Calcium-catalyzed direct amination of π-activated alcohols

Article abstract of DOI:10.1002/adsc.201000768

A calcium-catalyzed direct amination of π-activated alcohols with different nitrogen nucleophiles under very mild reaction conditions is presented. The high reactivity of the calcium catalyst allows for an efficient conversion of secondary and tertiary benzylic and allylic as well as tertiary propargylic alcohols. Nitrogen nucleophiles such as carbamates, tosylamides and anilines are readily alkylated at room temperature.

Full text of DOI:10.1002/adsc.201000768

FULL PAPERS
DOI: 10.1002/adsc.201000768
Calcium-Catalyzed Direct Amination of p-Activated Alcohols
Stefan Haubenreissera and Meike Niggemanna,*
a
Institute for Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany
Fax : (+49)-241-809-2127; phone: (+49)-241-809-4688; e-mail: niggemann@oc.rwth-aachen.de
Received: October 12, 2010; Revised: December 3, 2010; Published online: February 16, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000768.
Abstract: A calcium-catalyzed direct amination of p- sylamides and anilines are readily alkylated at room
activated alcohols with different nitrogen nucleo- temperature.
philes under very mild reaction conditions is present-
ed. The high reactivity of the calcium catalyst allows
for an efficient conversion of secondary and tertiary Keywords: p-activated alcohols; calcium catalysts;
benzylic and allylic as well as tertiary propargylic al- direct amination; room temperature alkylation of
cohols. Nitrogen nucleophiles such as carbamates, to- amines
Introduction
ciently catalyze this type of reaction. Very recently, a
first enantioselective direct amination of an allylic al-
Due to their continuing relevance for the synthesis of cohol has been realized in an intramolecular fashion
pharmaceuticals and fine chemicals, the efficient and via the action of an HgACHTNUGRTNEUNG
(OTf)2 catalyst.[9] Some, mostly
environmentally benign preparation of amines re- heterogeneous, Brønsted acid catalysts[10] were report-
mains a highly active research area in organic synthe- ed as well as rare earth metal-,[11] Bi-,[12,13] and iodine-
sis.[1] In this context, the direct nucleophilic substitu- catalyzed[14] protocols. However, the generally re-
tion of alcohols (Scheme 1) represents an elegant ap- quired harsh reaction conditions and extended reac-
proach as it combines several advantages.
tion times limit the applicability of this type of trans-
Many alcohols are readily available and water is formation.
formed as the only by-product during the conversion.
The development of sustainable methods using eco-
Preactivation of the hydroxy group by transformation logically benign and cheap catalysts becomes nowa-
into the corresponding halides, carboxylates, carbo- days more and more important. Precious metals, play-
nates, phosphates or related compounds is omitted ing a fundamental role in homogeneous catalysis, are
and salt loads, thus, substantially reduced. Ideally, the expensive and are becoming increasingly rare as we
process is effective under mild reaction conditions al- use up natural resources. The development of main
lowing for an energy-balanced transformation and group metal-catalyzed reactions has drawn only little
high substrate compatibility.
attention in the past despite its enormous economical
The first Pd(II)-catalyzed amination reaction of al- and ecological benefits. Among other alkaline earth
lylic alcohols in the absence of further stoichiometric metals, calcium seems to be an ideal main group
or substoichiometric activators was reported by metal catalyst,[15] as it is essentially free of toxicity,
Ozawa in 2002.[2] Following this pioneering work, a very cheap and the fifth most frequent element of the
number of transition metals such as Mo,[3] Fe,[4] Pd,[5] earth’s crust. We recently reported a novel calcium-
Pt,[6] Au[7] and Ag[8] have been demonstrated to effi- based, highly Lewis acidic system that catalyzes the
substitution of benzylic, allylic and propargylic alco-
hols with nucleophilic arenes under very mild reaction
conditions.[16,17] In continuation of our interest in de-
veloping main group metal-catalyzed organic transfor-
mations providing an alternative to traditionally used
transition metal catalysts, we herein report a novel
À
method for C N bond formation via the direct amina-
tion of alcohols.
Scheme 1. Direct amination of alcohols.
Adv. Synth. Catal. 2011, 353, 469 – 474
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469

Stefan Haubenreisser and Meike Niggemann
FULL PAPERS
Results and Discussion
similar yields were obtained, except for the potassium
salt (entry 7), the reaction times were substantially
In analogy to the previously presented arylation reac- longer than in the presence of Bu4NPF6. We assume
tion, we hypothesized that the highly active calcium that these findings can be ascribed to solubility ef-
catalyst would promote the reaction of N-nucleo- fects, as the tetrabutylammonium salt solubilized best
philes with different types of p-activated alcohols. in DCM. The shorter reaction time resulted in a
Thus, the reaction of allylic alcohol 1 with phenyl car- cleaner formation of product giving a slightly better
bamate (2) was examined (Table 1). In the presence yield. No reaction occurred in the presence of only
of 5 mol% of Ca
tution of the hydroxy group in 1 by the nucleophile 2 anion exchange (entry 9). In the presence of
was complete after 75 min at room temperature and Ca(OTf)2/Bu4NPF6 and in toluene or diethyl ether as
ACHTUNGTRENNUNG(NTf2)2 and 5 mol% Bu4NPF6 substi- Bu4NNTf2, that is formed as the side product of the
AHCTUNGTRENNUNG
the desired product 3 was isolated in 89% yield. Suit- solvents (entries 10–12) a competing decomposition
able reaction media were aprotic, non-coordinating of the carbamate was observed leading to the forma-
and polar, such as dichloromethane and dichloro- tion of significant amounts of phenol in the reaction
ethane. The reaction rate was significantly lower in mixture.
toluene (entry 11) or diethyl ether (entry 12) and con-
version ceased completely when coordinating solvents substrate was investigated applying the optimized
such as tetrahydrofuran or acetonitrile were used. conditions (Table 2). A series of different carbamate
Comparison with other alkaline earth metal salts derivatives was readily alkylated in good to excellent
such as Mg(NTf2)2 or Ba(NTf2)2 (entries 1/2) clearly yields (entries 1–6).
demonstrates the superior reactivity of the Ca(NTf2)2/ The required reaction times generally decreased
The scope of the reaction with respect to the amide
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
Bu4NPF6 system. As already mentioned in previous with increasing alkyl chain length in the carbamate.
publications,[12,17] the formation of the highly reactive This can be assigned to the thereby enhanced solubili-
CaNTf2PF6 species via anion exchange appears to be ty of the carbamate substrate and its slightly higher
critical for an efficient conversion. Accordingly, a 1:1 nucleophilicity. The poor reactivity of the secondary
stoichiometry of CaACTHUNTRGNE(UNG NTf2)2 and Bu4NPF6 gives the carbamate 4f is reflected in a significant drop of the
best results. To gain a more detailed insight into this reaction rate. Hence, heating of the reaction mixture
phenomenon we studied the reaction in the presence to 908C under microwave conditions was necessary to
of a series of different PF6 salts. Even though very achieve an acceptable degree of conversion to the de-
sired product 5f. Likewise, transformation of the car-
boxamides 4g and 4h to the amides 5g and 5h was ac-
Table 1. Optimization of the reaction conditions.
complished in only moderate yields (entries 7/8) at
elevated temperatures. In all three cases, the dimin-
ished yields are due to a competing background reac-
tion of the cyclohexenol derived intermediate carbo-
cation, yielding oligomeric products. This oligomeriza-
tion reaction becomes predominant when the nucleo-
Entry[a] Lewis
acid
Additive
[mol%]
t
Yield[b]
[%]
philicity of the amine substrate is too low.
Both aromatic and aliphatic sulfonamides 4i–4l
proved to be excellent N-nucleophiles, and the prod-
ucts 5i–5l were obtained in high yields without any
difficulties (entries 9–12). Furthermore, we were
pleased to find that the calcium-based catalyst system
was suitable for the alkylation of anilines, such as 4m,
4n and 4o (entries 13–15). The slow reaction rates ob-
served for these highly nucleophilic amines can be ex-
plained by a partial poisoning of the catalyst, occur-
ring through coordination of the aniline nitrogen
atom to the Lewis acid. Therefore, only electron-poor
anilines were suitable substrates, as their coordination
ability is comparatively low.
In Table 3 the amination of different secondary and
tertiary allylic alcohols with phenyl carbamate (2) and
aniline 4m is exemplified. All products were obtained
in good to excellent yields at room temperature. The
tertiary allylic alcohol 10 was found to be significantly
more reactive than the secondary alcohol 1, reflected
1
2
3
4
5
6
7
8
Mg
G
16 h
16 h
75 min 89
200 min 78
225 min 84
260 min 81
16 h
16 h
16 h
16 h
16 h
16 h
–
–
Ba
Ca
Ca
Ca
Ca
Ca
Ca
–
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
G
–
–
–
75
75
50
A
–
9
Bu4NNTf2
10
11[c]
12[d]
Ca
Ca
Ca
G
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
[a]
Reaction conditions: 5 mol% Lewis acid and 5 mol% ad-
ditive were added at room temperature to cyclohexenol
1 (0.5 mmol) and phenyl carbamate 2 (0.75 mmol) in
CH2Cl2 (1.5 mL) and stirred for the time indicated.
Isolated yield.
[b]
[c]
[d]
Reaction in toluene.
Reaction in Et2O.
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Adv. Synth. Catal. 2011, 353, 469 – 474

Calcium-Catalyzed Direct Amination of p-Activated Alcohols
Table 2. Addition of N-nucleophiles to cyclohexenol 1.
Table 3. Addition of N-nucleophiles to allylic alcohols.
Entry[a] Alcohol
Product[b]
t
Yield[c]
[%]
1
2
75 min 64
135 min 92
Entry[a] N-Nucleophile
Products t
Yield[e]
[%]
1
2
3
4
5
4a 5a 21 h
93
4b 5b 15 min[b] 87
3
4
1 h
86
89
4c 5c 3.5 h
4d 5d 3.5 h
4e 5e 3 h
83
91
89
2.5 h
5
1 h
72
6
7
4f 5f 30 min[d] 62
6
15 min 88
4g 5g 10 min[d] 58
4h 5h 105 min[d] 39
[a]
8
Reaction conditions: 5 mol% Bu4NPF6 and 5 mol%
Ca(NTf2)2 were added at room temperature to the alco-
AHCTUNGTRENNUNG
hol (0.5 mmol) and the nucleophile (0.75 mmol) in
CH2Cl2 (1.5 mL) and stirred for the time indicated.
Ar: p-NO2C6H4.
9
4i 5i 10 min
4j 5j 14 h
93
97
[b]
[c]
10
Isolated yield.
11
12
4k 5k 20 min
4l 5l 25 min
86
93
alcohol 8 reacted exclusively to allyl carbamate 9a
and allylamine 9b. The tertiary allylic alcohol 10, on
the contrary, fully isomerized during the reaction to
give carbamate 11a and amine 11b as the sole prod-
ucts.
13
4m 5m 16 h
85
14
4n 5n 30 min[c] 75
As shown in Table 4, different benzylic (12, 14 and
16) and even propargylic alcohol 18 reacted to afford
the desired products in high yields. As these alcohols
were generally less reactive than allylic alcohols, heat-
ing under thermal or microwave conditions was nec-
essary to obtain satisfactory yields in some cases. As
no difference was observed for samples heated under
thermal vs. microwave conditions, the latter were ap-
plied for better practicability. As already observed for
the conversion of allylic alcohol 10, the tertiary ben-
zylic alcohol 16 was more reactive than its secondary
counterpart 12. This observation was the most signifi-
cant for the propargylic alcohols. Here, the tertiary
15
4o 5o 1 h[b]
64
[a]
Reaction conditions: 5 mol% Bu4NPF6 and 5 mol%
Ca(NTf2)2 were added at room temperature to cyclohex-
ACHTUNGTRENNUNG
enol 1 (0.5 mmol) and the nucleophile 4 (0.75 mmol) in
CH2Cl2 (1.5 mL) and stirred for the time indicated.
Reaction at 408C under microwave (mw) conditions.
Reaction at 508C under mw conditions.
Reaction at 908C under mw conditions.
Isolated yield.
[b]
[c]
[d]
[e]
by a much higher conversion rate (entries 5/6). Where propargylic alcohol 18 was readily converted to carba-
the formation of regioisomers was prone to occur, the mate 19a and amine 19b, whereas the corresponding
selectivity of the substitution was high. Thus, allylic secondary propargylic alcohol displayed no reactivity.
Adv. Synth. Catal. 2011, 353, 469 – 474
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Stefan Haubenreisser and Meike Niggemann
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Table 4. Addition of N-nucleophiles to benzylic and propar-
Table 5. Influence of the water content.
gylic alcohols.
Entry[a] Alcohol
Product[b]
t
Yield[c]
[%]
Entry[a]
Water content
t
T[b]
Yield[c] [%]
1
2
90 min[d] 82
1
2
3
4
5
6
7
8
9
“extra dry”
no added
9 mL
16 h
75 min
16 h
8 min
16 h
12 min
16 h
r.t.
r.t.
r.t.
50
r.t.
50
r.t.
50
70
–
89
–
83
–
85
–
–
4 h[d]
16 h
75
80
18 mL
45 mL
3
4
30 min
15 min
84
[a]
Reaction conditions: Bu4NPF6 und Lewis acid were
added at room temperature to cyclohexenol
1 h[d]
87
1
(0.5 mmol), phenyl carbamate 2 (0.75 mmol) and H2O in
CH2Cl2 (1.5 mL) and stirred for the time indicated.
Heating under microwave (mw) conditions.
Isolated yield.
[b]
[c]
5
6
3.25 h
16 h
68
87
Interestingly, the reaction ceased when both of the
starting materials and the solvent were thoroughly
dried (entry 1). In the presence of 1, 2 and 5 equiva-
lents of water, with respect to cyclohexenol 1, no re-
action occurred at room temperature. At 508C, the
reaction was complete in the presence of 1 and
2 equivalents of water in 8 or 12 min, respectively,
whereas in the presence of 5 equivalents of water
15 min reaction time at 708C were necessary to
achieve complete conversion. We believe that the
higher reaction temperature accelerates the coordina-
tion-decoordination process of competing water and
alcohol molecules at the calcium cation, thus increas-
ing the reactivity.
14
1.5 h
5 h[d]
71
75
15
[a]
Reaction conditions: 5 mol% Bu4NPF6 and 5 mol%
Ca(NTf2)2 were added at room temperature to the alco-
hol (0.5 mmol) and the nucleophile (0.75 mmol) in
CH2Cl2 (1.5 mL) and stirred for the time indicated.
Ar: p-NO2C6H4.
ACHTUNGTRENNUNG
Furthermore, the reaction was found to be reversi-
ble (Scheme 2), which is in accordance with observa-
tions made by Shibasaki for a similar bismuth-cata-
lyzed amination of activated alcohols with N-nucleo-
philes.[12] When the carbamate 5d was treated with
[b]
[c]
[d]
Isolated yield.
Reaction at 508C under microwave (mw) conditions.
5 mol% CaACTHNUGTRNEUNG(NTf2)2 and 5 mol% Bu4NPF6 in the pres-
Limitation of the reaction clearly unfurls when it ence of one equivalent of the tosylamide 4i an equi-
comes to primary alcohols. Allyl alcohol, benzyl alco- librium between carbamate 4d and tosylamide 5i es-
hol, as well as propargyl alcohol were unreactive tablishes over time.
under the reaction conditions. The attempted reaction
of cinnamyl alcohol yielded a complex mixture of in- Table 3 and Table 4, another interesting observation
separable reaction products. was made underlining that the interplay of different
During most transformations listed in Table 2,
In addition to the evaluation of scope and limita- equilibrating species in the reaction mixture is even
tions of the calcium-catalyzed direct amination, some more complex. The reaction was found to proceed via
preliminary mechanistic investigations were per- the intermediate formation of a self-condensation
formed. For a better understanding of the moisture product 20 of the p-activated alcohol 1, which is sub-
tolerance of the calcium catalyst we ran a series of re- sequently transformed into the desired amination
actions in the presence of different amounts of water product (Figure 1).[17] The amination step might occur
(Table 5).
through direct attack of the N-nucleophile at the bi-
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Adv. Synth. Catal. 2011, 353, 469 – 474

Calcium-Catalyzed Direct Amination of p-Activated Alcohols
Finally, the aboÀve-mentioned anion displacement of
one of the NTf2 anions of the Ca
(NTf2)2 calcium
À
source by a PF6 anion originating from the additive
was analyzed by 19F NMR spectroscopy (Figure 2).
For this purpose, solutions of the additive Bu4NPF6
and the 1:1 mixture of additive and calcium-source
CaACTHUNTGRNENUG(NTf2)2 in CDCl3 were prepared in concentrations
similar to the reaction conditions and spectroscopical-
ly analyzed. The 19F NMR spectrum of the additive
À
alone has, as expected, a doublet for the PF6 anion
centered at À72 ppm (JP, F =711 Hz), which is consis-
tent with the corresponding 31P NMR spectrum (not
shown). Both 31P NMR and 19F NMR spectra of the
1:1 mixture of CaACTHNUTRGNE(NUG NTf2)2 and Bu4NPF6 do not show
Scheme 2. Reversibility of the reaction. Reaction conditions:
5 mol% Bu4NPF6 and 5 mol% Ca(NTf2)2 were added at
room temperature to 5d (0.5 mmol) and 4i (0.5 mmol) in
CH2Cl2 (1.5 mL) and stirred for the time indicated. The
equilibrium distribution was determined by GC analysis.
À
any of the afore-mentioned PF6 signals. The
31P NMR spectrum is devoid of any signal (not
shown) and the 19F NMR spectrum has a new singlet
at À79 ppm, which corresponds very accurately to the
signal for the CF3 moiety in Bu4NNTf2 (see Figure 2).
We assume that the NMR-signals of the counterions
AHCTUNGTRENNUNG
sallyl ether 20 or, as the formation of this ether is as- of the calcium cation disappear due to microaggrega-
sumed to be yet another equilibrium, the addition of tion effects, thus clearly indicating for the accom-
a molecule of water resulting in the regeneration of plished anion exchange.
cyclohexenol 1 followed by amination is also conceiv-
able as a reaction pathway.
Conclusions
In conclusion, we have developed a new and efficient
calcium-catalyzed direct amination of p-activated al-
Figure 1. Intermediary formation of self-condensation prod-
uct 20. Reaction conditions: 5 mol% Bu4NPF6 and 5 mol%
Figure 2. 19F NMR-spectroscopic analysis of the formation
of CaNTf2PF6 via anion exchange. Reaction conditions:
4.8 mg Bu4NPF6, 4.8 mg Bu4NPF6 and 7.5 mg CaACTHNUGRETNNU(G NTf2)2, as
Ca
A
and 4d (1.5 mmol) in CH2Cl2 (3 mL) and stirred for the time
indicated. Reaction control was performed by GC analysis.
well as 6.5 mg Bu4NNTf2 were stirred for 1 h at room tem-
perature in CDCl3 prior to 19F NMR spectroscopic analysis.
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Stefan Haubenreisser and Meike Niggemann
FULL PAPERS
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very mild reaction conditions. The high reactivity of
the calcium catalyst allows for an efficient conversion
of secondary and tertiary benzylic and allylic as well
as tertiary propargylic alcohols. Typical reactions pro-
ceed at room temperature, with no added strong acids
or bases, and special precautions for exclusion of
moisture or air are unnecessary.
Experimental Section
General Procedure for the Amination of Alcohols
The alcohol (0.5 mmol) and the nucleophile (0.75 mmol)
were dissolved in 1.5 mL of dichloromethane. Bu4NPF6
(5 mol%) and CaACHTUNGTRENNUNG(NTf2)2 (5 mol%) were added at room
temperature and stirred until conversion of the alcohol was
complete (monitored by TLC). For the isolation of the prod-
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aqueous phase was extracted with dichloromethane, the
combined organic extracts dried over Na2SO4 and concen-
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Adv. Synth. Catal. 2011, 353, 469 – 474