Formation of a dihydroborole by catalytic isomerization of a divinylborane

Article abstract of DOI:10.1039/c4dt01423e

Diphenylamino(divinyl)borane (1a) adds two molar equivalents of Piers' borane [HB(C₆F₅)₂] to give the expected double hydroboration product. In contrast diisopropylamino(divinyl)borane (1b) reacts cleanly already with one molar equivalent of HB(C₆F₅) ₂ to give the α-borylated tetrahydroborole derivative 10 in good yield. Subsequent treatment of 10 with benzaldehyde proceeded by retro-hydroboration to give the hydroboration product of the aldehyde plus the dihydroborole 3b. We were able to achieve the divinylborane to dihydroborole isomerization (1b to 3b) catalytically: treatment of diisopropylamino(divinyl) borane (1b) with 15 mol% of Piers' borane at elevated temperature gave (diisopropylamino)dihydroborole 3b in good yield.

Full text of DOI:10.1039/c4dt01423e

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Formation of a dihydroborole by catalytic
isomerization of a divinylborane†
Cite this: Dalton Trans., 2014, 43,
10794
Raúl A. Adler Yañez, Gerald Kehr, Constantin G. Daniliuc,‡ Birgitta Schirmer§ and
Gerhard Erker*
Diphenylamino(divinyl)borane (1a) adds two molar equivalents of Piers’ borane [HB(C6F5)2] to give the
expected double hydroboration product. In contrast diisopropylamino(divinyl)borane (1b) reacts cleanly
already with one molar equivalent of HB(C6F5)2 to give the α-borylated tetrahydroborole derivative 10 in
good yield. Subsequent treatment of 10 with benzaldehyde proceeded by retro-hydroboration to give the
hydroboration product of the aldehyde plus the dihydroborole 3b. We were able to achieve the divinyl-
borane to dihydroborole isomerization (1b to 3b) catalytically: treatment of diisopropylamino(divinyl)
borane (1b) with 15 mol% of Piers’ borane at elevated temperature gave (diisopropylamino)dihydroborole
3b in good yield.
Received 14th May 2014,
Accepted 19th May 2014
DOI: 10.1039/c4dt01423e
www.rsc.org/dalton
Introduction
Organic pentadienyl cations can be cyclized to give their cyclo-
pentenyl cation isomers.1 This thermally induced electrocycli-
zation reaction provides the principal basis for a variety of
important synthetic protocols such as e.g. the acid induced for-
mation of many substituted cyclopentadienes from the
respective bis(alkenyl)carbinols2 or the Nazarov cyclization of
pentadienones to the corresponding cyclopentenone products
(Scheme 1) under strongly acidic reaction conditions.3
Boron containing heterocycles serve as important building
blocks e.g. in organic materials chemistry.4 Therefore, develop-
ing novel synthetic entries to cyclic boron derivatives e.g. dihy-
droboroles, is of an increasing interest.5 Since divinylboranes
are isoelectronic neutral organoelement analogues of the
organic pentadienyl cations one might have envisaged that
some such systems might be prone to isomerization to give the
dihydroborole heterocycles. A DFT study (see below) will show
for the example investigated in this study that the thermal
divinylborane to dihydroborole rearrangement is very unlikely
to become realized.6 However, we found a surprising solution
to carry out this interesting cycloisomerization reaction by
Scheme 1
taking a completely different catalytic pathway. These new
developments will be described in this article.
Results and discussion
The DFT study
Organisch-Chemisches Institut, Universitat Münster, Corrensstraße 40, 48149
Münster, Germany. E-mail: erker@uni-muenster.de; Fax: +49 251-83-36503
†Electronic supplementary information (ESI) available: Additional experimental We investigated the potential thermally induced ring closure
and analytical details, spectral data and crystallographic data for all compounds
reaction of (diisopropylamino)divinylborane (1b, preparation
see below) to its corresponding dihydroborole isomer (3b) via
the hypothetical intermediate 2b (see Scheme 1), the putative
described. CCDC 982551–982553 and 983316. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c4dt01423e
‡X-ray crystal structure analyses.
diisopropylaminoboron analogue of the cyclopentenyl cation
intermediate in the pentadienyl cation ring closure sequence.
§Computational chemistry. B. S. is member of the Center for Multiscale Theory
and Computation (CMTC) of the University of Münster.
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The calculation was carried out using the double hybrid equivalent. Compound 1a shows a planar arrangement of the
density functional B2PLYP with the D3 dispersion correction four carbon substituents at the central NvB unit9c,10 [mole-
and the large Gaussian AO basis set def2-TZVP. Thermal and cule A: N1A–B1A: 1.416(2) Å, ∑N1BCC = 360.0°, ∑B1NCC
=
solvent corrections (at 298.15 K with solvent CH2Cl2) were 360.0°, θ C11A–N1A–B1A–C1A: −4.5(2)°; molecule B: N1B–B1B:
included to yield ΔG values of about 1–2 kcal mol−1 accuracy 1.419(2) Å, ∑N1BCC = 360.0°, ∑B1NCC = 360.0°, θ C11B–N1B–
(for details see the ESI†).7 The overall 1b to 3b cyclization reac- B1B–C1B: 5.8(2)°]. The phenyl substituents at nitrogen are
tion is strongly exergonic, both in the gas phase and in solu- both rotated out of the central molecular plane. The divinyl-
tion (CH2Cl2). The overall conversion of a pair of CvC double borane subunit in 1a has attained a U-shaped conformation
bonds to a combination of a C–C single bond and a CvC [molecule A: B1A–C1A: 1.559(2) Å, C1A–C2A: 1.317(2) Å,
double bond in the product shows the expected high exergoni- B1A–C3A: 1.561(2) Å, C3A–C4A: 1.322(2) Å, θ C2A–C1A–B1A–
city of ca. 20 kcal mol−1. However according to this DFT calcu- C3A: −28.0(2)°, C1A–B1A–C3A–C4A: −29.0 (2)°; for further
lation the formation of the potential intermediate 2b in this details see the ESI†].
cyclization of the divinylborane (1b) is strongly endergonic,
We then reacted the divinylborane 1a with Piers’ borane
namely by ΔG(298 K) = +27.6 kcal mol−1, in the gas phase. In [HB(C6F5)2] in toluene in a 1 : 2 molar ratio. Hydroboration
solution the energetic situation is actually quite similar took place rapidly at both vinyl groups at r.t. Workup including
(CH2Cl2: ΔG(298 K) = +26.9 kcal mol−1). Since the activation crystallization from pentane at −32 °C over 2 d eventually gave
barrier of the 1b to 2b interconversion would probably be even the product 8 as a crystalline solid in a ca. 50% yield (Scheme 2).
markedly higher it is unlikely that a thermally induced overall In solution we have monitored a 11B NMR signal of the symmetry-
1b to 3b ring closing isomerization can easily be realized. We equivalent pair of newly introduced B(C6F5)2 groups at δ 70.7,
actually could confirm this experimentally. Even prolonged i.e. in a typical range of strongly Lewis acidic trigonal planar
heating of the divinylborane starting material 1b at 200 °C did RB(C6F5)2 boranes. The 11B NMR resonance of the Ph2NvB
not lead to the desired compound (for details see the ESI†). unit occurs at δ 47.5. In addition we have observed the typical
However, we found a different way for synthesizing 3b from 1b 19F NMR signals of the B(C6F5)2 substituents [δ −130.3 (o),
in a rather convenient way.
−149.2 (p), −162.2 (m)] and the 1H/13C NMR features of the
phenyl substituents at nitrogen. The pair of –CH2–CH2–
groups bridging between the boron atoms give rise to 1H NMR
signals at δ 1.97 and 0.95, respectively (13C: δ 25.8, 11.7).
We followed the reaction by in situ NMR spectroscopy. In
CD2Cl2 solution the reaction between 1a and HB(C6F5)2 was
complete after ca. 20 min at r.t. We monitored the formation
of two 1 : 2 addition products in a ca. 80 : 20 ratio, the major of
which we have tentatively assigned the structure of 8. The
minor product was not positively identified. From the NMR
spectra it could contain a unit derived from Markovnikov
borane addition. The reaction between 1a and HB(C6F5)2 in a
1 : 1 molar ratio was more complex: it gave a mixture contain-
ing 8 and remaining unreacted 1a as the major components
plus some unidentified minor products (for details including
the depicted spectra see the ESI†).
Experimental studies, stoichiometric reactions
We first used (diphenylamino)divinylborane (1a) as the starting
material that we reacted with Piers’ borane [HB(C6F5)2].8 It was
prepared by treatment of (Ph2N)BCl2 with a slight excess
9
(2.5 mol equivalent) of vinylmagnesium bromide in THF–
ether. The product 1a was obtained as a colorless crystalline
solid in 76% yield. In solution it shows the typical 1H/13C NMR
features of the pair of symmetry-equivalent vinyl substituents
at boron (1H: δ 6.02, 5.87, 5.85; 13C: δ 139.0, 134.1) and a
typical 11B NMR resonance at δ 39.0.
Compound 1a was characterized by an X-ray crystal struc-
ture analysis (see Fig. 1). There are two crystallographically
independent molecules in the crystal, but they are chemically
Compound 8 was characterized by X-ray diffraction (see
Fig. 2). In the crystal it features a close to C2-symmetric struc-
ture. The central Ph2NvB unit shows a short N1–B1 bond
(1.407(6) Å). The CH2–CH2–B(C6F5)2 moieties attached at the
boron atom B1 each show a close to antiperiplanar confor-
mational orientation (θ B1–C1–C2–B2 173.9(4)°, B1–C3–C4–B3
167.6(4)°, B1–C1 1.591(6) Å, B1–C3 1.568(6) Å, B2–C2 1.558(6) Å,
Fig. 1 Molecular structure of compound 1a (thermal ellipsoids are
shown with 30% probability).
Scheme 2
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Scheme 3
B(C6F5)2(pyridine) adduct 11 that was isolated as a colorless
solid in 70% yield. We obtained single crystals from a
benzene–pentane solution that allowed for the characteriz-
ation of compound 11 by X-ray diffraction. In the crystal com-
pound 11 shows a tetrahydroborole framework (see Fig. 3). It
has the B(C6F5)2(pyridine) unit bonded to carbon atom C1. Its
boron atom (B2) shows a typical pseudotetrahedral coordi-
nation geometry. The B2–N11 bond length is only slightly
shorter than the adjacent B2–C(pentafluorophenyl) bond
lengths. Boron atom B1 is part of the five-membered hetero-
cyclic core of compound 11. It shows a trigonal planar geo-
metry (sum of the bond angles at ∑B1NCC: 359.6°) as does the
adjacent nitrogen atom N1 (sum of the three bond angles at
∑N1BCC: 359.8°). The N1–B1 bond (1.404(2) Å) is much shorter
Fig. 2 A view of the molecular structure of compound 8 (thermal
ellipsoids are shown with 30% probability).
B3–C4 1.551(6) Å). The bonding geometry at the nitrogen N1
atom is trigonal planar (∑N1BCC = 360.0°) as well as the
bonding geometries at all three boron atoms in compound 8
(∑B1NCC = 360.0°, ∑B2CCC = 359.7°, ∑B3CCC = 359.8°).
We prepared the slightly more bulky starting material 1b
analogously by treatment of (iC3H7)2NBCl2 with a slight
9
excess (2.8 equiv.) of vinylmagnesium bromide in THF–ether.
The product 1b was obtained as a colorless oil in 43% yield. It
shows a typical 11B NMR feature at δ 37.7 in dichloromethane
1
solution and the typical H and 13C NMR signals of the pairs
of homotopic vinyl groups at boron and isopropyl substituents
at nitrogen (for details see the ESI†).
We treated compound 1b with Piers’ borane [HB(C6F5)2]. In
contrast to the corresponding reaction of 1a (see above) it
takes only 1 molar equivalent of the borane to achieve a com-
plete conversion of (diisopropylamino)divinylborane to a satu-
rated reaction product. The reaction was complete within ca.
15 min at ambient temperature and we isolated the product 10
in >90% yield as a colorless oil. Although we do not know the
mechanistic details of this reaction at this time, the formation
of the cyclic product 10 can formally be explained by an anti-
Markovnikov hydroboration of one of the vinyl units of the
starting material (to generate 9) followed by a formal 1,2-alkyl-
borane addition reaction to the remaining CvC double bond
(see Scheme 3).
Compound 10 shows the 1H/13C NMR signals of a pair of
chemically inequivalent isopropyl groups at nitrogen each fea-
turing a pair of diastereotopic methyl groups. The compound
is characterized by showing two 11B NMR resonances
[B(C6F5)2: δ 71.7, iPr2NB: δ 49.2].
Fig. 3 Molecular structure of compound 11 (thermal ellipsoids are
shown with 30% probability). Selected bond lengths (Å) and angles (°);
B1–C1 1.615(2), C1–C2 1.573(2), C2–C3 1.528(2), C3–C4 1.525(2), B1–C4
1.600(2), C1–B2 1.646(2), B2–N11 1.636(2), B2–C21 1.667(2), B2–C31
1.666(2), B1–N1 1.404(2), N1–C5 1.487(2), N1–C8 1.482(2), C1–B1–N1
For the purpose of further characterization we reacted com-
pound 10 with pyridine.11 This gave the corresponding 127.4(1), C1–B1–C4 107.6(1), C5–N1–C8 113.7(1), C5–N1–B1–C1 179.6(1).
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than the B2–N11 linkage (1.636(2) Å) indicating substantial
double bond character of the N1–B1 linkage by nitrogen to
boron π-back-donation.
In solution compound 11 shows 11B NMR signals at δ 52.0
and δ 1.9, respectively. The broad [B]CH[B] 13C NMR resonance
was observed at δ 23.1.
Scheme 5
We also reacted the product 10 with benzaldehyde
(r.t., benzene). This resulted in a retro-hydroboration reac-
tion and formation of the dihydroborole product 3b and
compound 12, the trapping product of the in situ liberated
borane [HB(C6F5)2] with benzaldehyde (Scheme 4; for the
characterization of 3b see below). Compound 12 was inde-
pendently synthesized by treatment of benzaldehyde with
Piers’ borane. Compound 12 was characterized spectroscopically
and by an X-ray crystal structure analysis (for details including
a view of the molecular structure in the crystal see the ESI†).
at δ 7.13 and 6.27 (3JHH = 8.1 Hz) with corresponding 13C NMR
signals at δ 162.0 and 135.4, respectively. The H NMR reson-
ances of the remaining CH2–CH2–moiety of the heterocycle are
detected at δ 2.29 and 1.03 (13C: 32.6 and 15.2). Compound 3b
features a 11B NMR resonance at δ 46.6.
1
Conclusions
Catalytic dihydroborole formation
Formally, divinylboranes are the neutral analogues of the pen-
tadienyl cations. Therefore, one might have expected some
divinylborane systems undergoing ring-closure reactions
similar to their pentadienyl cation analogues. Our DFT results
have, however, shown that the formation of 3b from 1b by a
route involving 2b is unfavorable because of the high energy
content of this zwitterionic borata–alkene/carbenium ion type
intermediate.12 Since the overall 1b to 3b isomerization is
markedly exergonic we had to search for a different kinetically
viable pathway of this ring-closure reaction. It was found in
the treatment of 1b with catalytic amounts of Piers’ borane
[HB(C6F5)2]. The reaction is likely to involve the intermediates
9 and 10. The later was isolated upon carrying out the reaction
stoichiometrically. It can be assumed that the retro-hydrobora-
tion step from 10 may close the cycle in the catalytic variant of
this reaction liberating the dihydroborole product 3b and
the chain-propagating borane [HB(C6F5)2]. Thereby the appar-
ently in this case unfavorable “bora-Nazarov” ring closing
rearrangement could successfully be circumvented by a com-
pletely different catalytic divinylborane to dihydroborole
isomerization.
The reaction of 10 with benzaldehyde giving 3b and 12 indi-
cated equilibration of 10 with 3b and [HB(C6F5)2] by rapid
reversible hydroboration/dehydroboration reactions. This
observation made it tempting to search for a formation of the
dihydroborole 3b by catalytic conversion of the divinylborane
1b. This isomerization reaction should be strongly exergonic
(see above). We found that compound 3b can indeed be
obtained from 1b by treatment with catalytic quantities of
[HB(C6F5)2] at elevated temperatures. Orientating experiments
showed that treatment of 1b with 5 mol% of [HB(C6F5)2] in
toluene solution at 100 °C resulted in a ca. 20% conversion to
the dihydroborole isomer 3b within ca. 1 d, but then the reac-
tion stopped. Utilization of 10 mol% of the [HB(C6F5)2] catalyst
resulted in ca. 70% conversion at 100 °C after 2 days, and we
obtained a close to quantitative ring closing isomerization of
1b to 3b within 1 h at 100 °C by using 15 mol% [HB(C6F5)2] as
catalyst. For the catalytic formation of 3b from 1b on a prepara-
tive scale we used even harsher conditions (i.e. 36 h, 200 °C in
a sealed ampoule using 8 mol% of the [HB(C6F5)2] catalyst)
since we knew about the high thermal stability of the dihydro-
borole 3b from separate experiments. Workup involving con-
densation of the product eventually gave the dihydroborole
product 3b in 62% yield. The reaction on a preparative scale
can also be performed using standard glassware, under milder
conditions (r.t., 4 days, 15 mol% [HB(C6F5)2]; for details see
the ESI†) (Scheme 5).
Experimental section
Preparation of 1-(diisopropylamino)-2,3-dihydroborole (3b)
(Diisopropylamino)divinylborane (1b) (350 mg, 2.12 mmol) in
toluene (8 mL) and bis(pentafluorophenyl)borane (60 mg,
0.17 mmol, 0.08 eq.) were stirred for 15 minutes in an
ampoule. Then the ampoule was sealed and taken into an
autoclave. After 36 h at 200 °C, all volatiles were carefully
removed in vacuum at room temperature. Compound 3b was
condensed from the obtained yellow residue at 8 × 10−3 mbar
and 100 °C as a colorless oil (218 mg, 62%). Elemental analy-
sis: calcd for C10H20BN: C, 72.76; H, 12.21; N, 8.48. Found:
C, 72.79; H, 12.19; N, 8.66. 1H NMR (500 MHz, [D8]-toluene,
1
Compound 3b shows the H/13C NMR resonances of a pair
of inequivalent isopropyl substituents at nitrogen. The endo-
cyclic CvC double bond at boron gives rise to 1H NMR signals
3
298 K): δ = 7.13 (br dm, JHH = 8.1 Hz, 1H, vCH), 6.27 (dm,
3
3JHH = 8.1 Hz, 1H, BCH), 3.48, 3.24 (each hept, JHH = 6.8,
Scheme 4
6.9 Hz, each 1H, isopropyl CH), 2.29 (m, 2H, CH2), 1.03
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(m, 2H, BCH2). 13C{1H} NMR (126 MHz): δ = 162.0 (vCH), Decomp.: ∼240 °C. Elemental analysis: calcd for
135.4 (br, BCHv), 50.5, 46.4 (isopropyl CH), 32.6 (CH2), 15.2 C27H26B2F10N2: C, 54.95; H, 4.44; N, 4.75. Found: C, 54.20; H,
(br, BCH2). 11B{1H} NMR (160 MHz): δ = 46.6.
4.21; N, 4.49. 1H NMR (500 MHz, CD2Cl2, 298 K): δ = 3.78, 3.23
(each hept, JHH = 6.7, 7.0 Hz, each 1H, CH), 1.91 (br, 1H,
3
Preparation of compound 8
BCH), 1.91/1.44, 1.23/0.25, 0.87/0.24 (each 1H, CH2), 1.17/1.15,
The reaction mixture of divinyl(diphenylamino)borane (1a) 1.02/0.44 (each d, JHH = 7.0, 6.7 Hz, each 3H, CH3). 13C{1H}
(70 mg, 0.3 mmol) and bis(pentafluorophenyl)borane (208 mg, NMR (126 MHz): δ = 52.6, 45.0 (CH), 31.6, 25.3, 21.1 (CH2),
0.6 mmol, 2 eq.) in toluene (2 mL) was stirred for 20 minutes. 24.4, 24.2, 22.5, 20.2 (CH3), 23.1 (br, BCH). 19F NMR
Then the suspension was filtered. The filtrate was concen- (470 MHz): δ = −129.1, −130.0 (each 2F), −159.4, −160.4 (each
trated, pentane (0.5 mL) was added and after 2 days at −32 °C 1F), −164.5, −165.4 (each 2F). 11B{1H} NMR (160 MHz):
compound 8 was obtained as long colorless needles (144 mg, δ = 52.0 (BN), 1.9 (py-B).
3
52%). The obtained crystals were suitable for the X-ray single
crystal structure analysis. Elemental analysis: calcd for
C40H18B3F20N: C, 51.94; H, 1.96; N, 1.51. Found: C, 51.89; H,
1.85; N, 1.43. In CD2Cl2 solution a mixture of 8 and 8′ was
Acknowledgements
detected (ca. 79 : 21). Major isomer 8: 1H NMR (500 MHz, Financial support from the Deutsche Forschungsgemeinschaft
CD2Cl2, 298 K): δ = 1.97 (m, 2H, CH2B), 0.95 (m, 2H, NBCH2). is gratefully acknowledged.
13C{1H} NMR (126 MHz): δ = 25.8 (br, CH2B), 11.7 (br, NBCH2).
19F NMR (470 MHz): δ = −130.3 (2F), −149.2 (1F), −162.2 (2F).
11B{1H} NMR (160 MHz): δ = 70.7 (B), 47.5 (BN). Minor isomer
Notes and references
3
8′: 1H NMR (500 MHz): δ = 3.08 (q, JHH = 6.4 Hz, 1H, CH),
2.00/1.66 (each m, each 1H, CH2B), 1.08/0.97 (each m, each
1 (a) T. S. Sorensen, Can. J. Chem., 1964, 42, 2768;
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(f) C. W. Shoppee, Heterocycles, 1976, 5, 605;
3
1H, NBCH2), 1.34 (d, JHH = 6.4 Hz, 3H, CH3). 13C{1H} NMR
(126 MHz): δ = 148.8, 148.4 (i), 129.41, 129.35 (m), 128.1, 127.7
(o), 126.31, 126.25 (p)(Ph), 37.0 (br, CH), 24.5 (br, CH2B), 10.8
(br, NBCH2), 12.2 (CH3). 19F NMR (470 MHz): δ = −130.1,
−131.3 (each 2F), −149.2t, −151.5 (each 1F), −162.1, −162.5
(each 2F). 11B{1H} NMR (160 MHz): δ = 70.7 (B), 47.5 (BN).
Generation of compound 10
A solution of bis(pentafluorophenyl)borane (69 mg, 0.2 mmol)
and (diisopropylamino)divinylborane (33 mg, 0.2 mmol) in
dichloromethane (1 mL) was stirred for 15 minutes at room
temperature. Then all volatiles were removed in vacuum and a
1
colorless oil was obtained (94.8 mg, 93%). H NMR (600 MHz,
3
CD2Cl2, 298 K): δ = 3.36, 3.28 (each hept, JHH = 6.7, 7.0 Hz,
3
each 1H, CH), 2.70 (t, JHH = 6.8 Hz, 1H, BCH), 1.95/1.74 (each
1H), 1.52 (2H), 1.28/1.11 (each 1H)(CH2), 1.24/1.23, 0.95/0.87
3
(each d, JHH = 7.0, 6.7 Hz, each 3H, CH3). 13C{1H} NMR
(151 MHz): δ = 56.5, 45.6 (CH), 46.5 (br, BCH), 31.9, 27.9, 21.2
(CH2), 24.2, 24.1, 22.0, 21.5 (CH3). 19F NMR (564 MHz): δ =
−131.0 (2F), −151.4 (1F), −162.4 (2F). 11B{1H} NMR (192 MHz):
δ = 71.7 (B), 49.2 (BN).
Preparation of compound 11
Compound 10 was generated in situ by reaction of (diisopropyl-
amino)divinylborane (1b) (33 mg, 0.2 mmol) and bis(penta-
fluorophenyl)borane (69 mg, 0.2 mmol) in dichloromethane
(2 mL). After 15 minutes stirring pyridine (16 µL, 0.2 mmol)
was added and the obtained reaction solution was stirred for
additional 10 minutes. Then it was concentrated to ca. 0.5 mL
and pentane (4 mL) was added. Compound 11 was obtained as
a colorless solid after storing the reaction mixture at −32 °C
for one day (82 mg, 70%). Crystals suitable for the X-ray single
crystal structure analysis were obtained from a benzene–
pentane solution of compound 11 at −32 °C. M.p.: 120 °C.
10798 | Dalton Trans., 2014, 43, 10794–10800
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