reactions (eqn. (3)), but it was not specifically identified.
Dibenzylamine has also been reported to be oxidized with O2 to
the imine over the more common catalyst Ru/Al2O3 under similar
condition (100 uC, 1 atm O2).7
In contrast to the amines in entries 1, 2, and 3, the 5-, 6-, and
7-membered cyclic amines (1a–c) gave imine products (4a–c) that
consist of two molecules of the amines (Scheme 1). Yields of the
imine products decreased significantly as the size of the ring
increased from 5 to 6 to 7. In the absence of the gold catalyst or
O2, piperidine (1b) did not react. It has been reported previously
that palladium black catalyzes the reaction of 1a to give the same
product under an Ar atmosphere at 80 uC.8
In Scheme 1 is a possible mechanism for the conversion of the
cyclic amines (1a–c) to their imines (4a–c). Step 1 involves the
oxidative dehydrogenation of the amine (1) to the monomolecular
imine (2), a reaction that stops at this stage for the more sterically
bulky amines in Table 1 (entries 1, 2, and 3). For the less hindered
cyclic amines, intermediate 2 reacts with amine 1 in step 2. That
this step could occur under the conditions of the reactions is
established by the reaction of D1-pyrroline (2a), prepared
according to the literature method,9 with amine 1a which gives
4a directly in 91% yield (eqn (4)). This reaction appears to be faster
than step 1 because it is nearly complete after only 20 h. This
reaction is also catalyzed by the Au as no reaction occurs in
Scheme 1 Proposed pathway for the gold-catalyzed formation of imines
(4a–c) from cyclic secondary amines (1a–c).
mechanism in Scheme 1 for the reactions of cyclic amines and also
suggest that step 1 in this mechanism is slower than steps 2 and 3.
To check for possible colloidal or soluble catalytic gold species in
these reactions, piperidine (0.20 mmol) was reacted with O2 in the
presence of 1.0 g of Au powder under the standard conditions
(60 uC in acetonitrile). The reaction was stopped after 6 h when the
imine (4b) yield was 9%. The reaction solution was decanted from
the Au powder and put under the standard O2 atmosphere.
Reheating the solution to 60 uC and stirring the solution for 15 h
gave no additional product (beyond the 9% produced in the
presence of the Au powder). This result indicates that soluble
species (ligated nanogold particles or Au complexes) are not the
catalytically active species in the reactions of secondary amines.
Considering the high catalytic activity of bulk gold, we
explored the possibility that its less expensive congeners would
also be catalysts in eqn (3). However, when 1.0 g of commercial Ag
(2–3.5 mm) or Cu (1–5 mm) powder was stirred with piperidine
(0.20 mmol) and O2 in acetonitrile (at 60 uC for 40 h), the
piperidine did not react and no imine product was detected.
In summary, bulk gold powder is a highly active catalyst for the
aerobic oxidation of secondary amines to imines under the mild
conditions of 1 atm O2 and 60 uC or 100 uC. Considering the
previously reported gold powder-catalyzed reactions of isocyanides
(eqn (1)) and carbon monoxide (eqn (2)), it appears that non-
nanogold is capable of catalyzing several types of reactions. None
of these reactions requires nano-sized gold particles or supported
gold. Although the primary purpose of this communication is to
highlight the catalytic activity of bulk gold in a new type of
reaction, the application of these results to the practical conversion
of amines to imines requires making more efficient use of the gold
metal by supporting it on a high surface area material; such studies
are in progress.
ð4Þ
the absence of the Au. Especially surprising is the observation that
no reaction occurs in the absence of O2, which suggests that step 3
provides the driving force for step 2 (and/or O2 is required to
activate the gold catalyst for step 2). As compounds 3a–c are not
known and 3a was not obtained in the reaction of 2a (eqn (4)), we
chose 5 (eqn (5)) as a model for 3b.10 Compound 5 contains amine
ð5Þ
groups in the same positions as in 3b. When the Au-catalyzed
oxidation of 5 was carried out under the standard conditions, the
expected imine 611 was produced in almost quantitative yield. This
reaction is almost complete within only 10 h, which is much faster
than the aerobic oxidation of the amines in Table 1. Thus, the
gold-catalyzed aerobic oxidation of an amine with the unit R2N–
CH–NHR9 to give the R2N–CLNR9 imine is faster than the
oxidative dehydrogenation of a normal RCH2–NHR9 amine to
give a RCHLNR9 imine. It has been noted previously that the
dehydrogenation (H2) of nitrogen-containing hydrocarbons is
thermodynamically more favourable than simple hydrocarbons
when catalyzed by heterogeneous catalysts like Pd/C and Rh/C at
110 uC.12 Results of the above studies (eqn (4) and (5)) support the
This work was supported by the U.S. Department of Energy
under Contract No. DE-AC02-07CH11358 with Iowa State
University.{
Notes and references
{ Acetonitrile and the amines (pyrrolidine, piperidine, hexamethyleneimine,
and diisopropylamine) were dried and purified according to a literature
procedure,13 Toluene, dibenzylamine, 1,2,3,4-tetrahydroisoquinoline, and
N-benzylidenebenzylamine were purchased from Sigma Aldrich or Fisher
and used as received. Authentic samples of 2-isopropyliminopropane14 and
2158 | Chem. Commun., 2007, 2157–2159
This journal is ß The Royal Society of Chemistry 2007