Catalytic transient leaving group for atom-economic synthesis of allenes from 2-alkynols

Article abstract of DOI:10.1039/c7cc06866b

An atom economic approach from readily available propargylic alcohols to allenes, the first carboxylation of propargylic alcohols, has been established. Through the cooperative binary catalysis of Pd and a phosphoric acid, the reaction afforded multi-substituted allenoates with a broad scope tolerating useful functional groups. The synthetic potential of the obtained products has been demonstrated.

Full text of DOI:10.1039/c7cc06866b

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
N@C2n (2n
=
68 and 80). Synthesis of the
3
N@D5h-C80
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Edge Article
Catalytic Transient Leaving Group for Atom-Economic Synthesis of
Allenes from 2-Alkynols
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
b
*b,c
Wanli Zhang, Chaofan Huang, Yuan Yuan, and Shengming Ma
DOI: 10.1039/x0xx00000x
www.rsc.org/
An atom economic approach from readily available propargylic from the hydroxy group in the readily available propargylic
alcohols to allenes, the first carboxylation of propargylic alcohols, alcohols, which would trigger an oxidative addition with a
has been established. Under the cooperative binary catalysis of Pd transition metal catalyst to cleave their propargylic carbon-
and a phosphoric acid, the reaction afforded multi-substituted oxygen bond for subsequent coupling reactions via a similar
allenoates with a broad scope tolerating useful functional groups. Int-1-type intermediate. The challenge is to identifying the
Synthetic potentials of obtained products have been cooperative working Brønsted acid and transition metal
7
+
demonstrated.
catalysts. Herein, we report the first Pd/H -cocatalyzed
preparation of tetra- or tri-substituted allenoates from 2-
alkynols.
The presence of two cumulated carbon-carbon double bonds
is the distinctive structural characteristic of allenes and
1
contributes to their extraordinary reactivities. Due to the
a) Carboxylation of 2-alkynylic mesylates and carbonates
4
R1
H
COOR4
GL
R1
cat. Pd; Ligand
H
Pd
+
R
OH, CO
broad existence of allene unit in natural products,
pharmaceutical molecules, and applications of allenes in
advanced molecules/materials, the chemistry of allenes has
R2
2
LG = OMs, OCOOR
R1
R2
R
Int-1
b) Oxidative addition of 2-alkynols with transition metal catalyts
2
,3
been catching increasing attention from chemists. Especially,
much attention has been focused on development of new
methodologies for accessing differently substituted allenes
HO
cat. TM
H
TM
R2
1
R1
R
R2
c) Carboxylation of readily available 2-alkynyols (this work):
+H
cat. Pd
R1
R3
COOR4
R2
2
R1
O
R4OH, CO
4
HO
cat. H+
recently. In addition to the well established coupling reactions
1
R2
R2
R
R3 = H or not H
R3
R3
H
2
O + H+
of alkynes with
a
propargylic leaving group with
4
organometallic reagents, the Pd-catalyzed carboxylative
coupling reactions of propargylic mesylates or carbonates with
1. First example of oxidative addition-based catalytic
activation of propargylic C-OH bond
+
2
. H /Pd(0) binary catalysis;
alcohols in the presence of CO afforded 2,3-allenoates via Int-
5
3. Atom-ecomonic allene synthesis
1.
However, such a coupling strategy utilizing the readily
Scheme 1. Carboxylation approaches to 2,3-allenoates with CO
available 2-alkynols has never been reported due to the
difficulty encountered in the step of oxidative addition of 2-
Initially, the study was launched using 2-phenyloct-3-yn-2-ol
alkynols with a transition metal catalyst although the S 2'-type
N
(
1a) as the starting material, [(π-allyl)PdCl]
BINAP as the ligand to identify the appropriate acid. When
CF COOH was added, the expected formations of enone 2a
2
as the catalyst and
reaction of 2-alkynols with a stoichiometric amount of TiCl for
4
6
the synthesis of 1,2-allenyl chlorides has been reported. We
3
would like to propose a new concept of utilizing a catalytic
and enyne 3a were detected (Table 1, entry 1): enone 2a was
amount of Brønsted acid to form a transient leaving group
formed through Meyer-Schuster rearrangement and 3a was
8
formed via dehydration with the help of CF COOH (Scheme 2).
3
A range of acids were then screened, however, the expected
product 4a was not formed (Table 1, entries 2-7). To our
delight, when 1,1'-binaphthyl-2,2'-diyl hydrogen phosphate
(
acid
% yield of 3a (Table 1, entry 8). Using Ph
results-4a was formed in only 2% yield (Table 1, entry 9).
Surprisingly, the reaction with (PhO) POOH afforded 27% yield
4
) was used, 22% yield of 4a was detected together with
7
2
POOH led to inferior
2
of allenoate 4a (Table 1, entry 10). Control experiment was
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also conducted: the formation of 2a, 3a, and 4a was not found that DPEphos was the most effective ligand, forming 4a
detected in the absence of an acid (Table 1, entry 11).
with 64% yield (Table 3, entry 8).
a
a
Table 1. Screening of acid with BINAP as the ligand
Table 2. The effect of Pd sources
b
NMR yield (%)
Recovery of
Entry
Acid
b
b
2
a
c
3a 4a
1a (%)
NMR yield (%)
Recovery of
Entry
[Pd]
b
1
2
3
4
5
6
7
8
9
CF
AgOTf
CH SO OH
3
COOH
6
27
40
-
8
-
-
-
-
-
2a
3a
-
4a
1a (%)
c
c
1
2
3
4
5
6
PdCl
PdCl (PPh
[PdCl(cinnamyl)]
2
-
-
-
-
-
-
-
2
2
-
-
3
2
-
-
-
)
-
2
3
2
BnCOOH
Acid 1
Acid 2
Acid 3
Acid 4
-
-
82
80
87
96
-
2
5
21
-
-
-
-
-
PdCl
PdCl
2
(DPPE)
(DPPF)
21
14
-
-
-
-
-
2
-
-
-
-
-
[(π-allyl)PdCl]
2
2
27
42
5
13
-
7
1
2
-
22
2
27
-
c
7
[(π-allyl)PdCl]
1
Ph
2
POOH
-
-
a
1
1
0
1
(PhO)
2
POOH
-
5
Reaction conditions: 1a (0.5 mmol), [Pd] (2 mol%), BINAP (6 mol%), (PhO)
2
POOH
o
-
-
71
(10 mol%), and MeOH (1.2 equiv.) in toluene (2.0 mL) at 80 C under 1 atm of CO
b
1
unless otherwise noted.
Determined by
H NMR analysis using 1,3,5-
c
trimethylbenzene as the internal standard. The reaction was carried out at 50
o
C.
a
Table 3. The effect of ligand
a
Reaction conditions: 1a (0.5 mmol), [(π-allyl)PdCl]
2
(2 mol%), BINAP (6 mol%),
o
[( -allyl)PdCl]2 (2 mol%)
Ligand (6 mol%)
(PhO)2POOH (10 mol%)
MeOH (1.2 equiv.)
acid (10 mol%), and MeOH (1.2 equiv.) in toluene (2.0 mL) at 80 C under 1 atm of
b
1
CO unless otherwise noted. Determined by H NMR analysis using 1,3,5-
O
HO
Ph
n-Bu
+
c
COOMe
trimethylbenzene as the internal standard. E:Z > 20:1.
n-Bu
n-Bu+
Ph
Toluene
Ph
Ph
n-Bu
CO balloon
0 oC, 10 h
1a
2a
3a
4a
5
b
NMR yield (%)
Recovery of 1a
Entry
Ligand
BINAP
b
2
a
3a
1
-
4a
42
35
37
-
(%)
1
2
3
4
5
6
7
8
-
13
47
-
PPh
3
-
50
-
c
P(2-furyl)
3
11
-
Zheda-phos
Sphos
62
66
100
44
10
-
Scheme 2 Mechanism for the formation of enone 2a and enyne 3a
-
-
30
-
DPPE
-
-
Encouraged by these results, different Pd sources were then
examined. PdCl , PdCl (PPh ) , and [PdCl(cinnamyl)] gave 2-
DPPF
-
6
-
20
64
86
2
2
3 2
2
DPEphos
DPEphos
-
21% yields of 4a (Table 2, entries 1-3). PdCl (DPPE) and
2
d
9
-
-
PdCl (DPPF) were ineffective-nothing but 14-21% yields of 3a
2
a
were detected (Table 2, entries 4 and 5). [(π-allyl)PdCl] was
2
Reaction conditions: 1a (0.5 mmol), [(π-allyl)PdCl]
PhO) POOH (10 mol%), and MeOH (1.2 equiv.) in toluene (2.0 mL) at 50 C under
1 atm of CO unless otherwise noted. Determined by H NMR analysis using
2
(2 mol%), Ligand (6 mol%),
o
(
2
the most ideal Pd catalyst, obtaining 4a with 27% yield (Table
o
, entry 6). Running the reaction at 50 C gave better results-a
b
1
2
c
d
1
,3,5-trimethylbenzene as the internal standard. E:Z
>
20:1.
5 mol%
higher yield of 4a with some recovery of starting material
alcohol 1a was observed (Table 2, entry 7).
(
PhO) POOH and 8.0 equiv. MeOH were used instead.
2
Ligands always play an important role in organometallic
catalysis. PPh3 and P(2-furyl)3 were able to accelerate the
After further optimization, it is apparent that when 2 mol%
[(π-allyl)PdCl] , 6 mol% DPEphos, 5 mol% (PhO) POOH, and 8.0
2
2
o
formation of allenoates, obtaining 4a with 35-37% yields
9
Table 3, entries 2 and 3), whereas Zheda-Phos led to nothing
equiv. MeOH were applied in toluene at 50 C under 1 atm of
CO (Table 3, entry 9, defined as the standard conditions), 2,3-
allenoates can be obtained with the highest yield from the 2-
alkynylic alcohols (for details, see Tables S2 and S3 in
Supporting Information). The substrate scope was then
(
but 62% recovery of 1a (Table 3, entry 4). When Sphos was
used, 30% yield of 4a and 66% recovery of 1a were detected
(
Table 3, entry 5). Diphosphine ligands were also investigated:
DPPE gave inferior result with 100% of 1a being recovered
Table 3, entry 6); the desired allenoate 4a was produced with
1
2
evaluated (Table 4). When R is an alkyl group, R may be alkyl
and phenyl or substituted phenyl (Table 4, 4a 4n). The steric
(
-
20% yield when applying DPPF (Table 3, entry 7); finally, it was
effect is obvious since 2-methylphenyl led to a much lower
2
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yield (compare 4b, 4c, and 4d); it is interesting to observe that
the current reaction conditions are selecttive enough to
In order to unveil the mechanism of this carboxylation
1
differentiate the reactivity of C-Br bond and the propargylic C- reaction, the reactions of 1u and 1u- O at 50 C were
8
o
1
O bond in 1e: the C-Br bond remained untouched for further conducted and analyzed via H-NMR analysis (For details, see
elaboration. In addition, functional groups such as chloro, and Supporting Information). A linear relationship was observed
methoxy are well tolerated under the optimized reaction for ln ([1u]) vs. reaction time, indicating a first-order
1
0
conditions (Table 4, 4f and 4g); heteroaryl groups such as furyl dependence of reaction rate with respect to substrate 1u
1
.
In
8
and thienyl made no difference, furnishing the target products addition, no
O was found in the product 4u. The kinetic
3
with moderate yields (Table 4, 4i and 4j). R may also be an isotopic effect was determined to be 1.40, indicating that the
alkyl or aryl group. Meanwhile, reactions of ethanol, cleavage of C-O bond is not the rate-determined step (eq. 3).
isopropanol, and trimethylsilylmethanol have also been
4
studied, providing allenoates possessing different kinds of R
group with good yields (Table 4, 4r-4t).
Table 4. Substrate scope for the synthesis of multi-substituted allenoates from
a
propargylic alcohols
Based on these results, a plausible mechanism is proposed
(Scheme 3): the reaction of propargylic alcohol with Brønsted acid
[
(
-allyl)PdCl]2 (2 mol%)
DPEphos (6 mol%)
(
HB) would yield the oxonium ion I, which would easily undergo
(
PhO)2POOH (5 mol%)
R1
R2
COOR4
R3
HO
4
R OH (8 equiv.)
1
R3
oxidative addition reaction with Pd(0) to produce the
allenylpalladium intermediate II. Its subsequent reaction with CO
and MeOH afforded intermediates III and/or IV while regenerating
R
R2
Toluene
1
4
CO balloon
5
0 oC, 14 h
COOMe
COOMe
COOMe
11
HB. Although intermediate III may be more viable, complete
COOMe
n-C6H13
n-Bu
n-C6H13
exclusion of intermediate IV is not possible. Subsequent reductive
elimination would afford 2,3-allenoate products and regenerate
active Pd(0) catalyst to fulfill the catalytic cycle. We reasoned that
Ph
n-Bu
4a
4c
85% yield c
4d
58% yield d
4
b
90% yield
8
4% yield
COOMe
n-C6H13
COOMe
COOMe
COOMe
the acidity of (PhO)
2
POOH matches the propargylic alcohols and the
n-Bu
n-Pr
n-Bu
o
o
o 12
large bite angle of DPEphos (104 ) (DPPE (86 ) and DPPF (99 ))
4
e
4h
78% yield
Br
Cl
4f
MeO
4g
55% yield
7
6% yield
accelerated the reductive elimination of intermediate III and/or IV.
3% yield e,f
8
COOMe
COOMe
COOMe
Ph
COOMe
Ph
n-Bu
n-Bu
n-C5H11
n-Pr
O
4
S
4
i
4
j
4k
4l
53% yield e,h
4% yield
5
7% yield
56% yield e,g
COOMe
n-Pr
COOMe
COOMe
Et
Ph
COOMe
Ph
2 4
(CH ) Cl
n-Pr
Ph
Ph
n-Pr
4o
80% yield
O
4m
4n
80% yield
4p
8
1% yield
O
2% yield i
6
n-Bu
COOMe
COOEt
i
COOPr
COOCH TMS
2
Ph
n-C6H13
8% yield
Ph
n-Bu
Ph
n-Bu
Ph
n-Bu
4r
4q
4s
4t
82% yield e,j
59% yield e,k
7
2% yield e
8
a
Reaction conditions: 1 (1.0 mmol), [(π-allyl)PdCl]
PhO) POOH (5 mol%) and MeOH (8 equiv.) in toluene (5.0 mL) at 50 C under 1
atm of CO unless otherwise noted. isolated yields. the reaction was carried out
2
(2 mol%), DPEphos (6 mol%),
o
(
2
b
c
d
e
for 10 h. 36% recovery of 1d was detected. the reaction was carried out with
4
f
1
6 equiv. of R OH. the reaction was carried out for 16 h, 8% recovery of 1f was
g
h
i
detected. 32% recovery of 1k was detected. 44% recovery of 1l was detected.
the reaction was carried out with 3 mol% [(π-allyl)PdCl] and 9 mol% DPEphos.
1% recovery of 1a was detected. 20% recovery of 1a was detected.
j
2
k
Scheme 3. A plausible mechanism
1
As a class of useful chemicals in organic synthesis, 2,3-
allenoates can be transferred into different versatile products
Apart from these, secondary propargylic alcohol may also be
utilized to give trisubstituted allenoates 4u with 68% yield (eq.
(
Scheme 4): treatment with CuBr
1
2
or I
2
afforded 4-bromo-
14
1
). The reaction may be easily conducted on 5 mmol scale of
3
2
(5H)-furanone (
5)
or 4-iodo-2(5H)-furanone (
6)
with good
1a
(eq. 2).
o
to excellent yields; hydrolysis with KOH at 50 C for 1.5 hours
afforded tetra-substituted allenic acid in nearly
quantitative yield; reduction with DIBAL-H at -78 C could
yield 2,3-allenol with 70% yield; 1,2-addition reaction with
allyl magnesium chloride furnished tertiary α-allenol with
7
a
o
1
5
8
9
1
6
8
5% yield.
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Chem. Int. Ed., 2004, 43, 1196; (b) P. Rivera-Fuentes and F.
Diederich, Angew. Chem. Int. Ed., 2012, 51, 2818.
3
4
5
For selected examples of using allenes as ligands or catalysts,
see: (a) I. Sato, Y. Matsueda, K. Kadowaki, S. Yonekubo, T.
Shibata and K. Soai, Helv. Chim. Acta., 2002, 85, 3383; (b) S.
Löhr, J. Averbeck, M. Schurmann and N. Krause, Eur. J. Inorg.
Chem., 2008, 552; (c) X. Pu, X. Qi and J. M. Ready, J. Am.
Chem. Soc., 2009, 131, 10364; (d) F. Cai, X. Pu, X. Qi, V. Lynch,
A. Radha and J. M. Ready, J. Am. Chem. Soc., 2011, 133
1
,
8066; (e) A. Vanitcha, C. Damelincourt, G. Gontard, N.
Vanthuyne, V. M. Mansuy and L. Fensterbank, Chem.
Commun., 2016, 52, 6785.
For reviews on the synthesis of allenes, see: (a) A. Hoffmann-
Röder and N. Krause, Angew. Chem. Int. Ed., 2002, 41, 2933;
Scheme 4. Synthetic applications of tetra-substituted allenotae 4a
(
1
b) N. Krause and A. Hoffmann-Röder, Tetrahedron, 2004, 60
,
1671; (c ) G. B. Hammond, ACS Symp. Ser., 2005, 911, 204;
In conclusion, a robust and atom-economic straightforward
protocol utilizing Brønsted acid and palladium as the catalysts
for the preparation of multi-substituted 2,3-allenoates from
the readily available propargylic alcohols with moderate to
good yields directly was developed. The synthetic potentials of
these allenoates have also been demonstrated forming other
useful building blocks. Mechanistic study shows that the
cleavage of C-O bond with the k16O/k18O of 1.40. This protocol
would open a new area for the synthesis of allenes directly
from propargylic alocols via the Int-1-based coupling strategy.
Further studies in this area including the role of each catalyst
and enantioselective carboxylation are being conducted in our
laboratory.
(
(
d) K. M. Brummond and J. E. DeForrest, Synthesis, 2007, 795;
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S. Yu and S. Ma, Chem. Commun., 2011, 47, 5384; (g) R. K.
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Ma, Org. Chem. Front., 2014, , 1210; (i) R. K. Neff and D. E.
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(
1
g) Y. Wang, W. Zhang and S. Ma, J. Am. Chem. Soc., 2013,
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1
8
35, 11517; (h) W. Zhang and S. Ma, Chem. Eur. J., 2017, 23
590.
,
,
6
7
G. V. Karunnakar and M. Periasamy, J. Org. Chem., 2006, 71
7463.
ACKNOWLEDGMENTS
For selected examples of cooperative catalysis using
transition metal and Bronsted acid, see: (a) M. Ruepng, A. P.
Antonchich and C. Brinkmann, Angew. Chem. Int. Ed., 2007,
46, 6903; (b) S. Mukherjee and B. List, J. Am. Chem. Soc.,
Financial support from National Natural Science Foundation of
China (Grant No. 21690063) and the National Basic Research
Program (2015CB856600) are greatly appreciated. We also
thank Mr. Zheng Yang in this group for reproducing the results
2
007, 129, 11336; (c) Z. Han, H. Xiao, X. Chen and L. Gong, J.
Am. Chem. Soc., 2009, 131, 9182. (d) J. Wu, Z. Yang, S. Zhang,
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for 4g, 4j, and 4l presented in Table 4.
5
,
6
8
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W. Yan, X. Ye, N. G. Akhmedov, J. L. Petersen and X. Shi, Org.
Lett., 2012, 14, 2358.
Conflicts of interest
There are no conflicts to declare.
9
1
P. Li, B. L
255.
ü, C. Fu and S. Ma, Adv. Synth. Catal., 2013, 355,
1
0 P. Atkins and J. d. Paula, Atkins’ Physical Chemistry, Oxford
University Press, Eighth Edition; Oxford Univ. Press: New
York, 2006, Chap. 22.
Notes and references
1
For selected reviews, see: (a) N. Krause and A. S. K. Hashimi,
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DOI: 10.1039/C7CC06866B
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An atom economic approach to multi-substituted allenes
from propargylic alcohols with water as the only byproduct
has been established.
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