Ruthenium-catalyzed hydroamination of aminoallenes: An approach to vinyl substituted heterocycles

Article abstract of DOI:10.1002/adsc.201400776

Heterosubstituted aminoallenes underwent smooth ruthenium-catalyzed intramolecular exo-hydroamination reactions yielding the corresponding five-, six-, or seven-membered 1,3-diaza- or 1,3-oxaza-heterocyclic structures. This procedure is a valuable and less expensive alternative to the already known transition metal-catalyzed hydroamination reactions of aminoallenes.

Full text of DOI:10.1002/adsc.201400776

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DOI: 10.1002/adsc.201400776
Ruthenium-Catalyzed Hydroamination of Aminoallenes: an
Approach to Vinyl Substituted Heterocycles
Gianluigi Broggini,a,* Giovanni Poli,b,c,* Egle M. Beccalli,d Filippo Brusa,a
Silvia Gazzola,a and Julie Obleb,c
a
Dipartimento di Scienza e Alta Tecnologia, Universitꢀ dell’Insubria, via Valleggio 11, 22100 Como, Italy
Fax : (+39)-031-238-6449; Phone: (+39)-031-238-6444; e-mail: gianluigi.broggini@uninsubria.it
Sorbonne Universitꢁs, UPMC Univ Paris 06, UMR 8232, Institut Parisien de Chimie Molꢁculaire, FR2769 Institut de
b
Chimie Molꢁculaire, 4 Place Jussieu, 75005 Paris, France
E-mail: giovanni.poli@upmc.fr
CNRS, UMR 8232, Institut Parisien de Chimie Molꢁculaire, 75005, Paris, France
c
d
DISFARM, Sezione di Chimica Organica “A. Marchesini”, Universitꢀ di Milano, via Venezian 21, 20133 Milano, Italy
Received: August 6, 2014; Revised: December 19, 2014; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400776.
Abstract: Heterosubstituted aminoallenes under-
went smooth ruthenium-catalyzed intramolecular
exo-hydroamination reactions yielding the corre-
sponding five-, six-, or seven-membered 1,3-diaza-
or 1,3-oxaza-heterocyclic structures. This procedure
is a valuable and less expensive alternative to the
already known transition metal-catalyzed hydroami-
nation reactions of aminoallenes.
gate cyclizations of aminoallenes through ruthenium
catalysis, as the cost of this metal favorably compares
with that of most other noble metals. These studies al-
lowed the development of a concise method toward
vinyl substituted 1,3-diaza- and 1,3-oxaza- heterocy-
clic structures, resulting from the selective intramolec-
ular hydroamination of aminoallenes at the internal
unsaturation of the allene function (Figure 1). To the
best of our knowledge, Ru-catalyzed hydroaminations
of allenes are still unknown.
Keywords: allenes; heterocycles; homogeneous cat-
alysis; hydroamination; ruthenium
Our first investigations were accomplished on O-al-
lenyl N-Boc-2-aminoethanol (1a), which was submit-
ted to the conditions collected in Table 1. Taking
a cue from literature data involving intramolecular
aminations of carbon-carbon multiple bonds, the first
Intramolecular hydroaminations, enabling the forma- experiments were carried out with the system of
tion of a carbon-nitrogen and a carbon-hydrogen RuCl3 (1 mol%), dppe (1 mol%), K2CO3 (2 equiv),
bond through N-H addition across a double bond, and allyl acetate (4 equiv) in acetonitrile at 608C.[13,14]
represent direct and efficient methods for the synthe- These conditions brought about a clean exo-hydroa-
sis of nitrogen-containing heterocycles.[1] In particular, mination of the internal allenic unsaturation of 1a, af-
transition metal-catalyzed aminoallene cyclizations[2] fording 2-vinyl-oxazolidine 2 in 80% yield (Table 1,
show a rich chemistry and represent an excellent entry 1). As proven by further experiments, the hy-
choice to this synthetic purpose,[3] with a special men- droamination did not proceed in the absence of either
tion to palladium and gold catalysis.[4,5,6]
the bidentate phosphine ligand (Table 1, entry 2), or
Ruthenium complexes have been recently recog- of allyl acetate (Table 1, entry 3). This latter was suc-
nized as useful tools for the synthesis and the func- cessfully replaced by a stoichiometric amount of
ꢀ
tionalisation of heterocycles through C H bond acti- CuCl2, with a slight yield improvement to 90%
vation.[7,8] Ru-catalyzed allene cyclizations have been
also reported.[9] However, only in rare cases intramo-
lecular reactions of aminoallenes were concerned,
such as in the case of cyclocarbonylations[10] and
domino reactions.[11]
In the context of our ongoing interest in the synthe-
sis of heterocycles through transition metal-catalyzed
[12]
ꢀ
C N bond-forming reactions, we decided to investi- Figure 1. Concept of this work.
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Table 1. Hydroamination of O-allenyl N-Boc-2-aminoethanol
Entry
Catalyst/ligand[a]
Additive
Solvent
T [8C]
t [h]
Yield of 2
1
2
3
4
5
6
7
8
RuCl3 (1 mol%), dppe
RuCl3 (1 mol%)
RuCl3 (1 mol%), dppe
RuCl3 (1mol%), dppe
dppe
K2CO3 (2 equiv), allyl acetate (4 equiv)
K2CO3 (2 equiv), allyl acetate (4 equiv)
K2CO3 (2 equiv)
K2CO3 (2 equiv), CuCl2 (1 equiv)
K2CO3 (2 equiv), allyl acetate (4 equiv)
K2CO3 (2 equiv), CuCl2 (1 equiv)
K2CO3 (2 equiv), N2
K2CO3 (2 equiv), CuCl2 (1 equiv)
K2CO3 (2 equiv), CuCl2 (1 equiv)
CuCl2 (1 equiv)
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
THF
dioxane
DMF
DCE
MeCN
MeCN
MeCN
60
60
60
60
60
60
60
60
60
60
60
60
90
100
80
60
60
60
2
80%
5%
ꢀ
24
24
2
24
2
2
2
24
2
2
18
6
18
18
24
2
90%
ꢀ
dppe
ꢀ
RuCl3 (1 equiv), dppe
RuCl3 (1 mol%), dppp
RuCl3 (1 mol%), PPh3
RuCl3 (1 mol%), dppe
RuCl3 (1 mol%), dppe
RuCl3 (1 mol%), dppe
RuCl3 (1 mol%), dppe
RuCl3 (1 mol%), dppe
RuCl3 (1 mol%), dppe
RuCl3 (1 mol%), dppe
RuCl3 (1 mol%), dppe
RuCl3 (1 mol%), chiraphos
ꢀ
86%
ꢀ
9
10
11
12
13
14
15
16
17
18
ꢀ
allyl acetate (4 equiv)
ꢀ
K2CO3 (2 equiv), CuCl2 (1 equiv)
K2CO3 (2 equiv), CuCl2 (1 equiv)
K2CO3 (2 equiv), CuCl2 (1 equiv)
K2CO3 (2 equiv), CuCl2 (1 equiv)
K2CO3 (2 equiv), O2
41%
8%
5%
5%
20%
69%
47%
K2CO3 (2 equiv), PhI(OAc)2 (1 equiv)
K2CO3 (2 equiv), CuCl2 (1 equiv)
2
[a]
1 mol%
(Table 1, entry 4). As expected, RuCl3 was also essen- imidazolidine derivatives 3 and 4. These conditions
tial (Table 1, entries 5 and 6). are effective also for N-tosyl-protected aminoallenes,
On the other hand, RuCl3 was inactive when em- as disclosed by the formation of the 2-vinyl-oxazole 5.
ployed in stoichiometric amounts in the absence of The same catalytic system allowed also 6-exo-allylic
allyl acetate or CuCl2 (Table 1, entry 7). While the use hydroaminations, as proven by the isolation of the N-
of dppp instead of dppe provided the product in anal- (tert-butoxycarbonyl)-2-vinyl-1,3-oxazine
6
from
ogous yield, the substitution of the diphosphine with allene 1e and by the formation of the quinazolin-4-
PPh3 totally precluded the reaction (Table 1, entries 8 one products 8–11 starting from anthranilic allena-
and 9). The presence of the base is also crucial for the mides 1g–j. The cyclization of allene 1 f to the 1,3-ox-
successful conversion of the substrate (Table 1, en- azepine 7 highlights that 7-exo-allylic processes are
tries 10 and 11). Dioxane, THF, DMF or dichloro- also possible.[16] Finally, internal allenes undergo the
ethane (DCE) instead of acetonitrile did not improve intramolecular hydroamination too, as evidenced by
the yield of the reaction (Table 1, entries 12-15).
transformation of 1k into (E)-alkenyl-1-oxazolidine
As a consequence, acetonitrile was chosen as the 12 as the sole product.
most suitable solvent. Other additives such as dioxy-
In contrast to the previously described heterosubsti-
gen, or PhI(OAc)2 combined with the system of tuted allenes, the corresponding C-substituted allenes
RuCl3/dppe/K2CO3 were able to promote the cycliza- failed to react when submitted to the same reaction
tion, although in lower yields (Table 1, entries 16-17). conditions as above. This was the case for allenes 1l,
Our best conditions, those of entry 4, were also 1m, and 1n (Scheme 2).[17] Curiously, unreactive
tested replacing dppe with two chiral enantiopure allene 1m and reactive allene 1e differ barely for the
phosphines. While (+)-diop gave total degradation, mutual exchange of the oxygen atom and methylene
(ꢀ)-chiraphos afforded the expected product 2 in group, which suggests that the unresponsiveness of
47% yield, but in a virtually racemic form (Table 1, the C-substituted allenes is likely due to electronic ef-
entry 18). Thus, 1,2-diphosphines confirmed to be the fects, more than conformational reasons.[18]
ligands of choice for this transformation, although
enantioselection could not be controlled.
To gain further information on the fundamental
role of the additives, some additional experiments
The optimized conditions (Table 1, entry 4) were were performed. First, we tested the cyclization using
then applied to the intramolecular hydroamination of a catalytic system based on a well-defined rutheniu-
a series of amino-allenamides or amino-alkoxyallenes m(II) species. Accordingly, allene 1a was submitted
(Scheme 1).[15] Accordingly, allenamides 1b,c gave to [RuCl2(p-cymene)]2 (1 mol% or 1 equiv) and dppe
2
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Ruthenium-Catalyzed Hydroamination
Scheme 1. Scope of the Ru-catalyzed intramolecular hydroamination of heterosubstituted allenes.
Scheme 2. Unreactive allenes.
(1 mol%) under the otherwise optimized conditions,
in the presence (3.0 equiv) as well as in the absence
of CuCl2 (Scheme 3, path a and path b). In these ex-
periments, only the latter conditions allowed the reac-
tion to take place (87% yield), which confirmed the
crucial role of CuCl2 as additive. A catalytic system
containing RuCl3 (1 mol%) and CuCl2 in sub-stoi-
chiometric amount (0.2 equiv) was subsequently
tested (Scheme 3, path c). Under these conditions the
reactivity was restored, although the reaction became
more sluggish and gave a reduced yield (48%). Final-
ly, CuCl2 was replaced by LiCl. Accordingly, use of
the system [RuCl2(p-cymene)]2 (1 mol%), dppe
Scheme 3. Investigating the role of the additive in the
cycloisomerization.
(1 mol%), LiCl (2 equiv), and K2CO3 (2 equiv) in
acetonitrile at 608C provided
(Scheme 3, path d).
2 in 51% yield
The ensemble of these results provide us with sev- competent starting catalyst is believed to be a RuII
eral hints about the mechanism of this transforma- complex; besides the metal complex, also the base,
tion: an active catalytic system could be obtained the phosphine, as well as CuCl2 (or LiCl) were all
either starting from [RuIICl2(p-cymene)]2 or using found indispensable for the success of the reaction.
RuCl3 in the presence of dppe, which is expected to The diphosphine[20] is expected to coordinate the
reduce the metal to RuII.[19] As a consequence, the metal via a k2 coordination, while the copper or the
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Gianluigi Broggini et al.
Scheme 4. Postulated mechanisms for the Ru-catalyzed cycloisomerization of aminoallenes.
lithium salt additives provide extra chloride coordina-
In summary, a ruthenium-catalyzed synthesis of
tion to Ru, so as to generate a monomeric reactive vinyl-substituted heterocycles involving intramolecu-
Ru complex. As for K2CO3, it very likely plays lar hydroamination of amino-allenamides and amino-
a proton shuttle role from the nitrogen to the metal alkoxyallenes has been developed. This synthetic pro-
atom.
tocol is a valuable and less expensive alternative to
Although additional studies will be needed to fur- transition metal catalytic systems already reported in
ther elucidate the details of this transformation, we the literature. Efforts are underway to extend this re-
can hypothesize the mechanism depicted in Scheme 4 action to allenes tethered to carbon atoms as well as
for the transformation 1a ! 2. Interaction between to obtain more information on the mechanistic details
the RuII source, the diphosphine and the chloride ad- of this transformation. Moreover, an asymmetric ver-
ditive generates the active monomeric catalyst sion of the cyclization by use of chiral ligands will be
LLRuIIMCl, and ensuing substrate complexation investigated.
through the allene function gives the zwitterionic
complex II, through complex I (steps a and b). In-
volvement of intermediate II is supported by the fact
that non-heterosubstituted allenes (for which such an
Experimental Section
intermediate is not plausible) are not reactive (see
Scheme 2). Subsequent intramolecular addition of the
nitrogen atom onto the oxycarbenium function produ-
General procedure for the ruthenium-catalyzed
hydroamination of the allenes 1a–k
ces the heterocyclic structure III (step c). The subse- RuCl3 (0.01 mmol, 1 mol%), dppe (0.01 mmol, 1 mol%),
K2CO3 (2 mmol, 2 equiv), and CuCl2 (1 mmol, 1 equiv) were
added to a solution of the corresponding allene (1a-k,
1 mmol, 1 equiv) in acetonitrile (5 mL). The solution was
heated to reflux for 2 h, and the solvent was removed under
reduced pressure. The crude product was purified by silica
gel column chromatography.
quent K2CO3 mediated proton transfer from the nitro-
gen atom could take place according to two alterna-
tive mechanisms. A nitrogen-to-ruthenium 1,3-hydro-
gen transfer generates the RuIV hydride IV,[21] whose
reductive elimination gives the product and regener-
ates the catalyst. Alternatively, the product can be
formed via nitrogen-to-carbon 1,3’-hydrogen transfer
from III (step f), to produce the Ru carbenoid species
V, followed by a 1,2-hydrogen shift (step g) and final
Ru decomplexation (step h).[22,23]
3-(tert-Butoxycarbonyl)-2-vinyl-oxazolidine (2): Eluent:
(7:3 light petroleum/AcOEt); Yield: 90%. Colourless oil.
1
IR: 1685 cmꢀ1; H NMR (400 MHz, CDCl3) d=1.40 (s, 9H),
3.30–3.35 (m, 1H), 3.56–3.60 (m, 1H), 3.89–3.98 (m, 2H),
5.21 (d, J=10.3 Hz, 1H), 5.29–5.40 (m, 2H), 5.70–5.77 (m,
4
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1H); 13C NMR (100 MHz, CDCl3) d=28.3 (q), 44.2 (t), 65.3
(t), 80.2 (s), 88.2 (d), 117.7 (t), 134.7 (d), 152.9 (s). MS: m/z
199 (M+). Anal. calcd for C10H17NO3: C, 60.28; H, 8.60; N,
7.03. Found: C, 60.67; H, 8.43; N, 7.39.
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1-(tert-Butoxycarbonyl)-3-methyl-2-vinyl-imidazolidin-4-
one (3): Eluent: (7:3 light petroleum/AcOEt); Yield: 80%.
Yellow oil. IR: 1708, 1671 cmꢀ1
;
1H NMR (400 MHz,
CDCl3) d=1.47 (s, 9H), 2.80 (s, 3H), 3.86 (dd, J=16.0,
0.4 Hz, 1H), 4.01 (d, J=16.0 Hz, 1H), 5.21 (d, J=7.2 Hz,
1H), 5.43 (dd, J=9.9, 0.4 Hz, 1H), 5.45 (dd, J=17.0, 0.4 Hz,
1H), 5.63 (ddd, J=17.0, 9.9, 7.2 Hz, 1H); 13C NMR
(100 MHz, CDCl3) d=26.5 (q), 28.3 (q), 47.8 (t), 76.2 (d),
81.2 (s), 121.3 (t), 133.9 (d), 152.0 (s), 167.7 (s). MS: m/z 226
(M+). Anal. calcd for C11H18N2O3: C, 58.39; H, 8.02; N,
12.38. Found: C, 58.71; H, 7.78; N, 11.94.
Acknowledgements
We thank Universitꢀ dell’Insubria and Sorbonne Universitꢁs
UPMC for financial support. MIUR (project PRIN
2012C5YJSK 001), COST Action CM1205 (CARISMA),
and Labex Michem are kindly acknowledged.
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[16] Although 1,3-oxazepine 7 was unequivocally detected
in the 1H NMR spectrum of the crude material, at-
tempted chromatographic purification brought about
substantial degradation.
[17] For the preparation of allenes 1l–n, obtained according
to Crabbꢁꢄs protocol and described in the Supporting
Information, see: a) P. Rona, P. Crabbꢁ, J. Am. Chem.
Soc. 1968, 90, 4733–4734; b) P. Rona, P. Crabbꢁ, J. Am.
Chem. Soc. 1969, 91, 3289–3292; c) P. Crabbꢁ, H. Fil-
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[18] When treated with Pd(PPh3)4 as catalyst and K2CO3 in
acetonitrile under microwave irradiation, allene 1n un-
derwent 5-exo-allylic cyclization (see Supporting Infor-
mation). This fortifies the idea that the inactivity of the
C-substituted allenes is not due to conformational rea-
sons.
[19] a) M. Rohr, J.-D. Grunwaldt, A. Baiker, J. Mol. Catal.
A 2005, 226, 253–257; b) P. S. Hallman, T. A. Stephen-
son, G. Wilkinson, Inorg. Synth. 1970, 12, 237–240. Ex-
trapolating from the literature, in our case, the follow-
ing redox is likely to be involved: 2RuCl3·(1/2)H2O +
3dppe ! 2k2(dppe)RuCl2 + dppe(O) + 2HCl. For a re-
lated (2eꢀ) redox between Pd(OAc)2 and dppp, see:
c) Amatore, B. Godin, A. Jutand, F. Lemaitre, Chem.
Eur. J. 2007, 13, 2002–2011.
[20] In the experiments with RuCl3, we assume that unoxi-
dized dppe remains in the medium, besides dppe(O)
generated during the metal reduction (see preceding
ref).
[21] H. Nagashima, K. Mukai, Y. Shiota, K. Yamaguchi, K.
Ara, T. Fukahori, H. Suzuki, M. Akita, Y. Morooka, K.
Itoh, Organometallics 1990, 9, 799–807.
[22] For a computational study pointing at such mechanism
in the context of a Au-catalyzed cycloisomerization,
see: R.-X. Zhu, D.-J. Zhang, J.-X. Guo, J.-L. Mu, C.-G.
Duan, C.-B. Liu, J. Phys. Chem. A 2010, 114, 4689–
4696.
[23] At this stage of the study, a different mechanism in-
volving intramolecular syn aminoruthenation of the
allene function from a transiently formed amidoruthe-
nium complex cannot be totally ruled out.
[9] a) B. M. Trost, A. B. Pinkerton, J. Am. Chem. Soc.
1999, 121, 10842–10843; b) B. M. Trost, A. B. Pinkerton,
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3629.
[10] S.-K. Kang, K.-J. Kim, C.-M. Yu, J.-W. Hwang, Y.-K.
Do, Org. Lett. 2001, 3, 2851–2853.
[11] B. M. Trost, A. B. Pinkerton, D. Kremzow, J. Am.
Chem. Soc. 2000, 122, 12007–12008.
[12] a) M. Rigamonti, G. Prestat, G. Broggini, G. Poli, J. Or-
ganomet. Chem. 2014, 760, 149–155; b) G. Broggini, V.
Barbera, E. Beccalli, U. Chiacchio, A. Fasana, S. Galli,
S. Gazzola, Adv. Synth. Catal. 2013, 355, 1640–1648;
c) E. Borsini, G. Broggini, A. Fasana, S. Galli, M.
Khansaa, U. Piarulli, M. Rigamonti, Adv. Synth. Catal.
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[13] RuII-catalyzed oxidative processes usually generate
a Ru dihydride complex. In this case, interaction with
allyl acetate forms propene and acetic acid, bringing
the metal back to its original oxidation state. See: a) T.
Kondo, T. Mukai, Y. Watanabe, J. Org. Chem. 1991, 56,
487–489; b) T. Kondo, T. Okada, T. Mitsudo, J. Am.
Chem. Soc. 2002, 124, 186–187.
[14] Preliminary experiments with other transition metals
met with failure. For example, treatment of 1a with
AgOAc (10 mol%) and K2CO3 in DCE, at reflux for
24 h, gave starting material, while treatment with AuCl
(2.5 mol%) and K2CO3 in DCE, at rt, 16 h, gave only
intractable products.
[15] For the preparation of allenes 1a-k, obtained by base-
promoted prototropic isomerization of the correspond-
ing propargyl derivatives and described in the Support-
ing Information, see: a) W. B. Dickinson, P. C. Lang,
Tetrahedron Lett. 1967, 8, 3035–3040; b) L.-L. Wei,
6
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Ruthenium-Catalyzed Hydroamination of Aminoallenes: an
Approach to Vinyl Substituted Heterocycles
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Adv. Synth. Catal. 2015, 357, 1 – 7
Gianluigi Broggini,* Giovanni Poli,* Egle M. Beccalli,
Filippo Brusa, Silvia Gazzola, Julie Oble
Adv. Synth. Catal. 0000, 000, 0 – 0
ꢂ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
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