Reduction of base-stabilized difluoroboranes to induce rearrangement reactions

Article abstract of DOI:10.1039/c0nj00363h

Lewis base-stabilized difluoroboranes 2, 4-pyr and 4-ⁱPr, having an oxazoline- or amine-tethered amide ligand, were synthesized and fully characterized. The treatment of 2 with KC₈ led to its complete consumption, and the rearranged

Full text of DOI:10.1039/c0nj00363h

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www.rsc.org/njc | New Journal of Chemistry
Reduction of base-stabilized difluoroboranes to induce rearrangement
reactionswz
Makoto Yamashita,* Yoshitaka Aramaki and Kyoko Nozaki*
Received (in Montpellier, France) 13th May 2010, Accepted 6th July 2010
DOI: 10.1039/c0nj00363h
Lewis base-stabilized difluoroboranes 2, 4-pyr and 4-iPr, having an oxazoline- or amine-tethered
amide ligand, were synthesized and fully characterized. The treatment of 2 with KC8 led to its
complete consumption, and the rearranged product 5-H, probably originating from C–O bond
cleavage and B–O bond formation, could be isolated as a major Dip-containing product in 18%
yield. From deuterium labelling experiments and diffusion control reactions, the formation of 5-H
could be explained by a radical mechanism. The reduction of 4-pyr and 4-iPr using one-electron
reducing agents also gave the rearranged products 13-pyr and 13-iPr in 21 and 19% yields,
respectively, via C–N bond cleavage and B–N bond formation. The mechanism for the formation
of 13-pyr and 13-iPr is suggested to contain a benzylic radical intermediate.
reports on the synthesis of N-heterocyclic haloboranes,8 no
Introduction
reduced product has been fully identified so far. Herein, we
Borylene is the boron analog of carbene and nitrene, and is also
report our attempts to synthesize base-stabilized borylenes. The
sometimes called borene or boranediyl. It includes a boron
reduction of base-stabilized difluoroboranes, however, resulted
atom with an oxidation state of +1 and is a highly reactive
in the isolation of unexpected rearrangement products.
species that has only ever been observed in inert gas matrices at
low temperature.1 There have been no reports on its isolation in
a condensed phase to date.2 Isolated examples are not borylene
ð2Þ
ð3Þ
itself but its adducts with a Lewis acid3 or with transition
metals.4 One may expect that the coordination of a Lewis base
to borylene would provide a base-stabilized borylene, an iso-
electronic species to carbene (eqn (1)), which has also never
been synthesized. Considering the well-developed chemistry of
heavy group 13 metal(I) (Al, Ga, In, Tl) compounds,5 electronic
and steric stabilization by using nitrogen-containing boracycles
could afford the chance to isolate a base-stabilized borylene.
Results and discussion
ð1Þ
Difluoroboranes 2, 4-pyr and 4-iPr (eqn (4) and eqn (5)) were
designed based on the following expectations. Intramolecular
coordination of the imine or amine lone pair to a vacant
p-orbital of the central boron atom stabilizes the borylene.
A nitrogen atom was introduced as a covalent ligand on the
boron atom so that the remaining lone pair on the amide
nitrogen would further stabilize the boron center by a mesomeric
effect.
Recently, boryl anions6 and NHC-stabilized diborenes,7
possessing an oxidation state of +1, have been synthesized by
reduction of the corresponding halogenated precursors (eqn (2)
and eqn (3)). The former can be considered an isoelectronic
species to base-stabilized borylene and the latter may be
regarded as a dimer of it. Based on our previous approach for
the synthesis of boryl anions,6 we conceived that N-containing
heterocyclic haloboranes could be good precursors of
base-stabilized borylenes. Although there have been many
ð4Þ
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-8656 Tokyo,
Japan. E-mail: makotoy@chembio.t.u-tokyo.ac.jp,
nozaki@chembio.t.u-tokyo.ac.jp; Fax: +81 3 5841 7262;
Tel: +81 3 5841 7263
ð5Þ
w This article is part of a themed issue on Main Group chemistry.
z CCDC reference numbers 777065–777070. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/c0nj00363h
Difluoroboranes 2, 4-pyr and 4-iPr were synthesized from
corresponding iminoamine 19 or diamines 3-pyr and 3-iPr via
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deprotonation, followed by a reaction with BF3ꢁOEt2, in
moderate yields (Scheme 1). The 11B NMR signal of the
difluoroboranes appeared as a broad triplet [dB = 1.2 (2),
1.6 (4-pyr) and 3.0 (4-iPr)], indicating the intramolecular
coordination of a nitrogen atom in the oxazoline or amine
to form an sp3-type borate structure. Two distinct doublets of
the methyl protons and one septet of the methine protons of
the Dip group may indicate restricted rotation of the Dip–N
bond in all cases. Additionally, two fluorine atoms and two
methyl groups of the oxazoline moiety in 2 are magnetically
equivalent in its 19F and 1H NMR spectra, supporting the
restricted rotation of the Dip group.
X-Ray crystallographic analyses revealed the solid state
structures of difluoroboranes 2, 4-pyr and 4-iPr (Fig. 1,
Fig. 2 and Fig. 3). All of the compounds contain a nitrogen-
coordinated sp3 aminodifluoroborane moiety. The relationship
between the shorter B1–N1 bonds [1.528(4) and 1.522(4) A for
2, 1.502(3) A for 4-pyr and 1.5162(19) A for 4-iPr] and the
longer B1–N2 bonds [1.560(4) and 1.560(4) A for 2, 1.612(3) A
for 4-pyr and 1.6554(19) A for 4-iPr] is analogous to the
shorter B–N single bond (1.50 A) and the longer B’N dative
bond (1.66 A) calculated in the molecule H2N–B(H)2’NH3.10
The B–N bond lengths in previously reported dihaloborane
b-diiminate complexes8d,i–m lie mid-way between the B1–N1
and B1–N2 bond lengths in 2, 4-pyr and 4-iPr because the
p-electrons in the diiminato moiety are delocalized by con-
jugation. The N1–B1–N2 bond angles around the central
boron atom [107.1(2) and 107.5(2)1 for 2, 108.58(17)1 for
4-pyr and 108.46(11)1 for 4-iPr] showed a nearly ideal sp3
hybridization of the boron center. In contrast, amidinato- or
guanizinato-dihaloborane compounds have a distorted sp3
boron atom [N–B–N 82–851] due to their four-membered
ring.8a–c,e–h The boron-containing six-membered rings in 2
are nearly coplanar, as the distances the boron atoms are
from the mean plane of the remaining five atoms are only
0.2086(45) and 0.2360(44) A. On the contrary, the N2 atom in
4-pyr and 4-iPr is located above the mean plane of the other
five atoms by 0.7211(29) and 0.4775(20) A, respectively, due to
repulsions among the substituents on the B1 and N2 atoms.
The reduction of difluoroborane 2 with KC8 in toluene led
to the formation of diaminoalkoxyborane 5-H in 18% yield
(Scheme 2). Although 5-H was not the predominant species, as
detected by NMR spectroscopy of the crude mixture, it is a
major Dip-containing product. None of the other side
products could be assigned. The rearranged structure of 5-H
Fig. 1 An ORTEP drawing of 2 (50% thermal ellipsoids; one of two
independent molecules and all hydrogen atoms are omitted for
clarity). Selected bond lengths (A) and angles (1): B1–N1 1.528(4),
1.522(4), B1–N2 1.560(4), 1.560(4), B1–F1 1.395(4), 1.404(4), B1–F2
1.389(4), 1.391(4), N1–C13 1.369(4), 1.373(4), C13–C14 1.427(4),
1.420(4), C14–C15 1.420(4), 1.432(4), N2–C15 1.306(4), 1.306(4),
N1–B1–N2 107.1(2), 107.5(2), F1–B1–F2 108.3(3), 107.8(2).
was characterized by the following analyses. No oxazoline sp2
carbon was detected in its 13C NMR spectrum and, instead,
benzylic 2H protons appeared in its 1H NMR spectrum.
Furthermore, the 11B NMR spectrum of the product showed
a slightly shifted signal at dB 25, indicating the existence of a
three-heteroatom substituted boron center. By using deuterated
toluene as the solvent, the benzylic position was selectively
deuterated to afford 5-D. This labelling experiment indicated
that the two benzylic protons came from the toluene. Additionally,
the reduction of 2 without stirring led to the formation of
dimeric compound 6 (Scheme 3), probably via dimerization of
radical intermediate 7b at the benzylic position (vide infra).
The dimeric structure of 6 was confirmed by ESI-TOF MS and
the preliminary result of an X-ray crystallographic analysis
(see the Experimental section). There may be a possibility to
form a diastereomeric isomer of 6. During the interaction
between two molecules of radical intermediate 7b, steric
repulsion among bulky Dip groups and dimethyloxazoline
moieties may help to produce one diastereomer of
exclusively.
6
Fig. 2 An ORTEP drawing of 4-pyr (50% thermal ellipsoids; hydrogen
atoms are omitted for clarity). Selected bond lengths (A) and angles (1):
B1–N1 1.502(3), B1–N2 1.612(3), B1–F1 1.401(3), B1–F2 1.390(3),
N1–C13 1.394(3), C13–C14 1.416(3), C14–C19 1.498(3), N2–C19
1.508(3), N1–B1–N2 108.58(17), F1–B1–F2 108.75(18).
Scheme 1 The syntheses of base-stabilized difluoroboranes 2 and 4.
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Scheme 3 The reduction of base-stabilized difluoroborane 2 with
KC8 under diffusion-controlled conditions.
Fig. 3 An ORTEP drawing of 4-iPr (50% thermal ellipsoids;
hydrogen atoms are omitted for clarity). Selected bond lengths (A)
and angles (1): B1–N1 1.5162(19), B1–N2 1.6554(19), B1–F1
1.3783(18), B1–F2 1.3917(18), N1–C1 1.3885(17), C1–C6 1.4069(19),
C6–C7 1.493(2), N2–C7 1.5143(18), N1–B1–N2 108.46(11), F1–B1–F2
108.90(11).
Based on the above observation, we propose a reaction
mechanism for the formation of 5-H and 6, as shown in
Scheme 4.11 A one-electron transfer from KC8 to 2 could
remove one fluoride ion to generate boron-centered radical
species 7a, which can also be drawn as a resonance structure of
benzylic radical 7b. A subsequent radical–radical coupling of
7b under diffusion-controlled conditions could afford dimer 6.
Under stirring conditions, 7b could abstract a hydrogen atom
from the solvent, toluene, at the benzylic position to form
diaminofluoroborane 8. This compound could be further
reduced, again eliminating fluoride, to form a second boron-
centered radical species, 9. Oxygen could then migrate to the
boron center via a 1,3-shift reaction to afford benzylic radical
10.12 The second hydrogen abstraction from the solvent by 10
could finally give product 5-H.
Amine-coordinated difluoroboranes 4-pyr and 4-iPr were
also reduced with a one-electron transfer reagent. The reaction
of 4-pyr with Li/DTBB (DTBB = 4,40-di-tert-butylbiphenyl)
gave amine-rearranged product 13-pyr in 21% yield
(Scheme 5). A similar rearrangement reaction was observed
for 13-iPr with KC8 to give 13-iPr in 19% yield (Scheme 6). As
observed in the reduction of 2, no other products could be
characterized, although the yields of the major Dip-containing
products 13-pyr and 13-iPr were rather low. The 13C NMR
spectra of 13-pyr and 13-iPr showed broad benzylic signals due
to a connection between the benzylic carbon and the quadru-
polar boron nucleus. The chemical shift of the 11B NMR signal
(dB 33 for both 13-pyr and 13-iPr) also indicated the formation
of a diaminocarbylborane. Finally, the structures of 13-pyr
Scheme 4 A plausible mechanism for the formation of 5-H and 6.
and 13-iPr were unambiguously determined by X-ray crystallo-
graphic analysis (Fig. 4 and Fig. 5). The boron center is sp2
hybridized within the resulting five-membered ring as the sum
of the angles around the central boron atom is 3601. Apparently
shorter B1–N1 [1.4387(19) A for 13-pyr and 1.4583(19) A for
13-iPr] and B1–N2 [1.395(2) A for 13-pyr and 1.4088(19) A for
13-iPr] bond lengths than those seen in 13-pyr and 13-iPr, and
the coplanarity among the boron plane and the two nitrogen
planes, reflect a strong p–p interaction between the boron and
the two nitrogen atoms. The longer B–N bond lengths in
13-iPr may reflect the steric bulkiness of the NiPr2 group
compared to those of 13-pyr.
We suggest a mechanism for the formations of 13-pyr and
13-iPr, as illustrated in Scheme 7. A one-electron reduction of
4-pyr and 4-iPr by Li/DTBB or KC8 results in the elimination
of one fluoride ion to form boron-centered radical species 14.
Subsequent C–N bond cleavage leads to the formation of
benzylic radical 15. The further reduction of 15 generates
benzylic anion 16, which undergoes intramolecular nucleo-
philic substitution on the boron center to afford the rearranged
products 13-pyr and 13-iPr.
Conclusion
Lewis base-stabilized difluoroboranes, 2, 4-pyr and 4-iPr,
having an oxazoline- or amine-tethered amide ligand, were
synthesized and fully characterized. The treatment of 2 with
KC8 led to its complete consumption. The reaction mixture
contained rearranged product 5-H, probably formed via C–O
bond cleavage and B–O bond formation, as a minor product.
From deuterium labelling experiments and diffusion control
reactions, the formation of 5-H could be explained by a radical
mechanism. We were able to isolate 13-pyr and 13-iPr as one
Scheme 2 The reduction of base-stabilized difluoroborane 2 with
KC8 in toluene or toluene-d8.
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Scheme 5 The reduction of 4-pyr with Li/DTBB.
Scheme 7 A plausible mechanism for the formation of 13.
Scheme 6 The reduction of 4-iPr with Li/DTBB.
of the reduction products of 4-pyr and 4-iPr using one-electron
reducing agents, formed via C–N bond cleavage and B–N
bond formation. The mechanism for the formation of 13-pyr
and 13-iPr is also suggested to contain a benzylic radical
intermediate.
Experimental section
General
All manipulations, except for the purification of difluoro-
boranes, were carried out by standard Schlenk techniques
under an argon atmosphere purified by passing it through a
hot column packed with a BASF R3-11 catalyst, or in an
argon-filled glove box (Miwa MFG), unless otherwise noted.
1H, 11B{1H}, 13C{1H} and 19F NMR spectra were recorded on
500 or 400 MHz spectrometers with residual protonated
solvent for 1H, deuterated solvent for 13C{1H}, external
BF3ꢁOEt2 for 11B{1H} and external CFCl3 for 19F being used
as references. Elemental analyses were performed by the
Microanalytical Laboratory of the Department of Chemistry,
Faculty of Science, Graduate School of Science, The University
of Tokyo. X-Ray crystallographic analyses were recorded on a
Rigaku Mercury CCD diffractometer. High resolution mass
spectra (ESI-TOF, THF solution) were measured on a JEOL
AccuTOF JMS-T100LP instrument with calibration using
PEG as an internal reference. Melting points were measured
on an MPA100 Optimelt automated melting point system and
are uncorrected. Toluene, THF, pentane and hexane were
purified by passing them through a solvent purification system.
Low temperature reactions at ꢂ35 1C were performed with an
SW-M01 small stirrer (Nissin Laboratory Instruments) in the
freezer of a glove box. Amine-oxazoline ligand 19 and 1-bromo-
2-(pyrrolidin-1-yl-methyl)benzene13 were synthesized according
to literature procedures. Purifications by GPC were performed
by an LC-928 recycling preparative HPLC (JAI) equipped
with a JAI GEL 1H-2H column; the solvent was CHCl3.
Fig. 4 An ORTEP drawing of 13-pyr (50% thermal ellipsoids;
hydrogen atoms are omitted for clarity). Selected bond lengths (A)
and angles (1): B1–N1 1.4387(19), B1–N2 1.395(2), B1–C7 1.5896(19),
N1–C8 1.4395(16), N1–B1–N2 127.51(12), N1–B1–C7 107.15(11),
N2–B1–C7 125.34(12).
Syntheses
2. To an ethereal solution (12 mL) of 1 (751 mg, 2.14 mmol)
in a 20 mL Schlenk flask was added a hexane solution of
n-BuLi (1.66 M, 1.42 mL, 2.35 mmol) at ꢂ78 1C under an
argon atmosphere. After the solution was stirred at the same
temperature for 5 min, the cooling bath was removed and the
Fig. 5 An ORTEP drawing of 13-iPr (50% thermal ellipsoids;
hydrogen atoms are omitted for clarity). Selected bond lengths (A)
and angles (1): B1–N1 1.4583(19), B1–N2 1.4088(19), B1–C7 1.603(2),
N1–C8 1.4357(17), N1–B1–N2 126.52(12), N1–B1–C7 105.30(11),
N2–B1–C7 128.15(12).
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flask was kept at room temperature for 1 h without any
heating. The resulting yellow-green suspension, containing a
yellow precipitate, was cooled again to ꢂ78 1C. Then, freshly
distilled BF3ꢁOEt2 (290 mL, 2.35 mmol) was added to the flask.
After the addition was complete, the cooling bath was imme-
diately removed to induce the color change of the resulting
suspension to orange. The resulting suspension was stirred at
room temperature without any heating to produce a yellowish-
green suspension. All of the volatiles were removed under
reduced pressure and the residue was purified by silica gel
column chromatography (CH2Cl2–hexane = 1 : 1) to give a
pale yellow solid of 2 (362 mg, 42%).
stirred at room temperature for 1.5 h without any heating. All
the volatiles were removed in vacuo. The crude product was
then recrystallized by the diffusion of hexane into a saturated
CHCl3 solution of the crude product to give colorless crystals
(755 mg, 37% yield).
1H NMR (CDCl3, 500 MHz): d 0.99 (d, J = 7 Hz, 6H), 1.21
(d, J = 7 Hz, 6H), 2.04–2.16 (br m, 4H), 3.05–3.14 (br m, 2H),
3.19 (dq, J = 7 Hz, 7 Hz, 2H), 3.50–3.60 (br m, 2H), 4.18
(s, 2H), 5.95 (d, J = 8 Hz, 1H), 6.54 (t, J = 7 Hz, 1H),
6.92–7.00 (m, 2H), 7.20–7.26 (m, 1H), 7.28–7.34 (m, 1H);
11B NMR (CDCl3, 160 MHz): d 1.4; 13C NMR (CDCl3,
100 MHz): d 22.6 (CH2), 24.3 (CH3), 25.0 (CH3), 27.9
(CH2), 53.9 (CH), 58.6 (CH2), 113.7 (CH), 113.8 (CH), 115.0
(CH), 124.4 (CH), 127.0 (41), 127.6 (CH), 129.3 (CH), 136.8
(41), 148.0 (41), 149.0 (41); 19F NMR (CDCl3, 470 MHz):
d ꢂ156.6.
1H NMR (CDCl3, 500 MHz): d 1.01 (d, J = 7 Hz, 6H), 1.26
(d, J = 7 Hz, 6H), 1.68 (s, 6H), 3.07 (dq, J = 7 Hz, 7 Hz, 2H),
4.48 (d, J = 1 Hz, 2H), 6.23 (d, J = 9 Hz, 1H), 6.63 (t, J =
7 Hz, 1H), 7.22–7.29 (m, 3H), 7.36 (t, J = 8 Hz, 1H), 7.67
(d, J = 8 Hz, 1H); 13C NMR (CDCl3, 125 MHz): d 24.1
(CH3), 24.9 (CH3), 26.6 (CH3), 28.1 (CH3), 65.1 (41), 81.8
(CH2), 101.1 (41), 115.0 (CH), 115.8 (CH), 124.2 (CH), 127.4
(CH), 127.6 (CH), 127.6 (CH), 135.7 (41), 135.9 (CH), 148.1
(41), 152.3 (41), 165.7 (41); 11B{1H} NMR (CDCl3, 160 MHz):
1H NMR (C6D6, 500 MHz): d 1.05 (br m, 2H), 1.17 (br m,
2H), 1.19 (d, J = 7 Hz, 6H), 1.50 (d, J = 7 Hz, 6H), 2.37
(br m, 2H), 3.19 (br m, 2H), 3.50 (s, 2H), 3.57 (sep, J = 7 Hz,
2H), 6.23 (d, J = 8 Hz, 1H), 6.58 (t, J = 7 Hz, 1H), 6.65
(d, J = 7 Hz, 1H), 6.94 (t, J = 8 Hz, 1H), 7.32 (m, 3H);
11B NMR (C6D6, 160 MHz): d 1.6 (br t); 19F NMR (C6D6,
470 MHz): d ꢂ158.1 (br).
1
d 1.2 (t, JFB = 25 Hz); 19F NMR (CDCl3, 471 MHz):
d ꢂ130.6 (br q); mp 193.8–198.8 1C (decomp.). Anal. calc.
for C23H29BF2N2O: C, 69.36; H, 7.34; N, 7.03. Found: C,
69.36; H, 7.46; N, 6.95%.
mp: 123.1–136.2 1C (decomp.). Anal. calc. for C23H31BF2N2:
C, 71.88; H, 8.13; N, 7.29. Found: C, 71.78; H, 8.21; N, 7.09%.
3-pyr. 1-Bromo-2-(pyrrolidin-1-yl-methyl)benzene (9.89 g,
3-iPr. To a 100 mL round-bottomed flask were added
2-bromobenzyl bromide (15.9 g, 63.6 mmol), toluene (ca. 30 mL)
and diisopropylamine (16.1 g, 159 mmol), and the solution
was refluxed for 2 d in air. After cooling the reaction mixture
to room temperature, the organic salts were filtered off
through a pad of Celites and the filtrate pumped to remove
toluene and excess amine. The residue was dissolved in
CH2Cl2 and the resulting solution passed through a short
silica gel column. The solution was evaporated, giving crude
2-bromobenzyldiisopropylamine. The product was then
brought into a glove box. In the glove box, to a 500 mL
three-necked flask equipped with a rubber septum and a three-
way stopcock were added the crude aryl bromide, 2,6-diiso-
propylaniline (11.6 g, 65.4 mmol), Pd(OAc)2 (1.17 g, 5.23 mmol),
tBuP (2.11 g, 10.5 mmol), NaOt-Bu (8.17 g, 85.0 mmol) and
300 mL of toluene. After the flask had been brought out from
the glove box, it was equipped with a condenser and the
reaction mixture was then refluxed for 46 h. After cooling
the flask to room temperature, the reaction mixture was
filtered through a pad of Celites to remove inorganic salts
and the filtrate evaporated in vacuo. The crude product was
recrystallized from refluxing hexane to give 3-iPr (17.2 g,
47.0 mmol, 73% over two steps).
41.4 mmol), 2,6-diisopropylaniline (7.34 g, 41.4 mmol),
t
Pd(OAc)2 (743 mg, 3.31 mmol), BuP (1.34 g, 6.62 mmol),
NaOtBu (5.17 g, 53.8 mmol) were placed into a 500 mL
Schlenk flask in a glove box. Toluene (150 mL) was added
to the mixture and the flask connected to a condenser. The
resulting apparatus was brought out from the glove box and
the reaction mixture refluxed at 130 1C for 2 d. After cooling
the flask to room temperature, the solution was filtrated
through a pad of Celites on Silica gel to remove inorganic
salts, and the residue was washed with toluene. All the
volatiles were evaporated in vacuo. The crude mixture was
heated at 260 1C under reduced pressure to remove unreacted
aryl bromide and 2,6-diisopropylaniline, giving a brown oil
(8.32 g, 495%, containing a trace amount of 2,6-diisopropyl-
aniline). The obtained material was used for the next step
without any further purification.
1H NMR (CDCl3, 500 MHz): d 1.11 (d, J = 7 Hz, 6H), 1.17
(d, J = 7 Hz, 6H), 1.79 (m, 4H), 2.54 (m, 4H), 3.13 (sept,
J = 7 Hz, 2H), 3.74 (s, 2H), 6.15 (d, J = 8 Hz, 1H), 6.62
(t, J = 7 Hz, 1H), 7.00 (t, J = 8 Hz, 1H), 7.08 (d, J = 7 Hz, 1H),
7.20–7.26 (m, 3H), 7.88 (br s, 1H).
4-pyr. To an ethereal solution (15 mL) of 3-pyr (1.97 g,
5.38 mmol) in an 80 mL Schlenk flask was added a hexane
solution of n-BuLi (1.57 M, 3.60 mL, 5.65 mmol) at ꢂ78 1C
under an argon atmosphere. After the solution had been
stirred at the same temperature for 5 min, the cooling bath
was removed and the flask kept at room temperature for 1 h
without any heating. The resulting suspension was cooled
again to 0 1C and BF3ꢁOEt2 (0.700 mL, 5.65 mmol) added
dropwise. After the addition was complete, the cooling bath
was immediately removed and the resulting reaction mixture
1H NMR (CDCl3, 500 MHz): d 1.08 (d, J = 7 Hz, 6H), 1.11
(d, J = 7 Hz, 12H), 1.16 (d, J = 7 Hz, 6H), 3.11–3.21 (m, 4H),
3.84 (s, 2H), 6.07 (d, J = 8 Hz, 1H), 6.62 (t, J = 7 Hz, 1H),
6.96 (d, J = 8 Hz, 1H), 7.09 (d, J = 6 Hz, 1H), 7.21 (m, 2H),
7.48 (s, 1H); 13C NMR (C6D6, 126 MHz): d 20.3 (CH3), 23.4
(CH3), 25.0 (CH3), 28.2 (CH), 46.9 (CH), 49.0 (CH2), 111.8
(CH), 116.6 (CH), 123.1 (41), 123.8 (CH), 126.4 (CH), 128.1
(CH), 130.5 (CH), 135.9 (41), 147.1 (41), 148.6 (41); mp:
124.5–126.4 1C (decomp.); HRMS-ESI (m/z): [M + H+] calc.
+
for C25H39N2 367.3113; found 367.3110.
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4-iPr. An ether (40 mL) solution of 3-iPr (3.13 g, 8.55 mmol)
in an 80 mL Schlenk flask was cooled to ꢂ78 1C under an
argon atmosphere. A hexane solution of n-BuLi (1.57 M,
5.71 mL, 8.98 mmol) was then added dropwise to the reaction
mixture. After the solution had been stirred at the same
temperature for 5 min, the cooling bath was removed and
the flask kept at room temperature for 1 h without any
heating. The resulting suspension was cooled again to 0 1C
and BF3ꢁOEt2 (1.13 mL, 8.98 mmol) added dropwise. After
the addition was complete, the cooling bath was immediately
removed and the resulting reaction mixture stirred at room
temperature overnight. All the volatiles were then removed
in vacuo. After the Schlenk flask had been brought into a glove
box, the crude product was triturated with hexane and the
resulting suspension filtered through a pad of Celites. After
the solvents had been removed from the filtrate, the crude
product was re-precipitated by adding its saturated THF
solution to an excess amount of hexane to give 4-iPr (2.24 g,
5.42 mmol, 63% yield).
1H NMR (C6D6, 500 MHz): d 0.89 (d, J = 6 Hz, 6H), 1.18
(d, J = 7 Hz, 6H), 1.20 (d, J = 7 Hz, 6H), 1.50 (d, J = 7 Hz,
6H), 3.31–3.42 (br m, 2H), 3.55 (dq, J = 7 Hz, 7 Hz, 2H), 3.78
(br s), 6.17 (d, J = 8 Hz, 1H), 6.59 (m, 1H), 6.65 (d, J = 6 Hz,
1H), 6.92 (m, 1H), 7.29–7.37 (m, 3H); 13C NMR (C6D6,
100 MHz): d 20.9 (CH3), 24.8 (CH3), 25.4 (CH3), 28.4 (CH),
55.7 (CH), 56.7 (CH2), 113.5 (CH), 114.7 (41), 115.4 (CH),
124.8 (CH), 127.5 (CH), 127.7 (CH), 129.5 (CH), 137.9 (41),
148.9 (41), 149.4 (41); 11B NMR (C6D6, 160 MHz): d 3.0
(br t); 19F NMR (C6D6, 470 MHz): d ꢂ138.0 (br); mp:
145.2–170.9 1C (decomp.). Anal. calc. for C25H37B F2N2:
C, 72.46; H, 9.00; N, 6.76. Found: C, 72.21; H, 9.25; N,
6.58%.
through a pad of Celites to remove inorganic salts and
excess KC8. All the volatiles were removed under reduced
pressure and the crude product purified by recrystallization
(twice) by the diffusion of hexane into a saturated CHCl3
solution of the crude product to give a colorless solid of 6
(9.8 mg, 47%).
1H NMR (CDCl3, 500 MHz): d 0.59 (s, 6H), 1.05 (d, J =
7 Hz, 6H), 1.12 (d, J = 8 Hz, 6H), 1.14 (d, J = 8 Hz, 6H), 1.15
(s, 6H), 1.29 (d, J = 7 Hz, 6H), 2.61 (sept, J = 7 Hz, 2H), 2.85
(d, J = 8 Hz, 2H), 3.30 (d, J = 8 Hz, 2H), 3.64 (sept, J =
7 Hz, 2H), 6.18 (dd, J = 8 Hz, 1 Hz, 2H), 6.97 (dt, J = 7 Hz,
1 Hz, 2H), 7.05 (dt, J = 8 Hz, 1 Hz, 2H), 7.29 (dd, J = 8 Hz,
1 Hz, 2H), 7.33 (dd, J = 8 Hz, 1 Hz, 2H), 7.41 (dt, J = 8 Hz, 1 Hz,
2H), 7.59 (dd, J = 8 Hz, 1 Hz, 2H); 13C NMR (CDCl3,
101 MHz). Two signals were observed as a doublet, probably
due to through-space 13C–19F coupling:14 d 23.8 (CH3), 24.5
(CH3), 24.7 (CH3), 25.2 (CH3), 25.4 (CH3), 27.2 (CH), 28.3
(d, J = 4 Hz, CH3), 28.4 (CH), 62.0 (41), 78.7 (CH2), 100.4
(d, J = 6 Hz, 41), 114.3 (CH), 120.4 (CH), 124.3 (CH), 124.4
(CH), 124.7 (41), 128.1 (CH), 128.4 (CH), 129.7 (CH), 134.5
(41), 144.4 (41), 147.6 (41), 148.1 (41); 11B{1H} NMR (CDCl3,
160 MHz): d 23 (br s), 19F NMR (CDCl3, 471 MHz): d ꢂ127.7
(s); mp: 207.8–210.7 1C (decomp.); HRMS-ESI (m/z):
[M + H+] calc. for C46H59B2F2N4O2 759.4792; found 759.4823.
The reduction of 4-pyr to form 13-pyr. A 20 mL vial equipped
with a glass stirrer bar was charged with 4-pyr (217 mg,
0.564 mmol), lithium powder (39.2 mg, 5.64 mmol) and
di-tert-butylbiphenyl (30.1 mg, 0.113 mmol) in a globe box.
To the vial, cooled to ꢂ35 1C, was added pre-cooled THF
(10 mL, ꢂ35 1C). After stirring the reaction mixture at ꢂ35 1C
for 15 h, the solution was filtered through a pad of Celites to
remove inorganic salts. The resulting filtrate was evaporated
under reduced pressure. The crude product was recrystallized
by slow evaporation of a benzene solution to give 13-pyr
(42.1 mg, 0.122 mmol, 21%).
1H NMR (C6D6, 500 MHz): d 1.06 (d, J = 7 Hz, 6H), 1.16
(d, J = 7 Hz, 6H), 1.24–1.27 (m, 2H), 1.34–1.39 (m, 2H), 2.30
(s, 2H), 2.62 (t, J = 7 Hz, 2H), 3.01 (t, J = 7 Hz, 2H), 3.22
(sep, J = 7 Hz, 2H), 6.22 (d, J = 8 Hz, 1H), 6.89 (t, J = 7 Hz,
1H), 7.01 (t, J = 8 Hz, 1H), 7.19 (d, J = 8 Hz, 2H), 7.27
(t, J = 8 Hz, 1H), 7.34 (d, J = 7 Hz 1H); 13C NMR (C6D6,
100 MHz): d 20.1 (br, CH2), 23.9 (CH3), 24.7 (CH3), 25.8
(CH2), 27.1 (CH2), 28.7 (CH), 46.3 (CH2), 49.9 (CH2), 109.8
(CH), 119.3 (CH), 124.0 (CH), 126.6 (CH), 127.3 (CH), 127.9
(CH), 131.3 (41), 138.1 (41), 147.5 (41), 156.0 (41); 11B NMR
(C6D6, 160 MHz): 33 (br); mp 58.1–65.1 1C (decomp.). Anal.
calc. for C23H31BN2: C, 79.77; H, 9.02; N, 8.09. Found: C,
79.55; H, 9.21; N, 7.70%.
The reduction of 2 to give 5-H. To a 50 mL vial containing a
glass stirrer bar was added 2 (376 mg, 0.944 mmol), KC8
(1.22 g, 9.00 mmol) and toluene (15 mL) at room temperature
in a glove box. The resulting mixture was stirred at room
temperature for 2 d and filtered through a pad of Celites. All
the volatiles were removed from the filtrate under reduced
pressure. The solid residue was extracted with hexane in six
times and the combined hexane solution evaporated. The
crude product was purified by recycling HPLC (CHCl3 eluent)
to give a solid of 5-H (61.0 mg, 18%).
1H NMR (CD2Cl2, 500 MHz): d 1.06 (d, J = 7 Hz, 6H),
1.14 (d, J = 7 Hz, 6H), 1.27 (s, 6H), 2.99 (dq, J = 7 Hz, 7 Hz,
2H), 3.87 (s, 2H), 4.42 (s, 2H), 6.07 (dd, J = 8 Hz, 1 Hz, 1H),
6.79 (dt, J = 7 Hz, 1 Hz, 1H), 6.91 (dt, J = 8 Hz, 1 Hz, 1H),
7.09 (d, J = 7 Hz, 1H), 7.24 (d, J = 8 Hz, 2H), 7.34 (dd, J =
8 Hz, 1 Hz, 1H); 13C NMR (CDCl3, 100 MHz): d 23.7 (CH3),
24.3 (CH3), 24.4 (CH3), 42.3 (CH2), 58.5 (41), 77.4 (CH), 79.8
(CH2), 114.8 (CH), 119.7 (CH), 122.0 (41), 124.2 (CH), 127.4
(CH), 127.9 (CH), 128.0 (CH), 135.0 (41), 144.3 (41), 147.6(41);
11B NMR (C6D6, 160 MHz): d 25 (br s); HRMS-ESI (m/z):
calc. for C23H30BN2O+ 361.2451; found 361.2450.
The reduction of 4-iPr to form 13-iPr. A 20 mL vial equipped
with a glass stirrer bar was charged with 4-iPr (170 mg,
0.410 mmol) and KC8 (160 mg, 4.10 mmol) in a globe box.
To the vial, cooled to ꢂ35 1C, was added pre-cooled THF
(3 mL, ꢂ35 1C). After stirring the reaction mixture at ꢂ35 1C
for 2.5 h, the solution was filtered through a pad of Celites to
remove inorganic salts. The resulting filtrate was evaporated
under reduced pressure. The crude product was then recrystallized
The reduction of 2 without stirring to give 6. 2 (21.8 mg,
54.7 mmol), KC8 (37.0 mg, 274 mmol) and 1.0 mL of toluene
were added to a 20 mL vial in a glove box. The mixture was
kept unstirred for 3 d at room temperature and then filtrated
ꢀc
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Table 1 Crystallographic data and structure refinement details for 2, 4-pyr, 4-iPr, 6, 13-pyr and 13-iPr
2
4-pyr
4-iPr
6
13-pyr
13-iPr
Formula
fw
T/K
C23H29BF2N2O
398.29
103(2)
0.71070
Triclinic
ꢀ
P1
11.866(5)
12.250(5)
15.775(7)
100.107(4)
100.940(4)
104.311(5)
2121.0(15)
4
C23H31BF2N2
384.31
103(2)
0.71070
Monoclinic
P21/a
13.975(6)
11.390(5)
14.556(7)
90
114.7635(19)
90
2104.0(16)
4
1.213
0.082
824
C25H37BF2N2
414.38
103(2)
0.71070
Monoclinic
P21/n
10.9370(13)
11.0234(10)
20.180(2)
90
101.4026
90
2384.9(4)
4
1.154
0.077
896
C46H58B2F2N4O2 C23H31BN2
C25H37BN2
376.38
103(2)
0.71070
Triclinic
ꢀ
P1
9.969(4)
10.684(5)
11.315(5)
85.714(13)
77.331(12)
83.324(13)
1166.4(9)
2
758.58
103(2)
0.71070
Monoclinic
P21/n
346.31
103(2)
0.71070
Triclinic
ꢀ
P1
l/A
Crystal system
Space group
a/A
b/A
c/A
a (1)
b (1)
10.393(8)
17.539(13)
11.564(9)
90
99.422(5)
90
2080(3)
2
1.211
0.079
812
9.440(5)
10.504(5)
10.892(5)
89.733(3)
107.726(6)
103.128(7)
999.4(8)
2
g (1)
V/A3
Z
Dc/g cmꢂ3
m/mmꢂ1
F(000)
1.247
0.087
848
1.151
0.066
376
1.072
0.061
412
Crystal size/mm
2y range (1)
Reflections collected
Independent reflections 7298
Rint
Parameter
GOF on F2
R1
wR2 [I 4 2s(I)]
R1
wR2 (all data)
0.40 ꢃ 0.15 ꢃ 0.13 0.35 ꢃ 0.25 ꢃ 0.10 0.60 ꢃ 0.55 ꢃ 0.55 0.40 ꢃ 0.40 ꢃ 0.10 0.60 ꢃ 0.45 ꢃ 0.35 0.40 ꢃ 0.35 ꢃ 0.20
3.32–25.00
13859
3.08–25.00
13221
3570
3.12–25.00
14829
4067
3.06–25.00
13022
3592
3.41–25.00
6464
3415
3.08–25.00
7627
4007
0.0325
535
0.0378
257
1.124
00263
279
1.117
0.0627
259
1.268
0.0147
239
1.069
0.0218
261
1.081
1.104
0.0797
0.1969
0.1021
0.2153
0.0585
0.1406
0.0697
0.1493
0.0449
0.1024
0.0495
0.1053
0.1355
0.3076
0.1656
0.3278
0.0401
0.0923
0.0471
0.0968
0.0430
0.1006
0.0520
0.1066
X-Ray crystallography
Details of the crystal data and a summary of the intensity data
collection parameters for 2, 4-pyr, 4-iPr, 6, 13-pyr and 13-iPr
are listed in Table 1. In each case, a suitable crystal was
mounted with mineral oil onto a glass fiber and transferred
to the goniometer of a Rigaku Mercury CCD diffractometer
with graphite-monochromated Mo-Ka radiation (l = 0.71070 A).
The structures were solved by direct methods using SIR-9715
and refined by full-matrix least-squares techniques against
F2 (SHELXL-97).16 The non-hydrogen atoms were refined
anisotropically. The hydrogen atoms were placed using AFIX
instructions. The quality of the data for 6 (Fig. 6) was not
good enough for it to be discussed in detail.
Fig. 6 An ORTEP drawing of dimeric compound 6 (50% thermal
ellipsoids; hydrogen atoms are omitted for clarity). Half of the
molecule is the asymmetric unit; the numbers with asterisks refer to
the second half of the molecule.
Acknowledgements
This work was supported by the Global COE Program
(Chemistry Innovation through Cooperation of Science and
Engineering), KAKENHI (21245023 and 21685006) from
MEXT, Japan and by a Grant for Basic Science Research
Projects from the Sumitomo Foundation.
by slow evaporation of a hexane solution to give 13-iPr
(30.0 mg, 0.0797 mmol, 19%).
1H NMR (C6D6, 500 MHz): d 0.77 (d, J = 7 Hz, 6H), 1.01
(d, J = 7 Hz, 6H), 1.18 (d, J = 7 Hz, 6H), 1.21 (d, J = 7 Hz,
6H), 2.52 (s, 2H), 3.03 (s, 1H), 3.20 (sep, J = 7 Hz, 2H), 3.60
(s, 1H), 6.12 (m, 1H), 6.90 (m, 1H), 6.95 (m, 1H), 7.18 (m, 2H),
7.24 (m, 1H), 7.30 (m, 1H). 13C NMR (C6D6,100 MHz):
d 22.1 (CH3), 22.2 (br CH2), 23.7 (CH3), 24.3 (CH3), 25.4
(CH3), 28.6 (CH), 44.3 (CH), 47.4 (CH), 111.0 (CH), 119.4
(CH), 124.5 (CH), 126.0 (CH), 126.7 (CH), 127.7 (CH), 131.5
(41), 139.6 (41), 147.1 (41), 155.4 (41); 11B NMR (C6D6, 160
MHz): 33 (br); mp 98.2–100.1 1C. Anal. calc. for C25H37BN2:
C, 79.78; H, 9.91; N, 7.44. Found: C, 79.67; H, 10.01;
N, 7.20%.
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11 There is another possibility for
a mechanism involving the
formation of borylene 11 by reduction. A subsequent hydrogen
abstraction from its resonance form, biradical 12, could afford
diaminoborylradical species 9. However, the borylene mechanism
can be ruled out based on the formation of dimeric compound 6.
12 There is another possibility for an initial elimination of alkoxide
from intermediate 8, followed by the nucleophilic attack of
alkoxide towards the B–F moiety to form intermediate 10.
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1782 | New J. Chem., 2010, 34, 1774–1782