17 Copper–Aluminum Hydrotalcite
Hydrotalcites are a class of layered materials of current interest. They are
represented by a general formula [M(II) 1Àx M(III) x (OH) 2 ]
nÀ A
nÀ
x/n.y H 2 O,
where M(II) and M(III) are divalent and trivalent cations such as Cu
2+ , Mg
2+ , and
Al
3+ , respectively. The compensating anions may be OH
À , Cl
À , NO 3
À , CO 3
2À
, etc.
(see for a recent review [66]). Copper-containing LDHs act as the best heterogeneous catalysts for the production of substituted amines via Ullmann-type crosscoupling reactions of unreactive aryl chlorides with aryl and aliphatic amines under
ligand-free conditions [67]. This method tolerates a variety of functional groups and
does not require any additive (Schemes 14 and 15).
Chlorobenzenes with electron-withdrawing groups substituted at para and ortho
provided moderate to good yields (Scheme 14, entries 1–7). In the case of 2,4dichlorobenzaldehyde, the chloro group at the ortho position was N-arylated
(Scheme 14, entry 4). The C–N coupling product bearing an aldehyde group is
oxidized to an acid (Scheme 14, entry 3) while in the case of 2,4dichlorobenzaldehyde, no oxidation of the aldehyde group occurred (Scheme 14,
entry 3 vs. 4). Electron-neutral and electron-donating chloroarenes such as chlorobenzene and 4-chloroanisole (Scheme 14, entries 8 and 9) did not form any coupled
products under these conditions.
This protocol was successfully applied to cycloalkylamines (Scheme 15) using
the amine as self-solvent, with good isolated yields and negligible amount of Cu
leaching. Cu/Al-HT was recovered quantitatively by simple filtration and reused.
Consistent activity is observed even after the fourth cycle (91% in the first cycle and
90% in the fourth cycle) (Scheme 15, entry 1) for the coupling reaction between
o-nitrochlorobenzene and cyclohexylamine. A similar observation was made when
a C–N coupling product bearing an aldehyde in the para-position was oxidized
to an acid, while in the case of 2,4-dichlorobenzaldehyde, no oxidation of the
aldehyde group occurred (Scheme 15, entry 3 vs. 6).
In 2009, Sreedhar et al. [68] expanded the scope of amination with various
amines and aryl halides using Cu/Al-HT. In this study, the authors had chosen a
coupling reaction between octylamine and 2-nitro-chlorobenzene (Scheme 16) with
a catalytic amount of Cu catalyst under ligand-free conditions. From Table 25, it
can be seen that Cu/Al-HTB is the best catalyst among the catalysts screened
(Table 25, entry 6). The catalysts screened were (Cu-(HAP) copper hydroxyapatite,
Cu-(FAP) copper flouroapatite, and Cu/Al-HTs (HT, Hydrotalcite of Cu/Al in
different ratios) Cu/Al-HTA, 3:1(Cu: Al); Cu/Al-HTB, 2.5:1; and Cu/Al-HTC, 2:1)
The scope of the methodology was examined on many aryl chlorides using the
Cu/Al-HTB as catalyst. Selected examples are listed in Table 26. Although this
method works for variety of amines, some of the aryl chlorides, for example
chlorobenzene and 2-methyl chlorobenzene, did not participate in the coupling of
cyclopentyl and octylamine under the same reaction conditions. As can be seen,
aryl chlorides with electron-donating and electron-withdrawing groups coupled
with equal ease in many cases.
Recent Developments in Recyclable Copper Catalyst Systems for C–N Bond. . .
149
Hydrotalcites are a class of layered materials of current interest. They are
represented by a general formula [M(II) 1Àx M(III) x (OH) 2 ]
nÀ A
nÀ
x/n.y H 2 O,
where M(II) and M(III) are divalent and trivalent cations such as Cu
2+ , Mg
2+ , and
Al
3+ , respectively. The compensating anions may be OH
À , Cl
À , NO 3
À , CO 3
2À
, etc.
(see for a recent review [66]). Copper-containing LDHs act as the best heterogeneous catalysts for the production of substituted amines via Ullmann-type crosscoupling reactions of unreactive aryl chlorides with aryl and aliphatic amines under
ligand-free conditions [67]. This method tolerates a variety of functional groups and
does not require any additive (Schemes 14 and 15).
Chlorobenzenes with electron-withdrawing groups substituted at para and ortho
provided moderate to good yields (Scheme 14, entries 1–7). In the case of 2,4dichlorobenzaldehyde, the chloro group at the ortho position was N-arylated
(Scheme 14, entry 4). The C–N coupling product bearing an aldehyde group is
oxidized to an acid (Scheme 14, entry 3) while in the case of 2,4dichlorobenzaldehyde, no oxidation of the aldehyde group occurred (Scheme 14,
entry 3 vs. 4). Electron-neutral and electron-donating chloroarenes such as chlorobenzene and 4-chloroanisole (Scheme 14, entries 8 and 9) did not form any coupled
products under these conditions.
This protocol was successfully applied to cycloalkylamines (Scheme 15) using
the amine as self-solvent, with good isolated yields and negligible amount of Cu
leaching. Cu/Al-HT was recovered quantitatively by simple filtration and reused.
Consistent activity is observed even after the fourth cycle (91% in the first cycle and
90% in the fourth cycle) (Scheme 15, entry 1) for the coupling reaction between
o-nitrochlorobenzene and cyclohexylamine. A similar observation was made when
a C–N coupling product bearing an aldehyde in the para-position was oxidized
to an acid, while in the case of 2,4-dichlorobenzaldehyde, no oxidation of the
aldehyde group occurred (Scheme 15, entry 3 vs. 6).
In 2009, Sreedhar et al. [68] expanded the scope of amination with various
amines and aryl halides using Cu/Al-HT. In this study, the authors had chosen a
coupling reaction between octylamine and 2-nitro-chlorobenzene (Scheme 16) with
a catalytic amount of Cu catalyst under ligand-free conditions. From Table 25, it
can be seen that Cu/Al-HTB is the best catalyst among the catalysts screened
(Table 25, entry 6). The catalysts screened were (Cu-(HAP) copper hydroxyapatite,
Cu-(FAP) copper flouroapatite, and Cu/Al-HTs (HT, Hydrotalcite of Cu/Al in
different ratios) Cu/Al-HTA, 3:1(Cu: Al); Cu/Al-HTB, 2.5:1; and Cu/Al-HTC, 2:1)
The scope of the methodology was examined on many aryl chlorides using the
Cu/Al-HTB as catalyst. Selected examples are listed in Table 26. Although this
method works for variety of amines, some of the aryl chlorides, for example
chlorobenzene and 2-methyl chlorobenzene, did not participate in the coupling of
cyclopentyl and octylamine under the same reaction conditions. As can be seen,
aryl chlorides with electron-donating and electron-withdrawing groups coupled
with equal ease in many cases.
Recent Developments in Recyclable Copper Catalyst Systems for C–N Bond. . .
149