Control of selectivity in palladium-catalyzed oxidative carbocyclization/borylation of allenynes

Article abstract of DOI:10.1002/anie.201301167

In control: A highly selective carbocyclization/borylation of allenynes with bis(pinacolato)diboron (B₂pin₂) under palladium catalysis and with p-benzoquinone (BQ) as the oxidant was developed. The use of either LiOAc×2 H₂O with 1,2-dichloroethane (DCE) as the solvent or BF₃×Et₂O together with THF is crucial for the selective formation of borylated trienes and vinylallenes, respectively.

Full text of DOI:10.1002/anie.201301167

Angewandte
Chemie
DOI: 10.1002/anie.201301167
Synthetic Methods
Control of Selectivity in Palladium-Catalyzed Oxidative
Carbocyclization/Borylation of Allenynes**
Youqian Deng, Teresa Bartholomeyzik, and Jan-E. Bꢀckvall*
Organoboronates are convenient and versatile reagents
owing to their comparatively low toxicity, high functional
group compatibility, and good stability.[1] Moreover, these
compounds can easily be oxidized to alcohols[2] or used to
À
construct new C C bonds by Suzuki–Miyaura cross-cou-
plings.[3] Because of the broad applications of these boronates,
many borylation methods have been developed.[4–7] Amongst
À
the routes reported for C B bond formation the most
common are Miyaura borylation,[4] hydroboration,[5] and the
reaction of lithium or magnesium organometallic compounds
with borate esters.[1] In addition, recent developments in
À
transition-metal-catalyzed C H borylation reactions have
also provided efficient access to boronates.[6]
À
Furthermore, by combining the borylation with a C C
bond-forming cyclization, complex molecules suitable for
various further functionalizations could be obtained in one
step.[8–11,13b,d] Such borylating carbocyclizations have been
successfully developed by the group of Cꢀrdenas.[9–11] Starting
from unsaturated compounds, such as enynes,[9] enediynes,[10]
enallenes,[11] and allenynes,[11] homoallylic or allylic boronates
were prepared under palladium(0) catalysis. For instance, the
non-oxidative borylating carbocyclization of allenynes 1 in
the presence of bis(pinacolato)diboron (B2pin2) yielded two
isomers (2 and 3), where borylation occurred at the allene
(Scheme 1a).[11]
Scheme 1. Palladium-catalyzed borylating carbocyclizations of alle-
nynes: a) under non-oxidative conditions;[11] b) under oxidative condi-
tions;[13b] c) under selective oxidative conditions. E=CO2Me.
In ongoing investigations our research group has been
studying oxidative PdII-catalyzed carbocyclizations of various
unsaturated molecules.[12–15] Recently we accomplished the
carbocyclization/arylation of allenynes with arylboronic
acids.[13b] Also, some preliminary results regarding carbocyc-
lization/borylation were obtained with differently substituted
1,5-allenynes (1; R = H, Me, Ar), which only gave borylated
triene products 4 (Scheme 1b).[13b] However, under carbocy-
clization/arylation conditions alkyl-substituted allenynes
afforded two different constitutional isomers (arylated trienes
and arylated vinylallenes) in a ratio determined by the
substitution on the starting allenyne.[13b] The aim of the
present study was to develop a carbocyclization/borylation
that can be directed towards either a borylated triene or
a borylated vinyllallene by control of the reaction conditions
(Scheme 1c). We now report a highly selective oxidative
carbocyclization/borylation of allenynes 1 with B2pin2 under
PdII catalysis with p-benzoquinone (BQ) as the oxidant. The
use of LiOAc·2H2O in 1,2-dichloroethane (DCE) or
BF3·Et2O in THF addressed the issue of selectivity, to give
either borylated trienes 4 or borylated vinylallenes 5,
respectively.
[*] Dr. Y. Deng, T. Bartholomeyzik, Prof. Dr. J.-E. Bꢀckvall
Department of Organic Chemistry, Arrhenius Laboratory
Stockholm University
SE-106 91 Stockholm (Sweden)
E-mail: jeb@organ.su.se
We first studied the reaction of ethyl-substituted allenyne
1a with B2pin2 under the original carbocyclization/borylation
conditions (Scheme 1b).[13b] The use of a catalytic amount of
palladium acetate (2 mol%) and stoichiometric amounts of
BQ (1.1 equiv) in THF at 508C led to an isomeric mixture of
borylated triene 4a and borylated vinylallene 5a in 28% and
14% yield, respectively (Table 1, entry 1). Analyzing the
effect of different solvents showed that a higher selectivity for
4a was obtained when DCE was used as the solvent (in
Table 1, entry 4 vs. entries 1–3). Furthermore, upon the
addition of catalytic amounts (20 mol%) of a basic salt,
[**] Financial support from the European Research Council (ERC AdG
247014), the Swedish Research Council, the Berzelii Centre
EXSELENT, the Knut and Alice Wallenberg Foundation, and the
Wenner-Gren Foundation (Y.D.) is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201301167.
ꢁ 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution Non-Commercial NoDerivs License, which
permits use and distribution in any medium, provided the original
work is properly cited, the use is non-commercial and no
modifications or adaptations are made.
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Table 1: Solvent and additive effect in the selective formation of triene 4a
or vinylallene 5a.[a]
Table 2: Selective carbocyclization of allenynes 1 yielding borylated
trienes 4.[a]
Entry
Solvent
Additive
(20 mol%)
Time
[h]
Yield of
4a/5a
4/5[b]
Yield of 4 [%],[c]
4a/5a [%][b]
Entry Allenyne
Product
ratio[b]
1
2
3
4
5
6
7
THF
–
–
–
–
15
15
15
15
15
15
15
15
20
20
20
20
20
28:14
33:34
39:35
61:13
70:7
67:6
73:7
71:10
19:16
0
2:1
1:1
1:1
5:1
10:1
11:1
10:1
7:1
ca. 1:1
–
1:26
1:12
1:3
cyclohexane
CH2Cl2
DCE
DCE
DCE
DCE
DCE
THF
THF
1
2
10:1
99:1
73
92
Na2CO3
NaOAc
LiOAc·2H2O
LiOAc·2H2O
HOAc
p-TSA
BF3·Et2O
–
8[c]
9
10
11
12[d]
13
THF
THF
THF
3:78
5:60
8:24
Et3B
55
3
4
99:1
4c/4c’=2.4:1
[a] Unless otherwise noted, 1a, B2pin2 (1.3 equiv), Pd(OAc)2 (2 mol%),
BQ (1.1 equiv), and indicated additive (20 mol%) were dissolved in the
indicated solvent (5 mLmmolÀ1) and stirred at 508C in a sealed tube.
[b] Yield was determined by 1H NMR spectroscopy using anisole as
internal standard. [c] 50 mol% of LiOAc·2H2O was added. [d] 2 mol% of
[Pd(CH3CN)4][(BF4)2] was used in place of Pd(OAc)2. E=CO2Me.
9:1
81
>11:1 92[d]
such as Na2CO3, NaOAc, or LiOAc·2H2O, formation of triene
4a was favored (Table 1, entries 5–7). Boronate 4a was
obtained in high selectivity in 73% yield with LiOAc·2H2O
as the base additive and with DCE as the solvent (Table 1,
entry 7; defined as Method A). An increase of the amount of
LiOAc·2H2O to 50 mol% gave no additional improvement in
selectivity or yield (Table 1, entry 8).
65
5
18:1
4e/4e’=2.3:1
The finding that the addition of a basic salt substantially
enhanced the selective formation of alkenyl boronate 4
encouraged us to study the effect of acidic reaction conditions.
To our surprise, the addition of a Brønsted acid, such as
HOAc, generated an approximately 1:1 mixture of 4a and 5a
in moderate yields (Table 1, entry 9) and the use of p-
toluenesulfonic acid (p-TSA) even did not afford any
borylation products (Table 1, entry 10). However, the use of
a Lewis acid, BF3·Et2O, resulted in a high selectivity for 5 and
afforded products 4a and 5a in 3% and 78% yield,
respectively (Table 1, entry 11; defined as Method B). Nota-
bly when the cationic palladium catalyst [Pd(CH3CN)4]-
[(BF4)2] was used the same trend in selectivity was seen but
a lower yield was obtained (Table 1, entry 12 vs. entry 11).[16]
The structurally similar Lewis acid BEt3 was also tried and
moderate selectivity for 5a over 4a was seen with low yields
of products (Table 1, entry 13).
With the optimized conditions for the selective formation
of borylated triene 4a established, we applied them to
differently substituted allenynes (Table 2). The allenynes
bearing a methyl group on the alkyne moiety (1b and 1c)
afforded the borylated trienes as the sole products (Table 2,
entries 2 and 3). For substrates with a longer alkyl group (1d
and 1 f) on the alkyne moiety, the competing allene formation
took place to a notable extent (Table 2, entries 4 and 6), but
48
6
7
5:1
4 f/4 f’=1.4:1
>20:1 57
[a] Unless otherwise noted, 1 (0.1–0.2 mmol), B2pin2 (1.3 equiv), Pd-
(OAc)2 (2 mol%), BQ (1.1 equiv), and LiOAc·2H2O (20 mol%) were
dissolved in DCE (5 mLmmolÀ1) and stirred at 508C for 15 h. [b] The
ratio was determined by H NMR analysis of the reaction mixture.
[c] Yield of the isolated product. [d] 1 mmol of 1d was used. E=CO2Me.
1
the corresponding triene products 4d and 4 f/4 f’ could be
isolated in good to moderate yields. In those cases where the
substrates are unsymmetrically substituted at the allene
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moiety (1c and 1e) a comparatively high selectivity for the
triene products was observed (Table 2, entries 3 and 5).
However, a mixture of borylated triene products was
obtained, with a preference for formation of the products
with the more substituted double bond. Products 4c and 4e
were obtained as a mixture of Z/E isomers in a ratio of 3.3:1
and 3.5:1, respectively. The allenyne 1 f with a cyclohexylidene
group on the allene cyclized to give a mixture of isomers 4 f
and 4 f’, where the formation of 4 f’ could be explained by
a Pd-catalyzed isomerization of 4 f (Table 2, entry 6; for the
detailed mechanism, please see the Supporting Information).
Moreover, the reaction of allenyne 1g, having two benzyl
ether groups on the linker part X, also gave borylated triene
product 4g selectively in 57% yield (Table 2, entry 7), thus
proving that the malonate group of linker X is not necessary
for a successful transformation.
Method B) were applied to various allenynes (Table 3).
Allenynes 1a–1 f were transformed into vinylallenic boro-
nates 5a–5 f; for most cases the yield was between 70% and
80% and the formation of the corresponding triene isomers
4a–4 f was efficiently suppressed. Even the methyl-substi-
tuted substrate 1b, which intrinsically favors formation of
triene 4b,[13b] displayed opposite selectivity under these
reaction conditions, that is, favoring vinylallene formation
(Table 3, entry 2). The reaction of allenyne 1g under the
standard conditions of Table 3 was sluggish and did not give
the desired product 5g, probably because of the incompati-
bility between the benzyl ether group and BF3·Et2O. How-
ever, by switching the palladium catalyst to [Pd(CH3CN)4]-
[(BF4)2] and in the absence of BF3·Et2O, product 5g was
obtained in 37% yield (entry 7).
To gain further insights into the mechanism of the
oxidative carbocyclization/borylation, kinetic deuterium iso-
tope effects were studied (Scheme 2). An intermolecular
competition experiment using 1d and its hexadeuterated
derivative [D6]-1d under the conditions for selective triene
formation for 1 h provided a large intermolecular KIE value
The optimized reaction conditions for the selective
formation of borylated vinylallene 5 (20 mol% of BF3·Et2O,
Table 3: Selective carbocyclization of allenynes 1 yielding borylated
vinylallenes 5.[a]
of 6.7[17] (Scheme 2a). This result indicates that the allylic C
À
H bond cleavage involved has to occur prior to any
irreversible step of the reaction, for example, the carbocy-
clization step.[18] On the other hand, when a 1:1 mixture of 1d
and [D2]-1d was subjected to the conditions for selective
vinylallene formation for 1 h the ratio between 5d and [D1]-
5d was 2.4, from which the KIE was determined to 2.7[19]
(Scheme 2b).[17] The intrinsic KIE from intramolecular com-
petition for vinylallene formation was determined to 5.3[17] by
the use of [D1]-1d as the allenyne substrate (Scheme 2c). The
Entry
1
Allenyne
Product
5/4[b]
Yield of
5 [%][c]
>20:1
77
À
results in Scheme 2b and 2c indicate that the propargylic C
H bond cleavage does not fully determine the selectivity
2
3
>20:1
>20:1
73
56
79
4
5
>20:1
87[d]
20:1
77
6
>20:1
>20:1
70
37
7[e]
[a] Unless otherwise noted, 1 (0.1–0.2 mmol), B2pin2 (1.3 equiv), Pd-
(OAc)2 (2 mol%), BQ (1.1 equiv), and BF3·Et2O (20 mol%) were
dissolved in THF (5 mLmmolÀ1) and stirred at 508C for 20 h. [b] Ratio
determined by 1H NMR analysis of the crude reaction mixture. [c] Yield of
the isolated product. [d] 1 mmol of 1d was used. [e] 2 mol% of
[Pd(CH3CN)4][(BF4)2] was used in place of Pd(OAc)2 and BF3·Et2O.
E=CO2Me.
Scheme 2. Kinetic isotope effect study. Reaction conditions: i) B2pin2
(1.3 equiv), Pd(OAc)2 (2 mol%), BQ (1.1 equiv), LiOAc·2H2O
(20 mol%), DCE, 50 8C, 1 h. ii) B2pin2 (1.3 equiv), Pd(OAc)2 (2 mol%),
BQ (1.1 equiv), BF3·Et2O (20 mol%), THF, 50 8C, 1 h for b) and 20 h
for c). E=CO2Me.
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between 5d and [D1]-5d in the competitive experiment
(Scheme 2b).[18]
Transmetalation of C with B2pin2 and reductive elimination
would form product 4. The competing alkyne attack through
À
Three control experiments with allenyne 1d and the
corresponding deuterium-labeled allenynes [D2]-1d and [D6]-
1d were conducted under palladium catalysis in the absence
of any additional basic or acidic additive and using DCE as
the solvent (Scheme 3). Under these conditions the reaction
propargylic C H bond cleavage in A would produce allenyl-
palladium intermediate D. Intramolecular vinylpalladation of
the allene moiety would generate (p-allyl)palladium inter-
mediate E. Transmetalation with B2pin2 and subsequent
reductive elimination would give 5. The mechanism in
Scheme 4 is supported by the kinetic isotope effects and the
experiments with deuterium-labeled compounds (Scheme 2
and Scheme 3). The lower kinetic isotope effect observed for
the competitive experiment in Scheme 2b compared to the
intramolecular experiment in Scheme 2c may reflect that
1 and A are not in full equilibrium under the conditions for
formation of 5. In the path for formation of 5 it is likely that
BF3·Et2O creates a cationic palladium species, which interacts
better with the acetylene compared to the allene in A.[21]
In summary, we have developed an unprecedented
selective PdII-catalyzed carbocyclization/borylation of alle-
nynes under oxidative conditions. By controlling the reaction
conditions the reaction can be directed to either the triene 4
or the vinylallene 5. On the basis of the results of deuterium-
labeling experiments, we propose that the reactions of
allenynes proceed through competing allylic and propargylic
À
C H bond cleavage pathways to give borylated trienes and
borylated vinylallenes, respectively.
Scheme 3. Effect of isotope substitution on product distribution.
Reaction conditions: i) B2pin2 (1.3 equiv), Pd(OAc)2 (2 mol%), BQ
(1.1 equiv), DCE, 50 8C, 15 h. ii) B2pin2 (1.3 equiv), Pd(OAc)2
(5 mol%), BQ (1.1 equiv), DCE, 50 8C, 15 h. E=CO2Me.
Experimental Section
Typical experimental procedure for palladium-catalyzed oxidative
borylating carbocyclization of allenyne 1 to boronate 4: 1a (26.0 mg,
0.10 mmol) and 0.5 mL of DCE were added to a mixture of B2pin2
(33.1 mg, 0.13 mmol), BQ (12.2 mg, 0.11 mmol), Pd(OAc)2 (0.5 mg,
0.002 mmol), and LiOAc·2H2O (1.8 mg, 0.02 mmol) at RT. The
reaction was stirred at 508C for 15 h. After the reaction was complete,
as monitored by TLC, evaporation and column chromatography on
silica gel (pentane/ethyl acetate = 10:1) afforded 4a (27.9 mg, 73%)
of 1d gave a mixture of 4d and 5d in a ratio of 3.8:1
(Scheme 3a). When substrate [D2]-1d was employed
(Scheme 3b) under the same reaction conditions, the ratio
increased to 12.3:1.[20] Allenyne [D6]-1d showed the opposite
selectivity, with [D5]-4d and [D6]-5d being formed in a ratio of
1:5.1[20] (Scheme 3c).
1
as a liquid; H NMR (500 MHz, CDCl3): d = 5.93 (s, 1H), 5.02–5.00
(m, 2H), 3.72 (s, 6H), 3.20 (s, 2H), 2.19 (q, J = 7.5 Hz, 2H), 1.95 (s,
3H), 1.26 (s, 12H), 1.02 ppm (t, J = 7.5 Hz, 3H); 13C NMR (125 MHz,
CDCl3): d = 171.1, 151.1, 147.9, 139.8, 129.1, 116.6, 83.3, 63.0, 52.9,
37.5, 27.0, 25.2, 23.8, 13.4 ppm; HRMS (ESI): calc. for C21H31BNaO6
[M + Na]+: 413.2110; found: 413.2113.
The results in Scheme 2 and Scheme 3 indicate that
À
competing allylic and propargylic C H bond cleavage
Typical experimental procedure for palladium-catalyzed oxida-
tive borylating carbocyclization of allenyne 1 to boronate 5: 1a
(52.7 mg, 0.20 mmol) and 1.0 mL of THF were added to a mixture of
B2pin2 (66.2 mg, 0.26 mmol), BQ (24.0 mg, 0.22 mmol), Pd(OAc)2
(1.0 mg, 0.004 mmol), and BF3·Et2O (6 mL, 0.04 mmol) at RT. The
reaction was stirred at 508C for 20 h. After the reaction was complete,
as monitored by TLC, evaporation and column chromatography on
silica gel (pentane/ethyl acetate = 10/1) afforded 5a (59.6 mg, 77%)
as a liquid; 1H NMR (400 MHz, CDCl3): d = 5.55 (d, J = 1.6 Hz, 1H),
5.34–5.23 (m, 1H), 3.716 (s, 3H), 3.715 (s, 3H), 3.19–3.17 (m, 2H),
1.68 (d, J = 7.2 Hz, 3H), 1.18 (s, 12H), 1.17 (s, 3H), 1.13 ppm (s, 3H);
13C NMR (100 MHz, CDCl3): d = 199.1, 171.5, 171.3, 153.8, 122.9,
107.1, 91.2, 83.1, 63.5, 52.7, 36.5, 25.0, 24.7, 24.5, 23.9, 23.8, 14.8 ppm;
HRMS (ESI): calcd for C21H31BNaO6 [M + Na]+: 413.2110; found:
413.2103.
occurs in 1, and this determines the ratio of boronates 4 and
5 (Scheme 4). The allene attack on PdII complex A through
[12,13a–d]
À
allylic C H bond cleavage
would give B and subse-
quent alkyne insertion would generate intermediate C.
Received: February 8, 2013
Published online: May 17, 2013
Keywords: allenes · boronates · cyclization · oxidation ·
palladium
Scheme 4. Proposed mechanism for palladium-catalyzed oxidative
selective carbocyclization/borylation of allenyne 1.
.
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the isotope effect calculated from the product ratio and the
change of the starting material ratio is approximately 2.43 ꢈ [(1 +
1.2)/2] = 2.7 (see the Supporting Information).
À
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À
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extent.
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[21] For another possible mechanism involving a pallada(IV)cyclo-
pentene intermediate[13a,b] (see the Supporting Information).
Angew. Chem. Int. Ed. 2013, 52, 6283 –6287
ꢀ 2013 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 6287