Lewis acid-catalyzed direct amination of benzhydryl alcohols

Article abstract of DOI:10.1002/adsc.200600236

The Lewis acid-mediated direct amination of benzylic alcohols is described, providing various benzylic amine derivatives in good yields under mild and environmentally benign conditions. Among the different Lewis acids tested, gold(III) proved to be the catalyst of choice for both chemical (yield, conversion) and practical reasons (a filtration over a silica pad is generally sufficient to obtain the corresponding benzylic amine in analytically pure form).

Full text of DOI:10.1002/adsc.200600236

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DOI: 10.1002/adsc.200600236
Lewis Acid-Catalyzed Direct Amination of Benzhydryl Alcohols
Vincent Terrasson,a Sylvain Marque,a Marie Georgy,b Jean-Marc Campagne,b,*
and Damien Prima,*
a
Institut Lavoisier UMR CNRS 8180, UniversitØ de Versailles-Saint-Quentin-en-Yvelines, 45 avenue des Etats-Unis,
78035 Versailles, France
Fax : (+33)-1-3925-4452; e-mail: prim@chimie.uvsq.fr
Institut de Chimie des Substances Naturelles, Avenue de la Terrasse, 91198 Gif-sur-Yvette, France
b
Fax : (+33)-4-6714-7212; e-mail: jean-marc.campagne@enscm.fr
Received: May 23, 2006; Accepted: July 21, 2006
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: The Lewis acid-mediated direct amina-
tion of benzylic alcohols is described, providing var-
ious benzylic amine derivatives in good yields
under mild and environmentally benign conditions.
As part of a program directed towards the prepara-
tion and applications of new mono- and diamines to
the design of new transition metal ligands,[4] we have
been interested in developing a rapid and simple
access to these compounds. Starting from previously
Among the different Lewis acids tested, gold(III)
N
proved to be the catalyst of choice for both chemi-
cal (yield, conversion) and practical reasons (a fil-
tration over a silica pad is generally sufficient to
obtain the corresponding benzylic amine in analyti-
cally pure form).
described gold
(III)[5] mediated Friedel–Crafts-type re-
A
actions[6] and our own experience on gold
(III) in-
G
duced propargylic substitution,[7] we have been ex-
ploring the possibility of achieving the direct amina-
tion of the corresponding alcohols 1 [Scheme 1, Eq.
(c)].
Keywords: amination; diarylmethylamine; gold;
indium; Lewis acid
Thus, we began our investigations with benzhydryl
alcohol (1a; Ar1 =Ar2 =Ph) in the presence of tosyla-
mine (TsNH2) (Scheme 2). Gratifyingly in the pres-
ence of NaAuCl4·2H2O the amination product 2 was
obtained in 95% yield. Due to a complete conversion
of the starting material combined with the absence of
Homo- and hetero-biarylamines are found in a side-products, analytically pure 2 was easily isolated
number of biologically active compounds such as ce- by simple filtration over a silica pad. In the absence
tirizine hydrochloride (histamine H1-receptor)[1] or of tosylamine (Table 1, entry 2), the corresponding
SNC80 (an opioid receptor agonist).[2] Traditionally, ether 3 was obtained in quantitative yield. Other salts
the syntheses of such compounds have been achieved (Table 1, entries 3–8) were then tested. As previously
by the addition of an organometallic compound on a observed in other examples,[7,8] the less Lewis acidic
preformed imine [Scheme 1, Eq. (a)][3] or, more re- gold(I) complex was inefficient in this reaction
cently, by the reaction of organoboranes with hydrox- (entry 3, Table 1). With
a
Ti(III)-base catalyst
A
ylamine-O-sulfonic acid [Scheme 1, Eq (b)].[1]
(entry 4, Table 1), only traces of ether 3 could be de-
1
tected by H NMR in the crude mixture (entry 4).
Other Lewis acids were also tested. Ti(IV) afforded
the expected amination product 2 in a poor 37%
yield (entry 5), In
(III)[9] and BF3·OEt2 were equally
G
efficient leading to 2 in 87 and 85% yields, respective-
ly (entries 6 and 7). Slightly lower yields were ob-
Scheme 1. Synthesis of diarylamines.
Scheme 2. Catalytic amination of diphenylmethanol.
Adv. Synth. Catal. 2006, 348, 2063 – 2067
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Vincent Terrasson et al.
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Table 1. Catalyzed amination of benzylic alcohols.
philes (vide infra). Gold(III) catalysis then seems,
A
from both chemical (complete conversion, high yield)
and practical (isolation by filtration) considerations,
to be the catalyst of choice in these reactions.[10]
Entry Cat (5%) Recovered 1a Yield 2 [%] Yield 3 [%]
1
2
3
4
5
6
7
8
NaAuCl4[a]
NaAuCl4
Ph3PAuCl 100
TiCl3
TiCl4
InCl3
BF3-OEt2 Traces
PTSA
-
-
95
-
-
-
A recent publication by Sanz and Rodriguez,[11] de-
scribing propargylic substitution in the presence of
PTSA,[12] prompted us to test these conditions on our
model reaction (entry 8). Again conversion was
almost complete (5% starting material recovered)
and the expected product 2 was obtained along with
ether 3 in 31 and 50% yields, respectively, thus pre-
cluding the use of these reaction conditions.
Quant.
-
>95
-
Traces
-
Traces
[b]
37
87
85
31
-
-
Traces
50
5
[a]
[b]
Reaction carried out without TsNH2.
Unidentified by-products.
We next turned our attention to the introduction of
other nitrogen nucleophiles (Table 2). Such nitrogen
nucleophiles can be roughly divided in three main
classes: (1) basic nitrogen nucleophiles that may inter-
tained with indium and BF3·OEt2 compared to those act with and/or poison the catalyst leading to the re-
obtained with NaAuCl4, mainly due to the presence covery of the starting material (Table 2, entries 2–4,
of traces of both starting material and ether 3 in the 14 and 15); (2) non-basic and poor nucleophiles
crude product (thus also requiring purification by where the aryl ether 3 is obtained as the major prod-
flash chromatography). In addition, such behavior uct (Table 2, entries 5–13); (3) moderately-basic and
might be problematic with more demanding nucleo- better nucleophiles such as 4-nitroaniline, TMSN3 and
Table 2. Amination of benzhydryl alcohol 1a.[a]
Entry
Amine
Catalyst
Conditions
Product No. (Yield %)
3
1
2
3
4
5
6
7
8
PhSO2NH2
BnNHBoc
BnNHBoc
BnNHBoc
BnNHBoc
BnNHTos
NaAuCl4
NaAuCl4
NaAuCl4
NaAuCl4
InCl3
NaAuCl4
NaAuCl4
NaAuCl4
InCl3
NaAuCl4
InCl3
NaAuCl4
NaAuCl4
NaAuCl4
InCl3
NaAuCl4
InCl3
BF3·OEt2
NaAuCl4
InCl3
BF3·OEt2
NaAuCl4
InCl3
DCM, rt, 16 h
DCM, rt ,16 h
MeCN, 608C, 72 h
DCM, reflux, 16 h
DCM, rt, 16 h
DCM, rt, 16 h
DCM, rt, 72 h
1108C, MW,[g] 1 h
DCM, rt, 16 h
DCM, rt, 16 h
DCM, rt, 16 h
DCM, rt, 16 h
DCM, rt, 16 h
DCM, rt, 16 h
DCM, rt, 16 h
DCM, rt, 16 h
DCM, rt, 16 h
DCM, rt, 16 h
DCM, rt, 16 h
DCM, rt, 16 h
DCM, rt, 16 h
DCM, rt, 16 h
DCM, rt, 16 h
DCM, rt, 16 h
4 (95)
RSM[b]
RSM[b]
RSM[b]
-
5 (10)
5 (8)
-
-
-
-
-
-
94
80
78
Quant
91
BnNHTos[c]
BnNHTos
9
BnNHTos
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Phtalimide
Phtalimide
BnNHCOCF3
Potassium phthalimidate
Bn2NH
90
89
85[c]
RSM[b]
RSM[b]
RSM[b]
6 (91)
6 (51)[f]
6 (86)
7 (60)
-
Bn2NH
4-Nitroaniline
4-Nitroaniline
4-Nitroaniline
2,4-DNPH
2,4-DNPH
2,4-DNPH
TMSN3
-
-
10
15
Quant
-
-
7 (<5)[e]
8 (93)
8 (64)
8 (84)
TMSN3
TMSN3
Traces
6
BF3·OEt2
[a]
Reaction of benzhydryl alcohol 1a with the amine (2 equivs.) in the presence of 5% of catalyst in CH2Cl2 (0.2M) at room
temperature.
The starting diphenylmethanol material was recovered.
[b]
[c]
[d]
[e]
[f]
10% of the starting diphenylmethanol was also recovered.
Carried out with 10 equivs. of amine and mixture 10-fold diluted.
Identified by 1H NMR and mass analysis but obtained as a mixture with unidentified by-products.
44% of the starting diphenylmethanol was also recovered.
[g]
Closed vessel, P=10 bar in CEM Discover device.
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Lewis Acid-Catalyzed Direct Amination of Benzhydryl Alcohols
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À
2,4-DNPH allowing the formation of the benzylic C
N bond (Table 2, entries 16–24).
The former class of amines, N-benzyl-N-bocamine,
potassium phthalimide and N,N-dibenzylamine (en-
tries 2–4 and 13–15), led to a complete recovery of
the starting material. No traces of ether 3 could be
detected, clearly indicating the lack of catalytic activi-
ty. Unexpectedly, moving from Au
N
N
the reaction between N-benzyl-N-bocamine and benz-
hydryl alcohol leads to the exclusive formation of
ether 3 (entry 5), evidencing the complementarity of
both catalytic systems. In contrast to these results, the
use of phthalimide and N-benzyltrifluoromethylaceta-
mide under both gold and indium catalysis showed
catalytic activity (entries 10–12). Indeed, complete
conversions of the starting material were observed,
however leading to ether 3 as the sole product in high
yields.
Concerning the NaAuCl4-catalyzed reactions of N-
benzyl-N-tosylamine with benzhydryl alcohol (en-
tries 6–9), it is worth noting that the expected amina-
tion product 5 was obtained in low yield. Attempts to
increase the amount of compound 5 failed even by
using prolonged heating times or microwave activa-
tion where ether
(entry 8).
3
is obtained quantitatively
Finally, several other N nucleophiles including 2,4-
dinitrophenylhydrazine, 4-nitroaniline and trimethyl-
silyl azide (entries 16–24) were successfully tested, al-
lowing the introduction of hydrazine- and aniline-
based as well as azido moieties at the benzylic posi-
tion.
Scheme 3. Gold-catalyzed amination on various aromatic
and heteroaromatic substrates. NO: not obtained.
Among the nucleophiles, TMSN3 (Table 2, en-
tries 22–24) is particularly attractive since the azido
Starting from benzhydryl alcohol as reference, we
group can be reduced leading to the free amino group next examined the influence of the substitution by
(vide infra). As previously observed with TsNH2 (see electron-withdrawing or electron-donating groups on
Table 1), under identical reaction conditions and after the formation of the carbon-nitrogen bond. As ex-
complete conversion of the starting material as moni- pected, poor results were obtained in the presence of
tored by TLC, comparisons in various examples (com- electron-withdrawing groups. Indeed, as shown in
pare entries 16 with 17/18, 19 with 20/21 and 22 with Scheme 3, the presence of a strong electron-withdraw-
23/24), clearly evidenced the superiority of gold
over indium
gen bond formation reactions.
C
À
A
Scheme 3 contains the results of an aromatic substi- sponding ether could be obtained. The influence of a
tution survey for this reaction. Compounds 9–14, mild electron-withdrawing group, such as CF3, was
based on one or two p-excessive thiophene units, next studied. Under classical conditions (16 h, room
were efficiently prepared in 63 to 93% yields. As ex- temperature), a poor conversion (20%) was observed.
pected, no reaction takes place when moving from However, increasing the reaction time to 48 h at room
thiophene to usual transition metal ligands such as temperature led to 90% conversion, indicating that
pyridine. Indeed, the lack of reactivity of pyridine- mild electron-withdrawing groups even when ar-
based substrates might be explained either by possible ranged in an ortho fashion are tolerated. N-Tosyl-1-
complexation of pyridine by gold or by a lesser stabi- (2-trifluoromethylphenyl)-1-phenylmethylamine (19)
lization of the electrophilic intermediate due to the was isolated in 78% yield. In sharp contrast with
presence of an electron-deficient pyridine heterocycle. these results, donating groups favored the formation
Substrates involving the basic thiazolyl and the acid- of the carbon-nitrogen bond as illustrated in the effi-
sensitive furyl residues only lead to degradation.
cient synthesis of compounds 15–18 which were isolat-
ed in high to quantitative yields.
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Vincent Terrasson et al.
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In conclusion, we have developed an efficient pro-
cedure to obtain benzylic amines starting from the
corresponding alcohols under mild and environmen-
tally benign catalytic conditions. This methodology
tolerates various aromatic or heterocyclic substrates
bearing electron-donating but also mild electron-with-
drawing groups. Several catalytic systems were com-
Scheme 4. Direct access to diarylmethylamines.
Table 3. Access to diarylmethylamines.
pared evidencing the higher selectivity of Au(III)
A
Entry
Ar1
Ar2
Compound
Yield (%) over others. Among the nitrogen-based nucleophiles
Two steps
used, TMSN3 proved useful as intermediate in the
amination-reduction sequence leading to the direct
preparation of diarylmethylamines.
1
2
3
4
5
Phenyl
Thienyl
Thienyl
4-Cl-C6H4
2-CF3-C6H4
Phenyl
Phenyl
Thienyl
Phenyl
Phenyl
20
21
22
23
24
86
55
49
54
49
Experimental Section
General Remarks
Reactions were carried out in round-bottom flasks equipped
with a magnetic stirring bar and capped with a septum. Di-
chloromethane was distilled over CaH2 and run over neutral
alumina to remove any residual alcohol. TLC analyses were
performed on Merck silica gel 60 F254 TLC plates. FT-IR
spectra were recorded with a Perkin–Elmer Spectrum BX
1
spectrometer and H and 13C NMR spectra were recorded
with Bruker 200, Avance-300 and 500 spectrometers and ref-
erenced to CDCl3 unless otherwise noted. Mass spectra
were obtained from the mass spectrometry facilities operat-
ed by the Institut de Chimie des Substances Naturelles.
Scheme 5. Amination of alkylarylmethyl alcohols.
General Procedure for Gold-Catalyzed Amination of
Benzylic Alcohols
A direct access to di(hetero)arylmethylamines is
also valuable in the context of the synthesis of bioac-
tive products. Attempts to develop a one-pot access
G
To a solution of the benzylic alcohol (1 equiv.) in dichloro-
methane (0.2M) was added the nucleophile (3 equivs.) fol-
lowed by the catalyst (5 mol%). The reaction mixture was
stirred at room temperature for 16 h. Upon completion, as
to di(hetero)arylmethylamines involving TMSN3 ami-
G
nation followed by in situ reduction of the azido
group led to disappointing results. However, we found
that filtration of gold salts over a silica pad after the
amination step, followed by the reduction of the azido
intermediate using LAH, afforded the desired methyl-
amines. Thus, various diarylmethylamines have been
successfully prepared using LAH as the reducing
agent in 49 to 82% yields (Scheme 4) and the results
are gathered in Table 3.
Finally, the reaction was tested on other benzylic
substrates. When 1-phenylethanol was reacted with
tosylamine in the presence of NaAuCl4, the desired
amination product 25 was obtained in 87% yield. In
contrast, benzyl alcohol was recovered unchanged
even after prolonged reactions times (Scheme 5).
The formation of the carbon-nitrogen bond is as-
sumed to proceed through an overall SN1-like mecha-
nism including the generation of a benzylic cation as
reactive intermediate, in a first step, and subsequent
nucleophilic attack in a second step. This kind of
mechanism and intermediate are consistent with the
influence of the electronic effects of the substituents
as we observed and mechanisms suggested by other
related studies.[6]
1
monitored by H NMR, the mixture was concentrated under
vacuum and loaded on to a silica gel column and chomato-
graphed with an appropriate mixture of solvents to give the
amination products described below.
N-Tosyl-1,1’-diphenylmethylamine (2): Rf =0.38 (20%
AcOEt in cyclohexane); mp 1578C; 1H NMR (300 MHz,
CDCl3): d=2.38 (s, 3H), 5.24 (d, 1H, J=7.1 Hz), 5.58 (d,
1H, J=7.1 Hz), 7.09–7.23 (m, 12H), 7.57 (d, 2H, J=
8.3 Hz); 13C NMR (75 MHz, CDCl3): d=21.4, 61.3, 127.2
(2C), 127.3 (4C), 127.5 (2C), 128.5 (4C), 129.3 (2C), 137.3,
140.5 (2C), 143.2; IR (KBr): n=3242, 2656, 2358, 1594,
1491, 1454, 1317, 1161, 1095, 1054, 1021, 938, 818, 698, 674
cmÀ1; HR-MS (ESI): m/z= 360.1036, calcd. for [M+Na]+:
360.1034; anal. calcd.: C 71.19, H 5.68; found: C 71.07, H
5.75.
General Procedure for Reduction of Methyl Azides
To a stirring solution of diarylazidomethane (0.141 mmol) in
7.0 mL of dry THF were added 14 mg of lithium aluminium
hydride (0.351 mmol, 2.5 equivs.). The solution was refluxed
for 16 h and the reaction monitored by TLC until disappear-
ance of the starting azide. The yellow solution was allowed
to cool to room temperature. The reaction mixture was di-
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Lewis Acid-Catalyzed Direct Amination of Benzhydryl Alcohols
COMMUNICATIONS
luted with 15 mL of ethyl acetate, quenched with 5 mL of 1
M HCl and the phases were separated. The aqueous phase
was basified with 10 mL of a 1 M solution of NaOH, and
the organic phase was extracted with 3”20 mL of ethyl ace-
tate. The combined organic layers were dried over MgSO4,
filtered and concentrated under vacuum. The crude product
was purified by flash chromatography.
Biomol. Chem. 2005, 3, 387; b) A. S. K. Hashmi, Gold
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A
K. Merz, G. Dyker, Chem. Commun. 2006, 661;
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genbateer, Y. Yamamoto, J. Org. Chem. 2005, 70, 3682;
k) T. Yao, X. Zhang, R. C. Larock, J. Am. Chem. Soc.
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2005, 127, 10500.
1-Phenyl-1’-(2-fluorophenyl)-methylamine (24): Rf =0.27
1
(30% EtOAc in heptane); H NMR (300 MHz, CDCl3): d=
1.87 (bs, 2H), 5.70 (s, 1H), 7.23–7.57 (m, 8H), 7.66 (d, J=
8 Hz, 1H); 13C-{1H} NMR (CDCl3, 75 MHz): d=54.1, 115.9,
125.6, 125.7, 126.9, 127.1, 128.5, 132.5, 136.6, 144.3, 145.3; IR
(neat): n=3374, 3303, 3062, 3028, 2924, 1605, 1583, 1493,
1450, 1308, 1271, 1153, 1105, 1060, 1033, 958, 891, 798, 766,
732, 697, 658, 643 cmÀ1; HR-MS (ESI): m/z=235.0723,
calcd. for [Ph(o-CF3-C6H4)CH]+: 235.0735; anal. calcd.: C
66.93, H 4.81; found: C 66.98, H 4.97.
Acknowledgements
[6] a) J. Liu, E. Muth, U. Flçrke, G. Henkel, K. Merz, J.
Sauvageau, E. Schwake, G. Dyker, Adv. Synth. Catal.
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Zapf, M. Beller, Adv. Synth. Catal. 2006, 348, 691;
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We are grateful to MENRT-France for a grant (V.T. and
M.G.) and to CNRS and the UniversitØ de Versailles-Saint-
Quentin-en-Yvelines for financial support.
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Adv. Synth. Catal. 2006, 348, 2063 – 2067
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.asc.wiley-vch.de
2067