Chemo-, regio-, and stereoselective Heck-Matsuda arylation of allylic alcohols under mild conditions

Article abstract of DOI:10.1039/c5ob00235d

Heck arylation with allylic alcohol is extremely challenging due to chemo-, regio-, and stereoselective scrambling. Here we report a mild protocol for the alcohol selective β- and α-arylation of allylic and cinnamyl alcohols respectively with aryldiazonium salts. The steric and electronic parameters of the alkene play a prominent role in the regioselectivity.

Full text of DOI:10.1039/c5ob00235d

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Chemoꢀ, Regioꢀ, and Stereoselective HeckꢀMatsuda
Arylation of Allylic Alcohols under Mild Conditions
Cite this: DOI: 10.1039/x0xx00000x
Chaudhari Tohasib Yusub, Asik Hossian, Manash Kumar Manna, and Ranjan Jana*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/obc
Mechanistically, after oxidative addition of palladium(0)ꢀ to
the aryl electrophile, a migratory βꢀalkene insertion occurs to
generate an unstable σꢀalkylꢀPd species. Subsequently, random
βꢀhydride elimination (either Ha or Hb, Scheme 1) leads to the
formation of a mixture of products, which limits its synthetic
utility. Therefore, a mild reaction protocol that controls βꢀ
hydride elimination will be extremely useful in regioꢀ, and
chemoselective arylation of allylic alcohols. In general,
oxidative addition to the aryl electrophiles such as aryl halides,
triflates etc. is the rate limiting step, which requires high
temperature, ligands, bases etc.6 The harsh reaction conditions
lead to the nonꢀspecific arylation, βꢀ hydride elimination, and
double bond isomerizations. After the first report by Matsuda
and coworkers in 1977, arenediazonium salts have been
exploited as reactive aryl halide surrogates in Pdꢀcatalyzed
crossꢀcoupling reactions.7 The Sengupta,8 Genêt,9 Correia,10
Felpin11 and other groups12 have contributed substantially to the
HeckꢀMatsuda arylation of olefins. Recently, the Sigman group
has accomplished styrenyl selective arylation of electronically
nonbiased olefins with aryl diazonium salts.13 Due to the facile
oxidative addition to the aryldiazonium salts, the crossꢀcoupling
took place under mild reaction conditions, without any
additional ligand or base. Therefore, we decided to optimize
HeckꢀMatsuda arylation of allylic alcohols with aryldiazonium
salts. Although, there are few reports of Heck arylation with
allyl or crotyl alcohol,14 but most of them provides a mixture of
aldehyde(or ketone) and alcohol. In addition, HeckꢀMatsuda
arylation with βꢀsubstituted alkenes especially cinnamyl
alcohols is underexplored.15 A Heck arylation of cinnamyl
alcohol with expensive aryl iodide has been reported by the
Cacchi group.16 However high reaction temperature,
stoichiometric base, and longer reaction time was required to
afford only a low to moderate yield of the desired products.
Here we report an alcohol selective, βꢀ and αꢀarylation of allyl
and cinnamyl alcohols respectively using aryldiazonium salts
(Scheme 2). From the systematic study, we have observed that
besides steric factors, electronic parameters of the alkene also
play an important role in regioselectivity.
Heck arylation with allylic alcohol is extremely challenging
due to chemoꢀ, regioꢀ, and stereoselective scrambling. Here
we report a mild protocol for the alcohol selective βꢀ and αꢀ
arylation of allylic and cinnamyl alcohols respectively with
aryldiazonium salts. The steric and electronic parameters of
the alkene play a prominent role in the regioselectivity.
Since its discovery, a significant effort has been dedicated to the
development of highly efficient and selective HeckꢀMizoroki
reaction for the arylation of alkenes.1 Consequently, it has been
recognized as one of the most powerful synthetic tools for CꢀC bond
formation.2 The extent of research on Heck reaction over the past
decades has inculcated the false notion that Heck chemistry is now a
mature area. However, the mechanistic understanding of subtle
changes that dictate regioselectivity and βꢀhydride elimination is still
limited.3 Despite its robustness and efficiency, the Heck reaction
involving aryl halides and triflates is mainly limited to the activated
olefins such as acrylates, styrenes etc. On the contrary, Heck
arylation of electronically nonbiased olefin is still challenging due to
sluggish alkene insertion and nonꢀspecific βꢀhydride elimination.4
Similarly, Heck arylation of allylic alcohol is extremely challenging
due to chemoꢀ, regioꢀ, and stereochemical scrambling and double
bond isomerization (Scheme 1).5
Scheme 1. Chemoselectivity in Heck arylation with allylic alcohol.
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A homoallylic alcohol also underwent the HeckꢀMatsuda
reaction affording moderate yield of the arylation product (2h
Table 1). Interestingly, protected allyl alcohols such as allyl
acetate (2i Table 1), allyl carbonate (2j Table 1) underwent
,
,
,
smooth arylation providing βꢀ arylation in high yields. In
general, these substrates are unstable under Pd(0)ꢀ catalysis e.g.
TsujiꢀTrost conditions.19 This demonstrates the mild nature of
this reaction conditions.
Table 2. Substrate scope with cinnamyl alcoholsa,b
OH
Pd2(dba)3
N2BF4
R
(5 mol %)
+
Scheme 2. Regioselective arylation with aryldiazonium salts.
Ar
DMA, 25 o
3 h
C
Ar
R
To begin, we tested simple allyl alcohol and
phenyldiazonium salt with a catalytic amount of Pd2dba3. A
rapid effervescence of nitrogen was observed after the addition
of the catalyst in DMF at room temperature. Unfortunately, the
yield of the desired cinnamyl alcohol was low (~20 %)
although, all starting material was consumed. Next, we
screened several solvents which are frequently used in Heckꢀ
Matsuda reaction, e.g. MeOH, CH3CN, DMA, NMP etc. It was
observed that amide solvents are superior and NMP is the best
solvent for this transformation. It was also found that
tetrafluoroborate counterion is crucial for the reaction outcome.
Presumably, in addition to the improved stability of the
diazonium moiety, its nonꢀcoordinating nature imparts
electrophilicity to the metal center for facile alkene insertion.17
The amide solvent stabilizes the incipient electrophilic Pdꢀ
species which is formed through oxidative addition.18 All of our
screening with simple allyl alcohol afforded only a moderate
OH
-
3a 3r
OMe
NO2
Ph
Ph
Ph
OH
: 62%
OH
: 65%
3b
OH
: 71%
3c
3a
OMe
NO2
NO2
Ph
Ph
Ph
OH
: 64%
OH
: 64%
Br
OH
: 76%
3f
3d
3e
Me
Br
Cl
yield of the desired product (2a, 2b, Table 1) and a substantial
Ph
Ph
amount of oligomerization was observed. However, using
secondary allylic alcohols, the yield of the arylation product
was increased. The arylation with 3ꢀbuteneꢀ2ꢀol afforded the
OH
: 64%
OH
: 58%
OH
: 52%
3i
3g
3h
corresponding alcohol in good to high yields (2c
ꢀ
2g
,
Table 1).
Cl
Table 1. Substrate scope with allylic alcoholsa,b
COOMe
I
Pd2(dba)3
N2BF4
R
OH
Ph
Ph
Ph
(5 mol %)
OH
R1
+
NMP, 25 o
C
R1
OH
: 50%
OH
OH
R
30 min
: 73%
3k
: 40%
3l
3j
-
2a 2j
Me
Me
OH
OH
OH
OH
OMe
Me
OH
: 38%
2a
: 37%
2b
: 68%
2c
O2N
OH
MeO
: 65%
Me
: 85%
Me
3m
3n
OH
OH
Me
Me
Br
: 75%
2d
: 57%
2e
: 66%
Ph
2f
OMe
Me
OH
Me
OH
O2N
Br
OH
OH
: 90%
3p
: 92%
3o
Me
Me
Me
Me
Cl
: 60%
2g
: 42%
2h
MeO
Me
OH
OCOCH3
Me
OH
OCOOPh
MeO
3q: 70%
3r: 90%
: 67%
2j
: 82%
2i
aThe reaction was performed with 0.2 mmol scale. byield refer to here
is the isolated pure product.
aThe reaction was performed with 0.2 mmol scale. byield refer to here
is the isolated pure product.
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( ) sequential diarylation
a
Next we turned our attention to the arylation of cinnamyl
alcohol derivatives. From our previous experience with allyl
alcohol we realized that cinnamyl alcohol may lead to the
N2BF4
1. Pd2(dba)3 (5 mol %)
OH
NMP, 25 oC, 30 min
2. sat. KOH, MeOH
40 oC, 1 h
OAc
selective
α
ꢀ arylation due to steric hindrance with the terminal
+
Br
phenyl group. That said, we investigated analogous Heckꢀ
Matsuda reaction with cinnamyl alcohol and arenediazonium
tetrafluoroborate. Gratifyingly, a good yield of the arylation
product was obtained simply by changing the solvent to DMA.
4
87% overall yield
Br
+
NO2
N2BF4
As anticipated, the
αꢀ arylation was obtained exclusively to
Pd2(dba)3 (5 mol %)
DMA, 25 oC, 30 min
afford ( )ꢀ2ꢀ3ꢀdiarylallylic alcohol. The stereochemistry of the
Z
4
arylation product was assigned by NOESY experiment and
comparing with the literature report (see the Supporting
Information).20 A number of cinnamyl alcohol derivatives and
arenediazonium salts were examined for the coupling reaction,
Br
OH
6% yield
NO2
:
3f 7
( ) sequential Heck/Suzuki coupling
b
and a wide range of functional groups such as methoxy (3b
Table 2), nitro (3c 3e 3f, Table 2), ester (3k, Table 2), bromo
3h, Table 2), chloro (3i 3j, Table 2), even iodo (3l, Table 2),
were compatible under the reaction conditions. However,
reaction with ꢀiodophenyldiazonium (3l) also gave ~10% of
, 3d,
OH
N2BF4
OH
,
,
Pd2(dba)3 (5 mol %)
(
,
NMP, 25 o
C
+
Br
p
30 min
: 57% yield
2e
the corresponding aldehyde via double bond isomerization.
These halogen substituents are useful for further crossꢀ
Br
+
OH
couplings.
mesitylenediazonium salt provided excellent yield of the
arylation product (3n 3r, Table 2). Interestingly, no arylation
Gratifyingly,
a
sterically
hindered
Pd(PPh3)4 (5 mol%)
K2CO3 (1.5 equiv)
DMF, 130 oC, 3 h
2e
PhB(OH)2
ꢀ
Ph
was observed with cinnamyl acetate which indicates that
besides steric effects the free hydroxyl group also may have
role in regioselectivity through coordination with the metal
center.
Finally, we carried out a sequential HeckꢀMatsuda arylation
to afford (Z)ꢀ2ꢀ3ꢀdiarylallylic alcohol. Since allyl alcohol
: 78% yield
5
( )
c
- vs
diaryl allylic alcohols
OMe
1. Pd(OAc)2 (10 mol%)
NaOAc (3.0 equiv)
CO2Et
Ph
MeOH, 25 o
ref. 21
C
provided low yield, we took allyl acetate for the βꢀ arylation
followed by hydrolysis to provide cinnamyl alcohol derivative,
+
2. DIBAL-H (2.2 equiv)
toluene, -78 to 25 o
MeO
N2BF4
Ph
4
. The subsequent
αꢀ arylation provided 3f in 76% yield
C
(
Scheme 3 ). Since halogens are compatible with our reaction
, a
6
3 h
OH
conditions, we have demonstrated a sequential Heck/Suzuki
coupling reaction. Initially, Heck reaction was performed with
selective -arylation
52% overall yield
3ꢀbuteneꢀ2ꢀol and 4ꢀbromoꢀphenyldiazonium salt to give 2e
The product was further crossꢀcoupled with phenyl boronic acid
to yield Scheme 3 ). We have also showed that
synthetically useful β, βꢀ and β, αꢀ diarylallylic alcohol can
easily be accessed through the HeckꢀMatsuda reactions. The
arylation of cinnamic ester occurs at the βꢀ position
selectively,21 and the ester is reduced to the corresponding
alcohol with DIBALꢀH. Whereas, in this present protocol
cinnamyl alcohol leads to the αꢀ arylation product selectively
.
OMe
Ph
OH
Pd2(dba)3 (5 mol %)
5
(
, b
DMA, 25 o
C
+
Ph
3 h
MeO
N2BF4
OH
: 65% yield
3b
selective -arylation
Scheme 2. Synthetic manipulations of the present protocol.
(
Scheme 3, c). This phenomenon demonstrates that besides
steric parameters, electronic nature of the alkene also plays an
important role in regioselective arylation.
Conclusions
We have developed a mild protocol for the alcoholꢀselective
HeckꢀMatsuda arylation of allylic alcohols in highly regioꢀ, and
stereoselective manner. In sharp contrast to the allylic alcohols,
a selective αꢀ arylation of the cinnamyl alcohols was observed
to afford (Z)ꢀ2ꢀ3ꢀdiarylallylic alcohol. Taking the advantage of
mild reaction conditions, we have also demonstrated the
sequential diarylation and Heck/Suzuki coupling. From the
systematic study, we observed that besides steric factors,
electronic parameters also play an important role in
regioselectivity. Therefore, selective β,βꢀ and β,αꢀ diaryl allylic
alcohols have been synthesized from cinnamic ester and
cinnamyl alcohol respectively.
Acknowledgements
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This work was supported by DST, award no. SR/S2/RJNꢀ
97/2012, Govt. of India. AH and MKM thank CSIR, and UGC
respectively for their fellowships.
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