2,5-Bis-trimethylsilyl substituted boroles

Article abstract of DOI:10.1039/d0dt00393j

This manuscript includes a comprehensive study of the synthesis and spectroscopic features of 2,5-disilyl boroles. Reacting boron trichloride BCl₃ with 2,3-Ph?₂-1,4-(SiMe₃)₂-1,4-dilithiobuta-1,3-diene (Ph? = 3,5

Full text of DOI:10.1039/d0dt00393j

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DOI: 10.1039/D0DT00393J
ARTICLE
2,5-bis-Trimethylsilyl substituted Boroles
Tobias Heitkemper,a Leonard Naß a and Christian P. Sindlinger *, a
Received 00th January 20xx,
Accepted 00th January 20xx
This manuscript includes a comprehensive study of the synthesis and spectroscopic features of 2,5-disilyl boroles. Reacting
boron trichloride BCl3 with 2,3-Ph*2-1,4-(SiMe3)2-1,4-dilithiobuta-1,3-diene (Ph* = 3,5-t-Bu2(C6H3)) allowed reliable access to
1-Chloro-2,5-(SiMe3)2-3,4-(Ph*)2-borole in good yields (60 %). Unlike 2,3,4,5-tetraphenyl haloboroles, this 2,5-bis-
trimethylsilyl substituted chloroborole is thermally stable in solution to up to 130°C. Metathesis reactions of the
chloroborole with metal aryls or the dilithiobutadiene with arylboron dihalides grants access to 1-Ar-2,5-(SiMe3)2-3,4-(Ph*)2
boroles (Ar = Ph, Mes, Ph*, C6F5). Unlike the generally intensely blue-green 2,3,4,5-tetraaryl boroles, brightly orange/red
2,5-bis-trimethylsilyl substituted boroles reveal blue-shifted π/π*-transitions due to a lack of π-system interaction between
borole and 2,5-bound aryls. Light is shed on the synthetic peculiarities for the synthesis of 2,5-disilyl-boroles. While direct
treatment of the respective 1,1-dimethyl-stannole with ArBCl2 via otherwise well-established B/Sn exchange reactions fails,
the selectivity of reactions of 2,3-Ph*2-1,4-(SiMe3)2-1,4-dilithiobuta-1,3-diene with ArBCl2 is solvent dependent and leads to
rearranged 3-borolenes in hydrocarbons. Gutmann-Beckett analysis reveal reduced Lewis-acidity of disilylboroles compared
to pentaphenyl borole.
DOI: 10.1039/x0xx00000x
to provide 1,4-dilithiobuta-1,3-dienes as valuable precursor for
6
further derivatization.5, This allowed for the synthesis of
Introduction
tetraphenylboroles with varying boron-bound substituents in
Among unsaturated five-membered main group heterocycles,
1H-boroles adopt a unique position. Isoelectronic to the elusive
cyclopentadienyl cation, with four π-electrons in cyclic
conjugation via the empty p-orbital of boron, these systems
have been attributed a weakly anti-aromatic character.1-6 Free
boroles are very reactive species that add dihydrogen or silane
Si-H bonds across the π-system,7-10 react as potent Lewis-acids
also towards weak bases,2, 11-13 engage in various Diels-Alder
and ring-expansion reactions14-31 or readily accept two-
electrons to form dianionic 6π-electron systems.32-35
Borolediides are isoelectronic to cyclopentadienyl anions and
the past.2, 33, 55-60
The 2,5-bis-trimethylsilyl-butadiene backbone has recently
found application for s-/ and p-block element metaloles, mainly
driven by the groups of Xi (groups 2 and 13), Saito and Müller
(group 14).61-81 However, only very few examples of 2,5-bis-
trialkylsilyl-substituted boroles are known, all of which have
been accessed via synthetically rather unusual routes (Scheme
1). Saito and coworkers obtained free fluoroborole A via Pb/B
exchange reactions starting from donor-stabilized plumbole.69
Erker and coworkers reported formation of free 2,5-bis-
trimethylsilyl boroles B by 1,1-carboborations after treatment
have thus been used and studied as ligands in transition metal
coordination chemistry.32,
36-46
Only recently accounts of
Saito
Sekiguchi & Lee
boroles as π-ligands for p-block elements (Al, Ge) have been
reported.47, 48 Bearing rather small substituents, free boroles
undergo Diels-Alder dimerization. These dimers can provide a
source of monomeric borole synthons upon thermal
treatment.49-51
Me2tBuSi
Cl
Me2tBuSi
O
O
Li
Li
SiMe3
Ph
Ph
Ph
Ph
BF3
OEt2
Li
SiMe3
SiMe3
B
Me3Si
Me3Si
B
F
Pb
B
Cl
Me3Si
Me3Si
Me2tBuSi
Me2tBuSi
SiMe3
A
With free boroles being so reactive, to date, their synthesis and
successful isolation is limited to relatively few substituents
Erker
Müller & Albers
SiMe3
B(PhF)3
Ge
SiMe3
PhF
(PhF)2B
PhF= C6F5
SiMe3
SiMe3
around the central C4B cycle.52 Most reports on free borole
Cl2BNR'2
B
R
B
R
B
NR'2
K2
Ge
53, 54
chemistry discuss pentaaryl boroles34,
and particularly
SiMe3
SiMe3
SiMe3
R = Ph, NPh2
SiMe3
B
(PhC)4BAr for which reliable synthetic protocols exist. Here, a
key reaction involves the commercially available diphenyl
acetylene which can be readily reductively coupled with lithium
NR'2 = N(SiMe3)2, TMP
TMP = 2,2',6,6'-tetramethylpiperidine
This work:
SiMe3
Me3Si
Me3Si
Ph*
Ph*
Ph*
Ph*
Ph*
ArBCl2
[Ar-M]
Li
Li
B
Cl
B
Ar
a. Institut für Anorganische Chemie. Tammannstr. 4, 37077 Göttingen, Germany
Electronic Supplementary Information (ESI) available: Experimental and Synthetic
Details, depiction of spectra and crystal structures, computational details See
DOI: 10.1039/x0xx00000x
Ph*
Me3Si
SiMe3
Me3Si
Scheme 1 Examples of 2,5-Disilyl-Boroles.
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Journal Name
of bis-alkynylboranes with tris(pentafluorophenyl)borane metal and thus we had to take the detour via established
View Article Online
BPhF .82 Other than these free boroles, the borole motive can zirconacyclopentadiene protocols using Negishi’s or Rosenthal’s
DOI: 10.1039/D0DT00393J
3
be found in Sekiguchi’s and Lee’s dilithio borolediide obtained reagents.89,
2,5-Disilyl-zirconacyclopentadiene 1 forms in
90
from reduction of chlorobora pyramidane as well as Müller’s excellent yields (>95%) (Scheme 2). However, likely due to steric
and Alber’s Ge(II) complex of a borolediide.47, 83
pressure of the substituents 1 reversibly eliminates alkyne over
We are eager to extend the library of accessible free boroles time (see Supporting information).91 Our attempts for direct
with regards to the substituents attached to boron and carbon Zr/Sn transmetallation of 1 with Me2SnCl2 to stannole 5 failed
atoms of the central moiety and to study the electronic impacts (see ESI). Reactions of 1 with boron halides RBCl2 neither
of each individual substituent.84 Here we report on the produced boroles 6 or boryl-borolenes 7 (vide infra) but led to
synthetic access to 2,5-bis-trimethylsilyl substituted boroles.
formation of yet unidentified products.
CuCl-supported iodination of
in
situ
generated
zirconacyclopentadiene 1 reliably yields the anticipated (Z,Z)-
isomer of 1,4-diiodo-butadiene 2.92 In our hands, iodination of
previously isolated 1 under identical conditions lead to
unfavourable mixtures of (Z,Z) and (E,Z)-isomers of 2 (see SI). 2
is conveniently transformed into the 1,4-dilithiobuta-1,3-diene
3 with tert-butylithium in essentially quantitative yield (97%).
1,4-Dilithiobuta-1,3-diene 3 is obtained as an intensely orange
crystalline solid which revealed a dimeric structure in the solid
state (Figure 1). The central distorted [Li4]-tetrahedron reveals
its longest Li–Li distances between Li-atoms that are connected
to the same butadiene-dianion. All lithium atoms feature a
further short contact (Li–CTMS 2.423(3) – 2.472(3)) to one silicon-
bound methyl group thus also providing steric protection of the
reactive nucleus. A related structure was previously reported by
Saito and coworkers.88
Results and Discussion
Precursor Synthesis
Free boroles (RC)4BR’ are usually accessed via salt metathesis
reaction of 1,4-dilithiobuta-1,3-dienes or B/Sn exchange
reactions of the respective stannole with boron dihalides RBX2.
Stannoles are again either obtained from 1,4-dilithiobuta-1,3-
dienes or Fagan-Nugent-type Sn/Zr transmetallations from the
respective zirconacyclopentadiene.85-87 1,4-Dilithiobuta-1,3-
dienes are accessed by reductive coupling of acetylenes with
lithium or from 1,4-diiodobuta-1,3-dienes. The latter are
generally derived from zirconacyclopentadienes. However,
access to any of these (cyclic) 1,4-dimetalla-1,3-butadiene
precursors strongly depends on the nature of the substituents.
Especially coupling of silylacetylenes over lithium is heavily
depending on the substituents.88
When Li/I exchange was attempted applying n-butyllithium
followed by subsequent quenching of in situ generated 3 with
Me2SnCl2 at 0°C, rearrangements were observed to take place
and we isolated and identified silole 4 as a major product.93
Under these conditions, butyl iodide apparently reacts with
We are keen to provide systems that are soluble in apolar
hydrocarbon solvents and therefore anticipated 3,5-di-t-
butylphenyl-trimethylsilyl acetylene C Ph*CCSiMe3 (Ph* = 3,5-t-
Bu2(C6H3)) to be a suitable building block to start from.
Acetylene C however did not reveal any reaction with lithium
Si
Me3
Cp2ZrCl2
2 equiv. n-BuLi
2 equiv.
or
Cp2Zr(TMS2C2)(py)
Rosenthal's reagent
Ph*
tBu
tBu
C
Li
Si
Si
Me3
Si
Me3
Si
Me3
2.1 equiv.
I2
4 equiv.
t-BuLi
Ph*
Ph*
Ph*
Ph*
Ph*
Ph*
Li
I
I
Zr
Cp2
Figure 1 ORTEP plot of the solid-state structure of dimeric 1,4-dilithiobuta-1,3-diene (3)2.
Anisotropic displacement parameters are drawn at 50% probability level. Selected bond
length [Å] are given: Li1–C1 2.105(3), C1–Li2 2.173(3), Li1–C4 2.169(3), Li1–C39 2.198(3),
Li2–C4 2.112(3), Li2–C42 2.214(3), Li3–C39 2.114(3), Li3–C42 2.170(3), Li4–C42 2.100(3),
Li4–C39 2.165(3), Li4–C4 2.195(3), C1–Li3 2.215(3), Li1–Li2 2.721(3), Li1–Li3 2.390(3),
Li1–Li4 2.496(3), Li2–Li4 2.386(3), Li2–Li3 2.482(3), Li3–Li4 2.696(3), C1–C2 1.365(2), C2–
C3 1.542(2), C3–C4 1.368(2), C39–C40 1.366(2), C40–C41 1.538(2), C41–C42 1.367(2),
Li4–C10 2.423(3).
CuCl
Li
Si
Me3
Me3
Si
Me3
2
(98 %)
85 %
97 %
3
2
1
2 equiv.
1 equiv.
Me2SnCl2
work-up n-BuLi
Si
Me3
Bu
Ph*
Ph*
Ph*
Sn
Me2
Si
Me2
Ph*
Si
Me3
R = Ph*
Si
R = Me3
R
Ph*
Ph*
Ph*
Ph*
Ph*
Ph*
Ph*
B
Ph Cl2
B
Ar Cl2
Si
Me3
Si
Me3
B
Ph
B
Ar
Sn
Me2
- Me2SnCl2
(58 %)
5
(30 %)
4
Ar = Ph, XylF
Ph*
Si
Me3
R
Scheme 2 Precursor syntheses.
> 95 % (NMR)
0 % (NMR)
Scheme 3 Attempted Boron-Tin-Exchange Reactions.
2 | J. Name., 2012, 00, 1-3
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dilithio-butadiene 3. Thus, using t-BuLi to produce 3 remained
the more reliable approach. In the presence of HMPA
(OP(NMe2)3), formation of 1,1-dimethylsiloles from
View Article Online
DOI: 10.1039/D0DT00393J
rearrangements
of
1,4-dilithio-1,4-disilylbuta-1,3-dienes,
presumably via 2-lithiosilole and MeLi elimination, was
described earlier by Xi and coworkers.94, 95
Reaction of 3 with a mild electrophile such as Me2SnCl2 yielded
the 1,1-dimethylstannole 5 in moderate crystalline yields
(58%).96,71 Unfortunately however, the well-established boron-
tin exchange reaction with arylboron dichlorides RBCl2 (R = Ph,
XylF {XylF = 3,5-(CF3)2(C6H3)}), that allows convenient and clean
access to boroles with 2,3,4,5-tetraaryl-butadiene backbones5,
33, 84 does not occur in the case of this 2,5-disilyl system (Scheme
3). No formation of Me2SnCl2 is observed in NMR screening
reactions. Side reactions leading to intractable product mixtures
likely involve methyl abstraction from the tin atom in 5. An
abnormal B/Sn exchange reaction behaviour was recently
reported by Braunschweig and coworkers.97
Figure 2 a) ORTEP plot of the solid-state structure of 2-boryl-3-borolene (7-Ph).
Anisotropic displacement parameters are drawn at 50% probability level. Only one
moleculare within the asymmetric unit is shown. Selected bond length [Å] are given: B1–
Cl1 1.766(4), B1–C1 1.569(6), C1–C2 1.533(5), C2–C3 1.353(6), C3–C4 1.545(5), B1–C4
1.586(6), C4–B2 1.553(5), B2–Cl2 1.814(4).
With MesBCl2 the THF route fails completely. In Et2O, borole
formation was much less selective and did not provide
satisfyingly pure material. However, when 3 was analogously
reacted with phenylboron dichloride in hexane or benzene in
order to access free organoboroles, irrespective of the applied
stoichiometric ratio of the reagents, a clean consumption of 2
equiv. PhBCl2 per 3 took place yielding a 2-borylated 3-borolene
7-Ph. The structure of racemic mixtures of 7-Ph is confirmed by
an X-ray structure (Figure 2) and reveals a double bond between
C2 and C3. We reason, that formation of 7-Ph involves the
intermediate generation of the anticipated phenyl-borole and
rapid subsequent addition of a second equivalent of PhBCl2 via
an electrophilic attack of PhBCl2 at one Cα atom in the borole
(Scheme 5).
These Cα positions should be considerably nucleophilic given
they bear two electropositive atom substituents (-SiMe3 and –
BR2) and the Si β-effect should further moderate the α-addition
of the B-electrophile in these special cases of vinylsilanes.
Within the rearrangement sequence, the B-bound Ph-residue
then migrates to the other Cα when a chloride adds to the
borole B-atom. The addition of a second equivalent ArBCl2 to 6-
Ar is facilitated by the removal of anti-aromatic cyclic 4π-
electron delocalization and stabilizing lonepair π-donation from
Cl into the empty p-orbital of boron. Addition of B-H moieties
across a borole to give 2-boryl-3-borolenes where reported
earlier.10 Somewhat related chemistry was described for the
reaction of azobenzene with (PhC)4BPh.98 Similar reaction
behavior consuming 2 equiv. of aryl boron dihalides per 3 are
also observed for the reactions with ArBCl2 (Ar = Ph*, XylF). To
support the proposed mechanism via formation of the free
borole, 1 equiv. of isolated free borole 6-Ph* was treated with
the respective Ph*BCl2 to give the same asymmetric compound
2-boryl-3-borolene 7-Ph* as the reaction of 3 with 2 equiv. of
Ph*BCl2, thus corroborating the proposed mechanism (see ESI).
Notably, with Ar = Mes (Mes = 2,4,6-Me3(C6H2), no reaction
occurred at all, while (PhC)4BMes is reported to be readily
formed from (PhC)4Li2 and MesBCl2.58 Also when the
electrophilicity of the boron species is reduced such as in i-
Being short of one major synthetic pathway to boroles, we thus
turned to direct treatment of 1,4-dilithiobutadiene 3 with
organoboron dihalides. Reactions of 3 and 1 equiv. of ArBCl2 in
THF revealed to cleanly (>90% by NMR) form the aryl boroles
and allowed isolation of 6-Ph and 6-Ph* in good yields (Scheme
4). However, quantitative removal of THF from the resulting
Lewis-acidic boroles in vacuo (10-3 mbar) for several days was
found to be tedious. The application of THF is therefore usually
avoided when free boroles are targeted. While 6-Ph* was
obtained free of donor solvent after drying in vacuo, 6-Ph
always contained residual amounts of THF, despite their Lewis-
acidities were found to be identical (Gutmann-Beckett method,
see below). At mildly elevated temperatures (40°C) THF is
liberated from 6-Ph, however substantial decomposition
occurs.
Si
Me3
Si
Me3
Ph*
Ph*
1 equiv.
ArBCl2
Ph*
Ph*
Li
B
Ar
0.5
THF
Li
Ar = Ph, Ph*, Mes
Si
Me3
Si
Me3
2
(56 %)
(6-Ph)
•(THF)0.65
3
(46 %)
(0 %)
6-Ph*
6-Mes
Scheme 4 Reaction of 3 towards boron electrophiles in THF (yields after crystallisation).
Ar
Si
Me3
Si
Me3
Si
Me3
Ph*
Ph*
Ph*
Ph*
2 equiv.
ArBCl2
1 equiv.
Ph*
Ph*
Li
B
Cl
B
Cl3
B
0.5
Cl
pentane or
benzene
hexane
Li
Si
B
Me3
Cl
Si
Me3
Si
Me3
2
Ar
(rac)
7-Ar
60 %
(
)
3
8
quant. (NMR)
Ar = Ph, Ph*
1 equiv.
ArBCl2
Ar
Si
Si
Me3
Me3
1 equiv.
PhBCl2
Ph*
Ph*
Cl
B
B
Ar
Cl
Ph*
Ph*
B
Ar
Si
Si
Me3
Me3
putative transition state
putative intermediate
Scheme 5 Reaction of 3 towards boron electrophiles in hydrocarbons.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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Pr2NBCl2 or in the Lewis-adduct PhBCl2(DMAP) (DMAP = 4- While substituents at C4 and C3 essentially lie within the plane,
View Article Online
DOI: 10.1039/D0DT00393J
(N’,N’-dimethylamino)pyridine), dilithio-butadiene 3 did not substituents at B1, C1 and C2 alternatingly bend out above or
reveal any reaction in C6H6 or Et2O.
below the central plane by ca. 11(1)° each. The amount of
To our surprise, when a solution of 3 was treated with 1 equiv. known 1-haloboroles in total is limited to four stable examples.
of boron trichloride in hexane, chloroborole 8 was isolated in Eisch’s and Braunschweig’s previously reported 2,3,4,5-
moderate yields of about 60% as an orange-red crystalline solid. tetraphenyl-haloboroles either dimerize at 40°C ((PhC)4B-Cl)55
Unlike in the previously discussed reactions of arylboron or 55°C ((PhC)4B-Br)57 or even decompose at ambient
dihalides, feasability of the isolation of 8 likely stems from temperature over time. Marder reported on a transient
stabilizing np-π-donation interactions reducing both the Lewis- chloroborole that readily dimerizes.99 This 2,5-disilyl
acidity and anti-aromatic character of chloroborole 8. An X-ray chloroborole 8 is remarkably thermally and even forcing
crystallographic examination revealed the structure of 8 as only conditions in toluene at 130°C (in a teflon-valve sealed NMR-
the third example of a crystallographically characterized tube) for several hours did not indicate any decomposition of 6.
haloborole (Figure 3).
Its thermal stability thus more resembles Piers’ perfluorinated
Bond lengths within the C4B-ring are essentially identical to A 2,3,4,5-tetraphenyl bromoborole (PhFC)4B-Br (PhF = (C6F5)).54
and (PhC)4B-Cl with the B1-Cl1 bond of 1.7543(17) Å in 8 being Stable haloboroles are a key synthetic precursor to borolyl-
slightly longer than in the latter (1.7433(13) Å).55 Unlike these substituted systems. By functionalization of (PhC)4BCl,
haloboroles, the substituents around the central C4B-ring Braunschweig and coworkers previously accessed amino-
slightly bend out of the least-square plane through the borole boroles or even oxidatively added the B-Cl bond across the zero-
atoms (Figure 3b).
valent metal in [Pt(PCy3)2].2, 56
The 11B-NMR signal of 8 is found as a broad resonance (ω1/2
=
1370 Hz) at δ11B = 70.8 ppm. This is more lowfield shifted than
in (PhC)4BCl (δ11B = 66.4 ppm) and (PhFC)4B-Br (δ11B = 67 ppm).
Saito’s structurally related example of 1-F-2,5-(SiMe2t-Bu)2-3,4-
(Ph2)-borole (A), however, features an 11B-NMR resonance at
δ11B = 54.9 ppm in line with a pronounced B–F π-interaction.
Figure 3 a) ORTEP plot of the solid-state structure of chloroborole (8). Anisotropic
displacement parameters are drawn at 50% probability level. b) Excerpt of the central
substituted C4B-ring. Selected bond length [Å] are given: B1–Cl1 1.7543(17), B1–C1
1.564(2), C1–C2 1.354(2), C2–C3 1.538(2), C3–C4 1.353(2), B1–C4 1.568(2), C1–Si1
1.8683(15), Si2–C4 1.8725(16).
Si
Me3
Si
Me3
Ph*
Ph*
Ph*
Ph*
[ArM]
B
Cl
B
Ar
toluene
Si
Me3
Si
Me3
1/n (Ph*Li)n or
0.5 ZnPh*2
8
[ArM] =
(30 %)
(73 %)
(75# %)
6-Ph*
[ArM] = 0.5 (MesLi)2
6-Mes
6-PhF
[ArM] = 0.5 ZnPhF
2
#) repeatedly recrystallised product contained ca. 5-10% of an
impurity that could not be separated.
Figure
4 ORTEP plots of the solid-state structure of boroles (6-Ar). Anisotropic
displacement parameters are drawn at 50% probability level. Lattice solvent molecules
and disordered t-Bu groups are omitted for the sake of clarity. Top: 6-Ph*, Middle: 6-
Mes, Bottom: 6-PhF. Key structural features are summarized in Table 1.
Scheme 6 Formation of 6-Ar from 8 (yields after crystallization).
4 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/D0DT00393J
Table 1 Structural, spectroscopic and computational details of 2,5-(SiMe3)2-Boroles and some references.
a
entry
B-Cα,Cα-Cβ, Cβ-Cβ
C4B-Ar
torsionb
-
δ 13
C
δ 11B c
70.8
76.6
77.1
79.9
77.4
65.4
-
λexp,d (λcalc)e,
HOMO/LUMO/
gapg
NICSh
δ 31Pi
AN
f
(Cα,Cβ) c
ελ
8
1.568(2), 1.353(2), 1.538(2)
134.8, 182.1
447, (450)
258
480, (475)
-
473, (469)
130
≈470, (462)
310
-5.33/-3.55
1.78
-5.16/-3.50
1.66
-5.11/-3.41
1.70
-5.17/-3.45
1.72
-5.37/-3.83
1.54
-4.84/-3.64
1.20
-5.72/-3.95
1.77
14.0
6.5
15.1
7.4
14.7
7.1
14.6
6.8
16.7
8.4
13.6
7.2
20.6
11.5
73.0
70.7
73.6
72.1
73.6
72.1
46.1
11.3
74.4
73.8
76.6 100
78.7
-
chloroborole
6-Ph
1.564(2), 1.354(2)
-
-
139.2, 183.1
139.2, 182.8
138.8, 181.4
137.6, 184.8
137.9, 162.1
-
6-Ph*
6-Mes
6-PhF
1.587(3),1.355(3), 1.540(4)
51
85
59c
33
-
1.590(3), 1.360(4), 1.538(4)
1.594(4), 1.359(3)
1.577(4), 1.359(2), 1.539 (2)
≈505, (510)
121
560, (577)
D1
(PhC)4BPh
E
1.526(2), 1.428(2), 1.470(2)
1.539(2), 1.425(2)j
1.585, 1.348, 1.520g
- , (466)
(HC)4BH
a) In [Å]; b) Torsion angle in [°]; c) in parts per million, ppm; d) Absorption bands of lowest energy in nm; e) TD-DFT: RIJCOSX-CAM-B3LYP\def2-SVP\\RI-BP86-D3BJ\def2-
TZVP; f) in L mol-1 cm-1, g) RI-BP86\def2-TZVP, energies in eV; h) NICSiso(0) and NICSiso(1) GIAO PBE0\def2-TZVP; i) Gutmann-Beckett Lewis-acidity scale parameters
derived from mixtures with Et3PO in C6D6. Acceptor Numbers AN = 2.21 × (δ31P –41); j) Bond length as reported in [1]. Note that C4B bond length in this structure markedly
deviate from other known pentaaryl boroles.
To explore the synthetic potential of chloroborole 8, we probed The 11B chemical shifts in 6-Ar are found at comparatively low
the accessibility of boroles by salt-metathesis approaches field (Ar = Ph: 76.6 ppm; = Ph*: 77.1 ppm; = Mes: 79.9 ppm; =
starting from 8. When (Ph*Li)n, (MesLi)2 or Zn(PhF)2 are added PhF: 77.4 ppm). Pentaaryl boroles 11B resonances are usually
30, 54, 84
to toluene solutions of 8, NMR monitoring indicates the crudes found between 65 and 75 ppm1,
and Erker’s B-Ph
to primarily contain the respective aryl boroles 6-Ar (>80%), (Scheme 1) at 74.7 ppm.82 Previously, only sterically congested
however for 6-Ph* and 6-PhF isolated crystalline yields were mesityl substituted (ArC)4BMes boroles revealed 11B resonances
lower. Notably, attempts to obtain 6-Ph from this approach that low-field shifted (Ar = Ph: 79 ppm; Ar = thienyl: 77 ppm)
with PhLi or Ph2Zn in toluene failed and gave intractable product indicating, that increasing perpendicularity of the B-bound aryl
mixtures, that contained intensely colored side products. An correlates with low-field shifts.
intensely green side product (<1 % NMR) of unknown
constitution was also observed in case of 6-Ph* when prepared Properties of 2,5-Disilylboroles
via the Ph*Li route. Only reactions of 8 with 0.5 equiv MgPh2 in
NICS101-103 values for 6-Ar and 8 were calculated and NICS(0) and
THF led to clean formation of 6-Ph which was again
contaminated by THF. X-ray structures were obtained for 6-Ar
(Ar = Ph*, Mes, PhF) (Figure 4). Crystals of 6-Ph were repeatedly
found to be thin needles that diffracted poorly. Comparable
bond length within the central C4B ring of all boroles 6-Ar are
NICS(1) are tabulated in Table 1. The impact of the 2,5-disilyl
substitution pattern on the (anti)aromaticity of boroles causes
the NICS(0) values to be lower than the parent E (20.6), but
higher (with a maximum of 16.7 for 6-PhF) than D (PhC)4BPh
(13.6). However the NICS-profiles of 6-Ar drop steeper and
NICS(1) values do not differ significantly from D (see SI). The
virtually identical and are well in the range of Erker’s related 1-
Ph-2,5-Disilylborole (B-Ph, Scheme 1)82 and pentaaryl boroles34,
Lewis acidity of 2,5-disilylboroles was probed by means of the
54, 84 (except for (PhC)4BPh D)1.
Gutmann-Becket approach that correlates the 31P chemical shift
of Et3P=O interacting with Lewis acids with their acidity.104-106
As observed for chloroborole 8, in all cases the silyl-groups, and
For steric reasons the Lewis-acidity of 6-Mes is poorly
in some cases but much less pronounced Cβ-Ph*, bend out of
accounted for by this method but 8 (70.7), 6-Ph* (72.1), 6-Ph
(72.1) and even 6-PhF (73.8) all reveal similar acceptor numbers
(AN) well below those previously determined under identical
conditions for D (78.7).100 The bulky and electropositive silyl
the central C4B-plane by 9-15°. Even for aryls (such as Ph or Ph*)
that do not feature substituents in ortho-position that would
directly govern this torsion angle, the boron-bound aryls reveal
rather large torsion angles between the respective C4B- and
groups seem to lower the Lewis-acidity, however the method
aryl-planes of >50°. This is likely owing to the bulky silyl groups.
cannot provide insight which influence is dominating.
With pentaaryl boroles, this torsion angle usually lies between
Since the isolation of deeply-blue (PhC)4BPh (λmax 560 nm),5 a
striking feature of the hitherto well-characterized examples of
pentaaryl boroles is their intense color with broad absorptions
in the visible spectra roughly spanning from λmax 530 nm (purple
(PhFC)4BPhF)54 to 635 nm (green (PhC)4BPhF)30. Substitution with
15-30°.The torsion of the Cβ-bound aryls is much less affected.
For 6-Ar these torsions are found between 52-58° not much
different from the torsions in (PhC)4BAr (45-55°)1, 34 or (Ph*C)4B-
Ar (52-58°).84
heteroaryls (e.g. thiophene) allowed even stronger shifts of
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DOI: 10.1039/D0DT00393J
Figure 5 Difference Density plots for the computationally predicted lowest energy
excitations of a series of boroles at an isovalue of 0.001 a.u.; a) (HC)4BH E, b) (PhC)4BPh
D, c) 6-Mes, d) 6-Ph*.(positive: green; negative: magenta). TD-DFT RIJCOSX-CAM-
B3LYP\def2-SVP.
Figure 6 TD-DFT predicted shifts in excitation wavelength of (PhC)4BPh depending on the
torsion angle between C4B-plane and B-bound, Cα-bound or Cβ-bound phenyls.
λmax.53, 60 The 2,5-disilyl-substituted boroles reported here are all
orange to red both in solid state and in solution with absorption
bands around 470 nm. We originally anticipated that having
electropositive substituents, such as the SiMe3 group, attached
to the Cα-carbons in boroles should narrow the π/π* gap, which
is dominatingly responsible for the intense colorization, and
thus induce a red-shifted absorption. However, the opposite
seems to be the case. We therefore computationally probed the
photochemical properties of the elusive parent borole (HC)4BH
(E), the 2,5-disilyl-boroles 6-Ar and pentaphenyl borole
(PhC)4BPh (D) (Figure 5).
TD-DFT calculations on 8 and 6-Ar reproduced the experimental
absorption spectra very well. In all cases the lowest energy
absorption is dominated by the π/π* transition between the
borole C4B based HOMO and LUMO. The absorption for the
π/π* transition in parent (HC)4BH E is predicted at 466 nm well
in the range where the respective resonances for 6-Ar are
found. The difference density plots for the respective
excitations of E and 6-Mes are very similar and highlight only
minor contributions from substituents (Figure 5).
Starting from a hypothetical structure of (PhC)4BPh with all
phenyl groups perpendicular to the C4B plane, TD-DFT predicts
the absorption at 504 nm. Minimizing effective π-interaction
thus apparently results in blueshifted absorptions closer to
unperturbed (HC)4BH (E). From there on, gradually reducing the
C4B-Phi torsion angle at B-, Cα- and (to a reduced extent due to
eventual collapsing overlap) Cβ-bound aryls to coplanarity
allows a qualitative insight into the individual effects. While co-
planarily bound B-Ph groups lead to only mild blue-shifted
absorptions likely due to a LUMO raise on account of more
effective π-interaction, particularly co-planarily arranged Cα-
bound phenyls reveal a strong red-shifting effect. This further
corroborates the essential impact of Cα-bound aryls on the color
and HOMO/LUMO gap of boroles that we identified from their
substitution by silyl groups. The vast influence of the Cα aryl
torsion may also be a reason for the broad bands (ω1/2 ≈ 200
nm) commonly observed for these transitions in pentaphenyl
boroles. Some previous studies on heteroaryl substituted
boroles already suggested the importance of torsion-angle
dependent π-interaction affecting the spectroscopic features.53,
60
Pentaphenylborole (PhC)4BPh however reveals major
contributions both from the boron-bound (accepting) and Cα-
bound (donating) phenyl π-systems. From comparison with our
2,5-disilylboroles we reason, that particularly the π-interaction
of C4B with Cα-bound aryls causes the red-shifted absorption
that leads to the intense purple to green colors of (substituted)
pentaphenyl boroles. Thus the 2,5-disilylboroles without
perturbation of the C4B π-system by arene ligands represent a
much more accurate synthetic model of the parent, but elusive,
(HC)4BH E when it comes to direct comparisons of the frontier
orbital situation. This is also reflected in the HOMO-LUMO
energies (Table 1).
Conclusion
In summary we presented various synthetic approaches to 2,5-
disilyl boroles and shed light on their limitations. While
traditional routes via boron-tin exchange reactions from
stannoles fail, the reaction of the 1,4-dilithiobutadiene with aryl
borondichlorides provides access to substituted boroles when
the reaction is conducted in THF. In hydrocarbons two
equivalents of arylboron dihalides react with 1,4-
dilithiobutadienes to afford 2-boryl-3-borolenes, putatively via
the free borole. Treatment of 1,4-dilithiobutadiene with BCl3
grants access to a thermally robust chloroborole. The reactions
of the chloroborole with common available aryl-carbon
nucleophiles such as aryllithium, Grignard reagents and arylzinc
To shed further light on the influence of aryl π-system
interaction in boroles, in a qualitative computational approach,
we probed the individual influence of C4B bound aryls on the
computationally predicted π/π* absorption band (Figure 6).
6 | J. Name., 2012, 00, 1-3
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π/π*-absorption features of 2,5-disilyl-boroles are distinctive
from pentaaryl boroles in that they do not reveal deeply blue
colors. The differences were routed back to absence of π-
interaction contributions stemming from Cα-bound aryl groups
that are the foundation of the purple-to-blue color of pentaaryl
boroles. Spectroscopically and regarding the frontier orbital
situation, 2,5-disilylboroles are much closer to the parent, yet
elusive borole (HC)4BH. Given the field of free borole chemistry
has been dominated by tetraphenyl-butadiene systems for 50
years, the new boroles and their access routes reported in this
contribution represent a significant extension to the existing
library of substitution patterns that allow the handling of free
boroles. We are currently exploring the chemical potential of
these boroles.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
Experimental
Extensive synthetic and analytical details are given in the
Electronic Supplementary Information.
Conflicts of interest
Content of this manuscript is or will be included in the PhD/BSc
theses of T.H. and L.N. There are no conflicts to declare.
J. H. Barnard, S. Yruegas, K. Huang and C. D. Martin, Chem.
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Acknowledgements
Prof. Dietmar Stalke is most gratefully acknowledged for
generous and supportive mentorship. We are grateful to the
Fonds der chemischen Industrie for funding (fellowships to CPS
and TH). The institute of inorganic chemistry is thanked for the
provision of excellent conditions. We are grateful to LANXESS
Organometallics GmbH for a chemical donation. MSc Paul
Niklas Ruth is acknowledged for crystallographic advice and
expertise. This contribution was funded by the Deutsche
Forschungsgemeinschaft (DFG, German Research Foundation) -
389479699/GRK2455.
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