Palladium-catalyzed highly regioselective Mizoroki-heck arylation of allylamines with aryl chlorides

Article abstract of DOI:10.1002/cctc.201300755

Palladium catalyst systems for the regioselective Mizoroki-Heck arylation of N,N-diprotected and N-protected allylamines with aryl chlorides have been developed. Pd(OAc)₂ in combination with appropriate additives could efficiently catalyze the linear arylation of N,N-diprotected allylamines with aryl chlorides in toluene to give exclusively the γ-arylated products in good to excellent yields and with excellent regio- and stereocontrol and good functional group tolerance. With use of a catalytic mixture of Pd(OAc) ₂ and 1,1'-bis(diphenylphosphino)ferrocene, the arylation of N-protected allylamines with aryl chlorides could be accomplished in ethylene glycol/DMSO, which afforded complete β-regioselectivity with good functional group tolerance. The steric and electronic properties of allylamine substrates are found to be critical to the catalytic activity and regiocontrol in both linear and internal arylations. Copyright

Full text of DOI:10.1002/cctc.201300755

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DOI: 10.1002/cctc.201300755
Palladium-Catalyzed Highly Regioselective Mizoroki–Heck
Arylation of Allylamines with Aryl Chlorides
Lingjuan Zhang,[a] Zhen Jiang,[a] Chaonan Dong,[a] Xiao Xue,[a] Ruiying Qiu,[a] Weijun Tang,[b]
Huanrong Li,*[a] Jianliang Xiao,[b] and Lijin Xu*[a]
Palladium catalyst systems for the regioselective Mizoroki–
Heck arylation of N,N-diprotected and N-protected allylamines
with aryl chlorides have been developed. Pd(OAc)2 in combina-
tion with appropriate additives could efficiently catalyze the
linear arylation of N,N-diprotected allylamines with aryl chlor-
ides in toluene to give exclusively the g-arylated products in
good to excellent yields and with excellent regio- and stereo-
control and good functional group tolerance. With use of a cat-
alytic mixture of Pd(OAc)2 and 1,1’-bis(diphenylphosphino)fer-
rocene, the arylation of N-protected allylamines with aryl chlor-
ides could be accomplished in ethylene glycol/DMSO, which
afforded complete b-regioselectivity with good functional
group tolerance. The steric and electronic properties of allyla-
mine substrates are found to be critical to the catalytic activity
and regiocontrol in both linear and internal arylations.
Introduction
The palladium-catalyzed Mizoroki–Heck arylation of olefins
with aryl halides and pseudohalides has become one of the
most useful tools for constructing CÀC bonds in organic syn-
thesis.[1,2] Over the past several decades, significant advances
have been made through the development of highly efficient
palladium catalyst systems demonstrating high reactivities and
selectivities. However, most of the reports deal with the cou-
pling reactions of aryl bromides and aryl iodides with olefins,
and protocols involving aryl chlorides are relatively underex-
plored even though aryl chlorides are more readily available
and less expensive to access
efficient palladium catalysts that can activate the CÀCl bond of
chloroarenes have proved to be effective in the olefination of
aryl chlorides, the olefin substrates have thus far mostly been
limited to electron-deficient olefins, electron-neutral aliphatic
olefins, and styrenes, with products resulting from arylation at
the less substituted position of the olefin double bond.[4–7] In
the case of electron-rich or deactivated olefins, arylation is
often complicated by the formation of a mixture of regioiso-
mers.[4a,f,7a] This regioselectivity problem arises from two com-
peting reaction pathways (Scheme 1): one is neutral, which
than other aryl halides.[1,2] This
observation is mainly attributa-
ble to the higher strength of the
CÀCl bond, which results in the
well-known resistance of aryl
chlorides to undergo oxidative
addition with palladium cata-
lysts. Therefore, the recent re-
search focus in this area has
shifted to the use of less reactive
aryl chlorides as arylating re-
agents.[3] Although a number of
Scheme 1. Two competing pathways in the Mizoroki–Heck reaction.
[a] L. Zhang, Z. Jiang, C. Dong, X. Xue, R. Qiu, Dr. H. Li, Prof. L. Xu
Department of Chemistry
leads to the terminal arylation product, and the other is ionic,
which favors branched substitution.[8,9] Considerable research
efforts have been directed toward the efficient regiocontrol in
the Mizoroki–Heck arylation of electron-rich and deactivated
olefins;[1b,10,11] however, the literature provides only some ex-
amples of the regioselective coupling reaction of aryl chlorides
with electron-rich vinyl ethers and enamides.[12,13] Hence, an in-
centive to improve the olefin substrate scope clearly exists.
The efficient preparation of substituted allylamines has re-
ceived ever-increasing attention because these amines are im-
Renmin University of China
Beijing 1000872 (P.R. China)
Fax: (+86)10-62516444
E-mail: xulj@chem.ruc.edu.cn
[b] Dr. W. Tang, Prof. J. Xiao
Department of Chemistry
University of Liverpool
Liverpool L69 7ZD (UK)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300755.
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Results and Discussion
portant structural units in many natural products and pharma-
ceuticals and serve as versatile starting materials for the syn-
thesis of various functionalized compounds.[14,15] There has
been considerable interest in using palladium-catalyzed Mizor-
oki–Heck arylation of readily accessible allylamines to prepare
arylated allylamines, but the regiocontrol remains problemat-
ic.[11b,16,17] Hallberg,[16a,b] Wu,[16c] Baxter,[11e] and Zhou[11b] reported
that the catalyst prepared in situ from a diphosphine ligand
and Pd(OAc)2 could work well to catalyze the coupling reaction
of aryl triflates with allylamines to provide the b-arylated prod-
ucts with high regioselectivities and yields; however, the draw-
backs associated with the use of aryl triflates, such as base sen-
sitivity and thermal lability, make this method less attractive.
Ripin[17d] and Wilson[17e] reported that the palladium-catalyzed
highly regioselective and stereoselective terminal arylation of
allylamines could be accomplished in alcohol solvents under
ligand-free conditions; however, only one aryl iodide substrate
was tried. The groups of Sigman,[17h] Cacchi,[17i] and Correia[17j]
described Pd2(dba)3-catalyzed preferential g-arylation of allyla-
mines with arenediazonium salts in the absence of a ligand;
however, the synthetic utility of this chemistry could be re-
stricted by the intrinsic drawbacks of arenediazonium
For our initial investigations of suitable reaction conditions
that can deliver high linear (g-) regioselectivity, the reaction of
4-chloroacetophenone (1a) with N,N-(Boc)2 allylamine (2a) was
selected as the model reaction. The obvious advantages associ-
ated with ligand-free conditions urged us to perform the reac-
tion without using a ligand. We first investigated the solvent
effect by performing the coupling reaction with Pd(OAc)2 as
a precatalyst, tetrabutylammonium bromide (TBAB) as an addi-
tive, and K2CO3 as a base under ligand-free conditions for 12 h.
Solvents such as DMSO, EG, dioxane, and CH3CN proved to be
ineffective, and no coupling product was detected. The exclu-
sive formation of the linear arylation product 3aa was ob-
served with toluene, DMF, and p-xylene (Table 1, entries 1–3),
and the best yield of 55% was achieved with toluene (entry 1).
The fact that no b-arylated product was detected in these sol-
vents indicates that a neutral mechanism is operating and the
ionic pathway is completely suppressed. To improve the reac-
tion efficiency, the performance of other salts was explored
but none of them could work as effectively as TBAB (entries 4–
salts, such as instability and explosive potential. De-
spite these advances, it should be pointed out that
none of these precedents enable the efficient and se-
lective arylation of allylamines with aryl chlorides.
During our investigation of the regioselective aryla-
tion of allylamines,[18] we reported that the highly re-
gioselective internal Mizoroki–Heck arylation of N-
Boc-allylamine (Boc=tert-butyloxycarbonyl) with aryl
bromides could proceed smoothly with palladium–
Table 1. Screening conditions for the linear arylation of allylamine (2a) with 4-chlor-
oacetophenone (1a).[a]
Entry
Solvent
Base
Additive [equiv.]
Yield [%][b]
1
2
3
4
5
6
7
8
toluene
DMF
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
NEt3
TBAB (2)
TBAB (2)
55
12
30
n.d.
n.d.
10
n.d.
n.d.
24
22
89
n.d.
n.d.
n.d.
67
49
12
n.d.
n.d.
n.d.
69
dppp
(dppp=1,3-bis(diphenylphosphino)propane)
p-xylene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
TBAB
TBAB (2)
TBAC[c] (2)
catalysts in ethylene glycol (EG) and Pd(OAc)2 could
catalyze the coupling reaction of aryl bromides with
bulky N,N-diprotected allylamines under ligand-free
conditions in water or DMF, preferentially giving the
g-arylated products.[18a,b] More recently, we found that
the highly regioselective direct arylation of allyla-
mines with thiophenes and furans could be accom-
plished with Pd(OAc)2 and suitable additives.[18c] Nota-
bly, the electronic and steric properties of allylamine
substrates in these reactions have a significant effect
on the catalytic activity and regiocontrol. Encouraged
by these results and with our continued interest in
the regioselective arylation of olefins, herein we
report that a palladium catalyst system consisting of
Pd(OAc)2 and appropriate additives efficiently catalyz-
es the highly regioselective linear (g-)arylation of N,N-
diprotected allylamines with aryl chlorides under
ligand-free conditions whereas N-protected allyla-
mines could undergo a highly efficient internal (b-)ar-
ylation with aryl chlorides with the Pd-dppf catalyst
(dppf=1,1’-bis(diphenylphosphino)ferrocene) in EG/
DMSO.
TBAI[d] (2)
TBAA[e] (2)
Me4NBr (2)
–
TBAB (1.2)
9
10
11
12
13
14
15
16
17
18
19
20
21[f]
22[g]
23[h]
24[i]
25[j]
TBAB (2)/HQ (0.2)
TBAB (2)/TEMPO (0.2)
TBAB (2)/TEMPO (0.2)
TBAB (2)/TEMPO (0.2)
TBAB (2)/TEMPO (0.2)
TBAB (2)/TEMPO (0.2)
TBAB (2)/TEMPO (0.2)
TBAB (2)/TEMPO (0.2)
TBAB (2)/TEMPO (0.2)
TBAB (2)/TEMPO (0.2)
TBAB (2)/TEMPO (0.2)
TBAB (2)/TEMPO (0.2)
TBAB (2)/TEMPO (0.2)
TBAB (2)/TEMPO (0.2)
TBAB (2)/TEMPO (0.2)
–
NBu3
Cy2NMe
K3PO4
KOH
Cs2CO3
KOAc
tBuOK
EtONa
K2CO3
K2CO3
K2CO3
K2CO3
TBAA
32
67
68
21
[a] Reaction conditions: 1a (1.0 mmol), 2a (1.2 mmol), Pd(OAc)2 (5 mol%), base
(1.5 equiv.), additive, solvent (3.0 mL), T=1258C, t=12 h; b-arylated product was not
observed in any reaction, as determined by 1H NMR analysis. [b] Isolated yield. [c] Tet-
rabutylammonium chloride. [d] Tetrabutylammonium iodide. [e] Tetrabutylammonium
acetate. [f] PdCl2 was used instead of Pd(OAc)2. [g] Tris(dibenzylideneacetone)dipalladi-
um(0) was used instead of Pd(OAc)2. [h] Palladium(II) trifluoroacetate was used instead
of Pd(OAc)2. [i] T=1458C. [j] Reaction conditions: 1a (1.0 mmol), 2a (0.5 mmol),
Pd(OAc)2 (1.5 mol%), TBAB (1.0 g), TBAA (0.45 g, 1.5 mmol), 1208C, 12 h.
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7). The use of TBAB is critical to the reaction efficiency.[19] In
the absence of TBAB, no reaction occurred (entry 8). Decreas-
ing the amount of TBAB to 1.2 equiv. resulted in a lower yield
(24%; entry 9). Considering that we had demonstrated that
2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and hydroqui-
none (HQ) could greatly facilitate the linear arylation of allyla-
mines with aryl bromides,[18a,b] we now examined their effect.
The yield of 3aa was decreased considerably to 22% in the
presence of a catalytic amount of HQ (entry 10), whereas
TEMPO afforded a full conversion with an 89% isolated yield of
3aa (entry 11). A similar accelerating effect of the free-radical
scavenger in palladium-catalyzed cross-coupling reactions has
been reported in the literature.[20] Further study revealed that
the choice of a base strongly affected the reaction efficiency
(entries 12–20) and that K2CO3 proved to be superior to other
frequently used inorganic and organic bases. Notably, the or-
ganic bases totally inhibited the reaction (entries 12–14). The
palladium source was also important for this transformation.
Pd(OAc)2 proved to be the best choice, and catalysts derived
from other palladium precursors were not that effective (en-
tries 21–23). In the subsequent part of the study, the yield of
3aa decreased to 68% with the increase in the reaction tem-
perature to 1458C (entry 24). Furthermore, TBAB can undergo
SN2 substitution or Hofmann elimination to produce tributyla-
mine at a high reaction temperature.[21] The inhibitive effect of
tributylamine could be responsible for the decreased yield. The
Nacci protocol,[7a] reported to be highly effective for the Mizor-
oki–Heck reaction of aryl chlorides with deactivated olefins
under ligand-free conditions, was also tested; however, it gave
only a low yield (21%; entry 25). Thus, the optimal reaction
conditions were finally determined as follows: Pd(OAc)2
(5 mol%), TBAB (2 equiv.), TEMPO (20 mol%), and K2CO3
(1.5 equiv.), toluene, 1258C. It should be emphasized that allylic
migration or partial deprotection was not observed in any of
the cases.
Table 2. Linear arylation of 2a with aryl chlorides.[a]
Entry
Aryl chloride
Yield [%][b] (product)
84 (3ba)
1
2
3
4
1b
1c
1d
1e
90 (3ca)
88 (3da)
80 (3ea)
5
6
7
1 f
79 (3 fa)
85 (3ga)
85 (3ha)
1g
1h
8
9
1i
1j
82 (3ia)
87 (3ja)
10
11
12
13
1k
1l
79 (3ka)
80 (3la)
79 (3ma)
78 (3na)
1m
1n
14
15
1o
1p
75 (3oa)
72 (3pa)
16
1q
1r
1s
75 (3qa)
73 (3ra)
75 (3sa)
After determining the optimal reaction conditions, we exam-
ined the linear arylation of 2a with a range of activated and
unactivated aryl chlorides (Table 2). The reaction proceeded
well in all cases and provided the g-arylated linear (E)-allyla-
mine products in good to high yields and with excellent regio-
selectivities (terminal/internal >99:1) and stereoselectivities (E/
Z >99:1). Notably, the nature and the position of the substitu-
ents on the aromatic rings appeared to have no significant
effect on the isolated yields of the expected products, which is
evident from the fact that we encountered no problem in the
reactions involving aryl chlorides containing electron-donating/
electron-withdrawing groups or ortho substituents (Table 2, en-
tries 10 and 12–14). Furthermore, the heteroaryl chlorides dem-
onstrated similar reactivity to aryl chlorides, which gave the
corresponding allylamines in good yields (entries 15–17).
Under the current catalytic conditions, the polyhaloarene (1s)
underwent olefination to furnish the polyolefinated product
(3sa) in 75% yield (Table 2, entry 18). Notably, all the sub-
strates underwent clean conversions without the formation of
the allylic migration products or homocoupling products.
To expand the scope of the catalysis, we investigated the
linear arylation of other allylamine derivatives, and the results
17
18[c]
[a] Reaction conditions: 1 (1.0 mmol), 2 (1.2 mmol), Pd(OAc)2 (5 mol%),
K2CO3 (1.5 mmol), TBAB (2.0 mmol), TEMPO (0.2 mmol), toluene (3.0 mL),
T=1258C, t=12 h; b-arylated product was not observed in any reaction,
as determined by 1H NMR analysis. [b] Isolated yield. [c] 2a (2.4 mmol)
was used.
obtained are summarized in Table 3. The electron-deficient
N,N-diprotected allylamines 2b–d readily reacted with aryl hal-
ides regardless of the nature and position of the substituents
on the aryl ring, and high yields and selectivities of the expect-
ed products were observed (Table 3, entries 1–14). The aryla-
tion of a more electron-rich N,N-diprotected allylamines 2e
and 2 f proceeded with similar efficiency to exclusively afford
g-arylated linear (E)-allylamines in good yields (entries 15–18).
The frequently encountered partial deprotection in the aryla-
tion of N,N-diprotected allylamines did not occur in our
case.[17i,18b]
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preferential migration of the aryl moiety onto the g position
rather than the b position of the olefin, whereas sterically less
demanding substituents give rise to the formation of a mixture
of regioisomers, probably not owing to mechanistic alteration
between ionic and neutral pathways but owing to reduction of
steric hindrance. Furthermore, N-carbobenzyloxy allylamine
(2h) failed to give any detectable arylation product, probably
owing to the decomposition of the substrate under the reac-
tion conditions. The performance of the current catalyst
system was also examined in the arylation of a coordinating,
electron-rich N,N-diethyl allylamine (2i); however, no reaction
could be observed. The strong coordination of the nitrogen
atom of 2i to palladium results in catalyst poisoning, which in-
hibits the arylation.
Table 3. Linear arylation of allylamines with aryl chlorides.[a]
Entry
1
Aryl chloride
Olefin
Yield [%][c] (product)
1a
1b
2b
90 (3ab)
2
2b
87 (3bb)
3
4
5
6
7
8
9
1c
1h
1l
2b
2b
2b
2b
2b
2b
2b
91 (3cb)
88 (3hb)
81 (3lb)
80 (3mb)
78 (3nb)
83 (3tb)
81 (3ub)
Encouraged by the success of the linear arylation of allyla-
mines, we investigated the internal arylation of allylamines
with aryl chlorides. The products obtained are interesting be-
cause of their bioactivity as enzyme inhibitors.[22] However, as
in the case of g-arylation, no catalyst has been reported that
promotes the b-arylation of allylamines with aryl chlorides. The
initial screening studies were performed at 1458C by using 4-
chloroacetophenone (1a) and N-Boc-allylamine (2g) as model
coupling partners, with the catalyst prepared in situ from a bi-
dentate ligand and Pd(OAc)2. The effect of different solvents
was surveyed initially in the presence of the Pd-dppp catalyst
together with NaOAc as the base. Although EG has been iden-
tified as a useful solvent for the highly regioselective internal
arylation of electron-rich olefins,[10a,13c]the reaction performed
in EG in this case only led to the formation of the homocou-
pling product and the desired product 4ag was not detected.
The oxidative addition of ArCl to Pd0 occurred readily in EG;
however, the following olefin insert step was totally decelerat-
ed under the current catalytic conditions. Thus, the homocou-
pling of 1a was favored. Neither arylation nor homocoupling
was observed in DMF and DMSO, which indicates that no oxi-
dative addition occurred in these solvents. These results led us
to envision that a mixed solvent system of EG and DMF or
DMSO could inhibit the homocoupling while facilitating the in-
ternal arylation. With this in mind, we then examined the reac-
tion in EG/DMSO (1:1) and EG/DMF (1:1). No reaction hap-
pened in EG/DMF (Table 4, entry 1); however, the exclusive for-
mation of the internal arylation product 4ag was observed in
EG/DMSO, though in a low yield (5%; entry 2). The EG/DMSO
ratio plays an important role in this reaction. The homocou-
pling product dominated the reaction with the increase in the
EG/DMSO ratio to 5:1 (entry 3), whereas decreasing the
amount of EG resulted in a complete recovery of the starting
1m
1n
1t
1u
10
1b
2c
75 (3bc)
11
12
13
1c
1v
1a
2c
2c
2d
80 (3cc)
81 (3vc)
80 (3ad)
14
1b
2d
72 (3bd)
15
16
17
18
1a
1m
1a
1l
2e
2e
2 f
2 f
80 (3ae)
77 (3me)
76 (3af)
79 (3lf)
[a] Reaction conditions: 1 (1.0 mmol), 2 (1.2 mmol), Pd(OAc)2 (5 mol%),
K2CO3 (1.5 mmol), TBAB (2.0 mmol), TEMPO (0.2 mmol), toluene (3.0 mL),
T=1258C, t=12 h; b-arylated product was not observed in any reaction,
as determined by 1H NMR analysis. [b] N-carbobenzyloxy. [c] Isolated
yield.
However, when the less bulky N-Boc-allylamine (2g) was
tried, lower yields of g-arylated linear (E)-allylamines were ob-
tained owing to the formation of
the branched b-arylated prod-
ucts (Scheme 2). These results in-
dicate that the outcome of re-
gioselectivity is highly sensitive
to the bulkiness of the N-sub-
stituent on the allylamine. The
existence of two bulky N-sub-
stituents in the allylamine ap-
pears to be necessary for the Scheme 2. Mizoroki–Heck arylation of N-Boc-allylamine 2g.
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product from a potentially competing palladium-cat-
alyzed amidation reaction could be detected.[24]
Table 4. Screening conditions for the internal arylation of allylamines 2g with aryl
chloride 1a.[a]
After determining the optimal conditions for the
internal arylation of allylamine, we examined the sub-
strate scope of this process, and the results obtained
are summarized in Table 5. A number of aryl chlorides
underwent smooth olefination with allylamine 2g
with the Pd-dppf catalyst (entries 1–9, 12, and 13),
and good to high yields were obtained with good
tolerance of different substituents on the aromatic
ring. Excellent b-regioselectivities were achieved in all
cases and the occurrence of linear arylation was not
detected, which implies that the ionic pathway is op-
erating under the current reaction conditions. The
protocol is not limited to aryl chlorides; it could be
equally applied to heteroaryl chloride (entry 11). The
reaction involving electron-neutral chlorobenzene
(1l) did not proceed under the current reaction con-
ditions, but replacing dppf with 4-MeO-dppp resulted
in a good yield of the desired product (entry 10).
However, the electron-rich aryl chlorides failed to
give any detectable arylation product with either
dppf or 4-MeO-dppp as the ligand.
Entry
Solvent
Ligand
Base
Yield [%][b]
1
2
3
4
5
6
7
8
EG/DMF (1:1)
dppp
dppp
dppp
dppp
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NEt3
Cy2NMe
TBAA
K2CO3
Cs2CO3
KOH
KOAc
KOAc
KOAc
n.d.
5
n.d.
n.d.
24
EG/DMSO (1:1)
EG/DMSO (5:1)
EG/DMSO (1:2)
EG/DMSO (1:1)
EG/DMSO (1:1)
EG/DMSO (1:1)
EG/DMSO (1:1)
EG/DMSO (1:1)
EG/DMSO (1:1)
EG/DMSO (1:1)
EG/DMSO (1:1)
EG/DMSO (1:1)
EG/DMSO (1:1)
EG/DMSO (1:1)
EG/DMSO (1:1)
EG/DMSO (1:1)
EG/DMSO (1:1)
EG/DMSO (1:1)
EG/DMSO (1:1)
EG/DMSO (1:1)
EG/DMSO (1:1)
dppf
dppb[c]
4-MeO-dppp[d]
4-CF3-dppp[e]
BINAP[f]
PPh3
6
7
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
10
26
16
n.d.
n.d.
n.d.
46
9
10
11
12
13
14
15
16
17
18
19
20
21[j]
22[k]
phen[g]
dmphen[h]
bpy[i]
dppf
dppf
dppf
dppf
dppf
dppf
dppf
dppf
dppf
The combination of Pd(OAc)2 and dppf also ena-
bled the highly regioselective internal arylations of al-
lylamine 2h to exclusively provide the b-arylated
products in good yields (entries 15 and 16). Notably,
the activity and selectivity of the current catalyst
system appears to correlate with the steric character-
istics of the substituent on allylamine nitrogen, be-
cause no conversion was observed in the reaction in-
volving sterically demanding N,N-diprotected allyla-
mine 2a and 2e. Further study indicated that the
electron-rich allylamine 2i also failed to give any de-
16
83
[a] Reaction conditions: 1a (1.0 mmol), 2g (1.2 mmol), Pd(OAc)2 (5 mol%), ligand
(7.5 mol%), base (1.5 mmol), solvent (3.0 mL), T=1458C, t=14 h; g-arylated product
was not observed in any reaction, as determined by 1H NMR analysis. [b] Isolated
yield. [c] 1,4-bis(diphenylphosphino)butane. [d] 1,3-bis(bis(4-methoxyphenyl)phosphi-
no)propane. [e] 1,3-bis(bis(4-(trifluoromethyl)phenyl)phosphino)propane. [f] 2,2’-bis(di-
phenylphosphino)-1,1’-binaphthyl. [g] 1,10-phenanthroline. [h] 2,9-dimethyl-1,10-phe-
nanthroline. [i] 2,2’-bipyridine. [j] 1.5 equiv. of [H2N(iPr)2][BF4] were added. [k] t=24 h.
materials (entry 4). A similar promoting effect of DMSO has
been observed in the regioselective internal arylation of enam-
ides and allylic alcohol in ionic liquids.[23] To increase the reac-
tion efficiency, the catalytic performances of different bidentate
and monodentate ligands were investigated (entries 5–13). Of
the ligands tested, dppf was identified as the best choice
(entry 5) and other ligands, whether they are bidentate or
monodentate, were found to give worse results. Further stud-
ies revealed that the base had a dramatic effect on the yield
(entries 14–20). KOAc demonstrated the highest reactivity
(entry 20), and other bases were not that effective. Inspired by
the significant promoting effect of the hydrogen bond donor
[H2N(iPr)2][BF4] on the regioselectivity and reaction rate in the
arylation of electron-rich olefins with aryl bromides,[13a] we
then explored the effect of this salt but the catalytic reactivity
was reduced (entry 21). The yield of the product 4ag could be
increased significantly to 83% if the reaction time was in-
creased to 24 h (entry 22). Under the current reaction condi-
tions, no product from g-arylation was detected. The excellent
regioselectivity favoring b-arylation could be partly attributed
to the highly ionizing power of EG, which could facilitate the
formation of the cationic PdII–olefin species, which channels
the arylation into the ionic pathway (Scheme 1).[10a] Notably, no
tectable arylation product, presumably owing to catalyst poi-
soning due to the strong coordination of the nitrogen atom of
2i to PdII.
From the above studies, it is evident that both the steric
and electronic properties of allylamine substrates have a signifi-
cant effect on the catalytic activity and selectivity. In the case
of linear arylation, it appears that the use of bulky N,N-dipro-
tected allylamides as coupling partners is necessary for high
catalytic performance. The chelation between the carbonyl
oxygen atom and the palladium atom in the neutral intermedi-
ate A could promote a highly regioselective migration of the
aryl moiety onto the g, rather than b, position of the olefin to
give intermediate B, which undergoes regio- and stereoselec-
tive b-H elimination to furnish the expected product
[Scheme 3, reaction (1)]. With the decreased steric demand
from the amino moiety, b-arylation may become possible,
which leads to regioisomers in the case of 2g. Similar chela-
tion-assisted regio- and stereocontrol has also been reported
in the palladium-catalyzed linear arylation of allyl derivatives
and vinyl derivatives.[1b,g,17j,25,26] In contrast, the ionic pathway,
made possible by the chelating diphosphine and the ionizing
EG, may feature the formation of an ionic intermediate
C[16a,b,17b] as a result of the olefin chelation to the PdII, in which
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The failure of N,N-diprotected allylamides in internal b-arylation
may be traced to the bulkiness of the substituent on nitrogen,
which impedes the insertion of the olefin into the PdÀAr
bond. The ionic pathway could be possible with 2g even in
the absence of dppf, with 2g itself acting as a chelating
ligand, which thus promotes the formation of the ionic species
analogous to C and hence some branched arylation products.
Table 5. Internal arylation of allylamines 2g–h with aryl chlorides.[a]
Entry
Aryl chloride
Olefin
2g
Yield [%][c] (product)
86 (4bg)
1
2
3
1b
1c
1d
Conclusions
2g
83 (4cg)
We have developed two efficient palladium catalyst systems
for the highly regioselective Mizoroki–Heck reaction of aryl
chlorides with N,N-diprotected and N-protected allylamine de-
rivatives. In the presence of Pd(OAc)2 and appropriate addi-
tives, N,N-diprotected allylamines underwent the highly regio-
and stereoselective arylation with aryl chlorides to exclusively
give the g-arylated allylamines in good to excellent yields
under ligand-free conditions and a wide range of functionali-
ties in both coupling partners could be well tolerated. In con-
trast, the catalyst prepared in situ from Pd(OAc)2 and 1,1’-bis(-
diphenylphosphino)ferrocene worked effectively to catalyze
the highly regioselective internal arylation of N-protected allyl-
amines with aryl chlorides in ethylene glycol/DMSO, which af-
forded high yields of the b-arylated allylamines and good
group compatibility. Ethylene glycol/DMSO facilitates the disso-
ciation of chloride anions from PdII, which furnishes the key
ionic PdII–olefin intermediate responsible for the b-product, as
well as inhibits the homocoupling side reaction of aryl chlor-
ides. Our results reveal that the catalytic activity and regiocon-
trol in both linear and internal arylations demonstrate high
sensitivity to the steric and electronic properties of allylamines.
Further investigations on the mechanism of the reaction and
synthetic applications are underway in our laboratory and will
be reported in the future.
2g
76 (4dg)
4
5
6
1e
1g
1h
2g
2g
2g
80 (4eg)
79 (4gg)
75 (4hg)
7
1i
2g
78 (4ig)
8
9
1j
2g
2g
75 (4jg)
1k
73 (4 kg)
10[d]
11
1l
2g
2g
2g
2g
2g
2h
73 (4lg)
70 (4pg)
74 (4ug)
72 (4vg)
72 (4wg)
70 (4ch)
1p
1u
1v
1w
1c
12
13
14
15
16
1p
2h
68 (4ph)
Experimental Section
[a] Reaction conditions: 1 (1.0 mmol), 2 (1.2 mmol), Pd(OAc)2 (5 mol%),
dppf (7.5 mol%), KOAc (1.5 mmol), EG/DMSO (1:1, 3.0 mL), T=1458C, t=
24 h; g-arylated product was not observed in any reaction, as determined
by 1H NMR analysis. [b] N-carbobenzyloxy. [c] Isolated yield. [d] 4-MeO-
dppp was used instead of dppf.
General
The NMR spectra were recorded at 400 MHz. The chemical shifts
were recorded relative to tetramethylsilane and with the solvent
resonance as the internal standard. 13C NMR data were collected at
100 MHz with complete proton decoupling. Unless otherwise
noted, all experiments were performed in a nitrogen atmosphere.
The chloroarenes, palladium complexes, and other common mate-
rials and solvents are commercially available and were used as re-
ceived without further purification.
steric repulsion from the chelating ligands could facilitate
the formation of intermediate D, which thus furnishes the
branched product after b-H elimination [Scheme 3, Eq. (2)].
General procedure for the Miz-
oroki–Heck linear arylation of
allylamines
An oven-dried pressure tube con-
taining
a stir bar was charged
with an aryl chloride 1 (1.0 mmol),
allylamine (1.2 mmol), Pd(OAc)2
(11.2 mg,
(207.3 mg,
(644.8 mg,
0.05 mmol),
1.5 mmol),
2.0 mmol),
K2CO3
TBAB
TEMPO
Scheme 3. Possible pathways for high stereoselectivity and regioselectivity.
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a mixture of ethyl acetate and hexane to give the pure product.
General method for the Mizoroki–Heck internal arylation of
allylamines
An oven-dried pressure tube containing a stir bar was charged
with an aryl chloride 1 (1.0 mmol), allylamine (3 mmol), Pd(OAc)2
(11.2 mg, 0.05 mmol), dppf (41.6 mg, 0.075 mmol), K2CO3
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Keywords: allylamine · aryl chloride · Mizoroki–Heck reaction ·
palladium · regioselectivity
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Received: September 9, 2013
Published online on November 13, 2013
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