Trihydroborates and Dihydroboranes Bearing a Pentacoordinated Phosphorus Atom: Double Ring Expansion To Balance the Coordination States

Article abstract of DOI:10.1002/anie.201701718

The first trihydroborate bearing a pentacoordinated phosphorus atom was synthesized as a new P?B bonded compound. Hydride abstraction of the trihydroborate gave an intermediary dihydroborane, which showed hydroboration reactivity and was trapped with pyridine whilst maintaining the P?B bond. The dihydroborane underwent a rearrangement, which involved a double ring expansion to compensate for the unbalanced coordination states of the phosphorus and boron atoms, to give a new fused bicyclic phosphine-boronate.

Full text of DOI:10.1002/anie.201701718

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201701718
German Edition: DOI: 10.1002/ange.201701718
Main Group Chemistry
Trihydroborates and Dihydroboranes Bearing a Pentacoordinated
Phosphorus Atom: Double Ring Expansion To Balance the
Coordination States
Naokazu Kano,* Nathan J. OꢀBrien, Ryohei Uematsu, Romain Ramozzi, and Keiji Morokuma*
Dedicated to Professor Takayuki Kawashima on the occasion of his 70th birthday
Abstract: The first trihydroborate bearing a pentacoordinated
phosphorus atom was synthesized as a new PꢀB bonded
compound. Hydride abstraction of the trihydroborate gave an
intermediary dihydroborane, which showed hydroboration
reactivity and was trapped with pyridine whilst maintaining the
Figure 1. A diagram of some PꢀB bonded compounds. The total
PꢀB bond. The dihydroborane underwent a rearrangement,
which involved a double ring expansion to compensate for the
unbalanced coordination states of the phosphorus and boron
atoms, to give a new fused bicyclic phosphine-boronate.
charge for compounds of type III is ꢀ1. Charges are omitted.
[
9]
a phosphorus group-transfer reaction, and on a potential
[10]
product from the reduction of trimethyl phosphite-borane.
T
he combination of trivalent group 13 and group 15 ele-
Despite their promising applicabilities in the construction of
fundamental frameworks of new compounds containing both
phosphorus and boron atoms, formation of a compound with
ments conventionally forms a bond, which is isoelectronic to
[
1]
CꢀC bonded compounds, to make Lewis complexes. Among
the classical Lewis complexes, phosphine-boranes (R Pꢀ a PꢀB bond is difficult if the phosphorus atom is pentacoor-
3
BR’ ), which bear a PꢀB bond (Figure 1, type I), have been
dinated (type III), partly because of the insufficient polarizing
3
[
11]
used for the synthesis of P-chiral tertiary phosphines and
power of the boron moiety. A boron atom of low electro-
negativity is insufficient for the electron withdrawal which is
required, thus making it an obstacle for the stabilization of
[
2]
diphosphines, and as precursors for polymers containing
[3]
PꢀB bonds. Other types of PꢀB bonded compounds, such as
[4]
ꢀ
phosphino-boranes (R PꢀBR’ ; type II), have been shown
both phophoranyl-borates ([R PꢀBR’ ] ; type III) and phos-
2
2
4
3
to activate H by behaving as an intramolecular frustrated
phoranyl-boranes (R PꢀBR’ ; type IV).
2
4
2
Lewis pair and to coordinate to metal in a symmetric side-on
Herein, we report the synthesis, structure, and reactivities
[
5]
fashion. Further modification of the properties and reac-
tivities could be achieved if the coordination numbers of the
elements that constitute the bond were increased. Expanding
the coordination number of the group 15 elements from four
to five for these Lewis complexes, as achieved in several
stable pentacoordinated phosphorus compounds (phosphor-
of a new PꢀB bonded compound, a trihydroborate with
a pentacoordinated phosphorus atom. A pair of C,O-biden-
[12]
tate ligands, which is known to stabilize the hypercoordi-
[13]
nate phosphorus species, was used. Changes in the coordi-
nation numbers of the phosphorus and boron atoms caused
a unique rearrangement reaction to compensate the unbal-
anced coordination states of both the phosphorus and boron
atoms.
[
6]
anes), promises to enhance their properties. However, only
a few phosphoranes bearing a PꢀB bond appeared in previous
[
14]
reports on theoretical calculations of the parent anion [H Pꢀ
Reaction of phosphoranides 1a and 1b with BH ·THF
4
3
ꢀ
[7]
BH ] , on an equilibrium with a tetracoordinated phospho-
gave phosphoranyl-substituted trihydroborates 2a (M =
3
[
8]
rus-borane complex, on a proposal of intermediates for
K·18-crown-6) and 2b [M = (n-Bu) N], respectively, in good
4
yields (Scheme 1). Trihydroborates 2a,b were isolated as
colorless crystals and could be handled in air without taking
precautions. Both compounds were characterized by NMR
spectroscopy, mass spectrometry, and elemental analysis. The
crystal structure of 2b was determined by X-ray crystallo-
[
*] Prof. N. Kano, Dr. N. J. O’Brien
Department of Chemistry, Graduate School of Science
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
[15]
E-mail: kano@chem.s.u-tokyo.ac.jp
graphic analysis (Figure 2a).
The ion pair consists of
Dr. R. Uematsu, Dr. R. Ramozzi, Prof. K. Morokuma
Fukui Institute for Fundamental Chemistry, Kyoto University
Takano-Nishihiraki-cho 34-4, Sakyou-ku, Kyoto 606-8103 (Japan)
E-mail: morokuma@fukui.kyoto-u.ac.jp
a tetrabutylammonium cation and an anion, which contains
a PꢀB bond. The phosphorus and boron atoms are penta- and
tetracoordinated, respectively. The phosphorus coordination
polyhedra is described as a distorted trigonal bipyramid
Supporting information (experimental procedures, NMR spectral
charts, reaction mechanism, X-ray crystallographic analysis, and
computational details) and the ORCID identification number(s) for
the author(s) of this article can be found under:
[
OꢀPꢀO angle, 171.43(8)8]. The equatorial positions are
occupied by one boron and two carbon atoms, and the apical
positions are occupied by two oxygen atoms. The PꢀB bond
lengths [1.959(3) ꢀ] are similar to that of Ph PꢀBH [1.924-
https://doi.org/10.1002/anie.201701718.
3
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The density-functional theory (DFT) calculations for the
anion of 2, at the B3PW91/6-31 + G(d,p) level, were con-
ducted to evaluate the bond polarity and molecular orbitals.
[
20]
The Wiberg bond index
suggesting single-bond character. The bond energy of the PꢀB
of the PꢀB bond is 0.92, thus
ꢀ
1
bond in the anion of 2 was calculated to be ꢀ33.8 kcalmol
by energy decomposition analysis, and is larger than the
[
21]
ꢀ
1
ꢀ
22.8 kcalmol of phosphine-borane (H PꢀBH ). The nat-
3
3
ural charges of the phosphorus (+ 1.65) and boron (ꢀ0.67)
atoms indicate a highly polarized PꢀB bond character. The
Scheme 1. Synthesis of trihydroborates 2a and 2b, and hydride reduc-
tion of imines. THF=tetrahydrofuran.
natural charges of the oxygen (ꢀ0.82) and carbon (ꢀ0.34)
atoms suggest that the PꢀO and PꢀC bonds are more and less
polarized, respectively, than the PꢀB bond. All three hydro-
gen atoms on the boron show natural charges of either 0.00 or
+
0.01 as a result of the strong electron-withdrawing effect of
the bidentate ligands on the phosphorus. The HOMO is
localized on the s orbitals of the BꢀH and PꢀB bonds as well
as the lone pairs of the oxygen atoms (Figure 3a), the former
indicating hydride reactivity. The LUMO is localized mainly
on the p* orbitals of the aromatic rings (Figure 3b), suggest-
ing its stability against nucleophiles.
Figure 3. The HOMO (ꢀ3.81 eV; a) and LUMO (1.52 eV; b) of the
anion of 2.
Figure 2. ORTEP drawings of 2b (a), 4 (b), and 5 (c) with thermal
ellipsoids at the 50% probability level.
Reflecting the neutrality of the hydrogen atoms on the
boron atom, 2a showed persistence towards base hydrolysis
and alcoholysis. It was also intact when exposed to diethyl-
amine, which is known to cleave the PꢀB bond of phosphine-
[
16]
(
3) ꢀ], thus showing that the pentacoordinated state of the
[
2]
phosphorus has little influence on the PꢀB bond length. The
PꢀO bond lengths [1.8102(17)–1.8406(17) ꢀ] are consider-
ably longer than those in the hydroxyphosphorane [1.7059-
boranes. However, 2a was unstable towards acid and
treatment with 1m hydrochloric acid instantaneously gave
[
14]
the hydrophosphorane, [2-C H C(CF ) O] PH,
quantita-
6
4
3
2
2
[17]
(
18)–1.7421(18) ꢀ], but are similar to the average of those
tively. In a further study, 2a was found to reduce two molar
equivalents of N-benzylidenemethylamine and diphenyl-
methanimine to give the corresponding amines and hydro-
phosphorane (Scheme 1). It showed no reactivity with
benzaldehyde, acetophenone, and benzophenone oxime at
room temperature. The chemoselective reducing reactivity of
[
1.780(3), 1.851(3) ꢀ] in the phosphoranide-palladium com-
[
18]
plex, [(2-C H C(CF ) O) PPd(PhCHO)(dppe)](SbF ). The
6
4
3
2
2
6
long PꢀO bond lengths indicate localization of substantially
negative charges on the oxygen atoms and weakening of the
polar PꢀO bonds.
3
1
[22]
In a [D ]THF solution, the proton-decoupled P NMR
2a is similar to that of sodium cyanoborohydride. Consid-
8
spectrum of 2a showed a broad doublet at d =+ 9.0 ppm and
ering that 1,3-dimethylimidazol-2-ylidene borane shows sim-
ilar reducing reactivity, the effect of the phosphoranide on
P
1
1
1
[23]
coupling with the B nucleus ( J = 69 Hz). The chemical
PB
shift is in a downfield region compared to that of hydroxy-
the reactivity of the borane moiety is comparable to that of an
N-heterocyclic carbene (NHC).
phosphorane (d = ꢀ17.6 ppm) and chlorophosphorane (d =
P
P
ꢀ
7.6 ppm), thus reflecting the difference in charge and
The hydride-donor reactivity of 2a prompted us to
[
19]
ꢀ
electronegativity of the equatorial element. The coupling
convert the phosphoranyl-borate [R P-BH ] into the phos-
4
3
1
1
constant J is slightly larger than that of Ph PꢀBH ( J
=
phoranyl-borane [R P-BH ], which can be recognized as
4 2
PB
16]
3
3
PB
[
11
[24]
ꢀ
6
1.5 Hz). The non-decoupled B NMR spectrum showed
a phosphoranide-coordinating borenium complex [R P !
4
+
a broad singlet at d = ꢀ28.6 ppm. The downfield shift from
BH ]. A hydride was abstracted from 2a using tropylium
B
2
[
16]
that in Ph PꢀBH (d = ꢀ39.9 ppm) suggests an increased
tetrafluoroborate in THF to generate dihydroborane 3, which
was trapped by pyridine to afford stable pyridine complex 4
3
3
B
electron density on the boron atom.
2
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DFT calculations for 5 were conducted similarly to those for
the anion of 2. Natural charges of the phosphorus (+ 0.96) and
boron (+ 0.50) atoms showed a much less polarized PꢀB bond
character compared to those in the anion of 2 (see the
Supporting Information). The formation of 5 from 3 should
involve a twofold migration of the alkoxy groups from the
phosphorus to boron in addition to the migration of a hydro-
gen atom from the boron to phosphorus.
The solvent exchange process was conducted again using
[
D ]2a as the starting compound, and the corresponding [D ]5
3 2
bearing both the PꢀD and BꢀD bonds was obtained. This
result asserts the migration of the hydrogen from the boron in
2
a to phosphorus in 5, rather than hydrogen abstraction from
Scheme 2. Reactions of intermediary dihydroborane 3.
the solvent. The rearrangement to form 5 was also achieved
simply by hydride abstraction of 2a in benzene without the
solvent exchange. Furthermore, oxidation of 2a with half an
(
Scheme 2). The X-ray crystallographic analysis of 4 showed
structural similarities to the anion moiety of 2a despite the
different total charges (Figure 2b). However, the difference
in the charges between 4 and 2a is reflected in the chemical
equivalent of I in toluene also gave 5. Although the reaction
2
mechanism has not been established yet, the first step of this
rearrangement should be the migration of an alkoxy group
from the hypercoordinate phosphorus atom to the boron
atom in 3, and was confirmed by the trapping reaction with
pyridine (see the Supporting Information). The migration
compensates the vacant 2p orbital of the boron atom and
leads to a phosphonium-borate intermediate. The hydride-
donor and hydride-acceptor properties of the resulting
dihydroborate and phosphonium moieties in the intermediate
1
1
31
shifts in the B and P NMR spectra (4: d = ꢀ8.8 ppm; d =
B
P
ꢀ
21.1 ppm), which show large downfield and upfield shifts,
respectively, from 2a. These shifts agree with the downfield
shift of LiMeBH3 (d = ꢀ30.8 ppm) relative to MeBH -
B
2
[25]
pyridine (d =+ 5.8 ppm) and the upfield shift of a benzyl-
B
phosphorane (d = ꢀ21 ppm) relative to an alkali metal salt of
P
[
13b]
the corresponding anion (d = ꢀ34 ppm).
Compound 4 is
P
[26]
noteworthy as it is the first isolated neutral phosphorene
should facilitate the hydride migration, followed by another
oxygen migration to yield 5. As a result, both the phosphorus
and boron atoms are in balanced tetracoordinated states. This
rearrangement reactivity, which has not been found in other
PꢀB bonded compounds and phosphoranes, is meaningful
containing a PꢀB bond, as far as we know.
Dihydroborane 3 in a THF solution was observed directly
by NMR spectroscopy before the trapping. The proton-
3
1
decoupled P NMR spectra of the intermediate showed
a broad singlet at a chemical shift of d = ꢀ21.0 ppm, which
because of the following reasons: 1) the unique rearrange-
ment is accompanied by the double ring expansion, which
P
was very close to that of 4 (d = ꢀ21.1 ppm), whilst the
P
1
1
[27]
proton-decoupled B NMR spectra showed a broad singlet at
recalls reactivity of NHCꢀBH , 2) the rare bicyclic struc-
3
d = ꢀ1.7 ppm. These chemical shifts presented large differ-
ture fused on the PꢀB bond is constructed simply by the
B
ences from those of 2a. The most reasonable structure of the
observed intermediate is the THF complex of 3 with a B–O
interaction. In the ESI mass spectrum, a molecular ion peak at
m/z 600, which corresponds to the THF complex of 3,
supported its identity. Its high reactivity towards moisture and
oxygen hampered isolation and resulted in the formation of
the hydrophosphorane. Furthermore, exchange of the solvent
solvent exchange process, and 3) the synclinal conformation
of the HꢀPꢀBꢀH linkage is promising for a precursor for
oligomers and polymers with a moderately rigid PꢀB bond
backbone.
The rearrangement reactivity was also studied on a hydro-
(vinyl)borane derivative. Treatment of 3, generated from 2a,
1
31
with 2-butyne gave 6 (Scheme 2). The H and P NMR
from THF to Et O enabled isolation of colorless solid, but its
spectra of 6 are similar to that of 5. However, the non-
2
3
1
11
P NMR spectrum showed a drastic upfield shift and
revealed a transformation of the intermediate into a new
decoupled B NMR spectrum showed a broad singlet at d =
B
+ 4.2 ppm, which indicates no BꢀH bond. Formation of 6
shows the hydroboration reactivity of 3 and that only one
hydrogen atom was effective for the hydroboration even
though an excess of the alkyne was used. This result reveals
the hydroboration with 3 affords the corresponding phos-
phoranyl-substituted 2-butenyl(hydro)borane followed by
a fast rearrangement of one hydrogen and two oxygen
3
1
product. In the non-decoupled P NMR spectrum, the
product 5, resonated at d = ꢀ72.2 ppm as a doublet, thus
P
1
showing coupling with a phosphorus-bound proton ( J
91 Hz), while the non-decoupled
=
PH
1
1
3
B NMR spectrum
1
showed a broad doublet at d = ꢀ0.7 ppm ( J = 162 Hz).
B
BH
1
The H NMR spectrum showed a doublet at d = 4.8 ppm
H
1
with a large coupling constant ( J = 397 Hz) for the
atoms, similarly to 3 in Et O (see the Supporting Informa-
PH
2
phosphorus-bound proton and a broad doublet at d
tion). The rearrangement is so fast that the second hydro-
boration did not occur. This would be advantageous for
a selective single hydroboration reaction.
In summary, we have created a trihydroborate and
a Lewis-base stabilized dihydroborane that both bear a pen-
tacoordinated phosphorus atom. Decreasing the coordination
number of the boron from four to three caused a unique
H
1
ꢁ
4.39 ppm ( J = 166 Hz) for the boron-bound proton.
The spectral data suggest a HꢀPꢀBꢀH linkage, which was
BH
confirmed by the X-ray crystallographic analysis (Figure 2c).
The crystal structure of 5 shows the HꢀPꢀBꢀH linkage in
a synclinal conformation [torsion angle, 47(2)8] as well as two
fused six-membered rings each containing a BꢀO bond. The
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Angewandte
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rearrangement involving a double ring expansion to give the
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Science Foundation, Asahi Glass Foundation, Izumi Science
and Technology Foundation, and Grants-in-Aid for Scientific
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JSPS) and the Fukui Institute for the fellowships. The
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Manuscript received: February 16, 2017
Revised: March 24, 2017
Final Article published: && &&, &&&&
4
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Communications
Main Group Chemistry
N. Kano,* N. J. O’Brien, R. Uematsu,
R. Ramozzi,
K. Morokuma*
&&&& — &&&&
Trihydroborates and Dihydroboranes
Bearing a Pentacoordinated Phosphorus
Atom: Double Ring Expansion To Balance
the Coordination States
A lasting bond: The first trihydroborate
bearing a pentacoordinated phosphorus
atom was synthesized as a new PꢀB
rangement. The rearrangement involved
a double ring expansion, to compensate
for the unbalanced coordination states of
the phosphorus and boron atoms, to give
a new fused bicyclic phosphine-boronate.
bonded compound. The corresponding
dihydroborane, generated by the hydride
abstraction, underwent a unique rear-
Angew. Chem. Int. Ed. 2017, 56, 1 – 5
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!