Pd-neocuproine

Primary alcohols afforded the corresponding carboxylic acids via further oxidation of the aldehyde intermediate, e.g. 1-hexanol afforded 1-hexanoic acid in 95% yield. It is important to note, however, that this was achieved without the requirement of one equivalent of base to neutralize the carboxylic acid product (which is the case with supported noble metal catalysts). In contrast, when 1 mol% TEMPO (4 equivalents per Pd) was added, the aldehyde was obtained in high yield, e.g. 1-hexanol afforded 1-hexanal in 97% yield. Under cosolvent conditions using water/ethylene carbonate, Pd-neocuproine was found to be even more active (Fig. 4.65) [174]. This system is exceptional because of its activity (TOF 500h-1 could be reached for 2-octanol) and functional group tolerance, such as C=C bonds, C = C bonds, halides, a-carbonyls, ethers, amines etc. Thereby this system is expected to have a broad synthetic utility. 

Fig. 4.65 Pd-neocuproine as catalyst for alcohol oxidation in water/cosolvent mixtures.

Under cosolvent conditions using water/ethylene carbonate or water/DMSO, Pd-neocuproin was found to be even more active (Figure 5.16) [84]. This system is exceptional because of its activity (a TOF of 1800 h was observed with 2-hexanol) and functional group tolerance, such as C=C bonds, C = C bonds, halides, a-carbo-nyls, ethers, amines, and so on, giving it broad synthetic utility. 

Figure 5.16 Pd-neocuproin as catalyst for alcohol oxidation under water/cosolvent conditions...

Figure 5.17 Selectivity in oxidation of unsaturated alcohol oxidation using the Pd-bathophenanthroline catalyst [82] versus the Pd-neocuproin catalyst [84]. For conditions see Eq. (5.12) and Figure 5.16.

However, a more detailed examination of the results obtained with the Pd(II) bathophenthroline and Pd(II) neocuproin complexes revealed a remarkable difference in the oxidation of the unsaturated alcohol substrate shown in Figure 5.17. With the former, the major product was derived from oxidation of the alkene double bond, while the latter afforded >99% selective oxidation of the alcohol moiety. This suggested that we were concerned with totally different types of catalyst. Indeed, further investigation revealed that the Pd(II) neocuproin complex dissociates completely to afford Pd nanoclusters which are the actual catalyst [85]. 

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