From C,N- and N,N-chelated chloroboranes to substituted 1H-2,1-benzazaboroles and 1H-pyrrolo[1,2-c][1,3,2]diazaborolidines: A straightforward route to five-membered rings containing the B-N or N-B-N moiety

Article abstract of DOI:10.1039/c4dt01445f

A set of 1H-2,1-benzazaboroles as B-N analogues of 1H-indene and a set of 1H-pyrrolo[1,2-c][1,3,2]diazaborolidines as B-N analogues of 1H-pyrrolizine were prepared via nucleophilic addition of selected alkyl(aryl)lithiums (MeLi, tBuLi or PhLi) to, via N→B intramolecular interactions, the activated imino CN functionality in the structure of C,N- or N,N-chelated chloroboranes. All compounds were characterized by elemental analysis and ¹H, ¹¹B, ¹³C and ¹⁵N NMR spectroscopy, and molecular structures of isolated compounds were on several occasions established by means of single-crystal X-ray diffraction analysis. The presence of three adjacently bonded substituents and their systematic alternation on five-membered C₃BN (1H-2,1-benzazaboroles) or C₂BN₂ (1H-pyrrolo[1,2-c][1,3,2]diazaborolidines) rings allowed us to follow both the influence of the steric repulsion and limitations for the formation of respective annulated heterocyclic systems. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt01445f

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Journal Name
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ARTICLE
From C,N- and N,N-chelated chloroboranes to
substituted 1H-2,1-benzazaboroles and 1H-
pyrrolo[1,2-c][1,3,2]diazaborolidines: A
Cite this: DOI:
10.1039/x0xx00000x
straightforward route to five-membered rings
containing B-N or N-B-N moiety
†
Received 00th January 2012,
Accepted 00th January 2012
Martin Hejdaa Antonín Lyčkab Roman Jambora Aleš Růžičkaa
and Libor Dostál*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
A set of 1Hꢀ2,1ꢀbenzazaboroles as BꢀN analogues of 1Hꢀindene and 1Hꢀ
pyrrolo[1,2ꢀc][1,3,2]diazaborolidines as BꢀN analogues of 1Hꢀpyrrolizine
were prepared via nucleophilic addition of selected alkyl(aryl)lithiums
(MeLi, tBuLi or PhLi) to, via N
→B intramolecular interaction activated,
imino C=N functionality in the structure of C,Nꢀ or N,Nꢀchelated
chloroboranes. All compounds were characterized by the help of elemental
analysis; 1H, 11B, 13C and 15N NMR spectroscopy and molecular structures of
isolated compounds were on several occasions established by means of
singleꢀcrystal Xꢀray diffraction analysis. The presence of three
adjacently bonded substituents and their systematic alternation on fiveꢀ
membered C3BN (1Hꢀ2,1ꢀbenzazaboroles) or C2BN2 (1Hꢀpyrrolo[1,2ꢀ
c][1,3,2]diazaborolidines) rings allowed to follow both the influence of
the steric repulsion and limits for the formation of respective
annulated heterocyclic systems.
mechanism of this reaction involves nucleophilic addition of
anilides across the imino C=N bond, that is activated by the
Introduction
The chemistry of intramolecularly Nꢀdonor coordinated boranes
strong N→B intramolecular interaction.8 This procedure has
been successfully tested with variety of substrates, thereby
suggesting new possible application of C,Nꢀchelated boranes
i.e. preparation of BꢀN heterocyclic compounds. In this paper,
two major problems are targeted.
containing various types of ligands has received considerable
interest in recent years.1 These complexes often exhibit
desirable and tuneable photophysical and electronic properties
such as strong fluorescence with high quantum yields.2ꢀ5
Therefore, they are studied in relation to their utilization as
materials for fluorescent labels in biology,2 laser dyes,3 organic
lightꢀemitting diodes,4 molecular photoswitches5 etc. It has also
i) Is it possible to use other nucleophiles in similar reactions to
that illustrated in Scheme 1? For this purpose, the reactivity of
alkyl(aryl)lithiums (MeLi,
nucleophilic carbanions, toward chloroboranes L1,2BCl2 (
and L1,2BPhCl ( ) (where L1 = [2ꢀ(CH=N Bu)C6H4], L2 = [2ꢀ
(CH=Nꢀ2,6ꢀ Pr2C6H3)C6H4]) was studied with the aim of the
preparation of substituted 1 ꢀ2,1ꢀbenzazaboroles. Intended 1
2,1ꢀbenzazaboroles would also be interesting precursors for the
preparation of 1 ꢀ2,1ꢀbenzazaborolyl anions as analogues of
tBuLi or PhLi), as a source of
been shown that Nꢀdonor ligands are able to stabilize borenium
1, 3)
cations, which are able to activate molecular hydrogen.6 Liu et
al. have recently achieved isolation of 1,2ꢀBꢀN cyclopentane
and 1,2ꢀBꢀN cyclohexane and some of these compounds have
been studied as promising hydrogen storage materials.7 We
have recently reported on an unusual reactivity of C,Nꢀchelated
(di)chloroboranes with sterically encumbered lithium anilides.8
These reactions led to the facile formation of highly substituted
2
,
4
t
i
H
Hꢀ
H
indenyl anion (Scheme 2A) as we have recently
demonstrated.10
ii) The second important point is, whether this type of reaction
(i.e. nucleophilic attack at the activated imino C=N bond) is
1H
ꢀ2,1ꢀbenzazaboroles7 (Scheme 1), which were usually
accessible by complicated reaction paths.9 The plausible
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also applicable for the formation of other BꢀN heterocyclic Compounds 1-3 were prepared according to the literature
systems. In this context, the reactivity of N,Nꢀchelated boranes procedure7 and compound
was synthesized in an analogous
L3,4BPhCl ( ) derived from the pyrrole backbone (where L3 manner via the reaction of PhBCl2 and L2Li. Compound
was
[2ꢀ(CH=N Bu)C4H3N], [2ꢀ(CH=Nꢀ2,6ꢀ observed as a pale yellow crystalline solid in 69 % yield, which
L4
Pr2C6H3)C4H3N]) with alkyl(aryl)lithiums was investigated is well soluble in dichloromethane and aromatic solvents and
targeting the formation of ꢀpyrrolo[1,2ꢀ shows only limited solubility in hexane and pentane. The
c][1,3,2]diazaborolidines (Scheme 2B). To the best of our identity of was established by the help of elemental analysis,
knowledge, isolated 1
ꢀpyrrolo[1,2ꢀc][1,3,2]diazaborolidines 1H, 11B, 13C and 15N NMR spectroscopy (see Experimental
are unknown. These compounds would be BꢀN analogues11 of section and ESI†). Molecular structures of
(not determined
ꢀpyrrolizine being isoelectronic. More importantly, their before) and were determined using singleꢀcrystal Xꢀray
deprotonation should afford 10πꢀelectron ꢀpyrrolo[1,2ꢀ diffraction analysis (Figure 1). Compounds crystallized in the
c][1,3,2]diazaborolidinyl anion (Scheme 2B) with potentially chiral P212121 space group, while crystallized in the P21/c
interesting coordination behaviour as was proven for analogues space group. The molecular structures of and are closely
. Intramolecular N→B interactions
are characterized by the N(1)ꢀB(1) bond distances 1.620(5) and
4
5
,
6
4
=
t
=
i
1
H
4
H
3
1
H
4
1
H
3
4
3
4
namely 4ꢀazapentalenyl anion12 or pentalenediyl dianion related to those of
(Scheme 2C, D) earlier.13
1
and
2
1.632(2) Å for
3 and 4, respectively and both values approach
the Σcov(N,B) = 1.56 Å.14 The central boron atoms are
tetrahedrally coordinated with the percent tetrahedral character
(%THC)15 of 74.7 % (
to the values found for
3
) and 70.0 % (
(79.0 %) and
4
), which are comparable
(66.6 %).8
1
2
Scheme 1 Reported reactivity of C,N-chelated (di)chloroboranes with lithium
anilides (ref. 7).
Figure 1 Molecular structures of
dichloromethane molecule (in both cases) are omitted for clarity. Selected bond
lengths [Å] and bonding angles [deg]: For B(1)-C(1) 1.587(6), B(1)-N(1)
3 (left) and 4 (right). Hydrogen atoms and the
3:
1.620(5), B(1)-Cl(1) 1.852(5), B(1)-Cl(2) 1.849(4), N(1)-B(1)-C(1) 98.7(3), N(1)-
B(1)-Cl(1) 109.9(3), N(1)-B(1)-Cl(2) 109.7(2), C(1)-B(1)-Cl(1) 114.3(3), C(1)-B(1)-
Cl(2) 115.0(3), Cl(1)-B(1)-Cl(2) 108.7(2). For 4: B(1)-C(1) 1.595(4), B(1)-N(1)
1.632(2), B(1)-Cl(1) 1.897(3), B(1)-C(20) 1.598(3), N(1)-B(1)-C(1) 97.44(18), N(1)-
B(1)-Cl(1) 108.53(16), N(1)-B(1)-C(20) 111.18(19), C(1)-B(1)-Cl(1) 108.52(17),
C(1)-B(1)-C(20) 119.7(2), Cl(1)-B(1)-C(20) 109.62(17).
N,Nꢀchelated boranes
5 and 6 were obtained in reasonable
yields of 72% and 78%, respectively, by the treatment of
lithium precursors L3,4Li with PhBCl2 in hexane and subsequent
workup. Compounds
in aromatic solvents, but show only limited solubility in
hexane. Both compounds slowly decompose in
5 and 6 are crystalline solids well soluble
dichloromethane solutions. Their identity was approved by the
help of elemental analysis, 1H, 11B, 13C and 15N NMR
spectroscopy (see Experimental section and ESI†) and the
molecular structure of
ray diffraction analysis (Figure 2).
5
was established using singleꢀcrystal Xꢀ
crystallized as a racemate
5
in the P21/c space group as a consequence of the stereogenic
centre at the B(1) atom. The central boron atom is chelated by
two nitrogen atoms. The bond distance B(1)ꢀN(1) 1.536(3) Å is
significantly shorter than the B(1)ꢀN(2) bond 1.607(4) Å.
Nevertheless, both values approach the sum of respective
covalent radii [Σcov(N,B) = 1.56 Å].14 These values are
comparable to those found in related N,Nꢀchelated boranes
Scheme 2 Structures of discussed heterocyclic compounds and related anions.
Results and Discussion
Syntheses and structures of starting compounds 1–6
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reported earlier.16 The coordination polyhedron of the boron range from ꢀ242.1 to ꢀ222.9 ppm) and the obtained values of
atom is a distorted tetrahedron with the %THC14 value 72.1 %. 15N NMR chemical shifts are comparable to those reported by
The results of UV/Vis absorption spectroscopy of are us for related 1
ꢀ2,1ꢀbenzazaboroles earlier.8 The results of
summarized in Table 1. All compounds show two main UV/Vis absorption spectroscopy of 13 are summarized in
absorption bands in the region 230 – 368 nm. These data are Table 1. All compounds show two or three absorption bands in
comparable to those found in related C,Nꢀchelated boranes.1k,l
the region 207 ꢀ 271 nm.
1
ꢀ
6
H
7
-
Figure 2 Molecular structure of 5. Hydrogen atoms are omitted for clarity. Only
one position of the disordered tBu group is shown. Selected bond lengths [Å]
and bonding angles [deg]: B(1)-N(1) 1.536(2), B(1)-N(2) 1.606(3), B(1)-C(10)
1.596(3), B(1)-Cl(1) 1.876(3), N(1)-B(1)-N(2) 97.10(16), N(1)-B(1)-Cl(1) 109.20(15),
N(1)-B(1)-C(10) 112.75(15), N(2)-B(1)-Cl(1) 108.91(13), N(2)-B(1)-C(10)
113.59(18), Cl(1)-B(1)-C(10) 113.99(16).
Reactivity of 1–6 toward lithium reagents
Scheme 3 Preparation of 7–13 together with isolated yields.
The reaction of boranes 1-4 with one or two equivalents of
respective lithium reagents (MeLi,
afforded substituted 1 ꢀ2,1ꢀbenzazaboroles
Scheme 3. All compounds were obtained in reasonable to
excellent isolated yields (45ꢀ96%) as oily products ( and 11
or crystalline solids, which are well soluble in aromatic solvents
and are stable for a long time under an inert atmosphere.
t
BuLi or PhLi) smoothly
In a similar way, N,Nꢀchelated boranes
5 and 6 were treated
H
7ꢀ
13 according to
with one equivalent of lithium reagents (Scheme 4). These
reactions involved the nucleophilic attack of the respective
carbanion to the imino C=N functionality, thus, giving a set of
7
)
1
H
ꢀpyrrolo[1,2ꢀc][1,3,2]diazaborolidines 14
isolated yields (61ꢀ86%) as crystalline solids (except 14, which
was obtained only as honeyꢀlike material and was
ꢀ18 in good
Importantly, it turned out that the reactions between
and two equivalents of BuLi or and one equivalent of
did not afforded expected 1
ꢀ2,1ꢀbenzazaboroles.17 This is
probably a result of a significant steric repulsion between the
bulky Bu groups or Bu and 2,6ꢀ Pr2C6H3 group (vide infra).
1
(or
3)
a
t
4
t
BuLi
characterized mainly by NMR spectroscopy, see the
Experimental section and ESI†). All compounds are well
soluble in aromatic solvents and are stable for a long time under
an inert atmosphere. Analogously to the attempted synthesis of
H
t
t
i
Compounds 7-13 were characterized using elemental analysis,
1H, 11B, 13C and 15N NMR spectroscopy (for a detailed
assignment of the NMR spectra see the Experimental section
and ESI†). All observed NMR data are entirely consistent with
the proposed structures of 7-13 and are similar to those reported
before by us for related compounds.7 Thus, the 1H NMR spectra
of 7-13 revealed one signal with expected multiplicity for the
sterically overcrowded 1
between the borane and
expected 1 ꢀpyrrolo[1,2ꢀc][1,3,2]diazaborolidines instead a
complicated mixture of products was obtained. The formation
H
ꢀ2,1ꢀbenzazaboroles, the reaction
6
tBuLi did not lead to the formation of
H
of
1Hꢀpyrrolo[1,2ꢀc][1,3,2]diazaborolidines backbones was
unambiguously proven by the help of 1H, 11B, 13C and 15N
NMR spectroscopy (for a detailed assignment of the NMR
spectra see the Experimental section and ESI†). The presence
aliphatic C
(in the range 4.41ꢀ5.58 ppm). These values are significantly
upfield shifted in comparison with the imine C group in the
starting compounds (observed in the range 7.55ꢀ8.45 ppm).
Similarly, the 13C NMR spectra established the presence of this
H(R´) group by a signal with the chemical shift laying in the
interval 63.6ꢀ77.1 ppm for
13. The 11B NMR spectra showed
in each case one signal (in the range 38.8ꢀ43.8 ppm), that are
upfield shifted ( δ ~ 33 ppm) in comparison with the starting
compounds reflecting a change of the hybridization of the
H(R´) (Table 2) of the 1Hꢀ2,1ꢀbenzazaborole core
of the CH(R´) group 1Hꢀpyrrolo[1,2ꢀc][1,3,2]diazaborolidine
core (Table 2) was reflected by the observation of one signal (in
H
1ꢀ4
the range 4.24ꢀ5.74 ppm for 14-18) with expected multiplicity
1
in the H NMR spectra and also one signal (in the range 55.5ꢀ
C
68.5 ppm) was detected in corresponding 13C NMR spectra. All
these signals are significantly upfield shifted in comparison
7
ꢀ
with the imine C
H group found in the parent boranes 5 and 6.
Δ
The 11B NMR spectra showed one signal (in the range 29.8ꢀ
1ꢀ4
34.0 ppm for 14-18) that is upfield shifted in comparison with
values observed for 1Hꢀ2,1ꢀbenzazaboroles 7ꢀ13 (38.8ꢀ43.8
boron atom from sp3 ( ) to sp2 ( 13).18 As expected, one
1ꢀ4 7ꢀ
signal also appeared in the 15N NMR spectra of
7ꢀ13 (in the
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ppm). This fact should be ascribed to the presence of two
adjacently bonded nitrogen atoms resulting in a higher electron
density at the boron atom in 14
in the 15N NMR spectra of 14
18 (in the range from ꢀ198.1 to ꢀ
ꢀ . Two signals were detected
18 18
ꢀ
186.8 ppm for the pyrrole nitrogen atom and from ꢀ264.3 to ꢀ
246.3 ppm for the second nitrogen atom). This distinction
between chemical shifts of both nitrogen atoms in 14ꢀ18 is
remarkable and probably stems from a different degree of π
interaction between them and the boron atom. The chemical
shifts for the pyrrole nitrogen atom suggest rather week π
interaction with the boron centre, which is also corroborated by
corresponding B(1)ꢀN(1) bond lengths found in the solid state
Figure 3 Molecular structures of
9
(left) and 10 (right). Hydrogen atoms, except
C(7)H, are omitted for clarity. Selected bond lengths [Å] and bonding angles
[deg]: For : B(1)-C(1) 1.560(2), B(1)-N(1) 1.418(2), B(1)-C(13) 1.574(2), C(1)-C(2)
9
(
vide infra). Similar conclusion was made by Wrackmeyer et
al. on related
ꢀpyrrolyl boranes.19 Nevertheless, chemical
shifts of the pyrrole nitrogen atom in 14-18 are similar to those
observed in the starting compounds and (ꢀ174.4 and ꢀ178.5
1.397(2), C(2)-C(7) 1.507(2), C(7)-N(1) 1.4889(19), C(1)-B(1)-C(13) 122.55(14),
C(1)-B(1)-N(1) 106.16(13), C(13)-B(1)-N(1) 131.26(14), C(7)-N(1)-B(1) 111.23(13).
For 10: B(1)-C(1) 1.564(2), B(1)-N(1) 1.419(2), B(1)-C(16) 1.573(2), C(1)-C(2)
1.399(2), C(2)-C(7) 1.519(2), C(7)-N(1) 1.4953(19), C(1)-B(1)-C(16) 123.00(13),
C(1)-B(1)-N(1) 106.13(13), C(16)-B(1)-N(1) 130.84(14), C(7)-N(1)-B(1) 110.03(11).
N
5
6
ppm, respectively). On the contrary, the signals of the second
nitrogen atom in 14-18 are significantly shifted in comparison
with corresponding imino nitrogen atom in the starting
The molecular structures of
formation of benzene fused azaborole ring. The central rings
are essentially planar in all cases (except for 10 vide infra).
Both boron B(1) and nitrogen N(1) atoms adopt planar
environment consistent with the sp2 hybridization within the
fiveꢀmembered ring as the sum of the angles around both atoms
is close to the ideal value of 360°. The B(1)ꢀN(1) bond lengths
9, 10, 12 and 13 proved the
compound
pyrrolo[1,2ꢀc][1,3,2]diazaborolidine core. The results of
UV/Vis absorption spectroscopy of 15 18 are summarized in
5
(ꢀ150.1 ppm)20, reflecting closure of the
1Hꢀ
,
–
Table 1. All compounds show two or three absorption bands in
the region 211 – 260 nm.
laying in the interval 1.403(4)-1.419(2) Å for 9, 10, 12 and 13
are apparently shorter than the sum of covalent radii for single
bond Σcov(N,B) = 1.56 Å14 and correspond to the value for the
respective double bond (1.48 Å).14 This fact proves a multiple
character of these bonds and reflects strong
πꢀπ interaction
between both bonding partners. The geometry around the C(7)
atom is tetrahedral and, thus, the C(7) bonded substituents are
bent out of the plane defined by the benzazaborole core. On the
contrary, the Cꢀipso carbon atoms of the substituents bonded to
the B(1) and N(1) remain coplanar with the center
benzazaborole core, but this situation changes significantly in
Scheme 4 Preparation of 14–18 together with isolated yields.
10, where two bulky
positions (vide infra).
tBu groups are bonded in neighbouring
Solid state structures
Molecular structures of
9, 10, 12, 13, 15, 17 and 18 were
determined by the help of singleꢀcrystal Xꢀray diffraction
analysis and are depicted in Figures 3ꢀ5 together with relevant
structural parameters. The crystallographic data are
summarized in the Experimental section. All compounds
(except
as racemates reflecting the presence of the stereogenic carbon
centres at C(7) (in 10 12 and 13) and C(5) (in 15 17 and 18),
respectively. Compound crystallized in Pꢀ1 space group. The
9) crystalized in the centrosymmetric P21/c space group
,
,
9
unit cell in the case of 12 contained three independent
molecules, but they are structurally closely related so only one
of them is discussed below in more detail.
Figure 4 Molecular structures of 12 (left - one of three independent molecules
obtained in the unit cell) and 13 (right). Hydrogen atoms, except C(7)H, are
omitted for clarity. Selected bond lengths [Å] and bonding angles [deg]: For 12
–
data for only one of three independent molecules within the unit cell: B(1)-C(1)
1.559(4), B(1)-N(1) 1.403(4), B(1)-C(20) 1.555(4), C(1)-C(2) 1.401(3), C(2)-C(7)
1.513(3), C(7)-N(1) 1.490(3), C(1)-B(1)-C(20) 129.2(2), C(1)-B(1)-N(1) 105.7(2),
C(20)-B(1)-N(1) 125.1(2), C(7)-N(1)-B(1) 113.02(19). For 13: B(1)-C(1) 1.562(2),
B(1)-N(1) 1.404(2), B(1)-C(20) 1.561(2), C(1)-C(2) 1.397(2), C(2)-C(7) 1.509(2),
C(7)-N(1) 1.479(2), C(1)-B(1)-C(20) 128.40(14), C(1)-B(1)-N(1) 105.54(14), C(20)-
B(1)-N(1) 125.87(14), C(7)-N(1)-B(1) 112.91(12).
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The molecular structures of 15
established the formation
c][1,3,2]diazaborolidine cores. The B(1)ꢀN(1) and B(1)ꢀN(2) and 15 (Table 2), where the values of chemical shifts observed
bond lengths laying in the intervals 1.459(2) 1.4608(17) Å and for 10 and 15 in comparison with other compounds suggest
1.403(2)ꢀ1.4164(17) Å, respectively, are different and it lower degree of interaction between boron and nitrogen
probably reflects stronger interaction between both bonding atoms. This steric repulsion is also most probably responsible
partners in the case of B(1)ꢀN(2) bonds (as suggested before in for the fact that other ꢀ2,1ꢀbenzazaboroles and
solution based on the 15N NMR data). Nevertheless, all BꢀN pyrrolo[1,2ꢀc][1,3,2]diazaborolidine with adjacently bonded
,
17 and 18 unambiguously stress between the
t
Bu moieties. This phenomenon is also
of
1H
ꢀpyrrolo[1,2ꢀ evident in the corresponding 11B and 15N NMR spectra of 10
-
πꢀπ
πꢀπ
1
H
1Hꢀ
bond lengths correspond to the value for the respective double bulky group (tBu or 2,6ꢀiPr2C6H3) couldn’t be prepared as
bond (1.48 Å).14 The geometry around the boron B(1) as well mentioned above or were obtained as nonꢀseparable mixtures of
as N(1) and N(2) atoms is trigonal planar, while the tetrahedral isomers.17
geometry is found around the C(5) atom. The whole central 1Hꢀ
pyrrolo[1,2ꢀc][1,3,2]diazaborolidine cores in 17 and 18 remain
essentially planar, while this is significantly puckered in the
case of 15 (vide infra).
Figure 6 Views on the central cores of 10 (top) and 15 (bottom) demonstrating
their significant puckering caused by the presence of adjacently bonded bulky
tBu moieties. Molecules are intersected by the plane (red colour) defined by
atoms of the benzene (10) and pyrrole (15) annulated ring.
Table 1 UV/Vis absorption maxima of 1-13 and 15-18 (in CH2Cl2
for 1-4, CCl4 for 5 and 6 and hexane for 7-13, 15–18) (λ = [nm], ε =
[l·mol-1·cm-1], sh = shoulder).
Figure 5 Molecular structures of 15 (left), 17 (right) and 18 (bottom). Hydrogen
atoms, except C(5)H, are omitted for clarity. Selected bond lengths [Å] and
bonding angles [deg]: For 15: B(1)-N(1) 1.4608(17), B(1)-N(2) 1.4164(17), B(1)-
C(14) 1.574(2), N(2)-C(5) 1.4957(16), C(5)-C(1) 1.5065(17), C(1)-N(1) 1.3778(17),
N(1)-B(1)-N(2) 106.74(14), N(1)-B(1)-C(14) 120.70(11), N(2)-B(1)-C(14)
132.54(11), B(1)-N(2)-C(5) 109.23(10), B(1)-N(1)-C(1) 109.92(10). For 17: B(1)-
N(1) 1.460(2), B(1)-N(2) 1.404(2), B(1)-C(18) 1.555(3), N(2)-C(5) 1.488(2), C(5)-
C(1) 1.502(2), C(1)-N(1) 1.380(2), N(1)-B(1)-N(2) 105.90(16), N(1)-B(1)-C(18)
126.35(16), N(2)-B(1)-C(18) 127.45(16), B(1)-N(2)-C(5) 112.06(14), B(1)-N(1)-C(1)
110.07(15). For 18: B(1)-N(1) 1.459(2), B(1)-N(2) 1.403(2), B(1)-C(18) 1.555(2),
N(2)-C(5) 1.4947(18), C(5)-C(1) 1.502(2), C(1)-N(1) 1.3760(19), N(1)-B(1)-N(2)
105.90(13), N(1)-B(1)-C(18) 126.44(14), N(2)-B(1)-C(18) 127.66(14), B(1)-N(2)-
C(5) 112.33(12), B(1)-N(1)-C(1) 109.99(13).
Compound
λmax1 (ε)
λmax2 (ε)
λmax3 (ε)
1
2
3
230 (8700)
230 (13600)
231 (9000)
296 (9500)
292 (9500)
301 (8900)
330 sh (2400)
330 sh (2300)
ꢀ
4
5
6
7
8
9
10
11
12
13
15
16
17
18
231 (10800)
303 sh (5700)
298 (5500)
296 (12100)
362 (13800)
368 (16600)
245 (8200)
251 sh (7900)
235 (16800)
241 (10400)
243 (12100)
237 (22600)
239 (26300)
256 (3600)
ꢀ
ꢀ
ꢀ
ꢀ
211 (13300)
214 (27500)
209 (32000)
207 (21800)
215 (14900)
207 (38800)
207 (38200)
217 (9800)
ꢀ
249 sh (12400)
271 (4300)
ꢀ
261 (7400)
260 (10800)
ꢀ
As mentioned above, molecular structures of 10 and 15 are of
particular interest as they contain two tBu groups bonded in
216 (12700)
211 (14300)
215 (27400)
245 (5500)
229 sh (10500)
229 sh (24700)
ꢀ
adjacent positions. This leads to a significant distortion of the
central heterocyclic cores as illustrated in Figure 6. This is
obviously a result of slight pyramidalization of the coordination
geometry around the nitrogen atoms N(1) (10) and N(2) (15).
Similarly, B(1) and C(12) atoms in the case of 10 and B(1),
C(5) and C(10) atoms for 15 are bent out of the mean planes
defined by atoms of annulated benzene and pyrrole ring,
respectively. This distortion obviously helps to relax the steric
259 (4200)
260 (8800)
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 5

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View Article Online
ARTICLE
Journal Name
DOI: 10.1039/C4DT01445F
on the bulkiness of attached organic substituents. All here
reported derivatives hold a remarkable potential to become
ligands for the coordination of transition metals after their
deprotonation. The investigation targeting this field as well as
dealing with the preparation of related pꢀblock element
containing heterocycles is currently underway in our labs.
Conclusions
We have recognized new and facile synthetic protocol for the
formation of
c][1,3,2]diazaborolidines, which were accessible by quite
complicated reaction procedures or were not known at all in the
latter case. The detailed study also revealed an important
dependence of the formation of these heterocyclic compounds
1Hꢀ2,1ꢀbenzazaboroles and 1Hꢀpyrrolo[1,2ꢀ
Table 2 1H, 11B, 13C and 15N NMR data (δ = [ppm]) of the central cores of 7-18 in C6D6 at 298 K with numbering schemes of the 1H-2,1-
benzazaborole and the 1H-pyrrolo[1,2-c][1,3,2]diazaborolidine cycle.
Compound
Position Nucleus
7
41.8
ꢀ227.5
4.41 (q)
(63.6)
156.6
8
41.9
ꢀ225.8
5.58 (s)
(72.8)
156.2
9
40.2
ꢀ222.9
4.54 (q)
(64.0)
156.7
10
43.8
ꢀ230.9
4.53 (s)
(75.8)
156.4
11
42.0
ꢀ241.9
4.53 (q)
(67.7)
155.9
12
39.0
ꢀ239.5
5.86 (s)
(77.1)
155.4
13
38.8
ꢀ242.1
4.78 (q)
(67.9)
156.7
1
2
3
δ 11
δ 15
B
N
δ 1H
(δ 13C)
3a
4
δ 13
C
δ 1H
(δ 13C)
δ 1H
7.13 (m) 6.97 (dq) 7.15 (dq) 7.24 (m) 7.20 (d) 7.20 (dq) 7.27 (dq)
(121.2)
7.30 (td) 7.10 (m)
(129.1) (130.0)
7.26 (td) 7.10 (m)
(126.8) (127.2)
7.70 (m) 7.52 (m)
(122.7)
(121.2)
(123.6)
(122.1)
(124.3)
(122.4)
7.37 (t)
(129.8)
7.34 (t)
(127.7)
8.13 (dt)
7: R1 = R3 = Me, R2 = tBu
8: R1 = R3 = Ph, R2 = tBu
9: R1 = Ph, R2 = tBu, R3 = Me
10: R1 = Ph, R2 = R3 = tBu
5
6
7.29 (td) 7.14 (m) 7.34 (m) 7.30 (td)
(δ 13C)
δ 1H
(129.4)
7.17 (tt)
(127.1)
(127.7)
7.16 (m) 7.32 (m) 7.38 (tq)
(126.8) (127.5) (127.9)
(129.8)
(130.2)
(δ 13C)
δ 1H
11: R1 = R3 = Me, R2 = 2,6ꢀiPr2C6H3
12: R1 = R3 = Ph, R2 = 2,6ꢀiPr2C6H3
13: R1 = Ph, R2 = 2,6ꢀiPr2C6H3, R3 = Me
7
7.50 (dt) 7.56 (m) 7.83 (m) 8.23 (dt)
(δ 13C)
(130.2)
(131.3)
(131.5)
(130.7)
(131.3)
(132.3)
(132.5)
a
a
a
a
a
7a
δ 13
C
143.4b
142.0b
Compound
Position Nucleus
14
30.8
ꢀ244.9
4.39 (q)
(55.5)
15
34.0
ꢀ255.8
4.24 (s)
(68.4)
16
31.9
ꢀ246.3
5.44 (s)
(63.9)
145.1
17
29.8
ꢀ264.3
4.67 (q)
(59.9)
145.9
18
30.2
ꢀ261.1
5.74 (s)
(68.5)
143.4
1
2
3
δ 11
δ 15
B
N
δ 1H
(δ 13C)
3a
4
δ 13
C
145.5
144.4
δ 1H
(δ 13C)
δ 1H
6.12 (m)
(101.2)
6.51 (t)
(114.9)
6.69 (d)
(116.5)
ꢀ191.9
6.16 (d)
(105.5)
6.45 (t)
(114.4)
6.78 (d)
(116.8)
ꢀ186.8
5.96 (dt) 6.30 (m) 6.26 (m)
(102.9)
6.42 (t)
(115.6)
6.71 (dt)
(116.6)
ꢀ192.1
(102.5)
6.67 (t)
(115.8)
7.30 (d)
(117.7)
ꢀ198.1
(104.9)
6.70 (t)
(116.2)
7.40 (d)
(118.4)
ꢀ195.8
14: R1 = Ph, R2 = tBu, R3 = Me
15: R1 = Ph, R2 = R3 = tBu,
16: R1 = R3 = Ph, R2 = tBu
5
6
(δ 13C)
δ 1H
17: R1 = Ph, R2 = 2,6ꢀiPr2C6H3, R3 = Me
18: R1 = R3 = Ph, R2 = 2,6ꢀiPr2C6H3
(δ 13C)
6a
δ 15
N
a signal of the carbon atom 7a was not observed, b observation of signal of the carbon atom 7a required application of 10^5 pulses due to its broadening.
s = singlet, d = doublet, t = triplet, q = quartet, dt = doublet of triplets, dq = doublet of quartets, td = triplet of doublets, tt = triplet of triplets, m = multiplet
1H, 11B, 13C and 15N NMR spectra were recorded on a Bruker
Experimental
Avance 500 or a Bruker Avance III 400 MHz spectrometers,
General procedures
using a 5 mm tunable broadꢀband probe. Appropriate chemical
1
shifts in H and 13C NMR spectra were related to the residual
All air and moisture sensitive manipulations were carried out
signals of the solvent (C6D6: δ(1H) = 7.16 ppm and δ(13C) =
under an argon atmosphere using standard Schlenk tube
128.39 ppm), 11B NMR spectra were related to external
technique. All solvents were dried using Pure Solv–Innovative
standard B(OMe)3 (δ(11B) = 18.1 ppm), 15N NMR spectra were
Technology equipment. The starting compounds: BCl3 (1 M
related to external neat nitromethane (δ(15N) = 0.0 ppm). For
solution in hexane) and PhBCl2 (97%) were obtained from the
compounds 1-18 the full assignment of all signals in all
measured NMR spectra was managed with the help of various
commercial suppliers and used as delivered. The ligandꢀ
precursors L1,2Br and L3,4H were prepared according to
1
1
1
techniques including H, 13C{1H} APT, Hꢀ1H COSY, Hꢀ13C
published procedures.21 Compounds
according to the literature.8 Compound
prepared by an alternative reaction.10
1
ꢀ
3
were synthesized
HMQC, 1Hꢀ13C HMBC and in several cases 1D 1H NOESY has
9
has also been recently
been used. 15N NMR chemical shifts were obtained from Hꢀ
1
15N HMBC spectra. Table 2 summarizes all NMR data of the
NMR spectroscopy
central cores for 7ꢀ18 including the multiplicities of the signals
6 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C4DT01445F
in the 1H NMR spectra. The full assignment including mL) at 0 °C and stirred for 1 h at this temperature. A solution of
assignment of all bonded phenyl/alkyl groups bonded to central PhBCl2 (2.62 g, 16.5 mmol) in hexane (10 mL) was added
cores are available in the ESI†, thus these data are not under intensive stirring to the ivory suspension of lithium
mentioned in this section.
compound at 0 °C. The obtained dense ginger mixture was
allowed to reach r.t. and stirred overnight. Then the mixture
was heated to boil and filtered. The remaining insoluble
UV-VIS spectroscopy
Electronic absorption spectra were run on a BlackꢀComet Cꢀ material was extracted by additional 3x50 mL of boiling hexane
SRꢀ100 concave grating spectrometer in quartz cuvette (region and then filtered. All filtrates were collected and volume was
200 ꢀ 1080 nm, optical pathway 1 cm, concentration 10ꢀ4 ꢀ 10ꢀ5 reduced to one half. Storing at ꢀ30 °C gave
6
as ginger
crystalline solid (isolated yield 4.88 g, 78 %), decomp. p. 147
18. The spectra of 14 could not be obtained as it was not °C.
isolated as an analytically pure compound and therefore the
mol.lꢀ1) in CH2Cl2 for
13 15
1ꢀ4, CCl4 for 5 and 6 and hexane for 7ꢀ
,
ꢀ
Anal. calc. for C23H26BClN2 (MW 376.73): C, 73.3; H, 7.0;
results may be confusing. All observed data are summarized in Found: C, 73.2; H, 7.2 %. For NMR data see Table 2 and ESI†.
Table 1.
Synthesis
benzazaborole (7). MeLi (4.0 mL of 1.6M solution in Et2O,
Pr2C6H3)C6H4]B(Ph)Cl (4). 6.4 mmol) was added to a solution of (772 mg, 3.2 mmol) in
BuLi (30.4 mL of 1.6M solution in hexane, 48.7 mmol) was toluene (50 mL) at 0 °C. The obtained fine cream suspension
of
racemic
1,3-Me2-2-tBu-1H-2,1-
Syntheses
Synthesis of [2-(CH=N-2,6-
i
1
n
added to a solution of L2Br (16.759 g, 48.7 mmol) in hexane was stirred for 30 min at this temperature and then was allowed
(150 mL) at 0 °C and stirred for 1 h at this temperature. to reach r.t. and stirred overnight. The mixture was filtered and
Additional hexane (50 mL) was added to the resulting yellowꢀ the remaining insoluble material was extracted by additional 10
orange suspension of the lithium compound. A solution of mL of toluene. Paleꢀyellow filtrates were collected and
PhBCl2 (7.73 g, 48.7 mmol) in hexane (20 mL) was added evaporated in vacuo to give
7 as paleꢀyellow oil (isolated yield
under intensive stirring to the suspension of lithium compound 579 mg, 90 %). This product can be purified by vacuum
at 0 °C. The obtained ginger dense mixture was allowed to distillation at 190 °C, 1 mm Hg to give colourless oil.
reach r.t. and stirred overnight. The mixture was filtered in the
Anal. calc. for C13H20BN (MW 201.12): C, 77.6; H, 10.0;
air via the fine frit with layer of Celite and remaining insoluble Found: C, 77.6; H, 9.9 %. For NMR data see Table 2 and ESI†.
material was washed by 2x100 mL of undried dichloromethane.
Both portion of filtrate were collected and then evaporated to
give crude orange product, which was recrystallized from benzazaborole (8). Procedure A) PhLi (2.4 mL of 1.8M
minimal amount of boiling dichloromethane to give as paleꢀ solution in Bu2O, 4.4 mmol) was added to a solution of (528
yellow crystalline solid (isolated yield 13.01 g, 69 %), m. p. mg, 2.2 mmol) in toluene (30 mL) at 0 °C. The obtained fine
174 °C. Compound is air stable. ginger suspension was stirred for 30 min at this temperature and
Synthesis
of
racemic
1,3-Ph2-2-tBu-1H-2,1-
4
n
1
Anal. calc. for C25H27BClN (MW 387.75): C, 77.4; H, 7.0; then was allowed to reach r.t. and stirred overnight. The
Found: C, 77.3; H, 7.0 %. For NMR data see Table 2 and ESI†. mixture was filtered and filtrate was evaporated in vacuo to
give paleꢀyellow honeyꢀlike matter. This matter was dissolved
Synthesis of [2-(CH=N-
(24.2 mL of 1.6M solution in hexane, 38.7 mmol) was added to powder (isolated yield 565 mg, 80 %).
a solution of L3H (5.82 g, 38.7 mmol) in hexane (160 mL) at ꢀ
Procedure B) Analogously to the procedure A), using PhLi
60 °C and stirred for 1 h at this temperature. A solution of (1.7 mL of 1.8M solution in Bu2O, 3.0 mmol) that was reacted
tBu)C4H3N]B(Ph)Cl (5). nBuLi in hot pentane (5 mL). Storing at ꢀ30 °C gave 8 as white
n
PhBCl2 (6.15 g, 38.7 mmol) in hexane (20 mL) was added with
2
(854 mg, 3.0 mmol). Isolated yield 796 mg, 81 %, m. p.
under intensive stirring to the ivory suspension of lithium 83 °C.
compound at ꢀ30 °C. The obtained dense dark pink mixture was
Anal. calc. for C23H24BN (MW 325.25): C, 84.9; H, 7.4;
allowed to reach r.t. and stirred overnight. The mixture was Found: C, 84.8; H, 7.4 %. For NMR data see Table 2 and ESI†.
filtered, the pink filtrate with intensive green fluorescence was
discarded and the remaining insoluble material was extracted
by 200 mL of boiling mixture of hexane and toluene (3:1). The benzazaborole (9). MeLi (5.7 mL of 1.6M solution in Et2O,
resulting suspension was filtered and the dark pink filtrate was 9.1 mmol) was added to a solution of (2.59 g, 9.1 mmol) in
stored at ꢀ30 °C which gave as pink crystalline solid (isolated toluene (90 mL) at 0 °C. The obtained fine paleꢀyellow
yield 7.61 g, 72 %), m. p. 123 °C. suspension was stirred for 30 min at this temperature and then
Synthesis
of
racemic
1-Ph-2-tBu-3-Me-1H-2,1-
2
5
Anal. calc. for C15H18BClN2 (MW 272.58): C, 66.1; H, 6.7; was allowed to reach r.t. and stirred overnight. The mixture was
Found: C, 66.1; H, 6.7 %. For NMR data see Table 2 and ESI†. filtered and the remaining insoluble material was extracted by
additional 2x10 mL of toluene and then filtered. Paleꢀyellow
Synthesis of [2-(CH=N-2,6-
BuLi (10.3 mL of 1.6M solution in hexane, 16.5 mmol) was as paleꢀyellow crystalline solid. Obtained crude product was
added to a solution of L4H (4.20 g, 16.5 mmol) in hexane (150 recrystallized from hot hexane (40 mL). Storing at ꢀ30 °C gave
iPr2C6H3)C4H3N]B(Ph)Cl (6). filtrates were collected and evaporated in vacuo to give crude 9
n
This journal is © The Royal Society of Chemistry 2012
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VJieowuArrtnicalelONnlainme e
ARTICLE
DOI: 10.1039/C4DT01445F
9
in form of colourless singleꢀcrystals (isolated yield 2.31 g, 96
%), m. p. 90 °C.
Anal. calc. for C18H22BN (MW 263.18): C, 82.1; H, 8.4; Et2O, 0.9 mmol) was added to a solution of
Synthesis of racemic 1-Ph-2-(2,6-
2,1-benzazaborole (13). MeLi (0.5 mL of 1.6M solution in
(330 mg, 0.9
iPr2C6H3)-3-Me-1H-
4
Found: C, 82.0; H, 8.5 %. For NMR data see Table 2 and ESI†. mmol) in toluene (30 mL) at 0 °C. The obtained fine ivory
suspension was stirred for 30 min at this temperature and then
Synthesis
benzazaborole (10).
pentane, 7.2 mmol) was added to a solution of
of
racemic
BuLi (4.2 mL of 1.7M solution in filtered and obtained filtrate was evaporated in vacuo. The
(2.04 g, 7.2 crude product was recrystallized from hexane (30 mL). Storing
1-Ph-2,3-tBu2-1H-2,1- was allowed to reach r.t. and stirred overnight. The mixture was
t
2
mmol) in toluene (90 mL) at 0 °C. The obtained fine orange at ꢀ30 °C gave 13 as colourless singleꢀcrystals (isolated yield
suspension was stirred for 30 min at this temperature and then 289 mg, 92 %), m. p. 185 °C.
was allowed to reach r.t. and stirred overnight. The mixture was
Anal. calc. for C26H30BN (MW 367.33): C, 85.0; H, 8.2;
filtered and the remaining insoluble material was extracted by Found: C, 84.9; H, 8.3 %. For NMR data see Table 2 and ESI†.
additional 2x10 mL of toluene. Orange filtrates were collected
and evaporated in vacuo to give orange honeyꢀlike matter. This
matter was recrystallized from hot hexane (30 mL). Storing at ꢀ c][1,3,2]diazaborolidine (14). MeLi (2.4 mL of 1.6M solution
30 °C gave colourless crystalline solid polluted by yellow oil. in Et2O, 3.8 mmol) was added to a solution of (1.03 g, 3.8
Synthesis of racemic 1-Ph-2-tBu-3-Me-1H-pyrrolo[1,2-
5
Multiple recrystallization of this crude product from hot hexane mmol) in toluene (30 mL) at 0 °C. The obtained pink
and storing at ꢀ30 °C gave 10 as colourless singleꢀcrystals suspension was stirred for 30 min at this temperature and then
(isolated yield 983 mg, 45 %), m. p. 118 °C. Compound 10 is was allowed to reach r.t. and stirred overnight. The mixture was
stable for a long time (several months) in argon atmosphere but filtered and obtained red filtrate was evaporated in vacuo to
decomposes in solution slowly over time.
give reddish honeyꢀlike matter, which contained product 14 as
1
Anal. calc. for C21H28BN (MW 305.26): C, 82.6; H, 9.3; the major product (˃95%, according to the H NMR spectra).
Found: C, 82.5; H, 9.2 %. For NMR data see Table 2 and ESI†. Neither Xꢀray diffraction analysis nor melting point could be
obtained, because any attempt to crystalize 14 failed. Isolated
Synthesis of racemic 1,3-Me2-2-(2,6-
benzazaborole (11). MeLi (3.8 mL of 1.6M solution in Et2O, natural daylight conditions. For NMR data see Table 2 and
6.1 mmol) was added to a solution of (1.05 mg, 3.0 mmol) in ESI†.
iPr2C6H3)-1H-2,1- yield of crude product 820 mg, 86 %). 14 is unstable under
3
toluene (60 mL) at 0 °C. The obtained fine dark ginger
suspension was stirred for 30 min at this temperature and then
was allowed to reach r.t. and stirred overnight. The mixture was c][1,3,2]diazaborolidine (15).
Synthesis of racemic 1-Ph-2,3-
BuLi (2.4 mL of 1.7M solution
filtered and the remaining insoluble material was extracted by in pentane, 4.0 mmol) was added to a solution of (1.10 g, 4.0
tBu2-1H-pyrrolo[1,2-
t
5
additional 10 mL of toluene. Paleꢀorange filtrates were mmol) in toluene (30 mL) at 0 °C. The obtained orange
collected and evaporated in vacuo to give 11 as paleꢀorange suspension was stirred for 30 min at this temperature and then
honeyꢀlike matter (isolated yield 802 mg, 86 %). Neither Xꢀray was allowed to reach r.t. and stirred overnight, while colour of
diffraction analysis nor melting point could be obtained, reaction mixture changed to red. The mixture was filtered and
because any attempt to crystalize 11 failed.
volume of obtained red filtrate was reduced to one half. Storing
Anal. calc. for C21H28BN (MW 305.26): C, 82.6; H, 9.3; at ꢀ30 °C gave colourless crystalline solid polluted by red oil,
Found: C, 82.3; H, 9.2 %. For NMR data see Table 2 and ESI†. which was washed off by hexane (2 mL). Obtained crude
product was recrystallized from hexane (30 mL). Storing at ꢀ30
Synthesis of racemic 1,3-Ph2-2-(2,6-
benzazaborole (12). Procedure A) PhLi (1.3 mL of 1.8M 66 %), m. p. 110 °C.
solution in Bu2O, 2.2 mmol) was added to a solution of (390 Anal. calc. for C19H27BN2 (MW 294.24): C, 77.6; H, 9.3;
iPr2C6H3)-1H-2,1- °C gave 15 as colourless singleꢀcrystals (isolated yield 789 mg,
n
3
mg, 1.1 mmol) in toluene (40 mL) at 0 °C. The obtained fine Found: C, 77.5; H, 9.3 %. For NMR data see Table 2 and ESI†.
ginger suspension was stirred for 30 min at this temperature and
then was allowed to reach r.t. and stirred overnight. The
mixture was evaporated in vacuo and extracted by boiling c][1,3,2]diazaborolidine (16). PhLi (2.6 mL of 1.8M solution
hexane (30 mL). Resulting suspension was filtered and in Bu2O, 4.7 mmol) was added to a solution of (1.29 g, 4.7
obtained ginger filtrate was stored at ꢀ30 °C to give 12 as mmol) in toluene (30 mL) at 0 °C. The obtained pink
colourless crystalline solid (isolated yield 321 mg, 66 %). suspension was stirred for 30 min at this temperature and then
Procedure B) Analogously to the procedure A), using PhLi was allowed to reach r.t. and stirred overnight. The mixture was
(0.6 mL of 1.8M solution in Bu2O, 1.0 mmol) that was reacted filtered and volume of obtained red filtrate was reduced to one
with (390 mg, 1.0 mmol). Isolated yield 283 mg, 66 %, m. p. quarter. Storing at ꢀ30 °C gave colourless microcrystalline solid
180 °C.
Anal. calc. for C31H32BN (MW 429.40): C, 86.7; H, 7.5;
Synthesis of racemic 1,3-Ph2-2-tBu-1H-pyrrolo[1,2-
n
5
n
4
16. Isolated yield 909 mg, 61 %), m. p. 101 °C.
Anal. calc. for C21H23BN2 (MW 314.23): C, 80.3; H, 7.4;
Found: C, 86.7; H, 7.4 %. For NMR data see Table 2 and ESI†. Found: C, 80.5; H, 7.5 %. 16 is unstable under natural daylight
conditions. For NMR data see Table 2 and ESI†.
8 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C4DT01445F
150(1) K, 16082 total reflections, 4755 independent (Rint
0.038, 1 (obs. data) = 0.043, w
pyrrolo[1,2-c][1,3,2]diazaborolidine (17). MeLi (1.2 mL of ∆ρ, max., min. [e Åꢀ3] 0.262, ꢀ0.325, CCDC 999433.
1.6M solution in Et2O, 1.9 mmol) was added to a solution of Crystallographic data for 5. C15H18BClN2,
(715 mg, 1.9 mmol) in hexane (40 mL) at 0 °C. The obtained monoclinic, P21/c = 10.9790(8),
red suspension was stirred for 30 min at this temperature and 17.0041(10) Å, = 127.218(6)°,
= 1453.67(18) Å3,
then was allowed to reach r.t. and stirred overnight. The 150(1) K, 10875 total reflections, 2623 independent (Rint
mixture was filtered and obtained red filtrate was evaporated in 0.036, 1 (obs. data) = 0.045, w
vacuo to give red crystalline solid, which was recrystallized ∆ρ, max., min. [e Åꢀ3] 0.256, ꢀ0.300, CCDC 999430.
from hexane (20 mL). Storing at ꢀ30 °C gave 17 as pink singleꢀ Crystallographic data for 9. C18H22BN, = 263.18, triclinic,
=
Synthesis of racemic 1-Ph-2-(2,6-
iPr2C6H3)-3-Me-1
H-
R
R2 (all data) 0.081), S = 1.196,
6
M
= 272.57,
,
a
b
= 9.7780(3),
c
=
=
=
β
V
Z
= 4, T
R
R2 (all data) 0.095), S = 1.106,
M
crystals (isolated yield 573 mg, 85 %), m. p. 154 °C.
Anal. calc. for C24H29BN2 (MW 356.31): C, 80.9; H, 8.2; 74.766(3),
Found: C, 80.8; H, 8.1 %. For NMR data see Table 2 and ESI†. 2,
0.040,
∆ρ, max., min. [e Åꢀ3] 0.366, ꢀ0.203, CCDC 999426.
pyrrolo[1,2-c][1,3,2]diazaborolidine (18). PhLi (0.6 mL of Crystallographic data for 10. C21H28BN,
1.8M solution in Bu2O, 1.1 mmol) was added to a solution of monoclinic, P21/c = 13.5601(10), = 11.2810(9),
(432 mg, 1.1 mmol) in hexane (40 mL) at 0 °C. Resulting 12.2599(7) Å, = 107.667(6)°, = 4,
= 1787.0(2) Å3,
pink suspension was stirred for 30 min at this temperature and 150(1) K, 26459 total reflections, 3159 independent (Rint
then was allowed to reach r.t. and stirred overnight. The 0.031, 1 (obs. data) = 0.049, w
mixture was filtered and volume of pink filtrate was reduced to ∆ρ, max., min. [e Åꢀ3] 0.306, ꢀ0.249, CCDC 999425.
one half. Storing at ꢀ30 °C gave 18 as pink singleꢀcrystals Crystallographic data for 12. C31H32BN,
(isolated yield 332 mg, 69 %), m. p. 172 °C. monoclinic, P21/c = 20.1420(19), = 9.9090(11),
Anal. calc. for C29H31BN2 (MW 418.38): C, 83.3; H, 7.5; 37.433(3) Å, = 97.702(6)°, = 12,
= 7403.7(12) Å3,
Found: C, 83.4; H, 7.6 %. For NMR data see Table 2 and ESI†. 150(1) K, 45732 total reflections, 9787 independent (Rint
0.046, 1 (obs. data) = 0.066, w
∆ρ, max., min. [e Åꢀ3] 0.363, ꢀ0.302, CCDC 999428.
The suitable single crystals were mounted on a glass fibre Crystallographic data for 13. C26H30BN,
with an oil and measured on a fourꢀcircle diffractometer monoclinic, P21/c = 10.4121(8), = 12.0880(6),
KappaCCD with a CCD area detector by monochromatized 17.2750(9) Å, = 101.555(6)°, = 4,
= 2130.2(2) Å3,
MoKα radiation (λ = 0.71073 Å) at 150(1) K. The numerical22 150(1) K, 21672 total reflections, 3721 independent (Rint
absorption corrections from the crystal shape were applied for 0.035, 1 (obs. data) = 0.052, w
all crystals. The structures were solved by the direct method ∆ρ, max., min. [e Åꢀ3] 0.250, ꢀ0.240, CCDC 999429.
(SIR92)23 and refined by a full matrix least squares procedure Crystallographic data for 15. C19H27BN2,
based on F2 (SHELXL97).24 Hydrogen atoms were fixed into monoclinic, P21/c
= 13.3330(7), = 11.1231(10),
idealized positions (riding model) and assigned temperature 12.0420(8) Å, = 106.788(5)°, = 4,
= 1709.8(2) Å3,
factors Hiso (H) = 1.2 Ueq (pivot atom) or of 1.5 Ueq for the 150(1) K, 23195 total reflections, 3229 independent (Rint
methyl moiety with C–H = 0.96, 0.97, and 0.93 Å for methyl, 0.019, 1 (obs. data) = 0.042 w
methylene, and hydrogen atoms in the aromatic ring, ∆ρ, max., min. [e Åꢀ3] 0.281, ꢀ0.211, CCDC 999432.
respectively. The Bu group in is disordered. This disorder of Crystallographic data for 17. C24H29BN2,
three carbon atoms has been treated by splitting them to two monoclinic, P21/c = 10.3910(9), = 11.9710(8),
nearly equivalent positions. There is a slightly higher residual 17.1839(12) Å, = 103.239(7)°, = 4,
= 2080.7(3) Å3,
electron density peak detected at about 0.8 Å from the Cl(2) 150(1) K, 15236 total reflections, 3387 independent (Rint
atom in the structure of . This particular peak has no chemical 0.042, 1 (obs. data) = 0.056, w
significance. Crystallographic data for structural analysis has ∆ρ, max., min. [e Åꢀ3] 0.254, ꢀ0.217, CCDC 999431.
been deposited with the Cambridge Crystallographic Data Crystallographic data for 18. C29H31BN2,
Pꢀ1
,
a
= 8.8771(3),
b
= 9.2700(3),
c
= 9.7179(3) Å,
V
α
Z
=
=
=
β
= 76.770(4),
γ
= 82.477(4)°,
= 748.98(4) Å3,
T
= 150(1) K, 7972 total reflections, 2747 independent (Rint
1 (obs. data) = 0.049, w
R
R
2 (all data) 0.111), S = 1.113,
Synthesis of racemic 1,3-Ph2-2-(2,6-iPr2C6H3)-1H-
M
= 305.25,
n
,
a
b
c
T
=
=
=
6
β
V
Z
R
R
2 (all data) 0.104), S = 1.099,
M
= 429.39,
,
a
b
c
T
=
=
=
β
V
Z
R
R
2 (all data) 0.117), S = 1.180,
X-ray crystallography
M
= 367.32,
,
a
b
c
T
=
=
=
β
V
Z
R
R
2 (all data) 0.106), S = 1.169,
M
= 294.24,
,
a
b
c
T
=
=
=
β
V
Z
R
R
2 (all data) 0.101), S = 1.088,
t
5
M
= 356.30,
,
a
b
c
T
=
=
=
β
V
Z
3
R
R
2 (all data) 0.111), S = 1.143,
M
= 418.37,
Centre, CCDC nos. 999425ꢀ999434.
Crystallographic data for 3. C19H22BCl2N.CH2Cl2,
monoclinic, P21/c
,
a
= 16.5759(8),
b
= 10.0011(11),
= 4,
c
T
=
=
=
M
=
=
=
=
14.4270(7) Å, = 91.314(4)°,
β
V
= 2391.0(3) Å3,
Z
431.01, monoclinic, P21/c
14.1703(4) Å, = 108.492(3)°,
150(1) K, 18640 total reflections, 3863 independent (Rint
,
a
= 12.3332(4),
b
= 13.0120(2),
c
150(1) K, 23632 total reflections, 4207 independent (Rint
0.036, R1 (obs. data) = 0.051, wR2 (all data) 0.106), S = 1.113,
∆ρ, max., min. [e Åꢀ3] 0.287, ꢀ0.251, CCDC 999427.
β
V
= 2156.63(11) Å3,
Z
= 4,
T
0.039, R1 (obs. data) = 0.076, wR2 (all data) 0.182), S = 1.050,
∆ρ, max., min. [e Åꢀ3] 1.235, ꢀ0.866, CCDC 999434.
Crystallographic data for 4. C25H27BClN.CH2Cl2,
Acknowledgements
The authors thank the Grant agency of the Czech Republic
project no. P207/12/0223.
M
b
=
=
=
472.66, orthorhombic, P212121
13.0260(12), = 18.498(2) Å,
,
a
=
10.3590(6),
c
V
= 2496.1(4) Å3,
Z
= 4,
T
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 9

Dalton Transactions
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VJieowuArrtnicalelONnlainme e
ARTICLE
DOI: 10.1039/C4DT01445F
Notes and references
Department of General and Inorganic Chemistry, Faculty of Chemical
Trans., 2013, 42, 611. (c) W. Luo, P.G. Campbell, L.N. Zakharov
and S.ꢀY. Liu, J. Am. Chem. Soc., 2011, 133, 19326.
a
Technology, University of Pardubice, Studentská 573, CZ ꢀ 532 10,
Pardubice, Czech Republic Fax: +420466037068; Tel: +420466037163;
Eꢀmail: libor.dostal@upce.cz
8
9
M. Hejda, A. Lyčka, R. Jambor, A. Růžička and L. Dostál, Dalton
Trans., 2013, 42, 6417.
(a) R. Köster, K. Iwasaki, S. Hattori and Y. Morita, Liebigs Ann.
Chem. 1968, 720, 23; (b) R. Köster, K. Iwasaki, Advances in
Chemistry, 1964, 42, Chapter 16, 148; (c) P. T. Hawkins and A. U.
Blackham, J. Org. Chem., 1967, 32, 597; (d) H. E. Dunn, J. C. Catlin
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M. Nagy, G. E. Salnikov and V. G. Shubin, Chem. Commun., 2000,
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Research Institute for Organic Syntheses, Rybitví 296, CZꢀ533 54
Pardubice, Czech Republic
†
Electronic Supplementary Information (ESI) available: Full
assignment of NMR data of all compounds. See DOI: 10.1039/b000000x/
1
Selected examples of works dealing with intramolecularly Nꢀdonor
coordinated boranes derived from iminopyrrole: (a) M. J. Calhorda,
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Trans., 2012, 41, 13210. (b) D. Suresh, C. S. B. Gomes, P. T. Gomes, 10 M. Hejda, R. Jambor, A. Růžička, A. Lyčka and L. Dostál, Dalton
R. E. Di Paolo, A. L. Maçanita, M. J. Calhorda, A. Charas, J. Trans., 2014, DOI: 10.1039/c4dt00812j.
Morgado and M. T. Duarte, Dalton Trans. 2012, 41 8502. 11 A review concerning BꢀN analogues of aromatic hydrocarbons: M. J.
dipyrrolomethene (soꢀcalled BODIPYs): (c) R. Ziessel, G. Ulrich D. Bosdet and W. E. Piers, Can. J. Chem., 2008, 86, 8.
and A. Harriman, New J. Chem., 2007, 31, 496ꢀ501. (d) G. Ulrich, R. 12 (a) M. J. Bermingham, F. G. N. Cloke, M. G. Gardiner, P. B.
,
Ziessel, A. Harriman, Angew. Chem., Int. Ed. Engl., 2008, 47, 1184.;
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Cheng, K.S. Chen, P.ꢀT. Chou, S.ꢀM. Peng, G.ꢀH. Lee, A. J. Carty,
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Hitchcock, L. E. Wise and B. F. Yates, Dalton Trans., 2005, 1157.
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(g) A. Weiss, M. C. Hodgson, P. D. W. Boyd, W. Siebert and P. J. 13 O. T. Summerscales and F. G. N. Cloke, Coord. Chem. Rev., 2006,
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Harel and Z. Gross, Coord. Chem. Rev., 2010, 255, 717. C,N
250, 1122.
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14 (a) P. Pyykkö and M. Atsumi, Chem. Eur. J., 2009, 15, 186; (b) P.
Pyykkö and M. Atsumi, Chem. Eur. J., 2009, 15, 12770; (c) P.
Pyykkö, S. Riedel and M. Patzschke Chem. Eur. J., 2005, 11, 3511.
15 This criterion was developed by Toyota and coꢀworkers and later on
by Höpfl to gauge the strength of donorꢀacceptor bonds in amineꢀ
borane complexes: (a) S. Toyota and M. Oki, Bull. Chem. Soc. Jpn.
1992, 65, 1832; (b) S. Toyota and M. Oki, Bull. Chem. Soc. Jpn.
1992, 65, 2215; (c) H. Höpfl, J. Organomet. Chem. 1999, 581, 129.
16 J. J. Klappa, S. A. Geers, S. J. Schmidtke, L. A. MacManusꢀSpencer
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Soc. Jpn., 2004, 77, 2081. (j) M. Yamashita, Y. Yamamoto, K.
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Org. Chem., 2012, 77, 2176. (l) H. Amarne, C. Baik, S. K. Murphy
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,
,
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17 1H and 13C NMR spectra of these reaction mixtures revealed some
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typical signals in the region for CH(R’) group within the 1Hꢀ2,1ꢀ
benzazaborole core, thereby proving the closure of this ring.
Nevertheless, the mixture unambiguously contained several
compounds of this type that could not be isolated in a pure form.
These compounds may be probably atropisomers, whose existence is
caused by the presence of the adjacently bonded bulky groups on the
puckered C3BN ring (see the discussion about molecular structure of
10 in the main text).
2
(a) M. H. Vuori, R. L. Uronen, J. Repakova, E. Salonen, I.
Vattulainen, P. Panula, Z. Li, R. Bittman and E. Ikonen, Traffic
,
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4
19 B. Wrackmeyer and B. Schwarze, J. Organomet. Chem., 1997, 534
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compound
done.
6 could not be observed in spite of many experiments
7
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10 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C4DT01445F
22 P. Coppens, in Crystallographic Computing, ed. F. R. Ahmed, S. R.
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DOI: 10.1039/C4DT01445F
The reaction of C,N- and N,N-chelated boranes with lithium reagents resulted in the formation of
substituted 1H-2,1-benzazaboroles and 1H-pyrrolo[1,2-c][1,3,2]diazaborolidines.