Internal B-O Bond-Facilitated Photoisomerization of Boranes: Ring Expansion Versus Oxyborane Elimination/Intramolecular Diels-Alder Addition

Article abstract of DOI:10.1021/acs.orglett.9b01892

Boron compounds (1-4) containing an internal B-O bond have been found to undergo facile multistructural transformations upon irradiation at 365 or 410 nm, generating rare 8-membered B,O-heterocycles (1c-4c). In addition, 2 and 3 also undergo an intramolecular Diels-Alder addition and oxyborane elimination concomitantly, via intermediates 2b/3b, producing 2d/3d. The pathways to isomer c and product d were found to be a thermal process and a photo process, respectively.

Full text of DOI:10.1021/acs.orglett.9b01892

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Internal B−O Bond-Facilitated Photoisomerization of Boranes: Ring
Expansion Versus Oxyborane Elimination/Intramolecular Diels−
Alder Addition
Guo-Fei Hu,† Hai-Jun Li,‡ Chao Zeng,† Xiang Wang,‡ Nan Wang,† Tai Peng,† and Suning Wang*,†,‡
†Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of Photoelectronic/Electrophotonic
Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing, China
‡Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
S
* Supporting Information
ABSTRACT: Boron compounds (1−4) containing an internal B−
O bond have been found to undergo facile multistructural
transformations upon irradiation at 365 or 410 nm, generating
rare 8-membered B,O-heterocycles (1c−4c). In addition, 2 and 3
also undergo an intramolecular Diels−Alder addition and oxy-
borane elimination concomitantly, via intermediates 2b/3b,
producing 2d/3d. The pathways to isomer c and product d were
found to be a thermal process and a photo process, respectively.
Scheme 1. (a) Azole, (b) Imine, and (c) Carbonyl-
Supported Photoreactive Borane Systems
oron-supported substrates are widely used in chemical
1
Bmodification and synthesis. A variety of bond formations
facilitated by a boron unit are well-known in the literature.2,3
Metal-free coupling reactions and other chemical reactions
facilitated by boron have attracted much research attention.2,3
Recent studies on photoisomerization of boranes, especially
those of triarylboranes and their derivatives, demonstrated the
possibility of using light to achieve the rare intramolecular
structural transformation of boranes.4 Photochemical trans-
formation of internal-donor-supported four-coordinated bor-
anes often leads to rare and functional boron heterocycles in a
quantitative manner that are difficult to access via conventional
approaches.5 Most previously studied photoreactive four-
coordinate borane systems rely on an aryl or a heteroaryl
moiety as such pyridyl or thiazolyl as the supporting donor to
boron (type I in Figure 16b and Scheme 1a).6 For a longtime,
such diaryl chelate units were thought to be necessary in order
to achieve structural integrity throughout the photoreaction
and intramolecular charge transfer (CT) from Ar to the
chelate, which initiates the photoisomerization. Recently the
imine-supported type II system in Scheme 1b was found to
undergo a highly efficient photoisomerization, following a
Figure 1. Type I and II photochromic four-coordinated borane
systems.
Received: June 2, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01892
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Organic Letters
Letter
similar pattern as does the type I analogue,7 illustrating that a
simple conjugated donor is capable of supporting borane
phototransformation. The strong electron-donating ability of
the imine group was thought to play an important role in the
efficient photoisomerization of the imine-supported system. To
expand the scope of the type II photoreactive system, we
studied a carbonyl-supported borane system (Scheme 1c). The
O atom on a carbonyl is a much weaker donor to boron,
compared to the imine N atom. Therefore, it is very surprising
that the carbonyl-supported molecules were found to display a
similar phototransformation as the imine analogues and are
much more reactive toward light with divergent reaction
pathways. In addition to the formation of B,O-heterocycles (c),
unusual elimination of oxyborane (MesBO) and the formation
of species d, which is a rare carbonyl to alkane conversion
product, was also observed. The details of these interesting
new findings are presented herein.
The absorption spectra of 1−3 have a broad and intense
band near 400 nm (λmax = 397 nm (1); λmax = 388 nm (2 and
3)), whereas, in contrast, compound 4 has a very weaker band
at a much lower energy (λmax = 412 nm), as shown in Figure 2.
The NMe2 unit on the chelate backbone is clearly responsible
for the intense absorption bands of 1−3.
Compounds 1−4 were prepared in good yields, according to
the general procedure shown in Scheme 2. Compound 1 is a
Scheme 2. Synthetic Procedures for Compounds 1−4
Figure 2. Absorption spectra of 1−4 in toluene (1.0 × 10−5 M, left)
and the diagrams of key orbitals involved in vertical excitations to S1,
S2, and S5 states of 1 (right). Inset shows an enlarged absorption
spectrum of 4.
To elucidate the origin of the absorption spectra, time-
dependent density functional theory (TD-DFT) calculations
(B3LYP/6-31G(d)) for 1, 2, and 4 were performed; the details
are given in the SI. For 1 and 2, the vertical excitations to the
S1 states (HOMO (H) to LUMO (L), 98%−99%, f = 0.0087,
0.0089) and the S2 states (H-1 to L, 96%−97%, f = 0.0256,
0.0246) are similar in energy with appreciable oscillator
strengths, and can be ascribed to charge transfer (CT)
transitions from the Mes groups to the chelate backbone
(Figure 2). The vertical excitations to the S3 and S4 states of 1/
2 are similar CT transitions involving H-2 (98%) and H-3 to L
(97%), respectively, but much smaller oscillator strengths
(0.0008 and 0.0025−0.0027). The vertical excitation to the S5
state is mainly a π → π* (H-4 to L, > 83%) localized on the
chelate backbone, with the amino to the carbonyl CT
character, and a very large oscillator strength (0.3621 and
0.3677 for 1 and 2, respectively).The orbitals involved in
vertical excitations to the S1 to S5 states for 4 are similar to
those of 1 and 2, except they lack the amino group, and the
transitions having very small oscillator strengths. The
calculated trend of vertical excitation energies to S1 and S2
states for 1, 2, and 4 matches well that observed in the
absorption spectra. Compounds 1−4 are weakly emissive with
λem ≈ 540 nm for 1, λem ≈ 520 nm for 2 and 3, and λem ≈ 570
nm for 4, and a large Stokes shift (>100 nm; see the SI). The
fact that the S1−S4 states for all four compounds are CT
transitions from Mes to the chelate backbone is important,
because such CT transitions drive the photoisomerization of
donor-supported chelate borane systems.6,7
previously known molecule used as a precursor molecule for a
donor−acceptor functionalized stilbene that we recently
reported.8 The introduction of the NMe2 para to the
aldehyde/keto group in 1−3 is to enhance the nucleophilicity
of the carbonyl group. The variation of the R2 group from H
atom to ethyl is for the purpose of examining the impact of
both electronic and steric factors on the photoreactivity of this
class of compounds and the possibility of converting the
carbonyl to a stereogenic center via photoisomerization. To
compare the impact of the amino group, molecule 4, which
lacks the p-amino group with R2 = methyl was also prepared.
Full characterization details for compounds 2−4 are provided
in the Supporting Information.
The 11B NMR chemical shifts of 1−4 in benzene are 13.6,
13.5, 13.4, and 16.3 ppm, respectively, supporting their four-
coordinate structures in solution. The ∼3 ppm difference of
the 11B NMR signal between 2 and 4 indicates that the p-
NMe2 group indeed enhances the keto-donating ability to the
B center. Replacing the H atom on the R2 site by a methyl or
an ethyl group appears to have little effect on the electron
density on boron. It is noteworthy that the isomer of 1 with the
NMe2 group being para to BMes2 and meta to the aldehyde is
three-coordinated with a 11B NMR signal at ∼60 ppm, and is
not photochemically active.9 Therefore, the location of the
amino group is important in modulating the structure and the
properties of this class of borane molecules. The crystal
structures of 1−3 were determined by X-ray diffraction (XRD)
analyses. Their B−O bond lengths are similar (1.607(2),
1.612(6), and 1.624(4) Å for 1−3, respectively; see the
Supporting Information (SI)).
Compounds 1−4 are all responsive to irradiation at 365 nm
(1−3) or 410 nm (4), undergoing quantitative isomerization,
reaching completion in ∼2 h (0.05 M in C6D6), as shown by
the NMR spectral tracking data (see the SI). At low
temperature (−78 °C), upon irradiation, compounds 1−4
transform to dark-colored species 1a−4a, which most likely
possess structures similar to the boratanorcaradiene (borirane)
isomers a, identified for the azole- or imine-supported systems,
shown in Figure 3. Similar to the imine analogues, 1a−4a are
B
DOI: 10.1021/acs.orglett.9b01892
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Organic Letters
Letter
Figure 3. Proposed structure and photographs of 1a−4a in benzene,
taken immediately after the NMR tubes irradiated by UV light (365
nm) were pulled out of the dry ice bath.
thermally unstable. However, unlike the imine analogues a,
which can be characterized by NMR at low temperature and
converted to the H-atom transfer (HAT) isomer c at room
temperature (rt), 1a−4a could not be characterized by NMR,
because of their rapid reversal back to 1−4 at temperatures
above −40 °C.
When the photoreaction is performed at ambient temper-
ature, 1−4 isomerize rapidly to 1b−4b, respectively, which
Figure 5. 11B NMR spectra showing the photoconversion of 1−4
1
were observed in the H NMR spectra, albeit short-lived (see
(0.05 M in C6D6).
Figure 6, presented later in this work). Therefore, the a → b
isomerization for the carbonyl-supported system is driven by
light. 1b−4b further isomerize to 1c−4c, forming the ring-
expansion products, which are similar to the imine analogues
and fully characterized by nuclear magnetic resonance (NMR),
high-resolution mass spectroscopy (HRMS), and XRD
analyses (3c and 4c; see Figure 4). For 1 and 4, the formation
O bonds in 3c and 4c are quite short, 1.369(8) and 1.348(6)
Å, respectively, which is an indication of the presence of a
partial π-bond between the B and O atoms.10 The most
important feature of 3c and 4c is the stereogeometry of the
chiral C7 atom. Consistent with those observed in the azole-
supported system, the H atom on the C7 atom of 3c and 4c is
syn to the C−C bond of the neighboring benzene ring, while
the anti-diastereomer was not observed. This further supports
that the HAT process in the carbonyl-supported system is a
concerted intramolecular process, following the same mecha-
nistic pathway as the azole-supported borane system.6b
Compounds 1c−4c are air-stable and thermally stable. Heating
their toluene solutions to 100 °C did not produce any obvious
change of their NMR spectra.
Interestingly, for 2 and 3, in addition to the formation of 2c
and 3c, which have a 11B NMR peak at ∼48 and ∼50 ppm,
respectively (Figure 5), a new species 2d/3d was also observed
in the photoreaction mixture, formed concomitantly with 2c/
3c, as shown by Figure 6. Furthermore, a distinct 11B NMR
peak at ∼33 ppm was observed for the photoreaction of 2/3,
which was identified as a boroxine (MesBO)3 (see Figure S6F
in the SI).11 2c and 3c were separated from the products by
crystallization (NMR yields for both compounds are ∼70%;
isolated yields are ∼50%), while 2d and 3d were isolated by
column chromatography as colorless oils in 15% and 21%
yield, respectively. HRMS and 2D NMR spectroscopic
analyses established that 2d and 3d are N,N,5,7,10-
pentamethyl-9,10-dihydro-phenanthren-3-amine and 10-ethyl-
N,N,5,7-tetramethyl-9,10-dihydrophenanthren-3-amine, re-
spectively, as shown in Figure 6.
Mechanistically, 2c/3c is formed via a 1,3-sigmatropic shift
of the BMes unit in 2b/3b (the red pathway) similar to that
established previously for azole- and imine-supported
systems.6b,7 2d/3d is likely formed via a retro-Diels−Alder
process, eliminating a MesBO unit, followed by an intra-
molecular Diels−Alder cycloaddition of the vinyl group to the
chiral atom in 2b/3b (the blue pathway). Oxyboranes that are
not stabilized by an internal donor (e.g., an O,X-chelate) and
an acid are highly unstable and readily undergo dimerization or
trimerization, forming boroxine-like species.12 Therefore, the
Figure 4. Crystal structures of 3c and 4c with H atoms omitted for
clarity. Selected bond lengths for 3c: B1−O1, 1.369(8) Å; B1−C14,
1.584(9) Å; B1−C21, 1.571(8) Å; and O1−C7, 1.451(7) Å. Selected
bond lengths for 4c: B1−O1, 1.348(6) Å; B1−C9, 1.579(6) Å; B1−
C18, 1.579(6) Å; and O1−C7, 1.454(6) Å.
of the c isomer is quantitative. 1c is identified by its
characteristic methylene groups (CH2−O and CH2−B),
showing two sets of AB splitting patterns, because of the
unsymmetrical structure. The CH2−B in 4c displays an AB
splitting pattern similar to that of 1c, while the H atom and the
CH3 group on the chiral C7 atom display a quartet and a
doublet splitting pattern, respectively (see the SI). Signifi-
1
cantly, no diastereomer peaks were observed in the H NMR
spectrum of 4c, indicating that the HAT step is stereoselective,
similar to that observed in the azoly-supported system in
Scheme 1a. 1c and 4c have a broad 11B NMR peak at ∼52 ppm
(Figure 5), which is ∼7 ppm downfield from those of the imine
analogues; this is a reflection of a reduced donor strength of
the carbonyl group. As shown in Figure 4, the 8-membered
rings in 3c and 4c have an approximate boat structure. The B−
C
DOI: 10.1021/acs.orglett.9b01892
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Letter
to fully understand the role of the carbonyl group in the facile
phototransformation of borane compounds.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01892.
Synthetic details, characterization data, TD-DFT data,
photo and thermal elimination experiments, UV-vis
spectra, and crystal data of 1−4 (PDF)
Accession Codes
CCDC 1916845−1916849 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Figure 6. (Top) Scheme showing the two competing isomerization
pathways of 2, the DFT calculated energy (kcal/mol, B3LYP/6-
31G(d), relative to 2) and the ratio of 3c:3d observed by NMR for
photoreactions at different temperatures. (Bottom) 11H NMR spectra
showing the photoconversion of 2 (green) → 2b (blue box) → 2c
(red)/2d (yellow) + (MesBO)3 (blue) in C6D6, (0.05 M). (The
vertical scale of the two top spectra was increased to show some of the
small peaks.)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: sw17@queensu.ca.
ORCID
Nan Wang: 0000-0001-5973-4496
Suning Wang: 0000-0003-0889-251X
Notes
The authors declare no competing financial interest.
isolation of (MesBO)3 is consistent with the generation of
oxyborane in the photoreaction of 2 and 3. The fact that
oxyborane elimination was not observed for 1 and 4 could be
attributed to the lack of a R group in 1 and an p-NMe2 group
in 4, which reduces the electron density on the C atom bound
to the O atom, making the departure of the oxyborane in 1b
and 4b more difficult, compared to that in 2b and 3b. The
relative thermodynamic stability of different species involved in
the transformation of 2 was examined by DFT computational
analysis and shown in Figure 6. The products formed via either
the red or blue pathways are all thermodynamically favored,
compared to 2. The relatively higher distribution of 2c in the
products may be caused by a smaller activation barrier for the
red pathway, relative to the blue pathway, if both pathways
were thermal processes. To verify this, the photoreactions of 2
and 3 were repeated at 100 and 0 °C, respectively. Surprisingly,
2c/3c was obtained as a clean product at 100 °C, while the
yield of 2d/3d increased substantially at 0 °C (see the box in
Figure 6, as well as Figure S3X in the SI). This led us to
suggest that the red pathway forming c is a thermal process,
while the blue pathway, forming d and oxyborane/boroxine, is
a photochemical pathway, which is likely initiated by the
rupture of the C−O bond in 2b. The fact that the imine-
supported system does not undergo a similar deborylation
process may be explained by the relatively high propensity of
benzylic C−O bonds toward photochemical cleavage.13
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
for financial support (Grant Nos. 21571017, 21701011, and
21761132020) and the Natural Science and Engineering
Research Council of Canada (No. RGPIN1-2018-03866).
We are also grateful to Compute Canada for the use of their
computational facilities.
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