Selective Cascade Reaction of Bisallenes via Palladium-Catalyzed Aerobic Oxidative Carbocyclization–Borylation and Aldehyde Trapping

Article abstract of DOI:10.1002/anie.201610524

A cascade reaction, consisting of a palladium-catalyzed regioselective aerobic oxidative carbocyclization–borylation of bisallenes and a final aldehyde trapping, afforded triene alcohols with high diastereoselectivity. The cascade reaction occurs under mild reaction conditions and proceeds via an allylboron intermediate that is trapped by the aldehyde in a stereoselective manner.

Full text of DOI:10.1002/anie.201610524

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201610524
German Edition: DOI: 10.1002/ange.201610524
Aerobic Oxidation
Selective Cascade Reaction of Bisallenes via Palladium-Catalyzed
Aerobic Oxidative Carbocyclization–Borylation and Aldehyde
Trapping
Veluru Ramesh Naidu, Daniels Posevins, Chandra M. R. Volla, and Jan-E. Bꢀckvall*
Abstract: A cascade reaction, consisting of a palladium-
catalyzed regioselective aerobic oxidative carbocyclization–
borylation of bisallenes and a final aldehyde trapping, afforded
triene alcohols with high diastereoselectivity. The cascade
reaction occurs under mild reaction conditions and proceeds
via an allylboron intermediate that is trapped by the aldehyde
in a stereoselective manner.
I
n synthetic organic chemistry, cyclization of allenes is an
important class of reactions that can be applied for the
synthesis of a variety of carbocyclic and heterocyclic com-
pounds.[1,2] In particular, transition metal-catalyzed cycliza-
tions of conjugated and non-conjugated bisallenes provide
efficient and atom-economical routes to polyunsaturated
molecules.[3] In the synthesis of carbocyclic molecules,
Scheme 1. Previous work and proposal for this work.
À
metal-catalyzed cascade reactions with C C bond formation
lizations towards new applications. Recently, we have devel-
oped a carbocyclization–arylation of bisallenes.[3c] It would be
highly interesting to use a diboronate (e.g. B2pin2) instead of
an arylboronic acid in the latter reaction of bisallene 1 leading
to allylborane 2, since the latter may undergo an additional
are of great interest for obtaining complex structures with
high stereoselectivity.[4,5]
Organoboron compounds can be obtained through differ-
ent routes such as Brown hydroboration and the palladium-
catalyzed Miyaura borylation etc., and they are versatile
building blocks in the synthesis of natural products and
bioactive compounds.[6–8] In recent years, metal-catalyzed
non-oxidative or oxidative carbocyclization studies with
unsaturated systems have been well explored.[9,10,11] There
are recent reports on non-oxidative palladium-catalyzed
borylative cyclization of enynes and enallenes employing
bis(pinacolato)diboron (B2pin2) as the borylating agent.[9]
In the past decade, our group has focused on the
understanding of the mechanistic pathways in palladium-
catalyzed oxidative carbocyclization reactions of enallenes,
and allenynes (Scheme 1a).[3b,c,10] These studies include palla-
dium-catalyzed oxidative carbocyclization–borylation and
carbocyclization–arylation of enallenes by the use of B2pin2
and arylboronic acids, respectively.[10b,c] In a similar fashion we
have studied the palladium-catalyzed oxidative carbocycliza-
tion–borylation of allenynes in a regioselective manner.[10d]
There is a demand of exploring these allene-based carbocyc-
À
C C bond formation with an aldehyde (Scheme 1b). It is
expected that the reaction of the cyclic allylboron intermedi-
ate 2 with an aldehyde is highly stereoselective.[12] Here we
report on a one-pot cascade reaction of bisallenes involving
oxidative carbocyclization–borylation and final trapping with
an aldehyde (Scheme 1b).
In palladium-catalyzed oxidative carbocyclizations, the
use of an aerobic biomimetic oxidation system is an environ-
mentally benign procedure associated with high atom econ-
omy.[10a,11] We have applied an aerobic oxidation process via
multistep electron transfer involving three redox systems:
PdII/Pd0-benzoquinone
(BQ)/hydroquinone
(HQ)-Co-
(salophen)ox/Co(salophen). In the carbocyclization reaction
the substrate is oxidized to the desired product by the
substrate-selective catalyst[11a] Pd(OAc)2, which becomes
reduced to Pd0. The Pd0 is subsequently reoxidized to PdII
by catalytic amounts of BQ, which is reduced to HQ. The HQ
is reoxidized to BQ by the catalytic electron transfer mediator
(i.e. Co(salophen)) with molecular oxygen as the oxi-
dant.[10a,11,13–15]
[*] Dr. V. R. Naidu, D. Posevins, Dr. C. M. R. Volla, Prof. Dr. J.-E. Bꢀckvall
Department of Organic Chemistry, Arrhenius Laboratory
Stockholm University
Bisallenes 1a–1g were prepared according to literature
procedures.[16] In an initial investigation to optimize the
reaction, we employed bisallene 1a (Table 1) as a model
substrate, and a series of experiments were performed in
combination with B2pin2 to examine the oxidative palladium-
catalyzed reactions for carbocyclization and borylation using
Pd(OAc)2 (5 mol%) and BQ. Different amounts of BQ and
B2pin2 were employed in the Pd-catalyzed carbocyclization of
1a. The use of 1.5 equiv of BQ in the absence of B2pin2 at
10691 Stockholm (Sweden)
E-mail: jeb@organ.su.se
Dr. C. M. R. Volla
Present address: Department of Chemistry, Indian Institute of
Technology-Bombay, Powai, Mumbai 400076 (India)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201610524.
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Table 1: Results of initial study.[a]
Table 2: Optimization of the reaction conditions.
Yield [%][b,c]
Entry
Additives
Ratio of 2a:3a
Yield [%][a]
Entry
BQ
B2pin2
(equiv)
(equiv)
A
3a
B
1
2
3
4
5
6
7
–
74:26
99
0
0
44
34
97
99
99
99
PTSA
Et3B
NR[b]
1
2
3
4
1.5
2.5
3
–
72(70)
Traces
Traces
60
–
25
Traces
5
–
62
75(71)
10
Decompose
70:30
100:0
79:21
74:26
1.5
1.0
0.1
AcOH
BF3·Et2O
LiOAc·H2O
NaOAc
Na2CO3
Na2CO3
3
[a] Reaction conditions: 1a (1.0 equiv) (0.2 mmol), 5 mol% of Pd(OAc)2,
anhydrous DCE (2.0 mL). [b] Yield determined by 1H NMR using
mesitylene as an internal standard. [c] Isolated yield in parenthesis.
8
84:16
79:21
9[c]
[a] Yield determined by 1H NMR analysis using mesitylene as an internal
standard. [b] NR: no reaction. [c] 50 mol% of additive.
508C for 7 h in DCE (1,2-dichloroethane), afforded product
A in 72% yield (Table 1, entry 1) most likely via isomer-
ization of Int-3 (Scheme 1b) to the secondary s-allyl-Pd
complex followed by b-hydride elimination. Surprisingly, with
an increased amount of BQ (2.5 to 3 equiv) in the presence of
B2pin2 (1.5 to 1 equiv), BQ derivative B was obtained in 62 to
75% yield (Table 1, entries 2 and 3). Interestingly, when
B2pin2 was reduced to 0.1 equiv in the presence of BQ
(3 equiv), compound A (60%) was formed as the main
product together with small amounts of secondary boronic
ester triene derivative 3a (5%) and quinone derivative B
(10%) (Table 1, entry 4). In this initial study, we were not able
to obtain the desired carbocyclic primary boron compound
2a, probably because of its reaction with BQ.
We therefore decided to carry out the carbocyclization/
borylation reaction with different oxidants with the aim of
isolating 2a. After trying various oxidants, it was finally found
that tetrafluoro-1,4-benzoquinone (F4-BQ) (1.5 equiv) with
bisallene 1a using catalytic amount of Pd(OAc)2 (5 mol%) in
the presence of B2pin2 (2 equiv) at room temperature in DCE
provided allylic primary and secondary carbocyclic boronic
esters 2a (74%) and 3a (26%) as major and minor products,
respectively (Table 2, entry 1).
We then turned our attention towards optimizing the yield
of the primary boronic ester 2a by addition of various
additives. It was found that addition of base substantially
improved the yield of allylic primary carbocyclic boronic ester
2a. The employment of p-toluenesulfonic acid (PTSA) or
Et3B additives was ineffective as well as the addition of
catalytic amounts of AcOH (Table 2, entries 2–4). Addition
of BF3·Et2O gave a high selectivity (100:0) but the yield was
low. With LiOAc·H2O the yield was high and the selectivity
2a:3a was 79:21 (Table 2, entry 6). NaOAc decreased the
ratio 2a:3a (entry 7). Finally, with 20 mol% of Na2CO3, 2a
was formed with good selectivity (84/16 ratio) and yield
(entry 8). Increasing the amount of Na2CO3 to 50 mol%
decreased the selectivity (79:21 ratio, entry 9). Moreover, the
use of various PdII catalysts did not improve the regioselec-
tivity (see Supporting Information).
With these improved reaction conditions for obtaining 2a
as the major isomer, we studied various substrates 1a–1g for
the borylative carbocyclization (Table 3). The corresponding
allylboronic compounds 2a–2g were obtained in good yields
(64–84%). It was desirable to apply an aerobic oxidative
carbocyclization–borylation process. Pd-catalyzed reaction of
1a with B2pin2 (2 equiv) at room temperature using catalytic
amounts of F4-BQ (20 mol%) and Co(salophen) (5 mol%) as
electron transfer mediators (ETMs) under 1 atm of molecular
oxygen (O2) in DCE (Table 3, entry 1, method B) afforded 2a
in 83% yield. A similar yield (Table 3, entry 1, method A,
84%) was obtained in the non-aerobic oxidative reaction.
In the aerobic oxidative carbocyclization–borylation reac-
tion, Co(salophen) was the best ETM for obtaining product
2a in high yield. Similar results were obtained for substrates
1a and 1b, with either stoichiometric amounts of F4-BQ
(method A) or under aerobic conditions (method B) afford-
ing products 2a and 2b, respectively, in good yields (Table 3,
entries 1 and 2).
With the results in Table 3, we now can provide an
explanation for the formation of carbocyclic benzoquinone
derivative B (Table 1, entry 3). In this case the allylboron
compound 2a generated will react with the carbonyl group of
the quinone followed by a [3,3’] rearrangement (Scheme 2).[17]
Subsequent tautomerization and oxidation lead to derivative
B. This cascade reaction inspired us to develop a cascade
reaction where the generated allylboron compound is con-
tinuously trapped by a carbonyl compound (such as an
aldehyde) present in the reaction mixture.
In the planned cascade reaction, an allyl-Bpin intermedi-
ate 2a is generated via an in situ carbocyclization–borylation
and this intermediate can continuously be trapped by an
aldehyde present in solution. Normally, it is difficult to control
the relative configuration at two adjacent stereocenters in
a single reaction but the cascade reaction sequence proceeded
in a highly stereoselective manner. This high stereoselectivity
is due to the fact that the reaction between the allylboron
intermediate and the aldehyde involves a Zimmerman–Trax-
2
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Table 3: Scope of substrates.
Table 4: Palladium-catalyzed cascade reaction.[a,b]
Entry
Substrate
Product
Yield of 2 [%][a]
(E=CO2Me)
(method A/B)
A:84
B:83
1
2
A:84
B:82
3
4
A:81
A:80
5
6
7
A:85
A:78
A:64
[a] Isolated yield after column chromatography. [b] The ratio in paren-
thesis is the ratio 4:5. Method A: Same as in Table 3 but with 1.5 equiv of
an aldehyde. Method B: Same as in Table 3 but with 1.5 equiv of an
aldehyde.
ler (ZT) transition state[12a,b,18] (see Supporting Information).
The cascade reaction of 1a with benzaldehyde by method A
or B afforded 4aa (Table 4) as the major product in a mixture
with small amounts of regioisomers 5.
In the one-pot reaction using method A or B, 1b was
allowed to react with various substituted benzaldehydes (6a–
6 f) with electron-rich or electron-deficient substituents to
give of 4bb–4bf, in good yields (79–91%) with minor
amounts of regioisomers 5 (Table 4). Under biomimetic
oxidative conditions (method B), aliphatic aldehydes such
as phenylpropionaldehyde (6g) and cyclohexylcarboxalde-
hyde (6h) with 1b gave the corresponding derivatives 4bg
and 4bh in 71% and 66% yields, respectively. Similarly,
heterocyclic aldehydes, for example 2-thiophenecarboxalde-
hyde (6i) and furfural (6j), were also allowed to react under
aerobic conditions (method B) producing the corresponding
derivatives 4bi and 4bj in 70% and 80% yields, respectively,
with small amounts of regioisomers 5bi and 5bj (4bi:5bi =
93:7 and 4bj:5bj = 92:8). In all these cases two stereocenters
were created in the reaction that gave isomer 4 but only one
diastereomer of 4 was observed (> 99% d.r.).
[a] Isolated yield after column chromatography. Method A: Pd(OAc)2
(5 mol%), F4-BQ (1.5 equiv), B2pin2 (2 equiv), Na2CO3 (20 mol%), DCE,
rt, 7 h. Method B: Pd(OAc)2 (5 mol%), F4-BQ (20 mol%), Co(salophen)
(5 mol%), B2pin2 (2 equiv), Na2CO3 (20 mol%), DCE, rt, O2 balloon, 7 h.
Scheme 2. Reaction of allylboron derivative 2a with BQ.
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In the cascade reaction described in Table 4, we noticed
that triene alcohol derivatives 4 undergo lactone formation on
prolonged reaction times. This lactonization was faster when
dimethyl malonate derivative 1a was used. When 1a was used
together with 4-(trifluoromethyl)benzaldehyde (6 f) a crystal-
line bicyclic lactone product 7 (see Ref. [19]) was obtained on
prolonged reaction time. The bicyclic lactone product was
submitted to X-ray crystallography analysis, which allowed
stereochemical assignment of the trienol product 4 from the
cascade reaction.[19]
diastereoselectivity. A wide range of bisallenes having differ-
ent functionality and aldehydes with various functional
groups are tolerated under biomimetic or nonaerobic con-
ditions. The efficient construction of stereoselective complex
structures via cyclic allylboronates and their applications in
a one-pot reaction should be useful in natural product
synthesis. Further investigations directed towards synthetic
applications and asymmetric variants of this one-pot cascade
reaction are underway in our laboratory.
Acknowledgements
Financial support from the European Research Council
(ERC AdG 247014) and the Swedish Research Council is
gratefully acknowledged. C.M.R.V. thanks the Wenner-Gren
foundation.
Conflict of interest
The authors declare no conflict of interest.
Keywords: biomimetic oxidation · bisallene · cascade reaction ·
catalysis · palladium
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subsequent allene attack on the PdII would generate vinyl-
palladium intermediate (Int-2). The latter complex reacts via
insertion of allene into the Pd-vinyl bond affording (s-
allyl)PdII species Int-3, which can rearrange to Int-5 via the
more stable p-allyl complex (not shown). The s-allyl inter-
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In conclusion, we have developed a PdII-catalyzed aerobic
oxidative carbocyclization–borylation–aldehyde trapping cas-
cade reaction of bisallenes to give triene alcohols with high
4
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[19] Lactone formation of 4af gives
a
crystalline lactone 7.
(CCDC 1486074).
Manuscript received: October 27, 2016
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Aerobic Oxidation
V. R. Naidu, D. Posevins, C. M. R. Volla,
J.-E. Bꢁckvall*
&&& — &&&
Selective Cascade Reaction of Bisallenes
via Palladium-Catalyzed Aerobic
Oxidative Carbocyclization–Borylation
and Aldehyde Trapping
Efficient cascade: A palladium-catalyzed
regioselective aerobic oxidative carbo-
cyclization–borylation of bisallenes gen-
erates a reactive allylboron intermediate
that is trapped by aldehyde present in the
reaction mixture. The cascade reaction
proceeds with high diastereoselectivity.
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!