Rhodium- and palladium-catalyzed hydroarylation of propargylic amines with arylboronic acids

Article abstract of DOI:10.1002/adsc.200900773

The hydroarylation of 3-arylprop-2-yn-1-amine derivatives with arylboronic acids has been studied in the presence of rhodium or palladium catalysts. By using a rhodium-based catalytic system, β,γ-diarylallylamines were isolated in good yields. By contrast, the formation of the regiosomeric γ,γ-diar-ylallylamines was achieved by means of a palladium catalyst in the presence of acetic acid. The complementary regioselectivity displayed by these two processes is a consequence of different catalytic cycles, involving respectively carborhodation or hydropalladation of the coordinated alkyne as key steps. Calculated charge distributions in the π-complexes are in accord with the results obtained.

Full text of DOI:10.1002/adsc.200900773

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DOI: 10.1002/adsc.200900773
Rhodium- and Palladium-Catalyzed Hydroarylation of
Propargylic Amines with Arylboronic Acids
Antonio Arcadi,a Massimiliano Aschi,a Marco Chiarini,b Giovanni Ferrara,a
and Fabio Marinellia,*
a
Dipartimento di Chimica, Ingegneria Chimica e Materiali, Universitꢀ dell’Aquila, via Vetoio, I-67100 L’Aquila, Italy
Fax : (+39)-086-243-3752; phone: (+39)-347-983-6282; e-mail: fabio.marinelli@univaq.it
Dipartimento di Scienze degli Alimenti, Universitꢀ degli Studi di Teramo, via Carlo R. Lerici 1, I-64023 Mosciano
b
Sant’Angelo (TE), Italy
Received: November 6, 2009; Published online: February 10, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900773.
Abstract: The hydroarylation of 3-arylprop-2-yn-1- cesses is a consequence of different catalytic cycles,
amine derivatives with arylboronic acids has been involving respectively carborhodation or hydropalla-
studied in the presence of rhodium or palladium cat- dation of the coordinated alkyne as key steps. Calcu-
alysts. By using a rhodium-based catalytic system, lated charge distributions in the p-complexes are in
b,g-diarylallylamines were isolated in good yields. By accord with the results obtained.
contrast, the formation of the regiosomeric g,g-diar-
ACHTUNGTRENNUNGylallylamines was achieved by means of a palladium
catalyst in the presence of acetic acid. The comple- Keywords: alkynes; arylboronic acids; hydroaryla-
mentary regioselectivity displayed by these two pro- tion; palladium; propargylic amines; rhodium
Introduction
egy to the synthesis of butenolides[8] and quinolines[9].
Although some variation on the details of the catalyt-
The regio- and stereoselective synthesis of multisub- ic cycle have been proposed by different authors (i.e.,
stituted olefins represents an important theme in or- the occurrence of a 1-4 rhodium shift has been report-
ganic synthesis. The transition metal-catalyzed hydro- ed to occur in dioxane-water),[4] the reaction product
arylation of carbon-carbon triple bonds is one of the is generally considered to be the result of protonolysis
most efficient procedures towards this goal.[1]
of the s-vinylrhodium complex derived by the addi-
Besides unsaturated halides/triflates[2] and arene- tion of an arylrhodium (I) intermediate across the
ꢀ
ACHTUNGTRENNUNG
agents in this methodology, due to their high stability
The regiochemical outcome of the rhodium-cata-
and functional group compatibility. The rhodium-cata- lyzed hydroarylation of unsymmetrical disubstituted
lyzed hydroarylation of disubstituted alkynes with ar- alkynes has been an object of study. As for phenyl-n-
ylboronic acids 1 in dioxane/water was first reported alkylacetylenes, the major[4] or unique[6] regioisomer
by Hayashiꢁs group[4] and, since then, the reaction has isolated is derived from the addition of the aryl group
been carried out in water using aza-heteroaromatic al- of ArB(OH)2 at the b-position with respect to the
kynes as substrates[5] and in a water-toluene biphasic phenyl group. The replacement of the n-alkyl group
system using a water-soluble ligand;[6] more recently, by a SiMe3 group led to a loss of regioselectivity,
a phosphine-free rhodium complex has been explored whereas silicon has been reported to direct the aryla-
as an effective catalyst.[7] As a consequence of the syn tion of trimethylsilyl-n-alkylacetylenes in the b-posi-
stereochemistry of the rhodium-catalyzed hydroaryla- tion respect to the SiMe3 group.[5] Moreover, alkynoic
tion reaction of disubstituted alkynes, sequential het- acid esters and alkynones were arylated exclusively at
erocyclization can occur in the presence of both a nu- the b-position with respect to carbonyl group, likely
cleophilic and an electrophilic functional group close as a consequence of electronic effects.[4,8,9] With 2-pyr-
ꢀ
to the C C triple bond. We have applied such a strat- idyl substituents on the triple bond, chelation deter-
Adv. Synth. Catal. 2010, 352, 493 – 498
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493

Antonio Arcadi et al.
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mined the regioselective rhodation at the position
closer to the nitrogen atom.[5]
The palladium-catalyzed hyroarylation of alkynes
with arylboronic acids 1 has also been investigated.[10]
Usually, the reaction was carried out in the presence
of acetic acid, and it was proposed that the catalytic
cycle should involve an oxidative addition of a Pd(0)
[11]
À
species to the O H bond of AcOH to give an hy-
dridopalladium acetate. Subsequent hydropalladation
of the alkyne generates a s-vinyl palladium complex,
which undergoes a transmetallation with 1 to give the
reaction product after reductive elimination[10] (see
Scheme 3 in the Results and Discussion section). ESI-
FT-MS data concerning a related process, that is, Pd-
catalyzed hydroarylation of allenes with 1 in the pres-
ence of AcOH, were in agreement with this reaction
mechanism.[12] An alternative reaction path, involving
À
the oxidative addition of Pd(0) into the C B bond of
Ar B(OH)2 followed by arylpalladation of the
À
alkyne/protonolysis/reductive elimination, has also
been suggested.[13] The stereochemical outcome of the
reaction, as in the case of the Rh-catalyzed process,
allowed sequential cyclization to occur.[14] The regio-
selectivity of the addition to unsymmetrical disubsti-
tuted alkynes was found to be influenced by steric
factors and by the coordination with neighbouring
groups (OH or 2-pyridyl);[13] the presence of an EWG
substituent such as a keto[14b] or ester[15] group in the
starting alkyne resulted in a selective b-arylation re-
spect to the carbonyl group, but when the other sub-
stituent was a bulky tertiary alcoholic group both re-
gioisomers were formed. A variety of ligand/solvent
combinations was used in this case to direct the reac-
tion towards the formation of one of the two regioiso-
mers.[14a]
Scheme 1. Hydroarylation of propargylic amines 2a–k with
arylboronic acids 1a–e.
As part of our ongoing research activity in this with arylboronic acids has not been previously ex-
field,[8,9,14b] we envisaged that the transition metal- plored.
mediated hydroarylation of propargylic amines 2
The task of our investigation was therefore to
could represent a versatile new approach to diarylal- achieve the synthesis of both b,g- and g,g-regioisom-
lylamines. It is worth noting that the reaction of prop- ers 3 and 4 by determining the factors which direct
argylic amines bearing a free -NH2 group failed to the regiochemical outcome of the reaction
give the corresponding allylamine in satisfactory yield (Scheme 1).
when aryl iodides were employed as organic electro-
philes in the presence of formate salts and Pd cata-
lysts.[16] Furthermore, the reaction of their correspond- Results and Discussion
ing N-tosylamide or N-trifluoromethanesulfonamide
derivatives led to the isolation of allenes through a We began our study by investigating the rhodium-cat-
completely different reaction pathway involving a b- alyzed hydroarylation of 2a with phenylboronic acid
À
N Pd elimination. The lack of a suitable methodology 1a.
for the direct intermolecular hydroarylation made
The reaction was carried out using different cata-
necessary, for example, the use of a two-step process lyst/solvent/temperature combinations (Table 1). All
(hydrostannylation/Stille coupling with aryl halides) reactions afforded b,g-diarylallylamine 3aa as a single
for the conversion of a propargylic amine derivative isomer (purity >95% by GC-MS analysis), and its
into the g-arylallylamine derivatives that are direct structure was unambiguously assigned by NOE ex-
precursors of a new class of potent antagonists of the periments (see Supporting Information). Under the
human CCR5 receptor.[17] The rhodium- or palladi- hydroarylation conditions reported by Hayashi and
um-catalyzed hydroarylation of propargylic amines co-workers,[4] 3aa was obtained in moderate yield
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Rhodium- and Palladium-Catalyzed Hydroarylation of Propargylic Amines
Table 1. Rh-catalyzed hydroarylation of 2a with PhB(OH)2 1a.[a]
Entry
Catalyst
Rh(acac)
Ligand
Equiv. of 1a
Solvent
3aa% yield[c]
1
2
3
4
5
6
7
8
G
N
dppf
dppf
dppf
dppf
dppf
dppf
dppf
dppp
dppp
dppb
dppe
binap
5
5
3
3
3
3
3
3
2
3
3
3
dioxane/water[b]
dioxane/water[b]
dioxane/water[b]
EtOH
50[d]
70
68
87
88
Rh(acac)
Rh(acac)
Rh(acac)
Rh(acac)
[Rh(cod)OH]2
[Rh(cod)OH]2
[Rh(cod)OH]2
[Rh(cod)OH]2
[Rh(cod)OH]2
[Rh(cod)OH]2
[Rh(cod)OH]2
R
E
R
R
EtOH/water[b]
EtOH/water[b]
EtOH/water[b]
EtOH/water[b]
EtOH/water[b]
EtOH/water[b]
EtOH/water[b]
EtOH/water[b]
N
T
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
N
A
86
82[e]
92[f]
67 g]
76
9
10
11
12
85
70
[a]
Unless otherwise stated reactions were carried out on a 0.25 mmol scale with 3.5 mol% of Rh and 3.5 mol% of ligand,
using 1.5 mL of solvent at 808C under an N2 atmosphere for 8 h.
95:5 v/v.
HPLC yields.
Carried out at 1008C.
Carried out at 608C for 24 h.
Isolated yield.
[b]
[c]
[d]
[e]
[f]
[g]
Reaction time: 16 h.
(entry 1). Carrying out the process at 808C afforded Table 2. Rh-catalyzed hydroarylation of propargylicamines 2
with arylboronic acids 1.[a]
better results (entry 2). The molar excess of 1a can be
reduced to 300% without significative yield lowering
Entry Propargylic
Arylboronic Time Product (Yield
(entry 3). Interestingly, dioxane can be replaced by
the more environmentally friendly ethanol or ethanol/
water mixture 95:5 v/v (entries 4 and 5). The cheaper
amine 2
acid 1
[h]
[%])[b]
1
2
3
4
5
6
7
8
2a
2a
2a
2b
2b
2b
2b
2c
2d
2f
2g
2h
2i
1b
1c
1d
1a
1d
1d
1e
1a
1a
1a
1a
1a
1a
1a
1a
8
24
7
5
4
4
3.5
6
11
5
24
5.5
6
3ab (88)
3ac (84)
3ad (83)
3ba (86)
3bc (81)
3bd (95)
3be (97)
3ca (78)
3da (40)[d]
3fa (89)
3ga (42)[c]
3ha (73)
3ia (49)
3ja (77)
3ka (47)
binuclear [Rh
ACHTUNGTRENNUNG(cod)OH]2 was as efficient as RhACHTUGNTREN(NUGN acac)-
ACHTUNGTRENNUNG
ried out also at 608C with longer reaction times
(entry 7). A screening of some chelating phosphines
revealed that all those tested were effective, and 1,2-
bis(diphenylphosphino)propane (dppp) afforded the
best result (entry 8). Using the latter ligand, a good
yield was obtained even with further reduction of the
molar excess of 1a (entry 9).
The procedure was then extended to include differ-
ent propargylic amine/boronic acid combinations, and
the results are summarized in Table 2.
9
10
11
12
13
14
15
2j
2k
22
4
The rhodium-catalyzed reaction of tertiary, secon-
dary and primary propargylic amine derivatives with
various arylboronic acids afforded the b,g-diarylallyl-
[a]
Reactions were carried out on a 0.5 mmol scale in 3 mL
of ethanol/water 95:5 v/v at 808C under N2 atmosphere
amines 3 in moderate to excellent yields. According
using the following molar ratios: 2:1:[Rh
dppp=1:3:0.0017:0.0035.
Isolated yields.
ACHTUNGTRNEN(UNG COD)OH]2:
[b]
[c]
[d]
2:1: [Rh
ACHTUNGTNER(NUNG COD)OH]2:dppp=1:3:0.0035:0.007.
The regioisomer 4da was obtained in 20% yield
droarylation of 2d was the regioisomer 4da detected a tertiary carbon) the regioselectivity control was ac-
in a significative amount (entry 9). Interestingly, the complished as well as with the substrate 2k, whose
hindrance of propargylic groups did not affect the re- substitution pattern around the triple bond resembles
gioselectivity: that is, with substrates 2j (bearing as that of the aforementioned n-alkylacetylenes.[4,6] Our
substituent an N-benzyl group bonded to a secondary results can be rationalized by taking into account vari-
carbon) and 2i (bearing a free amino group bonded to ous considerations. The general predominance of re-
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Antonio Arcadi et al.
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Scheme 2. Proposed catalytic cycle for the Rh-catalyzed hy-
droarylation of propargylic amines 2.
Scheme 3. Proposed catalytic cycle for the Pd-catalyzed hy-
droarylation of propargylic amines 2.
gioisomer 3 suggests that the reaction outcome is not
under the influence of steric factors: it is conceivable
that, if those factors were determinant, the formation
of regioisomers 4 should be favoured, according to results, using b-(2-aminophenyl)-a,b-ynones as sub-
the general trend observed, for example, in the Pd- strates in the hydroarylation reaction with organobor-
catalyzed hydroarylation of alkynes with aryl halides on compounds, revealed that palladium catalysts in
or vinyl triflates[2] (in which, as in the present process, the presence of AcOH were as efficient as rhodium
carbometallation of coordinated alkyne is the regiose- catalysts.[9,14b] The calculations applied to the p-com-
lectivity-determining step). Coordination of the neigh- plex 8a, deriving from the coordination of propargyla-
bouring amino group to rhodium[5] would, also, result mine 2a to hydridopalladium acetate, showed a strong
in the formation of 4 through the intermediate 7.
positive charge on the palladium atom, while the neg-
As reported in the Introduction section, the pres- ative charge on Cg was higher than that on Cb
ence of a carbonyl group conjugated to the triple (Scheme 3).
bond determined the regioselectivity of the rhodium-
On the basis of this charge distribution, we envis-
catalyzed hydroarylation of internal alkynes;[4,8,9] aged that the use of palladium catalysis could result
showing that electronic effects can be of primary im- in an inversion of the regiochemistry, providing a
portance in controlling the carborhodation step. We straightforward entry into regioisomers 4. To verify
therefore carried out quantum-chemical calculations, this hypothesis, the reaction of 2a with 1a was then
with the aim to evaluate the charge distribution in the carried out, and the results are reported in Table 3.
p-complexes 5 that undergo carborhodation in the The use of 0.15 equivalents of acetic acid with
present case. Atomic point charges of 5aa (derived PdACHTUNRGTNEUNG(OAc)2/dppe in ethanol (Table 3, entry 1) did not
from 2a) are shown in Scheme 2, while the results rel- produce any detectable amount of product, although
ative to other p-complexes are reported in the Sup- these reaction conditions were found to be suitable
porting Information. In all cases the calculated nega- for the hydroarylation of b-(2-aminophenyl)-a,b-
tive charge on Cg was higher than that on Cb, whereas ynones. Likely, the higher basicity of 2a could account
the rhodium atom showed a positive charge and the for this result, determining the deprotonation of
CPh a negative charge. As a consequence, the forma- acetic acid and the reaction inhibition. We therefore
À
À
tion of Cg Rh and Cb Ph bonds should be favoured, increased the amount of AcOH (Table 3, entries 2
giving rise preferentially to s-vinylrhodium intermedi- and 3), and we were pleased to find that 4aa was ob-
ates 6, in accord with the experimental results.
tained as the sole product, although with unsatisfacto-
We next turned out to consider charge distribution ry yields.
for p- complex 8 that should be formed in the palladi-
The structure of 4aa was assigned on the basis of
um-catalyzed hydroarylation of 2 with 1. Our previous the chemical shift of the vinylic proton (that occurs at
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Rhodium- and Palladium-Catalyzed Hydroarylation of Propargylic Amines
Table 3. Pd-catalyzed hydroarylation of 2a with 1a.[a]
served in the presence of tertiary propargylic groups
(entries 1–8). This observation points out that, at least
within the examples tested, steric effects play a minor
role in controlling the reaction outcome.
Entry Catalyst
(equiv. %)
Ligand
(equiv. %) acetic acid
Equiv. of
4aa
Yield[b]
[%]
The results obtained in the Rh- and Pd-catalyzed
reactions are in agreement with the hypothesis that,
in both cases, the main product is generated through
migration of the metal atom towards Cg. According to
this view, the inversion of regioselectivity observed
can be rationalized taking into account the difference
1
2
3
4
P
P
P
P
U
0.1
1.3
1.3
–
20
17[c]
82
[a]
Reactions were carried out on a 0.25 mmol scale in etha-
nol at 808C using 3 equiv. of 1a, unless otherwise stated. between the two catalytic cycles, involving respective-
[b]
[c]
Isolated yield.
ly arylrhodation and hydropalladation of the coordi-
nated triple bond: in the former case the aryl group
migrates to the Cb position, while in the latter it is in-
troduced in the position occupied by the Pd, that is,
Cg, by subsequent transmetallation.
Carried out at 1008C with 5 equiv. of 1a.
higher field with respect to 3aa)[2c] and was confirmed
unambiguously by NOE experiments (see Supporting
Information). Finally, the use of tricyclohexylphos- Conclusions
phine as ligand instead of dppe resulted in a dramatic
improvement of the yield of 4aa (entry 4).
In conclusion, we have developed an efficient proce-
The Pd-catalyzed hydroarylation was then extended dure for the hydroarylation of propargylic amines 1,
to include different propargylic amines and boronic opening a new route towards b,g-diarylallylamines 3
acids (Table 4). Allylamines 4 were isolated in moder- and g,g-diarylallylamines 4. The reaction tolerates a
ate to good yields, and in all the experiments 4:3 variety of functional groups, and can be applied to
ratios (GC/MS analysis) were higher than 96:4. Pd(0) primary, secondary and tertiary propargylic amines.
can be used as catalyst in place of Pd(II) with similar While in the presence of Rh the arylation occurred
results (entries 10 and 11).
mainly on the sp carbon closer to the nitrogen, the
In addition to charge distribution, steric hindrance use of a Pd catalyst under acidic conditions resulted
around the propargylic carbon could be responsible in the complete inversion of regioselectivity, showing
for the observed regioselectivity. Hydroarylation of the complementary character of the two processes. In
propargylic amines 2j and 2k, bearing secondary and both cases, the calculated charge distribution in the
primary propargylic groups afforded, however, prod- intermediate p-complexes that undergo carborhoda-
uct distributions (entries 9–11) similar to that ob- tion or hydropalladation is in accord with the ob-
served regioselectivity.
Table 4. Pd-catalyzed hydroarylation of propargylic amines 2
with arylboronic acids 1.[a]
Experimental Section
Entry Propargylic
Arylboronic Time Product (Yield
General
amine 2
acid 1
[h]
[%])[b]
All reactions were carried out under a nitrogen atmosphere,
and were monitored using T.L.C. and/or GC/MS analysis.
The syntheses of propargylic amines 2a, 2d, 2f and 2h were
previously described.[18] Propargylic amines 2b, 2c, 2e, 2g, 2i,
2j and 2k were obtained in a similar manner by Sonogashira
coupling of the corresponding terminal alkynes with aryl
halides (see Supporting Information).
1
2
3
4
5
6
7
8
2a
2a
2b
2c
2d
2e
2f
2i
1b
1c
1e
1a
1a
1a
1a
1a
1a
1a
1a
3.5
3.5
6
6.5
3.5
8
36
36
4.5
2
4ab (77)
4ac (78)
4be (43)
4ca (85)
4da (78)
4ea (60)
4fa (69)
4ia (54)
4ja (70)
4ka (78)
4ka (80)[c]
General Procedure for the Rh-Catalyzed
Hydroarylation
9
10
11
2j
2k
2k
4
To a solution of propargylic amine 2 (0.5 mmol) in 3 mL of
ethanol/water (95:5 v/v) were added the arylboronic acid 1
[a]
Reactions were carried out on a 0.5 mmol scale in 3 mL
of ethanol at 808C under N2 atmosphere using the fol-
(1.5 mmol), [RhACHTUNGTERNNU(G COD)OH]2 (0.0035 mmol) and dppp
lowing molar ratios: 2:1:AcOH:Pd
1:2:1.2:0.05:0.10.
Isolated yields.
(OAc)2:(Cy)3P=
(0.017 mmol). The mixture was stirred at 808C under N2 at-
mosphere for the appropriate time and monitored by TLC
and/or GC-MS analysis. After completion, the mixture was
purified by chromatography on silica gel eluting with hex-
[b]
[c]
Carried out using 0.0025 equiv. of Pd2ACTHNUTRGNE(UNG dba)3 as Pd source.
Adv. Synth. Catal. 2010, 352, 493 – 498
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Antonio Arcadi et al.
FULL PAPERS
anes/ethyl acetate mixtures to afford the pure derivative 3.
Characterization data for products 3 can be found in the
Supporting Information.
pp 1335–1360; b) A. Arcadi, S. Cacchi, G. Fabrizi, F.
Marinelli, M. Verdecchia, Synlett 2006, 909–915; c) A.
Arcadi, S. Cacchi, F. Marinelli, Tetrahedron 1985, 41,
5121–5131.
[3] S. Cacchi, G. Fabrizi, A. Goggimani, D. Persiani, Org.
Lett. 2008, 10, 1597–1600.
[4] T. Hayashi, K. Inoue, N. Taniguchi, M. Ogasawara, J.
General Procedure for the Pd-Catalyzed
Hydroarylation
To a solution of propargylic amine 2 (0.5 mmol) in 3 mL of
ethanol were added the arylboronic acid 1 (1.5 mmol),
acetic acid (0.6 mmol), PdACTHNUTRGNE(NUG OAc)2 (0.025 mmol) and tricyclo-
hexylphosphine (0.05 mmol). The mixture was stirred at
808C under N2 atmosphere for the appropriate time and
monitored by TLC and/or GC-MS analysis. After comple-
tion, the mixture was purified by chromatography on silica
gel eluting with hexanes/ethyl acetate mixtures to afford the
pure derivative 4. Characterization data for products 4 can
be found in the Supporting Information.
Am. Chem. Soc. 2001, 123, 9918–9919.
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Acknowledgements
This work was supported by the Ministero dell’Universitꢀ e
della Ricerca Scientifica e Tecnologica, Rome, and by the
University of L’Aquila. The authors would like to thank
CASPUR (Roma) for the use of Gaussian 03 program.
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