2CO 3<br />
160 °C<br />
Ph O +<br />
( ) 4<br />
OH<br />
OH<br />
OH<br />
0 20 40 60 80 100 120 140 160<br />
Ph NH + H<br />
2<br />
2 Ph N<br />
TOF (h -1 )<br />
+ H 2<br />
Ph<br />
Ph N H<br />
Ph