Frustrated Lewis Pair Behavior of an Open, Noninteracting Phosphane/Borane Pair at a Rigid Organic Framework: Exploring Decisive Factors for FLP Activity

Article abstract of DOI:10.1021/acs.organomet.7b00819

The open, noninteracting vicinal P/B frustrated Lewis pair (FLP) at the very rigid, dibenzobicyclo[2.2.2]octadiene ( semi-triptycene ) framework shows a trans 1,2-arrangement of the Mes₂P- and (C₆F₅)₂B- Lewis base/Lewis acid pair. It is an active dihydrogen splitting reagent and shows a great variety of typical frustrated Lewis pair reactions under mild reaction conditions. This includes the 1,1-addition reaction to nitric oxide (NO) to form a persistent FLPNO^? nitroxide radical as well as CO reduction by Piers' borane [HB(C₆F₅)₂] at the reactive FLP template. The new FLP undergoes thermodynamically favorable 1,4-addition reactions to 1,2-diketones. Most products were characterized by X-ray diffraction, and the 1,4-diketone addition reaction was analyzed by DFT calculation. A comparison with the internally interacting parent vicinal Mes₂PCH₂CH₂B(C₆F₅)₂ system revealed some decisive indications for very active FLPs.

Full text of DOI:10.1021/acs.organomet.7b00819

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Frustrated Lewis Pair Behavior of an Open, Noninteracting
Phosphane/Borane Pair at a Rigid Organic Framework: Exploring
Decisive Factors for FLP Activity
Ke-Yin Ye, Constantin G. Daniliuc, Shunxi Dong, Gerald Kehr, and Gerhard Erker*
Organisch-Chemisches Institut der Universitat Munster, Corrensstr. 40, 48149 Munster (Germany)
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* Supporting Information
ABSTRACT: The open, noninteracting vicinal P/B frustrated Lewis pair
(FLP) at the very rigid, dibenzobicyclo[2.2.2]octadiene (“semi-tripty-
cene”) framework shows a trans 1,2-arrangement of the Mes2P− and
(C6F5)2B− Lewis base/Lewis acid pair. It is an active dihydrogen splitting
reagent and shows a great variety of typical frustrated Lewis pair reactions
under mild reaction conditions. This includes the 1,1-addition reaction to
nitric oxide (NO) to form a persistent FLPNO• nitroxide radical as well as
CO reduction by Piers’ borane [HB(C6F5)2] at the reactive FLP template.
The new FLP undergoes thermodynamically favorable 1,4-addition
reactions to 1,2-diketones. Most products were characterized by X-ray
diffraction, and the 1,4-diketone addition reaction was analyzed by DFT
calculation. A comparison with the internally interacting “parent” vicinal
Mes2PCH2CH2B(C6F5)2 system revealed some decisive indications for
very active FLPs.
Scheme 1. Examples of Interacting and Noninteracting FLPs
INTRODUCTION
■
Intramolecular Lewis acid/Lewis base pairs have played a
significant role in the development of frustrated Lewis pair
(FLP) chemistry.1 From early on, some such systems have
exhibited high reactivities in small-molecule binding and/or
activation.2,3 In order to serve as active FLPs, the Lewis acid/
base combinations must not be hindered from their cooperative
action by any LA/LB adduct formation.4 This is often not the
case in intramolecular systems.5,6 They often show global
minimum structures that indicate some direct Lewis adduct
formation. In order to show cooperative FLP behavior, the LA/
LB pair must dissociate in a preequilibrium step in order to
become active. Fortunately, for many intramolecular FLPs
adduct formation is weak. Nevertheless, the endergonic LA-LB
⇄ LA+LB equilibrium becomes part of the overall kinetic
expression of FLP action and principally reduces the rate of
FLP small-molecule reactions. Scheme 1 shows a typical
example.
Geometric features and steric hindrance may make the LA/
LB interaction in intramolecular FLPs impossible and eliminate
the preequilibrium situation altogether. The FLPs 3−5 depicted
in Scheme 1 are some typical examples.4,8 The geometrically
constrained noninteracting P/B systems 3 and 4 are rather
reactive frustrated Lewis pairs; however, they are not well suited
for a direct comparison with 1 because compound 3 shows an
unsymmetrical environment of the Lewis acid/base pair and
compound 4 is unstable with regard to isomerization. We
recently found that the N/B pair 5 is an excellent acceptor for
the H+/H− pair from the dihydrogen activation product 2 and
can, therefore, not be used reliably for the competition
experiment.4b We have now prepared a new open, non-
interacting vicinal P/B FLP at the very rigid symmetrical
dibenzobicyclo[2.2.2]octadiene framework. Our system con-
tains a trans-1,2-attachment of the reactive −PMes2/−B(C6F5)2
pair. In this paper, we will describe the formation, character-
ization, and chemistry of this very rigid new member of the
intramolecular FLP family of compounds and compare its
typical FLP action with the internally interacting “parent”
Mes2PCH2CH2B(C6F5)2 FLP 1.7
Received: November 10, 2017
© XXXX American Chemical Society
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RESULTS AND DISCUSSION
■
Synthesis of the New P/B FLP System. The synthesis of
the trans-1,2-P/B FLP 8 was carried out by a synthetic
sequence that followed the reported preparation of the
diphenylphosphino analogue of our −PMes2 phosphane
starting material 7.9 That in turn was prepared starting from
dimesitylphosphinoacetylene (HCC−PMes2), which was
oxidized to the respective acetylenic phosphinoxide HCC−
P(O)Mes2. This we used as a potent dienophile for the Diels−
Alder reaction with anthracene (200 °C, 48 h in toluene in an
autoclave) to give the dibenzobicyclo[2.2.2]-
octatrienylphosphinoxide 6, which we isolated as a solid in
84% yield after chromatographic purification (31P NMR: δ
+24.9). The phosphinoxide 6 was then reduced to the
respective alkenyldimesitylphosphane with trichlorosilane/
triethyl phosphite (50 °C, 48 h), according to a literature
procedure,9 to give compound 7 as a yellow solid in 75% after
chromatography. The phosphane was characterized by C,H
1
elemental analysis, by spectroscopy [31P NMR δ −27.7, H
Figure 1. Molecular structure of the open, noninteracting P/B FLP 8
(thermal ellipsoids are shown with 15% probability). Selected bond
lengths (Å) and angles (deg): P1−C2 1.901(4), C1−C2 1.589(5),
C2−C3 1.577(5), C3−C4 1.613(5), C3−B1 1.546(6); C1−C2−P1
107.6(2), C1−C2−C3 108.1(3), C2−C3−C4 108.9(3), C4−C3−B1
106.3(3).
NMR δ 5.06 (3JPH = 7.4 Hz) (1-H), δ 6.53 (3-H), δ 5.10 (4JPH
= 1.1 Hz) (4-H)], and by X-ray diffraction (for the X-ray crystal
structure analysis and spectroscopic details, see the Supporting
Information)
The hydroboration reaction of the alkenylphosphane with
Piers’ borane [HB(C6F5)2]10 went smoothly and gave the P/B
FLP 8 as a yellow solid in 78% yield after workup (see Scheme
2). Single crystals of compound 8 were obtained from a
These 11B and 19F NMR parameters are both typical for the
presence of a strongly Lewis acidic trigonal-planar R-B(C6F5)2
structure.12 The mesityl substituents at phosphorus (31P NMR:
δ −16.2) are diastereotopic. In addition there is hindered
rotation around the P−C(Mes) vectors at low temperature.
Therefore, the 1H NMR spectrum at 213 K shows a total of six
Scheme 2. Synthesis of the Noninteracting P/B FLP 8
1
mesityl methyl signals and four arene CH resonances. The H
NMR signals of the hydrogen atoms at the C(sp3)−C(sp3)
bridge occur at δ 4.62 (2-H) and 2.16 (3-H), and the
bridgehead 1H NMR signals of the tricyclic P/B FLP
framework were located at δ 4.33 and 4.29, respectively (for
further details see the Supporting Information).
Reactions of Compound 8 with Phenylacetylene and
with Dihydrogen. Compound 8 is a very reactive FLP, as
anticipated. It undergoes a variety of typical FLP reactions
under mild conditions with high reaction rates. For these
reactions the FLP 8 was usually not isolated prior to use but
was generated in situ by reacting the phosphane 7 with a
stoichiometric quantity of HB(C6F5)2, and then we used these
freshly prepared solutions for the subsequent FLP reactions.
Many frustrated Lewis pairs react with the C−H bonds of
terminal alkynes,13 and so does compound 8. Treatment of
compound 8 with phenylacetylene (room temperature,
dichloromethane, overnight) gave the zwitterionic phospho-
nium/alkynylborate product 9 in good yield (see Scheme 3). It
shows a typical 11B NMR signal (δ −17.6) of a tetravalent
borate and a 31P NMR doublet of the [P]−H phosphonium
solution in 1/5 (v/v) dichloromethane/pentane at −30 °C.
The X-ray crystal structure analysis (see Figure 1) shows the
formation of the doubly functionalized “semi-triptycene”11
framework. The saturated section of the molecule has a pair of
bulky PMes2 and B(C6F5)2 substituents attached to it. These
substituents are found trans attached at the C2−C3 carbon−
carbon single bond, as was expected from the regio- and
stereoselective hydroboration reaction. The dihedral angle
between the C2−P1 and C3−B1 vectors amounts to θ(P1−
C2−C3−B1) = −106.0(4)°. Consequently, the boron atom
1
unit at δ 1.6 (1JPH ≈ 490 Hz, corresponding H NMR doublet
features a trigonal-planar coordination geometry (∑B1CCC
=
at δ 8.05). Both the phosphonium and the borate section are
prochiral, and in addition, there is hindered rotation around
both pairs of P−C(aryl) and B−C6F5 vectors. Consequently,
we have observed six 1H NMR methyl signals of the
−P(H)Mes2 unit and five (two overlapping) 19F NMR C6F5
signals of the −B(alkynyl)(C6F5)2 substituent in compound 9.
359.0°) with both C6F5 substituents oriented slightly to the
outside. The phosphorus atom shows the typical distorted-
trigonal-pyramidal geometry with ∑P1CCC = 321.6°. The P1−
B1 separation in compound 8 amounts to 3.964 Å.
The NMR spectra of compound 8 were obtained from our in
situ prepared sample in d2-dichloromethane. It shows a 11B
NMR resonance at δ 74.7 and three 19F NMR signals (at 299
K) of the o,p,m-fluorine nuclei of the pair of C6F5 groups at
boron with a large Δ19Fm,p = 11.7 ppm chemical shift difference.
1
The H NMR signals of the C1−C4 core are found at δ 4.61/
4.52 (1-H, 4-H), 3.99 (2-H), and 2.72 (3-H); the last signal
3
features a characteristically large JPH = 35.2 Hz coupling
constant.
B
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Scheme 3. Reaction of the FLP 8 with Phenylacetylene and
with Dihydrogen
The structural assignment of compound 9 was confirmed by
its X-ray crystal structure analysis. The dihedral angle of the
P1−C2−C3−B1 unit amounts to θ = −86.9(3)°. The ∑P1CCC
value (sum of angles at the phosphonium phosphorus atom P1)
amounts to 344.3°. The boron atom has the linear phenyl-
acetylide substituent attached to it (for details of the structural
characterization of compound 9, including the depicted
molecular structure, see the Supporting Information).
We carried out a competition experiment of the phenyl-
acetylene C−H activation reaction between the parent
interacting P/B FLP 1 and the new open, noninteracting P/B
FLP 8. It turned out that the system 8 is much more reactive
toward the C−H acidic terminal acetylene. It reacts >20 times
faster than the system 1 (conventionally estimated, for details
see the Supporting Information).
Figure 2. View of the dihydrogen splitting product 10 (thermal
ellipsoids are shown with 30% probability). Selected bond lengths (Å)
and angles (deg): P1−C2 1.833(2), C1−C2 1.573(2), C2−C3
1.575(2), C3−C4 1.574(2), C3−B1 1.643(2); C1−C2−P1 115.1(1),
C1−C2−C3 110.8(1), C2−C3−C4 106.5(1), C4−C3−B1 114.9(1).
2
signals in the H NMR spectrum as a broad [P]-D doublet (δ
1
6.5; JPD = 72.0 Hz) and a broad unstructured [B]-D signal at
ca. δ 2.0. The 31P NMR spectrum of compound 10-D2 shows a
2
1:1:1 intensity triplet as expected (the H NMR spectra are
The FLP 8 activates dihydrogen.3,14 The in situ generated
open, noninteracting P/B frustrated Lewis pair reacts rapidly
with H2 under nearly ambient conditions (1.5 bar, room
temperature) to give the dihydrogen splitting product 10.
On a preparative scale we stirred the reaction mixture for 2 h
and isolated the product of heterolytic H2 cleavage as a white
solid in 77% yield. The X-ray crystal structure analysis (see
Figure 2) shows the typical dibenzobicyclo[2.2.2]octadiene
framework with the newly generated P(H)Mes2 and BH-
(C6F5)2 phosphonium cation/borate anion units attached
(θ(P1−C2−C3−B1) = −98.2(1)°). The [P]−H vector is
directed inside toward the boron atom, whereas the [B]−H
vector is pointing upward almost normal to the [P]−H unit.
We note that a mesityl ring at phosphorus and a C6F5 ring at
boron are almost oriented parallel to each other.
depicted in the Supporting Information section).
We determined the relative rate of the dihydrogen splitting
reaction between the pairs 1 and 8. From a competition
experiment we determined that 8 actives H2 under these
conditions only marginally faster that the reference compound
(krel at 263 K is ca. 2−3). This is surprising at first sight that the
open nonbridged FLP 8 is about equal in H2-splitting activity as
the closed interacting FLP 1, which needs to open first in a
preequilibrium step in order to be prepared to react with the
dihydrogen molecule at all. In a separate experiment we assured
that the hydrogen transfer reaction between the 2/8 and 1/10
pairs was much slower than the respective dihydrogen splitting
reactions (for details including the depicted NMR spectra see
the Supporting Information).
We note a principal difference in the behavior of the FLPs 1
and 8 from the competition experiments between the terminal
acetylene and the dihydrogen splitting reaction. It seems that
the phosphane in the FLP 8 just serves as an independent base
in the deprotonation reaction of phenylacetylene with a
subsequent separate addition step of the resulting alkynyl
anion to the pendent borane Lewis acid. This makes the
reaction of the FLP 8 with the terminal alkyne quite fast. In
contrast, the H2 splitting reaction of either of the frustrated
Lewis pairs requires a cooperative action of the phosphane
Lewis base and borane Lewis acid protagonists. A close
inspection of the DFT calculated transition state geometry of
the 1 + H2 reaction (1-TS in Scheme 1) had shown a P−C−
C−B dihedral angle of the FLP backbone of −54.0°, which is
actually not far away from that of the open gauche-like
intermediate (1-open: 54.4°).15 In contrast, the P−C−C−B
angle in the open, nonbridged FLP 8 is found (by X-ray
diffraction) at −106° (see above), which is probably far away
In solution the hydrogen splitting product 10 shows a 11B
NMR hydridoborate [B]−H doublet at δ −20.3 (1JBH ≈ 90 Hz)
and a corresponding 31P NMR phosphonium signal at δ −6.5
with a typical large 1JPH ≈ 480 Hz coupling constant (1H NMR
1
[P]−H signal at δ 6.50). The 1-H/4-H bridgehead H NMR
signals of compound 10 occur at δ 4.34 and 4.47, respectively,
and their neighboring 2-H and 3-H resonances appear at δ 3.65
and δ 1.78, respectively. The mesityl groups at the
phosphonium moiety are diastereotopic, and they show
hindered rotation around the P−C(aryl) vectors at room
temperature. Consequently, we have observed a total of six
1
mesityl methyl H/13C NMR resonances.
We have carried out the dihydrogen splitting reaction with
dideuterium and obtained the respective [P]D/[B]D com-
pound 10-D2. The corresponding phosphonium and hydrido-
borate 1H NMR resonances are missing in the 1H NMR
spectrum of compound 10-D2, but we observe the respective
C
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from an ideal situation for the cooperative H−H splitting
reaction. Therefore, it seems that the system 8 pays a price for
its increased separate Lewis acid and Lewis base reactivity in the
H2-splitting situation where their cooperative action is
apparently essential. We note that the degree of “frustration”
is apparently not the sole decisive parameter in cooperative
frustrated Lewis pair reactions; geometric and conformational
features are at least of equal importance.
Other Typical FLP Reactions of the Noninteracting P/
B Pair 8. Many alkylboranes react with carbon monoxide by
formation of acylboranes. This reaction has found frequent use
for the preparation of ketones, aldehydes, or alcohols.16 In
contrast, CO is not reduced by [B]−H boranes except when it
is catalyzed.17,18 It has long been known that diborane B2H6
just forms borane carbonyl (H3B−CO) on exposure to CO at
high temperature and pressure. We had recently shown that
even Piers’ borane HB(C6F5)2 shows this specific reaction
pathway: it forms Piers’ borane carbonyl (C6F5)2B(H)−CO,19
a compound that we had characterized by X-ray diffraction.19
However, CO is reduced to the formylborane stage by
HB(C6F5)2 at a variety of vicinal P/B FLP templates. We
could show that by involvement of the FLP framework the
thermodynamic restriction of the (C6F5)2B(H)−CO to
formylborane conversion could be overcome, and we have
isolated a number of FLP-η2-formylboranes.8,20 Compound 8 is
no exception. Treatment of the FLP 8/HB(C6F5)2 mixture with
CO (1.5 bar) at room temperature overnight gave the η2-
formylborane P/B FLP adduct 11, which we isolated as a white
solid in 67% yield. We assume that the (C6F5)2B(H)−CO
adduct is added to the P/B FLP 8 across the CO π system.
The resulting activated carbonyl is then readily reduced by the
adjacent in situ generated hydridoborate to directly give the
product 11.
Figure 3. Projection of the molecular structure of the (η2-
formylborane)FLP compound 11 (thermal ellipsoids are shown with
15% probability). Selected bond lengths (Å) and angles (deg): C5−
O1 1.482(8), O1−B1 1.637(8), C5−B2 1.603(10), P1−C5 1.839(7),
B2−O1 1.587(8), P1−C2 1.840(6), C1−C2 1.567(9), C3−B1
1.614(10); C5−B2−O1 55.4(4), B2−C5−O1 61.8(4), B2−O1−C5
62.9(4), C5−P1−C2 97.1(3), O1−B1−C3 100.7(5), P1−C2−C3−B1
−81.3(5).
Scheme 4. Formylborane Chemistry at the FLP 8 Template
The X-ray crystal structure analysis of compound 11 shows
the newly formed six-membered P/B-containing heterocycle
trans-fused with the dibenzobicyclo[2.2.2]octadiene framework
(θ(P1−C2−C3−B1) = −81.3(5)°). At its distal position it
contains the C5−O1 bond of the newly formed formyl group
which is η2 bonded to the B(C6F5)2 moiety originating from the
HB(C6F5)2 reducing agent (see Figure 3).
In solution compound 11 features heteroatom NMR
resonances at δ 30.6 (31P) and 7.5 (11B). It shows the 19F
NMR resonances of four pairwise diastereotopic C6F5
substituents at the pair of boron atoms, giving rise to four p-
C6F5 signals and (at 213 K) a total of eight o-C6F5 resonances.
the applied conditions to give the intermediate 12. This
contains an oxonium moiety and a neighboring hydridoborate
nucleophile, and consequently, hydride attack at the adjacent
carbon atom results in opening of the activated three-
membered heterocycle to eventually yield the observed product
13 (see Scheme 4).
The X-ray crystal structure analysis of compound 13
confirmed the formation of the seven-membered heterocycle
annulated with the “semi-triptycene” framework. It contains the
trans-fused phosphonium and borate moieties (θ(P1−C2−
C3−B1) = 91.6(2)°). The newly formed −CH2− group is
found bridging between the phosphorus atom P1 and the
boron atom B2. The former CO oxygen atom is attached to the
pair of B1/B2 atoms and it is found to be protonated (see
Figure 4).
Two o-C6F5 resonances show through-space coupling at 213 K
TS
(through-space coupling constant
J
FF
≈ 144 Hz; for details
21
1
see the Supporting Information ). The H NMR spectrum of
compound 11 shows the formyl signal at δ 5.01 (d, 2JPH = 12.7
Hz), and the 13C NMR spectrum shows this signal at δ 60.2
(1JPC = 60.7 Hz). The pair of diastereotopic mesityl groups at
phosphorus shows hindered rotation around both P−C(aryl)
vectors; consequently, we observed six mesityl methyl 1H NMR
signals at 299 K (for further details see the Supporting
Information)
Some (η2-formylborane)FLP compounds showed a high
reactivity toward H2,8,20 and so does compound 11. Exposure
of the η2-formylborane FLP adduct 11 to H2 at 20 bar for 48 h
at room temperature resulted in a clean reduction of the CO
derived −CHO group to methylene. We isolated the product
13 (see Scheme 4) in 67% yield. We assume that facile
reversible opening of the O−B1 linkage in 11 generates a
reactive O/B frustrated Lewis pair that splits dihydrogen under
The newly formed [P]−CH2−[B] methylene group shows
up as a tight AB pattern in the 1H NMR spectrum at δ 3.24 and
a
13C NMR feature at δ 38.0. The corresponding [B]−OH−
1
[B] H NMR resonance occurs at δ 5.47 as a doublet with a
D
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Figure 5. Molecular structure of persistent P/B FLPNO• radical 14
(thermal ellipsoids are shown with 15% probability). Selected bond
lengths (Å) and angles (deg): C2−P1 1.800(4), P1−N1 1.731(3),
N1−B1 1.618(5), B1−C3 1.623(5), C2−C3 1.567(5); C2−P1−N1
90.3(2), P1−N1−B1 115.6(2), N1−B1−C3 96.7(3), P1−C2−C3−B1
−60.6(3).
Figure 4. Molecular structure of compound 13 (thermal ellipsoids are
shown with 30% probability). Selected bond lengths (Å) and angles
(deg): P1−C5 1.810(2), C5−B2 1.635(4), B2−O1 1.550(3), O1−B1
1.599(3), B1−C3 1.634(3), C2−C3 1.572(3), C2−P1 1.855(2); P1−
C5−B2 119.4(2), B2−O1−B1 130.3(2), C5−B2−O1 106.4(2), O1−
B1−C3 107.0(2), C2−P1−C5 104.4(1), P1−C5−B2−O1 54.3(2),
C5−B2−O1−B1 45.8(3), B1−C3−C2−P1 91.6(2).
Scheme 5. Reaction of the FLP 8 with Nitrogen Oxides
4JPH = 13.3 Hz coupling constant. Again, rotation of the
diastereotopic pair of the bulky mesityl groups around their P−
C(aryl) σ bonds is hindered, leading to the observation of six
methyl 1H NMR (as well as six 13C NMR) signals at 299 K, and
we have monitored the 19F NMR signals of four separate C6F5
groups of the pair of B(C6F5)2 moieties; two of the o-C6F5
resonances show through-space coupling at 299 K (TSJFF ≈ 112
Hz; for details see the Supporting Information).
Many FLPs add to heterocumulenes.28 The reactions with N-
sulfinylaniline are special in that different types of products are
sometimes observed. We had found the cationic metal FLP
based PhNSO addition products 16 and 17,29 and
Stephan et al. discovered the 1,3-P/B FLP addition product
18 and its chemistry.30 We reacted the P/B FLP 8 with
sulfinylaniline in a 1:1 molar ratio at room temperature
overnight and isolated the addition product 19 as a yellow solid
in 64% yield (see Scheme 6). It shows 1H NMR resonances of
the four saturated CH bonds of the framework at δ 4.64/4.80
(bridgehead portions 1-H, 4-H, with corresponding 13C NMR
signals at δ 47.0 (C1) and 47.9 (C4), respectively), 4.60 (2-H;
We had shown that a variety of vicinal P/B FLPs undergo
metal-reminiscent 1,1-addition reactions with typical organic
ligands such as CO,22 isonitriles,23 and NO24,25 to give five-
membered heterocyclic products. In these reactions the boron
atom serves as a typical Lewis acceptor to the lone pair and the
phosphorus donor is able to mimic a back-bonding reactivity to
the π* orbital of these reagents. Compound 8 reacts
accordingly with nitric oxide (NO). The in situ generated P/
B FLP 8 reacts rapidly in a dichloromethane/n-pentane mixture
at room temperature with NO (1.5 bar, 1 h) to give a blue
precipitate of the P/B FLPNO• nitroxide radical.24 The
paramagnetic compound was characterized by C,H,N elemental
analysis and by X-ray diffraction (see Figure 5). The X-ray
crystal structure analysis shows the five-membered heterocycle
that was formed by N,N addition of the P/B pair of 8 to nitric
oxide. The NO attachment to the Lewis pair is rather
symmetrical (angles P1−N1−O1 = 119.3(2)°, B1−N1−O1 =
125.0(3)°). The N1−O1 bond length is 1.288(3) Å. This is
longer than the N−O distance in free nitric oxide (1.15 Å)26
but still rather short, which indicates some significant
delocalization of the oxygen and nitrogen in this persistent
nitroxide radical.24,25 The nitrogen atom in the radical 14 has a
typical planar coordination geometry (∑N1PBO = 359.9°).
Some FLPs add to N2O,27 and others are easily oxidized. The
latter occurs upon treatment of the P/B FLP 8 with N2O. We
isolated the oxidation product 15 in good yield (see Scheme 5).
See the Supporting Information for its characterization
(including the X-ray crystal structure analysis).
3
3
13C δ 51.9), and 3.59 (dd, JPH = 24.7 Hz, JHH = 9.1 Hz, 3-H;
13C δ 37.1). Compound 19 features heteroatom NMR
Scheme 6. FLP Reactions with N-Sulfinylaniline
E
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resonances at δ 3.0 (11B) and 55.0 (31P) and two equal-
intensity sets of 19F NMR signals of the diastereotopic C6F5
substituents at boron.
Scheme 7. Reactions of the P/B FLP 8 with 1,2-Diketones
Compound 19 was characterized by an X-ray crystal
structure analysis (see Figure 6). It revealed that 1,3-addition
FLP 8 with the acenaphthene quinone 20 in a 1:1 molar ratio in
dichloromethane solution at room temperature (overnight).
Workup then gave the 1,4-addition product 21 as a yellow solid
in 58% yield.
Compound 21 was characterized by C,H elemental analysis,
spectroscopy, and X-ray diffraction. The X-ray crystal structure
analysis (see Figure 7) shows the formation of the eight-
Figure 6. View of the molecular structure of compound 19 (thermal
ellipsoids are shown with 30% probability). Selected bond lengths (Å)
and angles (deg): P1−N1 1.695(2), N1−S1 1.710(2), S1−O1
1.608(2), O1−B1 1.522(3), B1−C3 1.647(4), C2−C3 1.579(3),
C2−P1 1.859(2); C2−P1−N1 108.5(1), C3−B1−O1 110.6(2), P1−
C2−C3−B1 −88.8(2).
of the P/B pair to the heterocumulene had taken place, forming
a seven-membered heterocycle. The phosphorus atom was
attached to the nitrogen atom of the reagent and the boron
atom ended up bonded to oxygen. This resulted in a bent
geometry at both oxygen (angle B1−O1−S1 = 114.0(2)°) and
also sulfur (angle O1−S1−N1 = 103.9(1)°). The boron and
phosphorus atoms in compound 19 both show pseudotetrahe-
dral coordination geometries, and the nitrogen atom N1 is
planar tricoordinate (∑N1PSC = 356.8°).
1,4-P/B FLP Addition to 1,2-Dicarbonyl Compounds.
P/B FLPs often add to carbonyl compounds. This usually gives
the 1,2-addition products, which feature a new pair of B−O and
P−C bonds.31 P/B FLPs may also undergo 1,2-carbonyl
addition reactions to conjugated enones and ynones, but in
many cases conjugated 1,4-addition prevails.32,33 Especially, a
number of vicinal P/B FLPs were shown to yield the respective
eight-membered heterocyclic product from such 1,4-addition
reactions. This posed a question of the outcome of the
exposure of the reactive vicinal P/B FLP 8 to 1,2-dicarbonyl
compounds. In principle, one might envisage a clear case of 1,2-
addition to one activated CO group. However, a DFT
calculation34 revealed that the 1,4-addition might be much
more favorable. The calculation was carried out on the reaction
of the P/B FLP 8 with the 1,2-diketone 20 by using two density
functional methods: i.e., optimization by the PBEh-3c
composite approach followed by PW6B95-D3/def2-TZVP
single-point energies. It showed an exergonic formation of
the 1,2-addition product 22, but the formation of the 1,4-
addition product 21 was ca. 10−12 kcal mol−1 more favorable
(see Scheme 7 and the Supporting Information). We
performed this reaction experimentally by reacting the P/B
Figure 7. View of the 1,4-dicarbonyl addition product 21 (thermal
ellipsoids are shown with 50% probability). Selected bond lengths (Å)
and angles (deg): P1−O1 1.590(1), O1−C71 1.400(2), C71−C81
1.357(3), C81−O2 1.344(2), O2−B1 1.526(2), B1−C3 1.647(3),
C2−C3 1.565(3), C2−P1 1.831(2); C2−P1−O1 109.2(1), P1−O1−
C71 131.2(1), O1−C71−C81 128.6(2), C71−C81−O2 128.6(2),
C81−O2−B1 118.7(1), O2−B1−C3 109.7(2), P1−C2−C3−B1
−91.4(2).
membered heterocycle by formation of a new pair of B−O and
P−O σ bonds. There is now a CC double bond between the
former carbonyl carbon atoms of the introduced reagent 20,
now completing the acenaphthene π system inside the addition
product.
In CD2Cl2 solution compound 21 features a phosphonium
31P NMR resonance at δ 86.8 and a broad 11B NMR signal at δ
F
DOI: 10.1021/acs.organomet.7b00819
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Article
1.4. It shows the 19F NMR signals of a pair of diastereotopic
t
Scheme 8. 1,4-Addition of the Bu3P/B(C6F5)3 FLP with a
1,2-Diketone
1
C6F5 substituents at boron. We monitored the H/13C NMR
signals of a pair of diastereotopic mesityl groups at phosphorus,
which show hindered rotation around both the P−C(mesityl)
vectors at 299 K (giving rise to each six CH3 1H NMR and 13
C
NMR resonances at room temperature). The former pair of
carbonyl carbon atoms now has formed the acenaphthene C
C double bond. We have assigned to them the 13C NMR
resonances at δ 149.2 (3JPC = 5.3 Hz) and 131.6 (2JPC = 12.4
Hz), respectively.
The reaction of the P/B FLP 8 with benzil takes a similar
course. Workup of the reaction mixture after stirring at room
temperature overnight gave the 1,4-dicarbonyl addition product
23, which we isolated in 83% yield. The X-ray crystal structure
analysis (see Figure 8) of compound 23 shows the presence of
calculation revealed that the formation of the observed 1,4-
addition product 24 was favored over the (not experimentally
observed) 1,2-addition product by ca. 24 kcal mol−1.
The X-ray crystal structure analysis confirmed the 1,4-P/B
addition with formation of the completed acenaphthene π
system with vicinal (C6F5)3B−O− and tBu3P−O− substituents
at the CC double bond at the five-membered subunit (see
Figure 9). Both of these substituents are strongly bent at their
oxygen atoms, and the P−O and B−O vectors are oriented
toward opposite faces of the acenaphthene framework.
Figure 8. Molecular structure of the 1,4-addition product 23 of the
vicinal P/B FLP 8 to benzil (thermal ellipsoids are shown with 15%
probability). Selected bond lengths (Å) and angles (deg): P1−O1
1.597(2), O1−C71 1.448(4), C71−C72 1.342(5), C72−O2 1.362(4),
O2−B1 1.526(5), B1−C3 1.658(6), C2−C3 1.565(5), C2−P1
1.851(4); C2−P1−O1 106.6(1), P1−O1−C71 121.9(2), O1−C71−
C72 117.6(3), C71−C72−O2 120.7(3), C72−O2−B1 122.4(3), O2−
B1−C3 105.8(3), P1−C2−C3−B1 88.9(3).
Figure 9. Molecular structure of compound 24. Selected bond lengths
(Å) and angles (deg): P1−O1 1.589(2), O1−C1 1.408(3), C1−C2
1.367(4), C2−O2 1.346(3), O2−B1 1.517(3); P1−O1−C1 136.6(2),
C2−O2−B1 123.2(2), P1−O1···O2−B1 124.3.
In solution compound 24 shows the typical NMR features of
the acenaphthene framework with the CC unit inside the
five-membered ring giving rise to 13C NMR signals at δ 149.7
the [P]−O−C(Ph)C(Ph)−O−[B] section of the newly
formed eight-membered ring and, thus, confirms that this
reaction of the P/B FLP with the 1,2-dicarbonyl reagent also
occurred by 1,4-addition.
3
2
(“BOC”, JPC = 4.1 Hz) and 132.7 (“POC”, JPC = 18.5 Hz):
i.e., NMR features similar to those observed for the related
compounds 21 and 23 (see above). Compound 24 shows a 11
B
In solution compound 23 features the 13C NMR signals of
the newly formed carbon−carbon linkage at δ 148.8 (“BOC”,
NMR signal at δ −2.0 in the typical borate range and a
phosphonium 31P NMR resonance at δ 110.3. The 19F NMR
pattern is rather complicated, apparently showing three
different sets of C6F5 signals due to a probably frozen propeller
geometry (for details see the Supporting Information).
2
3JPC = 5.3 Hz) and 136.1 (“POC”, JPC = 13.5 Hz). The
heteroatom NMR signals of compound 23 occur at δ 0.6 (11B)
and 85.3 (31P), respectively, and compound 23 shows a total of
ten well-separated 19F NMR resonances of the diastereotopic
pair of C6F5 substituents at boron.
CONCLUSIONS
Since the reactivity of the new intramolecular nonbridged
vicinal P/B FLP 8 resembled the reactivity of intermolecular
FLP systems, we treated the acenaphthene quinone reagent 20
■
By exploring the new open P/B FLP 8 we found a new FLP
reaction motif, namely the 1,4-P/B addition to 1,2-dicarbonyl
compounds.36 Our DFT study has revealed a thermodynamic
preference of 12−14 kcal mol−1 of the 1,4- over the
conventional 1,2-addition for our first example, which makes
the 1,4-product favored by far even under equilibrium
conditions. The preferred formation of the 1,4-addition
35
t
with the intermolecular Bu3P/B(C6F5)3 FLP under analo-
gous conditions (dichloromethane solution, room temperature,
overnight). We observed the analogous 1,4-addition reaction to
this 1,2-diketone. Workup gave the product 24 in 85% yield as
an orange solid (see Scheme 8). In this case the DFT
G
DOI: 10.1021/acs.organomet.7b00819
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
products (21, 23, 24) may indicate a special reactivity of the
FLP 8, making this an intramolecular P/B FLP with a reactivity
pattern resembling an intermolecular case.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00819.
■
S
Many intermolecular FLPs show high reactivities in small-
molecule activation.35 They circumvent the kinetic trap of
termolecularity (i.e., the necessity of three reagents coming
together to allow for a reaction to take place between them),
e.g. in the H2 splitting reaction, by forming an “encounter
complex” in a preequilibrium step from which the actual
cooperative H−H cleavage can take place.37 Computational
studies have merged to ascribe the intermolecular Lewis acid/
base interaction based on a sum of van der Waals contacts in a
dynamic situation,38,39 which leads to a transition state of the
H2 splitting reaction with a side-on arrangement of the H−H
molecule at the boron atom with a nearly linear arrangement of
the H−H−LB vector.15,37
Intramolecular FLPs do not have the termolecularity
problem, since both the Lewis acid and the Lewis base are
intramolecularly connected. The H2 splitting transition state,
nevertheless, has computationally been found to be quite
similar to the intermolecular case, namely showing a side-on
arrangement of the H−H vector to the borane and a close to
linear orientation of the H−H−Lewis base unit (see Scheme 1
for an example).40
Many intramolecular FLPs, however, may pay a price by
featuring an internal interaction between the core atoms of the
Lewis pair along the connecting organic framework. This
especially holds for the many vicinal P/B FLPs that have been
reported so far. In order to function as active frustrated Lewis
pairs they must dissociate, breaking the P···B interaction, in a
preequilibrium step, before they can undergo any of their
typical cooperative small-molecule reactions. Fortunately, the
LA−LB dissociation of many intramolecular P/B FLPs is
thermodynamically mildly endergonic (e.g., 7−8 kcal mol−1 for
our “parent” ethylene-bridged P/B FLP 1,7 see above) and the
activation barriers of equilibration are low;41 nevertheless these
equilibrium parameters contribute to the overall situation of
any subsequent FLP reaction.
The development of open, nonbridged intramolecular FLPs
might, therefore, be of importance. However, we found out that
the example described here of the trans-vicinal P/B FLP 8 at
the symmetrical “semi-triptycene” framework shows a similar
activity in the H2 splitting reaction as the interacting FLP 1.
Compound 8 does surprisingly not profit much from having a
kinetic advantage over 1 in featuring an open-nonbridged
structure. This probably means that the intrinsic reactivity of
the high-lying 1-open isomer must be much higher than that of
the open ground state structure of 8, since it must kinetically
compensate for the unfavorable 1-closed ⇄ 1-open equilibrium
situation. It is likely that this is due to the much higher
flexibility of the 1-open isomer over the rather rigid system 8,
resulting in an effective lowering of the decisive structurally
rather demanding FLP-H2 splitting transition state. Con-
sequently, we must conclude that frustration (i.e., the absence
of an effective Lewis acid/Lewis base interaction) alone is not
an exclusive criterion for FLP reactivity. It seems that geometric
factors and conformational flexibility to reach the decisive
transition state geometries is also of great importance. In our
special case this conformational feature seems to largely
compensate for the kinetic disadvantage that the 1-closed
system has over our new open, nonbridged FLP 8 in the
heterolytic H2 splitting reaction, so that they both show similar
overall reactivities toward dihydrogen.
Experimental, analytical, and DFT calculation details
(PDF)
Accession Codes
CCDC 1576800−1576811 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 for G.E.: 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 European Research Council is
gratefully acknowledged. We cordially thank Prof. Stefan
Grimme for doing the DFT calculations for us. Our
contribution is dedicated to Professor Peter Eilbracht on the
occasion of his 75th birthday.
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