Substituent Directed Phototransformations of BN-Heterocycles: Elimination vs Isomerization via Selective B-C Bond Cleavage

Article abstract of DOI:10.1021/jacs.6b07899

Electron-rich and -poor BN-heterocycles with benzyl-pyridyl backbones and two bulky aryls on the boron (Ar = tipp, BN-1, Ar = MesF, BN-2) have been found to display distinct molecular transformations upon irradiation by UV light. BN-1 undergoes an efficient photoelimination reaction forming a BN-phenanthrene with _PE = 0.25, whereas BN-2 undergoes a thermally reversible, stereoselective, and quantitative isomerization to a dark colored BN-1,3,5-cyclooctatriene (BN-1,3,5-COT, BN-2a). This unusual photoisomerization persists for other BN-heterocycles with electron-deficient aryls such as BN-3 with a benzyl-benzothiazolyl backbone and Mes^F substituents or BN-4 with a benzyl-pyridyl backbone and two C₆F₅ groups on the boron. The photoisomerization of BN-4 goes beyond BN-1,3,5-COT (BN-4a), forming a new species (BN-1,3,6-COT, BN-4b) via C-F bond cleavage and [1,3]-F atom sigmatropic migration. Computational studies support that BN-4a is an intermediate in the formation of BN-4b. This work establishes that steric and electronic factors can effectively control the transformations of BN-heterocycles, allowing access to important and previously unknown BN-embedded species.

Full text of DOI:10.1021/jacs.6b07899

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Substituent Directed Photo-Transformations of BN-Heterocycles:
Elimination vs Isomerization via Selective B-C Bond Cleavage
Deng-Tao Yang, Soren K Mellerup, Jin-Bao Peng, Xiang Wang, Quan-Song Li, and Suning Wang
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b07899 • Publication Date (Web): 31 Aug 2016
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Substituent Directed Photo-Transformations of BN-Heterocycles:
Elimination vs Isomerization via Selective B-C Bond Cleavage
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DengꢀTao Yang,† Soren K. Mellerup,† JinꢀBao Peng,† Xiang Wang,† QuanꢀSong Li,*‡ and Suning
Wang*†‡
†Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada
‡Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry, Beijing Institute of
Technology, Beijing 100081, P. R. China
Supporting Information Placeholder
as OLEDs based on such BNꢀsystems,7 the low quantum efficienꢀ
ABSTRACT: Electronꢀrich and electronꢀpoor BNꢀheterocycles
cy of the photoreaction (ΦPE ≤ 0.044) severely limits its synthetic
and practical usefulness. Inspection of the DFTꢀcalculated transiꢀ
tion state for this reaction revealed that B–CAr bond breaking is
likely the rateꢀlimiting step.6f,g Therefore, we postulated that
weakening the B–CAr bond through either greater steric encumꢀ
berment, i.e. replacing the Mes groups in BN-0 with 2,4,6ꢀ
triisopropylphenyl (tipp, BN-1) or bulky/strongly electronꢀ
withdrawing groups such as 2,4,6ꢀtrifluoromethylphenyl (MesF,
BN-2), could improve the efficiency of the photoelimination.
Indeed, this strategy proved successful with respect to BN-1,
which eliminated much more efficiently than BN-0. Remarkably,
however, instead of elimination BN-2 exhibited an unprecedented
photochromic switching around the boron core, generating an
intensely colored and previously unknown BNꢀ1,3,5ꢀ
cyclooctatriene (COT). Further investigation revealed that the
photochemical formation of BNꢀ1,3,5ꢀCOT is a general phenomꢀ
enon for BNꢀheterocycles that contain electronꢀdeficient aryl
groups on boron, thus establishing the possibility to control such
molecular transformations by tuning the steric and electronic
properties of the molecule. The details of the two distinct photoꢀ
transformations and mechanistic insights are presented herein.
with benzylꢀpyridyl chelate backbones and two bulky aryl groups
on the boron center (Ar = tipp, BN-1, Ar = MesF, BN-2) have
been found to display distinct molecular transformations upon
irradiation by UV light. BN-1 undergoes an exceptionally effiꢀ
cient photoelimination reaction forming a BNꢀphenanthrene (BN-
phen-1) with ΦPE = 0.25, while BN-2 undergoes a thermally reꢀ
versible, stereoselective, and quantitative isomerization to a dark
colored BNꢀ1,3,5ꢀcyclooctatriene (BNꢀ1,3,5ꢀCOT, BN-2a). This
unusual photoisomerization persists for other BNꢀheterocycles
with electronꢀdeficient aryl groups such as BN-3 with a benzylꢀ
benzothiazolyl backbone and MesF substituents or BN-4 with a
benzylꢀpyridyl backbone and two C6F5 groups on the boron center.
The photoisomerization of BN-4 goes beyond BNꢀ1,3,5ꢀCOT
(BN-4a), forming a new BNꢀ1,3,6ꢀcyclooctatriene species (BNꢀ
1,3,6ꢀCOT, BN-4b) via C–F bond cleavage and [1,3]ꢀF atom sigꢀ
matropic migration. DFT and TDꢀDFT mechanistic studies supꢀ
port that BN-4a is an intermediate in the formation of BN-4b.
This work establishes that steric and electronic factors can effecꢀ
tively control the molecular transformations of BNꢀheterocycles,
allowing access to important and previously unknown BNꢀ
embedded species.
Scheme 1. Substituent-Directed Transformations of BN-
heterocycles
Controlling the fate of chemical transformations is one of the
cornerstones of modern chemistry and biology. While nature has
achieved this feat by employing complicated and unique molecuꢀ
lar architectures (e.g. enzymes),1 chemists rely heavily on maꢀ
nipulating the steric and electronic factors of small moleꢀ
cules/complexes2 to promote desired reaction outcomes. For exꢀ
ample, steric/electronic considerations have proven highly effecꢀ
tive in both the stabilization of rare chemical entities3 as well as
the facilitation of unique molecular transformations.4 Recently,
similar strategies have been applied to organoboron systems,
which has demonstrated the remarkable breadth of varying reacꢀ
tivity and functions available to these species5 (e.g. optoelectronꢀ
ics,5bꢀh catalytic components,5iꢀl organic synthons,5mꢀq etc). To this
end, we have had a longꢀstanding interest in the photoꢀresponsive
properties of organoboron compounds,6,7 where we have previꢀ
ously shown that BNꢀheterocycles such as BN-0 (R = Me, X = H;
Scheme 1) are capable of undergoing photoelimination of mesityꢀ
lene to produce fully conjugated and highly luminescent BNꢀ
phenanthrenes (BN-phen).6d While this transformation does proꢀ
vide a new method of preparing these BNꢀphenanthrene isomers
and allow for the in-situ generation of optoelectronic devices such
h
ν
BN-0, R = Me, X = H
BN-1, R = i-pr, X = H
BN-2, R = CF3, X = H
BN-4, R = X = F
Isomerization
R = CF3
X = H
Elimination
R = alkyl
X = H
BN-1,3,5-COT
(BN-2a/4a)
BN-phen-0
BN-phen-1
R = X = F
BN-1,3,6-COT (BN-4b)
Compounds BN-1 and BN-2 were prepared using a modified
procedure6d reported previously for BN-0 and the corresponding
in-situ generated Ar2BX (X = F, Cl) reagents. Both were fully
characterized by H, 13C, 11B, and 19F NMR, HRMS, and single
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crystal Xꢀray diffraction analyses (Figure 1). Crystal data revealed
that the B–CAr bonds of BN-1 and BN-2 are about 0.02 Å longer
than those of BN-0, while all three BꢀCH2 bond lengths are comꢀ
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parable. The DFTꢀcalculated B–CAr lengths match well with those
with similar length compared to previously reported B=C bonds.8
determined experimentally, with the calculated BꢀMesF lengths in
BN-2 being ~0.01 Å longer than those of Bꢀtipp in BN-1. This
difference highlights the strong withdrawing nature of MesF in
addition to its steric bulk.
Comparing the structure of BN-2a to the photoproduct of previꢀ
ously reported B(ppy)Mes2,6aꢀ6d which forms a tricyclic 1,2ꢀ
azaboratabisnorcaradiene, it is clear that the switching described
for BN-2 represents a brand new class of photochromic materials.
The ΦPE of BN-2 to BN-2a transformation was determined to be
0.23 (see SI). BNꢀ1,3,5ꢀCOTs are a previously unknown class of
BNꢀembedded molecules. The clean and quantitative generation
of BN-2a via the photoisomerization of BN-2 provides a convenꢀ
ient method for accessing such unusual species. Furthermore,
despite BN-2a having a chiral carbon atom (C(14)) and chiral
COT structure, only one diastereoisomer was observed by NMR
in which the CF3 (C(19)) is syn to the py ring while the phenyl
and the cyclohexadienyl of the COT unit are syn to each other.
Thus, the BN-2 to BN-2a transformation proceeds with high steꢀ
reoselectivity.
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Figure 1. Crystal structure of BN-1 (left) and BN-2 (right). Key
bond lengths (Å) for BN-1: B(1) – C(13)/C(28) 1.680(2)/1.682(2),
B(1) – C(1)/N(1) 1.639(2)/1.684(2). For BN-2: B(1) – C(13)/
C(22) 1.681(3)/1.689(2), B(1) – C(1)/N(1) 1.642(2)/1.636(2).
The photoreactivity of BN-1 in solution and polymer films is
similar to that of BN-0, with the brightly green fluorescent BN-
phen-1 (λem = 500 nm, ΦFL = 0.27) formed quantitatively upon
irradiation at 300 nm as evidenced by UVꢀVis, NMR, and crystal
structure data (see SI). The most striking difference between BN-
0 and BN-1 is the rate of photoelimination. Because both comꢀ
pounds have identical absorption spectra and form similar prodꢀ
ucts, the ΦPE of BN-1 relative to that of BN-0 was determined to
be 0.25 using competitive NMR photolysis experiments (see SI).
The ΦPE of BN-1 is ca. 6 times greater than that of BN-0 (0.044),
exceptionally high for a photoelimination reaction, as they tend to
have a very low ΦPE. The weaker Bꢀtipp bonds of BN-1 are clearꢀ
ly responsible for this significant enhancement in photoeliminaꢀ
tion efficiency. This demonstrates that the use of bulkier electron
rich aryl groups on boron is an effective strategy for achieving
highly efficient arylꢀelimination in such systems, thus making the
use of BNꢀheterocycles as precursors for in-situ fabrication of
azaborineꢀbased optoelectronic devices7 more viable.
Despite a similarly congested structure and long BꢀMesF bonds,
BN-2 does not undergo photoelimination at all. Irradiation (300
nm) of BN-2 in C6D6 or THF resulted in the rapid and clean conꢀ
version of BN-2 to a new species, BN-2a, which has sharp and
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wellꢀresolved peaks in the H NMR spectrum and a 11B chemical
Figure 2. a) A scheme showing the interconversion of BN-2/BN-
3 and BN-2a/BNꢀ3a. b) 1H NMR spectra showing the photo (300
nm) and thermal (90 °C) interconversion of BN-2 and BN-2a in
C6D6. c) A diagram showing the fatigue resistance of the BN-2
and BN-2a with the irradiation and heating cycles using integrated
shift at ~35 ppm, typical of a threeꢀcoordinated boron with one
unsaturated bond8 (Figure 2b and SI). Accompanying the NMR
spectral change was a dramatic change in solution colour from
colorless to dark orangeꢀbrown as shown in Figure 2. In the UVꢀ
Vis spectrum, a broad low energy absorption band at λmax = 433
nm (Figure 2d) appears with irradiation. TDꢀDFT data suggests
that the low energy band of BN-2a originates from a charge transꢀ
fer (CT) transition from HOMO located on the πꢀsystem of the
B=C and the cyclohexandiene moiety to LUMO (π*) located on
the py ring of the backbone (see SI). Remarkably, heating the
orangeꢀbrown solution at 90 °C resulted in full recovery of BN-2
by NMR and restoration of the original solution colour. The actiꢀ
vation energy of this process was determined to be ~33.3 kcal/mol
(see SI). BN-2a forms dark orangeꢀbrown crystals, allowing us to
establish its structure unequivocally by Xꢀray diffraction.
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peak intensity relative to the C6D6 peak in H NMR spectra (see
Fig. S3.3). d) UVꢀVis spectra showing the conversion of BN-2 to
BN-2a in THF (1 × 10ꢀ4 M) and photographs showing the crystals
of BN-2a and solution color of BN-2 and BN-2a.
To demonstrate the robustness of this newly discovered photoꢀ
chromic system, the fatigue resistance of BN-2 was examined by
cycling through the BN-2→BN-2a→BN-2 transformations reꢀ
peatedly. Quite remarkably, the continuous photoꢀthermal cycling
between these two states did not yield any appreciable decomposiꢀ
tion until the 13th cycle as shown by 1H NMR data (Figure 2c and
SI), indicating that BN-2/BN-2a possess excellent fatigue reꢀ
sistance. This feature makes these systems promising for future
applications involving molecular switches.
The crystal structure of BN-2a (Figure 3) revealed a previously
unknown BNꢀembedded cyclic structure, which is formally a 4,5ꢀ
dihydroꢀ1,2ꢀazaborocine but will be described as BNꢀ1,3,5ꢀ
cyclooctatriene (BNꢀ1,3,5ꢀCOT) for convenience. The CH2 group
in BN-2a is no longer bound to the boron atom, but instead forms
a CꢀC bond with one CF3ꢀsubstituted quaternary carbon atom
(C(14)) of a MesF ring. This converts the MesF to a 1,3ꢀ
cyclohexadiene and the BꢀC bond to a B=C bond (1.465(3) Å),
To establish if the new photoꢀtransformation is available to othꢀ
er BNꢀheterocycles with different chelate backbones, BN-3, an
analogue of BN-2 with a 2ꢀbenzylꢀbenzothiazolyl backbone was
prepared (Figure 2). Upon irradiation at 300 nm, BN-3 undergoes
a similar photoꢀswitching as observed for BN-2 (see SI), forming
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a deep brown colored BN-3a, an analogue of BN-2a, according to
(see SI). The B atom in BN-4b is a chiral center, which, combined
its diagnostic 1H, 11B, and 19F NMR spectra. Again, only one diaꢀ
stereoisomer was observed. BN-3a possesses a low energy abꢀ
sorption band with λmax = 458 nm in its UVꢀVis spectrum, which
is redꢀshifted by ~25 nm compared to BN-2a, consistent with the
greater πꢀconjugation of the backbone in BN-3a. The ΦPE of BN-3
to BN-3a transformation was determined to be 0.10. Thermally,
BN-3a converts back to the colorless BN-3 at 90 °C with ~5%
decomposition observed in the process (see SI). This example
shows that the new photochromic phenomenon is not limited to
systems with a benzylꢀpyridyl backbone.
with the chiral COT structure, creates the possibility of diastereꢀ
omers. The fact that only one diastereomer was observed in which
the BꢀF bond is syn to the py ring as shown in Figure 3 supports
that the BN-4 to BN-4b transformation also has high stereoselecꢀ
tivity. Unlike BN-2a/3a which can thermally revert to BN-2/3,
BN-4b is thermally stable and does not convert back to BN-4.
Given the bond energy of the CꢀF bond,9 its cleavage in the BN-4
to BN-4b transformation is highly unusual and interesting, which
may be driven by the stability of BN-4b.
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Figure 3. Crystal structures of BN-2a (left), BN-4b (right), and
the BNꢀCOT cores. Important bond lengths (Å) for BN-2a: B(1) –
C(13)/C(22)/N(1) 1.465(3)/1.627(3)/1.585(3), C(8) – N(1)/C(3)
1.372(2)/1.477(3), C(2) – C(1)/C(3) 1.498(3)/1.390(3); C(14) – C
(1)/C(13)/C(15)/C(19) 1.567(3)/1.564(3)/1.509(3)/1.528(3), C(16)
– C(15)/C(17) 1.322(3)/1.427(3), C(18) – C(13)/C(17) 1.461(3)
/1.352(3). For BN-4b: B(1)–F(1)/N(1)/C(13)/C(19) 1.408(2)
/1.658(2)/1.628(2)/1.636(2), C(8) – N(1)/C(7) 1.356(2)/1.480(2);
C(2) – C(1)/C(7) 1.510(2)/1.397(2), C(14) – C(1)/C(13) 1.514(2)
/1.408(2).
Figure 5. Calculated potential energy profile for the photoisomerꢀ
ization of BN-4. The energies of the all key species, as well as the
intersection point of the S0 and S1 state ((S1/S0)X) are relative to
BN-4 and given in the parentheses. Some key distances (in Å) and
the HOMO/LUMO orbitals of BN-4 are also shown.
To understand the unusual photoisomerization of BN-2/4,
DFT and TDꢀDFT calculations were performed for each with the
complete reaction pathway of BN-4 shown in Figure 5. Excitation
at 300 nm populates the S1 state of BN-4 at the FranckꢀCondon
(FC) structure, which is of ππ* character and mainly arises from
its HOMO to LUMO transition. Relaxation leads to a minimum in
the S1 state, which is close in energy (~5 kcal molꢀ1) to a conical
intersection point (S1/S0)X between the S1 and S0 states, therefore
allowing access to the ground state from the excited state.10 In the
S0, S1, and (S1/S0)X structures, the BꢀCH2 distances are 1.6, 2.0,
and 2.7 Å, respectively, indicating that the BꢀCH2 is broken upon
photoꢀexcitation. At (S1/S0)X, the excited state decays to the
ground state and the reaction path leads to an intermediate (Int1)
with a BꢀCH2 distance of 3.3 Å, a negatively charged (ꢀ0.44 e)
CH2 carbon atom, and a positively charged (+0.29 e) o-carbon of
one C6F5 ring. The latter two moieties have a separation distance
of 2.3 Å at the TS1 state, with perfect orientation for CꢀC bond
formation or electrocyclization similar to that observed in a phoꢀ
tochromic boron system reported by Yamaguchi,11 generating Int2
(BN-4a), an analogue of BN-2a. The TS1 state lies only 2
kcal/mol above Int1, suggesting that the reaction from Int1 to BN-
4a occurs readily. Unlike BN-2a, which does not undergo further
isomerization, the F atom of BN-4a can migrate from carbon to
boron by overcoming a barrier of ~15 kcal/mol at the TS2 state
forming BN-4b, which resembles the classic transition state strucꢀ
ture proposed for 1,3ꢀsigmatropic shifts, leading to the selective
formation of one diastereoisomer.9c The attraction between the F
atom and the electron deficient B atom clearly favors the rearꢀ
rangement from BN-4a to BN-4b, which is ꢀ33.5 kcal/mol lower
in energy than BN-4. These data support that the formation of
Figure 4. Irreversible photoisomerization of BN-4 and the 11B
NMR spectra in C6D6 showing the full conversion of BN-4 (blue)
to BN-4b (red).
Given the bulky nature of the aryl groups in BN-1, BN-2, and
BN-3, but their distinctly different photoreactivity, the electronꢀ
deficiency of MesF appears to be pivotal for the new photochromꢀ
ism. To probe this hypothesis, the photoreactivity of BN-4,6f
which has the same backbone as BN-2 and two less sterically
congested but comparably electronꢀwithdrawing C6F5 substituꢀ
ents, was examined. Upon irradiation of BN-4 at 300 nm, no obꢀ
vious color change was observed. However, 1H, 19F, and 11B
NMR revealed the quantitative formation of a new species, BN-
4b, with a typical fourꢀcoordinated 11B chemical shift of 5.6 ppm
(Figure 4 and SI). Singleꢀcrystal Xꢀray diffraction analysis estabꢀ
lished that BN-4b shares some structural similarities with BN-
2a/3a and may be described as a BNꢀembedded 1,3,6ꢀCOT, an
isomer of BNꢀ1,3,5ꢀCOT, as shown in Figure 3. The key differꢀ
ence between BN-2a/3a and BN-4b is that the 3rd double bond in
the latter is not conjugated with the other two in the COT ring,
and the boron center is fourꢀcoordinated with a BꢀF bond. As a
consequence, BN-4b has no low energy absorption band in the
visible region, which is consistent with TDꢀDFT calculated data
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BN-4b is thermodynamically favorable and the reverse reaction is
The authors thank the Natural Science and Engineering Research
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prohibited due to the large reverse barrier of 96.3 kcal/mol. Due to
the large size of BN-2 and its analogous isomerization pathway,
only the groundꢀstate reaction pathway was determined. The key
transition state of BN-2 has a similar TS1 structure as BN-4, with
the CꢀC distance between CH2 and CCF3 being 2.4 Å (see SI). The
calculated barrier for BN-2a reverting back to BN-2 is 34.2
kcal/mol, which agrees very well with the experimental value of
33.3 kcal/mol. The fact that the BNꢀ1,3,6ꢀCOT isomer was not
observed for BN-2 may be explained by an unfavorable transition
state caused by steric crowding of the bulky CF3 groups. The
calculated reaction pathway also corroborates the stereoselective
formation of BN2a and BN-4b.
Council of Canada (RGPIN1193993ꢀ2013) and the National Natuꢀ
ral Science Foundation of China for Financial support (grants
21571017 and 21303007). S.K.M. thanks the Canadian Governꢀ
ment for the Vanier Scholarship.
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Ed.; Pergamon Press, Willowdale, 1980. (c) Immobilized Enzymes for
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One key feature revealed by DFT data is that the HOMO of
BN-2 and BN-4 is localized mainly on the benzyl ring of the cheꢀ
late backbone and the B atom with very little contributions from
the MesF or C6F5 groups. This is in sharp contrast to BN-0 and
BN-1, in which the HOMO is made of almost exclusively π orbitꢀ
als on the Mes and tipp groups (see Fig. S6.1e). The substituent
groups on the aryl rings are clearly responsible for this distinct
difference between the electronꢀrich and electronꢀpoor BNꢀ
heterocycles, which led to their distinct photoreactivity (i.e. arylꢀ
elimination vs. –CH2 migration/isomerization). The electronꢀ
withdrawing MesF and C6F5 likely contribute to the stabilization
of the TS1 state by strengthening the B=N bond. This stabilization
effect by MesF substitution is reminiscent of that reported by
Marder in borole systems12a and Jäkle in conjugated thienylborane
systems12b. Further evidence to support the importance of elecꢀ
tronꢀdeficient aryl groups in the photoisomerization of BNꢀ
heterocycles are provided by the photostability of BN-5 (replacing
one C6F5 in BN-4 by a phenyl ring) and BN-6 (replacing both
C6F5 in BN-4 by two phenyl rings), which do not show any
change under 300 nm irradiation for up to 48 hrs (see SI).
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In summary, both electronic and steric effects have been
found to have a significant and distinct impact on the photoreacꢀ
tion pathways of BNꢀheterocycles. With bulky, electronꢀdonating
aryl rings on the B atom, the photoelimination efficiency is greatꢀ
ly enhanced. In contrast, with sufficiently electronꢀwithdrawing
aryl groups on the B atom, bulky or not, photoisomerization ocꢀ
curs exclusively with high stereoselectivity. Bulky electronꢀ
withdrawing aryl groups such as MesF appear to be the key for
reversible photoꢀthermal isomerization of this new class of photoꢀ
chromic molecules. Balancing steric and electronic factors is
found to be critical for achieving the desired molecular transforꢀ
mations based on BNꢀheterocycles. The first examples of BNꢀ
1,3,5ꢀCOT and BNꢀ1,3,6ꢀCOT molecules have been obtained and
their relationship in structural transformation has been demonꢀ
strated.
ASSOCIATED CONTENT
Supporting Information
Spectroscopic data for new compounds, experimental and
computational details, crystal structural data, and additional data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
wangs@chem.queensu.ca, liquansong@bit.edu.cn
Notes
The authors declare no competing financial interests.
(11) Iida, A.; Saito, S.; Sasamori, T.; Yamaguchi, S. Angew. Chem. Int.
Ed. 2013, 52, 3760.
(12) (a) Zhang, Z.; Edkins, R. M.; Haehnel, M.; Wehner, M.; Eichhorn,
A.; Mailӓnder, L.; Meier, M.; Brand, J.; Brede, F.; MüllerꢀBuschbaum, K.;
Braunschweig, H.; Marder, T. B. Chem. Sci. 2015, 6, 5922. (b) Yin, X.;
Chen, J.; Lalancette, R. A.; Marder, T. B.; Jäkle, F. Angew. Chem. Int. Ed.
2014, 53, 9761.
ACKNOWLEDGMENT
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