Undeniable Confirmation of the syn-Addition Mechanism for Metal-Free Diboration by Using the Crystalline Sponge Method

Article abstract of DOI:10.1002/chem.201600392

The stereochemical outcome of the recently developed metal-free 1,2-diboration of aliphatic alkenes has, until now, only been elucidated by indirect means (e.g. derivatization). This is because classical conformational analysis of the resulting 1,2-diboranes is not viable; in the ¹H NMR spectrum the relevant ¹H resonances are broadened by ¹¹B, and the occurrence of the products as oily compounds precludes X-ray crystallographic analysis. Herein, the crystalline sponge method is used to display the crystal structures of the diboronic esters formed from internal E and Z olefins, evidencing the stereospecific syn addition mechanism of the reaction, which is fully consistent with the prediction from DFT calculations. The crystalline sponge method provides conclusive proof of the stereospecific addition in the metal-free diboration of internal aliphatic alkenes (see scheme). The compatibility of common boryl groups with a monoporous ZnI₂ network gives access to the crystal structures of the oily organoborane compounds. The relative stereochemistry of the metal-free diboration products was unequivocally assigned and fully consistent with the DFT calculations.

Full text of DOI:10.1002/chem.201600392

DOI: 10.1002/chem.201600392
Communication
&
Organocatalysis
Undeniable Confirmation of the syn-Addition Mechanism for
Metal-Free Diboration by Using the Crystalline Sponge Method
[
a]
[b]
[b]
[b]
[b]
Ana B. Cuenca, Nicolas Zigon, Vincent Duplan, Manabu Hoshino, Makoto Fujita,*
[a]
and Elena Fernµndez*
to establish the stereochemistry of these species. Thus, an ac-
curate, nondestructive and direct method for the observation
of the stereochemistry of oily diboronate ester compounds
would be greatly welcomed by the scientific community.
In this context, the crystalline sponge method has now been
established as a reliable technique for single-crystal X-ray dif-
fraction studies and does not require crystallization of the sam-
Abstract: The stereochemical outcome of the recently de-
veloped metal-free 1,2-diboration of aliphatic alkenes has,
until now, only been elucidated by indirect means (e.g.
derivatization). This is because classical conformational
analysis of the resulting 1,2-diboranes is not viable; in the
1
1
H NMR spectrum the relevant H resonances are broad-
11
ened by B, and the occurrence of the products as oily
compounds precludes X-ray crystallographic analysis.
Herein, the crystalline sponge method is used to display
the crystal structures of the diboronic esters formed from
internal E and Z olefins, evidencing the stereospecific syn
addition mechanism of the reaction, which is fully consis-
tent with the prediction from DFT calculations.
[9]
ples. Incorporating guests into porous coordination networks
causes them to become uniformly oriented, and their molecu-
lar structure can then be obtained by conventional X-ray analy-
[10]
sis. The porous network used as a crystalline sponge typical-
ly consists of pyridine-based linkers assembled around ZnI2
nodes, and features large pores capable of binding small to
medium-sized organic molecules. In this work, we report that
the crystalline sponge method is a direct and convenient way
to gain access to precise configurational information about
1,2-diboron compounds.
The synthesis of aliphatic 1,1- and 1,2-diboronic esters is of
great interest because of their potential for selective CÀB di-
functionalization in Suzuki–Miyaura cross-coupling processes,
To establish a suitable inclusion method, a target diboron
analyte that fulfilled certain pre-requisites was required. The
analyte must be chemically inert towards Zn–pyridine-type
ligand complexes, and the sizes of the pore and the diboron
species would ideally be commensurate. With this in mind,
a simple diboron pinacolato ester featuring a 1,1-geminal ar-
rangement of pinacolboryl (Bpin) moieties (1) was chosen to
check the inclusion feasibility. Interestingly, this 1,1-gem dibor-
on compound has also been prepared by metal-free diboration
[
1]
for example. Along with the well-known metal-catalyzed 1,1-
[
2]
[3,4]
diboration and 1,2-diboration
protocols, organocatalytic
diboration is emerging as an alternative, as it combines effi-
[5]
ciency and simplicity. However, the lack of a well-defined or-
ganocatalytic cycle brings about the inevitable question of the
stereochemistry of the final aliphatic diboron species. Determi-
nation of the stereochemistry of these species by traditional X-
ray analysis is difficult because many of them are obtained as
protocols. For the synthesis of 1, we followed the Wang proce-
[
6]
[5e,11]
oily products. In addition, the unambiguous assignment of
dure
based on the insertion of B pin (bis(pinacolato)dibor-
2 2
1
the configuration of aliphatic 1,2-diborated esters by H NMR
on) into
(Scheme 1).
a diazo-derived carbene synthon, PhCH CHN2
2
spectroscopy is nontrivial. This is due, in part, to fast quadru-
11
polar relaxation (spin for B=3/2, that is, substantial peak
A tiny, high-quality single crystal of crystalline sponge 2 with
the formula [(ZnI ) (tpt) ·x(cyclohexane)] (tpt=2,4,6-tris(4-pyri-
[
7]
broadening), which obfuscates the J coupling information.
2
3
2
n
[
12]
Even in cases for which Jvic values are available, the conforma-
dyl)triazine)
was treated with a cyclohexane solution of
À1
[10]
tional uncertainty for such species still hinders the application
1 (5 mg: 1 mgmL , 5 mL). After evaporation of the solution
at 508C for 24 h, the resulting guest-included crystal 1·2 was
subjected to X-ray diffraction analysis as a proof of concept.
The crystallographic analysis of the 1·2 complex revealed the
[8]
of the Karplus equation. As a result, derivatization, mainly by
oxidation to alcohols, is still one of the principal methods used
[
a] Dr. A. B. Cuenca, Prof. Dr. E. Fernµndez
Department Quimica Física i Inorgànica, University Rovira i Virgili
C/Marcel·lí Domingo s/n 43007 Tarragona (Spain)
E-mail: mariaelena.fernandez@urv.cat
[
b] Dr. N. Zigon, Dr. V. Duplan, Dr. M. Hoshino, Prof. Dr. M. Fujita
Department of Applied Chemistry, The University of Tokyo
7
-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
E-mail: mfujita@appchem.t.u-tokyo.ac.jp
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under http://dx.doi.org/10.1002/
chem.201600392.
Scheme 1. Metal-free diboration reaction to prepare compound 1.
Chem. Eur. J. 2016, 22, 4723 – 4726
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Communication
effective trapping of diboron ester 1 in the network pores
[
13]
(
Figure 1). Importantly, the successful inclusion of 1 under-
lined the compatibility of the reactive gem-diboronates with
the crystalline sponge system. It is noteworthy that this
method required sample amounts lower than those required
for regular TLC analysis.
Scheme 2. DFT-supported syn addition mechanism for the metal-free dibora-
tion reaction. The DFT calculation was carried out with R
ref [5a]).
1 2
=Me and R =H
(
Despite the theoretical prediction of the stereospecific vici-
nal CÀB bond formation, experimental evidence had not been
1
provided because of the ambiguity in the H NMR spectrum.
À
Based on the organocatalytic addition of the MeO !bis(pina-
colato)diboron adduct to (Z)-2-octene (3a) and (E)-2-nonene
(
3b) we synthesized the corresponding vicinal-diborated prod-
ucts. Our goal was to determine the relative stereochemistries
of the aliphatic 1,2-diboron species by single-crystal diffraction
of the products. (Z)-2-Octene (3a) was easily transformed into
2
,3-bis(pinacolato) diboron ester 4a (colorless oil) in 81% yield.
A cyclohexane solution of diboron 4a was then included into
À1
2
under the following inclusion conditions: 1 mgmL , 5 mL,
5
08C, 24 h. Guest 4a was surely included in 2 as revealed by
[
14]
X-ray analysis. In full agreement with the theoretical predic-
[
5a]
tions, compound 4a exhibited a relative 2R*,3S* configura-
tion of the two CÀB bonds, an outcome expected for the theo-
retically predicted syn addition (Figure 2, structure 4a·2). The
stereochemical attribution in 4a was valid by direct observa-
tion of their molecular structure using X-ray crystallography.
Under the same organocatalytic reaction conditions, 2,3-bis-
(pinacolato) diboron ester 4b was isolated as a colorless oil in
Figure 1. Top: Crystal structure of guest 1 (space-filling representation) in
crystalline sponge 2 (stick representation). Hydrogen atoms and solvent mol-
ecules have been removed for clarity. Bottom: ORTEP drawing of guest
1
with 30% probability.
7
5% yield from (E)-2-nonene (3b), and was also tested using
the crystalline sponge method. Diboron ester 4b was allowed
to diffuse into sponge 2, after which guest-absorbed crystal
4b·2 was examined by X-ray analysis. As shown in Figure 3,
4b·2 presents a 2R*,3R* configuration that correlates with the
After having demonstrated the potential of the crystalline
sponge method for the determination of the molecular struc-
ture of geminal diboronate esters, we proceeded to apply the
method to 1,2-diboron species stemming from internal olefins,
which posed challenges in the unambiguous assignment of
their configurational structure. A technique allowing the direct
observation of the X-ray structure of the 1,2-diborated prod-
ucts arising from a regioisomerically pure internal olefin would
be particularly beneficial because it would allow mechanistic
information about the stereochemical outcome of the reaction
to be confirmed. For this reason, we previously conducted
a comprehensive study of the metal-free 1,2-diboration of non-
[
15]
syn B pin addition to (E)-olefin 3b. X-ray structures of 4a·2
2
2
and 4b·2 constitute clear proof of the stereospecificity of the
metal-free diboration reaction of internal olefins, and are con-
sistent with the previously suggested mechanism.
In summary, we have demonstrated a straightforward way
to characterize the stereospecific outcome of the metal-free di-
boration of nonactivated olefins by X-ray crystallography, and
we can conclude that the mechanism is based on stereospecif-
ic syn-addition. The present study represents the application of
the crystalline sponge method to the mechanistic study of an
organic transformation, in which empirical stereochemical as-
signment of the products by classical NMR techniques is not
obvious, and in which the products are available only as oily
compounds. We truly believe that the combination of DFT cal-
culations and the crystalline sponge method is a powerful tool
for the confirmation of reaction pathways.
[5a]
activated alkenes by means of DFT calculations, and predict-
ed stereospecific CÀB bond formation by syn addition. Based
on this theoretical prediction, the metal-free diboration of
a (Z)-olefin would give rise to one diastereoisomer of the 1,2-
bis pinacolato diboron ester compound (Scheme 2a), whereas
the E olefin would provide the complementary diastereoisomer
(
Scheme 2b) in a stereospecific fashion.
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Communication
(
4 mmol, 1 equiv) was added dropwise. The Schlenk tube was then
sealed and heated at 708C in an oil bath for 16 h. After the reac-
tion was cooled to room temperature, the obtained mixture was
filtered over a small pad of Celiteꢁ and the solvent was gently con-
centrated on a rotary evaporator. After all the volatiles had been
removed, the crude residue was purified by silica gel flash chroma-
tography to afford the 1,2-diboron compounds.
General procedure for guest inclusion into crystalline
sponge
To a microvial containing a piece of a rod-shaped crystal of 2 and
cyclohexane (5 mL), a cyclohexane solution of the target compound
(
1 mg/1 mL, 22 mL) was added. The crystal-containing microvial
was then capped and pierced with a needle for solvent evapora-
tion. The microvial was kept at 508C and the solvent was gradually
evaporated over 1 day. Typically, 5 to 10 vials were used for this
procedure.
Acknowledgements
The present research was supported by the Spanish Ministerio
de Economía y Competitividad (MINECO) through projects
CTQ2013-43395P and CTQ2013-50219-EXP, and also in part by
JST-ACCEL. We thank AllyChem for the gift of diborane.
Figure 2. Top: Metal-free diboration of 3a. Bottom: Crystal structure of 2,3-
bis(pinacolato) diboron ester 4a trapped in the pore of crystalline sponge 2
(
ORTEP drawing with 30% probability).
Keywords: 1,2-diboron · configuration · crystalline sponge
method · diboron inclusion · organocatalysis
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À3
101.922(2)8, V=15790.8(6) Š , Z=8,
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Received: January 28, 2016
Published online on February 18, 2016
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim