Atom-Economical Ni-Catalyzed Diborylative Cyclization of Enynes: Preparation of Unsymmetrical Diboronates

Article abstract of DOI:10.1021/acs.orglett.9b02485

We report a Ni-catalyzed diborylative cyclization of enynes that affords carbo- and heterocycles containing both alkyl- and alkenylboronates. The reaction is fully atom-economical, shows a broad scope, and employs a powerful and inexpensive catalytic Ni-b

Full text of DOI:10.1021/acs.orglett.9b02485

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Atom-Economical Ni-Catalyzed Diborylative Cyclization of Enynes:
Preparation of Unsymmetrical Diboronates
†
†
‡
Natalia Cabrera-Lobera, M. Teresa Quiros, William W. Brennessel, Michael L. Neidig,‡
́
,†
,†
́
̃
Elena Bunuel,* and Diego J. Cardenas*
†
́
́
Departamento de Química Organica, Facultad de Ciencias, Universidad Autonoma de Madrid, Institute for Advanced Research in
Chemical Sciences (IAdChem), Av. Francisco Tomas y Valiente 7, Cantoblanco 28049, Madrid, Spain
‡Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
́
S
* Supporting Information
ABSTRACT: We report a Ni-catalyzed diborylative cyclization
of enynes that affords carbo- and heterocycles containing both
alkyl- and alkenylboronates. The reaction is fully atom-
economical, shows a broad scope, and employs a powerful and
inexpensive catalytic Ni-based system. The reaction mechanism
seems to involve activation of the enyne by Ni(0) through
oxidative cyclometalation of the enyne prior to diboron reagent
activation. An unprecedented dinuclear bis(organometallic) Ni(I)
intermediate complex was isolated.
he development novel efficient synthetic methodology
cannot ignore environmental and economic aspects. This
catalysts, and possibility of developing enantioselective trans-
formations.
T
is one of the reasons for the renaissance of the Earth-abundant
first-row transition metals as catalysts during the last few years.
Recently, Ni derivatives have demonstrated their utility in a
wide variety of synthetically useful reactions.1 The ability of Ni
to access a variety of oxidation states is related to the rich
reactivity of Ni derivatives and the discovery of unprecedented
activation pathways.2 In addition, cooperative catalysis or a
combination of metal-catalyzed and photoredox reactions
constitute novel promising approaches to solve synthetic
problems.3 On the other hand, boronates are especially
relevant synthons due to their stability, functional group
tolerance, low toxicity, versatility, and wide applicability.4
Preparation of boronates from alkenes, alkynes, and other
unsaturated compounds by diboration reactions has been
performed with a variety of heavy metals.5 More recently, Fe,6
Co,7 Ni,8 and Cu9 have demonstrated to be useful for these
purposes as well. Carboboration10,11 and borylative cyclization
reactions12 of polyunsaturated compounds constitute powerful
methods for the formation of complex boron-containing carbo-
and heterocycles by formation of C−C and C−B bonds in
cascade reactions. These products are useful synthetic
intermediates. Pd-catalyzed borylative cyclizations have proven
to be valuable for the preparation of cyclic boronates from di-
and polyunsaturated substrates (enynes, enediynes, allenynes,
enallenes, etc.) and diboron derivatives,13,14 but have some
drawbacks related to the loss of one boryl unit and the difficult
catalyst tuning in the necessary ligandless conditions. On the
other hand, recently reported Co-,15 Fe-,16 and Ni-catalyzed17
hydroborylative cyclizations of enynes with HBpin are far more
convenient for their high atom economy, use of inexpensive
Herein we wish to report a full atom-economical, versatile,
widely applicable synthetic method for the preparation of
cyclic compounds containing two different kinds of boronate
units, starting from an ample variety of simple enynes by
cooperative homometallic catalysis involving Ni complexes.
We report the structure of the first unsymmetrical bis-
[organonickel(I)] complex, which we propose as the key
intermediate for this transformation (Scheme 1).
Our aim was to develop diborylative cyclizations that led to
diboronates containing two different boryl units that could be
sequentially functionalized. To the best of our knowledge, this
process has not been achieved yet. Enynes are especially
interesting in this respect because they may provide
diboronates containing to different boryl units. We began
our study by searching reaction conditions for the diboration
of enyne 1a. After much experimentation, the desired product
2a could be obtained using B2pin2 (1.05 equiv), Ni(cod)2 (5
mol %), and 5,5′′-dimethylterpyridine (L1, 5 mol %) in
diisopropylether at 60 °C (see Supporting Information for
more details on the optimization of the reaction conditions).
Ni(II) salts were ineffective and led to recovery of the
unaltered starting enyne. Although the yield obtained for the
model reaction was moderate, the process is very general and
proceeds with excellent yields for a wide variety of substrates,
providing boron-functionalized carbo- and heterocycles
(Scheme 2).
Received: July 18, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02485
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Letter
Scheme 1. Metal-Catalyzed Mono- and Diborylative
Cyclizations
Scheme 2. Ni-Catalyzed Diborylative Cyclization of
Enynes
a
Thus, enynes containing arylalkynes afforded the corre-
sponding products in moderate to good yields for oxygen-
(2a−e), nitrogen- (2f−j, 2r), or malonate-derived (2k−n)
derivatives within the tethering chains. The reaction tolerates a
variety of functional groups with different electronic properties
on the aromatic ring (Me, Cl, MeO, CN, and CF3 in meta- and
para- positions). Simple carbocycles are also accessible in good
yields for substrates containing both aryl and alkyl substituents
on the alkyne (2o−q, 2s). Substrates containing methyl-
substituted alkynes were also effective and provided high to
excellent yields, regardless of the kind of the tethering chain
(2t−x). The presence of other primary or secondary alkyl
chains was also possible to give diboronates 2y−ab. TMS
derivative 1ac gave rise to the corresponding-1-silylalkenylbor-
onates 2ac in high yield. The process also tolerates substitution
on the propargyl and allyl carbon atoms, affording the expected
products in excellent to quantitative yields (2af−ah). However,
no stereocontrol on the formation of the new stereogenic
center was observed. The poorest results were obtained for the
reactions of enynes 1ad, containing a terminal alkyne, and for
1,7-enyne 1ae, which afforded the six-membered ring product
in just 18%.
With the aim of illustrating the synthetic utility of the
obtained diboronates, we performed some functionalization
reactions. Thus, oxidation of both boronate groups gave the
corresponding hydroxy-ketone 3 in high yield (Scheme 3).
More interestingly, we succeeded at performing an
orthogonal derivatization of diboronates 2u and 2k by selective
Suzuki cross-coupling reactions. First, we coupled the
alkenylboronate moiety with iodobenzene to obtain 4. Then,
the remaining alkylboronate group was converted into the
trifluoroborate salts 5, which were used in subsequent Suzuki
cross-couplings to give compounds 6 (Scheme 3). There is no
need to have different boronates such as Bpin and Bdan5g,7b
To get insight into the reaction mechanism, we performed
some experiments. Thus, when simple alkenes or alkynes were
subjected to the optimized conditions, we did not observe
borylation of any of the substrates. On the other hand, the
stoichiometric reaction of enynes 1f or 1k with Ni(cod)2 and
L1 in the absence of boron reagent followed by hydrolysis with
a
Conditions: enyne 1 (0.2 mmol), B2pin2 (0.21 mmol), Ni(cod)2 (5
mol %), L1 (5 mol %), and i-Pr2O (1 mL) at 60 °C for 5 h. Isolated
b
yields by column chromatography. Toluene was used instead of i-
Pr2O. A mixture of two diastereoisomers was obtained d.r. = 1:1.7
(2af), d.r. = 1:3.7 (2ag), d.r. = 1:1.2 (2ah).
c
DCl in D2O led to the corresponding dideuterated cyclic
derivatives 1f′ and 1k′ (Scheme 4). These results suggest the
formation of an intermediate complex containing both alkyl-
and alkenyl-Ni bonds. In fact, we obtained dark red single
crystals resulting from the stoichiometric reaction of 1a and
Ni(cod)2-tpy.
X-ray diffraction analysis revealed the structure of a
fascinating bimetallic complex containing a carbocycle and
two Ni(I)-tpy units bound to alkyl and alkenyl carbon ligands
(7, Figure 1).
B
DOI: 10.1021/acs.orglett.9b02485
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Organic Letters
Letter
Scheme 3. Functionalization of Diboronates
Scheme 5. (Top) Calculated Possible Reaction Pathways for
the Formation of 7 and (Bottom) Reaction Energies for the
Reaction of II with Diboron Reagent
a
Scheme 4. Experiments To Probe the Reaction Pathway
a
ΔG (kcal mol−1) values were calculated with the M06-2x functional
using 6-31G(d) (C, H, N, O), LANL2DZ (Ni) basis set, in
diisopropylehter (PCM) (activation energies are in italics).
energy (28.2 kcal mol−1) compatible with a process taking
place at 60 °C. Alternatively, oxidative cyclometalation could
occur from a mononuclear Ni species with both the alkyne and
the alkene coordinated to the metal (I). Activation energy for
the formation of metallacycle II from this complex is lower
(17.3 kcal mol−1), albeit the corresponding transition state lies
above the above-mentioned one. On the other hand, a triplet
transition state, TS(I−I)t, lying 1.1 kcal mol−1 below TS(III-
7), would lead to the formation of II. Subsequent
comproportionation with N(tpy) would afford species 7 as
well. Complex 7 shows a triplet ground state according to its
EPR spectrum (μeff = 3.1, see Supporting Information). The
calculated triplet state for 7 is only 3.1 kcal mol−1 more stable
compared to the singlet. When we subjected complex 7 to
reaction with an excess of B2pin2, simulating the reaction
relative concentrations, bis(boronate) 2a was formed, but in
moderate yield (41% NMR yield, Scheme 6).
Figure 1. Two different views of the crystal structure of dinuclear Ni
complex 7 derived from 1a.
The short Ni−Ni distance (2.97 Å) indicates a strong
interaction among both metals (van der Waals radius for Ni is
1.63 Å). This is reminiscent of the structure of MeNi(tpy), in
which a weaker intermolecular interaction between the Ni
atom was observed in the solid state (3.18 Å).18 Both metal
atoms are formally Ni(I), and therefore, the structure of the
complex corresponds to an oxidative cyclometalation with each
metal increasing its oxidation state by one unit.
Computational and experimental results are not conclusive
to decide whether the reaction takes place through
monometallic intermediate II or by direct formation of 7
We envisioned two possible mechanisms for the formation
of 7, and calculations at the DFT level were performed on 1a
as a model to probe their feasibility (Scheme 5, see Supporting
Information for details).
Scheme 6. Formation of Complex 7 with a Stoichiometric
Amount of Catalyst and Subsequent Reaction with B2pin2
Thus, the process could involve the cyclization of a
bimetallic precursor containing two Ni(tpy) units coordinated
to each unsaturation (III). Formation of III is exoergic, and its
evolution to 7 would take place with a moderate activation
C
DOI: 10.1021/acs.orglett.9b02485
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because DFT methods overestimate the stability of high-spin
states, and therefore, the computed energy for TS(I−II)t is not
fully reliable.19 In addition, the low energy of dinuclear
complex 7 suggests that it could be a resting state rather than a
reaction intermediate. Moreover, inspection of the structure of
7 makes it difficult to guess how this complex could react with
B2pin2. We studied the interaction of II with the boron reagent
(Scheme 5, bottom). Exploration of the potential energy
surface strongly suggests that oxidative addition of the B−B
bond to give a Ni(IV) intermediate does not take place.
Instead, we propose a σ-bond metathesis, which may involve
both Ni−C bonds to give either intermediate IV or V.
Reaction involving cleavage of the alkenyl−Ni bond to provide
IV seems more feasible because it is exoergic (−3.4 kcal
mol−1), whereas formation of V is endoergic (+3.5 kcal mol−1).
Finally, downhill C−B reductive elimination would give the
final diboronate and would regenerate the Ni(0) catalyst.
In conclusion, synthetically useful carbo- and heterocycles
containing and alkyl- and alkenylboronate units can be
efficiently synthesized from enynes with inexpensive catalyst.
Activation of the organic substrate takes place prior to reaction
with the borylation reagent. Further studies to ascertain the
reaction mechanism, especially regarding the activation of
diboron derivatives with novel dinuclear Ni(I)−Ni(I)
complexes, are underway.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02485.
Experimental procedures and analytical data for all
compounds; X-ray diffraction details; and computational
methods, energies, and coordinates (PDF)
Accession Codes
CCDC 1834503 and 1891623 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: diego.cardenas@uam.es; Tel.: +34 914974358
*E-mail: elena.bunnel@uam.es; Tel.: 914973879
ORCID
William W. Brennessel: 0000-0001-5461-1825
Michael L. Neidig: 0000-0002-2300-3867
́
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Diborylalkenes and Concomitant Stereoselective Reactivity. Eur. J.
Org. Chem. 2018, 2018, 2728−2739.
́
Diego J. Cardenas: 0000-0002-1707-6445
Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We thank the Spanish MINECO for funding (Grant
CTQ2016-79826-R) and a Juan de la Cierva fellowship to
M.T.Q., the MECD for a FPU fellowship to N.C.-L., the
́
́
Centro de Computacion Cientifica-UAM, and the U.S.
National Institutes of Health (Grant R01GM111480 to
M.L.N.).
D
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