Rhodium-catalyzed B-H activation of 1,2-azaborines: Synthesis and characterization of BN isosteres of stilbenes

Article abstract of DOI:10.1021/ol501362w

The first example of catalytic B-H activation of azaborines leading to a new family of stilbene derivatives through dehydrogenative borylation is reported. Ten 1,2-azaborine-based BN isosteres of stilbenes have been synthesized using this method, including a BN isostere of a biologically active stilbene. It is demonstrated that BN/CC isosterism in the context of stilbenes can lead to significant changes in the observed photophysical properties such as higher quantum yield and a larger Stokes shift. Direct comparative analysis of BN stilbene 3g and its carbonaceous counterpart 6g is consistent with a stronger charge-transfer character of the excited state exhibited by 3g in which the 1,2-azaborine heterocycle serves as a better electron donor than the corresponding arene.

Full text of DOI:10.1021/ol501362w

Letter
pubs.acs.org/OrgLett
Rhodium-Catalyzed B−H Activation of 1,2-Azaborines: Synthesis and
Characterization of BN Isosteres of Stilbenes
Alec N. Brown,†,‡ Lev N. Zakharov,‡ Tanya Mikulas,§ David A. Dixon,§ and Shih-Yuan Liu*,†,‡
†Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 02467, United States
‡Department of Chemistry, University of Oregon, Eugene, Oregon 97403-1253, United States
§Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487, United States
S
* Supporting Information
ABSTRACT: The first example of catalytic B−H activation of
azaborines leading to a new family of stilbene derivatives
through dehydrogenative borylation is reported. Ten 1,2-
azaborine-based BN isosteres of stilbenes have been
synthesized using this method, including a BN isostere of a
biologically active stilbene. It is demonstrated that BN/CC
isosterism in the context of stilbenes can lead to significant
changes in the observed photophysical properties such as
higher quantum yield and a larger Stokes shift. Direct comparative analysis of BN stilbene 3g and its carbonaceous counterpart
6g is consistent with a stronger charge-transfer character of the excited state exhibited by 3g in which the 1,2-azaborine
heterocycle serves as a better electron donor than the corresponding arene.
Scheme 1. BN Stilbene and Its Retrosynthetic Analysis
tilbene and its derivatives are ubiquitous in biologically active
1
Smolecules and materials science applications. For instance,
stilbenoid compounds such as resveratrol have demonstrated
anti-inflammatory, anticancer, and cardioprotective activities.2
Similarly, the interaction of radiation with stilbenoid compounds
has been intensively investigated3 for applications in nonlinear
optics4 and as light-emitting diodes,5 scintillators,6 photoresists,7
and optical brighteners.8,9 Our group has been focusing on
developing BN/CC isosterism10 as a strategy to expand the
chemical space of compounds relevant to biomedical research11
and materials science.12−14 In view of the versatile applications
exhibited by stilbene derivatives, we sought to develop a general
method for the synthesis of BN isosteres of stilbene. In this paper,
we report the first examples of 1,2-azaborine-based BN stilbenes
including a BN isostere of 4-methoxy-trans-stilbene, a compound
with in vitro activity against MCF-7/6 mammary carcinoma15
and human colon adenocarcinoma cancer (HT-29)16 cells.
Furthermore, we demonstrate that 1,2-azaborine-based BN
stilbenes exhibit photophysical properties that are distinct from
their carbonaceous counterparts.
the absence of promoters even at elevated temperatures (up to
110 °C). We thus surveyed a number of commercially available
catalysts known to promote B−H activation and dehydrogen-
ative borylations21 to facilitate the formation of BN stilbene 3a.22
As can be seen from Table 1, Wilkinson’s catalyst gave the
desired stilbene as the major product in just 15% yield (Table 1,
entry 1). Other phosphine-ligated rhodium complexes generally
used in B−H activation reactions were ineffective for this
reaction (Table 1, entries 2 and 3). On the other hand, Crabtree’s
complex furnished the product in 51% yield (Table 1, entry 4).
Gratifyingly, we found that phosphine-free rhodium alkene
complexes such as [Rh(cod)2]BF4 or [Rh(nbd)(Cl)]2 are
suitable catalysts for our model dehydrogenative borylation
reaction (Table 1, entries 5 and 6). We thus chose those two
complexes for further solvent optimization studies. We
determined that methylene chloride was the best performing
solvent for both rhodium complexes (Table 1, entries 7−12),
providing the desired product in up to 98% HPLC yield (83%
isolated yield) in the case of [Rh(nbd)(Cl)]2 (Table 1, entry 12).
A reduction of the catalyst loading to 2.5 mol % [Rh(nbd)(Cl)]2
was not detrimental with respect to the isolated yield of 3a
A retrosynthetic analysis revealed that B−H activation of 1,2-
azaborines coupled with C−H activation of styrenes through
dehydrogenative borylation could furnish the desired 1,2-
azaborine-based BN stilbene compounds (Scheme 1). The use
of styrenes as direct coupling partners appears particularly
attractive due to their ready commercial availability.17 However,
despite some recent progress in the late-stage functionalization of
1,2-azaborines,18 no systematic study of the B−H activation
chemistry of azaborines has been performed.19 We reported
previously that the B−H bond in 1,2-azaborines is relatively
nonhydridic.20 Indeed, no reaction is observed between our
model N-benzyl-B-H-1,2-azaborine 1 and p-methoxystyrene 2 in
Received: May 12, 2014
Published: June 11, 2014
© 2014 American Chemical Society
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Letter
products as both p- and o-tolylstyrene are suitable substrates (3d
and 3e). Electron-withdrawing substituents are tolerated (3f, 3g,
and 3h). Finally, p-bromostyrene couples with 1 to generate a
BN stilbene that can be potentially further functionalized
through cross-coupling (3i23).
Table 1. Survey of Catalysts and Solvents for the
Dehydrogenative Borylation Reaction between 1,2-Azaborine
a
1 and Styrene 2
We sought to demonstrate the utility of this dehydrogenative
coupling in the synthesis of a BN isostere of a biologically active
molecule. We identified the BN isostere of 4-methoxy-trans-
stilbene as a suitable target.15,16 Treatment of the parent 1,2-
azaborine 4 with p-methoxystyrene under the optimized
conditions results in the formation of the BN stilbene 5 in 82%
isolated yield (Scheme 3). Thus, the reaction conditions are
b
entry
catalyst
RhCl(PPh3)3
solvent
THF
yield (%)
1
15
0
2
RhH(CO)(PPh3)3
Rh(dppb)(cod)BF4
Ir(cod)(py)(PCy3)PF6
[Rh(cod)2]BF4
THF
3
THF
0
4
THF
51
64
80
60
23
75
94
52
Scheme 3. Synthesis of a BN Isostere of 4-Methoxy-trans-
stilbene
5
THF
6
[Rh(nbd)(Cl)]2
THF
7
[Rh(cod)2]BF4
toluene
acetonitrile
CH2Cl2
toluene
acetonitrile
CH2Cl2
CH2Cl2
8
[Rh(cod)2]BF4
9
[Rh(cod)2]BF4
10
11
12
13
[Rh(nbd)(Cl)]2
[Rh(nbd)(Cl)]2
c
[Rh(nbd)(Cl)]2
98 (83)
c,d
[Rh(nbd)Cl]2 (2.5 mol %)
(86)
a
Abbreviations: dppb (diphenylphosphinobutane), cod (cycloocta-
b
diene), py (pyridine), nbd (norbornadiene). Determined by HPLC
versus a calibrated internal standard, average of two runs. Isolated
yields in parentheses, average of two runs. 20 h reaction time.
c
d
compatible with the protic N-H functional group in 4. We
performed oxygen and water stability studies of compound 5
using published procedures.24 BN stilbene 5 showed <5%
decomposition under an oxygen atmosphere in C6D6 at 50 °C for
4 h or at room temperature under ambient atmosphere for 16 h.
The compound was also stable (<5% decomposition) to 8.5
equivalents of water in DMSO-d6 at room temperature for 2 h,
then 50 °C for 2 h.
We were able to isolate only the trans-BN stilbene isomers
among possible reaction products under our optimized
conditions (e.g., hydroboration adducts and cis-BN stilbene
isomers).25 Single-crystal X-ray diffraction analysis unambigu-
ously confirms our structural assignment for 3a, which is also
consistent with the relatively large 3JHH coupling constant of 18
Hz observed by 1H NMR.26
(Table 1, entry 13). Two equivalents of the styrene were
necessary to achieve full conversion to the BN stilbene because
one equivalent of styrene was hydrogenated during the catalytic
cycle.
With the optimized conditions determined, we next turned
our attention to the scope of the dehydrogenative borylation
reaction (Scheme 2). The reaction with the parent styrene
furnished 3b in 77% yield. While the electron-rich p-
methoxystyrene was a suitable substrate for the reaction (3a,
86% yield), p-(dimethylamino)styrene produced the corre-
sponding BN stilbene 3c in moderate 56% yield. Steric influences
on styrene do not appear to affect the yield of BN stilbene
Figure 1 illustrates the ORTEP of BN stilbene 3a along with
selected bond distances. The molecule adopts a relatively planar
Scheme 2. Synthesis of BN Stilbenes through Rh-Catalyzed
Dehydrogenative Borylation
a
Figure 1. ORTEP illustration, with thermal ellipsoids drawn at the 35%
probability level, of BN stilbene 3a. All hydrogens except for the alkenyl
hydrogens at C12 and C13 have been omitted for clarity.
structure in the solid state. The ∠N−B−C12−C13 and ∠C12−
C13−C14−C15 dihedral angles are 170.4(1)° and 2.3(2)°,
respectively. The heterocyclic intraring distances are consistent
with a typical 1,2-azaborine motif.27 The C12−C13 distance of
1.333(2) Å is also consistent with typical distances observed in
stilbenes.28 On the other hand, the B−C12 distance of 1.555(2)
Å is significantly longer than a typical arene C(sp2)−alkene
C(sp2) single-bond distance of ∼1.47 Å.29 On the other hand, the
B−C12 distance observed in 5 is slightly shorter compared to a
a
The yields of isolated products are given as the average of two runs.
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Letter
perpendicularly oriented 1,2-azaborine B−C(sp2)−arene single
DMSO).30 The solvatochromic study is consistent with a
stronger charge transfer character of the excited state for 3g
than for 6g, which also presumably involves a larger structural
reorganization from ground state leading to the large observed
Stokes shift for 3g. The relatively high photoluminescence
quantum yield for BN stilbene 3g may be due to destabilization
of biradical states relative to the charge-transfer state and
subsequent deactivation of nonradiative decay pathways
associated with the biradical states.31 Time-dependent density
functional theory (TD-DFT) calculations for compound 3g
reveal that the HOMO−LUMO transition is the predominant
one with an oscillator strength of 0.57 (see Supporting
Information for details of the calculations).
bond distance of ∼1.57 Å.18
We then investigated the impact of BN/CC isosterism on the
photophysical properties of stilbene derivatives. We chose a
representative set of BN stilbenes containing an electron-
withdrawing (3g, (−CF3) black in Figure 2), electron-neutral
Figure 4 shows that the N-benzyl group does not significantly
impact the HOMO−LUMO transition. The HOMO−LUMO
Figure 2. Normalized absorption and emission spectra (in MeCN) of
select BN stilbenes in direct comparison with their corresponding
carbonaceous counterparts. Quantum yields were determined in EtOH
at room temperature.
(3b, (−H) green in Figure 2), and electron-rich (3a, (−OMe)
blue in Figure 2) arene for direct comparison with the
corresponding carbonaceous stilbene analogues (6g (−CF3,
pink), 6b (−H, orange), 6a (−OMe, red)). As can be seen from
Figure 2, BN stilbenes exhibit a bathochromic shift in both the
absorption and emission spectra. The substituent effect is more
pronounced in the emission relative to the absorption. In
particular, the effect of the substituents on the emission peak is
Figure 4. HOMO and LUMO (B3LYP Kohn−Sham orbitals) of 3g and
6g.
significantly stronger in BN stilbenes (3a(λem): 390 nm; 3g(λem)
:
431 nm; Δ: 2439 cm−1) than in all-carbon series 6b(λem): 357
nm; 6a(λem): 380 nm; Δ: 1695 cm−1). The photophysical
properties of BN stilbene 3g (−CF3, black trace) is particularly
intriguing. Compound 3g exhibits a large Stokes shift (8340
cm−1) and the highest quantum yield (ΦPL: 0.29) among the
stilbene compounds that we have investigated.
diagrams are also consistent with 3g exhibiting a stronger charge
transfer character than the carbonaceous 6g. Thus, our studies
suggest that BN/CC isosterism in the context of stilbenes can
promote charge transfer transitions in which the 1,2-azaborine
heterocycle can be considered a better electron donor than the
corresponding arene.
In summary, we have established the first example of catalytic
B−H activation of azaborines leading to a family of BN stilbenes
through dehydrogenative borylation of styrenes. Ten examples of
BN stilbenes have been synthesized using this method, including
a BN isostere of a biologically active stilbene. Catalytic activation
of the relatively unreactive aromatic B−H bond in these systems
expands the basic synthetic utility of the 1,2-azaborines. This
study has enabled the discovery that BN/CC isosterism in the
context of stilbenes can lead to significant changes in observed
photophysical properties such as higher quantum yield and a
larger Stokes shift. Direct comparative analysis of BN stilbene 3g
and its carbonaceous counterpart 6g is consistent with a stronger
charge transfer character of the excited state exhibited by 3g in
which the 1,2-azaborine heterocycle serves as a better electron
donor than the corresponding arene.
We performed emission solvatochromism studies on BN
stilbene 3g and its carbonaceous analogue to elucidate the nature
of the excited state. Figure 3 illustrates a significant positive
solvatochromism on the emission maxima of BN stilbene 3g (a
red shift of 2457 cm−1 going from cyclohexane (λem: 393 nm) to
DMSO (λem: 435 nm)) as compared to the carbonaceous
stilbene 6g (a red shift of 674 cm−1 from cyclohexane to
ASSOCIATED CONTENT
* Supporting Information
■
S
Figure 3. Normalized emission spectra of (a) BN stilbene 3g and (b)
carbonaceous analogue 6g in various solvents (DMSO, orange; MeCN,
red; black, CH2Cl2; pink, EtOH; blue, THF; green, Et2O; cyclohexane,
pale blue). All measurements were taken at 1 × 10−5 M.
Experimental procedures, spectroscopic data, electronic struc-
ture calculations, and crystallographic data (CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: shihyuan.liu@bc.edu.
Notes
The authors declare no competing financial interest.
Correspondence concerning electronic structure calculations
should be directed to D.A.D. (dadixon@as.ua.edu). Corre-
spondence concerning X-ray crystallography should be directed
to L.N.Z. (lev@uoregon.edu).
Geetharani, K.; Jimenez-Halla, J. O.; Schafer, M. Angew. Chem., Int. Ed.
̈
2014, 53, 3500−4504.
́ ̀
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ACKNOWLEDGMENTS
T.; Vanhoecke, B. W.; Katritzky, A. R.; Bracke, M. E.; Stevens, C. V.
Bioorg. Med. Chem. 2013, 21, 5054−5063.
■
Support has been provided by the National Institutes of Health
(National Institute General Medical Sciences, R01-GM094541).
Funding for the University of Oregon Chemistry Research and
Instrumentation Services has been provided in part by the NSF
(CHE-0923589). D.A.D thanks the Robert Ramsay Chair Fund
of The University of Alabama for support.
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