Photoswitching of the Lewis acidity of a catecholborane bearing an azo group based on the change in coordination number of boron

Article abstract of DOI:10.1021/ol051337e

(Chemical Equation Presented) Photoisomerization of a catecholborane bearing a 2-(phenylazo)phenyl group with an N-B dative bond caused photoswitching of the coordination number of boron between 3 and 4. The Lewis acidity of the catecholborane was switched by photoirradiation, and the complexation ability of the (E)- and the (Z)-isomers of the catecholborane with pyridine differs by more than a factor of 300.

Full text of DOI:10.1021/ol051337e

ORGANIC
LETTERS
2005
Vol. 7, No. 18
3909-3911
Photoswitching of the Lewis Acidity of a
Catecholborane Bearing an Azo Group
Based on the Change in Coordination
Number of Boron
Naokazu Kano, Junro Yoshino, and Takayuki Kawashima*
Department of Chemistry, Graduate School of Science, The UniVersity of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
takayuki@chem.s.u-tokyo.ac.jp
Received June 8, 2005
ABSTRACT
Photoisomerization of a catecholborane bearing a 2-(phenylazo)phenyl group with an N−B dative bond caused photoswitching of the coordination
number of boron between 3 and 4. The Lewis acidity of the catecholborane was switched by photoirradiation, and the complexation ability of
the (E)- and the (Z)-isomers of the catecholborane with pyridine differs by more than a factor of 300.
Tricoordinate organoboron compounds are known to be
useful Lewis acids in organic synthesis.1 Their Lewis acidity
is attributable to the existence of the vacant 2p orbital of
boron. In several organoboron compounds bearing a hetero-
atom tether in the vicinity of the boron atom, the 2p orbital
is filled by coordination with the lone pair of a heteroatom
such as nitrogen or oxygen.2 Although these tetracoordinate
species do not have Lewis acidity, most of them are in
equilibrium with the tricoordinate species that are not
coordinated with the heteroatom, which show high Lewis
acid characteristics. Usually, the coordination number of
boron is switched by the addition of external reagents or
solvents with high Lewis basicity. There have been several
reports on the switching of some functions of organoboron
compounds by taking advantage of the change in coordina-
tion number of boron caused by the addition of external
reagents.3 If the coordination number of boron could be
changed by external stimuli such as light and magnetism,
the structure, reactivities, and Lewis acidity of the organo-
boron compounds could be controlled without addition of
any external reagent. However, there has been no report on
such control of the coordination number of boron by
photoirradiation.
(1) (a) Ishihara, K. In Lewis Acid Reagents; Yamamoto, H., Ed.; Oxford
University Press: Oxford, 1999; Chapter 3, pp 31-63. (b) Ishihara, K.;
Yamamoto, H. Eur. J. Org. Chem. 1999, 527. (c) Ishihara, K. In Lewis
Acids in Organic Synthesis; Yamamoto, H., Ed.; Wiley-VCH: New York,
2000; Vol. 1, Chapter 4, pp 89-133.
(2) (a) Lauer, M.; Wulff, G. J. Organomet. Chem. 1983, 256, 1. (b) Lauer,
M.; Bo¨hnke, H.; Grotstollen, R.; Salehnia, M.; Wulff, G. Chem. Ber. 1985,
118, 246. (c) Kirby, A. J.; Percy, J. M. Tetrahedron 1988, 44, 6903. (d)
Liu, X.-C.; Hubbard, J. L.; Scouten, W. H. J. Organomet. Chem. 1995,
493, 91. (e) Brown, D. S.; Carmalt, C. J.; Cowley, A. H.; Decken, A.; Isom,
H. S. Heteroat. Chem. 1998, 9, 79. (f) Toyota, S.; Asakura, M.; Futawaka,
T.; Oki, M. Bull. Chem. Soc. Jpn. 1999, 72, 1879. (g) Vedejs, E.; Chapman,
R. W.; Lin, S.; Mu¨ller, M.; Powell, D. R. J. Am. Chem. Soc. 2000, 122,
3047. (h) Wiskur, S. L.; Lavigne, J. J.; Ait-Haddou, H.; Lynch, V.; Chiu,
Y. H.; Canary, J. W.; Anslyn, E. V. Org. Lett. 2001, 3, 1311. (i) Yamashita,
M.; Kamura, K.; Yamamoto, Y.; Akiba, K.-y. Chem. Eur. J. 2002, 8, 2976.
(j) Norman, D. W.; Edwards, J. P.; Vogels, C. M.; Decken, A.; Westcott,
S. A. Can. J. Chem. 2002, 80, 31. (k) Norrild, J. C.; Sotofte, I. J. Chem.
Soc., Perkin Trans. 2 2002, 303. (l) Giles, R. L.; Howard, J. A. K.; Patrick,
L. G. F.; Probert, M. R.; Smith, G. E.; Whiting, A. J. Organomet. Chem.
2003, 680, 257. (m) Toyota, S.; Ito, F.; Nitta, N.; Hakamata, T. Bull. Chem.
Soc. Jpn. 2004, 77, 2081.
Azobenzene moieties, which have been widely utilized as
switches as a result of their geometrical change caused by
photoirradiation,4 are expected to coordinate to a boron center
and work as a photoswitching unit of the coordination
(3) (a) James, T. D.; Sandanayake, K. R. A. S.; Shinkai, S. J. Chem.
Soc., Chem. Commun. 1994, 477. (b) James, T. D.; Sandanayake, K. R. A.
S.; Iguchi, R.; Shinkai, S. J. Am. Chem. Soc. 1995, 117, 8982. (c) James,
T. D.; Sandanayake, K. R. A. S.; Shinkai, S. Nature 1995, 374, 345.
10.1021/ol051337e CCC: $30.25
© 2005 American Chemical Society
Published on Web 08/11/2005

number of the central boron atom.5 We report here the change
of the Lewis acidity of a catecholborane bearing an azo group
based on the change in coordination number of the boron
atom induced by photoirradiation.
(E)-2-Iodoazobenzene ((E)-1) in ether was allowed to react
successively with n-BuLi (1.10 equiv), trimethyl borate (1.1
equiv) at -112 °C, and diluted sulfuric acid at 0 °C to give
boronic acid (E)-2 (70%) (Scheme 1). The dehydration
the bond angles [94.1(3)-118.3(4)°] around the B1 atom
and the sum of bond angles of O1-B1-O2, O1-B1-C1,
and O2-B1-C1 (343°), (E)-4b presented a distorted tetra-
hedral structure around the tetracoordinate boron atom with
the N2-B1 dative bond. The N1-N2 bond length [1.280-
(5) Å (1.264(5) Å)7] exhibited only slight elongation
compared to the mean value (1.25 Å)8 of the previously
reported (E)-azobenzenes, suggesting no formation of a
zwitterionic inner borate-amido complex, but rather retention
of the double-bond character of the N-N bond despite the
formation of the N-B dative bond. This is the first example
of organoboron compounds with the coordination of an azo
group to the boron atom.
Scheme 1
In the UV-vis spectrum in cyclohexane, (E)-4a showed
its absorption maximum at 339 nm, assignable to the π-π*
transition of the azo group. Irradiation (λ ) 360 nm) of
(E)-4a in C6D6 for 2 h caused its photoisomerization to give
(Z)-4a in 35% yield (Scheme 2). Irradiation of (E)-4a in
Scheme 2
reaction of (E)-2 and catechols 3a-c (1.0 equiv) at 100 °C
in toluene afforded catecholboranes (E)-4a-c in good yields.
In the 11B NMR spectra, all of the catecholboranes (E)-4a-c
showed a broad peak at higher fields [(E)-4a, δB 21.8; (E)-
4b, δB 18.6; (E)-4c, δB 23.6] than phenylcatecholborane [δB
32.0] in C6D6. They are considered to have tetrahedral
structures around the boron atom in the solution state. The
structure of (E)-4b was finally determined by X-ray crystal-
lographic analysis.6 Figure 1 shows the ORTEP drawing of
cyclohexane gave a better yield of (Z)-4a (51%).9 Upon
irradiation of a cyclohexane solution of (E)-4a, a decrease
in the absorption maximum at 339 nm and a corresponding
increase in a new maximum at 460 nm were observed. The
absorption maximum at 460 nm is assigned to the n-π*
transition of the azo group of (Z)-4a. Irradiation (λ ) 431
nm) of (Z)-4a caused recovery of (E)-4a in 98% yield.
In the 11B NMR spectra of (Z)-4a, the signal was observed
at δB 30.8 in C6D6, which is almost the same value as that
Figure 1. ORTEP drawing of (E)-4b with thermal ellipsoid plot
(50% probability). One of two independent molecules in the cell
was omitted. Selected bond lengths (Å) and bond angles (deg):
B1-O1, 1.459(6); B1-O2, 1.445(6); B1-C1, 1.570(6); B1-N2,
1.721(6); N1-N2, 1.280(5); O1-B1-O2, 107.0(4); O1-B1-C1,
117.7(4); O1-B1-N2, 108.4(3); O2-B1-C1, 118.3(4); O2-B1-
N2, 110.3(4); C1-B1-N2, 94.1(3).
(5) For photoswitching of the coordination number of silicon in orga-
nosilicon compounds, see: (a) Kano, N.; Komatsu, F.; Kawashima, T. J.
Am. Chem. Soc. 2001, 123, 10778. (b) Kano, N.; Yamamura, M.; Komatsu,
F.; Kawashima, T. J. Organomet. Chem. 2003, 686, 192. (c) Kano, N.;
Yamamura, M.; Kawashima, T. J. Am. Chem. Soc. 2004, 126, 6250.
(6) Crystallographic data. (E)-4b: C18H11BCl2N2O2, monoclinic, P21, a
) 8.809(4), b ) 18.028(8), c ) 10.368(5) Å, â ) 96.8388(19)°, V ) 1634.8-
(13) Å3, Z ) 4, MW ) 369.00, T ) 120 K, R1(I > 2σ(I)) ) 0.045, wR2-
(all data) ) 0.0939, GOF ) 0.898. The unit cell contains two independent
molecules of (E)-4b, which have a similar conformation around the boron
atom.
one of the two independent molecules of (E)-4b in the unit
cell. The N2 atom of the azobenzene unit is directed to the
B1 atom [N2-B1, 1.721(6) Å (1.734(6) Å)7]. Considering
(7) Data in parentheses are those for another independent molecule.
(8) Allmann, R. In The Chemistry of the Hydrazo, Azo and Azoxy Groups;
Patai, S., Ed.; John Wiley & Sons: London, 1975; Chapter 2, p 43.
(9) The yields of photoisomerization of 4a and 4c were determined by
(4) In Molecular Switches; Feringa, B. L., Ed.; Wiley-VCH: Weinheim,
2001.
1
the integral of the H NMR spectra.
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Org. Lett., Vol. 7, No. 18, 2005

of phenylcatecholborane (δB 32.0). The downfield shift
indicates the tricoordination state of (Z)-4a in the solution
state. The coordination number of the boron atom in
catecholborane 4a is switched between 3 in the (Z)-isomer
and 4 in the (E)-isomer by photoirradiation, because the
azobenzene moiety isomerizes reversibly between the
(E)- and (Z)-isomers. Catecholborane (E)-4c similarly isomer-
ized to give (Z)-4c (δB 30.8) in 31% yield in C6D6, whereas
(E)-4b did not isomerize under similar conditions.9
The change in coordination number induced by photoir-
radiation resulted in a change in the Lewis acidity of
catecholborane 4a. Reactions of the (E)- and (Z)-isomers of
4a with pyridine were examined to estimate the Lewis acidity
of these two isomers of 4a. Catecholborane (E)-4a was
treated with 1 equiv of pyridine in C6D6, and the chemical
shift (δB 21.8) in the 11B NMR spectrum showed no change
from that in the absence of pyridine. Irradiation (λ ) 360
nm) of a mixed solution of (E)-4a and pyridine (1 equiv)
for 2 h caused a large upfield shift (δB 13.8) at 11B NMR
spectrum. This chemical shift is at higher field than those
of (E)-4a and (Z)-4a. These results show that the equilibrium
in eq 1 favors (E)-4a rather than its pyridine complex
(E)-4a‚py, in which pyridine coordinates to the boron atom
instead of the nitrogen atom of the azobenzene moiety, as
shown in Scheme 2. In contrast, the upfield shift after
irradiation indicates that (Z)-4a generated by photoisomer-
ization and pyridine form the complex (Z)-4a‚py, and that
the equilibrium in eq 2 lies to the right:
Figure 2. Changes of the 11B NMR chemical shifts upon addition
of pyridine to a solution of (E)- and (Z)-4a.
nm) of the mixture of (Z)-4a and 1 equiv of pyridine in C6D6
changed the 11B chemical shift from δB 13.8 to 21.4, which
is the same as that of the mixture of (E)-4a and pyridine,
indicating the complete photoisomerization to (E)-4a. These
results demonstrate that the complexation ability of a
catecholborane with pyridine can be reversibly changed by
photoirradiation.
In summary, we have succeeded in photoswitching the
coordination number of boron in a catecholborane and
changing the Lewis acidity toward pyridine induced by
irradiation. This concept of switching the Lewis acidity based
on the photoinduced change in coordination number will
enable photocontrol in the future of some reaction processes
catalyzed by a Lewis acid.
(E)-4a + py a (E)-4a‚py
(Z)-4a + py a (Z)-4a‚py
(1)
(2)
Acknowledgment. We thank Tosoh Finechem Corp. for
the gift of alkyllithiums. This work was partially supported
by Grants-in-Aid for The 21st Century COE Program for
Frontiers in Fundamental Chemistry (T.K.) and for Scientific
Research nos. 14740395 (N.K.), 15105001 (T.K.), and
16033216 (T.K. and N.K.) from Ministry of Education,
Culture, Sports, Science and Technology, Japan and Japan
Society for the Promotion of Science. We also thank
Dainippon Ink and Chemicals, Inc.
Changes of the 11B NMR chemical shifts upon addition
of 1-10 equiv of pyridine to a solution of (E)-4a are shown
in Figure 2, in which 9 and ] denote the values with and
without irradiation, respectively. The signal of (E)-4a was
gradually shifted upfield with an increase in the ratio of
pyridine to (E)-4a and reached δB 18.0, which is far from
that (δB 11.7) in pyridine. By contrast, the signal of (Z)-4a
formed under irradiation showed a large upfield shift by
addition of 1 equiv of pyridine from δB 30.8 to 13.8, which
is almost the same as that (δB 11.2) of (Z)-4a in pyridine.
The equilibrium constants of eqs 1 and 2, K1 and K2, were
calculated by curve fitting to be (1.45 ( 0.08) × 10 and
(5.3 ( 0.9) × 103 M-1, respectively (Figure 2). These values
show that irradiation of catecholborane 4a changed its Lewis
acidity by a factor of 300. Furthermore, irradiation (λ ) 431
Supporting Information Available: Synthetic procedures
and spectral data for (E)-2, (E)-4a-c, and (Z)-4a and the
X-ray crystallographic data in CIF format for (E)-4b. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL051337E
Org. Lett., Vol. 7, No. 18, 2005
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