ChemComm
Communication
obtain a good yield with p-chloro-substituted amine (entry 3). Total
selectivity toward the oxidation products (imine and nitrile) was close
to 100 C-% in each case. In addition, oxidation of the aliphatic pri-
mary amines, butylamine and isobutylamine, gave the corresponding
imines in high yields within 1 and 2 h, respectively (entries 4 and 5). A
series of aromatic secondary amines (N-alkylbenzylamines; R0 = Me,
Et, iPr, tBu, and Bn, entries 6–10) were converted into dehydrogenated
imines in excellent yields in 1 or 1.5 h. In this series, as the steric
hindrance around the N atoms decreased (tBu - iPr - Et - Me),
the selectivity slightly decreased by the formation of benzaldehyde and
N-benzylidenebenzylamine, which probably resulted from the C–N
bond cleavage. This result may reflect the fact that an undesired C–N
bond cleavage becomes slightly allowed in a sterically unhindered
environment. However, emphasis should be placed on the fact that
selectivities greater than 90 C-% were maintained even at almost
complete conversions. The obtained yield in each case (entries 6–10) is
the highest value ever reported.7b,9,10b,14 Surprisingly, however, the
oxidation of N-phenylbenzylamine did not proceed at all over Pd3Pb/
Al2O3-L (data not shown). This reaction was effectively catalyzed by
Pd3Pb/MgO (entry 11). Furthermore, the aliphatic secondary amines,
dibutylamine and diisobutylamine, were converted into the corre-
sponding dehydrogenated imines with good yields within a few hours
Scheme 1 Possible reaction mechanisms of oxidation of secondary amines
over Pd3Pb supported on (a) SiO2 and (b) TiO2, Al2O3, and MgO. A portion of
the atomic arrangement on the Pd3Pb {111} facets is illustrated.
(entries 12 and 13). To the best of our knowledge, this work represents activation occurred (step 2). These results imply that the rate-
the first report of the successful aerobic oxidation of aliphatic determining step over Pd/SiO2 is the desorption of imine (step 3).
secondary amines using a heterogeneous catalyst. The oxidation of
On the basis of the obtained results, we concluded that the higher
cyclic amines such as 1,2,3,4-tetrahydroquinoline and indoline gave catalytic activity of Pd3Pb/SiO2 compared to that of Pd/SiO2 was due to
aromatized quinolone and indole, respectively, with excellent yields promotion of the desorption rate; i.e., the intermetallic Pd3Pb phase
(entries 14 and 15). Particularly, in the latter case, the reaction or the Pb atoms themselves provide favorable desorption sites. In the
occurred quantitatively in only 0.3 h, which also afforded higher chemistry of alcohol oxidation, the addition of Pb or Bi to Pt-group
TOF (119 hꢀ1) than those of Ru (18 hꢀ1)6 and Au (92 hꢀ1 7b catalysts. metals is known to improve their catalytic activity or selectivity.15,16
) A
The catalyst used in indoline oxidation was easily separated from the number of explanations to these positive effects have been advocated
reaction mixture by simple decantation and was reused at least twice, and are still under debate.15–17 However, the effect revealed in the
with the catalytic activity being almost maintained (entries 16 and 17). present study, i.e., promotion of desorption, is completely different
Thus, Pd3Pb/Al2O3-L exhibited not only excellent catalytic activity and from that previously proposed for alcohol oxidation. Thus, the
selectivity but also a wide substrate scope and reusability.
findings in the present study provide not only a highly efficient
We then investigated the reason for the great enhancement in catalytic system but also an entirely novel insight into Pd-based
catalytic activity by the formation of the Pd3Pb intermetallic phase. oxidation chemistry.
Analogous to the well-studied oxidation of alcohols over supported
The ability of the basicity of the support to enhance the
Pd catalysts,15 the aerobic oxidation of amines appears to proceed catalytic activity can be attributed to the acceleration of amide
via a two-step dehydrogenation and oxygenation of the hydrogen formation facilitated by basic sites adjacent to the Pd3Pb
to form a water molecule. A possible reaction mechanism for the particles (Scheme 1b). Such base-mediated deprotonation is
oxidation of a secondary amine over Pd3Pb/SiO2 is shown in common in alcohol oxidation over Pt-group metals and over Au
Scheme 1a: (1) adsorption of the amine (N–H activation), catalysts.15 In the case of amine oxidation over Pd/SiO2, however,
(2) C–H activation, (3) desorption of the imine, and (4) removal such a promotion effect does not appear because the reaction
of the hydrogen atoms by oxygen. A kinetic study of the oxidation rate is limited at the desorption step. Thus, the intermetallic
of dibenzylamine over Pd3Pb/SiO2 revealed a first-order relation- Pd3Pb phase is also necessary to exert the base-promotion effect
ship with the amine concentration ([A] = 0.10–0.28 M) and a zero- during amine oxidation.
order dependence of the reaction rate on the partial pressure of
In conclusion, intermetallic Pd3Pb exhibits catalytic activity
oxygen (PO : 2.0–10 kPa). Moreover, a primary kinetic isotope effect that is remarkably higher than that of a monometallic Pd
2
was observed with N-deuterated dibenzylamine (kH/kD = 2.2). catalyst in the oxidation of amines to imines. Nanoparticulate
These results strongly indicate that the dissociative adsorption of Pd3Pb supported on Al2O3 can act as a highly efficient hetero-
dibenzylamine (step 1) is the rate-determining step over Pd3Pb/ geneous catalyst for the oxidation of various amines. The obtained
SiO2. A similar kinetic study on Pd/SiO2 showed that the reaction catalytic activities, selectivities, and the substrate scope are super-
orders with [A] and PO were both close to zero. Furthermore, no ior to those of existing heterogeneous catalysts. The enhanced
2
kinetic isotope effect was observed (kH/kD = 1.0) for deuteration catalytic activity of Pd3Pb is attributed to its ability to promote
at the benzylic positions (dibenzylamine-a,a,a,a-d4) where C–H imine desorption.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 3277--3280 | 3279