Synthesis of multi-substituted dihydrofurans via palladium-catalysed coupling between 2,3-alkadienols and pronucleophiles

Article abstract of DOI:10.1039/c8cc02589d

Multi-substituted dihydrofurans were obtained from a palladium-catalysed coupling reaction between 2,3-alkadienols and ketones bearing an electron-withdrawing group at the α-position. Methanol as a solvent was essential for the initial dehydrative substitution to suppress the competitive hydroalkylation of the diene moiety. The substitution would be followed by intramolecular hydroalkoxylation under the same catalysis.

Full text of DOI:10.1039/c8cc02589d

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
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Synthesis of multiple-substituted dihydrofurans via palladium-
catalysed coupling between 2,3-alkadienols and pronucleophiles
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Hirokazu Tsukamoto,* Kazuya Ito and Takayuki Doi
www.rsc.org/
Multiple-substituted dihydrofurans were obtained by palladium- of transformations of
1
into 1,3-diene
4
have been reported
would be
catalysed coupling reaction between 2,3-alkadienols and ketones (Scheme 1).9–11 The dehydrative allenylation of
2
bearing an electron-withdrawing group at the α-position. more difficult than simple allylation owing to two possible side
Methanol as a solvent was essential for the initial dehydrative reactions: 1) hydroalkylation of allene under palladium
substitution to suppress competitive hydroalkylation of the diene catalysis to give 9 14–16
;
2) insertion of allene
moiety. The substitution would be followed by intramolecular dimerisation product 10 17
Herein, we report the reaction
conditions for the dehydrative allenylation of , which can
suppress the side reactions. Moreover, we also demonstrate
that the dehydrative allenylation of ketone , substituted by
an electron-withdrawing group at the -position, accompanied
the cyclisation of the resulting allenic ketone 5' to give
multiple-substituted dihydrofuran in a single step. Here, it
1 into 3 to give
.
hydroalkoxylation under the same catalysis.
2
A nucleophilic substitution of a hydroxyl group without
transforming it into a leaving group such as halide and
sulphonate is very attractive in modern organic synthesis in
terms of step-economy and waste minimisation.1 Although
Mitsunobu reaction2 is classified as a dehydrative substitution
2
α
6
should be noted that other possible carbocyclic products 11
and 12 were hardly obtained. The single-step procedure has a
applicable to
a wide range of alcohols, it generates a
stoichiometric amount of side products that are difficult to
remove. On the other hand, Friedel-Crafts and Tsuji-Trost
reactions, using transition metal-catalysed dehydrative
substitutions of π-activated alcohols including allylic,
propargylic and benzylic ones have recently received
considerable attention because these reactions form only
great advantage over a three-step synthesis of dihydrofuran
1
6
from the common allenic alcohol through 1)
phosphorylation, 2) palladium-catalysed substitution of the
phosphate with sodium salt of activated ketone and 3)
intramolecular hydroalkoxylation of the resulting allenic β-
ketoesters under the catalysis of mercury oxide and p-
toluenesulfonic acid, as reported by Delair and Doutheau.18, 19
4
water as a byproduct.3, Tsuji-Trost reaction using allylic
alcohol, instead of its acetate that is commonly utilised for this
EWG
R
OH
reaction, can exclude
a base additive for the catalyst
R1
Pd0
R1
previous
reports
Pd+ OH-
R1
turnover but requires certain reaction conditions including
special ligands5, acidic additives6, or protic media7 to improve
the low leaving ability of hydroxide ion. In contrast to allylic
alcohol4–8, the transformation of allenic alcohol, which can also
lead to a π-allylpalladium intermediate upon activation,9–12 has
received only scattered attention (Scheme 1). To the best of
our knowledge, Tsuji-Trost-type substitution reaction of allenic
Nu
H
R1
•
O
oxidative
addition
R2 R3
R2 R3
R2 R3
R2 R3
1
3
4
6
Nu-H 2
hydropalladation
this report
–H2O
cyclisation
insertion of 1
OH
OH
R1
H
R1
Nu- Pd+
R2
R1
R1
EWG
R
Pd+ OH-
R3
Nu
R1
•
•
alcohol
1 with pronucleophile 2 leading to the formation of
H
O
R2
when Nu =
CH(COR)(EWG)
R3
R2 R3
R2 R3
R2 R3
dehydration product
5
via exo-alkylidene-π-allylpalladium
intermediate 313 has never been developed, although a couple
7
8
5
5´
cyclisation
Nu = CH(EWG)2
H-Nu 2, –H2O
OH
R1
R1
H
OH
H
R1
EWG
EWG
EWG
R1
EWG
H
Nu
R3
R3
R1
Nu
R2 R3
11
R2
12
R2
R2 R3
R2 R3
9
10
Scheme 1 Coupling Reactions between Allenic Alcohol 1 and 2.
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At first, 2-methyl-2,3-butadien-1-ol (1a, 1 equiv) was examined (entries 3 and 4). The use of 2,4,4-trisubstituted allenic alcohol
as an allenylating reagent for benzoylacetonitrile (2A, 2 equiv) 1f also resulted in dehydrative allenylation of 2A and
on heating at 65 °C in the presence of
5 mol% concomitant cyclisation to give 6fA in 65% yield (entry 5). The
tetrakis(triphenylphosphine)palladium [Pd(PPh3)4] (Scheme 2, use of secondary alcohol 1g resulted in the formation of 4-
Table 1, entries 1–5).9 Aprotic solvents including toluene, THF, substituted 4,5-dihydrofuran 6gA as a diastereomeric mixture
1,4-dioxane,
and
dichloromethane
resulted
in
a
(entry 6). It should be noted that the parent primary alcohol 1h
hydroalkylation of 1a to give ca. 1:1 isomeric mixture of allylic was converted into C-cyclisation product 12hA instead of O-
alcohol 9aA in moderate to good yield (Table 1, entries 1–4). alkylation product 6hA (entry 7, vide infra). Unfortunately,
Interestingly, the use of methanol as a solvent switched the unsubstituted 2,3-butadien-1-ol
reaction mode from addition to substitution to afford dehydrative allenylation of 2A at all (entry 8).
dihydrofuran 6aA as
major product (entry 5).20 The
(
1i
)
did not undergo
a
Table 2 Scope of Allenic Alcoholsa
formation of allenylated product 5aA' was not observed and
would be followed by the intramolecular hydroalkoxylation of
allene to give dihydrofuran 6aA instead (vide infra). The
reaction temperature was also a major reason for preferring
the substitution reaction with 80 °C, leading to the best yield
of 6aA (entries 5–7). The molar ratio of pronucleophile 2A to
allenic alcohol 1a was also crucial, and the use of 2 equiv of 2A
to 1a turned out to be the best for the predominant formation
of 6aA (entries 7–10). In contrast, the use of an excess amount
of 1a to 2A completely shut the reaction (entry 10). Instead of
triphenylphosphine ligand, biaryl-based diphosphines such as
BINAP and MeO-BIPHEP with allyl(cyclopentadienyl)
palladium(II) led to the formation of a trace amount of 6aA
(data not shown).
entry
substrate
product
yield (%)
71
CN
OH
Ph
Ph
Ph
1
O
•
6bA
1b
2
3
4
47
45
43
O
NC
Ph
OH
Me
CN
(2A, X equiv.)
Pd(PPh3)4 (5 mol%)
OH
Me
CN
Ph
Me
Me
+
Ph
H
•
•
solvent (0.1 M)
temp, time
O
O
Ph
NC
9aA
5
6
65
1a
O
6aA
5aA´
Scheme 2 Pd(0)-Catalysed Coupling Reaction between 1a and 2A
Table 1 Optimisation of Reaction Conditions for the Coupling Reaction
between 1a and 2A
74
(dr = 1 : 1)
entry
solvent
X temp time
(equiv) (°C) (h)
yield of
9aA (%)a
yield of
6aA (%)
1
2
toluene
THF
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.5
3.0
0.2
65
65
65
65
65
50
80
80
80
80
4
2
54
70
64
63
18
7
trace
trace
trace
5
80
(dr = 1.3 : 1)
7
8
3
1,4-dioxane
CH2Cl2
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
2
4
2
CN
OH
5
4
58
22
Ph
6
36
1.5
28
1
0
O
•
7
12
12
25
0
68
38
8
6iA
1i
9
10
42
0
a
Reaction conditions: 1b–i (1 equiv), 2A (2 equiv), Pd(PPh3)4 (5 mol%),
MeOH (0.1 M), 80 °C, 1. 5 h (entries 1–6), 2 h (entry 7), or 24 h (entry 8).
24
a E- and Z-9aA were obtained in the ratio of ca. 1.2:1 in entries 1–9.
Next, the scope of pronucleophiles was also investigated
With the optimised reaction conditions in hand (Table 1, entry
7), the scope of allenic alcohols 1b–i was investigated (Table
2). Substitution of the methyl group at C-2 in 1a by a phenyl
group did not affect the efficiency of the coupling reaction
with 2A to give 2,5-diphenyl-5-vinyl-4,5-dihydrofuran-3-
carbonitrile (6bA) in 71% yield (entry 1). Diphenylphosphine
oxide as the substituent was also compatible with the reaction
conditions to give 6cA in moderate yield (entry 2). Two
(Table 3). Instead of benzoylacetonitrile (2A), acetylacetone
(
2B) and methyl acetoacetate (2C) also underwent dehydrative
allenylation with 1a and concomitant cyclisation to provide 3-
substituted 2,5-dimethyl-2-vinyl-2,3-dihydrofurans 6aB and
6aC in fair yields (entries 1 and 2). Cyclic 1,3-diketone 2D also
participated
in
the
tandem
reaction
to
give
tetrahydrobenzofuranone 6aD in 43% yield (entry 3).
α
-
substituted cyclic ketones 2E and 2F, as well as active
methylene compounds 2G–I bearing no ketone functionality,
underwent dehydrative substitution of 1b, which was not
substituents at C-2 and C-4 in 2,3-butadien-1-ol
1 were also
tolerated and transferred to the C-5 position and the terminal
carbon of vinyl group at C-5 of 4,5-dihydrofuran, respectively
2 | J. Name., 2012, 00, 1-3
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followed by cyclisation to furnish 1,1-disubtituted allenes 5bE– To reveal the requirements for the concomitant cyclisation,
in moderate to good yields (entries 4–8). In contrast, the allenylated ketone 5aA', prepared by allenylation of 2A with
I
coupling reaction between 2,3-butadienol (1i) and dimethyl methanesulfonate of 1a under basic conditions, was subjected
malonate (2G) took place, but the major product was not to the reaction conditions shown in Scheme 3. The O-
allene 5iG but triene 10iG (entry 9).
cyclisation of 5aA' proceeded under the palladium catalysis in
either methanol or THF as a solvent, whereas no reaction took
place in the absence of the catalyst (see supporting
information). Hence, dihydrofuran 6aA would be formed by
intramolecular hydroalkoxylation of allene 5aA' via either π-
Table 3 Scope of Pronucleophilesa
entry
1
substrate
pronucleophile
product
yield (%)
49
COMe
Me
allylpalladium intermediates 14 or 15 21, 22
On the contrary, the
.
Me
Me
CH3COCH2COCH3
(2B)
1a
O
palladium-catalysed cyclisation of phenyl-substituted allene
6aB
CO2Me
5hA' was dependent on the solvent with THF and methanol,
leading to dihydrofuran 6hA and cyclopentene 12hA
,
CH3COCH2CO2CH3
(2C)
Me
2
3
1a
1a
57
43
O
respectively. In addition, the exposure of 6hA to the catalyst in
methanol caused rearrangement to 12hA. Although it is not
clear yet, the exceptional C-cyclisation of 5hA' takes place only
in methanol through syn,anti-π-allylpalladium 15 with properly
arranged substituents (R1=Ph, R2≠H, R3=H)(Table 2, entry 7 vs.
1, 3, 5, 6).23
6aC
O
dimedone
(2D)
Me
O
6aD
R1
H
CN
R1
CN
5 mol% Pd(PPh3)4
R1
Ph
O
+
•
4
5
1b
51
53
Ph
H
O
Ph
solvent, 80 °C, 1.5 h
O
R2
R2
CN
R2
5aA´: R1 = Me, R2 = H
5hA´: R1 = Ph, R2 = Me
5aA´
A) THF
B) MeOH
6aA
97%
quant
12aA
0%
0%
condition B
70%
5hA´
A) THF
B) MeOH
6hA
78%
0%
12hA
0%
quant
1b
oxidative
addition
reductive
coupling
–Pd(0)
Ph
H
CN
Pd
hydropalladation
R1
R2
R1
CN
Ph
CN
CH2(CO2Me)2
(2G)
6
7
8
9
1b
1b
1b
1i
60
58
48
58
H
O-
H
Ph
R1
•
O
Pd+
Pd+
syn, anti-15
O-
R2
R2
13
syn, syn-14
Scheme 3 Pd(0)-Catalysed Cyclisation of 5aA' and 5hA' Leading to
Dihydrofuran 6aA·6hA and Cyclopentene 12hA
CH2(CN)2
(2H)
In summary, we have developed a Tsuji-Trost-type reaction
using allenic alcohols with pronucleophiles under neutral
conditions. Both methanol solvent and a substituent at C-2 in
2,3-butadienols turned out to be essential for the dehydrative
coupling reaction. Palladium complex plays a dual role in the
dihydrofuran synthesis to catalyse not only allenylation of
enolisable ketone pronucleophiles but also the following O-
cyclisation. Further studies on the asymmetric variant of the
reaction are underway.
CH2(SO2Ph)2
(2I)
OH
CO2Me
CO2Me
CH2(CO2Me)2
(2G)
10jG
a
Reaction conditions: 1 (1 equiv), 2B–I (2 equiv), Pd(PPh3)4 (5 mol%),
MeOH (0.1 M), 80 °C, 1. 5 h (entries 1–5, 8) or 2 h (entries 6, 7, 9).
This work was partly supported by JSPS KAKENHI Grant
Number JP15K07849, The Research Foundation for
Pharmaceutical Sciences, SUNTRY FOUNDATION for LIFE
SCIENCES and Platform Project for Supporting Drug Discovery
As reported in the literature on Tsuji-Trost reaction using allylic
alcohols in protic media,7 methanol is the best solvent for
dehydrative coupling reaction between allenic alcohol
which activate the poor leaving ability of the hydroxyl group in
via hydrogen-bond (Scheme 1). In methanol, the oxidative
addition of allenic alcohol to palladium(0) could predominate
over that of pronucleophile , and the latter leads to the
formation of hydroalkylation product
. The substituent R1 at
C-2 would help to avoid the undesired carbopalladation of
with exo-alkylidene-π-allylpalladium intermediate to give
dimerisation product 10
1 and 2,
and
Life
Science
Research
(Basis
for
Supporting Innovative Drug Discovery and Life Science
Research (BINDS)) from AMED under Grant Number
JP18am0101095 and JP18am0101100.
1
1
2
9
1
Conflicts of interest
3
.
There are no conflicts of interest to declare.
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20 The product structure was established by conventional
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5aA', which was prepared by alkylation of sodium salt of 2A
with methanesulfonate of 1a (see Supporting Information).
21 (a) I. Kadota, L. M. Lutete, A. Shibuya and Y. Yamamoto,
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,
,
22 There is a single report on Pd(0)-catalysed O-cyclisation of
,
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9
Pd-catalysed coupling between allenic alcohols and
arylboronic acids. (a) M. Yoshida, T. Gotou and M. Ihara,
23 Even a catalysis of HgO-TsOH transformed 5hA' into 12hA
and 6hA as major and minor products, respectively (see
Supporting Information).
4 | J. Name., 2012, 00, 1-3
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