Reductive amination using a combination of CaH2 and noble metal

Article abstract of DOI:10.1039/c4ra14808h

Amines were prepared by a reductive amination reaction in the presence of calcium hydride and Pt/C. The in situ formation of water seems to be the key to activate CaH₂ to reduce the intermediate imine.

Full text of DOI:10.1039/c4ra14808h

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Reductive amination using a combination of CaH2
and noble metal†
Cite this: RSC Adv., 2015, 5, 2292
Carole Guyon,‡ Eric Da Silva, Romain Lafon, Estelle Metay* and Marc Lemaire*
´
Received 18th November 2014
Accepted 28th November 2014
DOI: 10.1039/c4ra14808h
www.rsc.org/advances
Amines were prepared by a reductive amination reaction in the pres- several methods to reduce organic functions with 1,1,3,3-tetra-
11
12
ence of calcium hydride and Pt/C. The in situ formation of water seems methylsiloxane (TMDS) and hypophosphite derivatives. Aer
to be the key to activate CaH
2
to reduce the intermediate imine.
an analysis of the literature data, we were curious to notice that
the calcium hydride was poorly explored for the reduction of
organic functions. In fact, the calcium hydride alone is reported
Occurring in nature, amines are important building blocks in
13
14
to reduce disulde, carbon monoxide or tetrauorosilane at
1
organic synthesis. One way amongst others to prepare these
ꢀ
15
temperatures up to 200 C. With a mechanical activation
hexachlorobenzene could be dehalogenated and in the pres-
compounds is the reductive amination reaction. This trans-
formation, which supposes the reaction between a carbonyl and
an amine then the reduction of the intermediate imine, is
16
ence of a chelating agent silanes were prepared. In an ionic
17
liquid, AlCl –Et SBr, benzophenone was reduced. The reduc-
3
3
2
already well described. Two main approaches are considered
tion of ketones and imines were performed with Lewis acids
for the reductive amination: the rst one supposes the forma-
tion of the imine before the addition of the reducing agent and
in the second one all the reactants are present at the beginning
18
such as zinc halides with chlorosilane or Ti(Oi-Pr)
4
.
The
authors generally mentioned the low reactivity of calcium
19
hydride. Probably for this reason, Harder reported the
3
of the reaction. To perform this reaction several reducing
synthesis of an organic calcium hydride complex which is able
agents have been employed: the most used are sodium boro-
20
to reduce several organic functions.
4
5,6
hydride derivatives and hydrogen. Unfortunately, it is well
established now that even if boron and aluminium hydrides are
efficient for the reduction of organic functions they should be
substituted for security and environmental reasons. Hydro-
In our laboratory, we observed that the reaction of a 1 : 1
molar ratio of cyclohexanone 1 and hexylamine 2 in the pres-
ꢀ
ence of 4 mol% of Pd/C and 1.1 equivalent of CaH at 130 C in a
2
sealed tube afforded the N-hexylcyclohexylamine 3 (Scheme 1).
More precisely, analysis of the crude showed a complete
conversion of the starting material toward the formation of
several products (Fig. 1). The two major products were identi-
7
8
silanes and hydrosiloxanes or formates have been associated
with a metal complex or with an organocatalyst to realize
reductive amination. Several reports are also dealing with
9
Hantzsch esters. Recently, amines were prepared via a

ed by GC-MS as N-hexylcyclohexylamine 3 and N-hexyl-
10
hydrogen autotransfer in the presence of a metal catalyst.
In the course to nd alternative reducing agents to
aluminium and boron hydrides, we have previously developed
cyclohexylimine 4. The di- or tri-substituted amine 5, 7 and 8
were formed from hexylamine by condensation reactions. The
formation of bicyclic compounds 9 came from aldol reactions.
The compounds 6 were identied as products from dehydro-
genation reaction. This dehydrogenation reaction was already
Equipe Catalyse Synth `e se Environnement, Institut de Chimie et Biochimie
Mol ´e culaires et Supramol ´e culaires, UMR-CNRS 5246, Universit ´e de Lyon, Universit ´e described in the presence of Pd/C including by our group.
21
Claude Bernard-Lyon 1, B aˆ timent Curien, 43 boulevard du 11 Novembre 1918,
F-69622 Villeurbanne Cedex, France. E-mail: estelle.metay@univ-lyon1.fr; marc.
lemaire.chimie@univ-lyon1.fr; Fax: +33 (0)4 72 43 14 08; Tel: +33 (0)4 72 44 85
In order to increase the selectivity of the reaction, the
different parameters were modied. decrease of the
A
0
†
1
‡
7, +33 (0)4 72 43 14 07
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra14808h
Carole Guyon held a doctoral fellowship from La R ´e gion Rh ˆo ne-Alpes nanced
to the amount of 32 116 euros. The authors are grateful for the access to the MS
analysis at the Centre Commun de Spectroscopie de Masse and NMR facilities
at the Universit ´e Lyon 1.
Scheme 1 Reductive amination of cyclohexanone and hexylamine.
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benzaldehyde 10 into imine 16 was complete however reduction
of the in situ formed imine 16 yielded only 5% of amine 11
(Table 1, entry 1). The reductive amination in the presence of
palladium or platinum catalysts led to the formation of the
amine 11 with 71–79% GC yields (Table 1, entries 2–6). The
platinum catalysts had the advantage to afford less toluene 17
than palladium catalysts respectively 2% against 5–10% GC
2
yields. The conditions using Pt/SiO as catalyst were applied to
the reductive amination of acetophenone 12 and hexylamine 2.
Aer 16 h, only 45% GC yield of the corresponding amine 13
was observed (Table 1, entry 9). Screening of the catalysts
showed a strong effect of the support in this case. Palladium
and platinum on carbon led to the complete conversion of
acetophenone 12 and 76–77% GC yields into the amine 13
against 37–45% with palladium and platinum on silica (Table 1,
entries 7–10). In addition, with palladium on silica incomplete
conversion of acetophenone 12 was observed while with plat-
inum on silica 20% GC yield of unreduced imine 15 was
detected with 21% GC yield of 1-phenylethanol 20 (Table 1,
entries 8 and 9).
Fig. 1 Gas chromatogram of the crude.
The following conditions were retained for the study of the
ꢀ
temperature (60 C) and the load of the catalyst (1 mol% of scope and limitations of the reaction: platinum on carbon
ꢀ
Pd/C) allowed the isolation of the compound 3 with 68% (1 mol%), CaH (60 mol%), 60 C for 16 h (Table 2).
2
yield. These conditions were then applied to the formation of
amines from benzaldehyde 10 or acetophenone 12 (Scheme 21 proceeded with an excellent isolated yield of 87% in diben-
a). The reaction of benzaldehyde 10 and hexylamine 2 gave zylamine 22 (Table 2, entry 1). The reaction with secondary
Reductive amination of benzaldehyde 10 with benzylamine
2
the corresponding amine 11 with a 72% of isolated yield. amines such as dibutylamine 23, morpholine 25 and piperidine
Similar results were obtained with acetophenone 12 as 27 afforded the corresponding tertiary amines 24, 26 and 28
compound 13 was isolated with 74% yield.
with moderate to good isolated yields of 40–82% (Table 2,
When chloride derivative 14 reacted with hexylamine the entries 2–4). The formation of benzyl alcohol 18 (12–17% GC
desired product was not formed. Only dehalogenated yields), as co-product, was observed when morpholine 25 and
compounds 12, 13 and 15 were obtained as shown in Scheme piperidine 27 were used (Table 2, entries 3 and 4). The reaction
2b. As a consequence, the parameters of the reaction were of benzaldehyde 10 with aniline 29, a less nucleophilic amine
investigated. The reductive amination of benzaldehyde 10 and than aliphatic ones, allowed the formation of 30 with moderate
hexylamine 2 in the presence of calcium hydride (1.2 equiv.), 58% yield (Table 2, entry 5). The benzamide 31, poor nucleo-
ꢀ
Pd/C (1 mol%), 60 C, 16 h led to the incomplete reduction of phile and poorly soluble, did not react. Benzaldehyde 10 was
the formed imine 16 (8% GC yield), the formation of the amine detected in 84% GC yield aer reaction with the formation of
1
1 (72% GC yield) and toluene 17 (9% GC yield). The reduction 10% GC yield of benzyl alcohol 18 (Table 2, entry 6). The reac-
of CaH equivalent from 1.5 to 0.6 did not affect the conversion tion with phenylalanine ester hydrochloride salt 33 led to the
into the amine 11 and reduced the formed quantity of imine amine 34 with 71% isolated yield (Table 2, entry 7). The reaction
5% GC yield) and toluene (5% GC yield).
with ammonium acetate led to the formation of dibenzylamine
In order to reduce the amount of toluene 17, metal catalysts 22 with 67% isolated yield (Table 2, entry 8). The reactions
2
(
were screened with calcium hydride (60 mol%), metal (1 mol%) starting from benzaldehyde 10 and acetophenone 12 as
ꢀ
at 60 C for 16 h (Table 1). With Ru/C the conversion of carbonyl compounds gave good isolated yields of respectively
88% and 80% into amine 11 and 13 (Table 2, entries 9 and 11).
p-Chlorobenzaldehyde 36 led efficiently to the corresponding
product of reductive amination 37 in 67% isolated yield without
formation of dehalogenated products. The GC analysis of the
crude showed a proportion between amine and imine of 78/22
explaining the moderate yield. The reaction of p-nitro-
benzaldehyde with hexylamine led to complete conversion of
the starting materials and a mixture of imines: imine from
p-nitrobenzaldehyde with hexylamine and imine from p-for-
mylaniline. No product of reductive amination has been
observed. Aer column chromatography, 58% of the p-nitro-
benzaldehyde was recovered. p-Formylbenzaldehyde and
p-nitrosobenzaldehyde were isolated respectively with 8% and
Scheme 2 (a) Reductive amination of carbonyls; (b) limitation of the
reductive amination reaction.
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Table 1 Screening of catalyst
a
GC yields
Entry
Metal
Ru/C
Conversion of benzaldehyde 10
Amine 11
Imine 16
Toluene 17
Benzylalcohol 18
1
2
3
4
5
6
100
100
100
100
100
100
5
80
6
5
5
3
0
5
5
10
2
2
0
0
0
1
0
Pd/Al
Pd/C
2
O
3
76
71
71
79
73
Pd/SiO
Pt/SiO
Pt/C
2
2
4
2
a
Conversion of acetophenone 12
GC yields
Amine 13
Imine 15
Ethylbenzene 19
1-Phenylethanol 20
7
8
9
Pd/C
Pd/SiO
Pt/SiO
Pt/C
100
66
96
76
37
45
77
0.5
4
20
4
N.D.
N.D.
N.D.
N.D.
0
2
12
21
7
2
1
0
100
a
GC yields were determined by GC using dodecane as internal standard; N.D. ¼ not determined.
27
1
8% yield. Electro donating group on acetophenone such as thanks to carbon and NOESY RMN and supported by the
28
para methoxy did not impair the reactivity with 81% isolated literature.
yield of 39 compared with acetophenone where 13 was obtained
with 80% yield (Table 2, entries 11 and 12). The reaction in the number of pathways. The alkylation reaction remains difficult
presence of a-tetralone 40 led to the amine 41 with an average as the monomethylated product is more reactive than the
The monomethylation of amines is a challenge in spite of the
29
30
yield of 62% (Table 2, entry 13). The crude of the reaction starting materials. Reductive amination reactions issue from
contained an important quantity of unreduced imine (GC Eschweiler–Clarke reaction oen affords a mixture of prod-
31
proportion: amine/imine ¼ 81/19). However, no other side- ucts. In this case, under the optimized conditions in toluene
product was observed. The reductive amination of hexylamine (1 M) in the presence of Pt/C and CaH , the reaction of para-
with aliphatic carbonyl compounds such as cyclohexanone 1 formaldehyde 48 (1 equiv.) and hexylamine (1.06 equiv.) gave a
2
2
and 3-phenylpropionaldehyde 42 proceeded well leading to the mixture of hexylamine 2 (11%), N-methylhexylamine 49 (67%)
corresponding amines 3 and 43 in moderate yields of 62 and and N,N-dimethylhexylamine 50 (22%) with a global yield of
58% (Table 2, entries 14 and 15).
95% (Scheme 5). The products have not been separated. The
The reductive amination of p-cyanobenzaldehyde 44 with distillation is difficult due to close boiling points: hexylamine 2
ꢀ
ꢀ
hexylamine 2 in toluene under the optimized conditions did not (130 C), N-methylhexylamine 49 (135 C), N,N-dimethylhexyl-
ꢀ
lead to the expected product of reductive amination but led to amine 50 (145 C). Even if the major product is the mono-
the diamine 45 resulting from the pinacol reaction of the methylated and this result is similar to the literature, the
intermediate imine (Scheme 3). The diamine 45 was isolated selectivity should be improved.
with 73% yield as a mixture of rac/meso mixture in a 50/50 ratio.
A mechanism is proposed to explain these results
This family of diamines have already been observed in the (Scheme 6). The amine reacts with the carbonyl group to form
radical reduction of imines of p-cyanobenzaldehyde 44 by the corresponding imine aer elimination of water. The water
22
23
NaTeH or by sodium metal. More generally, the pinacol would then react with calcium hydride to release one molecule
24
reaction of imine can take place in the presence of Zn/TMSCl.
of hydrogen. The hydrogen could be adsorbed on the metal
This method has been applied to the synthesis of ligand (R,R)- catalyst and hydrogenate the in situ formed imine into amine. A
0
and (S,S)-N,N -dimethyl-1,2-diphenylethylene-1,2-diamine on 10 control experiment was carried out on the reductive amination
25
g scale. Recently, this method has been selected for the of benzaldehyde and hexylamine 2 under the optimized
26
ꢀ
conditions in the absence of Pt/C: CaH (60 mol%), 60 C. Aer
2
synthesis on 100 g.
From 2,6-dimethylcyclohexanone 46 (83/17 cis/trans mixture) 16 h, only the imine 16 was observed showing the importance of
two diastereoisomers 47A and 47C were isolated respectively in the catalyst. The reaction of CaH (60 mol%) and Pt/C (1 mol%)
2
ꢀ
2
3% and 25% yield (Scheme 4). Three different products could on the dry imine 16 at 60 C for 16 h led to the formation of 44%
have been obtained: two diastereoisomers (47A and 47B) and of the amine 11. It has been attributed to the presence of water
25
one pair of enantiomers (47C). 47A and 47C had been assigned in the Pt/C. It has been observed with different batch of Pt/C.
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Table 2 Scope of the reductive amination catalyzed by Pt/C
Isolated yield
GC yield (%) amine product (%)
a
Entry Carbonyl derivative
1
Amine
Product
21
23
22 94
87
82
b
2
24 96
26 87
28 48
b
3
25
27
75
40
b
4
b
5
29
31
30 58
58
0
10
c
6
32
0
b,d,e
f
f
7
33
35
34 N.D.
71
b,d,e
8
NH
4
OAc
22 N.D.
67
88
9
11 89
b
f
1
0
36
12
37 N.D.
67
80
1
1
13 77
1
2
3
38
40
39 94
81
62
2
f
1
1
41 N.D.
4
1
3
63
62
58
1
5
42
43 62
a
b
c
d
e
GC yields were determined by GC using dodecane as internal standard. Toluene ([S] ¼ 2 M). Ethyl acetate ([S] ¼ 0.5 M). CaH
2
(1.2 equiv.). 2.2
f
ꢀ
equiv. of CaH
tube, 16 h, stirring 700 rpm.
2
.
N.D. ¼ not determined; general conditions: 1 equiv. of carbonyl, 1 equiv. of amine, Pt/C (1 mol%), CaH
2
(60 mol%), 60 C, sealed
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We have showed that calcium hydride can be used directly in
the reductive amination of carbonyl compounds. The calcium
hydride seems to react with the formed water and then liberate
2
hydrogen. CaH could be considered as a hydrogen reservoir.
This hydrogen adsorbed on the metal catalyst could reduce the
formed imine. The reaction leads to good yield in amines,
however, it is sensitive to the nucleophilicity of the amine and
the steric hindrance of the substrates.
This method allows an easy to handle reductive amination
without the need of dedicated equipment and selectivity issue
due to excess of hydrogen.
6
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