Homogeneous catalytic hydroamination of alkynes and allenes with ammonia

Article abstract of DOI:10.1002/anie.200801136

(Chemical Equation Presented) A golden ticket to the synthesis of reactive nitrogen-containing compounds, such as imines, enamines, and allyl amines, through the addition of NH₃ to unsaturated bonds is the cationic cyclic (alkyl)-(amino)carbene-gold(I) catalyst shown in blue (Dipp=diisopropylphenyl). An ideal initial step for the preparation of simple bulk chemicals, this reaction is also useful for the synthesis of more complex molecules (see examples).

Full text of DOI:10.1002/anie.200801136

Communications
DOI: 10.1002/anie.200801136
À
C N Bond Formation
Homogeneous Catalytic Hydroamination of Alkynes and Allenes with
Ammonia**
Vincent Lavallo, Guido D. Frey, Bruno Donnadieu, Michele Soleilhavoup, and Guy Bertrand*
Nitrogen–carbon bonds are ubiquitous in products ranging
from chemical feedstock to pharmaceuticals. As ammonia is
among the least expensive bulk chemicals produced in the
largest volume, one of the greatest challenges of synthetic
chemistry is to develop atom-efficient processes for the
combination of NH3 with simple organic molecules to create
nitrogen–carbon bonds. Transition-metal complexes can read-
ammonia was condensed into a sealable NMR tube contain-
ing A (5 mol%), 3-hexyne, and deuterated benzene. Upon
heating to 1608C for 3.5 h, the clean addition of NH3 afforded
the primary imine 2a, the expected tautomer of the corre-
sponding enamine (Table 1).[17]
Table 1: Catalytic hydroamination of 3-hexyne with ammonia.[a]
À
ily render a variety of N H bonds reactive enough to undergo
functionalization, including those of primary and secondary
amines. However, with a few exceptions,[1,2] metals react with
ammonia to afford supposedly inert Lewis acid–base com-
plexes, as first recognized in the late 19th century by
Werner.[3] Consequently, the homogeneous catalytic function-
alization of NH3 remained elusive[4] until the recent discovery
by Shen and Hartwig[5] and Surry and Buchwald[6] of the
palladium-catalyzed coupling of aryl halides with ammonia in
the presence of a stoichiometric amount of a base. An even
more appealing process would be the addition of NH3 to
carbon–carbon multiple bonds, a process that would occur
ideally with 100% atom economy.[7] Although various homo-
geneous catalysts, including alkali metals,[8] early[9] and late
transition metals,[10] and d-[11] and f-block elements,[12] have
been used to effect the so-called hydroamination reaction,
none of them were reported to be effective when NH3 is used
as the amine partner.[13] Herein we report that cationic gold(I)
complexes supported by a cyclic (alkyl)(amino)carbene
(CAAC)[14] ligand readily catalyze the addition of ammonia
to a variety of unactivated alkynes and allenes to provide a
diverse array of linear and cyclic nitrogen-containing com-
pounds.
Catalyst [mol%]
t [h]
T [8C]Conv. [%]
5.0
1.0
0.1
0.1
3.5
14
17
160
160
160
200
95
95
58
93
20
[a]Dipp =2,6-diisopropylphenyl.
Complex A does not have to be isolated; when it was
prepared in situ from an equimolar mixture of [(CAA-
C)AuCl]/KB(C6F5)4 (A1), identical results were obtained.
When the related silver complex [(CAAC)AgCl]/KB(C6F5)4
or NH4B(C6F5)4 was used as the catalyst, no reaction was
observed, which shows the importance of gold and rules out a
Brønsted acid mediated reaction.[18] Finally, as AuCl, AuCl/
KB(C6F5)4, and even [(CAAC)AuCl] do not induce the
hydroamination, it is clear that the gold center can only
catalyze the addition of NH3 if it is coordinated by the CAAC
ligand and rendered cationic by Cl abstraction.
We showed recently that the cationic CAAC–gold com-
plex A was very robust and exhibited unusual catalytic
reactivity towards alkynes.[15] This discovery prompted us to
investigate whether such a complex could activate alkynes
sufficiently to enable the addition of NH3.[16] Thus, excess
To gain insight into the catalytic process, we performed a
number of experiments: The addition of excess NH3 to
complex A gave the Werner complex B instantaneously; the
addition of 3-hexyne (1a; 1 equiv) to complex A gave the
h2-bound alkyne complex C instantaneously (Scheme 1).
Upon the exposure of a solution of C in benzene to excess
NH3, 3-hexyne was immediately displaced from the gold
center, and the Werner complex B was isolated in quantitative
yield. This result suggests that NH3 does not add to the alkyne
through an outer-sphere mechanism. Importantly, when a
solution of complex B in benzene was treated at room
temperature for 24 h with a large excess of 3-hexyne, the
imine complex D was obtained quantitatively, even when the
reaction vessel was open to a glovebox atmosphere. This
[*]V. Lavallo, Dr. G. D. Frey, B. Donnadieu, Dr. M. Soleilhavoup,
Prof. G. Bertrand
UCR-CNRS Joint Research Chemistry Laboratory (UMI 2957)
Department of Chemistry, University of California
Riverside, CA 92521-0403 (USA)
Fax: (+1)951-827-2725
E-mail: guy.bertrand@ucr.edu
Homepage: http://research.chem.ucr.edu/groups/bertrand/guy-
bertrandwebpage/
[**]We are grateful to the NIH (R01 GM 68825) and RHODIA Inc. for
financial support of this research, the Alexander von Humboldt
Foundation for a Postdoctoral Fellowship (G.D.F.), and the ACS for
a Graduate Fellowship (V.L.).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Angewandte
Chemie
Scheme 1. Experiments to probe the reaction mechanism. The results
suggest that an insertion process is involved, and that B is the resting
state of the catalyst.
experiment implies that NH3 does not dissociate from the
metal by a simple ligand exchange with the alkyne. Therefore,
an insertion mechanism, similar to that proposed by Tanaka
and co-workers[19] and Nishina and Yamamoto[20] for gold-
catalyzed hydroamination with aryl amines, is quite likely.
Finally, the addition of excess NH3 to D liberated the imine 2a
and regenerated complex B. From the results of these
experiments, it can be concluded that the Werner complex
B is the resting state of the catalyst. Indeed, B exhibits
identical catalytic activity to that of A/A1. Consequently, the
robust and readily available complex B (Figure 1)[21] was used
in subsequent experiments.
To test the scope of the reaction, the terminal alkyne 1b
and the diaryl alkyne 1c were treated with NH3 in the
presence of a catalytic amount of complex B (Scheme 2).
With 1b, the reaction took place even at 1108C to afford the
Markovnikov imine 2b exclusively in 60% yield. When
diphenyl acetylene (1c) was used, the 2-aza-1,3-diene 3c was
Scheme 2. Catalytic hydroamination of various alkynes with ammonia.
formed cleanly in 95% yield. The different outcome of the
reaction of 1c with respect to the results with substrates 1a,b
can be rationalized by the presence of acidic benzylic
hydrogen atoms in the imine. These acidic hydrogen atoms
favor the formation of the enamine tautomer, which can then
react further with a second molecule of the alkyne to afford
3c.
Nitrogen heterocycles are an important class of com-
pounds that occur widely in natural products and often
display potent biological activity. On the basis of the results
described above, we attempted the direct synthesis of hetero-
cycles from diynes and NH3. When 1,4-diphenylbuta-1,3-
diyne (1d) and hexa-1,5-diyne (1e) were used, the corre-
sponding 2,5-disubstituted pyrroles 4d and 4e were produced
in 87 and 96% yield, respectively. Both products result from
the Markovnikov addition of NH3, followed by ring-closing
hydroamination.[22,23] The treatment of the 1,4-diyne 1 f with
NH3 under similar conditions led to a 3:2 mixture of the five-
and six-membered heterocycles 5 f and 6 f in 88% yield. The
six-membered ring 6 f arises from two consecutive Markov-
nikov hydroamination reactions, whereas the formation of 5 f
involves an anti-Markovnikov addition of NH3 or ring-closing
step.
To expand the scope of the hydroamination reaction with
NH3, we next tested allenes as substrates. When 1,2-prop-
adiene (7a) was used, a mixture of mono- (8a), di- (9a) and
triallylamine (10a) was obtained in excellent yield. Allyl
amines are among the most versatile intermediates in syn-
thesis and are of industrial importance. For example, the
parent compound 8a, which is produced commercially from
ammonia and allyl chloride, is used in antifungal preparations
Figure 1. Molecular structure of complex B in the solid state. (Hydro-
gen atoms, except those at N1, and the (C6F5)4B anion are omitted for
clarity; ellipsoids are drawn at 50% probability).
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5225

Communications
were loaded into a Wilmad QPV thick-walled (1.4 mm) NMR tube.
C6D6 (0.4 mL) and the internal standard benzyl methyl ether (5 mg)
were added to the mixture. For experiments with a low catalyst
loading (0.1 mol%), 3 mg of the catalyst and 0.1 mL of C6D6 were
used. The NMR tube was connected to a high-vacuum manifold, and
excess NH3 (typically 3–6 equivalents) was carefully condensed at
À608C. For experiments with 1,2-propadiene, the allene was first
condensed, with the subsequent addition of NH3. The tube was sealed,
placed in an oil bath behind a blast shield, and heated at the specified
temperature. (Caution: Sealed NMR tubes containing NH3 and/or
1,2-propadiene are under high pressure and pose an explosion hazard.
Only new tubes with a wall thickness of at least 1.4 mm should be
used).
and the synthesis of polymers. By varying the NH3/allene ratio
it is possible to control the selectivity of this reaction
significantly (Table 2). In particular, the parent allylamine
(8a) and triallylamine (10a) can be obtained with 86 and 91%
Table 2: Catalytic hydroamination of allenes with ammonia.
B: Excess NH3 (approximately 1 mL) was condensed into a
solution of A (1.00 g, 0.74 mmol) in toluene (3 mL) at À508C. The
mixture was stirred for 1 min and then removed from the cold bath.
The excess NH3 was removed, hexane (50 mL) was added, and the
upper portion of the biphasic mixture was removed with a canula. The
oily residue was dried under a high vacuum to afford complex B
(0.90 g, 95%) as a colorless solid. M.p.: 112–1148C; 1H NMR
R
NH3/7 B [mol%] t [h] T [8C]Conv. [%]
8/9/10
1.4:1
40:1
1:4.5
1.2
4.3
1.4
22
16
36
175
175
165
98
96
91
1:4.3:3.7
6.2:1:–
1:6:73
3
(300 MHz, C6D6): d = 1.19 (d, J = 6.7 Hz, 6H, CH(CH3)2), 1.22 (d,
a
b
H
3J = 6.7 Hz, 6H, CH(CH3)2), 1.30 (s, 6H, C(CH3)2), 1.70–1.97 (m,
12H), 2.31 (s, 2H, CH2), 2.53 (br s, 3H, NH3), 2.60 (sept, 3J = 6.7 Hz,
2
2H, CH(CH3)2), 3.50 (d, J = 12.0 Hz, 2H, CH2), 7.18 (d, J = 7.7 Hz,
Me
0.8:1
1.3
22
155
87
1:18:47
2H), 7.35 ppm (t, J = 7.7 Hz, 1H); 13C NMR (75 MHz, C6D6): d =
22.8, 26.8, 27.4, 28.5, 28.8, 29.4, 34.4 (CH2), 35.9 (CH2), 37.3, 39.1
(CH2), 48.1 (CH2), 64.2 (Cq), 78.7 (NCq), 125.6 (CHm), 131.0 (CHp),
i
Ar
Ar
135.5 (c), 135.6 (m, B C ), 137.4 (m, B C ), 138.9 (m, B CAr), 140.8
À
À
À
(m, B C ), 145.1 (c ), 147.9 (m, B C ), 151.1 (m, B CAr), 236.7 ppm
(Ccarbene); HRMS (ESI; CH3CN): m/z calcd for C29H42AuN2: 615.3008
[(MÀNH3+CH3CN)]+; found: 615.3010; m/z calcd for C28H40AuN2:
601.2857 [(MÀNH3+HCN)]+; found: 601.2856.
Ar
o
Ar
À
À
À
selectivity, respectively, and further optimization of the
conditions should be possible. The addition of NH3 to 1,2-
dienes is not restricted to the parent allene 7a. The dialkyl-
substituted derivative 7b was also converted into the corre-
sponding allyl amines 8b–10b, with exclusive addition of the
NH2 group at the less-hindered terminus; however the
selectivity of this reaction for the mono-, di-, or trisubstituted
amine product needs some improvement. Interestingly, even
the tetrasubstituted allene 7c underwent hydroamination
with ammonia. Probably because of steric factors, a different
regioselectivity was observed, and only the monohydroami-
nation product 11c was formed.[24]
The results outlined herein demonstrate that (CAAC)-
gold(I) cations readily catalyze the addition of NH3 to non-
activated alkynes and allenes. This reaction leads to reactive
nitrogen-containing compounds, such as imines, enamines,
and allyl amines, and is therefore an ideal initial step for the
preparation of simple bulk chemicals, as well as rather
complex molecules, as illustrated by the preparation of
heterocycles 4–6. This study paves the way for the discovery
of catalysts that mediate the addition of ammonia to simple
alkenes, a process considered to be one of the ten greatest
challenges for catalytic chemistry.[25]
C: 3-Hexyne (1 equiv) was added to a solution of A (1.00 g,
0.74 mmol) in toluene (3 mL). The mixture was stirred for 1 min, and
then hexane (50 mL) was added. The upper portion of the biphasic
mixture was removed with a canula, and the oily residue was dried
under a high vacuum to afford complex C (0.95 g, 96%) as a colorless
1
3
solid. M.p.: 183–1848C; H NMR (300 MHz, C6D6): d = 0.93 (t, J =
7.5 Hz, 6H, CH2CH3), 1.24 (d, J = 6.7 Hz, 6H, CH(CH3)2), 1.26 (d,
3
3J = 6.7 Hz, 6H, CH(CH3)2), 1.37 (s, 6H, C(CH3)2), 1.73–1.98 (m,
12H), 2.12 (q, 3J = 7.5 Hz, 4H, CH2CH3), 2.38 (s, 2H, CH2), 2.69 (sept,
3J = 6.7 Hz, 2H, CH(CH3)2), 3.33 (d, 2J = 12.7 Hz, 2H, CH2), 7.26 (d,
J = 7.7 Hz, 2H), 7.41 ppm (t, J = 7.7 Hz, 1H); 13C NMR (75 MHz,
C6D6): d = 13.7 (CH2CH3), 15.4 (CH2CH3), 23.2, 26.5, 27.3, 28.4, 28.9,
29.5, 34.2 (CH2), 36.5 (CH2), 37.3, 38.8 (CH2), 48.5 (CH2), 65.2 (Cq),
q
m
p
79.9 (NC ), 87.5 (C C), 126.2 (CH ), 131.3 (CH ), 135.9 (m, B CAr),
ꢀ
À
135.8 (c), 137.5 (m, B C ), 139.1 (m, B C ), 141.0 (m, B CAr), 145.3
i
Ar
Ar
À
À
À
(c ), 148.0 (m, B C ), 151.1 (m, B CAr), 243.9 ppm (Ccarbene); HRMS
o
Ar
À
À
(ESI;
CH3CN):
m/z
calcd for
C29H42AuN2:
615.3008
[(MÀC6H10+CH3CN)]+; found: 615.3011.
D: 3-Hexyne (6.47 g, 78.7 mmol) was added to a solution of B
(0.50 g, 0.39 mmol) in benzene (3 mL). The mixture was stirred for
24 h, and then hexane (100 mL) was added. The upper portion of the
biphasic mixture was removed with a canula, and the oily residue was
dried under a high vacuum to afford complex D (0.50 g, 99%; cis/trans
1
55:45) as a colorless solid. H NMR (300 MHz, CDCl3): d = 0.80 (t,
J = 7.1 Hz, 3H, CH3), 0.88 (t, J = 7.8 Hz, 3H, CH3), 0.89 (t, J = 7.8 Hz,
3H, CH3), 1.07 (t, J = 7.1 Hz, 3H, CH3), 1.29 (d, 3J = 6.7 Hz, 6H,
CH(CH3)2), 1.30 (d, 3J = 6.7 Hz, 6H, CH(CH3)2), 1.33 (br d, 3J =
7.1 Hz, 12H, C(CH3)2), 1.41 (s, 6H, C(CH3)2), 1.42 (s, 6H, C(CH3)2),
1.45–1.55 (br m, J = 7.2 Hz, 4H, CH2), 1.75–2.18 (m, 24H), 2.29 (t, J =
8.2 Hz, 4H, HNC(CH2CH2CH3)), 2.40 (q, J = 7.1 Hz, 4H, HNC-
(CH2CH3), 2.42 (s, 4H, CH2), 2.76 (sept, 3J = 6.7 Hz, 4H, CH(CH3)2),
3.50–3.75 (br m, 4H), 7.30 (d, J = 7.7 Hz, 4H), 7.46 (t, J = 7.7 Hz, 2H),
8.30–8.40 ppm (br s, 2H, NH); 13C NMR (75 MHz, CDCl3): d = 8.6
(CH2CH3), 11.1 (CH2CH3), 13.1 (CH2CH3), 13.8 (CH2CH3), 18.5
(CH2CH3), 20.2 (CH2CH3), 23.0, 23.1, 26.6, 26.7, 26.9, 27.9, 29.2, 29.4,
Experimental Section
All manipulations were performed under an inert atmosphere of
argon by using standard Schlenk techniques. Water- and oxygen-free
solvents were employed.
General procedure: The catalyst B (15 mg) and the appropriate
amount (see Tables 1 and 2 and Scheme 2,) of an alkyne or allene
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Angewandte
Chemie
32.5 (CH2), 33.8 (CH2), 34.3 (CH2), 35.7 (CH2), 37.2, 38.8 (CH2), 40.7
(CH2), 42.4 (CH2), 48.5 (CH2), 64.3 (Cq), 64.4 (Cq), 78.6 (2 ” NCq),
c) N. Hazari, P. Mountford, Acc. Chem. Res. 2005, 38, 839 – 849;
d) I. Bytschkov, S. Doye, Eur. J. Org. Chem. 2003, 935 – 946; e) E.
Smolensky, M. Kapon, M. S. Eisen, Organometallics 2007, 26,
4510 – 4527; f) M. Dochnahl, K. Löhnwitz, J.-W. Pissarek, M.
Biyikal, S. R. Schulz, S. Schön, N. Meyer, P. W. Roesky, S.
Blechert, Chem. Eur. J. 2007, 13, 6654 – 6666.
m
p
p
125.5 (CH ), 130.5 (CH ), 130.6 (CH ), 134.9 (m, B CAr), 135.8 (br,
À
i
Ar
Ar
c), 136.8 (m, B C ), 138.0 (m, B C ), 140.0 (m, B CAr), 145.0 (co),
À
À
À
o
Ar
Ar
À
À
=
145.1 (c ), 146.6 (m, B C ), 150.1 (m, B C ), 199.6 (C N), 200.2
=
(C N), 238.1 (Ccarbene), 238.6 (Ccarbene).
[10] For recent examples, see: a) J. Zhang, C.-G. Yang, C. He, J. Am.
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P. W. Roesky, M. Dochnahl, S. Blechert, Organometallics 2006,
25, 3730 – 3734; b) X. Y. Liu, C. H. Li, C. M. Che, Org. Lett. 2006,
8, 2707 – 2710; c) A. Zulys, M. Dochnahl, D. Hollmann, K.
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[12] a) D. V. Gribkov, K. C. Hultzsch, F. Hampel, J. Am. Chem. Soc.
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Aillaud, J. Collin, J. Hannedouche, E. Schulz, Dalton Trans.
2007, 5105 – 5118; g) M. Rastätter, A. Zulys, P. W. Roesky,
Chem. Eur. J. 2007, 13, 3606 – 3616.
[13] The hydroamination of short-chain alkenes with NH3 has been
reported with zeolites[4d–g] and alkali metals as catalysts.[4h,i]
[14] a) V. Lavallo, Y. Canac, C. Präsang, B. Donnadieu, G. Bertrand,
Angew. Chem. 2005, 117, 5851 – 5855; Angew. Chem. Int. Ed.
2005, 44, 5705 – 5709; b) V. Lavallo, Y. Canac, A. DeHope, B.
Donnadieu, G. Bertrand, Angew. Chem. 2005, 117, 7402 – 7405;
Angew. Chem. Int. Ed. 2005, 44, 7236 – 7239; c) R. Jazzar, R. D.
Dewhurst, J. B. Bourg, B. Donnadieu, Y. Canac, G. Bertrand,
Angew. Chem. 2007, 119, 2957 – 2960; Angew. Chem. Int. Ed.
2007, 46, 2899 – 2902.
[15] a) V. Lavallo, G. D. Frey, S. Kousar, B. Donnadieu, G. Bertrand,
Proc. Natl. Acad. Sci. USA 2007, 104, 13569 – 13573; b) G. D.
Frey, R. D. Dewhurst, S. Kousar, B. Donnadieu, G. Bertrand, J.
Organomet. Chem. 2008, 693, 1674 – 1682..
[16] For recent reviews on gold chemistry, see: a) R. A. Widenhoefer,
X. Han, Eur. J. Org. Chem. 2006, 4555 – 4563; b) A. S. K.
Hashmi, Chem. Rev. 2007, 107, 3180 – 3211; c) A. Fürstner,
P. W. Davies, Angew. Chem. 2007, 119, 3478 – 3519; Angew.
Chem. Int. Ed. 2007, 46, 3410 – 3449; d) D. J. Gorin, F. D. Toste,
Nature 2007, 446, 395 – 403.
[17] Primary imines are valuable intermediates for a variety of
transformations, as exemplified by the dual-metal-catalyzed
hydroaminomethylation with ammonia to give terminal primary
amines via aldimines: B. Zimmermann, J. Herwig, M. Beller,
Angew. Chem. 1999, 111, 2515 – 2518; Angew. Chem. Int. Ed.
1999, 38, 2372 – 2375. Interestingly, the hydrogenation of keti-
mines 2a would lead to the complementary internal primary
amine.
[18] A fewexamples of acid-catalyzed hydroamination reactions
(without NH3) have been reported: a) B. Schlummer, J. F.
Hartwig, Org. Lett. 2002, 4, 1471 – 1474; b) K. Motokura, N.
Nakagiri, K. Mori, T. Mizugaki, K. Ebitani, K. Jitsukawa, K.
Kaneda, Org. Lett. 2006, 8, 4617 – 4620; c) D. C. Rosenfeld, S.
Shekhar, A. Takemiya, M. Utsunomiya, J. F. Hartwig, Org. Lett.
Received: March 8, 2008
Published online: June 4, 2008
Keywords: alkynes · ammonia · gold · hydroamination ·
.
nitrogen heterocycles
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[2] A nonmetallic system was reported recently to cleave NH3 under
mild experimental conditions: G. D. Frey, V. Lavallo, B. Donna-
dieu, W. W. Schoeller, G. Bertrand, Science 2007, 316, 439 – 441;
see also: a) A. L. Kenward, W. E. Piers, Angew. Chem. 2008, 120,
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