Synthesis, Structure and Reactivities of Pentacoordinated Phosphorus–Boron Bonded Compounds

Article abstract of DOI:10.1002/ejic.202000229

The isolation and reactivities of two pentacoordinated phosphorus–tetracoordinated boron bonded compounds were explored. A highly Lewis acidic boron reagent and electron-withdrawing ligand system were required to form the pentacoordinated phosphorus state of the P–B bond. The first compound, a phosphoranyl-trihydroborate, gave a THF stabilised phosphoranyl-borane intermediate upon a single hydride abstraction in THF. This compound could undergo a unique rearrangement, which involved a two-fold ring expansion, to give a fused bicyclic compound or it could act as a mono-hydroboration reagent. The hydroboration reactivity of the intermediate was found to be higher towards alkynes vs. alkenes, with good to moderate regioselectivity towards the terminal carbon. The second compound, a phosphoranyl-triarylborate, was found to have different reactivity as it was highly stable towards acids and bases. This is thought to be due to the large bulk around the P–B bond as shown in the crystal structure.

Full text of DOI:10.1002/ejic.202000229

A Journal of
Accepted Article
Title: Synthesis, Structure and Reactivities of Pentacoordinated
Phosphorus–Boron Bonded Compounds
Authors: Nathan James O'Brien, Naokazu Kano, Nizam Havare,
Ryohei Uematsu, Romain Ramozzi, and Keiji Morokuma
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202000229
Link to VoR: http://dx.doi.org/10.1002/ejic.202000229

10.1002/ejic.202000229
European Journal of Inorganic Chemistry
FULL PAPER
Synthesis, Structure and Reactivities of Pentacoordinated
Phosphorus–Boron Bonded Compounds
Nathan J. O’Brien,[a] Naokazu Kano,*[a,b] Nizam Havare,[a] Ryohei Uematsu,[c] Romain Ramozzi,[c] and
[c]
Keiji Morokuma†
shown to activate H2 and other small molecules[14–17] and to
Abstract: The isolation and reactivities of two pentacoordinated
coordinate to metals in a symmetric side-on fashion.[18] Moreover,
phosphorus–tetracoordinated boron bonded compounds were
explored.
A highly Lewis acidic boron reagent and electron-
phosphinoboronate
esters
(R2P–B(OR)2)
have
shown
withdrawing ligand system were required to form the
pentacoordinated phosphorus state of the P–B bond. The first
compound, a phosphoranyl-trihydroborate, gave a THF stabilised
phosphoranyl-borane intermediate upon a single hydride abstraction
in THF. This compound could undergo a unique rearrangement, which
involved a two-fold ring expansion, to give a fused bicyclic compound
or it could act as a mono-hydroboration reagent. The hydroboration
reactivity of the intermediate was found to be higher towards alkynes
versus alkenes, with good to moderate regioselectivity towards the
terminal carbon. The second compound, a phosphoranyl-triarylborate,
was found to have different reactivity as it was highly stable towards
acids and bases. This is thought to be due to the large bulk around
the P–B bond as shown in the crystal structure.
phosphinoboration reactivity to double-bond compounds and N-
heterocycles.[16,19–22] The different reactivities and properties of
phosphine-boranes, phosphino-boranes and phosphino-boronate
esters depends on the coordination number of the phosphorus
and boron atoms as well as the presence or absence of the
phosphorus lone pair and boron vacant 2p orbital.
Phosphoranes, pentacoordinated organophosphorus compounds,
have been known to show different reactivities, such as ligand
coupling,[23] from those of phosphines and phosphonium salts.[24–
26] Thus, incorporation of a pentacoordinated phosphorus atom for
P–B bonded compounds could show unique properties and
reactivities. Only a few examples of compounds bearing a P–B
bond between pentacoordinated phosphorus–tetracoordinated
boron atoms have appeared in either theoretical calculations, or
as a reaction intermediate in formation of a possible reduction
product.[9,26–32] Although these compounds have promising
application in the construction of new compounds containing
phosphorus–boron frameworks, formation of a stable P–B
bonded compound with a pentacoordinated phosphorus atom is
difficult. A boron atom of low electronegativity is insufficient for the
electron withdrawal that is required to stabilise the
pentacoordinated phosphorus atom.
Introduction
The combination of a phosphine and borane gives a phosphine-
borane (R3P–BR3) through P–B bond formation.[1–4] These
compounds have been used as protecting groups for P-chiral
tertiary phosphines and diphosphines,[5–9] as well as precursors
for P–B bond containing polymers.[10–12] More recently,
phosphino-boranes,[13] bearing the lower coordinated bond
between a tricoordinated phosphorus and a tricoordinated boron
atom (R2P–BR2), have regained attention as they have been
However, we recently reported the first examples of a stable
pentacoordinated phosphorus–tetracoordinated boron bonded
compound by isolating phosphoranyl-trihydroborates 1a (M =
K·18-crown-6) and 1b [M = (nBu)4N] (Scheme 1).[33] A pair of the
strongly electron-withdrawing C,O-bidentate ligand (-[2-
C6H4C(CF3)2O]-)[34,35] around the phosphorus atom, which has
been used to stabilise many other anionic hypercoordinated
phosphorus compounds,[36–40] contributes to the withdrawal of
electrons through the phosphorus atom for the stabilisation of
phosphoranyl-trihydroborates 1a,b. Here we report the full
investigation into pentacoordinated phosphorus–tetracoordinated
boron bonded compounds. A new phosphoranyl-borate was
synthesised, fully characterised and its structure and reactivity
explored. In addition, the reactivity of 1 was further explored,
[a]
[b]
Dr. N. J. O’Brien, Prof. N. Kano, Dr. N. Havare
Department of Chemistry, Graduate School of Science
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Prof. N. Kano
Current address: Department of Chemistry, Faculty of Science
Gakushuin University
1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588 (Japan)
E-mail: naokazu.kano@gakushuin.ac.jp
Dr. R.Uematsu, Dr. R. Ramozzi, Prof. K. Morokuma
Fukui Institute for Fundamental Chemistry
Kyoto University
[c]
Takano-Nishihiraki-cho 34-4, Sakyou-ku, Kyoto 606-8103 (Japan)
Deceased November 27, 2017.
†
Supporting Information (NMR spectral charts, X-ray crystallographic
analysis, and computational details), and the ORCID identification
number(s) for the author(s) of this article can be found under:
1
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10.1002/ejic.202000229
European Journal of Inorganic Chemistry
FULL PAPER
which showed it could act as a monohydroboration reagent with
good regioselectivity.
number may increase the steric hindrance around the phosphorus,
the appropriate combination of a borane reagent with the
phosphoranide, which is a strong base, can form the Lewis adduct
in this case. Thus, the strong Lewis acids of BH3 and B(C6F5)3 are
important in P–B bond formation despite the increase of the
coordination number. Sufficient electron withdrawing ability of
both the phosphorus and boron moieties could contribute to
stabilising the pentacoordinated state of the phosphorus atom of
phosphoranyl-borates 1a,b and 3a,b.
F3C CF3
O
H
H
H
M+
P
O
B
F3C CF3
1a M = K·18-crown-6
1b M = (nBu)4N
Scheme 1. Phosphoranyl-trihydroborates 1a and 1b.
F3C CF3
O
H
H
BH3·THF
H
M+
B
Results and Discussion
P
O
THF, r.t.
Synthesis
F3C CF3
F3C CF3
Phosphoranyl-trihydroborates 1a,b were synthesised by reacting
phosphoranide[41–43] 2a,b, respectively, with BH3·THF in THF and
were obtained in good yields (Scheme 2). Further investigation of
the reactivity of phosphoranide 2 and its ability to create a P–B
bond was undertaken through a borane reagent screen. Reaction
of 2 with boron reagents such as BPh3, BEt3, B(OMe)3, 9-
O
1a (91%)
1b (88%)
M+
P
O
F3C CF3
F3C CF3
O
2a M = K·18-crown-6
2b M = (nBu)4N
C6F5
C6F5
B(C6F5)3
M+
P
O
B
toluene/C6H6, r.t.
C6F5
borabicyclo[3.3.1]nonane,
catecholborane,
F3C CF3
bis(catecholato)diboron, pinacolborane, bis(pinacolato)diboron
and methoxypinacolborane gave no reaction and thus failed to
form a P–B bond. The failure of these reagents to form a P–B
bond with 2, in contrast with the formation of 1a,b, is most likely
due to the decreased Lewis acidity of their boron atom, which
prevents their reaction with the sterically hindered
tetracoordinated phosphorus of 2.
3a (94%)
3b (80%)
Scheme 2. Synthesis of phosphoranyl-borates 1a,b and 3a,b.
Phosphoranyl-trihydroborates
1a,b
and
phosphoranyl-
triarylborates 3a,b were isolated as colourless crystals. All the
compounds were fully characterised by X-ray crystallographic
analysis, NMR spectroscopy, mass spectrometry and elemental
analysis.
Phosphoranide 2 reacted with B-chlorocatecholborane, but gave
a mixture of hydrophosphorane, [C6H4-2-C(CF3)2O]2PH,[42,43] and
potassium
arylspiroborate
ester,
18-crown-6
(K[B(C6H4O2)2]·C12H24O6)[44,45] without formation of any P–B
bonded product (See Supporting Information, Figures S1-S3).
Moreover, reaction with BX3 (X = Cl and F), gave a mixture of an
unstable P–B bonded compound (dP = 66.3 ppm, JBP = 165 Hz; dB
X-ray Crystallographic Analysis
The crystal structure of 1b has been previously reported[33], and
the structure of 1a and 3a,b are now revealed. The X-ray
crystallographic analysis of 1a showed the existence of a P–B
bond featuring P1 and B1 atoms of trigonal bipyramidal and
tetrahedral geometries, respectively (Figure 1a). This structure of
1a is very similar to that of 1b, except an interaction between the
potassium ion, K1, and the hydrogens around the boron atom at
an average distance of 2.8 Å. An interaction with the counter-
cation was not observed for the tetrabutylammonium salt (Figure
1b) and has negligible effect on the structure as the P–B bond of
1a [1.942(2) Å] was similar to that of 1b [1.959(3) Å (1.937(3) Å
for the independent molecule in the unit cell)]. Moreover, one of
B–H bonds in both 1a and 1b is eclipsed with one of the P–O
bonds with torsion angle of 8°, regardless of the interaction.
=
2.2 ppm, JBP = 163 Hz), chlorophosphorane, [C6H4-2-
C(CF3)2O]2PCl,[46] and hydrophosphorane for BCl3 and
hydrophosphorane for BF3·OEt2 (See Supporting Information,
Figures S4-S9). The P–B bonded product was not isolated
because of its instability.
Lastly, the reagent screen revealed that phosphoranides 2a and
2b could react with B(C6F5)3 to give phosphoranyl-triarylborates
3a (M = K·18-crown-6) and 3b [M = (nBu)4N], respectively, in good
yields. Formation of phosphoranyl-borates 3a and 3b was
unexpected as the screening of the other reagents, such as BPh3,
indicated that the combination of the bulky phosphoranide anion
and B(C6F5)3 would form a frustrated Lewis pair rather than P–B
bond formation.[47,48] Although the increase of the coordination
2
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10.1002/ejic.202000229
European Journal of Inorganic Chemistry
FULL PAPER
Figure 1. ORTEPs of 1a (a) and 1b (b) with thermal ellipsoids at the 50%
probability level. One of two independent molecules in the unit cell of 1b is
shown. A molecule of benzene was omitted from the structure of 1a and 1b for
clarity. Selected bond lengths (Å) and bond angles (deg): 1a; P1–B1 1.942(2),
P1–O1 1.817(1), P1–O2 1.830(1); O1–P1–O2 171.53(5), O1–P1–B1 95.97(8),
O2–P1–B1 92.49(8). 1b; P1–B1 1.959(3), P1–O1 1.820(2), P1–O2 1.825(2);
O1–P1–O2 171.43(8), O1–P1–B1 95.34(11), O2–P1–B1 93.23(11).
Figure 2. ORTEPs of 3a (a) and 3b (b) with thermal ellipsoids at the 50%
probability level. (c) Space-filling model view of the anion of 3b. Selected bond
lengths (Å) and bond angles (deg): 3a; P1–B1 2.075(3), P1–O15 1.781(2), P1–
O17 1.812(2); O15–P1–O17 171.71(9), O15–P1–B1 97.9(1), O17–P1–B1
90.3(1). 3b; P1–B1 2.093(1), P1–O1 1.8071(9), P1–O2 1.8064(9); O1–P1–
O2 172.50(4), O1–P1–B1 89.38(5), O2–P1–B1 98.11(5).
The crystal structures of 3a and 3b are shown in Figure 2. Both
complexes 3a and 3b feature a P–B bond, in which the
phosphorus and boron atoms are penta- and tetracoordinated,
respectively. On one hand, the anion of 3a interacts with the
potassium ion, K1, to form an ion pair with a p-fluorine (F19) atom
of a C6F5 ring with a distance of 2.908(2) Å (3.040(2) Å for the
molecule in the unit cell) (Figure 2a). On the other hand, the anion
of 3b shows no contact with the tetrabutylammonium ion (Figure
2b). The P1 atom is bonded to one boron and two carbon atoms
in the equatorial positions, and two oxygens in the apical positions.
The phosphorus coordination polyhedral for 3b is described as a
distorted trigonal bipyramid [O–P–O angle, 172.50(5)°], whilst the
boron coordination polyhedral is described as a distorted
tetrahedral [C–B–C av. 110.0(4) Å]. The P–B bond length of 3b
[2.075(3)Å] is shorter to that of Ph3P–B(C6F5)3 [2.180(6) Å],[49]
showing that the pentacoordinated state of the phosphorus could
shorten the P–B bond length. Conversely, it is considerably longer
than that of 1b [1.959(3)Å],[33] reflecting the increased bulk around
the P–B bond. The P–O bond lengths [1.806(1)-1.807(1) Å] are
considerably longer than those found in the hydroxyphosphorane
[1.7059(18)-1.7421(18) Å],[50] but similar to those of 1b
The crystal structures of 3a and 3b were found to be in the O-
apical pseudorotamer. This pseudorotamer is thought to be more
stable and thus favoured due to high electronegativity of oxygen
and the large steric bulk around the P–B bond, which can be
observed in the space-filling model of the anion of 3b (Figure 2c).
This B-equatorial conformation and steric bulk led to one of the
C6F5 rings within close proximity to an apical oxygen. As a result,
one of the quaternary carbons (C31) of the C6F5 rings is eclipsed
with the apical oxygen (O2) bonded to the phosphorus with a
torsion angle of 0.4°. This eclipsed conformation is also found as
a key feature in the crystal structure of 3a.
NMR Spectroscopy
The structure of 3 in solution state was consistent with its crystal
structures. The 1H NMR spectra of 3b in THF-d8 showed the ortho
proton signal of the benzene ring of the ligand in a low-field region
of 8.71 ppm. This reflects the proximity and deshielding effect of
the apical bond of the trigonal bipyramidal structure, indicating the
C-equatorial conformation.[41] The 19F NMR showed three
quartets and one multiplet peaks for the CF3 groups on the two
sets of bidentate ligands. This result shows that 3a is
predominately the O-apical pseudorotamer in solution state, as
found in the crystal structure, due to the lack of pseudorotation of
the bidentate ligand groups around the pentacoordinated
phosphorus atom.[52] This is thought to be due to the large
equatorial bulk of the B(C6F5)3 moiety, which hinders rotation of
the bidentate ligands.
[1.8102(17)-1.8406(17) Å] and
a
phosphoranide-palladium
complex, [(2-C6H4C(CF3)2O)2P–Pd(PhCHO)(dppe)](SbF6), [av.
1.780(3), 1.851(3) Å].[51] The long P–O bond lengths indicated
localisation of substantially negative charges on the oxygen
atoms and weakening of the polar P–O bonds.
3
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10.1002/ejic.202000229
European Journal of Inorganic Chemistry
FULL PAPER
The lack of pseudorotation was also observed with 1a, even when
analysed at high temperatures (+30 °C to +60 °C) (See
Supporting Information, Figure S38), which showed a pair of
quartets for the CF3 groups in the 19F NMR spectra. This was
easily seen in the variable temperature 19F NMR analysis, in
which the two quartet peaks for each of the CF3 groups remained
unchanged until +60 °C. At lower temperatures (–45 to –80 °C),
only a little peak broadening, which is due to slow rotation of the
CF3 groups around the adjacent carbon at low temperatures, was
observed (See Supporting Information, Figure S39).
charges of the oxygen (–0.83) and carbon (–0.35 and –0.34)
atoms suggest that the P–O and P–C bonds are more polarised
than the P–B bond. All three carbon atoms on the boron show
negative natural charges (–0.35, –0.36 and –0.38) as a result of
their bonding to a less electronegative boron atom.
The HOMO is localised on the s orbital of the P–B bond and p
orbitals of the C6F5 aromatic rings (Figure 3a), whilst the LUMO is
localised mainly on the p* orbitals of the aromatic rings of the
bidentate ligand (Figure 3b). The contribution of the s orbital of
the P–B bond is small in the frontier orbitals, corresponding to the
stability of compounds 3a,b.
The non-decoupled 11B NMR spectrum gave a doublet peak at dB
= –10.3 ppm with a coupling to the phosphorus atom of 1JBP = 156
Hz. The chemical shift of the doublet peak is in an upfield shift
from that of Ph3P–B(C6F5)3 (–2.5 ppm).[49] The coupling constant
1JBP of 3b is larger than that of Ph3P–B(C6F5)3 (1JBP = 110 Hz) and
1a (1JBP = 69 Hz)[33], reflecting the large s-character of the P–B
bond featuring the trigonal bipyramidal phosphorus.[53] The
proton-decoupled 31P NMR spectrum of 3b showed a broad
quartet at dP –15.6 ppm, which is in a similar region compared to
that of hydroxyphosphorane, [C6H4-2-C(CF3)2O]2POH, (dP –17.6
ppm)[36] and chlorophosphorane, [C6H4-2-C(CF3)2O]2PCl, (dP –7.6
ppm).[46] However, it is somewhat a high-field shift to that of Ph3P–
B(C6F5)3 (dP –2.5 ppm),[49] reflecting the pentacoordinated
phosphorus state.
Anions 1 and 3 were further investigated by Bader’s Atoms in
Molecules (AIM) analysis[55] by mapping the topology of electron
density (ꢀ(r)). The bond critical points (BCPs) for the P–B bond of
anion 1 gave ꢀ(r) and –∇2ꢀ(r) values of 0.1087 e/bohr3 and 0.0151
e/bohr5, respectively, confirming its ionic character. However,
analysis of anion 3 showed that ꢀ(r) was 0.0956 e/bohr3 and –
∇2ꢀ(r) –0.151 e/bohr5, indicating the bond is of more covalent
character. The difference between the two bonds reflects the
stronger electron-withdrawing ability of the C6F5 groups on the
boron atom of 3 than the hydrogens on the boron atom of 1, and
suggests that the B(C6F5)3 moiety helps to decrease polarisation
of the P–B bond. Interestingly, bond paths were found between
fluorine atoms of each CF3 group with hydrogens on the boron for
1 (See Supporting Information, Figure S41). In addition, bond
paths were also observed for anion 3 between some fluorine
atoms of the CF3 groups and carbon atoms of C6F5 rings. These
interactions, each of which is very weak, could help the previously
mentioned eclipsed feature of a P–O bond with a B–H or B–C
bond observed in the X-ray crystal structure.
DFT Calculations
The density functional theory (DFT) calculations for the anion of
3, at B3PW91/6-31+G(d,p) level, were conducted to evaluate the
bond properties and molecular orbitals. The heterolytic bond
dissociation energy of the P–B bond of anion 3 was calculated to
be 18.1 kcal mol–1, which is almost the same as that of Me3P–
B(C6F5)3 (17.8 kcal mol–1), but smaller than that of the anion of 1
(33.8 kcal mol–1).[33] Moreover, it was found to be smaller than the
homolytic bond dissociation energy of the P–B bond (32.1 kcal
mol–1), suggesting preference of heterolysis of the P–B bond in 3.
Hydride Abstraction
It was found that a single hydride abstraction of phosphoranyl-
trihydroborate 1a with tropylium tetrafluoroborate would lead to
THF stabilised intermediate 4 (Scheme 3), which was confirmed
by trapping with pyridine.[33] Additionally, it was discovered that
intermediate 4 could rearrange to form fused bicyclic compound
5 through a unique double ring expansion rearrangement reaction
after solvent exchange from THF to Et2O. It is now revealed that
direct access to 5 can be achieved in good yield by treatment of
1a with methyl triflate in toluene at room temperature. The lack of
a coordination solvent allows the rearrangement to occur
spontaneously upon hydride abstraction accompanied by the
reduction of methyl triflate. It should be mentioned that the
coordinatively stabilised phosphine is readily accessible starting
from the corresponding phosphorane.[56]
Figure 3. The HOMO (–4.51 eV; a) and LUMO (+ 0.70 eV; b) of the anion of 3.
The natural charges of the phosphorus (+1.64) and boron (+0.19)
atoms of 3, which were obtained by natural population analysis,[54]
indicate a highly polarised P–B bond character. The natural
4
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10.1002/ejic.202000229
European Journal of Inorganic Chemistry
FULL PAPER
F3C
F3C CF3
BF4
the phosphorus to the boron atom gives the bicyclic ring structure
of 6. It was observed that only mono-hydroboration occurred even
though there is a second hydrogen atom on the borane moiety of
4 and a large excess of the substrate was used. This result shows
the rearrangement to give the fused bicyclic product 6, which is
accompanied with a double ring expansion, is faster than
secondary hydroboration.
CF3
CF3
O
H
H
O
O
1a
P
O
B
H
P
B
(20 eq.)
THF, r.t.
O
H
F3C CF3
F3C
6 (64%)
4
Et2O
F3C
F3C CF3
F3C
CF3
CF3
O
O
O
B
H
MeOTf
4
P
O
B
P
O
B
H
P
H
toluene, r.t.
H
H
O
H
CF3
F3C CF3
F3C CF3
F3C
5 (84%)
7
8
F3C
F3C
CF3
O
CF3
CF3
Scheme 3. Reactions of phosphoranyl-borate 1a.
O
H
P
B
H
P
B
O
O
H
H
Hydroboration Reactivity
F3C CF3
F3C
Intermediate 4 was also shown to effectively mono-hydroborate
2-butyne, even though a large excess was used, to give bicyclic
compound 6 (Scheme 3). The X-ray crystal structure of 6 showed
9
6
Scheme 4. Proposed reaction mechanism for the formation of 6 from 4 and 2-
butyne.
a
similar structure to the previously synthesised bicyclic
Conversion of 6 to the boronic acid ester as an isolable form was
thought to be a promising strategy because some boronic acid
esters have been utilised for Suzuki-Miyaura cross-coupling
reactions.[57,58] Treatment of 6 with H2O was found to cleave its P–
B bond to give (Z)-2-butene boronic acid (dB = +28.2 ppm), which
matched the literature value.[59] The boronic acid was successfully
converted to the corresponding N-methyliminodiacetic acid ester
(MIDA) to give 10, which was isolated in 60% yield after silica gel
column chromatography (Table 1, entry 1). To further explore the
hydroboration reactivity of 4, a study was undertaken on various
alkynes and alkenes. It was found to effectively hydroborate
terminal alkynes, 1-heptyne and ethynylbenzene, to give the
corresponding MIDA esters 11 and 12 in yields of 68 and 62%,
respectively, after purification (entries 2-3). The hydroboration
regioselectivity of 4 for 1-heptyne and ethynylbenzene was
predominately for the terminal carbon, but 30% and 40% of the
two-isomer was obtained for each alkyne, respectively. When 4
was reacted with terminal alkenes, 1-hexene and styrene, the
corresponding MIDA ester products 13 and 14 were isolated in
slightly lower yields of 49 and 38%, respectively, even after longer
reaction time (entries 4-5). These results show that 4 has higher
reactivity towards alkynes over alkenes. Although the overall yield
of the hydroboration was lower for alkenes, the regioselectivity
was improved giving 75% and 90% of the 1-isomer for 1-hexene
and 1-styrene, respectively. Hydroboration with 2,3-dimethyl-2-
butene, an internal alkene, was also undertaken, but
unfortunately, only ~5% of hydroborated bicyclic P–B
compound 5 (Figure 4). However, instead of a H–P–B–H linkage,
a H–P–B–(but-2-ene) linkage with a synclinal conformation
[torsion angle, 42.3°] was observed. The 2-butene moiety was
also found to be in a (Z)-conformation. The P–B bond of 6 [P–B
2.004(5) Å] was slightly longer than 5 [P–B 1.972(3) Å], reflecting
the decrease in electron density of the substituted boron atom.
Figure 4. ORTEPs of 5 (a) and 6 (b) with thermal ellipsoids at the 50%
probability level. Selected bond lengths (Å), bond angles (deg) and torsion
angles (deg): 5; P1–B1 1.971(3), P1–H1 1.33(2), B1–O1 1.469(3), B1–O2
1.441(3), B1–H2 1.09(2); C1–P1–H1 105(1), C10–P1–H1 107(1), O1–B1–
H2 116(1), O2–B1–H2 111(1); H1–P1–B1–H2 –47(2). 6; P1–B1 2.004(5), P1–
H1 1.34(6), B1–O1 1.477(5), B1–O2 1.466(5); C1–P1–H1 105(2), C10–P1–
H1 104(2), O1–B1–C19 121.4(3), O2–B1–C19 109.9(3); H1–P1–B1–C19 42(3).
The mechanism for the formation of 6 from the reaction of 4 and
2-butyne is thought to first undergo a regular hydroboration
reaction to give 7 (Scheme 4). Then, an oxygen shift occurs from
the phosphorus to the boron atom to give intermediate 8, which is
quickly followed by a hydride shift from the boron to the
phosphorus atom to give 9. Finally, a second oxygen shift from
5
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10.1002/ejic.202000229
European Journal of Inorganic Chemistry
FULL PAPER
intermediate could be observed by 19F NMR spectroscopy after
hydrophosphorane in solution over time. Considering that 1,3-
dimethylimidazol-2-ylidene borane shows similar hydroboration
reactivity,[69,70] the effect of the phosphoranide on the reactivity of
the borane moiety is comparable to that of a N-heterocyclic
carbene (NHC).[71] It should be noted that BH3·THF, thexylborane
and NHC-BH3 compounds can undergo secondary hydroboration,
which highlights the advantages of this reported mono-
hydroboration strategy.
72 hours (entry 6).
Table 1. Hydroboration of alkynes and alkenes using intermediate 4.
F3C
N
CF3
O
substrate
(20 eq.)
1. H2O
4
H
P
B
R
B
R
O
O
2. MIDA
O
O
O
CF3
= B—R (10-14)
F3C
Reaction
Time
Entry
1
Substrate
Product(s)
Ratio[a]
Yield
Stability
Phosphoranyl-trihydroborates
1a,b
and
phosphoranyl-
B
H
24 h
24 h
-
60%
triarylborates 3a,b are thermally stable at room temperature and
could be handled in air without precautions. It makes remarkable
contrast with the previously reported intermediary generated
pentacoordinated phosphorus–boron bonded species.[30–32] The
impressive stability of these new compounds is ascribed to the
stabilising effect of the electron-withdrawing bidentate ligand on
the phosphorus atom. Surprisingly, 3a showed persistence
towards base hydrolysis and alcoholysis even though it
possesses a long P–B bond. It was also stable towards
diisopropyl- and diethylamine, which are known to cleave P–B
bonds of phosphine-boranes.[5–8] However, 3a was found to have
good stability towards acid, since treatment with 1M hydrochloric
acid would give hydrophosphorane[42,43], quantitatively, after 4
days at room temperature. This is different to 1a as it reacted with
1M HCl instantaneously due to its hydride reactivity. Further
studies of 3a with tetrabutylammonium fluoride, methyl iodide and
methyl triflate gave no reaction. The stability of 3a could be
explained by the large bulk around the P–B bond (Figure 2c).
10
B
H
B
H
2
7:3
68%[b,c]
11a
11b
B
H
H
H
B
H
3
4
24 h
48 h
3:2
3:1
62%
49%
12a
12b
B
B
H
13b
13a
B
B
H
5
6
48 h
72 h
9:1
-
38%
14a
14b
-
<5%[d]
[a] Regioisomer ratio determined by 1H-NMR spectroscopy. [b] Combined yield of
Conclusion
[c]
the mixture of isomers.
Regio-isomers separated by flash column
chromatography. [d] Only 5% of hydroborated phosphonium borate intermediate
was observed by 19F NMR spectroscopy and conversion to the MIDA ester was
therefore not undertaken.
In summary, we isolated and explored the reactivities of two
pentacoordinated phosphorus–tetracoordinated boron bonded
compounds. This study revealed that a strong Lewis acidic boron
reagent and electron-withdrawing ligand system around the
pentacoordinated phosphorus was required to form the P–B bond
The decreased hydroboration reactivity of 4 towards alkenes is
similar to that of pinacol-[60,61] and catecholborane,[62–65] both of
which are known to react faster with alkynes over alkenes.[60,66]
Overall, hydroboration of the less substituted carbon was
favoured in all examples due to the increased bulk around the
pentacoordinated phosphorus, which provides some steric to the
boron atom. The regioselectivity of 4 is similar to that observed
with BH3·THF and other mono-substituted boranes such as
thexylborane,[67,68] however its rate of hydroboration is
considerably slower and the yield of the products are lower. In
addition, a large excess of the substrate was required to maximise
the yield of the hydroboration reaction as 4 slowly decomposes to
and stabilise the phosphoranyl-borates. Compound 1,
a
phosphoranyl-trihydroborate, gave phosphoranyl-borane
intermediate 4 upon a single hydride abstraction. This compound
could undergo a unique rearrangement reaction, that involved a
two-fold ring expansion, to give 5 or it could act as a mono-
hydroboration reagent. The hydroboration reactivity of 4 was
found to be more reactive towards alkynes over alkenes with good
to moderate regioselectivity towards the terminal carbon.
Compound 3, a phosphoranyl-triarylborate, was found to have a
long P–B bond and a vastly different reactivity to 1 as it was highly
stable towards acids and bases. This was due to the large bulk
6
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10.1002/ejic.202000229
European Journal of Inorganic Chemistry
FULL PAPER
around the P–B bond as shown in the crystal structure. The
potential impact of these compounds for P–B bond-containing
polymers are promising and further studies are currently
underway.
3a: Colourless crystals, m.p. 154-159 °C (decomp.). 1H NMR (300
MHz, THF-d8): d 3.59 (s, 24H, CH2), 7.08–7.28 (m, 2H, CH), 7.32–
1
7.63 (m, 5H, CH), 8.70 (t, JHH = 9.5 Hz, 1H, CH). 11B NMR (96
MHz, THF-d8): d –10.2 (d, JPB = 156 Hz). The 13C{1H} NMR data
could not be obtained to a satisfactory standard due to poor
solubility in THF-d8 or Acetonitrile-d3. 19F NMR (282 MHz, THF-
d8): d –171.79 to –172.09 (m, 1F), –171.46 to –171.48 (m, 1F), –
170.94 to –171.51 (m, 3F), –170.20 to –170.50 (m, 1F), –167.15
(t, JFF = 20.4 Hz, 1F), –166.94 (t, JFF = 20.4 Hz, 1F), –166.32 (td,
JFF = 20.4, 8.5 Hz, 1F), –133.31 to –133.81 (m, 1F), –130.62 to –
131.00 (m, 1F), –126.75 to –127.18 (m, 1F), –123.89 to –124.68
(m, 1F), –122.71 to –123.28 (m, 1F), –121.56 (dd, JFF = 44.4, 23.3
Hz, 1F), –76.31 to –76.80 (m, 6F), –74.76 (q, 4JFF = 11.0 Hz, 3F),
–74.43 (q, 4JFF = 11.0 Hz, 3F). 31P NMR (122 MHz, THF-d8): d –
13.7 (q, JPB = 147 Hz). 31P {1H} NMR (122 MHz, THF-d8): d –13.7
(q, JPB = 147 Hz). LRMS (ESI, negative) m/z = 1027 ([M –
C12H24KO6]–). Anal. Calcd for C48H32BF27KO8P+1.0THF: C, 44.53;
H, 2.87. Found: C, 44.16; H, 3.04.
Experimental Section
General Remarks
Solvents were pre-dried and purified before use by an MBRAUN
MB-SPS solvent purification system and further dried with
molecular sieves 4Å. All reactions were carried out under an
argon atmosphere. All NMR spectra were measured with Bruker
AV300 spectrometer. Solvents peaks were used as an internal
standard for the 1H NMR (300 MHz) and 13C NMR (75 MHz)
spectra. F3B·OEt2, CFCl3 and H3PO4 were used as external
standards for the 11B NMR (96.3 MHz), 19F NMR (282 MHz) and
31P NMR (121 MHz) spectra, respectively. The ESI-mass spectral
data were obtained on Thermo Scientific Exactive mass
spectrometer. Melting points were recorded with a Yanaco MP-
500D micro melting point apparatus and were uncorrected.
Elemental analysis was performed by the Microanalytical
Laboratory from the Department of Chemistry, Faculty of Science,
The University of Tokyo. Phosphoranides were prepared from
hydrophosphorane, [C6H4-2-C(CF3)2O]2PH, according to the
procedure in the literature.[42,43] Phosphoranyl-trihydroborates
were prepared according to our previous reported studies.[33]
Synthesis of Tetrabutylammonium Phosphoranyl-borate (3b)
Tetrabutylammonium phosphoranyl-borate 3b was synthesised
from tetrabutylammonium phosphoranide 2b (0.50 g, 0.66 mmol)
similarly to the procedure for 3a, but using C6H6 instead of toluene
as the reaction solvent. The crude product was recrystallised from
THF/Et2O at –35 °C to give 3b in (0.84 g, 80%) as a colourless
solid.
.
Tris(pentafluorophenyl)borane and BH3 THF were purchased
3b: Colourless crystals, m.p. 135-137 °C (decomp.). 1H NMR (300
MHz, THF-d8): d 0.99 (t, J = 7.4 Hz, 12H, CH3), 1.32–1.47 (m, 8H,
CH2), 1.60–1.76 (m, 8H, CH2), 3.18–3.29 (m, 8H, CH2), 7.10–7.27
(m, 2H, CH), 7.34–7.62 (m, 5H, CH), 8.71 (t, J = 9.3 Hz, 1H, CH).
from TCI Chemicals and Sigma-Aldrich, respectively, and used
without further purification. All alkynes and alkenes used in
hydroboration reactions were freshly distilled prior to use except
for 2-butyne, which was passed through a bed of basic alumina.
All isomer ratios for hydroboration reactions were determined by
1H NMR spectroscopy.
1
11B NMR (96.3 MHz, THF-d8): d –10.3 (d, JBP = 156 Hz). The
13C{1H} NMR data could not be obtained due to poor solubility in
THF-d8 or Acetonitrile-d3. 19F NMR (282 MHz, THF-d8): d –171.83
to –172.10 (m, 1F), –171.52 to –171.78 (m, 1F), –170.94 to –
171.51 (m, 3F), –170.22 to –170.49 (m, 1F), –167.15 (t, JFF = 20.2
Hz, 1F), –166.94 (t, JFF = 20.6 Hz, 1F), –166.32 (td, JFF = 20.6,
8.5 Hz, 1F), –133.33 to –133.78 (m, 1F), –130.62 to –130.98 (m,
1F), –126.79 to -127.17 (m, 1F), –123.90 to –124.67 (m, 1F), –
122.78 to –123.25 (m, 1F), –121.36 to –121.78 (m, 1F), –76.32
to –76.79 (m, 6F), –74.76 (q, 4JFF = 11.2 Hz, 3F), –74.43 (q, 4JFF
= 11.2 Hz, 3F). 31P NMR (122 MHz, THF-d8): d –15.6 (q, JPB = 146
Hz). 31P{1H} NMR (122 MHz, THF-d8): d –15.6 (q, JPB = 145 Hz).
MS (ESI, negative) m/z 1027 ([M – N(C4H9)4]–). Anal. Calcd for
C52H44BF27NO2P: C, 49.19; H, 3.49; N, 1.10. Found: C, 49.23; H,
3.64; N, 1.14.
Synthesis of Potassium Phosphoranyl-borate (3a)
To a suspension of potassium phosphoranide 2a (0.50 g, 0.61
mmol) in toluene (10 mL) was added a solution of B(C6F5)3 (0.31
g, 0.61 mmol) in toluene (10 mL) and the mixture was stirred at
room temperature for 1 h. The reaction mixture was concentrated
under reduced pressure and the resulting solid was dissolved in
THF, filtered through a pad of Celite and concentrated under
reduced pressure to give a solid. The solid was recrystallised from
THF/hexanes at –35 °C to give a 1:1 solvate of potassium
phosphoranyl-borate 18-crown-6 3a with a molecule of THF (0.80
g, 94%) as a colourless solid. The inclusion of a THF molecule in
the solid was confirmed by the 1H-NMR spectroscopy, X-ray
crystallography and elemental analysis.
7
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10.1002/ejic.202000229
European Journal of Inorganic Chemistry
FULL PAPER
11a, 11b: Colourless solid. 1H NMR (300 MHz, CDCl3): 11a d 0.87
(t, J = 6.8 Hz, 3H, CH3), 1.17–1.54 (m, 6H, CH2), 2.11 (q, J = 6.5
Hz, 2H, CH2), 2.82 (s, 3H, NCH3), 3.69 (d, J = 16.8 Hz, 2H, CH2),
4.02 (d, J = 16.8 Hz, 2H, CH2), 5.37 (d, J = 17.6 Hz, 1H, CH), 6.16
(dt, J = 17.6 Hz, 1H, CH); 11b d 0.87 (t, J = 6.8 Hz, 3H, CH3),
1.17–1.54 (m, 6H, CH2), 1.98 (t, J = 7.4 Hz, 2H, CH2), 2.81 (s, 3H,
NCH3), 3.74 (d, J = 16.8 Hz, 2H, CH2), 4.04 (d, J = 16.8 Hz, 2H,
CH2), 5.34 (br s, 1H, CH), 5.40 (br s, 1H, CH). 11B NMR (96.3 MHz,
CDCl3): d 12.6 (br s). 13C{1H} NMR (75 MHz, CDCl3): 11a d 14.1
(s, CH3), 22.6 (s, CH2), 28.5 (s, CH2), 31.6 (s, CH2), 35.6 (s, CH2),
47.0 (s, CH3), 61.5 (s, CH2), 123.6 (br s, CH), 148.0 (s, CH), 168.5
(s, Cq); 11b d 14.2 (s, CH3), 22.7 (s, CH2), 28.5 (s, CH2), 31.9 (s,
CH2), 34.7 (s, CH2), 46.9 (s, CH3), 61.9 (s, CH2), 123.9 (s, CH2),
168.6 (s, Cq). The carbon connected to the boron atom was not
observed for 11b due to quadrupolar line broadening. MS (ESI,
positive) m/z 254.1 ([M + H]+).
Synthesis of Phosphonium-borate 5 by Treatment of 1a with
Methyl Triflate
To a solution of 1a (0.50 g, 0.59 mmol) in toluene (20 mL) was
added methyl triflate (65 µL, 0.59 mmol) and the reaction mixture
was stirred at room temperature for 1 hour, then concentrated
under reduced pressure to give a solid. The solid was dissolved
in Et2O, filtered through a bed of Celite and concentrated under
reduced pressure to give a solid. The solid was recrystallised from
Et2O/hexanes to give compound 5 as a colourless solid (0.26 g,
84%). The spectra obtained on the product matched those
previously reported.[33]
General Procedure for Hydroboration of Alkynes and
Alkenes with 4
To a THF solution of 4, prepared from 1a (250 mg, 0.29 mmol) in
THF (15 mL) according to the previously reported procedure[33]
,
was added freshly distilled alkene/alkyne (20 eq.) and the reaction
mixture was stirred for 48 h at room temperature. The reaction
mixture was diluted with H2O (20 mL) and extracted with CHCl3
(x3). The combined organic extracts were washed with H2O, dried
over MgSO4 and concentrated under reduced pressure to give a
residue. The residue was dissolved in a toluene/DMSO mixture
(10:1, 20 mL) and N-methyliminodiacetic acid (44 mg, 0.3 mmol)
was added. The mixture was refluxed with a Dean–Stark
apparatus attached under N2 for 16 hours, then cooled to room
temperature. The solvent was removed under reduced pressure
and the resulting residue was purified by flash chromatography
(0-20% MeCN/Et2O) to give a colourless solid of compounds 10-
14.
(E)-Styryl MIDA boronate (12a) and 1-phenylvinyl MIDA
boronate (12b). Compound 4 was treated with phenylacetylene
(0.59 g, 5.8 mmol) according to the general procedure to give 12a
(28 mg, 36%) and 12b (20 mg, 26%) as solids.
12a: Colourless solid. 1H NMR (300 MHz, Acetone-d6): d 3.06 (s,
3H, CH3), 4.09 (d, J = 16.8 Hz, 2H, CH2), 4.26 (d, J = 16.8 Hz, 2H,
CH2), 6.35 (d, J = 18.2 Hz, 1H, CH), 6.96 (d, J = 18.2 Hz, 1H, CH),
7.21–7.38 (m, 3H), 7.48–7.55 (m, 2H). 11B NMR (96.3 MHz,
Acetone-d6): d 11.0 (br s). 13C{1H} NMR (75 MHz, Acetone-d6): d
47.4 (s, CH3), 62.4 (s, CH2) 127.5 (s, CH), 128.7 (s, CH), 129.4 (s,
CH), 139.2 (s, Cq), 142.9 (s, CH), 169.1 (s, Cq). The carbon
connected to the boron atom was not observed for 12a due to
quadrupolar line broadening. MS (ESI, positive) m/z 260.1 ([M +
H]+). Spectral data for 12a were consistent with those previously
reported in the literature.[72]
(Z)-But-2-en-2-yl MIDA boronate (10). Compound 4 was treated
with 2-butyne (0.31 g, 5.8 mmol) according to the general
procedure to give 10 as a solid (38 mg, 60%).
12b: Colourless solid. 1H NMR (300 MHz, Acetone-d6): d 2.82 (s,
3H, CH3), 3.84 (d, J = 16.8 Hz, 2H, CH2), 4.22 (d, J = 16.8 Hz, 2H,
CH2), 5.69 (br s, 1H, CH), 5.73 (br d, J = 3.0 Hz, 1H, CH), 7.19–
7.36 (m, 3H), 7.37–7.46 (m, 2H). 11B NMR (96.3 MHz, Acetone-
d6): d 10.7 (br s). 13C{1H} NMR (75 MHz, Acetone-d6): d 47.7 (s,
CH3), 62.8 (s, CH2) 127.4 (s, CH), 128.1 (s, CH2), 128.4 (s, CH),
129.2 (s, CH), 145.4 (s, Cq), 168.9 (s, Cq). The carbon connected
to the boron atom was not observed for 12b due to quadrupolar
line broadening. MS (ESI, positive) m/z 260.1 ([M + H]+). Spectral
data for 12b were consistent with those previously reported in the
literature.[73]
1
10: Colourless solid. H NMR (300 MHz, Acetone-d6): d 1.62 (s,
3H, CH3), 1.66 (d, J = 6.5 Hz, 3H, CH3), 2.94 (s, NCH3), 3.99 (d,
J = 16.8 Hz, 2H, CH2), 4.17 (d, J = 16.8 Hz, 2H, CH2), 5.91 (br q,
J = 6.5 Hz, 1H, CH). 11B{1H} NMR (96.3 MHz, Acetone-d6): d 11.1
(s). 13C{1H} NMR (75 MHz, Acetone-d6): d 14.1 (s, CH3), 14.3 (s,
CH3), 47.0 (s, CH3), 62.5 (CH2), 133.4 (CH), 169.3 (Cq). MS (ESI,
positive) m/z 212.1 ([M + H]+).
(E)-Hept-1-en-1-yl MIDA boronate (11a) and (E)-hept-1-en-2-yl
MIDA boronate (11b). Compound 4 was treated with 1-heptyne
(0.56 g, 5.8 mmol) according to the general procedure to give 11a
and 11b as a solid that contained an inseparable mixture of
isomers (7:3 ratio, 52 mg, 68%).
Hexan-1-yl MIDA boronate (13a) and hexan-2-yl MIDA
boronate (13b). Compound 4 was treated with 1-hexene (0.49 g,
5.8 mmol) according to the general procedure to give 13a and 13b
8
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10.1002/ejic.202000229
European Journal of Inorganic Chemistry
FULL PAPER
as a solid that contained an inseparable mixture of isomers (3:1
graphite-monochromated Mo-Ka radiation. Data was collected
and processed using CrystalClear (Rigaku). The data was
corrected for Lorentz and polarisation effects. The structures were
solved by direct methods (SHELXS-97) and expanded using
Fourier techniques.[74] The non-hydrogen atoms were refined
anisotropically. Hydrogen atoms on the boron atom of 1a and on
the phosphorus atom of 6 were assigned by Fourier method and
refined isotropically. All other hydrogen atoms were assigned by
calculation and refined isotropically by using a riding model.
Selected crystal data are summarised below.
ratio, 36 mg, 49%).
13a, 13b: Colourless solid. 1H NMR (300 MHz, Acetone-d6): 13a
d 0.63 (br t, J = 7.5 Hz, 2H, CH2), 0.88 (t, J = 7.0 Hz, 3H, CH3),
1.22–1.42 (m, 8H, CH2), 3.07 (s, 3H, NCH3), 3.99 (d, J = 16.8 Hz,
2H, CH2), 4.16 (d, J = 16.8 Hz, 2H, CH2); 13b d 0.84–0.93 (m, 7H,
2 x CH3, CH), 1.22–1.42 (m, 6H, CH2), 3.13 (s, 3H, NCH3), 4.01
(d, J = 16.8 Hz, 2H, CH2), 4.17 (d, J = 16.8 Hz, 2H, CH2). 11B NMR
(96.3 MHz, Acetone-d6): d 13.2 (br s). 13C{1H} NMR (75 MHz,
Acetone-d6): 13a d 14.4 (s, CH3), 23.3 (s, CH2), 25.0 (s, CH2), 32.5
1a: C30H35BF12KO8P·C6H6, M = 910.57, monoclinic, space group
P21/n, a = 8.400(3), b = 26.682(9), c = 18.660(6) Å, b = 92.259(3)°,
V = 4179(2) Å3, Z = 4, Dc = 1.447 g cm–3, µ = 0.266 mm–1, T =
133(2) K, 9506 reflections measured, 7824 unique, final R1 (I >
2s(I)) = 0.0393, wR2 (all data) = 0.0960, GOF = 1.060.
3a: C52H40BF27KO9P, M = 1402.72, triclinic, space group P–1, a =
14.8793(19), b = 18.318(2), c = 21.841(3) Å, a = 71.928(3), b =
78.948(4), g = 88.026(5)°, V = 5552.2(12) Å3, Z = 4, Dc = 1.678 g
cm–3, µ = 0.272 mm–1, T = 113(2) K, 24354 reflections measured,
18657 unique, final R1 (I > 2s(I)) = 0.0584, wR2 (all data) =
0.1551, GOF = 1.083.
(s, CH2), 33.4 (s, CH2), 46.2 (s, CH3), 62.7 (s, CH2), 63.4 (d, JBC
=
9.0 Hz, CH2), 168.8 (Cq); 13b d 14.5 (s, CH3), 14.7 (s, CH3), 23.7
(s, CH2), 31.2 (s, CH2), 32.7 (s, CH2), 46.2 (s, CH3), 62.7 (s, CH2),
168.8 (Cq). The carbon connected to the boron atom was not
observed for 13b due to quadrupolar line broadening. MS (ESI,
positive) m/z 242.1 ([M + H]+).
Phenethyl MIDA boronate (14a) and 1-phenylethyl MIDA
boronate (14b). Compound 4 was treated with styrene (0.60 g,
5.8 mmol) according to the general procedure to give 14a and 14b
as a solid that contained an inseparable mixture of isomers (9:1
ratio, 52 mg, 38%).
3b: C36H8BF27O2P·C16H36N, M = 1269.66, triclinic, space group
P–1, a = 11.0027(12), b = 13.1239(14), c = 18.828(2) Å, a =
95.2712(14)°, b = 101.9873(12)°, g = 98.0208(15)°, V = 2612.8(5)
Å3, Z = 2, Dc = 1.614 g cm–3, µ = 0.193 mm–1, T = 133(2) K, 11312
reflections measured, 9900 unique, final R1 (I > 2s(I)) = 0.0349,
wR2 (all data) = 0.0913, GOF = 1.032.
14a, 14b: Colourless solid. 1H NMR (300 MHz, Acetone-d6): 14a
d 0.92–1.05 (m, 2H, CH2), 2.62–2.72 (m, 2H, CH2), 3.12 (s, 3H,
NCH3), 4.05 (d, J = 16.8 Hz, 2H, CH2), 4.22 (d, J = 16.8 Hz, 2H,
CH2), 7.06–7.18 (m, 1H, CH), 7.19–7.34 (m, 4H, CH). 14b d 0.82–
0.92 (m, 3H, CH3), 3.01 (s, 3H, NCH3), 4.05 (d, J = 16.8 Hz, 2H,
CH2), 4.18 (d, J = 16.8 Hz, 2H, CH2), 7.06–7.18 (m, 1H, CH),
7.19–7.34 (m, 4H, CH). 11B NMR (96.3 MHz, Acetone-d6): d 12.9
(br s). 13C{1H} NMR (300 MHz, Acetone-d6): 14a d 31.2 (s, CH2),
46.4 (s, CH3), 62.8 (s, CH2), 63.3 (d, JBC = 8.4 Hz, CH2), 126.1 (s,
CH), 128.7 (s, CH), 129.1 (s, CH), 146.4 (s, Cq) 168.9 (s, Cq); 14b
d 19.0 (s, CH3), 46.3 (s, CH3), 62.8 (s, CH2), 125.9 (s, CH), 129.2
(s, CH), 129.3 (s, CH), 168.9 (s, Cq). MS (ESI, negative) m/z 262.1.
The CH peak of 14b in the 1H NMR spectrum could not be
observed due to it overlapping with the CH2 multiplet peak of 14a
between 0.92–1.05 ppm. The quaternary carbon of the aromatic
ring and the carbon next to the boron atom were not observed for
14b due to its low abundance in the sample and quadrupolar line
broadening, respectively.
6: C22H16BF12O2P, M = 582.13, monoclinic, space group P21/n, a
= 11.4558(14), b = 12.8592(13), c = 15.6289(19) Å, b = 94.821(7)°,
V = 2294.2(5) Å3, Z = 4, Dc = 1.685 g cm–3, µ = 0.238 mm–1, T =
113(2) K, 5101 reflections measured, 3784 unique, final R1 (I >
2s(I)) = 0.0781, wR2 (all data) = 0.2522, GOF = 1.132.
Additional crystal data are available at the Cambridge
Crystallographic Data Centre, deposition no. CCDC 1938562 (1a),
CCDC 1938563 (3a), CCDC 1938564 (3b) and CCDC 1938565
(6).
Variable
Temperature
19F
NMR
Experiment
with
Phosphoranyl-Trihydroborate 1a
Phosphoranyl-trihydroborate 1a (15 mg, 0.01 mmol) was
dissolved in THF-d8 (400 µL) and analysed by variable
temperature 19F NMR spectroscopy. The sample was first
analysed using 5 °C increments from +30 to +60 °C, and then
from –80 to –40 °C (See Supporting Information, Figures S38 and
S39).
X-ray Crystallographic Analysis
Colourless single crystals were obtained by recrystallisation from
C6H6 at room temperature for 1a, THF/Et2O at –35 °C for 3a and
at room temperature for 3b, and Et2O/hexanes at –35 °C for 6.
The single crystals were used for X-ray diffraction data collection
on a Rigaku Mercury charge-coupled device diffractometer with a
9
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10.1002/ejic.202000229
European Journal of Inorganic Chemistry
FULL PAPER
38, 551.
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10.1002/ejic.202000229
European Journal of Inorganic Chemistry
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Entry for the Table of Contents
Key Topic: Phosphorus–Boron Bond
Insert graphic for Table of Contents here.
Pentacoordinated phosphorus–tetracoordinated boron bonded compounds were isolated. Upon hydride abstraction of a phosphoranyl-
trihydroborate, a phosphoranyl-borane intermediate could undergo a rearrangement reaction to give a bicyclic compound or it could act as a
mono-hydroboration reagent. A phosphoranyl-triarylborate was stable to acids and bases due to the large bulk around the long P–B bond.
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