Diastereoselective Cyclobutenol Synthesis: A Heterogeneous Palladium-Catalyzed Oxidative Carbocyclization-Borylation of Enallenols

Article abstract of DOI:10.1002/chem.201805118

A highly selective and efficient oxidative carbocyclization/borylation of enallenols catalyzed by palladium immobilized on amino-functionalized siliceous mesocellular foam (Pd-AmP-MCF) was developed for diastereoselective cyclobutenol synthesis. The heter

Full text of DOI:10.1002/chem.201805118

A Journal of
Accepted Article
Title: Diastereoselective Cyclobutenol Synthesis: A Heterogeneous
Palladium-Catalyzed Oxidative Carbocyclization-Borylation of
Enallenols
Authors: Jan-E. Bäckvall, Man-bo Li, Daniels Posevins, Karl
Gustafson, Cheuk-Wai Tai, Andrey Shchukarev, and Youai
Qiu
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To be cited as: Chem. Eur. J. 10.1002/chem.201805118
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Diastereoselective Cyclobutenol Synthesis: A Heterogeneous
Palladium-Catalyzed Oxidative Carbocyclization-Borylation of
Enallenols
Man-Bo Li, Daniels Posevins, Karl P. J. Gustafson, Cheuk-Wai Tai, Andrey Shchukarev, Youai Qiu,*
and Jan-E. Bäckvall*
Abstract: A highly selective and efficient oxidative carbocyclization-
borylation of enallenols catalyzed by palladium immobilized on
amino-functionalized siliceous mesocellular foam (Pd-AmP-MCF)
was developed for diastereoselective cyclobutenol synthesis. The
heterogeneous palladium catalyst can be recovered and recycled
without any observed loss of activity or selectivity. The high
diastereoselectivity of the reaction is proposed to originate from a
directing effect of the enallenol hydroxyl group. Optically pure
cyclobutenol synthesis was achieved by the heterogeneous strategy
using chiral enallenol obtained from kinetic resolution.
methods for accessing cyclobutenols proceed via lengthy
syntheses involving subsequent functionalization of cyclo-
butenones.[7a] In addition, control of diastereoselectivity during
functionalization of cyclobutenones is challenging,[9] and to date
only few methods are known for the distereoselective synthesis
of cyclobutenols.[7b,7d] Therefore, the development of efficient
methods for the stereoselective synthesis of cyclobutenols is an
important and significant research task.
We have recently been involved in the use of palladium nano-
particles (NPs) immobilized on amino-functionalized siliceous
mesocellular foam (Pd0-AmP-MCF) as a heterogeneous nano-
palladium catalyst in various organic transformations such as
dynamic kinetic resolution,[10c] selective hydrogenation,[10d] and
water-splitting.[10e] Not only has this catalyst been shown to
display excellent activity, it has also proven to be associated
with high recyclability.[10] Our recent extensive involvement in Pd-
catalyzed oxidative carbocyclization of allenes for the
construction of synthetically important carbocyclic skeletons[3]
made us interested in exploring the Pd-AmP-MCF catalyst for
oxidative carbocyclizations. We envisioned that enallenol 1
could be transformed into cyclobutenol 2 via Int-A by a Pd-
catalyzed oxidative carbocyclization-borylation using Pd-AmP-
MCF (Scheme 1). The hydroxyl group of enallenol 1 could
potentially direct the reaction to proceed with control of
diastereoselectivity. A major challenge with this approach is to
activate the nanoparticle to provide access to the Pd(II) atoms
required for the oxidative C–C bond formation (via Int-A).
fter the discovery of the Wacker process in the late
1950’s,[1] homogeneous palladium-catalyzed oxidations
A
have developed rapidly and play an important role in both
academia and industry.[2] Our group has had a long-standing
interest in homogeneous palladium-catalyzed oxidation reac-
tions,[2a,3a,3b] which in recent work has involved the oxidative car-
bocyclization of allenes.[3] In these reactions a Pd(II) salt, in most
cases Pd(OAc)2, is used as a homogeneous catalyst together
with an oxidant (typically benzoquinone (BQ)). A drawback with
Pd(OAc)2 as the catalyst is that recycling is difficult, which limits
its use on larger scale. Another drawback of the homogeneous
Pd-catalyzed oxidations is the deactivation of active Pd catalysts
because of the generation of Pd black during the catalytic
cycle.[2e] Because of these limitations there is a need to develop
heterogeneous catalysts that are able to mimic Pd(OAc)2 in
oxidation reactions.
A
problem and
a
challenge when
transforming a Pd-catalyzed oxidation into a heterogeneous
version[4] is to maintain high selectivity and efficiency.
Cyclobutenols are highly versatile building blocks in organic
synthesis that can be readily transformed into a range of useful
compounds.[5] Furthermore, they are important structural
elements in a large number of biologically active compounds
and natural products.[6] However, synthetic methods for their
preparation are still limited.[7] For example, the most
straightforward route to cyclobutene derivatives, via a [2+2]-
cycloaddition[8] of an alkyne to an olefin, cannot be used for the
direct preparation of cyclobutenols. Most of the reported
[*]
Dr. M.-B. Li, D. Posevins, K. P. J. Gustafson, Dr. Y, Qiu, Prof. Dr. J.-
E. Bäckvall
Scheme 1. The strategy of heterogeneous Pd-catalyzed oxidative
carbocyclization for cyclobutenol synthesis. [11]
Department of Organic Chemistry, Arrhenius Laboratory, Stockholm
University, SE-10691, Stockholm, Sweden
E-mail: jeb@organ.su.se; qiuyouai@gmail.com
Dr. C.-W. Tai
Department of Materials and Environmental Chemistry, Arrhenius
Laboratory, Stockholm University, SE-10691, Stockholm, Sweden
Dr. A. Shchukarev
Herein, we report on the Pd-AmP-MCF-catalyzed oxidative
carbocyclization-borylation of enallenols for diastereoselective
synthesis of cyclobutenols. This recyclable catalyst shows high
activity and selectivity and to the best of our knowledge, it is an
Department of Chemistry, Umeå University, SE-901 87 Umeå,
Sweden
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11[c,d,e]
12[c,d]
13[c,d]
14[c,d]
15[c,d]
16[c,d]
PdII-AmP-MCF (1)
PdII-AmP-MCF (1)
PdII-AmP-MCF (1)
PdII-AmP-MCF (1)
PdII-AmP-MCF (1)
PdII-AmP-MCF (1)
TMEDA (0.1)
0
unprecedented report on
oxidative carbocyclization of allenes.
a
heterogeneous Pd-catalyzed
PhNMe2 (0.1)
Ph3N (0.1)
55
Nanopalladium immobilized on amino-functionalized siliceous
mesocellular foam (Pd0-AmP-MCF) was synthesized using our
previously reported procedure[10] (Supporting Information, p. S2).
We initiated the investigation by using enallenol 1a as substrate,
bis(pinacolato)diboron (B2pin2) as trapping agent, Pd0-AmP-MCF
as the catalyst, p-benzoquinone (BQ) as the oxidant, and MeOH
as solvent. To our delight, cyclobutenol 2a was obtained in 8%
yield with excellent regio- and diastereoselectivity (Table 1, entry
1). With these inspiring results in hand, we set out to optimize
the reaction conditions for the selective formation of 2a. After
screening other solvents, such as CH2Cl2, CHCl3, THF, EtOH, n-
PrOH, or t-BuOH, which did not increase the yield, we tried to
improve the outcome of the reaction by adding different
additives. Interestingly, the yield of cyclobutenol 2a increased to
45% by the use of 1.0 equiv of triethylamine (Et3N) as an
additive (Table 1, entry 2). Running the reaction more diluted
(from 0.1 M to 0.05 M) improved the yield to 50%, probably due
to the decrease of side-reactions (Table 1, entry 3). By lowering
the amount of catalyst from 5 mol% to 1 mol%, using less Et3N
(0.1 equiv), and shortening the reaction time to 2 h, the yield of
2a was further improved to 59% (Table 1, entry 4). To our
delight, the yield of 2a was improved to 82% (Table 1, entry 5)
with replacement of Pd0-AmP-MCF by its precursor PdII-AmP-
MCF (Supporting Information, p. S2). To understand the role of
Et3N in this reaction, we studied the effect of amines on the
formation of cyclobutenol 2a (Table 1, entries 6-16). We found
that trialkylamines improved the yield of cyclobutenol 2a to a
great extent (Table 1, entries 10, 14, 15). N,N,N’,N’-tetramethyl
ethylenediamine (TMEDA) as an additive completely shut down
the reaction and enallenol 1a was recovered in 92% yield (Table
1, entry 11). Interestingly, Chiral trialkylamine as an additive
gave the cyclobutenol 2a in 62% yield and 16% ee (Table 1,
entry 16). These results suggest that there is an interaction
between triethylamine and the Pd catalyst (e.g. coordination).[12]
To be noted, 1 mol% of Pd(OAc)2 as a catalyst with or without
Et3N only gave cyclobutenol 2a in 58% or 32% yields,
respectively (Table 1, entry 17 and 18), which demonstrate the
high efficiency of the heterogeneous catalyst.[13]
51
n-Bu3N (0.1)
n-Pr3N (0.1)
77
75
62 (16% ee)
(0.1)
17[c,d]
18[c,d]
Pd(OAc)2 (1)
Pd(OAc)2 (1)
Et3N (0.1)
58
32
none
[a] The reaction was conducted using 0.2 mmol of 1a, B2pin2 (1.3 equiv), BQ
(1.1 equiv), and catalyst (1 or 5 mol%) in 1.0 mL of MeOH. [b] Determined by
NMR using anisole as the internal standard. [c] 2.0 mL of MeOH was used. [d]
reaction time was shorten to 2 h. [e] 1a was recovered in 92% yield.
Furthermore, recycling and stoichiometric experiments of Pd-
AmP-MCF gave us insight into the activation mode of the
catalytic system. Interestingly, with Pd0-AmP-MCF the yield of
2a increased from 59% in the first run to 78% in the second run
and was maintained around 80% from the second run to seventh
run, while PdII-AmP-MCF gave more constant yield from the first
to seventh run in ~80% yield (Scheme 2a). These results
indicate that an activation step is required for the Pd0-AmP-MCF
catalyst. Transmission electron microscopy (TEM) images of
Pd0-AmP-MCF and PdII-AmP-MCF before the reaction are quite
different, but after several recyclings they became very similar to
one another (Supporting Information, p. S27). Furthermore, the
deconvoluted Pd3d X-ray photoelectrons pectroscopy (XPS)
spectrum of Pd0-AmP-MCF pretreated with BQ showed that the
surface of the palladium nanoparticles had a high proportion of
Pd(II) (Scheme 2b). These results suggest that oxidation of the
nanopalladium surface by BQ may be the activation step of Pd0-
AmP-MCF catalyst. Furthermore, the deconvoluted C1s XPS
spectrum of Pd0-AmP-MCF showed that after treatment with BQ,
the atomic concentrations (AC) of C–(C,H), C–O, C=O and π-π*
excitation increased from 8.88%, 2.76%, 0.98% and 0% to
11.17%, 7.33%, 1.13% and 0.63%, respectively (Scheme 2b).
This result demonstrates that BQ and the reduced hydroquinone
(HQ) are adsorbed on Pd-AmP-MCF after treatment with BQ.
Control experiments with 1a showed that neither Pd0-AmP-MCF
nor PdII-AmP-MCF in stoichiometric amounts did lead to any
detectable formation of 2a (Scheme 2c). However, after
treatment of these two materials with 2.0 equiv of BQ for 2 h in
MeOH followed by centrifugation and washing several times with
MeOH, they promoted the formation of 2a in 74% and 77% yield,
respectively (Scheme 2c). These results indicate that the
adsorption (or coordination) of BQ to Pd-AmP-MCF is essential
for the initiation of the reaction. Based on these observations,
we conclude that the active Pd(II) atoms required for the
oxidative carbocylization are generated through adsorption of
BQ to the palladium nanoparticles in Pd-AmP-MCF (Scheme 2d).
Inductively Coupled Plasma Optical Emission Spectroscopy
(ICP-OES) analysis of a liquid aliquot taken from the Pd-AmP-
MCF-catalyzed oxidative carbocyclization-borylation of enallenol
1a showed that there were no detectable amounts of Pd in the
reaction solution (< 0.1 ppm). In addition, leaching and hot filtra-
tion test (Supporting Information, p. S25) also showed that there
was no detectable leaching of Pd species during the reaction,
which rules out that the active catalyst for the oxidative carbo-
cyclization-borylation of enallenols had arised from leached
Table 1. Selected optimization of the reaction conditions.[a]
Entry
1
Catalyst (mol%)
Pd0-AmP-MCF (5)
Pd0-AmP-MCF (5)
Pd0-AmP-MCF (5)
Pd0-AmP-MCF (1)
PdII-AmP-MCF (1)
PdII-AmP-MCF (1)
PdII-AmP-MCF (1)
PdII-AmP-MCF (1)
PdII-AmP-MCF (1)
PdII-AmP-MCF (1)
Additive (equiv)
none
Yield (%)[b]
8
2
3[c]
Et3N (1.0)
45
50
59
82
42
46
50
44
76
Et3N (1.0)
4[c,d]
5[c,d]
6[c,d]
7[c,d]
8[c,d]
9[c,d]
10[c,d]
Et3N (0.1)
Et3N (0.1)
none
n-BuNH2 (0.1)
i-Pr2NH (0.1)
pyridine (0.1)
MeN(CH2)4 (0.1)
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Scheme 2. Recycling and stoichiometric experiments. a, Recycling of Pd-AmP-MCF. b, XPS spectra of Pd3d of Pd0-AmP-MCF after treatment with BQ, and C1s
of Pd0-AmP-MCF before and after treatment with BQ. c, Stoichiometric reactions. d, Schematic illustration of Pd atoms on Pd-AmP-MCF in catalysis. [11]
species.[14] Although we favor a heterogeneous mechanism we
cannot exclude the possibility of a (temporary) dissolution of
Pd(II) catalyzing a (pseudo-)homogeneous reaction inside the
pores.[15]
We next studied the substrate scope of the carbocyclization-
borylation of enallenols 1 to cyclobutenols 2 (Scheme 3). In
addition to butyl-substituted enallenol 1a, other alkyl groups in
the R1 position like methyl, benzyl, and phenethyl worked well to
give 2b, 2c and 2d in good yields. Even sterically demanding
alkyl groups, such as cyclopentyl and t-butyl afforded the corre-
sponding cyclobutenols 2e and 2f in 66% and 54% yields,
respectively. Aromatic substituted substrate 1g furnished
cyclobutenol 2g in 69% yield. The reaction tolerated functional
groups like ester groups or an additional hydroxyl group to fur-
nish the corresponding cyclobutenols, 2h, 2i and 2k. Cyclobu-
tylidene, cyclopentylidene and cyclooctylidene substituents on
the allene moiety of the enallenols in place of two methyl groups
also afforded the corresponding products 2j, 2k and 2l in good
yields. In addition, the reaction of an unsymmetrical enallenol
1m bearing methyl and phenyl groups afforded 2m in 77% yield.
To our delight, enallenol 1n with a tertiary alcohol afforded terti-
ary cyclobutenol 2n in 70% yield. Cyclobutenol 2o bearing an all
carbon-substituted quaternary carbon was obtained from 1o in
only 8% yield under the standard conditions, probably due to the
steric effects disfavoring olefin insertion to give a four-membered
ring. Interestingly, bicyclic compound 2p bearing three chiral
centers was obtained in 50% yield from 1p as a single diastereo-
mer. It is noteworthy that all of the cyclobutenol derivatives 2
were obtained as single diastereomers in high selectivity.
Scheme 3. Substrate scope for the diastereoselective synthesis of
cyclobutenols 2.
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Cyclobutenols constitute a versatile platform for further che-
mical transformations (Scheme 4a). Hydrogenation of 2a or 2c
using Pd/C as the catalyst afforded the corresponding tetra-sub-
stituted cyclobutane 3 or 4 bearing four chiral centers as a single
diastereomer in 90% and 94% yields, respectively. A selective
hydrogenation of 2a or 2c catalyzed by [Ir(cod)Cl]2 gave C=C
bond isomerized products 5 or 6 as a single diastereomer with
three chiral centers. Esterification or Dess-Martin oxidation of
the hydroxyl group of cyclobutenols 2a afforded the correspond-
ding cyclobutene ester 7 or cyclobutenone 8, respectively.
Oxidation of the boronic ester functionality in 2a afforded diol 9
with a cyclobutene framework. Enallenol (S)-1a (95% ee)
obtained by kinetic resolution of 1a with Candida antarctica
lipase B (CalB) was transformed to optically pure cyclobutenol
(1S, 4S)-2a in 83% yield without any loss of optical purity
(Scheme 4b). This transformation provides a simple and efficient
approach to optically pure cyclobutenol derivatives.
Scheme 5. Functional groups on the diastereoselective formation of 2.
further insight into the reaction mechanism (Scheme 6). An inter-
molecular competition experiment was conducted using a 1:1
mixture of 1a and 1a-d6 at 0 oC for 3 h (Scheme 6a). The product
ratio 2a/2a-d5 measured at 23% conversion was 4.5:1, with a
total 22% yield of 2a and 2a-d5. From this ratio, the competitive
KIE value was determined to be kH/kD = 5.5. Furthermore, paral-
o
lel KIE experiments (from initial rate) at 0 C afforded kH/kD = 3.0
(Scheme 6b and 6c). These results indicate that the initial allenic
C–H bond cleavage is partially rate-limiting, and the large com-
petitive isotope effect in the C–H bond cleavage (kH/kD = 5.5)
requires that this step is the first irreversible step of the reaction.
Scheme 4. Synthetic applications of cyclobutenols 2 and synthesis of optically
pure cyclobutenol 2a. a, (1) Pd/C (1 mol%), H2 (5 bar), MeOH, rt, 12 h; (2)
[Ir(cod)Cl]2 (1 mol%), H2 (5 bar), DCM, rt, 12 h. (3) 4-Br-C6H4COCl (1.2 equiv),
Et3N (1.2 equiv), DMAP (5 mol%), DCM, rt, 12 h. (4) Dess-Martin periodinane
(DMP) (1.1 equiv), DCM, rt, 2 h. (5) NaBO3•4H2O (4.0 equiv), THF/H2O, rt, 8 h.
b, Synthesis of optically pure cyclobutenol 2.
To obtain more information on the origin of the high diaste-
reoselectivity in the formation of cyclobutenols 2, we examined
the reactivity and selectivity of substrate 1ab in which the
hydroxyl group of 1a had been replaced by an acetoxy group.
Although the reactivity of 1ab was similar to that of enallenol 1a,
there was no diastereoselectivity in the formation of four-
membered ring product 2ab (Scheme 5a), indicating that the
hydroxyl group is crucial for the diastereoselective formation of
cyclobutenols 2. We then studied the reactivity and selectivity of
imide 1ac and sulfonamide 1ad (Scheme 5b and 5c). As with
1ab, substrate 1ac showed no diastereoselectivity, but
interestingly, the reaction of 1ad was diastereoselective and
afforded 2ad in a d.r. of 11:1. These results suggest that an
active hydrogen (OH or NH) is required in the functional group
for the control the diastereoselectivity.[16]
Scheme 6. Kinetic isotope effect studies.
Based on the experimental results and our previous work on
Pd-catalyzed oxidative carbocyclization of allene derivatives, we
propose a plausible mechanism for the formation of cyclo-
butenols 2 (Scheme 7). Simultaneous coordination of allene,
olefin, and hydroxyl group to Pd(II) center would form Int-1. The
additional coordination of the olefin unit to Pd(II) is essential for
the allene attack involving allenic C–H bond cleavage.[18] Int-2
generated from allene attack on Pd(II), bearing an axial hydroxyl
group, would promote the formation of Int-3 by face-selective
olefin insertion. Subsequent transmetallation of Int-3 with B2pin2
would produce intermediate Int-4, which on reductive elimination
gives the target cyclobutenol 2. The Pd(0) is subsequently re-
oxidized by BQ to active Pd(II) to close the catalytic cycle.
As the allenic C–H bond is cleaved during the reaction, we per-
formed deuterium kinetic isotope effect (KIE) studies[17] to gain
In conclusion, an efficient heterogeneous version of a Pd-
catalyzed oxidative carbocyclization of enallenols has been
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Scheme 7. Proposed reaction mechanism.
realized for the diastereoselective synthesis of cyclobutenols.
The heterogeneous Pd catalyst was used at least seven times
upon recycling without any loss of activity and selectivity. The
coordination of the hydroxyl group of the enallenols to a Pd(II)
atom on the nanoparticle is proposed to control the diastereo-
selectivity of the transformation. An optically pure cyclobutenol
[8] For selected reviews of [2+2]-cycloaddition, see: a) E. Lee-Ruff, G.
Mladenova, Chem. Rev. 2003, 103, 1449; b) Y. Xu, M. L. Conner, M. K.
Brown, Angew. Chem. Int. Ed. 2015, 54, 11918; Angew. Chem. 2015,
127, 12086.
was obtained from
a chiral enallenol. This cyclobutenol
[9] G. Chai, S. Wu, C. Fu, S. Ma, J. Am. Chem. Soc. 2011, 133, 3740.
[10] a) M. Shakeri, C.-W. Tai, E. Göthelid, S. Oscarsson, J.-E. Bäckvall,
Chem. Eur. J. 2011, 17, 13269; b) E. V. Johnston, O. Verho, M. D.
Kärkäs, M. Shakeri, C.-W. Tai, P. Palmgren, K. Eriksson, S. Oscarsson,
J.-E. Bäckvall, Chem. Eur. J. 2012, 18, 12202; c) K. Engström, E. V.
Johnston, O. Verho, K. P. J. Gustafson, M. Shakeri, C.-W. Tai, J.-E.
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preparation will be useful in synthetic chemistry, and the
heterogeneous catalyst applied here may open up novel
opportunities in oxidative C-C bond formation. Further studies on
the use of this heterogeneous nanopalladium catalyst as a
mimic for Pd(OAc)2 in other oxidative cabocyclizations are
currently under way in our laboratory.
Acknowledgements
Financial support from the European Research Council (ERC
AdG 247014), the Swedish Research Council (2016-03897), the
Berzelii Center EXSELENT, and the Knut and Alice Wallenberg
Foundation is gratefully acknowledged. Parts of this research
were carried out at PETRA III at DESY, a member of the
Helmholtz Association (HGF). We thank Vadim Murzin and
Wolfgang Caliebe for assistance in using the P-64 beamline.
[11] In the Scheme it looks like each nanoparticle (NP) bind to only one or two
NH2‘s. This is only a cartoon and in reality several NH2’s bind to each NP.
[12] a) M. Moreno, F. J. Ibañez, J. B. Jasinski, F. P. Zamborini, J. Am. Chem.
Soc. 2011, 133, 4389; b) G. M. Lari, B. Puértolas, M. Shahrokhi, N.
López, J. Pérez-Ramĺrez, Angew. Chem. Int. Ed. 2017, 56, 1775; Angew.
Chem. 2017, 129, 1801.
[13] We observed Pd black on the wall of reaction tubes by using Pd(OAc)2 as
the catalyst. However, There are no observed generation of Pd black by
using Pd-Amp-MCF as the catalyst.
Keywords: cyclobutenols • heterogeneous palladium catalysis •
diastereoselectivity • enallenols • oxidative carbocyclization
[14] When studying the catalyst using synchrotron based XAS, the XANES-
spectra of the catalysts before and after treatment with BQ appeared
virtually the same, confirming that no significant single-site atomic
palladium catalyst is formed (see Supporting Information, p. S28).
[15] If precipitation of Pd(II) on the nanoparticles is faster than diffusion in a
(pseudo)homogeneous reaction the reaction may occur inside the pores.
However, because of he large window size of Pd-Amp-MCF (13-14 nm)
we consider such a scenario less likely.
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[16] PdII-AmP-MCF catalyze the same reaction of a substrate bearing no
hydroxyl group as described in ref. 3e only in 28% yield, which indicates
that the hydroxyl group plays an important role not only on selectivity, but
also on reactivity.
a
discussion of the use of soluble nanoparticles formed from e.g.
Pd(OAc)2/DMSO in oxidation reactions see: e) D. Wang, A. B. Weinstein,
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10.1002/chem.201805118
Chemistry - A European Journal
COMMUNICATION
COMMUNICATION
An oxidative carbocyclization-
Man-Bo Li, Daniels Posevins, Karl
P. J. Gustafson, Cheuk-Wai Tai,
Andrey Shchukarev, Youai Qiu,*
and Jan-E. Bäckvall*
borylation
of
enallenols
catalyzed by recoverable and
recyclable heterogeneous Pd-
AmP-MCF
developed for the synthesis of
cyclobutenols with high
diastereoselectivity.
catalyst
was
Page No. – Page No.
Diastereoselective Cyclobutenol
Synthesis: A Heterogeneous
Palladium-Catalyzed Oxidative
Carbocyclization-Borylation of
Enallenols
This article is protected by copyright. All rights reserved.