The Chemistry of a Non-Interacting Vicinal Frustrated Phosphane/Borane Lewis Pair

Article abstract of DOI:10.1002/chem.201603954

The dimesitylphosphinocyclopentene/HB(C6F5)2-derived vicinal trans-1,2-P/B frustrated Lewis pair (FLP) 4 shows no direct phosphane–borane interaction. Toward some reagents it behaves similar to an intermolecular FLP; it cleaves dihydrogen, deprotonates terminal alkynes, and adds to organic carbonyl compounds including CO2. It shows typical intramolecular FLP reaction modes (cooperative 1,1-additions) to mesityl azide, to carbon monoxide, and to NO. The latter reaction yields a persistent P/B FLPNO nitroxide radical, which undergoes H-atom abstraction reactions. The FLP 4 serves as a template for the CO reduction by [HB(C6F5)2] to generate a FLP-η2-formylborane. The formylborane moiety is removed from the FLP template by reaction with pyridine to yield a genuine pyridine stabilized formylborane that undergoes characteristic borane carbaldehyde reactions (Wittig olefination, imine formation). Most new products were characterized by X-ray diffraction.

Full text of DOI:10.1002/chem.201603954

DOI: 10.1002/chem.201603954
Full Paper
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Frustrated Lewis Pairs
The Chemistry of a Non-Interacting Vicinal Frustrated Phosphane/
Borane Lewis Pair
Lisa-Maria Elmer,[a] Gerald Kehr,[a] Constantin G. Daniliuc,[a] Melanie Siedow,[b]
Hellmut Eckert,[b, d] Matthias Tesch,[a] Armido Studer,[a] Kamille Williams,[c]
Timothy H. Warren,[c] and Gerhard Erker*[a]
Dedicated to Professor Rolf Gleiter on the occasion of his 80th birthday
Abstract: The dimesitylphosphinocyclopentene/HB(C6F5)2-
derived vicinal trans-1,2-P/B frustrated Lewis pair (FLP) 4
shows no direct phosphane–borane interaction. Toward
some reagents it behaves similar to an intermolecular FLP; it
cleaves dihydrogen, deprotonates terminal alkynes, and
adds to organic carbonyl compounds including CO2. It
shows typical intramolecular FLP reaction modes (coopera-
tive 1,1-additions) to mesityl azide, to carbon monoxide, and
to NO. The latter reaction yields a persistent P/B FLPNO ni-
troxide radical, which undergoes H-atom abstraction reac-
tions. The FLP 4 serves as a template for the CO reduction
by [HB(C6F5)2] to generate a FLP-h2-formylborane. The for-
mylborane moiety is removed from the FLP template by re-
action with pyridine to yield a genuine pyridine stabilized
formylborane that undergoes characteristic borane carbalde-
hyde reactions (Wittig olefination, imine formation). Most
new products were characterized by X-ray diffraction.
Introduction
been employed in such reactions, for example, in dihydrogen
splitting and/or hydrogenation catalysis.[5–7]
Frustrated Lewis pair (FLP) chemistry has seen some remark-
able development in recent years. An increasing number of
FLP types has led to many new examples of small molecule
binding and/or activation, and sometimes to new reactions.[1,2]
Usually the Lewis acids and bases in FLPs are effectively hin-
dered from the usual neutralizing adduct formation by attach-
ing very bulky substituents at their core atoms. This can lead
to active intermolecular frustrated Lewis pairs.[3] Their reaction
with added reagents in solution is thought to initially involve
the formation of encounter complexes between the FLP com-
ponents[4] from which the actual FLP reaction then can take
place. Various types of intermolecular FLPs have successfully
In principle the reaction of intramolecular FLPs is less com-
plex since here the Lewis acid and Lewis base components are
connected by a linker in a single molecule and, therefore, a bi-
molecular reaction can just take place with an added sub-
strate.[8–10] Various types of bridging frameworks have success-
fully been used for generating active intramolecular frustrated
Lewis pairs, among them many vicinal P/B or N/B FLPs.[11]
However, here internal interaction between the FLP Lewis
acid and base components of various strengths may (and
often does) provide a serious complication. In such cases one
assumes efficient opening and closing of the LA/LB contact in
a rapid pre-equilibrium before the actual FLP reaction can
occur from the active open isomer.[12]
There are actually some intramolecular vicinal FLPs that do
not show a direct internal Lewis acid–base interaction. This can
be due to extreme steric hindrance as it is demonstrated by
Repo’s N/B FLP pair 1 (interacting, closed structure) and 2
(non-interacting, open)[10b] or it can be caused by a special
geometric orientation of the respective groups at a suitable
framework (Scheme 1). The norbornane-derived exo-B(C6F5)2/
endo-PMes2 FLP 3 is an example (P···B separation: 3.878(1) ꢀ).[13]
However, the very reactive P/B FLP 3 is not too convenient to
make; therefore, we had looked for alternatives and found that
the simple trans-1,2-B(C6F5)2/-PMes2 arrangement at the cyclo-
pentane ring in 4 provided a solution. It is an open system
and as such might show structural, spectroscopic, and chemi-
cal features that can either be reminiscent of those of the
many intermolecular FLP examples or be specific to vicinal in-
[a] Dr. L.-M. Elmer, Dr. G. Kehr, Dr. C. G. Daniliuc, M. Tesch, Prof. Dr. A. Studer,
Prof. Dr. G. Erker
Organisch-Chemisches Institut, Universitꢀt Mꢁnster
Corrensstraße 40, 48149 Mꢁnster (Germany)
E-mail: erker@uni-muenster.de
[b] M. Siedow, Prof. Dr. H. Eckert
Institut fꢁr Physikalische Chemie, Universutꢀt Mꢁnster
Corrensstraße 28/30, 48149 Mꢁnster (Germany)
[c] K. Williams, Prof. Dr. T. H. Warren
Georgetown University, Department of Chemistry
Box 571227, Washington DC 20057-1227 (USA)
[d] Prof. Dr. H. Eckert
Instituto de Fꢂsica Sao Carlos, Universidade de Sao Paulo, CP 369
Sao Carlos, SP, 13560-970 (Brasil)
Supporting information for this article can be found under
http://dx.doi.org/10.1002/chem.201603954.
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Scheme 1. Open and closed vicinal N/B and P/B frustrated Lewis pairs.
Figure 1. Molecular structure of the P/B FLP 4 (ellipsoids are set at 30%
probability). Selected bond lengths [ꢀ] and angles [8]: P1ꢀC1: 1.862(2), C1ꢀ
C2: 1.588(2), C2ꢀB1: 1.541(3), C2ꢀC3: 1.547(3), C3ꢀC4: 1.516(3), C4ꢀC5:
1.519(3), C5ꢀC1: 1.546(3), C11-P1-C1: 110.0(1), C2-B1-C31: 120.3(2), P1-C1-C2-
B1: 125.0(1), SBCCC: 360.0, SPCCC: 321.2, P1···B1: 3.84.
tramolecular systems. Our results on the FLP chemistry of the
open, non-internally stabilized FLP system 4 will be described
and discussed in this account.
125.0(1)8), so that we assume that there is no direct PꢀB inter-
action in this frustrated Lewis pair. Consequently, the boron
atom is planar-tricoordinate (SB1CCC =360.08); the phosphorus
atom features the typical trigonal pyramidal coordination ge-
ometry (SP1CCC =321.28). We note that the C41ꢀC46 and the
C21ꢀC29 C6F5/mesityl arene pair at B1 and P1 are oriented
close to parallel (angle between the planes: 4.48) with a dis-
tance between the centroids of the planes of ca. 3.6 ꢀ. The cy-
clopentane framework in 4 shows a distorted envelope confor-
mation.
Results and Discussion
Synthesis and characterization of the vicinal P/B FLP 4
We prepared the cyclopentane-derived P/B frustrated Lewis
pair 4 starting from dimesitylphosphinocyclopentene (5). This
in turn was conveniently prepared by treatment of
cyclopentanone with PCl5 followed by lithiation of the result-
ing chlorocyclopentene and reaction with Mes2PCl.[14] The cy-
clopentenylphosphane 5 was characterized by X-ray diffraction
(for details see the Supporting Information).
In solution, compound 4 shows a 11B NMR signal at d=
76.9 ppm, which is typical for a strongly Lewis acidic planar-tri-
coordinate RB(C6F5)2 borane. It shows three 19F NMR resonan-
ces with Dd19Fp,m =11.5 ppm. The 31P NMR resonance of com-
pound 4 is at d=ꢀ12.5 ppm.
Hydroboration of compound 5 with Piers’ borane [HB(C6F5)2]
is fast.[15] In solvents such as toluene, dichloromethane, or pen-
tane, the reaction is practically complete within a few minutes,
giving a yellow solution of the P/B FLP 4. We prepared 4 on
a preparative scale by reacting 5 with HB(C6F5)2 in pentane at
room temperature and then crystallized the product at ꢀ408C
to give 4 in 69% yield (Scheme 2).
Compound 4 is thermally sensitive, which is probably due to
a subsequent isomerization by means of retro-hydroboration/
hydroboration sequences. Within several hours at room tem-
perature this results in the formation of a mixture of 4 and
two isomers 6a,b; these were not positively identified, but we
assume that they are to be regarded as the respective cis- and
trans-1,3-disubstituted isomers. After heating for 5 h at 708C in
[D8]toluene a mixture of 4:6a:6b in a ratio of ca. 1:1.6:2.5 was
formed. To avoid complication due to the subsequent isomeri-
zation reaction we always generated the P/B FLP 4 by reacting
5 with HB(C6F5)2 for a few minutes in the respective solvent
and used these freshly prepared samples of 4 (after NMR con-
trol) for the subsequent FLP reactions described below.
Compound 4 is a reactive FLP. It reacted with dihydrogen
under mild conditions (2 bar, room temperature) to give the
product of heterolytic H2-splitting.[1,2] From the respective pen-
tane solution we isolated the [P]H+/[B]Hꢀ product 7 as a white
solid in 68% yield. Single crystals for the X-ray crystal structure
analysis were obtained from dichloromethane/pentane by the
diffusion method. It shows a trans-1,2-arrangement of the
Scheme 2. Formation of the P/B FLP 4 and its reaction with dihydrogen.
The X-ray crystal structure analysis (Figure 1) showed the
presence of the bulky B(C6F5)2 and PMes2 functionalities in
a trans-1,2-arrangement at the cyclopentane derived frame-
work. The B/P separation is large (3.84 ꢀ, q P1-C1-C2-B1=
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served with both intra- and intermolecular FLPs. Treatment of
4 with pyridine[17] (CH2Cl2/pentane) gave the crystalline adduct
8 in 75% yield (Scheme 3). The X-ray crystal structure analysis
showed that the pyridine donor had just added to the B(C6F5)2
group (B1ꢀN1 1.631(8) ꢀ, SB1CCC =336.78,
q P1-C1-C2-B1
153.3(4)8 (for details see the Supporting Information).
Compound 4 also cleanly formed a borane adduct 9 with
benzonitrile. It shows a 11B NMR resonance at d=ꢀ4.7 ppm
(31P: d=ꢀ16.2 ppm). The X-ray crystal structure analysis of 9
features a ꢀCꢂN bond length (C6ꢀN1 1.141(3) ꢀ) that is slightly
longer than the one observed in free benzonitrile (1.138 ꢀ).
This typical feature of a Lewis acid adduct of a RCꢂN donor is
complemented by the observed shifting of the IR n˜(CꢂN)
stretching band in 9 to larger wavenumbers (n˜ =2308 cmꢀ1)
relative to the free benzonitrile (n˜ =2230 cmꢀ1).[18] For further
details about the characterization of the adduct 9 see the Sup-
porting Information.
Figure 2. A view of the molecular structure of the PH+/BHꢀ zwitterion 7 (el-
lipsoids are set at 30% probability). Selected bond lengths [ꢀ] and angles
[8]: P1ꢀC1: 1.813(3), C1ꢀC2: 1.564(4), C2ꢀB1: 1.635(4), C11-P1-C1: 116.8(2),
C2-B1-C31: 114.5(2), P1-C1-C2-B1: 137.8, SBCCC: 334.2, SPCCC: 344.4, P1···B1:
4.08.
The FLP 4 also forms the respective borane Lewis acid
adduct with n-butyl isocyanide. The adduct 10 was isolated in
88% yield and characterized by X-ray diffraction (for details see
the Supporting Information). It also shows a carbon-nitrogen
triple-bond length of the isonitrile end-on bonded to the
strong boron Lewis acid of 1.146(7) ꢀ (N1ꢀC6) and an IR n˜(Cꢂ
N) stretch of n˜ =2297 cmꢀ1.
+
ꢀ
P(H)Mes2 and BH(C6F5)2 substituents at the five-membered
carbocyclic framework (see Figure 2). The P/B separation in
compound 7 is large at 4.08 ꢀ. Both the phosphorus and the
boron atom now have a hydrogen atom attached, which are
also oriented away from each other (PꢀH/HꢀB separation:
5.06 ꢀ). The boron coordination geometry in the zwitterionic
product 7 is pseudo-tetrahedral (SB1CCC =334.28).
We reacted the FLP 4 with phenylacetylene (Scheme 3) in n-
pentane solution and isolated the product 11 as a colorless
solid in 88% yield. Compound 11 was characterized by X-ray
diffraction. It shows that the terminal alkyne was deprotonated
by the phosphane base of the FLP 4 and the remaining acety-
lide moiety bonded to the borane Lewis acid (Figure 3).[18,19]
Consequently, compound 11 features a trans-1,2-arrangement
The 11B NMR signal of compound 7 is at d=ꢀ19.2 ppm. It
shows coupling to the attached hydrogen (1JBH ꢁ90 Hz). The
31P NMR resonance is at d=ꢀ3.2 ppm. It shows the typical
large 1JPH =463 Hz coupling constant (corresponding 1H NMR
[P]ꢀH signal at d=7.14 ppm). Owing to the molecular chirality,
both the pairs of mesityl substituents at phosphorus and the
C6F5 substituents at the tetracoordinated boron atom are dia-
stereotopic. The Dd19Fp,m separation of each C6F5 substituent is
small (2.0 and 2.8).
+
of the P(H)Mes2 and B(C6F5)2ꢀCꢂCꢀPhꢀ substituents at its
five-membered carbocyclic core.
Reaction of the P/B FLP 4 with D2 (1.7 bar) under similar con-
ditions gave the zwitterionic [P]D+/[B]Dꢀ product 7-D2. It
showed a [P]D+ doublet in the 2H NMR spectrum (d=
7.15 ppm, 1JPD =71.2 Hz) and a broad [B]Dꢀ signal at d=
2.67 ppm. The 11B NMR resonance of 7-D2 was located at d=
ꢀ19.3 ppm. The 31P NMR resonance is broadened, but features
a 1:1:1 equal intensity triplet upon proton decoupling (d=
1
ꢀ3.7 ppm, JPD ꢁ71 Hz in dichloromethane).
Both the P/B FLP 4 and the [P]H+/[B]Hꢀ hydrogen activation
product 7 have been used as metal-free hydrogenation cata-
lysts for typical organic substrates.[16] However, their per-
formance was not optimal as compared to other P/B FLPs,
probably because of complications due to the above men-
tioned isomerization processes that may take place under the
applied hydrogenation conditions (for details see the Support-
ing Information).
Scheme 3. Reactions of the P/B FLP 4 with donor molecules and with termi-
nal alkynes.
In solution, compound 11 shows a 11B NMR signal at d=
ꢀ16.5 ppm and a 31P NMR resonance at d=0.0 ppm with the
Addition reactions at the boron Lewis acid
1
The reactions of 4 with a few donor reagents and with termi-
nal alkynes proceeded similarly as they have often been ob-
typical large JPH coupling constant of ca. 470 Hz. The alkynyl
unit at boron shows 13C NMR signals at d=109.7 ppm ([B]ꢀCꢂ)
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Figure 4. A view of the major isomer rac-R, R, R 15a from the addition reac-
tion of benzaldehyde to FLP 4 (ellipsoids are set at 30% probability). Select-
ed bond lengths [ꢀ] and angles [8]: P1ꢀC1: 1.822(4), C1ꢀC2: 1.570(4), C2ꢀB1:
1.639(6), B1ꢀO1: 1.452(6), O1ꢀC6: 1.388(9), P1ꢀC6: 1.970(7), C6ꢀC51: 1.513
(11), C1-P1-C6: 98.0(2), C21-P1-C11: 109.9(5), O1-B1-C2: 106.1(3), C31-B1-C41:
Figure 3. Molecular structure of compound 11 (ellipsoids are set at 30%
probability). Selected bond lengths [ꢀ] and angles [8]: P1ꢀC1: 1.820(3), C1ꢀ
C2: 1.564(4), C2ꢀB1: 1.652(4), B1ꢀC31: 1.596(4), C31ꢀC32 :1.209(4), C32ꢀC33:
1.442(4), C32-C31-B1: 169.5(3), C31-C32-C33: 178.9(3), C31-B1-C2: 105.4(2),
105.3(3), P1-C1-C2-B1: 66.6(4), B1-O1-C6-P1: 83.4(7), SBCCC: 333.9, SPC1C11C21
:
C11-P1-C1: 117.6(1), C2-B1-C41: 115.1(2), P1-C1-C2-B1: ꢀ132.9(2), SBC2C41C51
:
330.0, SPCCC: 344.5.
332.5.
and d=95.3 ppm (ꢂCꢀPh), respectively. Both the pairs of mesi-
tyl groups at phosphorus and the C6F5 groups at boron in
compound 11 are diastereotopic and thus show 1:1 intensity
pairs of the respective NMR resonances.
namely irradiation at the OꢀC[H]ꢀP (6-H) signal of the major
isomer 15a gave a marked negative NOE response at the re-
spective (6-H) H resonance of the minor isomer 15b. This indi-
cates some 15aQ15b rearrangement on the NMR timescale,
a feature that had previously been observed of other benzal-
dehyde P/B FLP addition products as well.[21]
1
The reaction of the FLP 4 with the conjugated enyne 12
took a similar course (Scheme 3) and we isolated the respec-
tive zwitterionic phosphonium/enynyl borate product 13 in
55% yield. It shows similar spectroscopic features as its conge-
ner 11 [13, for example, 11B NMR: d=ꢀ16.6 ppm, 31P NMR: d=
The X-ray crystal structure analysis confirmed the formation
of the heterocyclic core of the product by BꢀO and PꢀC bond
formation of FLP 4 to the incoming benzaldehyde moiety. In
the crystal we found two diastereoisomers, the major isomer
rac-R, R, R 15a (53%) and the minor isomer rac-RC1, RC2, SC6
15b (47%). The core heterobicyclo[4.3.0]nonane framework of
compound 15 shows a trans-junction between the two rings
(Figure 4).
1
ꢀ0.4 ppm (1JPH ꢁ468 Hz)] and we observed the typical H NMR
signals of the ꢀCꢂCꢀC(CH3)=CH2 group at boron (for details
see the Supporting Information).
Addition reactions to carbonyl compounds
The NMR spectra of the system 15 show mostly well-sepa-
rated signals of the major (15a) and minor (15b) isomer. At
273 K both the rotation around the PꢀC(Mes) as well as the Bꢀ
C(C6F5) vectors are frozen on the NMR timescale. Consequently,
we observed a total of 6 mesityl CH3 1H NMR signals for each
isomer and pairs of o-, p-, m-19F signals of the C6F5 groups. The
1H NMR resonances of the OꢀCH[P] protons were located at
d=6.41 ppm (major isomer 15a) and d=5.89 ppm (minor
isomer 15b), respectively, and we find two 31P NMR resonan-
ces: d=43.2 ppm (15a), d=28.5 ppm (15b).
The FLP addition reaction to trans-cinnamic aldehyde pro-
ceeds similarly (Scheme 4). We isolated a circa 28:72 mixture of
the respective 1,2-carbonyl addition products 16a and 16b
from the pentane reaction mixture as a colorless solid in 62%
yield. The detailed NMR/X-ray diffraction analysis showed that
in this case the rac-RC1, RC2, SC6 diastereoisomer 16b was the
major product. It features the 6-H and 2-H protons in a trans-
arrangement. The ring junction is again trans (for details see
the Supporting Information).
The P/B FLP 4 undergoes 1,2-addition to benzaldehyde.[20] We
performed the reaction at room temperature in pentane and
isolated the product as a solid mixture of two isomers 15a/
15b in a ratio of ca. 62:38 (in CD2Cl2) (Scheme 4). Both isomers
showed the same principal relative atom connectivity with the
nucleophilic carbonyl oxygen having become connected to
boron and the electrophilic carbonyl carbon atom to phospho-
rus. The system 15 contains three stereogenic carbon atoms:
two at the five-membered ring, the stereochemistry of which
is dependent on each other as a result of their formation by
the hydroboration reaction, and the newly formed stereogenic
center derived from the binding of the former aldehyde re-
agent. From the X-ray crystal structure analysis (Figure 4) and
a series of NOE experiments we assigned the major diastereo-
isomer 15a the rac-(R, R, R) configuration; that is, the CꢀH vec-
tors at C6 and C2 inside the newly formed heterocyclic six-
membered ring are in a cis-arrangement. Actually we noticed
a dynamic behavior of the 15a/b pair in the NOE experiments,
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the P/B FLP has 1,2-added to one carbonyl group of carbon di-
oxide. The resulting heterobicyclic framework of compound 17
shows a trans-junction between the two rings (q B1-C2-C1-P1:
69.0(3)8). Both the boron and the phosphorus atoms show
pseudo-tetrahedral coordination geometries (SB1CCC: 336.88,
SP1C1C6C11: 326.08). The carbon atom C6 is planar-tricoordinate
(SC6OOP: 359.98).
In solution, compound 17 features the 13C NMR carbonyl
carbon atom signal at d=160.9 ppm (1JPC =87.9 Hz) and
a
31P NMR resonance at d=11.3 ppm. Compound 17 shows
hindered rotation around the BꢀC(C6F5) and the PꢀC(Mes) vec-
tors at low temperature leading to the observation of five (two
1
overlapping) H mesityl methyl NMR signals plus four mesityl
methine signals as well as four o-C6F5, two p- and four m-C6F5
19F NMR resonances. Compound 17 shows an IR carbonyl
stretching band at n˜ =1699 cmꢀ1.
Scheme 4. Reactions of the P/B FLP 4 with carbonyl compounds.
1,1-Addition reactions of the P/B FLP 4
1,1-Addition to a variety of reagents is a typical reaction fea-
ture of some vicinal P/B FLPs. Compound 4 shows a similar be-
havior towards some of those reagents. P/B Lewis pairs can un-
dergo 1,3-, 1,2-, or even 1,1-addition reactions to organic aryl
azides.[20,24] Some of the resulting products can be viewed as
internally Lewis acid trapped intermediates of the Staudinger
reaction.[25] Some such systems actually lose dinitrogen upon
heating and yield Staudinger reaction type products, but a few
undergo an anomalous Staudinger reaction.[26]
The trans-P/B FLP 4 reacts readily with carbon dioxide.[22,23]
We exposed a solution of 4 in pentane to a CO2 atmosphere at
room temperature. Reaction overnight furnished a precipitate
of the addition product 17 (Scheme 4), which was isolated as
a colorless solid in 80% yield. This CO2/FLP adduct is remarka-
bly stable. Its thermal stability is somewhere intermediate be-
tween typical intra- and intermolecular FLP–CO2 adducts.[22a] It
can even be handled for short periods of time at room temper-
ature, but we measured the NMR spectra conveniently at
lower temperatures to avoid any decomposition by CO2 elimi-
nation altogether. Compound 17 was characterized by C, H el-
emental analysis, by spectroscopy, and by X-ray diffraction
(Figure 5).
The reactive P/B FLP 4 rapidly undergoes a 1,1-addition reac-
tion to the terminal azide nitrogen atom of mesityl azide (N3ꢀ
Mes) (Scheme 5). The product 18 was isolated in 78% yield.
The X-ray crystal structure analysis (Figure 6) shows the newly
formed geminal pair of NꢀB and NꢀP bonds at the azide nitro-
gen atom N1. The resulting heterobicyclic framework again
shows a trans-junction. The remaining ꢀN=NꢀMes substituent
shows a trans-nitrogen–nitrogen double bond.
Single crystals of compound 17 were obtained by the diffu-
sion method. The X-ray crystal structure analysis shows that
Scheme 5. Reactions of the P/B FLP 4 with mesityl azide.
The periphery of compound 18 is congested. This led to the
observation of hindered rotation around the heteroatom–
C(aryl) bonds and thus we observed a total of ten 19F NMR C6F5
Figure 5. Molecular structure of the P/B FLP CO2 addition product 17 (only
molecule A of two independent molecules is shown; ellipsoids are set at
30% probability). Selected lengths [ꢀ] and angles [8]: P1ꢀC1: 1.817(3), C1ꢀ
C2: 1.547(4), C2ꢀB1: 1.623(4), B1ꢀO1: 1.548(4), O1ꢀC6: 1.287(4), O2ꢀC6:
1.209(4), P1ꢀC6: 1.900(3), C1-P1-C6: 102.2(1), C21-P1-C11: 110.5(1), O1-B1-C2:
111.3(3), C41-B1-C31: 108.4(3), O1-C6-P1: 119.6(2), O2-C6-P1: 115.8(2), O2-C6-
O1: 124.5(3), P1-C1-C1-B1: 69.0(3), B1-O1-C6-P1: 2.5(4), SBCCC: 336.8,
SPC1C6C11: 326.0, SC6OOP: 359.9.
1
resonances and six H NMR methyl signals plus four aromatic
methine resonances of the endocyclic PMes2 groups. In con-
trast, the peripheral ꢀN=N-mesityl substituent shows only
1
three H NMR signals (arom. CH, p-CH3, o-CH3) in a 2:3:6 inten-
sity ratio.
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precursor 18 (1.676(2) ꢀ), but due to its internal borane Lewis
acid coordination it is longer than typical phosphinimine P=Nꢀ
bonds (Ph3P=NH 1.524(3) ꢀ).[27] The B1ꢀN1 bond in 19 is also
rather short at 1.581(2) ꢀ: both these values indicate a delocal-
ized structure as described by the mesomeric formulae
19$19’ in Scheme 5.
Vicinal P/B Lewis pairs contain a phosphorus donor and
a boron acceptor center. Some such systems have been used
as ambiphilic ligands for metal complex coordination,[28] but P/
B Lewis pairs can apparently also undergo synergetic donor/
acceptor bonding,[29] with for example CO, in a way that is
itself reminiscent of metal coordination.[30]
Compound 4 reacted with carbon monoxide in dichlorome-
thane/pentane solution at 2 bar CO pressure at temperatures
below ꢀ408C to give the cooperative P/B FLP addition product
21 (Scheme 6). Crystallization gave the cyclic carbonyl in 52%
yield.
Figure 6. A view of the 1,1-addition product 18 of the P/B FLP 4 to mesityl
azide (ellipsoids are set at 30% probability). Selected bond lengths [ꢀ] and
angles [8]: P1ꢀC1: 1.809(2), C1ꢀC2: 1.554(3), C2ꢀB1: 1.629(4), P1ꢀN1:
1.676(2), B1ꢀN1: 1.629(3), N1ꢀN2: 1.386(3), N2ꢀN3: 1.246(3), N3ꢀC51:
1.439(3), N1-P1-C1: 93.5(1), C11-P1-C21: 107.1(1), N1-B1-C2: 96.6(2), C41-B1-
C31: 106.7(2), B1-N1-P1: 116.2(2), N2-N1-P1: 109.8(1), N2-N1-B1: 134.1(2), P1-
C1-C2-B1: ꢀ54.0(2), SBCCC: 334.7, SPCCC: 335.5, SN1PNB: 360.0.
Scheme 6. 1,1-P/B addition reaction of FLP 4 to CO.
Upon photolysis (HPK 125) in CH2Cl2, compound 18 under-
goes an anomalous Staudinger reaction[24,26] with formation of
the indazole derivative 20 (identified from the reaction mixture
by NMR and comparison with the data of the known com-
pound) and the phosphinimine/borane adduct 19. Compound
19 was isolated from the reaction mixture by column chroma-
tography.
The X-ray crystal structure analysis showed that both the
boron Lewis acid and the phosphorus Lewis base had added
to the carbonyl carbon of CO. The carbonyl carbon atom is
planar-tricoordinate (SC6POB =360.08) and the C6ꢀO1 linkage is
short. The C=O group is somewhat leaning over toward phos-
phorus as if the phosphorus Lewis base had trapped a borane
carbonyl (Figure 8). The P1/B1 separation in compound 21 is
2.99 ꢀ.
The X-ray crystal structure analysis shows the presence of
the endocyclic [P]=NꢀH phosphinimine moiety (Figure 7). Con-
sequently, the P1ꢀN1 bond of 19 is rather short (1.622(1) ꢀ),
which is shorter than the corresponding P1ꢀN1 bond of its
Compound 21 was also characterized by magic angle spin-
ning solid-state NMR spectroscopy (Figure 9; see the Support-
ing Information for details).[31] Consistent with the fourfold
boron coordination, the 11B MAS-NMR spectrum reveals
a rather small quadrupolar coupling constant of 0.85 MHz. The
31P MAS-NMR lineshape is not influenced by 11B–31P spin–spin
2
coupling even though J(11B–31P) amounts to 55 Hz as deter-
mined by heteronuclear J-resolved spectroscopy (for details
see the Supporting Information). The NMR observables mea-
sured on compound 21 are found to be in excellent agree-
ment with the values calculated by quantum chemical meth-
ods on DFT level. Finally, Figure 9 (top) shows the experimen-
tally observed 11B{31P} rotational echo double resonance
(REDOR) curve of 21. Based on simulations of the oscillatory
behavior measured at long dipolar mixing times, the 11B–31P in-
ternuclear distance is measured as 305ꢃ5 pm, which is found
to be in excellent agreement with the crystal structure. These
results demonstrate the ability of REDOR to characterize the
weaker 11B–31P dipole–dipole couplings of FLP adducts, which
generally occur over longer distance ranges.
Figure 7. Molecular structure of the product 19 of an anomalous Staudinger
reaction (ellipsoids are set at 30% probability). Selected bond lengths [ꢀ]
and angles [8]: P1ꢀC1: 1.809(2), C1ꢀC2: 1.563(2), C2ꢀB1: 1.630(2), P1ꢀN1:
1.622(1), N1ꢀB1: 1.581(2), N1-P1-C1: 95.1(1), C21-P1-C11: 107.4(1), N1-B1-C2:
Compound 21 shows a typical IR feature at n˜ =1774 cmꢀ1.
The solution-state 13C NMR C=O resonance of 21 was located
99.1(1), C31-B1-C41: 107.0(1), B1-N1-P1: 115.8(1), P1-C1-C2-B1: 51.6(1), SBCCC
336.1, SPCCC: 335.5.
:
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boron and the mesityl groups at phosphorus in 21 are diaste-
reotopic and both show hindered rotation around the hetero-
atom–C(aryl) vectors on the NMR timescale at 223 K.
The FLP 4 was found to react rapidly with NO. We stirred
a pale yellow solution of 4 in pentane at ꢀ788C under a NO
atmosphere (2 bar). Within minutes a precipitate formed and
the product 22 (Scheme 7) was isolated at room temperature
as a pale turquoise crystalline solid in 83% yield.
Scheme 7. Reactions of the P/B FLP 4 with nitric oxide.
Figure 8. A view of the molecular structure of the cooperative P/B FLP CO
addition product 21 (ellipsoids are set at 30% probability). Selected bond
lengths [ꢀ] and angles [8]: P1ꢀC1: 1.820(2), C1ꢀC2: 1.557(2), C2ꢀB1: 1.630(2),
P1ꢀC6: 2.071(2), C6ꢀ-B1: 1.670(2), O1ꢀC6: 1.181(2), C1-P1-C6: 90.8(1), C21-
P1-C11: 108.5(1), C2-B1-C6: 102.2(1), C41-B1-C31: 109.7(1), B1-C6-P1: 105.8(1),
O1-C6-P1: 119.4(1), O1-C6-B1: 134.9(2), P1-C1-C2-B1: ꢀ59.4(1), SC6POB: 360.0,
SBC2C31C41: 339.0, SPC1C11C21: 336.7.
Single crystals of the persistent P/B FLPNO nitroxide radical
22 were obtained from cyclopentane/dichloromethane by the
diffusion method. The X-ray crystal structure analysis (Figure 10
and Table 1) showed that the NO molecule had been captured
by forming a pair of BꢀN and PꢀN bonds. The resulting NꢀO
bond is short (ꢄ1.3 ꢀ), but markedly longer than the NꢀO dis-
tance in free NO (1.15 ꢀ).[33] The nitrogen coordination in com-
Figure 10. Molecular structure of the persistent P/B FLPNO nitroxide radical
22 (ellipsoids are set at 30% probability). Selected bond lengths [ꢀ] and
angles [8]: P1ꢀC1: 1.806(4), C1ꢀC2: 1.559(5), C2ꢀB1: 1.626(6), P1ꢀN1:
1.716(3), B1ꢀN1: 1.604(5), O1ꢀN1: 1.299(4), N1-P1-C1: 93.0(2), C11-P1-C21:
107.2(2), N1-B1-C2: 97.6(3), C41-B1-C31: 108.7(3), B1-N1-P1: 115.9(2), P1-C1-
C2-B1: 52.4(3), SBCCC: 339.9, SPCCC: 338.6.
Figure 9. Top : 11B{31P} REDOR data of 21: Squares show uncorrected experi-
mental data, while triangles show corrected data obtained with a compen-
sated REDOR method.[32] Simulations for three different B···P distances are
shown; the best agreement with the experimental data is found for a B···P
distance of 305 pm. Bottom: 11B (right) and 31P (left) MAS-NMR spectra of
compound 21. Upper traces show experimental data and lower traces are si-
mulated spectra. An impurity is marked by the symbol +.
Table 1. Selected structural parameters of the compounds 22–24[a]
[N]OX
22
23
24
[N]O
[N]OH
[N]OCH2Ph
N1ꢀO1
1.299(4)
1.716(3)
1.604(5)
125.7(3)
118.4(2)
360.0
1.432(3)
1.635(3)
1.553(4)
123.2(2)
117.4(2)
360.0
1.436(2)
1.647(2)
1.593(3)
123.4(2)
116.0(1)
356.9
P1ꢀN1
B1ꢀN1
B1-N1-O1
P1-N1-O1
SN1BOP
1
at d=229.8 ppm; it shows a large JPC coupling constant of ca.
102 Hz. The 31P NMR resonance of 21 is at d=ꢀ0.8 ppm and
P1-C1-C2-B1
52.4(3)
ꢀ51.1(3)
ꢀ52.6(2)
the 11B NMR signal at d=ꢀ12.2 ppm. The C6F5 substituents at
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pound 22 is trigonal planar, although the individual bond
angles around nitrogen are quite different from each other. It
again appears that the NꢀO unit is slightly leaning over
toward phosphorus (Table 1), similar to the way it had been
observed for the CO addition product 21.[34]
The FLPNO radical 22 was characterized by EPR spectrosco-
py in solution. It shows a multi-line pattern in dichloromethane
at room temperature (Figure 11). The spectrum is centered at
g=2.00644 and exhibits coupling to 31P [A(31P)=50.8 MHz],
14N [A(14N)=22.0 MHz], and 11B/10B [A(11B)=9.2 MHz]. A de-
tailed discussion of the solid-state EPR spectrum of compound
22 has been recently published separately.[35]
X-ray diffraction and by spectroscopy (for details, see the Sup-
porting Information).
The analogous reaction of 22 was carried out with ethylben-
zene to give a ca. 1:1 mixture of the diastereoisomers of 25
(Scheme 8). Unfortunately we could isolate this product only in
very low yield (3%). Therefore, we prepared 25 according to
a Matyjaszewski method[37] by treatment of 22 with phenyl-
ethylbromide and Cu/CuOTf in the presence of a substituted
dipyridyl ligand. This eventually gave the 25a/25b mixture of
diastereoisomers in a combined yield of 38%. One of the iso-
mers could be crystallized and characterized by X-ray diffrac-
tion (the structure is depicted in the Supporting Information,
see there also for the spectroscopic characterization of the
compounds).
Figure 11. X-band EPR spectrum (bottom) and simulation (top) of com-
pound 22 in dichloromethane at room temperature.
Scheme 8. Some typical reactions of the persistent P/B FLPNO·radical 22.
Compound 22 is a markedly oxygen-centered nitroxide radi-
cal. It undergoes the typical reactions of the general class of
compounds of the nitroxide radical type.[36] The P/B FLPNO
radical 22 undergoes H-atom abstraction reactions (Scheme 8).
With 1,4-cyclohexadiene it reacts at room temperature in di-
chloromethane to give the diamagnetic P/B FLPNOH product
23 (plus benzene). Compound 23 was characterized by X-ray
diffraction (the structure is depicted in the Supporting Informa-
The P/B FLPNO-CHMePh products 25a/25b were tested to
serve as initiators/regulators in the nitroxide mediated (radical)
polymerization (NMP) of styrene.[38] Experiments were conduct-
ed with 0.5 mol% of 25a/25b in neat styrene at 110–1508C
for 1–5 h in the presence of varying amounts of additional free
nitroxide 22 (for details, see the Supporting Information).
Under the tested conditions controlled styrene polymerization
was not achieved, as revealed by PDIs significantly above the
C
tion). The structural parameters show that the H addition to
give the [N]ꢀOꢀH product resulted in a marked lengthening of
the NꢀO bond (Table 1). Compared to the radical 22, the Nꢀ theoretical limit of 1.5 (1.59–1.81). For example, at 1208C after
O[H] bond in 23 is 0.133 ꢀ longer. At the same time the PꢀN
bond has become shorter by 0.081 ꢀ, which probably indicates
some phosphinimine character of 23.
3 h, poly(styrene) with an average molecular weight of Mn =
23.9 kDa and a PDI of 1.59 was isolated after precipitation into
methanol (30% yield).
The P/B FLPNOH product 23 shows an IR OꢀH band at n˜ =
3531 cmꢀ1. In solution it features heteronuclear magnetic reso-
nance signals at d= +46.3 ppm (31P) and d=ꢀ5.7 ppm (11B),
respectively. The 1H NMR [N]OH resonance occurs as a multiplet
at d=4.83 ppm. Compound 27 shows the signals of pairwise
diastereotopic C6F5 groups at boron (19F) and mesityl groups at
We had recently studied the application of a structurally re-
lated FLP-derived alkoxyamine in the NMP of styrene.[34b]
Unlike 25a/25b, the former system features a cyclohexane in-
stead of a cyclopentane ring annulated with the cyclic FLB-
alkoxyamine in its backbone. Under similar conditions, good
control of the polymerization was obtained with that homo-
phosphorus (1H). There is hindered rotation around the Bꢀ logue, and poly(styrene) with PDIs between 1.30 and 1.35 had
C(aryl) and PꢀC(aryl) s-bonds on the NMR timescale at 299 K.
The P/B FLPNO radical 22 activates a benzylic hydrogen of
toluene. At 608C the H-atom abstraction gives a mixture of the
P/B FLPNOH (23) and P/B FLPNO-benzyl (24) products. The re-
action is thought to be initiated by hydrogen atom abstraction
from toluene by 22 generating 23 and a very reactive benzyl
radical which is instantaneously trapped by additional nitroxide
radical 22 to give 24. Compound 24 was also characterized by
been isolated. Obviously, the ring size of the second ring in
these bicyclic P/B FLPs influences the effectivity of the FLPNO
radicals in controlling the radical polymerization reaction.
Carbon monoxide reactions and formylborane chemistry
We had previously shown that carbon monoxide can be re-
duced to the formyl stage by BH boranes at suited FLP tem-
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plates. The FLP template serves to overcome the thermody-
namic restrictions of CO reduction by [B]-H boranes.[39] In this
way we had prepared the FLP-h2-formylborane compound 26
from the FLP 4 generated in situ.[13] We now treated the prod-
uct 26 with H2 (50 bar, 30 h). This resulted in a further reduc-
tion of the carbon monoxide derived formyl group with cleav-
age of the strong CꢀO bond to the methylene stage
(Scheme 9).[40] Product 27 was eventually isolated as a colorless
crystalline solid in 51% yield.
resonances (d=4.3 and d=0.7 ppm) and a 31P NMR signal at
d=36.5 ppm. The newly formed CH2 group inside the seven-
membered ring shows diastereotopic hydrogen atoms (d=
3.36 and d=2.76 ppm) with coupling constants 2JPH =2JHH
=
14.5 Hz. We observe a resonance of relative intensity one at
d=5.78 ppm that was assigned to the OH proton; it appears
as a doublet due to coupling with a fluorine atom of one of
the adjacent C6F5 groups (JFH =12.2 Hz, shown by a selective
decoupling experiment).[41]
We had previously shown that FLP–h2-formylborane systems
can react in two different ways with pyridines depending on
the specific FLP framework. They can either undergo a nucleo-
philic aromatic formyl transfer to the pyridine framework, a re-
action that eventually yields a hydroxymethylation product,[42]
or liberate the genuine formylborane as the pyridine
adduct.[40,43] The latter reaction is favored with the FLP–h2-for-
mylborane 26. The reaction of 26 with excess pyridine in di-
chloromethane (3 h, room temperature) gave a ca. 1:1 mixture
of the pyridine-stabilized formylborane 28 and the FLP pyri-
dine adduct 8 (Scheme 10). Crystallization from toluene/n-pen-
Scheme 9. Reaction of the (h2-formylborane) FLP product 26 with dihydro-
gen.
The X-ray crystal structure analysis shows the newly formed
CH2 group bridging between phosphorus and boron and the
separated former carbonyl oxygen atom protonated bridging
between the pair of boron atoms (Figure 12). The B1-O1-B2
angle at this unit amounts to 129.1(2)8.
Scheme 10. Formation and carbonyl reactions of the pyridine stabilized for-
mylborane 28.
tane then gave the pure pyridine formylborane product 28 in
67% yield [[B]ꢀCHO: d=11.24 (1H), d=233.2 (13C), d=
ꢀ4.5 ppm (11B)]. It undergoes a variety of typical organic car-
bonyl reactions. The Wittig olefination reaction with Ph3P=
CHCH3 (29) gave the substituted vinylborane 30 (Scheme 10),
isolated as a viscous oil in 48% yield [1H NMR: d=6.16,
6.08 ppm, JHH =13.5 Hz, 13C NMR: d=138.4, 135.1 ppm (ꢀCH=
3
CHꢀ), d=17.2 (CH3), 11B NMR: d=ꢀ3.6 ppm].[40]
Figure 12. A view of the molecular structure of the CO-reduction product
27 (ellipsoids are set at 30% probability). Selected bond lengths [ꢀ] and
angles [8]: P1ꢀC1: 1.841(2), C1ꢀC2: 1.573(3), C2ꢀB1: 1.636(3), P1ꢀC6:
1.807(2), C6ꢀB2: 1.635(3), O1ꢀB1: 1.600(3), O1ꢀB2: 1.553(3), C6-P1-C1:
104.7(1), C21-P1-C11: 108.4(1), B2-C6-P1: 118.8(1), O1-B2-C6: 106.2(2), C61-
B2-C51: 105.7(2), B2-O1-B1: 129.1(2), O1-B2-C2: 107.6(2), C31-B1-C41:
Reaction of the formylborane 28 with aniline (dichlorome-
thane, 2d, room temperature) gave the aldimine 31
(Scheme 10), isolated as a crystalline solid in 54% yield. The X-
ray crystal structure analysis shows the presence of a trans-
configurated imine C=N double bond with a trigonal-planar
carbonyl carbon atom C1. The boron atom shows a pseudo-
tetrahedral coordination sphere. It contains the stabilizing pyri-
dine ligand as an essential structural element (Figure 13).
109.7(2), P1-C1-C2-B1: ꢀ96.7(2), SB1CCC: 337.0, SB2CCC: 331.9, SP1C1C11C21
:
333.5.
The 19F NMR spectrum of compound 27 in CD2Cl2 solution
shows the signals of four different C6F5 substituents at the pair
of boron atoms. The compound features diastereotopic mesityl
groups at phosphorus that show hindered rotation on the
1
Compound 31 shows a H NMR ꢀCH=Nꢀ carbaldimine reso-
nance at d=8.79 ppm (d13C: 182.5 ppm) and a 11B NMR signal
at d=ꢀ3.7 ppm. The IR n˜(C=N) stretching band of 31 was lo-
cated at n˜ =1647 cmꢀ1. The analogous reaction of compound
28 with n-propyl amine gave the borane carbaldimine product
1
NMR timescale, giving rise to six H NMR methyl signals and
four aromatic CH groups. Compound 27 shows two 11B NMR
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Experimental Section
For general information and details of the characterization of the
compounds, see the Supporting Information. Synthetic procedures
for the preparation of selected examples are given below.
4: Equivalent amounts of dimesitylcyclopentenylphosphane (5)
(100 mg, 0.297 mmol) and bis(pentafluorophenyl)borane (103 mg,
0.297 mmol) were dissolved in n-pentane (ca. 4 mL) and stirred at
r.t. for ca. 10 min. The bright yellow solution was stored at ꢀ408C
and after several weeks at ꢀ408C a yellowish crystalline solid was
formed (140 mg, 0.205 mmol, 69%, compound 4). The obtained
crystals were suitable for the X-ray crystal structure analysis. Ele-
mental analysis calcd for C35H30BF10P: C 61.60, H 4.43; found:
C 61.59, H 4.49.
Reactions with dihydrogen
7: Compound 4 was prepared in situ by dissolving equivalent
amounts of dimesitylcyclopentenylphosphane (5) (60.1 mg,
0.179 mmol)
and
bis(pentafluorophenyl)borane
(61.9 mg,
Figure 13. Molecular structure of the pyridine stabilized borane formaldi-
mine 31 (ellipsoids are set at 30% probability). Selected bond lengths [ꢀ]
and angles [8]: B1ꢀC1: 1.614(3), B1ꢀN41: 1.616(3), C1ꢀN1: 1.265(3), N1ꢀC11:
1.421(3), C1-B1-N41: 108.8(2), C31-B1-C21: 114.3(2), N1-C1-B1: 125.5(2), C1-
N1-C11: 118.7(2), C11-N1-C1-B1: 178.1(2), SB1CCC: 333.0.
0.179 mmol) in n-pentane (3 mL) and stirring at r.t. for ca. 10 min.
The solution was degassed and H2 gas (2 bar) pressure was applied
for a few minutes at r.t. The reaction mixture was stirred overnight
upon which a white precipitate had formed. The solvent was re-
moved by cannula filtration and the residue was washed with n-
pentane (2ꢂ2 mL) and was dried in vacuo. The phosphonium hy-
32 (Scheme 10), isolated as a colorless solid in 63% yield [IR:
dridoborate
7 was obtained as a white powder (83.4 mg,
n˜ =1644 cmꢀ1
;
1H/13C NMR: d=8.55/180.0 ppm (ꢀCH=Nꢀ),
0.122 mmol, 68%). Elemental analysis calcd for C35H32BF10P:
C 61.42, H 4.71; found: C 61.68, H 5.02.
11B NMR: d=ꢀ4.1 ppm].
Reactions with a terminal alkyne
11: Compound 4 was prepared in situ by dissolving equivalent
amounts of dimesitylcyclopentenylphosphane (5) (50.1 mg,
Conclusion
Intramolecular vicinal FLPs have considerably contributed to
the development of frustrated Lewis pair chemistry.[9–11] Many
such systems exhibit some internal Lewis acid–base interac-
tion, although it is thought that their typical reactions with
small molecules take place from energetically higher-lying
open isomers, reached in an endergonic equilibrium situation.
Therefore, it was an attractive concept to develop vicinal frus-
trated Lewis pair systems that are devoid of such an intramo-
lecular interaction. This can either be achieved by introducing
extreme steric bulk[10b] or by placing the Lewis acid and Lewis
base functionalities at specifically designed rigid frameworks
that serve to effectively separate the protagonists from each
other spatially. The norbornane derived FLP 3 is an example.
In this study we have shown that attaching the B(C6F5)2 and
PMes2 functionalities in a trans-1,2-fashion at the cyclopentane
backbone provides a solution to this problem. In contrast to
cyclohexane-derived trans-P/B or N/B FLPs, which show inter-
nal interaction between the pair of equatorially oriented Lewis
acid and Lewis base functions,[44] the corresponding cyclopent-
ane derived system 4 is a free, non-interacting P/B FLP. Conse-
quently, this is a reactive FLP which can undergo a variety of
reactions that may be typical for either intra- or intermolecular
FLPs. Some of the products show some interesting follow-up
chemistry. It seems that the typical properties of the open P/B
FLP 4 direct us toward an expansion of FLP structures and re-
activities by using strictly non-interacting Lewis acid–base
functionalities at suitably devised frameworks.
0.149 mmol)
and
bis(pentafluorophenyl)borane
(51.5 mg,
0.149 mmol) in n-pentane (2.5 mL) and stirring at r.t. for ca. 5 min.
Phenylacetylene (16.0 mg, 0.157 mmol, 1.05 equiv) dissolved in n-
pentane (1 mL) was added to the yellow solution, upon which
a white precipitate was formed. The solution was removed by filter
cannula, the residue washed with n-pentane (1ꢂ2 mL), and it was
dried in vacuo to give compound 11 as a white powder (103.1 mg,
0.131 mmol, 88%). Elemental analysis calcd for C43H36BF10P:
C 65.83, H 4.63; found: C 65.72, H 4.66.
Reactions with carbonyl compounds
15: FLP 4 was prepared in situ by dissolving equivalent amounts
of dimesitylcyclopentenylphosphane (5) (70.2 mg, 0.209 mmol) and
bis(pentafluorophenyl)borane (72.3 mg, 0.209 mmol) in n-pentane
(3.5 mL) and stirring at r.t. for ca. 5 min. Benzaldehyde (23.5 mg,
0.221 mmol, 1.05 equiv) dissolved in n-pentane (1 mL) was added
to the yellow solution to give a colorless suspension. The superna-
tant was removed by filter cannula, and the residue dried in vacuo,
to finally give a white powder (132.9 mg, 0.169 mmol, 81%). The
solution of compound 15 in [D2]dichloromethane showed a mixture
of 2 diastereoisomers 15a/15b in a ratio of 62:38 (31P, 273 K). Crys-
tals of compounds 15a and 15b suitable for the X-ray crystal
structure analysis were obtained by slow diffusion of n-pentane
into a solution of the obtained white solid in CH2Cl2 at ꢀ408C. The
obtained crystal contains compounds 15a and 15b (ratio: 53:47).
Elemental analysis calcd for C42H36BF10OP: C 63.98, H 4.60; found:
C 64.32, H 4.41.
16: FLP 4 was prepared in situ by dissolving equivalent amounts
of phosphane 5 (70.0 mg, 0.208 mmol) and bis(pentafluorophenyl)-
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borane (72.0 mg, 0.208 mmol) in n-pentane (3.5 mL) and stirring at
r.t. for ca. 5 min. trans-Cinnamic aldehyde (26.2 mL, 0.208 mmol)
was added to the yellow solution, upon which a white precipitate
was formed. The solution was removed by filter cannula, and the
residue dried in vacuo. Compound 16 was obtained as white
powder (103.9 mg, 0.128 mmol, 62%). NMR analysis of a solution
of the white solid in [D2]dichloromethane showed a mixture of 2
diastereoisomers 16a/16b in 1:2.5 ratio (1H NMR). Crystals of com-
pound 16b suitable for the X-ray crystal structure analysis were
obtained by slow diffusion of n-pentane into a solution of the ob-
tained white solid in CH2Cl2 at ꢀ408C. Elemental analysis calcd for
C44H38BF10OP: C 64.88, H, 4.70; found: C 64.83, H 4.60.
Reaction with nitrogen monoxide
22: Compound 4 was prepared in situ by dissolving equivalent
amounts of dimesitylcyclopentenylphosphane (5) (100.0 mg,
0.297 mmol)
and
bis(pentafluorophenyl)borane
(102.8 mg,
0.297 mmol) in n-pentane (6 mL) and stirring at r.t. for ca. 10 min.
The solution was degassed, cooled to ꢀ788C, and NO gas (2 bar)
pressure was applied for a few minutes, upon which a brownish
precipitate was formed. The reaction mixture was allowed to warm
to r.t. and stirred for another 5 min. Excess NO gas was removed
by flushing with argon gas, upon which the precipitate turned tur-
quoise. The solvent was removed by filter cannula, the residue
washed with n-pentane (2ꢂ3 mL), and dried in vacuo. Compound
22 was obtained as light turquoise powder (176.8 mg, 0.248 mmol,
83%). Single crystals of compound 22 suitable for the X-ray crystal
structure analysis were obtained by slow diffusion of cyclopentane
into a solution of the obtained light turquoise solid in CH2Cl2 at
ꢀ408C. Elemental analysis calcd for C35H30BF10NOP: C 59.01, H 4.24,
N 1.97; found: C 59.04, H 4.11, N 1.90.
17: FLP 4 was prepared in situ by dissolving equivalent amounts of
phosphane 5 (50.1 mg, 0.149 mmol) and bis(pentafluorophenyl)-
borane (51.3 mg, 0.149 mmol) in n-pentane (3 mL) and stirring at
r.t. for ca. 10 min. The solution was degassed, and CO2 gas (2 bar)
pressed on it at r.t. for a few minutes. The reaction mixture was
stirred overnight upon which a white precipitate had formed. The
solvent was removed by syringe, the residue washed with n-pen-
tane (2ꢂ2 mL), and dried in vacuo. Product 17 was obtained as
a white powder (86.6 mg, 0.119 mmol, 80%). Crystals suitable for
the X-ray crystal structure analysis were obtained by slow diffusion
of n-pentane into a concentrated solution of compound 17 in
Reaction with dihydrogen
27: Compound 26 (82.2 mg, 0.078 mmol) was suspended in CH2Cl2
(3.5 mL) and the reaction mixture stirred under an atmosphere of
H2 gas (50 bar) at r.t. for 30 h. Purification by filtration over silica
and crystallization from a concentrated CH2Cl2 solution at ꢀ408C
gave compound 27 as white crystalline material (42.3 mg,
0.040 mmol, 51%). The obtained crystals were suitable for the X-
ray crystal structure analysis. Elemental analysis calcd for
C48H33B2F20OP·CH2Cl2: C 51.48, H 3.09; found: C 51.44, H 3.00.
CH2Cl2
at
ꢀ408C.
Elemental
analysis
calcd
for
C36H30BF10O2P·0.5CH2Cl2: C 57.02, H 4.06; found: C 57.02, H 3.79.
1,1-Addition reactions
18: Compound 4 was prepared in situ by dissolving equivalent
amounts of phosphane 5 (70.0 mg, 0.208 mmol) and bis(penta-
fluorophenyl)borane (72.0 mg, 0.208 mmol) in n-pentane (4 mL)
and stirring at r.t. for ca. 5 min. Mesitylazide (33.8 mg, 210 mmol)
dissolved in n-pentane was added to the yellow solution, upon
which a white precipitate was formed. The solution was removed
by filter cannula, and the residue was dried in vacuo to give com-
pound 18 as a white powder (136.3 mg, 0.162, 78%). Crystals of
compound 18 suitable for the X-ray crystal structure analysis were
obtained by slow diffusion of n-pentane into a solution of com-
pound 18 in CH2Cl2 at ꢀ408C. Elemental analysis calcd for
C44H41BF10N3P: C 62.65, H 4.90, N 4.98; found: C 63.08, H 4.80,
N 5.01.
Formylborane chemistry
28: Compound 26 (354 mg, 0.335 mmol) was suspended in CH2Cl2,
(8 mL) and excess pyridine (0.15 mL, 1.86 mmol, 5.2 equiv) was
added, and the reaction mixture stirred at r.t. for 3 h. Separation of
the formylborane 28 and the FLP–pyridine adduct 8 by filtration
over silica and crystallization from a concentrated toluene/n-pen-
tane solution at ꢀ408C gave compound 28 as off-white crystalline
material (101.8 mg, 0.225 mmol, 67%).
Wittig olefination
19: Compound 18 (40.0 mg, 0.0474 mmol) was dissolved in CH2Cl2
(1.5 mL) and the solution submitted to UV irradiation for 90 h, after
which it was converted into a 1:1 mixture of phosphinimine com-
pound 19 and indazole derivative 20. The compounds were sepa-
rated by column chromatography, and crystallization from CH2Cl2
gave pure compound 19 as off-white crystalline material (25.2 mg,
0.0361 mmol, 76%). The obtained crystals were suitable for the X-
ray crystal structure analysis. Elemental analysis calcd for
C35H31BF10NP: C 60.28, H 4.48, N 2.01; found: C 60.27, H 4.61, N 2.25.
30: Compound 28 (54.2 mg, 0.120 mmol) and ethylidentriphenyl-
phosphorane (34.7 mg, 0.120 mmol) were dissolved in THF and
stirred at r.t. for 5.5 h. Purification by column chromatography
gave compound 30 as an off-white, highly viscous oil (26.5 mg,
0.0570 mmol, 48%).
Imine formation
31: Compound 28 (50.3 mg, 0.111 mmol) was dissolved in CD2Cl2
(0.7 mL), aniline (10.3 mg, 0.111 mmol) dissolved in CD2Cl2 (0.7 mL)
was added, and the reaction mixture stirred over MgSO4 for 2 days
at r.t. The suspension was filtered through a Whatman filter and
analyzed by NMR spectroscopy. Compound 31 was obtained by
crystallization from CD2Cl2/n-pentane at ꢀ408C as white crystalline
material (31.6 mg, 0.0598 mmol, 54%). The obtained crystals were
suitable for the X-ray crystal structure analysis. Elemental analysis
calcd for C24H11BF10N2: C 54.58, H 2.10, N 5.30; found: C 54.33,
H 2.05, N 5.22.
21: Compound 4 was prepared in situ by dissolving equivalent
amounts of dimesitylcyclopentenylphosphane (5) (100.0 mg,
0.297 mmol)
and
bis(pentafluorophenyl)borane
(102.5 mg,
0.296 mmol) in [D2]dichloromethane (4 mL) and stirring for ca.
5 min at r.t. The solution was cooled to ꢀ788C, the flask evacuated,
and CO gas (2 bar) pressed on it for 5 min with stirring. The stirring
was stopped, and the light yellow solution layered with n-pentane
(3 mL). The reaction mixture was kept under a CO atmosphere at
ꢀ408C. After several days product 21 was obtained as yellow crys-
talline material (110.4 mg, 0.155 mmol, 52%). The obtained crystals
were suitable for the X-ray crystal structure analysis. Elemental
analysis calcd for C36H30BF10OP: C 60.87, H 4.26; found: C 60.53,
H 4.67.
CCDC 1457831, 1457832, 1457833, 1457834, 1457835, 1457836,
1457837, 1457838, 1457839, 1457840, 1457841, 1457842, 1457843,
1457844, 1457845, 1457856, 1457847, 1457848, 1457849 and
Chem. Eur. J. 2016, 22, 1 – 14
www.chemeurj.org
11
ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
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Full Paper
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1457850 contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
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Financial support from the Deutsche Forschungsgemeinschaft
is gratefully acknowledged. T.H.W. is grateful to the US National
Science Foundation (CHE-1459090).
Keywords: boron · CO reduction · dihydrogen splitting ·
frustrated Lewis pairs · nitroxide radicals · phosphorus
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Received: August 22, 2016
Revised: September 23, 2016
Published online on && &&, 0000
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www.chemeurj.org
13
ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ

Full Paper
FULL PAPER
Poised to attack: The trans-1,2-attach-
ment of the B(C6F5)2 and PMes2 func-
tional groups at a cyclopentane frame-
work leads to a non-interacting frustrat-
ed P/B Lewis pair (see Scheme). The di-
verse reactivity of this system was stud-
ied in detail.
&
Frustrated Lewis Pairs
L.-M. Elmer, G. Kehr, C. G. Daniliuc,
M. Siedow, H. Eckert, M. Tesch, A. Studer,
K. Williams, T. H. Warren, G. Erker*
&& – &&
The Chemistry of a Non-Interacting
Vicinal Frustrated Phosphane/Borane
Lewis Pair
&
&
Chem. Eur. J. 2016, 22, 1 – 14
www.chemeurj.org
14
ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!