Metal-Free Carbonylation Route to a Reactive Borataepoxide System

Article abstract of DOI:10.1021/acs.organomet.8b00035

Hydroboration of N-allyl-cis-2,6-dimethylpiperidine with HB(C₆F₅)₂ gave the trimethylene-bridged frustrated N/B Lewis pair 7. It featured a trans-2,6-dimethyl substitution pattern at the piperidine unit which indicated preceding equilibration with its iminium cation/hydridoborate isomer 6 by means of an internal hydride transfer. In situ generated compound 6 is essential for the reaction with CO/HB(C₆F₅)₂ to give the borataepoxide product 12 at the [N]-(CH₂)₃-[B] framework. The borataepoxide 12 reacts rapidly with CO₂, cleaves the acidic C-H bond of a terminal alkyne, splits dihydrogen, and reacts with nitriles and benzaldehyde. Most products were characterized by X-ray diffraction.

Full text of DOI:10.1021/acs.organomet.8b00035

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Metal-Free Carbonylation Route to a Reactive Borataepoxide System
Tongdao Wang, Gerald Kehr, Constantin G. Daniliuc, and Gerhard Erker*
Organisch-Chemisches Institut, Westfalische Wilhelms-Universitat Munster, Corrensstraße 40, 48149 Munster, Germany
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* Supporting Information
ABSTRACT: Hydroboration of N-allyl-cis-2,6-dimethylpiper-
idine with HB(C6F5)2 gave the trimethylene-bridged frustrated
N/B Lewis pair 7. It featured a trans-2,6-dimethyl substitution
pattern at the piperidine unit which indicated preceding
equilibration with its iminium cation/hydridoborate isomer 6
by means of an internal hydride transfer. In situ generated
compound 6 is essential for the reaction with CO/HB(C6F5)2
to give the borataepoxide product 12 at the [N]−(CH2)3−[B]
framework. The borataepoxide 12 reacts rapidly with CO2,
cleaves the acidic C−H bond of a terminal alkyne, splits
dihydrogen, and reacts with nitriles and benzaldehyde. Most
products were characterized by X-ray diffraction.
Scheme 1. Formation of the [Zr]-formylhydridoborate 2
INTRODUCTION
■
Frustrated Lewis pairs (FLPs) are composed of Lewis acid and
base components that use sterically bulky substituents in order
to hinder their neutralizing Lewis adduct formation.1 The
resulting pairs of the active Lewis acid and base components
have attracted some current interest as they provide effective
means of small molecule activation in the absence of metal
containing components.2 In a way, FLPs often undergo
reactions reminiscent of metal complex chemistry, but in a
metal-free arrangement.3 The majority of the FLP systems
described so far contain P/B or N/B Lewis base/acid
combinations.2,4−6 Other combinations are less often encoun-
tered.7,8 This also holds for oxygen/boron FLPs. There have
been a few systems where the involvement of in situ generated
O/B pairs9 plays a role, but true isolated active O/B FLPs are
still rare.
We had recently disclosed a novel approach to frustrated
oxygen/borane Lewis pairs.10 For this purpose, we used the
reactive zirconium hydride reagent 1 (which we had readily
prepared from Cp*2ZrH211 and the sufficiently Brønsted acidic
2,4,6-trimethylphenol) and reacted it with carbon monoxide in
the presence of Piers’ borane [HB(C6F5)2].12 We assume that
Piers’ borane carbonyl [(C6F5)2B(H)CO]13 was formed in situ,
which then was reduced by the active [Zr]−H reagent to give
the zirconocene coordinated formyl hydridoborate product 2.
Compound 2 was characterized by X-ray diffraction and by
solid state NMR spectroscopy. However, the compound turned
out to show dynamic behavior in solution, indicating a very
rapid equilibration with the [Zr]−O−CH2−[B] isomer 3. The
in situ available system 3 turned out to be highly reactive. It
underwent rapid reactions with a variety of added substrates
acting as a reactive oxygen/borane FLP. The zirconocene unit
was just an innocent bystander that was necessary for the
effective generation of the O/B pair by the carbonylation route
(Scheme 1).10
It was tempting to speculate that related reactive geminal O/
B FLPs might become available by true metal-free carbon-
ylation routes. We found that this was indeed the case by
employing a special intramolecular N/B FLP for the reaction
with the CO/HB(C6F5)2 pair. The formation of the respective
metal-free −O−CH2−[B] system and first reactions are
described and discussed in this account.
RESULTS AND DISCUSSION
■
Formation and First Characterizing Reactions of the
N/B FLP 7. For the purpose of this study, we used a
trimethylene-bridged N/B FLP 714 with a specifically designed
amine Lewis base function, that, as we shall see, provides an
essential chemical feature for the generation of the −O−CH2−
[B] function (Scheme 2). The synthesis started from N-allyl-cis-
2,6-dimethylpiperidine (4) that we had made by N-allylation of
the respective piperidine derivative. The commercial starting
Received: January 18, 2018
© XXXX American Chemical Society
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geometry, but we note that the N1···B1 distance is long (1.735
(3) Å) and the N/B interaction is probably weak.5 That was
confirmed by the dynamic NMR spectra of compound 7 (see
below).
Scheme 2. Synthesis of the N/B FLP 7 and H2/D2-Splitting
Reactions
The 1H NMR spectrum of compound 7 in CD2Cl2 at 213 K
features the signals of a pair of inequivalent methyl substituents.
It shows their adjacent C−H methine resonances at δ 4.29 and
δ 2.68 ppm, respectively (with corresponding 13C NMR signals
at δ 64.4 and δ 53.2 ppm). Due to the chirality of the trans-2,6-
dimethylpiperidino unit, all CH2 hydrogens are diastereotopic.
1
So we observed a total of 12 respective H NMR signals and a
total of 6 methylene 13C NMR resonances (see the Supporting
Information for details). The 11B NMR resonance of
compound 7 occurs at δ 3.4 ppm. The 19F NMR spectrum
shows four o-, two p-, and four m-C6F5 signals of the pair of
diastereotopic C6F5 substituents at boron. Compound 7 shows
dynamic NMR spectra at high temperature. The 19F NMR
signals of the −B(C6F5)2 moiety eventually coalesce to a single
2:1:2 intensity set for the o-, p-, and m-fluorines at Tc ∼ 299 K,
which probably indicates rapid equilibration with the respective
“invisible” open N/B isomer. From this behavior, we have
estimated the Gibbs activation barrier of the (reversible) N···B
bond dissociation at ΔG⧧ (299 K) = 12.7 0.3 kcal·mol−1.17
The FLP 7 is an active dihydrogen splitting reagent.
Exposure of a solution of compound 7 in dichloromethane to
a dihydrogen atmosphere (2.0 bar, r.t.) resulted in the
appearance of a white precipitate within 10 min. The reaction
was stirred for some time to let it go to completion. We isolated
the ammonium/hydridoborate product 8 in 87% yield. It shows
material is usually derived from hydrogenation of lutidine15
which accounts for the cis-2,6-dimethyl substitution pattern.
Compound 4 was then treated with Piers’ borane HB(C6F5)2,
which resulted in a rapid and clean hydroboration reaction (r.t.,
20 min) to give the trimethylene-bridged N/B FLP 7 (isolated
in 89% yield). We note that, starting from the cis-
dimethylpiperidine precursor 4, we cleanly arrived at the
trans-2,6-dimethylpiperidinyl containing N/B FLP 7. We must,
therefore, assume that a selective isomerization has taken place
along the way to 7. In view of the high hydride abstracting
ability of the −B(C6F5)2 boranes from C−H positions adjacent
to nitrogen,16 we assume that the initially formed hydro-
boration product 5 underwent H− transfer to boron with
iminium ion formation. Readdition is apparently fast under our
typical reaction condition, and this probably provides a viable
pathway to the eventual formation of the observed product 7
(Scheme 2).
Compound 7 was characterized by C,H,N-elemental analysis,
by X-ray diffraction, and by NMR spectroscopy. The X-ray
crystal structure analysis shows the presence of the trans-2,6-
dimethylpiperidino group (Figure 1). It shows a chair
conformation with one methyl group axially and the other
equatorially oriented. Carbon atom C1 of the trimethylene
bridge to boron takes an axial position at nitrogen; this leaves
room for the N···B interaction in the equatorial site at nitrogen.
The boron atom shows a pseudo-tetrahedral coordination
1
the typical NMR features of the backbone plus a broad H
NMR NH signal at δ 7.76 and a 1:1:1:1 intensity BH 1H NMR
resonance at δ 2.87, with a corresponding 11B NMR feature at δ
1
−20.5 ppm (d, JBH ∼ 77 Hz). We also carried out the
analogous reaction with D2 and isolated the isotopologue 8-D2
in 86% yield. In the 1H NMR spectrum, the respective NH and
BH resonances are now missing, but two broad signals
2
appeared in the H NMR spectrum of compound 8-D2 at δ
7.62 (ND) and δ 2.87 ppm (BD), respectively (for further
details and the depicted NMR spectra of compound 8/8-D2,
see the Supporting Information).
The X-ray crystal structure analysis of compound 8 shows
the dimethylpiperidino moiety featuring one methyl group in
the axial position and the other equatorially oriented. The
trimethylene bridge is equatorially attached at nitrogen, which
brings the newly introduced ammonium hydrogen atom in an
axial position. The [N]−(CH2)3−[B] unit features a distorted
cisoid (close to doubly gauche) conformational arrangement
(dihedral angles N1−C1−C2−C3 −46.4(2)°, C1−C2−C3−B1
−45.5(2)°). The boron atom B1 bears the hydride in a pseudo-
tetrahedral arrangement (∑B1CCC 337.7°) (Figure 2).
Compound 8 is a metal-free hydrogenation catalyst. Both the
imine 9a and the enamine 10a were hydrogenated using 10 mol
% each of the NH+/BH− catalyst in CD2Cl2 solution under
mild conditions (r.t., 2.0 bar H2, 3 d) to give the respective
amine products 9b and 10b in practically quantitative
conversion.5a,18 Compound 8 is also a hydrogenation catalyst
for the α,β-unsaturated ketone chalcone 11a, albeit this
hydrogenation reaction is somewhat slower.19 With 10 mol %
of the catalyst 8, we achieved a 38% conversion under our
typical conditions. It took 20 mol % of 8 to achieve a close to
quantitative conversion to the saturated ketone 11b (94%,
Scheme 3; see the Supporting Information for further details).
Figure 1. Molecular structure of the N/B FLP 7 (thermal ellipsoids
are shown at the 15% probability level). Selected bond lengths (Å) and
angles (deg): N1−C1 1.523(3), C3−B1 1.638(3); C3−B1−C31
113.2(2), C3−B1−C21 107.8(2), ∑B1CCC 327.4, ∑N1CCC 323.9,
C1−N1−C11−C16 −70.6(6), C1−N1−C15−C17 176.0(2).
B
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Figure 2. A view of the molecular structure of the FLP dihydrogen
splitting product 8 (thermal ellipsoids are shown at the 30%
probability level). Selected bond lengths (Å) and angles (deg): N1−
C1 1.513(2), C1−C2 1.523(2), C2−C3 1.544(2), C3−B1 1.632(2);
∑N1CCC 340.0, C15−N1−C11−C16 66.1(2), C11−N1−C15−C17
−176.3(1).
Figure 3. A view of the FLP derived borataepoxide system 12 (thermal
ellipsoids are shown at the 15% probability level). Selected bond
lengths (Å) and angles (deg): N1−C11 1.282(4), N1−C15 1.526(4),
N1−C1 1.489(4), C1−C2 1.531(4), C2−C3 1.530(4), C3−B1
1.610(4), B1−O1 1.551(4), O1−C4 1.482(3), C4−B2 1.549(5),
B2−O1 1.553(4); C11−N1−C1 123.6(3), O1−C4−B2 61.6(2), C4−
B2−O1 57.1(2), B2−O1−C4 61.3(2), ∑N1CCC 360.1, ∑B1CCC
337.8, ∑O1CBB 326.4, N1−C1−C2−C3 −63.1(4), C1−C2−C3−B1
178.3(3).
Scheme 3. Catalytic Hydrogenation Using the Metal-Free
NH+/BH− Catalyst 8
oxide ring with its ring-opened isomeric form on the NMR time
scale.
[B]−H boranes usually do not reduce carbon monoxide,
unless catalyzed or induced by hydridoboranes.20,21 Instead,
they form borane carbonyls. Typical examples are H3B−CO20
or (C6F5)2B(H)CO (13).13 CO reduction by HB(C6F5)2 had
been achieved at P/B FLP templates. Some of these effects are
probably used in the CO reduction by HB(C6F5)2 at the N/B
FLP system 7, as well. We notice that the formation of the
borataepoxide from CO required two hydride equivalents. One
obviously originates from the added HB(C6F5)2 reagent, and
the other then must be provided by the FLP system 7. This lets
us to assume equilibration of 7 with the iminium/
hydridoborate isomer 6 (Schemes 2 and 4) by the well-
established hydride abstraction at the α-NCH by the adjacent
active −B(C6F5)2 borane.16 The hydridoborate functionality
might then reduce the CO group of in situ generated Piers’
borane carbonyl [(C6F5)2B(H)CO] (13) to generate the
borane coordinated formyl hydridoborate moiety10,22 in the
intermediate 14 (Scheme 4). Internal carbonyl reduction by the
boron-hydride then leads to 15. This can then probably
undergo facile ring closure to give the observed borataepoxide
ring system attached at the N/B FLP derived framework (12).
Compound 12 is thermally sensitive. Keeping it for 18 h in
CH2Cl2 solution at r.t. resulted in a complete rearrangement to
the product 16, which we isolated from the workup procedure
as a white solid in 95% yield. The 19F NMR spectrum showed
two sets of o,p,m-C6F5 signals in a 1:3 intensity ratio indicating
the presence of a −B(C6F5) and a −B(C6F5)3 unit. We detect a
sharp 11B NMR signal in the typical borate range (δ −14.3
ppm) and a broad 11B NMR feature at δ 44.7 ppm, which is
typical for an oxygen bound trivalent boron center in this
situation. There is a set of NMR signals of a [B]−O−CH2−[B]
unit at δ 4.43/4.32 (1H) and δ 71.6 ppm (13C). In addition, we
monitored the typical NMR signals of the trimethylene-bridged
N/B framework [e.g., δ 188.0 ppm (13C), iminium cation].
Formation of the Borataepoxide System by the FLP
Carbonylation Route. The borataepoxide system 12 turned
out to be an important compound of our study. It was prepared
from the N/B FLP 7 by treatment with carbon monoxide and
HB(C6F5)2 under mild conditions (CH2Cl2, r.t., 2.0 bar CO, 1
h). Workup gave the product 12 as a white solid in 91% yield.
The X-ray crystal structure analysis showed that the saturated
COB containing three-membered heterocycle had been formed
selectively (Figure 3). This formally anionic moiety is found
coordinated to the N/B FLP boron atom (B1) through its ring
oxygen atom. The oxygen coordination geometry in the solid
state is trigonal pyramidal. The trimethylene bridge connects
boron atom B1 with the dimethylpiperidine derived moiety, but
this has lost a hydrogen atom adjacent to nitrogen to generate
an endocyclic iminium ion moiety.
The 1H NMR spectrum of compound 12 in CD2Cl2 solution
shows the signal of the sole remaining α-hydrogen atom
adjacent to nitrogen at δ 4.11 ppm. There is the iminium ion
13C NMR resonance at δ 187.5 ppm. The borataepoxide CH2
group gives rise to signals at δ 3.17 (1H) and δ 57.5 ppm (13C),
respectively [cf. the −CH2−B1 resonances of the trimethylene
bridge occur at δ 1.12 (1H) and δ 21.3 ppm (13C)]. We found
two boron resonances (δ 3.5 and δ −10.5 ppm), both being in
the tetracoordinate boron range, and we monitored the 19F
NMR signals of a total of four −C6F5 substituents attached at
the pair of boron atoms of compound 12. We found that one
pair of p-C6F5 signals was beginning to broaden with increasing
the monitoring temperature from 263 to 299 K, which might
potentially indicate a beginning equilibration of the borataep-
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Scheme 4. Carbon Monoxide Reduction with the N/B FLP 7
Scheme 5. Reaction of the Borataepoxide System 12 with a
Terminal Alkyne and with Dihydrogen
The X-ray crystal structure analysis confirmed that the
product 16 was formed by C6F5 migration from boron atom B1
to B2 (Figure 4). Boron atom B1 consequently has one C6F5
Compound 12 reacts rapidly with phenylacetylene (r.t., 1 h)
in dichloromethane solution to give the C−H activation
product 17 (isolated as a white solid in 92% yield). The
compound is thermally sensitive, so the characterization was
done at low temperature. The 13C NMR spectrum of
compound 17 shows the typical iminium NC feature at δ
186.9 and a pair of [B]-acetylide carbon resonances at δ 103.7
(br, B−C) and δ 99.0 ppm (C−Ph), respectively. The
pair of 10B NMR signals at δ 4.3 and δ −19.6 ppm. We have
monitored four 19F NMR o,p,m-sets of signals for a total of four
diastereotopic C6F5 substituents at the pair of boron atoms (see
the Supporting Information for details).
1
compound shows a broad −OH H NMR signal at 6.85 and a
Single crystals suited for the X-ray crystal structure analysis of
compound 17 were obtained at −35 °C from a dichloro-
methane solution layered with n-pentane. It confirms cleavage
of the acetylene C−H bond. The proton has become bonded to
the [B]−O1 oxygen atom to generate an oxonium cation
moiety, and the remaining alkynyl anion has become bonded to
the boron atom B2 of the former borataepoxide. The B2−
acetylide moiety is close to linear as expected (Figure 5).
We performed the reaction of the borataepoxide 12 with
dihydrogen at 50 bar H2 (1 d) using a suspension in
dichloromethane. Under these conditions, the competing
rearrangement of 12 to 16 was sufficiently suppressed and we
isolated the H2-splitting product 21 in 83% yield. The X-ray
crystal structure analysis showed again the presence of the
[B1]−OH−[B2] bridged unit. In this case, the boron atom B2
has a methyl substituent bonded to it. The methyl group
eventually originated from carbon monoxide, which had
become first reduced to the boron bonded −O−CH2− moiety
and then further reduced by treatment with H2 all the way to
the [B2]−CH3 structural unit (Figure 6).24
Figure 4. Molecular structure of the thermodynamic CO reduction
product 16 (thermal ellipsoids are shown at the 15% probability level).
Selected bond lengths (Å) and angles (deg): B1−C3 1.565(4), B1−
O1 1.334(4), B1−C21 1.593(4), O1−C4 1.470(3), C4−B2 1.635(4);
B2−C4−O1 111.3(2), C4−O1−B1 123.0(2), O1−B1−C3 120.3(3),
∑B1OCC 360.0, N1−C1−C2−C3 −165.3(4), C2−C3−B1−O1
−6.6(4), B1−O1−C4−B2 134.5(3).
group, carbon atom C3 of the trimethylene bridge to nitrogen,
and the former carbonyl oxygen atom O1 bonded to it in a
planar-tricoordinated geometry. The dimethylpiperidine de-
rived nitrogen heterocycle contains an internal iminium ion
function. Boron atom B2 is tetracoordinated by three C6F5
substituents and the CH2 group (C4) of the [B1]−O−CH2−
[B2] bridge.
C−H Activation and H2-Splitting. Many frustrated Lewis
pairs react with the C−H acidic terminal alkynes by
deprotonation to give the respective cation/alkynylborate
salts,23 and many FLPs split dihydrogen under mild reaction
conditions.1,2 Our new borataepoxide system 12 undergoes
both of these reactions as well (Scheme 5).
In solution (THF-d8), compound 21 shows the broadened
1H NMR OH resonance at δ 6.04 and the [B2]−CH3 1H/13C
NMR signals at δ 0.42/δ 10.0 ppm. In principle, we should see
the 19F NMR signals of four diastereotopic C6F5 groups.
D
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using the good oxonium leaving group.9 Consequently,
cleavage of the O−C σ-bond would lead to a [B]−OH 19/
CH3−B(C6F5)2 20 pair of products. Capture of the Lewis acidic
methyl-bis(pentafluorophenyl)borane by the hydroxylborate
function may then directly lead to the observed product 21.
Carbon Dioxide Capture and Related Reactions. We
had shown that the reactive formyl(hydrido)borate zirconium
complex 2 added CO2.10,25,26 This reaction might have
proceeded via the zirconoxy/borane FLP 3/3′. The borataep-
oxide system 12 undergoes the analogous reaction in its metal-
free environment. The solution of compound 12 in dichloro-
methane was exposed to a CO2 atmosphere (2 bar) for 2 h at
r.t. Workup involving washing with n-pentane gave compound
23 as a white solid, which we isolated in a yield of 96%
(Scheme 6).
Scheme 6. Reaction of the Borataepoxide 12 with Carbon
Dioxide
Figure 5. A view of the molecular structure of the acetylene C−H
cleavage product 17 (thermal ellipsoids are shown at the 15%
probability level). Selected bond lengths (Å) and angles (deg): N1−
C11 1.498(6), N1−C15 1.313(6), N1−C1 1.469(5), B1−O1
1.589(6), O1−C4 1.496(5), C4−B2 1.622(6), B2−C5 1.597(6),
C5−C6 1.203(5), C6−C41 1.438(6); C11−N1−C15 123.4(4), B1−
O1−C4 124.2(3), O1−C4−B2 104.7(3), C4−B2−C5 104.4(3), B2−
C5−C6 168.7(4), C5−C6−C41 176.7(5), ∑N1CCC 359.8, ∑B1CCC
340.7, B1−O1−C4−B2 −177.1(3), N1−C1−C2−C3 −172.3(4),
C1−C2−C3−B1 176.3(4).
The X-ray crystal structure analysis shows that a borata-
ethylenecarbonate unit was formed. The five-membered
heterocyclic unit contains the B−O−CH2 building block
originating from the starting material 12 combined with a
molecule of carbon dioxide. In the product 23, the borata-
carbonate carbonyl oxygen atom O1 is found bonded to the
boron atom (B1) of the FLP framework (Figure 7).
Compound 23 shows the typical iminium 13C NMR
resonance in solution (THF-d8) at δ 188.7 ppm. The borata-
carbonate 13C NMR feature was located at δ 166.4, and the
endocyclic CH2 group of this newly formed five-membered
heterocycle shows 1H/13C NMR signals at δ 4.35/73.8 ppm. It
seems that the boron resonances are by chance isochronous; we
observed only a single broad resonance at δ 1.7 ppm in both
the 11B and 10B NMR spectra of compound 23. It shows two
equal intensity o,p,m-C6F5 sets of 19F NMR signals for the pair
of −B(C6F5)2 groups in this molecule (see the Supporting
Information for details).
We assume a reaction pathway that involves endergonic
equilibration of 12 with the open form 15. This then could
undergo the O/B FLP addition reaction to a CO moiety of
carbon dioxide to generate the borata-carbonate subunit in the
intermediate 22. Rearrangement to the favored carbonyl to
boron Lewis acid coordinated isomer then closes the reaction
cycle as depicted in Scheme 6.
Figure 6. Molecular structure of the borataepoxide reduction product
21 (thermal ellipsoids are shown at the 30% probability level).
Selected bond lengths (Å) and angles (deg): B1−O1 1.566(2), O1−
B2 1.563(2), B2−C4 1.606(2); B1−O1−B2 131.5(1), O1−B2−C4
109.6(1), B1−O1−B2−C4 30.3(2).
However, we have monitored two o,p,m-sets of signals at 299 K
(see the Supporting Information for details).
We do not know the mechanistic details of the formation of
the product 21. In principle, one might discuss two possible
pathways to explain the splitting of the H−H molecule by
compound 12. One would involve a direct reaction of H2 with
the O−C bond of the strained borataepoxide. A more
conventional alternative would involve the ring-opened form
15, and a subsequent frustrated Lewis pair (FLP) pathway. The
latter is depicted in Scheme 5, proceeding via the zwitterionic
oxonium/hydridoborate intermediate 18. This is set for internal
nucleophilic attack by hydride on the adjacent methylene group
The reaction of the 12 with acetonitrile (a) or benzonitrile
(b) proceeds similarly.27,28 The nitrile addition reactions yield
the products 24. We assume 1,2-addition of the −CN
E
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solid (isolated in 95% yield). From the n-pentane extract, we
isolated compound 24a (R = CH3) in 93% yield.
We obtained single crystals of compound 25 from the
acetonitrile experiment that were suited for its characterization
by X-ray diffraction. The compound shows a heterobicyclo-
[5.4.0]undecene-like structure. There is an iminium NC
double bond between the bridgeheads. Carbon atom C9 inside
the six-membered subunit bears the methyl substituent, and the
borate anion unit is part of the seven-membered substructure
(Figure 8).
Figure 7. Molecular structure of the borata-ethylenecarbonate product
23 from the reaction of the borataepoxide 12 with carbon dioxide
(thermal ellipsoids are shown at the 30% probability level). Selected
bond lengths (Å) and angles (deg): B1−O1 1.573(2), O1−C4
1.255(2), C4−O2 1.285(2), C4−O3 1.310(2), B2−O2 1.548(2), B2−
C5 1.644(3), C5−O3 1.480(2), N1−C11 1.300(3); B1−O1−C4
124.2(2), C4−O2−B2 110.3(2), C4−O3−C5 109.3(1), O2−B2−C5
98.5(1), B2−C5−O3 103.7(2), C5−O3−C4 109.3(1), ∑B1CCC 336.1,
∑C4OOO 360.0.
Figure 8. A projection of the molecular structure of compound 25
(thermal ellipsoids are shown at the 15% probability level). Selected
bond lengths (Å) and angles (deg): N1−C5 1.295(4), N1−C1
1.483(4), B1−C3 1.628(5), B1−C4 1.688(5); C4−B1−C3 107.7(3),
∑N1CCC 359.9, ∑C5NCC 360.0, N1−C1−C2−C3 −77.7(4), C2−-
C3−B1−C4 −51.8(4).
function to the O/B section of 12 to generate 26. Both the
systems in these cases then are not stable but undergo
deprotonation at the methyl group of the iminium ion part of
the system under our typical reaction conditions (r.t., 1 h,
CH2Cl2) to form the observed products 24 and generate the
reactive intermediate 27. This contains an enamine function-
ality alongside of an active borane Lewis acid. Enamine C-
addition to boron29 then probably represents the final step in
the formation of the second reaction product, the bicyclic
iminium/borate zwitterion 25 (Scheme 7). The products 24
and 25 were in both cases found in a ca. 1:1 ratio.
The 1H NMR spectrum of compound 25 in CD2Cl2 at 299 K
shows the methyl group signal at δ 1.33 ppm. The hydrogen
atoms of the ring CH2 groups are each pairwise diastereotopic
due to the C9 carbon chirality center (atom numbering as
depicted in Figure 8) as are the C6F5 groups at boron. These
show two sets of each o,p,m-C6F5 19F NMR signals at 299 K,
which both decoalesce upon lowering the monitoring temper-
ature. The dynamic behavior indicates freezing of the rotation
of the −C6F5 groups around the B−C vectors plus a lowering of
the conformational mobility of the framework ring systems on
the NMR time scale. The iminium 13C NMR carbon resonance
of compound 25 was located at δ 197.9 ppm.
In the acetonitrile series, we could separate the products by
treatment with n-pentane. This left compound 25 as a white
Scheme 7. Reaction of the O/B FLP with Nitriles
Compound 24a (R = CH3) was characterized by C,H,N-
elemental analysis and by NMR spectroscopy. The zwitterionic
five-membered heterocycle shows a very simple 1H NMR
spectrum showing three singlets at δ 7.95 (br, 1H, NH), δ 4.75
(2H, CH2; 13C: δ 79.4), and δ 2.31 (3H, CH3; 13C: δ 16.6),
respectively. The 11B NMR resonance of compound 24a was
located at δ −6.8 ppm, i.e., in the typical tetracoordinate borate
range, and there are three 19F NMR signals of the o,p,m-fluorine
atoms of the pair of symmetry equivalent C6F5 groups at boron.
The reaction of 12 with benzonitrile (1 h, r.t., CH2Cl2)
proceeded similarly. After removal of the solvent in vacuo,
treatment with n-pentane gave the insoluble compound 25,
which we isolated in 96% yield. The n-pentane extract was
concentrated. Crystallization at −78 °C gave compound 24b in
95% yield. Single crystals of 24b were obtained from a two-
layer dichloromethane/n-pentane mixture at −35 °C. This
allowed characterization of compound 24b by X-ray crystal
structure analysis. It shows a close to planar five-membered
F
DOI: 10.1021/acs.organomet.8b00035
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Article
heterocycle with a strong internal B−N linkage and an adjacent
iminium-type CN double bond. The phenyl substituent at
the ring carbon atom C2 is rotated slightly away from the
central five-membered core plane of compound 24b (Figure 9).
(THF-d8) of the trimethylene-bridged N/B backbone [δ 187.3
(CN+), δ 58.6, 26.2, 20.4 (−(CH2)3−), δ 0.1 ppm (11B)].
1
We monitored the H NMR signals of the newly introduced
−OCH2Ph group at boron at δ 4.29 [δ 66.2 (13C)] (CH2) and
δ 7.39, 7.17, 7.04 ppm (Ph).
The X-ray crystal structure analysis has confirmed the
structural assignment of the product 31. Its structural features
are unexceptional. The structure is depicted in the Supporting
Information. The X-ray crystal structure analysis of product 30
shows the presence of a planar five-membered heterocycle. The
phenyl substituent at carbon atom C2 is oriented almost in
plane. The adjacent carbon−oxygen bonds are almost equi-
distant (C2−O2: 1.268(2) Å, C2−O1: 1.294(3) Å).30 The B−
C6F5 vectors are oriented close to eclipsed with the C1−H
bonds of the adjacent CH2 group (Figure 10).
Figure 9. Molecular structure of compound 24b (R = Ph) (thermal
ellipsoids are shown at the 15% probability level). Selected bond
lengths (Å) and angles (deg): B1−N1 1.564(3), N1−C2 1.292(2),
C2−C31 1.462(3), C2−O1 1.310(2), O1−C1 1.494(2), C1−B1
1.645(3); N1−B1−C1 95.8(1), B1−C1−O1 105.3(1), C1−O1−C2
109.7(1), C2−N1−B1 113.7(2), ∑B1CCC 339.8, ∑C2ONC 360.0, N1−
C2−C31−C32 11.2(3).
The phenyl substituted compound 24b shows similar NMR
features as 24a. We observed the NH 1H NMR resonance at δ
8.31 and the endocyclic CH2 resonance at δ 4.96. The iminium
ion 13C NMR signal occurs at δ 175.0 and the 11B NMR
resonance is at δ −6.5 ppm.
Compound 12 reacts rapidly with benzaldehyde (1 h, r.t.,
CH2Cl2). Two equivalents of the aldehyde are consumed to let
the reaction go to completion. It gives a ca. 1:1 mixture of the
products 30 and 31 (Scheme 8). The compounds could be
Figure 10. Molecular structure of compound 30 (thermal ellipsoids
are shown at the 15% probability level). Selected bond lengths (Å) and
angles (deg): B1−C1 1.636(3), B1−O2 1.566(3), O2−C2 1.268(2),
C2−O1 1.294(3), O1−C1 1.493(3); B1−C1−O1 103.6(2), O1−C2−
O2 118.3(2), ∑B1CCC 342.4, O1−C2−C11−C12 −174.3(2).
1
In CD2Cl2 solution, compound 30 shows a H NMR singlet
Scheme 8. Reaction of the Borataepoxide with Benzaldehyde
of the endocyclic CH2 group at δ 4.92 (13C: δ 79.0) and the
resonances of the phenyl substituent at carbon C2. The
carbonyl carbon shows a 13C NMR signal at δ 181.1 ppm.
Compound 30 features a 11B NMR signal at 3.4 ppm and a
single set of three 19F NMR resonances of the pair of
symmetry-equivalent C6F5 substituents at boron (Δδ19Fm,p
=
6.6 ppm).
We assume that compound 12 adds to the benzaldehyde
carbonyl group21c−e,31 to generate 29. This would give the core
of the eventually observed five-membered heterocycle, albeit in
the wrong oxidation state. Therefore, it is likely that the active
borane Lewis acid at the intermediate stage 29 (Scheme 8)
serves to abstract hydride16 from it to form the final product 30.
This reaction generates the zwitterion 6 containing an active
hydridoborate function which is trapped by benzaldehyde to
give the second observed product 31.
CONCLUSIONS
■
[B]−H boranes usually do not reduce carbon monoxide.
Instead, they may form borane carbonyls. Typical examples are
the formation of the parent borane carbonyl 32 from
diborane20 and CO or the formation of Piers’ borane carbonyl
(13) from HB(C6F5)2 and carbon monoxide. We had recently
prepared this compound and characterized it by X-ray
diffraction.13 CO reduction by [B]−H boranes required a
catalyst,21 and hydridoborates had been shown to successfully
execute this task (Scheme 9).
separated by treatment with n-pentane. Stirring the mixture
with this solvent gave a suspension of the insoluble product 31.
It was collected by filtration and obtained in 93% yield. From
the n-pentane extract, compound 30 was crystallized at −78 °C
(1 h). The crystalline product was isolated in 91% yield. Both
products were characterized by NMR spectroscopy and by X-
ray diffraction. Compound 31 shows the typical NMR features
G
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Article
−35 °C. Anal. Calcd for C22H22BF10N (501.2 g mol−1): C, 52.72; H,
4.42; N, 2.79. Found: C, 52.62; H, 4.32; N, 2.81.
Scheme 9. Reactions of B−H Boranes with Carbon
Monoxide
Synthesis of Compound 8-D2. A solution of compound 7 (180.3
mg, 0.36 mmol) in CH2Cl2 (5 mL) was degassed by applying vacuum
at r.t. Then the solution was exposed to D2 (1.0 bar), which resulted in
the formation of a white precipitate within 10 min. The mixture was
stirred at r.t. for 5 h. Then the suspension was filtered, the obtained
solid material was washed with n-pentane (3 × 3 mL) and dried in
vacuo to give a white solid. Yield: 156.3 mg, 86%.
Catalytic Hydrogenation. Procedure A: In a glovebox with an
argon atmosphere, compound 8 (0.1 equiv) and a substrate (1.0
equiv) were dissolved in CD2Cl2 (1.5 mL), which was then transferred
to an autoclave. The solution was stirred at r.t. for 3 days under a H2
We had used the reductive power of a zirconocene hydride to
achieve the [B]−CO to [B]−formyl reduction, and this had
provided us with an entry to a geminal zirconoxy/B FLP system
(Scheme 1).10 Our present study was based on the necessity to
provide two hydride sources for the CO reduction at the boron
center under these restrictive conditions in order to achieve the
reduction of carbon monoxide to the geminal OCH2[B] unit in
a metal-free environment. One hydride was provided by the
HB(C6F5)2 reagent, but this could only be used in the second
step of the reduction sequence. We, therefore, used the specific
features of the trimethylene-bridged N/B FLP framework of
compound 7 to serve as a source for a reactive borohydride
reagent. This made use of the ubiquitous ability of strongly
electrophilic boranes to abstract hydride from the α-position of
amines to generate iminium cation/hydridoborate pairs,16 here
the respective isomer 6 from the N/B FLP 7 (Schemes 2 and
4). This sets the scene for external reduction of the in situ
generated (C6F5)2B(H)CO by the borohydride function of 6,
followed by subsequent internal formyl hydridoborate reduc-
tion to give the borataepoxide 12. It reacts with a variety of
reagents, this including CO2 capture to give the borata-
carbonate product, splitting of dihydrogen under mild
conditions, C−H cleavage of a terminal acetylene as well as
nitrile and carbonyl addition reactions. The study has shown
that the reactive borataepoxide can readily be generated under
metal-free conditions. Its chemistry adds to the manifold of
small molecule activation and binding reactions without the use
of transition metal derived reagents.
1
atmosphere (50 bar). Product conversion was estimated by H NMR
spectroscopy by integration. Procedure B: In a glovebox with an argon
atmosphere, compound 8 (0.1 equiv) and a substrate (1.0 equiv) were
dissolved in CD2Cl2 (1.5 mL), which was then transferred to a Schlenk
flask. The solution was degassed by applying vacuum carefully at −78
°C, then stirred at r.t. for 3 days under a H2 atmosphere (2.0 bar).
Product conversion was estimated by 1H NMR spectroscopy by
integration.
Synthesis of Compound 12. Compound 7 (237.5 mg, 0.48
mmol) and HB(C6F5)2 (165.1 mg, 0.48 mmol) were weighed together
and dissolved in CH2Cl2 (15 mL) (10 mL). The mixture was degassed
by applying vacuum carefully at r.t. Then the solution was exposed to
CO (2.0 bar). The mixture was stirred at r.t. for 1 h. After that, all the
volatiles were removed in vacuo, and the residue was washed with n-
pentane (3 × 5 mL) and dried in vacuo to give a white solid. Yield:
378.2 mg, 91%. Crystals suitable for the X-ray crystal structure analysis
were obtained from a two-layer procedure using a solution of
compound 12 in the mixture of CH2Cl2 and toluene (v/v = 1:1)
covered with n-pentane at −35 °C. Anal. Calcd for C35H21B2F20NO
(873.1 g mol−1): C, 48.15; H, 2.42; N, 1.60. Found: C, 48.07; H, 2.41;
N, 1.56.
Synthesis of Compound 16. A solution of compound 12 (503.6
mg, 0.50 mmol) in CH2Cl2 (25 mL) was stirred at room temperature
for 18 h. Then the solvent was removed in vacuo and the obtained
residue was washed with n-pentane (3 × 5 mL) and dried in vacuo to
give a white solid. Yield: 478.5 mg, 95%. Crystals suitable for the X-ray
crystal structure analysis were obtained from a two-layer procedure
using a solution of compound 16 in CH2Cl2 covered with n-pentane at
−35 °C. Anal. Calcd for C35H21B2F20NO (873.1 g mol−1): C, 48.15;
H, 2.42; N, 1.60. Found: C, 47.92; H, 2.51; N, 1.78.
Synthesis of Compound 17. Phenylacetylene (30.6 mg, 0.30
mmol) in CH2Cl2 (1 mL) was added to a suspension of compound 12
(236.3 mg, 0.27 mmol) in CH2Cl2 (10 mL). The mixture was stirred at
room temperature for 1 h. Then all the volatiles were removed in
vacuo. The obtained residue was washed with n-pentane (3 × 3 mL)
and dried in vacuo to give a white solid. Yield: 242.9 mg, 92%. Crystals
suitable for the X-ray crystal structure analysis were obtained from a
two-layer procedure using a solution of compound 17 in CH2Cl2
covered with n-pentane at −35 °C. Anal. Calcd for C43H27B2F20NO
(975.3 g mol−1): C, 52.96; H, 2.79; N, 1.44. Found: C, 51.79; H, 2.71;
N, 1.27.
Synthesis of Compound 21. Compound 12 (296.3 mg, 0.30
mmol) was suspended in CH2Cl2 (10 mL) and stirred at r.t. under a
H2 atmosphere (50 bar) in an autoclave for 1 day. Then the solution
was transferred into a Schlenk flask and n-pentane (30 mL) was added
to give a white precipitate. The suspension was filtered and the
obtained solid material was washed with n-pentane (3 × 3 mL) and
dried in vacuo to give a white solid. Yield: 246.5 mg, 83%. Crystals
suitable for the X-ray crystal structure analysis were obtained from a
two-layer procedure using a solution of compound 21 in CH2Cl2
covered with n-pentane at −35 °C. Anal. Calcd for C35H23B2F20NO
(875.2 g mol−1): C, 48.03; H, 2.65; N, 1.60. Found: C, 48.05; H, 2.33;
N, 1.48.
EXPERIMENTAL SECTION
■
For general information and the spectroscopic and structural data of
these new compounds, see the Supporting Information. Some of the
compounds were not obtained completely pure, as judged from the
results of the elemental analysis and the NMR spectra (for details, see
the Supporting Information).
Synthesis of Compound 7. A solution of compound 4 (154.1
mg, 1.0 mmol) in n-pentane (2 mL) was added to a suspension of
HB(C6F5)2 (345.6 mg, 1.0 mmol) in n-pentane (20 mL), which
resulted in a colorless solution within 5 min. The reaction mixture was
stirred at room temperature for another 20 min, and then stored at
−78 °C for 2 h to give a colorless crystalline solid, which was collected
by filtration and dried in vacuo. Yield: 443.9 mg, 89%. Crystals suitable
for the X-ray crystal structure analysis were obtained from a solution of
compound 7 in n-pentane at −35 °C. Anal. Calcd for C22H20BF10N
(499.2 g mol−1): C, 52.93; H, 4.04; N, 2.81. Found: C, 52.42; H, 4.13;
N, 2.96.
Synthesis of Compound 8. A solution of compound 7 (253.6
mg, 0.51 mmol) in CH2Cl2 (10 mL) was degassed by applying vacuum
at r.t. Then the solution was exposed to H2 (2.0 bar), which resulted in
the appearance of a white precipitate within 10 min. The mixture was
stirred at r.t. for 5 h. Then the suspension was filtered, the obtained
solid was washed with n-pentane (3 × 3 mL) and dried in vacuo to
give a white solid. Yield: 221.7 mg, 87%. Crystals suitable for the X-ray
crystal structure analysis were obtained from a two-layer procedure
using a solution of compound 8 in CH2Cl2 covered with n-pentane at
Synthesis of Compound 23. A solution of compound 12 (265.1
mg, 0.30 mmol) in CH2Cl2 (20 mL) was degassed by applying vacuum
carefully at r.t. Then the solution was exposed to CO2 (2.0 bar) and
the mixture was stirred at r.t. for 2 h. After removal of all the volatiles
H
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Article
in vacuo, the obtained residue was washed with n-pentane (3 × 3 mL)
and dried in vacuo to give a white solid. Yield: 267.3 mg, 96%. Crystals
suitable for the X-ray crystal structure analysis were obtained from a
two-layer procedure using a solution of compound 23 in THF covered
with n-pentane at −35 °C. Anal. Calcd for C36H21B2F20NO3 (917.1 g
mol−1): C, 47.14; H, 2.31; N, 1.53. Found: C, 46.73; H, 2.26; N, 1.53.
Synthesis of Compounds 24a and 25. Acetonitrile (29.6 mg,
0.72 mmol) in CH2Cl2 (1 mL) was added to a suspension of
compound 12 (573.2 mg, 0.66 mmol) in CH2Cl2 (10 mL). The
mixture was stirred at room temperature for 1 h. Then all the volatiles
were removed in vacuo. The obtained residue was extracted with n-
pentane (3 × 30 mL). The white solid was collected by filtration and
dried in vacuo. It was identified as compound 25 (Yield: 310.2 mg,
95%). Crystals suitable for the X-ray crystal structure analysis were
obtained from a two-layer procedure using a solution of compound 25
in CH2Cl2 covered with n-pentane at −35 °C. Anal. Calcd for
C22H18BF10N (compound 25, 497.2 g mol−1): C, 53.15; H, 3.65; N,
2.82. Found: C, 52.50; H, 3.37; N, 3.00. The n-pentane solution was
collected, concentrated to ca. 10 mL, and then stored at −78 °C for 1
h to give a colorless crystalline solid, which was collected by filtration
and dried in vacuo to give a white solid (compound 24a Yield: 254.6
mg, 93%). Anal. Calcd for C15H6BF10NO (compound 24a, 417.0 g
mol−1): C, 43.20; H, 1.45; N, 3.36. Found: C, 43.00; H, 1.36; N, 3.38.
Synthesis of Compounds 24b and 25. Benzonitrile (61.9 mg,
0.60 mmol) in CH2Cl2 (1 mL) was added to a suspension of
compound 12 (463.5 mg, 0.53 mmol) in CH2Cl2 (10 mL). The
mixture was stirred at room temperature for 1 h. Then all the volatiles
were removed in vacuo. The obtained residue was extracted with n-
pentane (3 × 30 mL). The white solid was obtained by filtration and
dried in vacuo, which was identified as compound 25 (Yield: 253.3 mg,
96%, characterization see above). The n-pentane solution was
collected, concentrated to ca. 10 mL, and then stored at −78 °C for
1 h to give a colorless crystalline solid, which was collected by filtration
and dried in vacuo to give a white solid (compound 24b, Yield: 241.6
mg, 95%). Crystals suitable for the X-ray crystal structure analysis were
obtained from a two-layer procedure using a solution of compound
24b in CH2Cl2 covered with n-pentane at −35 °C. Anal. Calcd for
C20H8BF10NO (compound 24b, 479.1 g mol−1): C, 50.14; H, 1.68; N,
2.92. Found: C, 50.08; H, 1.81; N, 2.73.
Synthesis of Compounds 30 and 31. Benzaldehyde (106.5 mg,
1.00 mmol) in CH2Cl2 (1 mL) was added to a suspension of
compound 12 (436.6 mg, 0.50 mmol) in CH2Cl2 (10 mL). The
mixture was stirred at room temperature for 1 h. Then all the volatiles
were removed in vacuo. The obtained residue was extracted with n-
pentane (3 × 30 mL). The white solid was obtained by filtration and
dried in vacuo, which was identified as compound 31 (Yield: 281.5 mg,
93%). Crystals suitable for the X-ray crystal structure analysis were
obtained from a two-layer procedure using a solution of compound 31
in CH2Cl2 covered with n-pentane at −35 °C. Anal. Calcd for
C29H26BF10NO (compound 31, 605.3 g mol−1): C, 57.54; H, 4.33; N,
2.31. Found: C, 56.77; H, 4.54; N, 2.47. The n-pentane solution was
collected, concentrated to ca. 10 mL, and then stored at −78 °C for 1
h to give a colorless crystalline solid, which was collected by filtration
and dried in vacuo to give a white solid (compound 30, Yield: 218.3
mg, 91%). Crystals suitable for the X-ray crystal structure analysis were
obtained from a two-layer procedure using a solution of compound 30
in CH2Cl2 covered with n-pentane at −35 °C. Anal. Calcd for
C20H7BF10O2 (compound 30, 480.1 g mol−1): C, 50.04; H, 1.47.
Found: C, 49.70; H, 1.28.
Accession Codes
CCDC 1589587−1589597 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing 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 Author
*E-mail: erker@uni-muenster.de.
■
ORCID
Gerald Kehr: 0000-0002-5196-2491
Gerhard Erker: 0000-0003-2488-3699
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support from the Deutsche Forschungsgemeinschaft
is gratefully acknowledged.
■
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DOI: 10.1021/acs.organomet.8b00035
Organometallics XXXX, XXX, XXX−XXX