Palladium-catalyzed intramolecular hydroamination of allenes coupled to aerobic alcohol oxidation

Article abstract of DOI:10.1002/chem.200802381

The palladium-catalyzed intramolecular hydroamination of allenes was investigated under a dioxygen atmosphere where bathocuproine was used as the ligand. The study tested the reaction of N-(y-allenyl)tosylamide with the proposed Pd-hydride species formed

Full text of DOI:10.1002/chem.200802381

COMMUNICATION
DOI: 10.1002/chem.200802381
Palladium-Catalyzed Intramolecular Hydroamination of Allenes Coupled to
Aerobic Alcohol Oxidation
Shuifa Qiu,[a, b] Yunyang Wei,[b] and Guosheng Liu*[a]
The direct addition of a nitrogen–hydrogen bond across
an unsaturated carbon–carbon bond (hydroamination) is a
powerful strategy to generate nitrogen–carbon bonds and an
effective way to synthesize biologically active nitrogen-con-
taining heterocycles.[1] The development of catalyst systems
for this type of transformation has been extensively studied.
Although lanthanides[2] and Group IV transition metals[3]
are among the most reactive catalyst systems, the synthetic
utility of these protocols is compromised by the poor func-
tional group compatibility and excessive air and moisture
sensitivity. In contrast, late transition metals have been
shown to have better tolerance. For instance, some late tran-
sition metals catalyze the addition of amines to vinylar-
enes;[4] the catalysis of the addition of amides and sulfona-
mides to alkenes, allenes, and dienes[5,6] has also been re-
ported recently. Most late-transition-metal-catalyzed hydro-
aminations require the use of phosphine ligands, and, hence,
are not compatible with aerobic reaction conditions owing
to rapid degradation of phosphine ligands. On the other
hand, nitrogen-containing ligands, which are generally stable
under oxidative reaction conditions,[7] have rarely been used
in hydroamination reactions.[8] Herein, we report the first
PdII-catalyzed intramolecular hydroamination of allenes
under a dioxygen atmosphere, in which bathocuproine[9] is
used as the ligand.
ligands, have been developed for the aerobic oxidation of al-
cohols[10] and in which a PdII–hydride species was generated
as an intermediate.[11] However, the PdII–hydride species is
generally considered to undergo rapid reductive elimination
to yield Pd0, especially in the presence of base (Scheme 1,
Path a).[12] We reasoned that if allenes could coordinate to
the PdII center before the PdII–hydride intermediate under-
goes reductive elimination, a p-allyl–PdII intermediate could
[13,14]
À
form through the allene insertion into Pd H bond.
Sub-
sequent intramolecular attack by a nitrogen nucleophile
(e.g., tosylamide) at the p-allyl–PdII intermediate would
then lead to the corresponding hydroamination product and
a Pd0 species, which could readily be reoxidized by molecu-
lar dioxygen to complete the catalytic cycle (cf. Scheme 1,
Path b).[15]
During the past ten years, many effective palladium cata-
lyst systems, which have been based on nitrogen-containing
[a] S. Qiu, Prof. Dr. G. Liu
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academic of Science, 354 Fengling Road
Shanghai, 200032 (China)
Scheme 1. Hypothesis of hydroamination of allenes.
Fax : (+86)21-64166128
E-mail: gliu@mail.sioc.ac.cn
We set out to examine the above hypothesis by testing the
reaction of N-(g-allenyl)tosylamide (1a) with the proposed
PdII–hydride species formed in aerobic oxidation of isopro-
pyl alcohol. Several nitrogen-containing ligands that have
been reported in palladium-catalyzed aerobic alcohol oxida-
tion were tested in our initial studies. Treatment of 1a
[b] S. Qiu, Prof. Dr. Y. Wei
Department School of Chemical Engineering
Nanjing University of Science and Technology
200 Xiaoling wei, Nanjing, 200094 (China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200802381.
(0.1 mmol) with PdACHTUNGTRENNUNG
Chem. Eur. J. 2009, 15, 2751 – 2754
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(10 mol%) in isopropyl alcohol (1 mL) under 1 atm O2 af-
forded cyclic hydroamination product E-2a with high regio-
selectivity in low yield (26%; Table 1, entry 1), along with
Based on the optimized reaction condition, the substrate
scope of this hydroamination reaction was then investigated
with a variety of allene substrates. Substrates bearing differ-
ent protecting groups were tested. The reaction with nosyl-
AHCTUNGTRENNUNG
Table 1. Screen results of palladium-mediated hydroamination of alle-
ne.[a]
AHCTUNGTRENNUNG
À
which has an acidic N H proton, was treated under stan-
dard conditions, no reaction occurred (entry 3). These obser-
vations could be attributed to the inhibitory effect of acidic
proton on aerobic alcohol oxidation.[18] A number of inter-
nal allene substrates bearing two or three substituents (1d–i)
underwent effective intramolecular hydroamination to
afford the corresponding products with high regioselectivity,
and the configuration of the resulting double bonds in the
products was exclusively trans (entries 4–9). For allene 1 f,
the mixture of 2 f and 2 f’ (ratio of 1:1.2) was generated due
to alkene isomerization (entry 6). It is worth noting that the
substrates 1j and 1k, bearing hydroxyl and ester groups, re-
spectively, both afforded hydroamination products in good
yields, indicating that this catalytic system has better func-
tional group tolerance (entries 10 and 11). For terminal
allene 1l, compound 2l was obtained in a slightly low yield
(41%; entry 12). However, terminal allene substrates bear-
ing two substituents (1m and 1n) afforded moderate yields
of the five-membered cylic products (entries 13 and 14). For
the reaction of 1o, which has one more carbon atom teth-
ered between amide and allene, the substituted piperidine
product 2o was obtained in 61% yield (entry 15).
To explore our initial mechanism hypothesis, two deuteri-
um-labeled solvents were used to determine the origin of
the proton incorporated into the cyclic product. When
(CH3)2CHOD was used as solvent, no deuterium was incor-
porated into the product [Eq. (1)]. In the case of
(CD3)2CDOD, the reaction afforded hydroamination prod-
uct [D1]-2a in 78% yield and 100% deuterium incorpora-
tion [Eq. (2)]. These observations are consistent with the hy-
pothesis that the intermediate PdII-hydride is generated
from b-hydride elimination of alkoxypalladium species in
Pd-catalyzed alcohol oxidation,[11] rather than from oxidative
addition of HX (acid proton) to Pd0 center.[13,19]
Entry [Pd]
Ligand
(OAc)2 (À)-sparteine
(OAc)2 BC[f]
(OAc)2 pyridine
(OAc)2 Et3N
(OAc)2 bipy
(OAc)2 phen[g]
Additive Solvent Yield [%][b]
1
2
3
4
5
Pd
Pd
Pd
Pd
Pd
Pd
Pd
U
–
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
iPrOH
MeOH
nPrOH
BnOH
toluene
26
24
0
–
–
E
–
0
N
–
trace
6
7
–
–
trace
0
A
–
8
9
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
BC
BC
BC
BC
BC
BC
BC
BC
BC
BC
BC
–
50
MA[h]
BQ[i]
NQ[j]
MA
MA
MA
MA
MA
MA
MA
67
35
15
83
56
0
27
69
10
0
10
11
12[c]
13[c,d]
14[c,e]
15
16
17
18
[a] Reaction conditions: 1a (0.1 mmol), [Pd] (5 mol%), ligand
(10 mol%), NaOAc (1 equiv), solvent (1 mL) at 608C. [b] GC yield, tet-
radecane as internal standard. [c] iPrOH (2 mL). [d] Under air. [e] Under
N2. [f] BC=bathocuproine. [g] phen=1,10-phenanthroline. [h] MA=
maleic anhydride. [i] BQ=1,4-benzoquinone. [j] NQ=1,4-naphthoqui-
none.
the formation of acetone. Utilization of bathocuproine (BC,
10 mol%) as a ligand led to the similar result (entry 2).
Other commonly used nitrogen-containing ligands such as
pyridine (Py), triethyl amine (TEA), bipyridine (Bipy) and
1,10-phenanthroline (phen) were not effective (entries 3–6).
No reaction occurred in the absence of ligand (entry 7).
During the optimization of reaction conditions, we found
that the reaction with bathocuproine afforded better repro-
ducibility than (À)-sparteine. Palladium source screen indi-
cated that PdCl2 was a more efficient catalyst than Pd-
ACHTUNGTRENNUNG(OAc)2 (entry 8). Alkenes with electron-withdrawing
groups[16] were tested as additives in this reaction; maleic an-
hydride (MA, 20 mol%) was found to give better result
(67%, entry 9) than other additives, such as benzoquinone
(BQ) and 1,4-naphthoquinone (NQ) (entries 10 and 11).
Lower substrate concentration seemed beneficial to this re-
action: the reaction of 1a gave the best yield (83%) when
the reaction was conducted in a 0.05m solution of isopropyl
alcohol (entry 12). The same reaction under air still afforded
the hydroamination product in moderate yield (entry 13).
No reaction occurred under
a
nitrogen atmosphere
(entry 14). Finally, isopropyl alcohol was proven to be supe-
rior to other alcohols, such as methanol, n-propanol and
benzyl alcohol (entries 15–17). It is also worth noting that
no reaction was observed in toluene (entry 18).[17]
To confirm the formation of the proposed intermediate p-
allyl–PdII species, chiral substrate (S)-1d was synthesized
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Chem. Eur. J. 2009, 15, 2751 – 2754

Palladium-Catalyzed Hydroamination
COMMUNICATION
Table 2. Pd-catalyzed hydroamination of allenes.[a]
allenes using nitrogen-based li-
Entry
Substrate (1)
Product (2)
Yield [%][b]
gands. The reaction is initiated
by PdII-mediated alcohol oxida-
tion, followed by allene inser-
tion into a PdII–hydride inter-
mediate to afford p-allyl–PdII
intermediate. Subsequent intra-
molecular nucleophilic attack
by amide leads to the corre-
sponding hydroamination prod-
1
2
1a Z=Ts
1b Z=Ns
2a
2b
82
20[g]
3
1c
2c
0
4
1d R1 =C5H11
1e R1 =iPr
2d
2e
72
71
5[c,d]
2 f+
2 f’
64 (1:1.2)[h]
uct and
transformation is performed
under aerobic conditions
a
Pd0 species. This
6[c]
1 f
wherein the reoxidation of Pd0
species by O2 is necessary to
complete the catalytic cycle. In-
vestigation of asymmetric hy-
droamination is in progress.
7
1g R3 =H
2g
2h
85
78
8[c]
1h R3 =Me
9[c,e]
1i
2i
67
10[c]
11
1j R3 =CH2OH
1k R3 =CH2OAc
2j
2k
62 (2:1)[i,j]
69 (5:3)[i]
Experimental Section
A
representative procedure: PdCl2
(0.005 mmol, 5 mol%), bathocuproine
(0.01 mmol, 10 mol%), maleic anhy-
dride (0.02 mmol, 20 mol%), NaOAc
(0.1 mmol, 100 mol%), and allene 1
(0.1 mmol) were combined in a glass
tube. The tube was evacuated under
reduce vacuo and refilled with O2 five
times. Then isopropyl alcohol (iPrOH,
2 mL) was added. The mixture was
stirred under O2 at room temperature
for 30 min and then heated up to
608C. The reaction was monitored by
thin-layer chromatography. After the
reaction was completed, the solvent
was concentrated and the residue was
purified by silica gel column chroma-
tography to give the corresponding hy-
droamination product 2.
12
1l R2 =H
2l
41
13[c]
14[c]
1m R2 =Me
2m
2n
69
52
1n R2 =C5H11
15[f]
1o
2o
61
[a] Reaction conditions: 1 (0.1 mmol), PdCl2 (5 mol%), BC (10 mol%), MA (20 mol%) and NaOAc (1 equiv)
in iPrOH (2 mL) at 608C under 1 atm O2. [b] Isolated yield; [c] MA (10 mol%). [d] 13% 1e was recovered.
[e] 508C; [f] 758C. [g] 50% 1b was recovered. [h] The ratio of 2 f and 2 f’. [i] The ratio of cis and trans isomers.
[j] 13% substituted tetrahydrofuran product was obtained.
and submitted under standard condition. A racemic product
is expected if the p-allyl–PdII species was involved as a key
intermediate.[20] The reaction of (S)-1d (85% ee) indeed af-
forded racemic product 2d in 75% yield [Eq. (3)], which
strongly supports our proposed mechanism which features a
Acknowledgements
p-allyl–PdII complex as a key intermediate (Scheme 1, path
We are grateful for financial support from the Chinese Academy of Sci-
ences, the National Nature Science Foundation of China (No. 20872155),
the National Basic Research Program of China (973-2009CB825300) and
the Science and Technology Commission of the Shanghai Municipality
(Grant No. 07JC14063 and 08J14116).
b).
Keywords: alcohols · allenes · hydroamination · aerobic
oxidation · palladium
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Chem. Eur. J. 2009, 15, 2751 – 2754
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www.chemeurj.org
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Received: November 17, 2008
Published online: February 9, 2009
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Chem. Eur. J. 2009, 15, 2751 – 2754