Using the FpXylBH2?SMe2reagent for the regioselective synthesis of cyclic bis(alkenyl)boranes

Article abstract of DOI:10.1039/d0cc05230b

The reactive borane reagent FpXylBH₂?SMe₂was prepared from 1,4-bis(trifluoromethyl)benzene by treatment withn-BuLi, followed by H₃B·SMe₂and subsequent removal of hydride. It undergoes a regioselective hydroborat

Full text of DOI:10.1039/d0cc05230b

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Using the FpXylBH2 SMe2 Reagent for the Regioselective Synthesis
of Cyclic Bis(alkenyl)boranes
Karel Škoch,a Constantin G. Daniliuc,a Gerald Kehra and Gerhard Erker*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The reactive borane reagent FpXyl-BH2·SMe2 was prepared from adduct (6·SMe2) that was isolated as a pale-yellow oil.9 It shows
1,4-bis(trifluoromethyl)benzene by treatment with n-BuLi, a pair of 19F NMR resonances (in CD2Cl2) of the inequivalent CF3
followed by H3B·SMe2 and subsequent removal of hydride. It groups. There is a broad ca 1:1:1:1 q 1H NMR resonance due to
undergoes
a
regioseletive hydroboration reaction with 1,2- the BH2 moiety with a corresponding 11B NMR feature at δ -9.8
bis(trimethylsilylethynyl)benzene to give the “dimeric” product 13a (t, 1JBH ~ 110 Hz). Vacuum sublimation of 6·SMe2 (80°C) followed
featuring a conjugated 14-membered core heterocyclic structure by crystallization from CH2Cl2 layered with pentane gave
that contains a pair of FpXylB units.
crystals of the SMe2 free FpXylBH2 dimer (Fig. 1). It shows a
doubly H-bridged four membered B2H2 core. Each boron atom
bears a bulky FpXyl arene substituent, oriented trans to each
other in the Ci-symmetric molecular arrangement.
Fluoroarene containing BH boranes are reactive and useful
reagents. A prominent example is Piers´ borane [(C6F5)2BH] 11
that has found many interesting synthetic applications, among
them the convenient preparations of intramolecular P/B Lewis
pairs by respective hydroboration routes.2 Similar, but less
frequently encountered are the corresponding Fmes2BH (2)3
and FXyl2BH (3)4 analogues (Scheme 1). Lancaster´s reagent
(C6F5)BH2·SMe2 (4·SMe2)5 is known for some years. (Fxyl)BH2
was mentioned in the literature.4 Recently, the related FmesBH2
borane was reported, either as dimethyl-sulfide stabilized
adduct 5·SMe2 or the doubly H-bridged donor-free dimer (5)2.6
The aryl boranes 4 and 5 were used in the synthesis of various
active FLPs and related systems. FmesBH2 was recently
employed as a reagent in multi-component syntheses of rare
dihydro-1,3-azaborinine compounds.7 We have now developed
a convenient synthesis of a new member in this series,8 namely
the BH2-borane FpXylBH2 (6) and found that it undergoes a
series of clean sequential two-fold hydroboration reactions
with some bis-alkynyl substrates that led to unusually
structured cyclic borane products.
CF3
F3C
(C6F5)2BH
F3C
BH
BH
2
2
1 (Piers´ borane)
CF3
F3C
2 (Fmes2BH)
3 (Fxyl2BH)
CF3
This work:
CF3
(C6F5)BH2
F3C
BH2
BH2
4 (Lancaster
reagent)
CF3
F3C
5 (FmesBH2)
6 (FpXylBH2)
Scheme 1 Some fluoroarene BHn boranes.
CF3
CF3
i),ii)
iii)
FpXylBH2
(83%)
F3C
BH3
Li
F3C
SMe2
6∙SMe2
The precursor 1,4-bis(trifluoromethyl)benzene (7) was mono-
lithiated by treatment with n-BuLi. Subsequent addition of
H3B·SMe2 was thought to generate the respective borate
intermediate 8 in situ (Scheme 2). Its treatment with
trimethylsilylchloride in the presence of dimethylsulfide in this
one pot reaction resulted in the formation of the FpXylBH2·SMe2
7
8
H
H
H
FpXyl
Xyl
Xyl
iv)
B
N
B
FpXyl
B
H
N
Xyl
N
FpXyl
H
H
10 (39%)
9
a. Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster,
Corrensstraße 40, 48149; Münster, Germany
Scheme 2 Synthesis of the FpXylBH2 reagent and its reaction with an isonitrile. [i)
n-BuLi, ii) H3B·SMe2, iii) Me3SiCl, SMe2, iv) XylNC, 80°C ].
Electronic Supplementary Information (ESI) available: [details of the preparation
and characterization of the new compounds]. See DOI: 10.1039/x0xx00000x
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geometrically precluded, so intermolecular hydroboration with
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DOI: 10.1039/D0CC05230B
a second equiv. of 12a (or an analogous unsymmetrical reagent
combination) then provides an attractive pathway to the
observed macrocyclic product 13a.
FpXyl
B
FpXyl
B
Me3Si
SiMe3
SiMe3
6∙SMe2
Me3Si
H
H H
H H
H
X
X
X
X
SiMe3
Me3Si
B
SiMe3
SiMe3
Figure 1 A view of the molecular structure of the FpXylBH2 dimer (6)2 obtained
from thermolysis of 6·SMe2 (thermal ellipsoids at 30 % probability).
12a,b
FpXyl
13a (48%),13b (60%)
11a (X: CH=CH),11b (X: S)
Scheme 3 Formation of macrocyclic products from the reaction of the
Compound 6·SMe2 is an active reducing reagent. It slowly FpXylBH2·SMe2 reagent with vic-bis-alkynes.
reacted with 2,6-dimethylphenyl isocyanide (Xyl-NC, 80°C,
The X-ray crystal structure analysis showed that a 14-
membered ring system had been formed. It features twelve sp2-
hybridized carbon atoms with a pair of Lewis acidic planar tri-
coordinate boron atoms incorporated. The system is
conjugated but not co-planar. It shows a twisted core structure
with approximate (but not crystallographically exact) C2-
symmetry. We note that the marked distortion from planarity
lets the “inner” olefinic hydrogen atoms become pairwise
oriented above and below the mean core plane. The distal pairs
of SiMe3 groups become similarly arranged towards positions
above and below the twisted ring structure (Fig. 3).
heptane, 15h) to give the dimeric [B]-CH2-[N] containing
product 10 that we isolated crystalline at -20°C. Compound 10
is a rare example of this class of compounds.10 The X-ray crystal
structure analysis revealed a dimeric structure with a planar
B2C2N2 core (B1-N1 1.385(3) Å). Both the boron and the nitrogen
atoms show planar tricoordinate geometries. The planes of the
N-Xyl substituents are oriented perpendicular to the central
core (Fig. 2). In solution we observed decoalescence of some 1H
and 19F NMR signals at low temperature (233 K) that indicated
the presence of ca 1:1 mixture of two conformational isomers,
probably caused by “freezing” of the B-FpXyl rotation. Both
these assumed syn- and anti-isomers show 1H NMR AB quartets
of their pairs of symmetry-equivalent core CH2 groups at 253 K
(see the ESI for details).
Figure 3 A projection of the molecular structure of the macrocyclic bis-acetylene
hydroboration product 13a (thermal ellipsoids at 30 % probability; for clarity the
hydrogen atoms are omitted and only the ipso-carbon atom of the FpXyl
substituent at boron is shown).
Figure 2
probability).
Molecular structure of compound 10 (thermal ellipsoids at 30%
We assume a reaction pathway of the formation of compound
10 that is initiated by formation of a borylated aldimine
intermediate 911 that is then subsequently undergoing
intermolecular B-H attack to form the final “dimeric” product.
FpXylBH2 is an active hydroboration reagent. It was reacted with
1,2-bis(trimethylsilylethynyl)benzene (11a) in a 1:1 molar
ratio.12 The reaction went to completion within 60 min at r.t. in
CH2Cl2 and we isolated the product 13a as a solid in 48% yield.
We assume a pathway of the formation of compound 13a as
depicted in Scheme 3. Regioselective cis-hydroboration of the
trimethylsilylacetylene unit of 11a would generate the
intermediate 12a. Subsequent intramolecular hydroboration is
Compound 13a shows temperature dependent dynamic NMR
spectra in solution. At low temperature (233 K, CD2Cl2) the
1
olefinic H NMR CH resonance splits into two equal intensity
singlets and we observed a similar behaviour for the SiMe3 1H
NMR signal. This spectroscopic behaviour probably indicates
“freezing” of the topomerization process of the 14-membered
ring system on the NMR time scale. The 11B NMR signal of
compound 13a occurs at δ 71.2.
The reaction of FpXyl-BH2·SMe2 with 3,4-bis(trimethylsilyleth-
ynyl)thiophene (11b) proceeds analogously. Workup after a
reaction time of 60 min at r.t. in dichloromethane involving
crystallization at -35°C gave the doubly thiophene-annulated
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14-membered macrocyclic product 13b. It was characterized by framework with a pair of =CHtBu groups symmetrically arranged
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X-ray diffraction (see the ESI for details). The NMR spectra in at the five-membered heterocyclic substDruOcIt:u10r.e1.039/D0CC05230B
CD2Cl2 solution also showed dynamic behaviour. The broad
olefinic =CH 1H NMR resonance, averaged at 299K, splits into an
equal intensity pair of singlets (δ 8.11 and 7.23). Similarly, we
observed a single 1H NMR SiMe3 resonance of compound 13b at
299K in CD2Cl2 at δ -0.21, which slightly shifted upon lowering
the measuring temperature and decoalesced to a pair of
singlets at δ -0.23 and δ -0.45.
We also examined the reaction of 11a with the Lancaster
reagent 4 and with the FmesBH2 borane 5. The former reaction
gave the macrocycle 13c (containing a pair of B-C6F5 groups); its
B-OH hydrolysis product was characterized by X-ray diffraction.
The latter reaction stopped at the mono-hydroboration stage;
its hydrolysis product was also characterized by an X-ray crystal
structure analysis (see the ESI for details).
The favoured reaction course in the FpXylBH2 alkyne
Figure 4 Molecular structure of compound 15·pyr (thermal ellipsoids at 15 %
hydroboration system is strongly substituent dependent. The
reaction of the doubly tert-butyl substituted bis-alkyne 11c with
probability).
one molar equivalent of borane 6·SMe2 initially (r.t., 10 min)
The 1:1 reaction of 1,8-bis(tert-butylethynyl)naphthalene (16)
gave
a compound which we tentatively assigned the
with the FpXylBH2 reagent 6·SMe2 took a similar course. In an
NMR experiment (CD2Cl2, 60 min, r.t.) we observed the
formation of the cyclization product 17. It showed single sets of
NMR signals of the symmetry equivalent halves. The 11B NMR
resonance at δ 60.2 indicated the presence of a Lewis acidic tri-
coordinate boron atom.
We trapped the cyclization product directly by adding pyridine
and isolated compound 17·pyr as a solid in 76% yield. The X-ray
crystal structure analysis (Fig. 5) showed that the hydroboration
had again brought the hydrogen atom to the alkynyl β-carbon
atoms, that had the tert-butyl groups attached. This resulted in
the formation of the planar 1,8-annulated boron-containing six-
membered ring in compound 17·pyr.
composition of the SMe2 adduct of the mono-hydroboration
1
product 14 (Scheme 4). Its H NMR spectrum shows a pair of
tert-butyl singlets, an ABCD like pattern of signals of the
phenylene moiety, the typical resonances of the FpXyl
substituent and a single =CH-signal (at δ 5.72). The 11B NMR
resonance of compound 14 was located at δ 3.4 and there is a
broad 1H NMR resonance of the BH unit at δ 3.45.
tBu
H
SMe2
FpXylBH2
tBu
FpXyl
SMe2
B
H
(6∙SMe2)
d8-toluene
(r.t., 10 min)
tBu
tBu
14
tBu
11c
tBu
tBu
FpXyl
FpXylBH2.SMe2
tBu
B
tBu
tBu
(6∙SMe2)
H
H
CH2Cl2, 1h, r.t.
FpXyl
100°C
60 min
pyridine
B
FpXyl
B
N
H
17
H
16
tBu
tBu
pyridine
15
15∙pyr (84% overall)
Scheme 4 Formation of the cycle borane 15 and its pyridine adduct.
N
FpXyl
B
Heating the sample (100 °C, 60 min) resulted to a complete
conversion to the cyclized product 15. It was in situ generated
tBu
tBu
1
and characterized by NMR spectroscopy. It shows a single H
NMR tert-butyl resonance, a AA´BB´ pattern of four phenylene
hydrogen signals and a singlet of rel. intensity two at δ 6.42 of
the pair of symmetry-equivalent =CH- moieties. Compound 15
shows a 11B NMR resonance of the Lewis acidic tri-coordinated
boron atom at δ 68.7.
We did not isolate compounds 14 or 15, but trapped 15 by
treatment with pyridine on a preparative scale. The pyridine
adduct 15·pyr was isolated in 84% yield. The X-ray crystal
structure analysis (Fig. 4) shows the benzo-borole derived
17∙pyr (76%)
Scheme 5 Formation of compound 17·pyr by the hydroboration route.
In solution (CD2Cl2, 299 K) compound 17·pyr showed a single 1H
NMR singlet of the pair symmetry-equivalent tert-butyl
substituents (rel. intensity 18) as well as a broad singlet of the
adjacent pair of =CH-hydrogen atoms (rel. intensity 2). The
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naphthalene nucleus shows a single ABM pattern of the arene
CH´s (rel. intensity 6). The 11B NMR signal of compound 17·pyr
was located at δ 2.7.
Conflicts of interest
There are no conflicts to declare.
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DOI: 10.1039/D0CC05230B
Notes and references
1
a) D.J. Parks, R.E. von Spence and W.E. Piers, Angew. Chem.
Int. Ed. Engl., 1995, 34, 809; Angew. Chem., 1995, 107, 895;
b) D.J. Parks, W.E. Piers and G.P.A. Yap, Organometallics,
1998, 17, 5492; c) M. Hoshi, K. Shirakawa and M. Okimoto,
Tetrahedron Lett., 2007, 48, 8475; d) X-S. Tu, N-N. Zeng, R-Y.
Li, Y-Q. Zhao, D-Z. Xie, Q. Peng and X-C. Wang, Angew. Chem.
Int. Ed., 2018, 57, 15096; Angew. Chem. 2018, 130, 15316; e)
E.A. Patrick and W.E. Piers, Chem. Commun. 2020, 56, 841.
a) P. Spies, G. Erker, G. Kehr, K. Bergander, R. Fröhlich, S.
Grimme and D.W. Stephan, Chem. Commun. 2007, 5072; b)
G. Kehr, S. Schwendemann and G. Erker, Top. Curr. Chem.
2013, 332, 45.
2
3
a) Z. Lu, Z. Cheng, Z. Chen, L. Weng, Z.H. Li and H. Wang,
Angew. Chem. Int. Ed., 2011, 50, 12227; Angew. Chem. 2011,
123, 12435; b) H. Ye, Z. Lu, D. You, Z. Chen, Z.H. Li and H.
Wang, Angew. Chem. Int. Ed. 2012, 51, 12047; Angew. Chem.
2012, 124, 12213; c) Z. Lu, Y. Wang, J. Liu, Y-J. Lin, Z.H. Li and
H. Wang, Organometallics 2013, 32, 6753.
a) K. Samigullin, M. Bolte, H-W. Lerner and M. Wagner,
Organometallics, 2014, 33, 3564; b) L. Wang, K. Samigullin,
M. Wagner, A.C. McQuilken, T.H. Warren, C.G. Daniliuc, G.
Kehr and G. Erker, Chem. Eur. J., 2016, 22, 11015.
A-M. Fuller, D.L. Hughes, S.J. Lancaster and C.M. White,
Organometallics, 2010, 29, 2194.
J. Li, C.G. Daniliuc, G. Kehr and G. Erker, Angew. Chem. Int.
Ed., 2019, 58, 6737; Angew. Chem., 2019, 131, 6809.
J. Li, C.G. Daniliuc, C. Mück-Lichtenfeld, G. Kehr and G. Erker,
Angew. Chem. Int. Ed., 2019, 58, 15377; Angew. Chem.,
2019, 131, 15521.
Figure 5 Molecular structure of compound 17·pyr (thermal ellipsoids at 15 %
probability).
4
We adopted the methodology outlined by Repo, Wagner and
others4,9 for the preparation of the new borane FpXylBH2 that
contains the readily available 2,5-bis(trifluoromethyl)phenyl
substituent. The reagent was usually generated as the stabilized
dimethylsulfide adduct 6·SMe2. The compound turned out to be
a reactive hydroboration reagent. The acetylene hydroboration
reactions investigated in this study showed a remarkable
dependency on the variation of the bulky substituents at the
alkynes. The SiMe3 group in the substrates 11a and 11b
apparently supported an “anti-Markovnikov” like regioselective
cis-1,2-BH addition reaction13 that resulted in the hydride
addition to the α-position to the arene and, consequently,
brought the B(H)FpXyl function to the β-carbon. This
arrangement geometrically precluded a use of the remaining BH
functionality for internal attack on the remaining acetylene and
directed the system into an intermolecular pathway toward the
formation of the unique fully conjugated 14-membered
macrocyclic π-products 13.
The hydroboration reactions of the FpXylBH2 reagent with the
closely related tert-butylethynyl substituted substrates
proceeded with the opposite regioselectivity. We assume that
steric hinderance here was a sufficient force to overcome any
weak electronic preference and, consequently, the -B(H)FpXyl
functionality ended up at the α-carbon atom. This enabled the
system for an efficient subsequent internal hydroboration
reaction with the pendant alkynyl substituent to form the
products 15 and 17 with five or six-membered heterocyclic ring
formation. The new FpXylBH2 reagent has, thus, shown some
interesting initial reaction modes, giving rise to the formation of
some unusually composed boron containing heterocycles with
hopefully more to come.
5
6
7
8
9
(FpXyl)3B had previously been reported: R.J. Blagg, E.J.
Lawrence, K. Resner, V.S. Oganesyan, T.J. Herrington, A.E.
Ashley and G.G. Wildgoose, Dalton Trans., 2016, 45, 6023.
Synthesis scheme analogous to: a) A. Schnurr., K. Samigullin,
J.M. Breunig, M. Bolte, H-W. Lerner and M. Wagner,
Organometallics, 2011,30, 2838; b) K. Chernichenko, B.
Kótai, I. Pápai, V. Zhivonitko, M. Nieger, M. Leskelä and T.
Repo, Angew. Chem. Int. Ed., 2015, 54, 1749; Angew. Chem.,
2015, 127, 1769; c) J. Li, C.G. Daniliuc, G. Kehr and G. Erker,
Chem. Commun., 2018, 54, 12606.
10 a) G. Hesse and H. Witte, Angew. Chem., Int. Ed. Engl., 1963,
2, 617; b) J. Tanaka and J. C. Carter, Tetrahedron Lett., 1965,
6, 329; c) J. Casanova Jr., H. R. Kiefer, D. Kuwada and A. H.
Boulton, Tetrahedron Lett., 1965, 6, 703; d) H. Witte,
Tetrahedron Lett., 1965, 6, 1127; e) S. Bresadola, F. Rossetto
and G. Puosi, Tetrahedron Lett., 1965, 6, 4775; f) G. Hesse
and Witte, Liebigs Ann. Chem., 1965, 687, 1; g) P. I. Paetzold
and G. Stohr, Chem. Ber., 1968, 101, 2874; h) J. Casanova Jr.
and H. R. Kiefer, J. Org. Chem., 1969, 34, 2579.
11 B.R. Barnett, C.E. Moore, A.L. Rheingold and J.S. Figueroa,
Chem. Commun., 2015, 51, 541.
12 See for a comparison: R. Liedke, M. Harhausen, R. Fröhlich,
G. Kehr and G. Erker, Org. Lett., 2012, 14, 1448.
13 On the -silicon effect in carbobocation stabilization see e.g.
a) J.B. Lambert, G-T. Wang, R.B. Finzel, D.H. Teramura, J. Am.
Chem. Soc., 1987, 109, 25; b) M.J. Bausch and Y. Gong, J. Am.
Chem. Soc., 1994, 116, 5963.
14 The CCDC numbers of the structures in this paper are
2016810-2016815, 2026639 and 2024709.
Financial Support from the Deutsche Forschungsgemeinschaft
is gratefully acknowledged.
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TOC
SiMe3
SiMe3
FpXyl
Me3Si
B
SiMe3
r.t.
H H
H H
+
CF3
Me3Si
B
SiMe3
F3C
BH2∙SMe2
FpXylBH2∙SMe2
FpXyl
(non-planar core)
The FpXylBH2 reagent undergoes a twofold hydroboration
reaction with 1,2-bis(trimethylsilylethynyl)benzene to give the
14-membered macrocyclic bis-borane product.
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