Persistent P-Stabilized Boryl Radicals with Bulky Substituents at Boron

Article abstract of DOI:10.1055/s-0037-1610151

Two new P-stabilized boryl radicals [Ph ₂ P(naph)BAr] (Ar = Tip and Ter) have been prepared and characterized by electron paramagnetic resonance spectroscopy. These radicals are persistent for several days in solution at room temperature. The h

Full text of DOI:10.1055/s-0037-1610151

SYNTHESIS
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© Georg Thieme Verlag Stuttgart · New York
2018, 50, A–H
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A. J. Rosenthal et al.
Feature
Synthesis
Persistent P-Stabilized Boryl Radicals with Bulky Substituents at
Boron
Amos J. Rosenthala
Sonia Mallet-Ladeirab
Ghenwa Bouhadira
Didier Bourissou*a
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a CNRS, Université Paul Sabatier, Laboratoire Hétérochimie Fon-
damentale et Appliquée (LHFA, UMR 5069), 118 route de Nar-
bonne, 31062 Toulouse Cedex 09, France
dbouriss@chimie.ups-tlse.fr
b Institut de Chimie de Toulouse (FR 2599), 118 route de Nar-
bonne, 31062 Toulouse Cedex 9, France
Published as part of the Special Section on the Main Group Metal
Chemistry Symposium
Received: 01.04.2018
Accepted after revision: 16.04.2018
Published online: 19.06.2018
B-centered reactivity was observed upon halogen transfer
with trityl chloride (Ph3CCl) and radical coupling with
2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO).
DOI: 10.1055/s-0037-1610151; Art ID: ss-2018-z0221-fa
Abstract Two new P-stabilized boryl radicals [Ph2P(naph)BAr] (Ar = Tip
and Ter) have been prepared and characterized by electron paramag-
netic resonance spectroscopy. These radicals are persistent for several
days in solution at room temperature. The high steric congestion at bo-
ron does not prevent radical reactivity. Two different types of dimeriza-
tion equilibrium have been observed. The structures of the dimers have
been unambiguously established by X-ray diffraction crystallography.
P
B
N
N
N
C
C
N
C
B
B
B
A
B
C
D
Figure 1 Persistent/stable boryl radicals A–D stabilized by C- or P-based
donors
Key words radicals, boron, boryl radicals, phosphorus, Gomberg-type
dimerization, m-terphenyl, EPR spectroscopy, X-ray diffraction
Since aryl substituents are known to largely influence
the stability and behavior of highly reactive main group
compounds, we sought to introduce sterically demanding
groups at boron. Accordingly, we report here studies on two
new P-stabilized boryl radicals.
Boron-centered radicals are attracting considerable in-
terest.1 On the one hand, the preparation of stable B radi-
cals is challenging and fundamentally interesting. On the
other hand, B radicals are valuable in organic and polymer
synthesis (e.g., as intermediates, initiators), as well as in
materials science (as π-conjugated open-shell species).
In particular, neutral radicals of general formula
(L→BR2)•, namely boryl radicals, have drawn much atten-
tion. A variety of persistent or even isolable boryl radicals
have been reported using C-based donors such as acri-
dines,2 NHCs3 and CAACs4 (see Figure 1 for representative
examples A–C). P-Based donors can also be used, as we
showed recently using a rigid naphthyl spacer which en-
forces strong intramolecular P→B interaction.5 The BMes
radicals D6 were found to be persistent in solution and to
undergo a Gomberg-type dimerization equilibrium. The
spin density at boron reaches 60–70% in D and, consistently,
The phosphine–bromoborane 1a was prepared in two
steps. 1-(Diphenylphosphino)-8-iodonaphthalene was suc-
cessively treated with nBuLi and (2,4,6-triisopropylphe-
nyl)dibromoborane (TipBBr2). The structure of 1a was un-
ambiguously ascertained by multinuclear NMR spectrosco-
py and X-ray diffraction analysis.7 The Tip group does not
prevent P and B binding [P–B = 2.024(2) Å], evidencing once
more the propensity of the naphthyl spacer to enforce P→B
interaction despite steric shielding. Bromoborane 1a was
chemically reduced with a slight excess of Na/Hg amalgam
in toluene (Scheme 1). The solution progressively turned
deep red. After stirring for 24 hours at room temperature,
the reaction medium was decanted and filtered several
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–H

B
A. J. Rosenthal et al.
Feature
Synthesis
times to remove the sodium salt (NaBr) and all traces of Hg.
After washing with pentane, the desired boryl radical 2a
was isolated as a red solid in 86% yield.
EPR characterization in toluene at room temperature
showed a well-resolved signal at giso = 2.0026 whose hyper-
fine structure could be nicely reproduced taking into ac-
count couplings to P, B and two H nuclei (Figure 2, top). The
obtained coupling constants are very similar to those ob-
served for the previously reported P-stabilized boryl radi-
cals.6 Thus, increasing the steric demand at boron does not
significantly perturb the electronic structure of the radical.
In particular, the a(11B) coupling remains large (10.9 G) in-
dicating high spin density at boron. The EPR signal re-
mained unchanged after several days at room temperature,
pointing out the persistent character of radical 2a. Ben-
zene-d6 solutions of 2a are not NMR silent. The 31P NMR
Biographical Sketches
Amos Jaakov Rosenthal re-
ceived his B.Sc. (honours) at the
VU University, The Netherlands,
in 2007 and earned his M.Sc.
degree in synthetic chemistry
from the same institution in
2009. In 2013, he was awarded
his Dr. degree in organometallic
chemistry under the supervision
of Prof. Hansjörg Grützmacher
from the ETH Zurich, Switzer-
land. The next year, he moved
to University Paul Sabatier, Tou-
louse, France to take up a post-
doctoral position under the
tutelage of Prof. Didier Bouris-
sou. Currently he is working in
the Salzman Group, Katzrin, Is-
rael on the synthesis of pharma-
ceutically active compounds.
Sonia Mallet-Ladeira is crys-
tallographer for nearly 10 years.
She received a Master degree in
Claude Bernard University, Lyon,
France in 2008. The same year,
she was appointed as CNRS en-
gineer at the Institut de Chimie
de Toulouse to work on single
crystal and powder X-Ray Dif-
fraction in Chemistry area.
Analytical
Chemistry
from
Ghenwa (Rinoi) Bouhadir
obtained her PhD degree in
1996 under the supervision of
G. Bertrand in Toulouse. After
three postdoctoral stays (S.
Zard, ICSN Gif/Yvette, 1996-
1998; B. Meunier, LCC Toulouse
1998-1999 and M. Ciufolini CPE
Lyon 1999-2000), she was ap-
pointed as lecturer at Paul Sa-
batier University, Toulouse,
France in 2000. Since 2003, she
is working on the chemistry of
ambiphilic derivatives, their use
as σ-acceptor ligands, as small
molecules trapping agents and
as bifunctional organocatalysts.
Didier Bourissou holds a Se-
nior Scientist position at the
CNRS and an Associate Profes-
sor position at the Ecole Poly-
technique in Palaiseau. He is
Director of the Laboratory of
Fundamental and Applied Het-
erochemistry at Paul Sabatier
University, Toulouse, France. His
research activities span a broad
range, from highly reactive
main group species and transi-
tion metal complexes, to homo-
ments. He has initiated the de-
velopment of ambiphilic ligands
in the mid 2000’s and moved
forward the concept of σ-accep-
tor ligands. Part of his research
also deals with non-innocent
pincer complexes and unusual
behavior of the coinage metals.
geneous
catalysis
and
biodegradable polymers. He is
particularly interested in new
bonding situations and reactivi-
ty patterns arising from the in-
terplay between transition
metals and main group ele-
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–H

C
A. J. Rosenthal et al.
Feature
Synthesis
Ph
Ph
Tip
P
B
Tip
Tip
Ph2P
B
Ph2P
B
1% Na(Hg)
Br
Ph
toluene
86%
Ph
P
H
B
Tip
2a
1a
[2a]2
Scheme 1 Synthesis and Gomberg-type dimerization of the P-stabi-
lized boryl radical 2a
spectrum showed two sets of signals attributed to two ste-
reoisomers of the unsymmetrical dimer [2a]2. The major
isomer displays 31P NMR resonances at δ 5.46 and 0.41 (br)
ppm. Crystals of [2a]2 suitable for X-ray diffraction analysis
were grown from a diethyl ether solution at room tempera-
ture (Figure 2, bottom). Accordingly, [2a]2 forms
a
Gomberg-type dimer by radical coupling between the bo-
ron center of one boryl unit and the naphthyl carbon atom
at the para position to boron of another boryl unit. The
metrical parameters of [2a]2 are comparable to those of the
related BMes compounds [D]2, albeit with slightly higher
steric congestion induced by the Tip group. The most no-
ticeable features are: the long B2–C4 bond formed upon
radical coupling [1.692(5) Å]; the long and short P–B bonds
to the tetracoordinate and tricoordinate boron centers, re-
spectively [2.111(5) and 1.940(4) Å]; the short B=C bond
between the B center and the naphthyl linker [1.446(7) Å].
The equilibrium between radical 2a and its Gomberg-
type dimer [2a]2 was analyzed by variable-temperature EPR
spectroscopy.7 The dimerization energy was thereby esti-
mated to be –9 kcal/mol. This is substantially smaller than
that of the corresponding BMes radical (–15 kcal/mol), indi-
cating that the steric demand of the Tip group noticeably
affects the thermodynamic stability of the boryl radical to-
wards dimerization.
It is important to note that the Tip-substituted radical
2a retains the B-centered reactivity evidenced for D.6b This
was clearly shown upon reacting 2a with Ph3CCl and TEMPO
(Scheme 2). In both cases, 2a reacted rapidly at room tem-
perature, as visible from the disappearance of the charac-
teristic red color. After workup, the products of chloride ab-
straction 4 and radical coupling 3 were isolated in 66% and
42% yield, respectively. The NMR and X-ray data of chlorob-
orane 4 [11B NMR: δ = 3.4 ppm, 31P NMR: δ = 2.9 ppm; P–B =
2.031(2) Å] are very similar to those of the bromo deriva-
tive 1a. In the TEMPO adduct 3 (Figure 3), the boron center
remains tricoordinate; it is stabilized by π-donation from
Figure 2 Experimental (blue) and simulated (red) X-band EPR spectra
of 2a (top) and X-ray crystal structure of [2a]2 (bottom; 50% probability
ellipsoids; Ph and Tip groups are simplified and H atoms are omitted for
clarity)
the nitrogen atom rather than P→B interaction [O–B =
1.377(3) Å, P–B = 3.091(3) Å] . The P and B centers strongly
deviate from the naphthyl plane [PC1C3B torsion angle =
45.6(1)° in 3] indicating important steric congestion be-
tween the PPh2 and B(Tip)(TEMPO) groups sitting at peri po-
sitions to each other.
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D
A. J. Rosenthal et al.
Feature
Synthesis
Tip
Cl
Tip
O
N
Ph2P
B
Ph3CCl
toluene
Ph2P
B
TEMPO
toluene
2a
Ph3C
4 (66%)
3 (42%)
Scheme 2 B-Centered reactivity of 2a: chloride abstraction and radical
coupling upon reaction with Ph3CCl and TEMPO, respectively
Figure 3 X-ray crystal structure of 3 (50% probability ellipsoids; Ph and
Tip groups are simplified and H atoms are omitted for clarity)
Figure 4 Experimental (blue) and simulated (red) X-band EPR spectra
of 2b (top) and X-ray crystal structure of [2b]2 (bottom; 50% probability
ellipsoids; Ph and Ter groups are simplified and H atoms are omitted for
clarity)
To further increase the steric demand, we then decided
to introduce an m-terphenyl substituent at boron. These
substituents are among the most sterically shielding groups
devised so far. Their geometry and rigidity create a 3D
pocket around the central element, with the two ortho aryl
groups acting as protecting wings. They are frequently used
in main group chemistry, in particular to stabilize radical
and low-coordinate species.8 For this study, we chose the
2,6-Mes2C6H3 group (referred to as Ter) which is easily pre-
pared and readily functionalized. The phosphine–bromobo-
rane precursor 1b was prepared following the same route
as for 1a. Of note, the P→B interaction is retained in 1b both
in solution (according to 31P and 11B NMR spectroscopy) and
in the solid state (according to X-ray diffraction analysis).7
To accommodate the steric congestion induced by the Ter
group, the P→B dative bond slightly elongates [2.080(7) Å],
but the largest distortion involves the naphthyl spacer. In
1b, the P and B atoms strongly deviate on both sides of the
mean plane, the PCCB torsion angle reaching 20.8(4)°, ver-
sus 7.3(1)° in 1a. Upon exposure to Na/Hg amalgam at room
temperature under vigorous stirring, toluene solutions of
1b turn dark brown. EPR spectroscopy indicated the forma-
tion of a persistent radical 2b (giso = 2.0024), but the hyper-
fine structure (Figure 4, top) differs noticeably from those
of the previously characterized P-stabilized boryl radicals
2a and D. Upon simulation, the couplings to 31P (31.3 G) and
11B (11.5 G) were found to exceed those previously ob-
served. The large coupling to 11B is particularly remarkable;
it is actually the largest reported so far for a persistent boryl
radical.9 Analysis of toluene solutions of 2b by EPR spec-
troscopy at different temperatures revealed variations of
the radical concentration, in line with a monomer/dimer
equilibrium, but compared to the other P-stabilized boryl
radicals, the kinetics are extremely slow, precluding the de-
termination of the dimerization energy.10 Nevertheless, we
tried to establish more about the structure of the dimer.
Layering a toluene solution of 2b with pentane afforded a
few microcrystals that were analyzed by CP MAS NMR
spectroscopy and X-ray diffraction crystallography. The 31P
and 11B NMR spectra both showed two signals (31P NMR:
δiso = 11.7 and –3.8 ppm, 11B NMR: δiso = 25.0 and 1.5 ppm)
suggesting the formation of an unsymmetrical dimer. The
crystallographic data are not of high quality, but good
enough to establish the connectivity (Figure 4, bottom). Ac-
cordingly, an original dimerization occurred, involving the
naphthyl carbon atom at the para position to boron of one
boryl unit and the terphenyl substituent of another unit
(Scheme 3). The boron atom of the second boryl unit cou-
ples with an ortho position of a Mes ring, creating a quater-
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–H

E
A. J. Rosenthal et al.
Feature
Synthesis
nary carbon atom and a 1,4-cyclohexadienyl moiety. The
concerned Mes group loses its aromaticity (the lateral
C48=C49 and C52=C51 bonds are short at ~1.33 Å) and a bo-
ron-centered spiro motif is generated. The newly formed C–C
gen transfer with Ph3CCl, radical coupling with TEMPO and
Gomberg-type dimerization of the BTip radical 2a. The ter-
phenyl-substituted radical 2b undergoes an original di-
merization process, as substantiated by X-ray crystallogra-
phy, which involves remote positions of the naphthyl spac-
er and of the Ter substituent, where spin density is low. This
process is probably favored by the spatial proximity be-
tween the boron center and the flanking Mes groups. Fu-
ture work will seek to further vary the nature of the substit-
uent at boron to modify the electronic structure of the bor-
yl radical.
bond is remarkably long for
a
CHsp3–CHsp3 bond
[1.61(1) Å].11 This value exceeds that of the Gomberg dimer
of the trityl radical Ph3C• [1.583(3) Å]12 and approaches the
upper limit of Csp3–Csp3 bond length reported for extremely
crowded hexaarylethanes Ar3C–CAr3 (1.67 Å).13
Ter
Ter
B
Ph2P
Ph2P
B
All reactions and manipulations were carried out under an atmo-
sphere of dry argon using standard Schlenk techniques or in a glove-
box. All solvents were sparged with argon, dried using an MBRAUN
Solvent Purification System (SPS) and degassed with three freeze–
pump–thaw cycles. Deuterated solvents were dried over alumina or
molecular sieves and degassed with three freeze–pump–thaw cycles.
NMR spectra were recorded on Bruker Avance 300, 400 or 500 sys-
tems. The chemical shifts are given as δ values and were referenced
B
2b
Ph2P
B
[2b]2
Ph2P
1
2b
against residual solvent signals for H [5.32 and 7.16 ppm for CD2Cl2
and C6D6 (benzene-d6), respectively] and 13C (53.84 and 128.06 ppm
for CD2Cl2 and C6D6, respectively). The 11B NMR spectra were refer-
enced to an external BF3·OEt2 sample. The 31P NMR spectra were ref-
erenced to an external 85% H3PO4 sample. Solid-state NMR experi-
ments were recorded on a Bruker Avance 400 spectrometer equipped
with a 3.2 mm probe. Samples were spun between 12 and 14 kHz at
the magic angle using ZrO2 rotors. Melting points were determined
with a Stuart SMP40 melting point apparatus and are not corrected.
Mass spectra were recorded on a Waters LCT mass spectrometer. X-
Band EPR data were recorded using an Elexsys ESP 500 spectrometer,
operating at a microwave frequency of approximately 9.5 GHz. All
spectra were recorded using a microwave power of 2 mW, across a
sweep width of 140 G (centered at 3400 G) with a modulation ampli-
tude of 1 G. At the end of the measurement, the first spectrum was
remeasured to verify that no decomposition occurred and to verify
the reliability of the integral values.
Scheme 3 Structure of the P-stabilized terphenylboryl radical 2b and
its dimerization equilibrium by remote radical coupling leading to [2b]2
The Ter group brings huge steric shielding to the boron
center, but radical 2b finds another way to dimerize. The
radical coupling involves remote positions of the naphthyl
spacer and of the Ter substituent. It occurs despite the low
spin density at these positions and the loss of aromaticity it
induces on one of the rings of the naphthyl and on one of
the Mes groups. We are not aware of any related radical di-
merization involving an m-terphenyl moiety, but the for-
mation of [2b]2 is somewhat reminiscent of the radical cou-
pling/cyclization observed upon crystallization of the per-
sistent diboryl biradical dianion E (Scheme 4).14 The latter
process induces dearomatization of the central ring of the
p-terphenyl spacer and results in a ladder-type diborate de-
rivative.
TipBBr2
Based on a poorly described literature procedure.15
To a stirred suspension of solvent-free (2,4,6-triisopropylphenyl)lithi-
um (1.42 g, 6.8 mmol) in pentane (10 mL) at –80 °C was added neat
BBr3 (0.64 mL, 1.69 g, 6.8 mmol). The suspension was warmed to
room temperature and stirred overnight. The beige suspension was
filtered and the solid was extracted with pentane (2 × 10 mL). The
combined organic phases were evaporated to dryness to yield 2 g of
crude product. This crude product was recrystallized from pentane to
yield 1.4 g (56%) of the product as colorless needles, suitable for a sin-
gle-crystal X-ray diffraction study.
Mes
Mes
Mes
H
Mes
B
B
crystallization
B
H
Mes
B
Mes
E
Mes
Mes
Scheme 4 Radical cyclization of diboryl biradical dianion E with dearo-
matization of the central ring of the p-terphenyl spacer
1H NMR (300 MHz, 298 K, C6D6): δ = 6.99 (s, 2 H, HAr), 2.87 (sept, 3J =
6.8 Hz, 2 H, CH(CH3)), 2.71 (sept, 3J = 6.7 Hz, 1 H, CH(CH3)), 1.20 (d, 3J =
6.8 Hz, 12 H, CH3), 1.16 (d, 3J = 6.7 Hz, 6 H, CH3).
11B{1H} NMR (96.3 MHz, 298 K, C6D6): δ = 62.6.
In conclusion, two new P-stabilized boryl radicals 2a,b
featuring a sterically hindered substituent at boron have
been prepared. According to EPR spectroscopy, these radi-
cals display high spin density at boron and are persistent for
days in solution at room temperature. Despite steric con-
gestion, B-centered reactivity is retained, as shown by halo-
1a
Under argon, (8-iodonaphthalen-1-yl)diphenylphosphane (1.8 g, 4.1
mmol) was suspended in Et2O (16 mL). At –80 °C and under stirring,
n-butyllithium (2.6 mL, 1.6 M solution in hexane, 4.2 mmol) was add-
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–H

F
A. J. Rosenthal et al.
Feature
Synthesis
ed. After 2 h at –80 °C, the yellow suspension was filtered out with a
cannula. The yellow solid was dissolved in toluene (10 mL) at –60 °C.
A solution of dibromide (2,4,6-triisopropylphenyl)borane (1.38 g, 3.7
mmol) in toluene (10 mL) was added slowly. The solution was allowed
to warm to room temperature and the red solution was stirred over-
night. The red solution was filtered off from the white solid. The
white solid was washed with toluene (5 mL) and the solid was ex-
tracted with CH2Cl2 (3 × 10 mL). The combined solutions were evapo-
rated to dryness to yield 643 mg (29%) of product as a white powder;
mp 207 °C.
1H NMR (400 MHz, 253 K, CD2Cl2/C6D6): δ = 8.29–8.24 (m, 1 H, CHnaph),
8.01 (d, 3JHH = 7.1 Hz, 1 H, CHnaph), 7.92 (br d, 3JHH = 8.3 Hz, JHH = 0.8 Hz,
JHP = 1.1 Hz, 1 H, CHnaph), 7.80–7.74 (m, 2 H, CHAr), 7.69–7.64 (m, 1 H,
CHAr), 7.62–7.46 (m, 8 H, CHAr), 7.32 (td, 3JHH = 7.9 Hz, JHP = 2.4 Hz, 2 H,
CHAr), 7.21 (s, 1 H, CHTip), 7.06 (s, 1 H, CHTip), 3.78 (sept, 3JHH = 6.8 Hz, 1
evaporated and the oil was dissolved in pentane (1 mL). The product
crystallized out at –20 °C to yield in total 11 mg (42%) of colorless
cubes; mp 148 °C.
1H NMR (500 MHz, 283 K, C6D6): δ = 9.66 (dt, 3JHH = 7.3 Hz, JHH = 1 Hz,
1 H, CHnaph), 7.66 (dd, 3JHH = 8.1 Hz, JHH = 1.0 Hz, 1 H, CHnaph), 7.62 (dd,
3
3JHH = 8.2 Hz, JHH = 1.2 Hz, 1 H, CHnaph), 7.48 (t, JHH = 7.6 Hz, 1 H,
CHnaph), 7.29 (d, JHH = 1.0 Hz, 1 H, CHTip), 7.27 (ddd, 3JHH = 7.1 Hz, JHH
=
1.3 Hz, JHP = 4.2 Hz, 1 H, CHnaph), 7.06–6.91 (m, 10 H, CHAr), 6.54 (ddd,
3JHH = 8.0 Hz, JHH = 1.4 Hz, JHP = 7 Hz, 2 H, CHnaph), 4.69 (sept, 3JHH = 6.7
Hz, 1 H, CH(CH3)2), 2.86 (sept, 3JHH = 6.9 Hz, 1 H, CH(CH3)2), 1.92 (sept,
3JHH = 6.8 Hz, 1 H, CH(CH3)2), 1.78 (d, 3JHH = 6.8 Hz, 3 H, CH3), 1.67–1.60
(m, 1 H, CHH), 1.53 (d, 3JHH = 6.8 Hz, 3 H, CH3), 1.51–1.44 (m, 5 H, CH2
and CH3), 1.42–1.35 (m, 4 H, CHH and CH3), 1.33 (s, 3 H, CH3), 1.32 (d,
3JHH = 6.9 Hz, 6 H, CH3), 1.29–1.10 (m, 2 H, CH2), 1.01 (d, 3JHH = 6.7 Hz, 3
H, CH3), 0.99 (s, 3 H, CH3), 0.18 (d, 3JHH = 6.6 Hz, 3 H, CH3).
13C{1H} NMR (125.8 MHz, 283 K, C6D6): δ = 154.8 (d, JCP = 4.4 Hz, CAr),
152.4 (d, JCP = 4.6 Hz, CAr), 148.7 (s, CAr), 142.3 (d, JCP = 31.1 Hz, CAr),
141.8 (br d, 3JCP = 8 Hz, CB), 139.9 (d, JCP = 12.5 Hz, CAr), 138.6 (br, CB),
137.6 (d, JCP = 15.4 Hz, CAr), 137.4 (d, JCP = 5.2 Hz, CHAr), 136.2 (s, CHAr),
135.2 (d, JCP = 5.2 Hz, CAr), 135.0 (d, JCP = 6.5 Hz, CAr), 133.7 (d, JCP = 18.6
H, CH(CH3)2), 3.01 (sept, 3JHH = 6.9 Hz, 1 H, CH(CH3)2), 2.41 (sept, 3JHH
=
=
6.3 Hz, 1 H, CH(CH3)2), 1.46 (d, 3JHH = 6.2 Hz, 3 H, CH3), 1.42 (d, 3JHH
3
6.9 Hz, 3 H, CH3), 1.41 (d, JHH = 6.9 Hz, 3 H, CH3), 1.08–1.04 (m, 6 H,
CH3), 0.26 (d, 3JHH = 6.7 Hz, 3 H, CH3).
13C{1H} NMR (100.6 MHz, 253 K, CD2Cl2/C6D6): δ = 155.8 (s, CAr), 154.0
(d, JCP = 8.2 Hz, CAr), 148.1 (d, JCP = 2.8 Hz, CAr), 140.3 (d, JCP = 30.0 Hz,
CAr), 134.1 (d, JCP = 8.3 Hz, CHAr), 134.0 (d, JCP = 8.9 Hz, CHAr), 132.5 (d,
JCP = 13.7 Hz, CHAr), 132.4 (d, JCP = 10.3 Hz, CAr), 132.3 (d, JCP = 2.5 Hz,
CHAr), 132.2 (d, JCP = 2.7 Hz, CHAr), 131.9 (d, JCP = 3.0 Hz, CHAr), 130.8 (s,
CHAr), 129.1 (d, JCP = 10.0 Hz, CHAr), 128.9 (d, JCP = 2.7 Hz, CHAr), 128.6
(d, JCP = 11.1 Hz, CHAr), 127.1 (d, 1JCP = 65.1 Hz, CAr), 126.6 (d, JCP = 8.9
Hz, CHAr), 132.6 (d, JCP = 13.8 Hz, CHAr), 131.4 (s, CHAr), 130.5 (d, JCP
=
3.7 Hz, CHAr), 128.5 (d, JCP = 6.5 Hz, CHAr), 128.4 (s, CHAr), 125.5 (s,
CHAr), 125.3 (s, CHAr), 122.9 (s, CHTip), 120.9 (s, CHTip), 61.0 (s, C–N),
60.5 (s, C–N), 40.6 (s, CH2), 40.4 (s, CH2), 34.7 (s, CH(CH3)2), 34.4 (d,
JCP = 11.7 Hz, CH(CH3)2), 34.2 (d, JCP = 6.6 Hz, CH3), 31.6 (s, CH3), 31.3
(s, CH(CH3)2), 29.8 (s, CH(CH3)2), 26.5 (s, CH(CH3)2), 24.5 (s, CH(CH3)2),
23.8 (d, JCP = 3.4 Hz, CH(CH3)2), 23.7 (s, CH(CH3)2), 22.6 (s, CH3), 22.3 (s,
CH3), 17.7 (s, CH2).
1
1
Hz, CHAr), 125.7 (d, JCP = 59.9 Hz, CAr), 124.7 (d, JCP = 53.8 Hz, CAr),
124.6 (d, JCP = 1.6 Hz, CHAr), 122.7 (s, CHAr), 122.3 (s, CHAr), 34.1 (s,
CH(CH3)2), 34.1 (s, CH(CH3)2), 32.1 (s, CH(CH3)2), 26.0 (s, CH3), 25.9 (s,
CH3), 25.2 (s, CH3), 24.4 (s, CH3), 24.0 (s, 2 CH3); some aromatic carbon
atoms were not observed.
11B{1H} NMR (160.5 MHz, 283 K, C6D6): δ = 35.6.
31P{1H} NMR (202.4 MHz, 283 K, C6D6): δ = –8.0.
HRMS (TOF ES): m/z calcd for [M + H]+: 682.4357; found: 682.4363
11B{1H} NMR (96.3 MHz, 298 K, CD2Cl2): δ = –0.1.
31P{1H} NMR (162.0 MHz, 253 K, CD2Cl2/C6D6): δ = 1.4.
(error 0.9 ppm) (100%); [M – C9H18N1]+ (12%), [M – C9H18N1O1]+ (10%).
HRMS (TOF ES): m/z calcd for [M – Br]+: 525.2889; found: 525.2894
(error 1 ppm) (100%).
4
To a red solution of 2a (18 mg, 0.038 mmol) in toluene (1 mL) was
added a solution of triphenylmethylchloride (6 mg, 0.038 mmol) in
Et2O (1 mL). The next day, the light orange solution was filtered and
evaporated to dryness. The resulting solid was washed with pentanes
(2 × 0.5 mL) to yield 14 mg (66%) of colorless solid; mp 161 °C.
[2a]2
To a solution of (8-(bromo(2,4,6 triisopropylphenyl)boryl)naphtha-
len-1-yl)diphenylphosphane (200 mg, 0.33 mmol) in toluene (4 mL)
was added 1% sodium amalgam (1.0 g, 0.43 mmol) and the reaction
was stirred vigorously. The next day, the stirring was stopped and 1 h
later the dark red solution was carefully decanted onto a glass filter.
After the filtration and washing with toluene (1 mL), the solution was
let to stand for 24 h and checked for the presence of grey mercury
particles; if these formed, the solution was filtered once more. The
solvent was evaporated to dryness and washed with pentanes (3 × 1
mL) to yield 150 mg (86%) of [2a]2 as a red solid. The product could be
further purified by recrystallization from Et-2O; mp 95 °C (dec.).
At room temperature, the 2,4,6-triisopropylphenyl group rotates
slowly and gives broad signals in the NMR spectra. Hence the 1H and
13C NMR spectra were recorded at 253 K.
1H NMR (500.3 MHz, 253 K, CD2Cl2): δ = 8.18–8.15 (m, 1 H, CHnaph),
3
3
7.82 (d, JHH = 7.5 Hz, 1 H, CHnaph), 7.76 (d, JHH = 7.5 Hz, 1 H, CHnaph),
3
4
7.69–7.63 (m, 3 H, CHAr), 7.56 (td, JHH = 7.5 Hz, JHH = 1.4 Hz, 1 H,
CHAr), 7.49–7.22 (m, 9 H, CHAr), 7.01 (d, 4JHH = 1.4 Hz, 1 H, CHTip), 6.84
(s, 1 H, CHTip), 3.56 (sept, 3JHH = 6.6 Hz, 1 H, CH(CH3)2), 2.84 (sept, 3JHH
=
6.5 Hz, 1 H, CH(CH3)2), 2.09 (sept, 3JHH = 6.6 Hz, 1 H, CH(CH3)2), 1.29 (d,
11B{1H} NMR (160.5 MHz, 298 K, C6D6): δ = 27.7, –3.6.
31P{1H} NMR (121.5 MHz, 298 K, C6D6): δ (major) = 5.46 (s), 0.41 (br);
δ (minor) = 9.58 (br), 4.98 (s).
3JHH = 6.6 Hz, 3 H, CH3), 1.25 (d, 3JHH = 6.6 Hz, 3 H, CH3), 1.24 (d, 3JHH
=
6.6 Hz, 3 H, CH3), 0.81 (d, 3JHH = 6.6 Hz, 3 H, CH3), 0.80 (d, 3JHH = 6.6 Hz,
3 H, CH3), 0.05 (d, 3JHH = 6.6 Hz, 3 H, CH3).
HRMS (TOF ES): m/z calcd for [monomer]+: 525.2889; found:
13C{1H} NMR (125.8 MHz, 253 K, CD2Cl2): δ = 155.5 (d, JCP = 2.6 Hz,
525.2885 (error 0.8 ppm).
CAr), 153.6 (d, JCP = 7.6 Hz, CAr), 147.8 (d, JCP = 2.8 Hz, CAr), 141.0 (d, JCP
30.6 Hz, CAr), 134.0 (d, JCP = 8.2 Hz, CHAr), 133.6 (d, JCP = 9.3 Hz, CHAr),
132.4 (d, JCP = 10.4 Hz, CAr), 132.1 (d, JCP = 2.2 Hz, CHAr), 132.0 (d, JCP
=
=
3
2.7 Hz, CHAr), 131.7 (d, JCP = 2.6 Hz, CHAr), 131.4 (d, JCP = 13.9 Hz, CHAr),
130.7 (s, CHAr), 128.9 (d, JCP = 10.3 Hz, CHAr), 128.8 (d, JCP = 10.6 Hz,
CHAr), 128.7 (d, JCP = 2.7 Hz, CHAr), 127.3 (d, JCP = 63.8 Hz, CAr), 126.5 (d,
JCP = 8.5 Hz, CHAr), 125.2 (d, JCP = 54.0 Hz, CAr), 124.6 (d, JCP = 1.5 Hz,
CHAr), 124.5 (d, JCP = 35.3 Hz, CAr), 122.5 (d, JCP = 2.2 Hz, CHAr), 121.8 (s,
To a stirred red solution of 2a (18 mg, 0.038 mmol) in toluene (1 mL)
was added a solution of TEMPO free radical (6 mg, 0.038 mmol) Et2O
(1 mL). The solution discolored in a few minutes. The solvent was
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–H

G
A. J. Rosenthal et al.
Feature
Synthesis
CHAr), 34.1 (s, CH(CH3)2), 33.6 (s, CH(CH3)2), 31.7 (s, CH(CH3)2), 26.2 (s,
CH3), 25.3 (s, CH3), 24.8 (s, CH3), 24.2 (s, CH3), 23.9 (s, CH3), 23.8 (s,
CH3); some aromatic carbon atoms were not observed.
concentrated by half and the solution layered with pentanes (2 mL) to
yield 11 mg (46%) of the product as beige-orange microcrystals (suit-
able for a single-crystal X-ray diffraction study).
11B{1H} NMR (96.3 MHz, 298 K, CD2Cl2): δ = 3.4.
31P{1H} NMR (202.5 MHz, 298 K, CD2Cl2): δ = 2.9.
HRMS (TOF ES): m/z calcd for [M – Cl]+: 525.2889; found: 525.2881
11B{1H} NMR (128.2 MHz, 298 K, MAS NMR, 10 kHz): δiso = 25.0 (CQ =
2050 kHz, η(quad) = 0.5), 1.50 (CQ = 2200 kHz, η(quad) = 0.1).
31P NMR (161.8 MHz, 298 K, MAS NMR, 20 kHz): δ = 11.67, –3.85.
(error 1.5 ppm).
HRMS (TOS ES): m/z calcd for M = 635.3047; found: 635.3047 (error 0
ppm)
1b
Under argon, (8-iodonaphthalen-1-yl)diphenylphosphane (0.475 g,
1.1 mmol) was suspended in Et2O (4 mL). At –60 °C and under stirring,
n-butyllithium (0.7 mL, 1.6 M solution in hexane, 1.1 mmol) was add-
ed. After an hour at –60 °C, the yellow suspension was cooled to –80
°C and the solvent was filtered out with a cannula. The yellow solid
was dissolved in toluene (5 mL) at –60 °C. A solution of dibromide (2,6
dimesitylphenyl)borane16 (0.500 g, 1.0 mmol) in 10 mL of toluene
was added slowly. The solution was allowed to warm to room tem-
perature and stirred overnight. The red solution was filtered off from
the white solid and the solid was extracted with toluene (10 mL). The
solvent was evaporated and the resulting solid was washed with pen-
tanes (3 × 5 mL) and toluene (3 × 1 mL). The white solid 0.20 g (28%)
was pure 1b; mp 122 °C.
Funding Information
The Centre National de la Recherche Scientifique (CNRS) and the Uni-
versité Paul Sabatier (UPS) are acknowledged for financial support of
this work.
)(
Acknowledgment
We are grateful to Lionel Rechignat (LCC Toulouse) and Yannick Cop-
pel (LCC Toulouse) for assistance with the EPR measurements and sol-
id-state NMR analyses, respectively.
3
1H NMR (500 MHz, 273 K, CD2Cl2): δ = 7.87 (d, JHH = 8.3 Hz, 1 H,
3
CHnaph), 7.67 (d, JHH = 7.0 Hz, 1 H, CHnaph), 7.54–7.40 (m, 5 H, CHAr),
Supporting Information
3
7.34–7.29 (m, 3 H, CHAr), 7.22–7.15 (m, 3 H, CHAr), 7.01 (d, JHH = 7.4
Hz, 1 H, CHTer), 6.92 (t, JHH = 8.9 Hz, 2 H, CHAr), 6.88 (s, 1 H, CHMes),
3
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1610151. Included are experimental
procedures, copies of EPR and NMR spectra, and crystallographic data
6.82 (s, 1 H, CHMes), 6.73 (d, 3JHH = 7.4 Hz, 1 H, CHAr), 6.61 (dd, 3JHH = 8.0
Hz, 3JHP = 10.6 Hz, 2 H, CHAr), 6.09 (s, 1 H, CHMes), 5.64 (s, 1 H, CHMes),
2.31 (s, 3 H, CH3), 2.13 (s, 3 H, CH3), 2.01 (s, 3 H, CH3), 1.94 (s, 3 H,
CH3), 1.80 (s, 3 H, CH3), 1.12 (s, 3 H, CH3).
for 1a, 1b, [2a]2, [2b]2, 3 and 4.
S
u
p
p
ortiInfogrmoaitn
S
u
p
p
o
nrtogI
frmoaitn
13C{1H} NMR (125.8 MHz, 273 K, CD2Cl2): δ = 151.6 (br, CB), 149.5 (d,
References
JCP = 4.9 Hz, CAr), 147.9 (d, JCP = 5.1 Hz, CAr), 144.6 (s, CAr), 141.8 (d, JCP
30.8 Hz, CAr), 141.6 (s, CAr), 139.5 (br, CB), 138.6 (s, CAr), 135.9 (d, JCP
8.1 Hz, CHAr), 135.6 (s, CAr), 134.3 (s, CAr), 134.2 (s, CAr), 133.2 (d, JCP
8.4 Hz, CHAr), 133.1 (s, CAr), 133.0 (d, JCP = 8.6 Hz, CAr), 132.6 (d, JCP
=
=
=
=
(1) (a) Power, P. P. Chem. Rev. 2003, 103, 789. (b) Chivers, T.; Konu, J.
In Comprehensive Inorganic Chemistry II: From Elements to Appli-
cations, Volume 1: Main-Group Elements, Including Noble Gases;
Chivers, T., Ed.; Elsevier: Amsterdam, 2013, 349.
14.6 Hz, CHAr), 132.3 (d, JCP = 2.5 Hz, CHAr), 131.3 (d, JCP = 2.2 Hz, CHAr),
131.2 (br, CHAr), 131.1 (d, JCP = 9.7 Hz, CAr), 130.9 (d, JCP = 3.0 Hz, CHAr),
130.1 (d, 1JCP = 58.9 Hz, CP), 128.9 (d, JCP = 9.7 Hz, CHAr), 128.4 (s, CHAr),
(2) Chiu, C. W.; Gabbai, F. P. Angew. Chem. Int. Ed. 2007, 46, 1723.
(3) (a) Ueng, S. H.; Solovyev, A.; Yuan, X.; Geib, S. J.; Fensterbank, L.;
Lacôte, E.; Malacria, M.; Newcomb, M.; Walton, J. C.; Curran, D.
P. J. Am. Chem. Soc. 2009, 131, 11256. (b) Walton, J. C.; Brahmi,
M. M.; Fensterbank, L.; Lacôte, E.; Malacria, M.; Chu, Q.; Ueng, S.
H.; Solovyev, A.; Curran, D. P. J. Am. Chem. Soc. 2010, 132, 2350.
(c) Matsumoto, T.; Gabbai, F. P. Organometallics 2009, 28, 4252.
(d) Martin, C. D.; Soleilhavoup, M.; Bertrand, G. Chem. Sci. 2013,
4, 3020. (e) Bertermann, R.; Braunschweig, H.; Dewhurst, R. D.;
Hörl, C.; Kramer, T.; Krummenacher, I. Angew. Chem. Int. Ed.
2014, 53, 5453. (f) Silva Valverde, M. F.; Schweyen, P.; Gisinger,
D.; Bannenberg, T.; Freytag, M.; Kleeberg, C.; Tamm, M. Angew.
Chem. Int. Ed. 2017, 56, 1135.
128.3 (d, JCP = 2.6 Hz, CHAr), 128.2 (d, JCP = 10.9 Hz, CHAr), 127.9 (d, JCP
=
4.0 Hz, CHAr), 127.7 (s, CHAr), 127.4 (s, CHAr), 127.0 (s, CHAr), 126.7 (d,
1
1JCP = 59.6 Hz, CP), 126.2 (d, JCP = 48.6 Hz, CHAr), 126.0 (s, CAr), 125.9
(d, JCP = 8.4 Hz, CHAr), 123.5 (d, JCP = 2.0 Hz, CHAr), 23.1 (s, CH3), 22.9 (s,
CH3), 22.7 (s, CH3), 21.5 (s, CH3), 21.0 (s, CH3), 20.5 (s, CH3); some aro-
matic carbon atoms were not observed.
11B{1H} NMR (96.3 MHz, 298 K, CD2Cl2): δ = 0.7.
31P{1H} NMR (202.4 MHz, 273 K, CD2Cl2): δ = 4.6.
HRMS (TOF ES): m/z calcd for [M – Br]+: 635.3074; found: 635.3053
(error 1 ppm).
(4) (a) Ahcheh, F.; Martin, D.; Stephan, D. W.; Bertrand, G. Angew.
Chem. Int. Ed. 2014, 53, 13159. (b) Bissinger, P.; Braunschweig,
H.; Damme, A.; Krummenacher, I.; Phukan, A. K.; Radacki, K.;
Sugawara, S. Angew. Chem. Int. Ed. 2014, 53, 7360.
(c) Braunschweig, H.; Krummenacher, I.; Légaré, M. A.; Matler,
A.; Radacki, K.; Ye, Q. J. Am. Chem. Soc. 2017, 139, 1802.
(5) (a) Bontemps, S.; Devillard, M.; Mallet-Ladeira, S.; Bouhadir, G.;
Miqueu, K.; Bourissou, D. Inorg. Chem. 2013, 52, 4714.
(b) Devillard, M.; Brousses, R.; Miqueu, K.; Bouhadir, G.;
[2b]2
To a suspension of 1b (27 mg, 0.038 mmol) in toluene (1 mL) was add-
ed 1% sodium amalgam (113 g, 0.049 mmol) and the reaction was
stirred vigorously for 24 h. The stirring was stopped and 1 h later the
dark brown solution was carefully decanted onto a glass filter. After
the filtration and washing with toluene (1 mL), the solution was let to
stand for 24 h and checked for the presence of grey mercury particles;
if these formed, the solution was filtered once more. The solvent was
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–H

H
A. J. Rosenthal et al.
Feature
Synthesis
Bourissou, D. Angew. Chem. Int. Ed. 2015, 54, 5722. (c) Devillard,
M.; Bouhadir, G.; Mallet-Ladeira, S.; Miqueu, K.; Bourissou, D.
Organometallics 2016, 35, 3788.
(10) Prohibitively long delays are needed to reach equilibration and
accurately estimate the radical concentration.
(11) Bond lengths of 1.55–1.59 Å were found in CHsp3–CHsp3
dimers of benzazaborolyl radicals; see: Hejda, M.; Lyčka, A.;
Mikysek, T.; Jambor, R.; Růžička, A.; Vinklárek, J.; Wilfer, C.;
Hoffmann, A.; Herres-Pawlis, S.; Dostál, L. Chem. Eur. J. 2016, 22,
15340.
(6) (a) Rosenthal, A. J.; Devillard, M.; Miqueu, K.; Bouhadir, G.;
Bourissou, D. Angew. Chem. Int. Ed. 2015, 54, 9198.
(b) Rosenthal, A. J.; Mallet-Ladeira, S.; Bouhadir, G.; Sosa-Car-
rizo, E. D.; Miqueu, K.; Bourissou, D. Organometallics 2018, 37,
755.
(12) Grimme, S.; Schreiner, P. R. Angew. Chem. Int. Ed. 2011, 50,
12639.
(7) See the Supporting Information.
(8) (a) Clyburne, J. A. C.; McMullen, N. Coord. Chem. Rev. 2000, 210,
73. (b) Rivard, E.; Power, P. P. Dalton Trans. 2008, 4336. (c) Jutzi,
P. Angew. Chem. Int. Ed. 2011, 50, 9020.
(13) Rösel, S.; Balestrieri, C.; Schreiner, P. R. Chem. Sci. 2017, 8, 405.
(14) Yu, L.; Li, Y.; Wang, X.; Wang, X.; Zhou, P.; Jiang, S.; Pan, X. Chem.
Commun. 2017, 53, 9737.
(15) Bartlett, R. A.; Rasika Dias, H. V.; Feng, X.; Power, P. P. J. Am.
Chem. Soc. 1989, 111, 1306.
(9) Couplings to 11B do not exceed 8.5 G, even for NHC-stabilized
boryl radicals which are considered among the leaders in terms
of boron spin density; see ref. 3f.
(16) Grigsby, W. J.; Power, P. P. J. Am. Chem. Soc. 1996, 118, 7981.
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2018, 50, A–H