Reductive Cleavage of the CO Molecule by a Reactive Vicinal Frustrated PH/BH Lewis Pair

Article abstract of DOI:10.1021/jacs.0c07161

A dimeric ethylene-bridged PH/BH system reduced carbon monoxide to the -CH2-O- state. In the presence of B(C6F5)3, the frustrated PH/BH Lewis pair reacted with carbon monoxide by reductive coupling of two CO molecules at the template. Removal of the B(C6F5)3 borane with pyridine liberated one equiv of carbon monoxide to give a cyclic five-membered P(=O)-CH2-B compound, the product of reductive cleavage of carbon monoxide. It reacted as a borylated Horner P(=O)CH2B carbon nucleophile with carbon dioxide to give a bicyclic product by P-CH2 attack on CO2 combined with internal P=O to boron coordination.

Full text of DOI:10.1021/jacs.0c07161

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Communication
Reductive Cleavage of the CO Molecule by a
Reactive Vicinal Frustrated PH/BH Lewis Pair.
Qiu Sun, Constantin G. G. Daniliuc, Christian Mück-
Lichtenfeld, Klaus Bergander, Gerald Kehr, and Gerhard Erker
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c07161 • Publication Date (Web): 16 Sep 2020
Downloaded from pubs.acs.org on September 16, 2020
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Reductive Cleavage of the CO Molecule by a Reactive Vicinal
Frustrated PH/BH Lewis Pair.
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Qiu Sun, Constantin G. Daniliuc, Christian Mück-Lichtenfeld, Klaus Bergander, Gerald Kehr, and
Gerhard Erker*
Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Corrensstraße 40, 48149 Münster, Germany.
Supporting Information Placeholder
reaction conditions we could observe compound 6 by NMR
spectroscopy on the way to 7 (see the Supporting Information for
details).
ABSTRACT:
A dimeric ethylene-bridged PH/BH system
reduced carbon monoxide to the -CH2-O- state. In the presence of
B(C6F5)3 the frustrated PH/BH Lewis pair reacted with carbon
monoxide by reductive coupling of two CO molecules at the
template. Removal of the B(C6F5)3 borane with pyridine liberated
one equiv. of carbon monoxide to give a cyclic five-membered
P(=O)-CH2-B compound, the product of reductive cleavage of
carbon monoxide. It reacted as a borylated Horner P(=O)CH2B
carbon nucleophile with carbon dioxide to give a bicyclic product
by P-CH2 attack on CO2 combined with internal P=O to boron
coordination.
Scheme 1. Formation of compound 3 and its reaction with
carbon monoxide.
Mes*-P
2
(Fmes)
1
H
(Fmes)
H
CO
Mes*
B
H
P
B
+
P
H
H
Mes*
2 (Fmes)BH2•SMe2
2
O
4
2
Mes*: 2,4,6-tri(tert-butyl)phenyl
Fmes: 2,4,6-tris(trifluoromethyl)phenyl
H
Mes*P
H
(Fmes)
P
H
B
Metal-free reduction of the carbon oxides has seen some major
Mes*
H
B(Fmes)
(Fmes)
H
developments in the recent years. For carbon dioxide
a
3
O
H2C
O
P
B
(Fmes)
B
considerable number of studies have been published involving the
formation of all essential CO2 reduction stages.1 A much smaller
number of reports on the selective reduction of carbon monoxide
under metal-free conditions has been published.2 This is to a
considerable extent due to the observation that BH boranes do not
reduce carbon monoxide unless catalyzed.3 Typically, B2H6 was
reported to form borane carbonyl H3B-CO when exposed to carbon
monoxide, and HB(C6F5)2 likewise forms Piers’ borane carbonyl
[(C6F5)2B(H)CO].4 BH-reduction of CO has, however, been
achieved by various combinations of reagents, most notably by the
use of frustrated Lewis pairs (FLPs) and related systems. Several
examples have achieved CO reduction in this way to the formyl
stage and beyond.5,6
Mes*
O
7
5
H
6
(Fmes)B
PMes*
PMes*
H
H
The essential compound of our study is the new ethylene-bridged
PH/BH FLP 3. It was prepared by hydroboration of the
aryl(vinyl)phosphane
1
with
our
recently
reported
(Fmes)BH2∙SMe2 reagent (2).7 We isolated the PH/BH FLP as a
doubly BH bridged dimer (3)2 [δ -66.0 (1JPH ~ 225 Hz) (31P), δ 23.9
(11B), δ 4.48, 3.03 (PH/BH)]. It was characterized by X-ray
diffraction (Fig 1).
Figure 1. A view of the molecular structure of the PH/BH FLP
dimer (3)2 (thermal ellipsoids at 30 % probability). Selected bond
lengths (Å) and angles (°): P1-C1 1.869(4), C1-C2 1.531(6), C2-
B1 1.585(6), B1-B1* 1.828(9), C1-P1-C11 99.8(2), C2-B1-C31
124.2(3), P1-C1-C2-B1 179.9(3).14
Compound (3)2 reacted slowly with carbon monoxide (toluene,
110 oC ,12 h, 2 bar CO) to give the product 7 (yield: 95% isolated).
The characterization by C, H elemental analysis, by X-ray
diffraction (see the Supporting Information) and by NMR
spectroscopy [δ 4.20/67.3 (-CH2-O-),δ -66.5, -67.3 (31P), δ 77.3,
49.5 (11B)] revealed that the CO molecule had been reduced to a
bridging [B]-CH2-O-[B] unit. We assume a reaction pathway that
may initially involve P/B activation of CO8 to generate 5, which
was then opened by the reaction with the BH function of a second
equivalent of the FLP 3 (Scheme 1). Under carefully controlled
In the presence of B(C6F5)3 the carbonylation reaction took a
different course. Treatment of the dimer (3)2 with CO and B(C6F5)3
(toluene, 2 bar CO, r.t., 12 h) gave an 85% yield of compound 8.
We assume that it is formed by the reaction of the in situ generated
compound 5 (see Scheme 1) with the (C6F5)3B-CO borane
carbonyl.9
Compound 8 was obtained as a mixture of two diastereoisomers
in a ca. 4:1 ratio. The major isomer showed NMR resonances at δ
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Page 2 of 6
36.0 (31P, 1JPH = 462 Hz, 3JPH = 45 Hz, 1H: δ 6.89, 253 K), δ 10.5,
We reacted this PO/B compound 12 with B(C6F5)3 for further
characterization (1 h, r.t. in dichloromethane). Workup gave the
[P]=O-B(C6F5)3 adduct 13 in 95% yield [δ 81.3, -1.2 (11B), δ 70.6
(31P), δ 2.60, 2.44 (1H of [P]-CH2-[B])]. The X-ray crystal structure
analysis (Fig 3) confirmed the formation of the five-membered
1
2
3
4
5
6
7
8
-17.7 (11B, 299 K), δ 5.17 (2JPH = 3.8 Hz, 253K)/ 81.8([P]CH[B],
253 K) and δ 208.6 (13C, C=O, 253 K). The X-ray crystal structure
analysis revealed the formation of a bicyclic framework featuring
a cis-junction between the pair of five-membered rings through C3
and B1. The phosphorus atom bears a hydrogen and the bulky Mes*
group. The bridgehead atoms C3 and B1 are each connected to
oxygen atoms. The distal O1-C4-O2 unit is delocalized; it has the -
B(C6F5)3 moiety bonded to the central carbon atom (Fig 2.).
PO/B
containing
ring
system
formed
by
the
fragmentation/rearrangement reaction of 8 with pyridine. The
central five-membered heterocycle has an envelope conformation
with P1 serving as its tip. The phosphanoxide moiety has the
B(C6F5)3 Lewis acid bonded at oxygen.
9
Scheme 2. Carbonylation of the FLP dimer (3)2 in the
presence of B(C6F5)3.
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3
½ (
)
2
Mes*
pyridine
(Fmes)
CO
r.t.
P
H
B
Mes*-P
B-(Fmes)
+
H
O
- (C6F5)3Bpyr
B(C6F5)3
O
C
O
H
O
H
9
10
8
B(C6F5)3
- CO
B(C6F5)3
Mes*
Mes*
O
P
B-(Fmes)
P
Mes*-P
B-(Fmes)
H
B-(Fmes)
O
O
H
H
H
11
H
H
13
12
B(C6F5)3
Figure 3. Molecular structure of compound 13 (thermal ellipsoids
shown at 30 %, only the Mes* ipso-C at P1 is shown for clarity)
Selected bond lengths (Å) and angles (°): P1-O1 1.525(1), O1-B2
1.549(3), C1-P1-C3 94.8(9), ∑B1CCC 360.0.14
A possible rationalization of a pathway of the formation of
compound 12 is shown in Scheme 2. The pyridine donor has
obviously served to remove the B(C6F5)3 unit from 8. This resulted
in cleavage of the oxygen to carbon linkage with proton transfer to
form the bridging -CH2- group, followed by PO bond formation by
nucleophilic phosphane attack at the B-O- oxygen with liberation
of CO. According to our DFT analysis (Scheme 4 below), the
alleged heterobicyclo[2.1.1]hexane intermediate 11 is energetically
within reach.10 It then undergoes ring-opening to form the observed
product 12.
Scheme 3. Formation of dimeric structures from 12 (with
DFT calculated relative Gibbs energies in kcal mol-1 [in
benzene] method: PW6B95-D3//TPSS-D3/def2-TZVP).
Mes*
(Fmes)
B
Figure 2. A projection of the core framework of compound 8
(thermal ellipsoids at 30 % probability). Selected bond lengths (Å)
and angles (°): B2-C4 1.643(4), C4-O1 1.315(3), C4-O2 1.269(3),
B1-O2 1.582(3), B1-C3 1.676(4), C3-O1 1.476(3), C1-P1-C3
95.9(1), C3-O1-C4 110.7(2), O1-C4-O2 115.3(2), C4-O2-B1
114.3(2).14
P
Mes*
O
crystallization
solution
P
B-(Fmes)
O
O
B
P
H
H
(Fmes)
Mes*
[+1.8]
12 [0.0]
pentane
3d, r.t.
14
Mes*
P
(Fmes)
Mes*
H3C
(Fmes)
B
B
P
H
Compound 8 reacts in an unusual way with pyridine. The
reaction in a ca 1:1 molar ratio in CD2Cl2 at r.t. resulted in a ca. 95
% conversion to the (C6F5)3B-pyridine adduct 9 and the PO/B
product 12. A gas evolution was observed, and we identified the
formation of carbon monoxide by in situ 13C NMR spectroscopy.
Compound 12 was separated from the mixture by pentane
extraction and obtained in 87% yield admixed with ca. 10% of the
B(C6F5)3 pyridine adduct 9. According to X-ray diffraction the
isolated crystalline material is probably the dimer 14 (see below);
however, it dissociates to the monomer 12 upon bringing it back
into solution. Compound 12 shows a 11B NMR feature at δ 82.2
{[P]CH2[B]: δ 58.7 (31P), δ 2.51, 2.23/39.1 (1H/13C, 1JPC ~ 68 Hz)}.
We note that compound 8 does not loose CO2 upon treatment with
pyridine but rather liberates CO, which seems to be the favored
pathway for this system.
O
O
O
O

P
Mes*
Mes*
P
H
-
B
H
B
(Fmes)
16
15
(Fmes)
Compound 12 formed two types of isolable dimers under
different conditions. Crystallization from a heptane solution at -35
oC gave the dimer 14. The X-ray crystal structure analysis showed
that it was formed by a pair of typical [P]=O-[B] contacts (Scheme
3, the structure is depicted in the Supporting Information).
According to the DFT calculation the formation of the dimer 14
from 12 is slightly endergonic. This is in accordance with the
observation that the dimer 14 dissociates to the monomer 12 upon
dissolving it at r.t.
Keeping a solution of compound 12 in pentane solution at r.t. for
3 days led to the slow formation of a different dimer, namely
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compound 16. The X-ray crystal structure (Fig. 4) showed that two
signal was located at δ 164.7 (2JPC = 5.9 Hz) (for further details see
1
2
3
4
5
6
7
8
five-membered rings had become connected by a P-O-B linkage.
One ring had retained its original heterocyclic structure, only the
original P-CH2-B unit had become converted to a delocalized P-
CH-B P-ylide/borata-alkene like moiety.11 The other five
membered ring had undergone a rearrangement process. The
external P=O phosphanoxide was converted to an internal P-O-B
unit and its phosphorus atom bears a methyl substituent.
the Supporting Information).
The P(O)-CH2-B functionality of compound 12 underwent an
analogous 1,2-addition to the CN group of the tosylcyanide reagent
to give the bicyclic product 18b. In this case the potent cyanation
reagent Ts-CN13 added a second cyanide unit to the initially formed
intermediate. Compound 18b also features the internal P=O
interaction with the adjacent boron Lewis acid. Compound 18b was
characterized by an X-ray crystal structure analysis (see the
Supporting Information for details).
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12
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Scheme 4. Carbonyl addition reactions of compound 12
(with DFT calculated relative Gibbs energies in kcal mol-1
[in dichloromethane] method: PW6B95-D3//TPSS-D3/def2-
TZVP).
(Fmes)
O
P
Mes*
H2C
Mes*
B
B-(Fmes)
P
P
B-(Fmes)
[0.0]
O
O
Mes*
12
11 [+3.8]
17
[+11.2]
Figure 4. A view of the core framework of the dimer 16 (thermal
ellipsoids is depicted at 30 %, for clarity only the ipso-carbon atoms
of the aryl substituents are shown). Selected bond lengths (Å) and
angles (°): P1-O1 1.566(1), P1-C1 1.830(2), P1-C3 1.726(2), B1-
C2 1.597(3), B1-C3 1.459(3), O1-B2 1.510(2), B2-O2 1.569(2),
O2-P2 1.562(1), P1-C3-B1 110.1(1), P1-O1-B2 138.7(1), B2-O2-
P2 114.8(1).14
CO2
Ts-CN
(Fmes)
PhCHO
O
O
(Fmes)
O
(Fmes)
B
P
B
P
B
P
NH
O
O
O
Mes*
O
S
tol
Mes*
Ph
Mes*
H
NC
18b
O
18a
18c
Compound 16 shows pairs each of 11B (δ 50.1, 13.4) and 31P (δ
83.7, 77.8) NMR resonances (in CD2Cl2 solution). It shows 1H/13C
signals of the P-CH-B moiety at δ 3.67 (2JPH = 13.1 Hz) and δ 82.5
(1JPC ~ 85 Hz) (see the Supporting Information for further details).
The pathway of the formation of compound 16 apparently involves
a single initial P=O coordination to the boron Lewis acid of a
second molecule of 12. The bridging P(O)-CH2-B-
phosphanoxide/borate unit may then serve as a base to abstract a
proton from the adjacent five-membered ring to generate the P-
ylide/borata-alkene hybrid found in the product 16 (see Scheme 3).
In solution at 299 K only the monomer 12 was observed by NMR
spectroscopy. However, in D8-toluene solution the 31P NMR
signals of three new compounds appeared below ca. 223 K with
increasing intensity. One could speculate that these might possibly
be dimers (e.g. of type 14 and diastereoisomers of 15) in a strongly
temperature dependent equilibrium with 12 (see the Supporting
Information for the depicted spectra).
Figure 5. A view of the molecular structure of the CO2 addition
product 18a (thermal ellipsoids shown at 30%, only the Mes* ipso-
C at P1 is shown for clarity). Selected bond lengths (Å) and angles
(°): P1-O1 1.565(1), O1-B1 1.538(2), B1-O2 1.517(2), O2-C4
1.311(2), C4-O3 1.213(2), C4-C3 1.533(2), C3-P1 1.823(2), P1-C1
1.808(2), C1-C2 1.554(2), C2-B1 1.637(3), P1-O1-B1 104.1(1),
O2-C4-C3 119.6(2).14
Since the seminal work by L. Horner et al. is well known that the
deprotonation of alkyl containing phosphanoxides and related
compounds generates stabilized carbanions that show carbonyl
olefination activity.12 Our P-oxide system 12 might actually be
looked at as a borylated analogue of such a “Horner [R2P(O)=CH2]-
” anion and, consequently, exhibit basic or nucleophilic properties
at the P/B bridging CH2 group. This showed up in the reaction of
12 with carbonyl and nitrile reagents.
Exposure of a solution of 12 in d6-benzene to CO2 (2 bar, 60 oC,
2 h) gave the product 18a, that was isolated in 88% yield after
workup. The X-ray crystal structure analysis revealed that the CO2
oxygen atom had become connected to boron, and the P-methylene
carbon atom had formed a C-C bond with the CO2 carbon to
generate a heterobicyclo[3.2.1]octane framework (Fig. 5). It has an
oxygen atom connecting the P and B heteroatoms and the ring
carbon atom C4 is part of the C=O double bond.
Compound 12 reacted with benzaldehyde (CD2Cl2, 50 °C, 48 h).
Crystallization from pentane at r.t. gave the product 18c in 85%
yield admixed with ca. 10% of the pyridine-borane adduct 9. The
X-ray crystal structure analysis of a recrystallized sample showed
that the nucleophilic P(=O)CH2 group had attacked the
benzaldehyde carbon atom and the oxygen atom is found bonded
to boron. In the crystal compound 18c shows a typical chair-like
conformation of the seven-membered ring core with the bulky
substituents oriented in pseudo-equatorial positions. The phenyl
substituent and the Mes* group are 1,3-cis-oriented, the P=O vector
is in a pseudo-axial position (see the Supporting Information for the
depicted structure).
We conclude that the reaction of 3 with carbon monoxide results
in the reductive cleavage of the CO molecule under metal-free
conditions. The resulting product 12 serves as a reactive borylated
Horner-like ylidic system. It adds as a carbon nucleophile to carbon
The NMR spectrum of compound 18a in CD2Cl2 solution shows
the core heteroatom resonances at δ 70.0 (31P) and δ 8.3 (11B),
respectively. The 13C NMR methylene resonances occur at δ 14.0,
35.8 (1JPC ~ 65 Hz) and 37.7 (1JPC ~ 45 Hz). The 13C NMR carbonyl
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Rathke, M. W.; Brown, H. C. A New Reaction of Diborane with Carbon
dioxide, to an aldehyde and to a nitrile functionality. Our DFT
analysis shows that in solution compound 12 is the energetically
preferred monomer in this system, but that its putative isomers,
namely 11 and 17 are thermodynamically possibly within reach
(see Scheme 4). So, it may be that the observed carbonyl reactions
of 12 may constitute an intrinsic reactivity of this borylated Horner
ylide analogue, or that alternatively the observed reactions were
taking place via any of these isomeric systems. At present we
cannot distinguish between these options. The observed reaction
sequences indicate that the presence of the PH and BH
functionalities at the internal P/B Lewis pair 3 serve to greatly
advance the possibilities for small molecule activation at frustrated
Lewis pairs.
Monoxide Catalyzed by Sodium Borohydride. A Convenient Synthesis of
Trimethylboroxide. J. Am. Chem. Soc. 88, 2606−2607. (c) Finze, M.;
Bernhardt, E.; Terheiden, A.; Berkei, M.; Willner, H.; Christen, D.;
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Ye, K.-Y.; Bursch, M.; Qu, Z.-W.; Daniliuc, C. G.; Grimme, S.; Kehr, G.;
Erker, G. Reversible Formylborane/SO2 Coupling at a Frustrated Lewis
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Wu, L.; Lin, Z.; Yamashita, M. Reaction of B2(o-tol)4 with CO and
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information contains details about the
experimental work, the characterization of the new compounds and
details about the DFT analysis. This material is available free of
charge via the Internet at http://pubs.acs.org
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(a) Sajid, M.; Elmer, L.-M.; Rosorius, C.; Daniliuc, C. G.;
Grimme, S.; Kehr, G.; Erker, G. Facile Carbon Monoxide Reduction at
Intramolecular Frustrated P/B Lewis Pair Templates. Angew. Chem. Int. Ed.
2013, 52, 2243-2246. (b) Sajid, M.; Kehr, G.; Daniliuc, C. G.; Erker, G.
α−Hydroxymethylation of Pyridines at a Frustrated Lewis Pair Template.
Chem. Eur. J. 2013, 21, 1454-1457.
AUTHOR INFORMATION
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Li, J.; Daniliuc, C. G.; Kehr, G.; Erker, G. Preparation of the
Corresponding Author
Borane (Fmes)BH2 and its Utilization in the FLP Reduction of Carbon
Monoxide and Carbon Dioxide. Angew. Chem. Int. Ed. 2019, 58, 6737–
6741.
erker@uni-muenster.de.
(8)
(a) Dureen, M. A.; Stephan, D. W. Reactions of Boron
Notes
Amidinates with CO2 and CO and Other Small Molecules. J Am Chem Soc.
2010, 132, 13559–13568. (b) Sajid, M.; Lawzer, A.; Dong, W.; Rosorius,
C.; Sander, W.; Schirmer, B.; Grimme, S.; Daniliuc, C. G.; Kehr, G.; Erker,
G. Carbonylation Reactions of Intramolecular Vicinal Frustrated
Phosphane/Borane Lewis Pairs. J. Am. Chem. Soc. 2013, 135, 18567–
18574. (c) Ueno, A.; Tao, X.; Daniliuc, C. G.; Kehr, G.; Erker, G. Unusual
1,1-Hydroboration Route to a Reactive Unsaturated Vicinal Frustrated
Phosphane/Borane Lewis Pair. Organometallics 2018 37, 2665–2668.
The authors declare no competing financial interests.
ACKNOWLEDGMENT
Financial support from the Deutsche Forschungsgemeinschaft and
the Alexander von Humboldt-Stiftung (stipend to Q.S.) is
gratefully acknowledged.
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See for a comparison: (a) Dobrovetsky, R.; Stephan, D. W.
Stoichiometric Metal-Free Reduction of CO in Syn-Gas. J. Am. Chem. Soc.
2013, 135, 4974–4977. (b) Wang, T.; Wang, L.; Daniliuc, C. G.;
Samigullin, K.; Wagner, M.; Kehr, G.; Erker, G. CO/CO, and NO/NO
Coupling at a Hidden Frustrated Lewis Pair Template. Chem. Sci. 2017, 8,
2457–2463.
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On the Preparation of Aryl Nitriles Uisng Tosyl Cyanide. J. Org. Chem.
1995, 60, 2948-2950.
(18c) contain the supplementary crystallographic data for this paper. These
data are provided free of charge by The Cambridge Crystallographic Data
Centre.
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(Fmes)
Mes*
(Fmes)
CO
Mes*
B
P
H
B
P
H
H
H
O
B(C6F5)3
2
O
B(C6F5)3
9
O
(Fmes)
pyridine
CO2
Mes*
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B
P
P
B-(Fmes)
O
O
- CO
- pyrB(C6F5)3
O
H
H
Mes*
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