Direct synthesis of imines from alcohols and amines with liberation of H2

Article abstract of DOI:10.1002/anie.200907018

"Chemical equation presented" A clean sweep: Aryl and aliphatic mines can be synthesized directly and efficiently from alcohols and amines under mild, neutral conditions with the liberation of only molecular hydrogen and water (see scheme; R=isopropyl, tert-butyl). This general, environmentally benign reaction is catalyzed by a de-aromatized ruthenium PNP pincer complex (0.2 mol%), and can proceed in toluene under an inert atmosphere or under air.

Full text of DOI:10.1002/anie.200907018

Communications
DOI: 10.1002/anie.200907018
Homogeneous Catalysis
Direct Synthesis of Imines from Alcohols and Amines with Liberation
of H2**
Boopathy Gnanaprakasam, Jing Zhang, and David Milstein*
Imines are important compounds because of their diverse
reactivity, which has led to widespread applications in
laboratory and industrial synthetic processes.[1] Traditionally,
imines are synthesized from the reaction of ketones or
aldehydes with amines in the presence of an acid catalyst.
Imines have also been prepared by the self-condensation of
amines upon oxidation[2] and by the oxidation of secondary
amines.[2b,c,3] The catalytic N-alkylation of amines[4] and
ammonia[5] with alcohols is thought to involve imines as
transient intermediates that undergo rapid hydrogenation.
The development of an efficient, general method for the
synthesis of imines from alcohols and amines is very desirable
because of its potential versatility and wide scope. The
formation of imines by coupling alcohols with amines in the
presence of stoichiometric amounts of oxidants has been
reported, but it is limited to activated alcohols and leads to the
generation of stoichiometric amounts of waste.[6] Recently,
interesting oxidative coupling reactions of alcohols with
primary amines under O2 were reported, but the reactions
were also limited to activated (benzylic) alcohols, and a
maximum turnover number of 50 was reported.[6,7]
We have previously reported the catalytic dehydrogen-
ative coupling of primary alcohols to give esters,[8] the
dehydrogenation of secondary alcohols to give ketones,[9]
and the hydrogenation of esters to give alcohols.[10] These
reactions are catalyzed by de-aromatized 2-(di-tert-butylphos-
phinomethyl)-6-(diethylaminomethyl)pyridine (PNN) and
2,6-bis(diisopropylphosphinomethyl)pyridine (PNP) pincer-
type RuII complexes 1 and 2, respectively (Scheme 1).
We now report a general, efficient, and environmentally
benign method for the direct synthesis of imines by the
reaction of alcohols with amines. This reaction occurs with
liberation of H2 gas and water, high turnover numbers, and no
waste products. Furthermore, the reaction proceeds under
neutral conditions and no hydrogen acceptor is needed
[Eq. (1)]. Remarkably, hydrogenation of the imine does not
take place.
Scheme 1. PNN- and PNP-type ruthenium pincer complexes.
Complex 1 is also an excellent catalyst for the coupling of
alcohols with amines to form amides with the liberation of
H2.[11] All these reactions are based on metal–ligand cooper-
ation during the reversible deprotonation of a pyridinyl
methylene group, in which de-aromatization/aromatization
are key catalytic steps.[8,10–14] Surprisingly, when the PNP
complex 3, analogous to the PNN complex 1 (except for
having a phosphine rather than an amine “arm”), was
employed in the reaction of alcohols with amines, dehydro-
genative coupling to form imines, rather than amides, took
place [Eq. (1)]. Only minor amounts of amides were detected.
It is noteworthy that hydrogenation of the imines to amines
was not observed.
[*] Dr. B. Gnanaprakasam, Dr. J. Zhang, Prof. D. Milstein
Department of Organic Chemistry
Weizmann Institute of Science, 76100 Rehovot (Israel)
Fax: (+972)8-934-4142
E-mail: david.milstein@weizmann.ac.il
Homepage: http://www.weizmann.ac.il/Organic_Chemistry/
milstein.shtml
[**] This research was supported by the Israel Science Foundation, the
DIP program for German-Israeli Cooperation, and the Kimmel
Center for Molecular Design. D.M. is the Israel Matz Professorial
Chair of Organic Chemistry.
Complex 3 was prepared by reaction of the PNP ligand
with [RuHCl(CO)(PPh3)3] to form the hydridochloride com-
plex 4b, followed by deprotonation of the latter with KOtBu,
in analogy to the preparation of complex 2[10] (see the
Supporting Information).
Supporting information for this article (general procedures for the
dehydrogenative coupling of amines with alcohols, spectroscopic
data for the product imines, and synthesis of complexes 2 and 4) is
available on the WWW under http://dx.doi.org/10.1002/anie.
200907018.
Heating a solution containing equimolar amounts of
benzyl alcohol and benzylamine in toluene at reflux in the
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presence of 0.2 mol% 3 under argon resulted in the formation
of water. After 56 h, GC analysis showed 94% conversion of
benzyl alcohol (85% conversion after 22 h) to form the imine
(87%) and a minor amount of the ester (3%). Analysis of the
gas phase revealed the formation of dihydrogen. The solvent
was evaporated and the residue was submitted to vacuum
distillation to give pure N-benzylidene-1-phenylmethanamine
in 79% yield (Table 1, entry 1). The NMR and GC-MS
spectra of the isolated product are consistent with the
reported data.[15]
A variety of alcohols and amines were examined to
explore the scope of this reaction (Table 1). Heating a
solution containing equimolar amounts of benzyl alcohol
and 1-hexylamine with 0.2 mol% of 3 in toluene at reflux for
52 h resulted in 100% conversion (90% conversion after
20 h) of the starting compounds, and N-benzylidenehexan-1-
amine was isolated in 82% yield after distillation. Only traces
of amide and ester were detected. The water formed during
the course of the reaction does not hinder the reaction,
although we previously reported that the PNN complex 1
adds water reversibly, with aromatization, to form a hydrido-
hydroxo complex.[13] Such a reaction probably also takes place
with complex 3, but considering the much larger concentra-
tion of the alcohol (as compared with water) in toluene, and
the reversibility of water addition, the presence of water does
not pose a problem.
A variety of substituted benzyl alcohols undergo efficient
dehydrogenative coupling with amines containing either
electron-donating or -withdrawing substituents. Thus, heating
4-methoxybenzyl alcohol with 1-hexylamine, or 3,4-dime-
thoxybenzyl alcohol with 2-phenylethylamine, in toluene at
reflux for 48 h with 0.2 mol% of 3 resulted in the formation of
the corresponding imines in 89 and 92% yields, respectively
(Table 1, entries 3 and 4). The bicyclic amine (À)-cis-myrta-
nylamine reacts effectively with 4-methylbenzyl alcohol with
97% consumption of the alcohol, and the corresponding
imine was isolated in 88% yield (entry 5). Complex 3 also
catalyzes effectively the reaction of 4-fluorobenzyl alcohol
with 4-fluorobenzylamine to afford the imine in 77% yield
after distillation (entry 6). Substitution of the amine at the
a position does not decrease the yield of the imine; for
example, the reaction of 4-methoxybenzyl alcohol with
2-heptylamine generated the corresponding imine in good
yield (entry 7). The reaction of 4-methoxybenzyl alcohol and
benzylamine furnished N-(4-methoxybenzylidene)-1-phenyl-
methanamine in 90% yield (entry 8). Notably, all the
aromatic primary alcohols gave the corresponding imines as
the primary products.
The synthesis of aliphatic imines is inherently more
challenging because of their instability and difficult isolation.
The possibility of expanding the scope of the new reaction to
the synthesis of these versatile compounds was also explored:
a solution of 1-hexanol and 1-hexylamine in toluene was
heated at reflux for 48 h in the presence of 0.2 mol% of
complex 3. A 90% conversion of the alcohol was achieved,
and the corresponding pure imine was isolated in 65% yield
(entry 9). Minor amounts of the corresponding amide (10%)
and ester (5%) were detected by GC analysis. The reactions
of 1-hexanol with benzylamine and with 4-methylbenzyl-
amine led to the corresponding imines in moderate yields.
Surprisingly, 18% of the corresponding amide and 7% of the
corresponding ester were formed in the reaction of 1-hexanol
and benzylamine (entry 10). GC analysis showed that the
reaction of 1-hexanol with 4-methylbenzylamine (entry 11)
afforded imine (62%), amide (12%), and ester (7%). The
imines were characterized by NMR spectroscopy and GC-MS
analysis. When these reactions were carried out with
1-pentanol, the yield of the imine improved significantly.
Thus, reaction of 1-pentanol with 1-hexylamine or 2-phene-
thylamine led to the corresponding imines in good yields
(entries 12 and 13). However, 16% of the corresponding
amide and 5% of the ester were also detected in the case of
1-hexylamine (entry 12). Monitoring the progress of the
reaction by GC analysis revealed that 30% of the alcohol
was consumed after 4 h, and the imine was formed as the sole
product. Longer reaction times led also to the formation of
minor amounts of the amide and ester. Interestingly, the
reaction of 1-butanol with 2-heptylamine exhibited excellent
selectivity for the formation of the imine, which was obtained
as the sole product after evaporation of the solvent (as
observed by NMR spectroscopy) and was isolated in 86%
yield (entry 14). The reaction of 1-butanol with 4-methyl-
benzylamine resulted in greater than 98% conversion
(entry 15) with formation of imine (63%), amide (14%),
and ester (6%), as observed by GC analysis. Lower con-
version was observed in the reaction of cyclohexylmethanol
with 1-hexylamine, even after heating at reflux for four days,
which led to the isolation of the imine in 57% yield (entry 16).
The reaction can also be carried out with secondary alcohols,
although it is slower. Thus, reaction of cyclohexanol and
benzylamine in the presence of 3 resulted in only 20.5%
conversion after 22 h at reflux, with the corresponding
ketimine formed in 20% yield (entry 17).
The dehydrogenative reaction of alcohols and amines was
also studied with the de-aromatized isopropyl-substituted
PNP catalyst 2. Thus, heating the solution of 4-methoxybenzyl
alcohol, benzylamine, and 0.2 mol% of 2 in toluene at reflux
for 48 h, followed by complete evaporation of the solvent and
excess amine under high vacuum at 608C (water bath)
provided the N-(4-methoxybenzylidene)-1-phenylmethana-
mine in 92% yield. Similarly, the reaction of 1-butanol and
2-heptylamine catalyzed by complex 2 gave N-butylidene-
heptan-2-amine as the only product (76% conversion) after
52 h at reflux.
Conveniently, the new catalytic reaction can be carried
out in air. Thus 4-methoxybenzyl alcohol, benzylamine,
catalyst 3, and toluene were placed into an open flask and
heated at reflux for 24 h under air. The conversion of the
alcohol was monitored by GC analysis, and after all the
alcohol was consumed, the solvent and excess of amine were
completely removed under high vacuum at 608C (water bath).
This led to an 89% yield of the respective imine as the sole
product, in an almost pure state (Table 1, entry 18).
While insufficient mechanistic data exist at present, a
likely mechanism for the direct imination of alcohols with
amines catalyzed by complexes 2 and 3 which accounts for the
strikingly different catalytic activity of these complexes
compared with that of complex 1 is presented in Scheme 2.
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Table 1: Direct synthesis of imines from alcohols and amines catalyzed by the dearomatized ruthenium complex 3.[a]
Conv. of
alcohol
Yield of isolated
product [%]
Entry R1CH2OH
R2NH2
t [h]
GC yield [%]
87 imine,
3 ester
1
56
52
48
>94
100
100
79
82
89
2[b]
3[b]
–
–
4[b]
48
48
100
97
–
–
92
88
5[b]
6[b]
56
48
90
97
–
–
–
77
84
7[b,c]
8[b,c]
48
48
56
56
46
100
90
90
65
67 imine,
10 amide,
5 ester
63 imine,
18 amide,
7 ester
62 imine,
12 amide,
7 ester
69 imine,
16 amide,
5 ester
9
57 imine
16 amide
10[d]
11[d]
12[c]
96
60 imine,
10 amide
94
>96
68
78 imine,
6 amide,
4 ester
13
52
56
32
56
>94
–
–
14(b,c]
15[d]
16
–
86
63 imine,
14 amide,
6 ester
>98
72
58 imine, 12 amide
traces of amide and ester 57
20 imine and
–
17(b]
18[e]
22
24
20.5
100
traces of ketone
–
89
[a] Complex 3 (0.02 mmol), alcohol (10 mmol), amine (10.1 mmol), m-xylene (1 mmol, internal standard), and toluene (3 mL) were heated at reflux in
a Schlenk tube. Conversion of alcohols and yields of products were determined by GC. [b] Only imine was observed by GC. [c] Crude yield, almost pure
by NMR spectroscopy. [d] Amide was completely separated by precipitation upon addition of n-hexane to the crude reaction mixture. [e] Under air.
À
Activation of the O H bond of the alcohol by complex 2 or 3
likely results in the aromatized intermediate A, which upon
b-H elimination (perhaps involving “arm” opening) yields the
coordinated aldehyde intermediate B. Dissociation of the
aldehyde leads to the known dihydride C which liberates H2
(formed from a hydride and a methylene proton) to
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Keywords: homogeneous catalysis · imines ·
.
pincer complexes · ruthenium ·
synthetic methods
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regenerate complex 2 or 3. Reaction of the aldehyde with the
amine generates an unstable hemiaminal D which loses water
to produce the product imine. If nucleophilic attack on the
coordinated aldehyde (in B) takes place, a hemiaminal
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complex 1, which bears a hemilabile amine “arm”, the
coordinated aldehyde is attacked which leads to an
amide,[11] while with the PNP complexes 2 or 3 closure of
the phosphine “arm” results in rapid dissociation of the
aldehyde. The fact that complexes 2 and 3 show similar
catalytic activity indicates that steric factors are not respon-
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amide products. This study represents an unusual case in
which structurally similar complexes lead to entirely different
catalytic reactions.
In conclusion, a new reaction has been discovered in
which imines are formed with high turnover from alcohols
and amines. The reaction occurs under neutral conditions
with liberation of molecular hydrogen. The reaction can be
applied to a variety of alcohols and amines and offers an
environmentally friendly, general method for the synthesis of
imines. Mechanistic studies are in progress.
À
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Received: December 13, 2009
Published online: January 25, 2010
Angew. Chem. Int. Ed. 2010, 49, 1468 –1471
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