9-Hetero-10-boraanthracene-derived borinic acid catalysts for regioselective activation of polyols

Article abstract of DOI:10.1039/c3sc51172c

Heteraborinine-derived borinic acids serve as efficient catalysts for regioselective monofunctionalization of di- and polyols. Arylborinic acids of this type, wherein the B-OH group is incorporated into a 6π electron system, display both improved catalytic activity for functionalization of diols and enhanced stability towards air oxidation relative to the 'parent' diphenylborinic acid (Ph₂BOH). These properties enable their applications at loadings as low as 0.1 mol% and without the need for a stabilizing precatalyst ligand (e.g., ethanolamine). Complexation studies, computation and kinetic data suggest that while the heteraborinine-derived borinic acids show significantly lower association constants with substrates than Ph₂BOH, this effect is more than compensated for by the increased nucleophilicity of their tetracoordinate diol adducts.

Full text of DOI:10.1039/c3sc51172c

Chemical Science
View Article Online
View Journal | View Issue
EDGE ARTICLE
9-Hetero-10-boraanthracene-derived borinic acid
catalysts for regioselective activation of polyols†
Cite this: Chem. Sci., 2013, 4, 3298
*
´
Elena Dimitrijevic and Mark S. Taylor
Heteraborinine-derived borinic acids serve as efficient catalysts for regioselective monofunctionalization of
di- and polyols. Arylborinic acids of this type, wherein the B–OH group is incorporated into a 6p electron
system, display both improved catalytic activity for functionalization of diols and enhanced stability
towards air oxidation relative to the ‘parent’ diphenylborinic acid (Ph2BOH). These properties enable
their applications at loadings as low as 0.1 mol% and without the need for a stabilizing precatalyst
ligand (e.g., ethanolamine). Complexation studies, computation and kinetic data suggest that while the
heteraborinine-derived borinic acids show significantly lower association constants with substrates
than Ph2BOH, this effect is more than compensated for by the increased nucleophilicity of their
tetracoordinate diol adducts.
Received 29th April 2013
Accepted 4th June 2013
DOI: 10.1039/c3sc51172c
www.rsc.org/chemicalscience
electrophiles for which Lewis base activation has been chal-
lenging (e.g., alkyl and glycosyl halides). It exploits the strong,
Introduction
The development of catalysts able to inuence the regio- or
chemoselectivity of organic reactions is an area of active
investigation.1 Selective functionalization of hydroxyl (OH)
groups in di- or polyols is of particular interest, with applica-
tions ranging from manipulation of simple hydroxylated feed-
stocks2 to the preparation of carbohydrate-3 and inositol-based
targets4 and the derivatization of complex natural products.5
Approaches for catalyst-controlled regioselective activation of
OH groups include Lewis base,6 Lewis acid7 and transition
metal catalysis.8
Our group has explored applications of organoboron
compounds as catalysts for regioselective activation of OH
groups in 1,2- and 1,3-diol motifs. The use of borinic (R2BOH)
rather than boronic acid (RB(OH)2) catalysts was an important
consideration, with the former providing direct access to the
tetracoordinate adducts that were proposed to serve as acti-
vated nucleophiles. Accordingly, aminoethyl diphenylborinate
1a is an efficient precatalyst (see below) for acylation,9 alkyl-
ation,10 sulfonylation11 and glycosylation12,13 of numerous
substrates, including carbohydrate derivatives (Scheme 1).14
This mode of catalysis results in high levels of selectivity
for equatorial OH groups belonging to cis-1,2-diol motifs
in pyranoside substrates and is applicable to classes of
selective and reversible covalent interactions between organo-
boron compounds and diols that have been employed exten-
sively in the molecular recognition eld.15
In considering how to identify organoboron compounds
having improved properties as diol activation catalysts, we faced
a set of issues that appeared to present divergent requirements.
Tosylation of cis-1,2-cyclohexanediol displayed apparent zero-
order (saturation) kinetics in substrate, rst-order kinetics in
catalyst 1a and rst-order kinetics in TsCl, consistent with sul-
fonylation of the borinate ester being turnover-limiting.11 This
nding suggested that a more electron-rich borinic acid would
display higher catalytic activity due to the enhanced nucleo-
philicity of its tetracoordinate borinate derivative. Experiments
employing substituted arylborinic acids, although consistent
with this hypothesis, did not lead to improvements in catalyst
activity: incorporation of para-methoxy substituents resulted in
a less active diarylborinic acid catalyst, apparently reecting the
inductively electron-withdrawing nature of these groups.11
Moreover, the magnitude of the observed substituent effects
Department of Chemistry, University of Toronto, 80 St. George St., Toronto, ON M5S
3H6, Canada. E-mail: mtaylor@chem.utoronto.ca; Fax: +1 416 978-8775; Tel: +1
416 946-0571
† Electronic supplementary information (ESI) available: Experimental procedures,
characterization data, crystallographic data for the diboroxanes derived from 2b
and 2c (.cif) and computational details. CCDC 935003 and 935004. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c3sc51172c
Scheme 1 (a) Borinic acid catalyzed diol activation; and (b) structures of borinic
ester precatalysts 1a–d.
3298 | Chem. Sci., 2013, 4, 3298–3303
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Edge Article
Chemical Science
was modest and it appeared that signicantly more electron- with 1,2-diols, display high activities as catalysts for the mon-
rich variants would be needed to effect useful levels of rate ofunctionalization of such substrates, allowing loadings as low
acceleration.
as 0.1 mol%. Their improved stability towards oxidation obvi-
On the other hand, the propensity of diarylborinic acids – ates the need for an amino alcohol-derived precatalyst (and the
especially electron-rich congeners – to undergo oxidation, generation of ligand-derived byproducts) and enables their
generating phenols and boronic acids, presented practical recovery upon completion of catalytic reactions. Modication of
limitations. Long-term storage of most diarylborinic acids in the heteroatom tether also provides a means to optimize turn-
their pure form is problematic, and instead they are generally over frequency and/or number for a given reaction type.
handled as their crystalline, tetracoordinate ethanolamine
esters.16 We have shown that the latter may be employed as
precatalysts for diol activation, as the ligand undergoes elec-
Results and discussion
Catalyst synthesis and properties
trophilic functionalization (e.g., N,O-bis-acetylation or -sulfo-
nylation) and displacement by substrate. However, avoiding the
use of a precatalyst and the generation of the resulting ligand-
derived by-products could be advantageous. In addition, the
rate proles of certain reactions (particularly those in which
Ag2O was employed as the halide-abstracting reagent), and our
inability to recover diphenylborinic acid from product mixtures,
suggested that the catalyst could be susceptible to oxidation
under the reaction conditions. Borinic acids displaying
improved stability towards oxidation might thus be useful
candidates as catalysts.
Our preliminary structure–activity relationships for borinic
acid catalysts illustrated the difficulties in simultaneously ful-
lling the criteria of enhanced borinate nucleophilicity and
improved stability towards oxidation. Unsymmetric borinic acid
Ph(CH3)BOH underwent more rapid oxidation than Ph2BOH,
necessitating careful exclusion of oxygen during workup and
immediate complexation with ethanolamine.17 The resulting
tetracoordinate adduct 1b was stable towards storage, but did
not show improved activity relative to 1a in acylation, sulfony-
lation, alkylation and glycosylation reactions of representative
substrates. Variation of the alkyl group in alkyl/arylborinate
esters Ph(R)B(OCH2CH2NH2) (1c: R ¼ n-Bu; 1d: R ¼ s-Bu)
did not provide improved results, and the challenges
associated with preparation and handling of the free borinic
acids rendered the systematic evaluation of such compounds
challenging.
We became interested in the possibility that borinic acids
based on oxa-, thia- or azaborinine frameworks might possess
unique properties that could simultaneously address the two
criteria discussed above. Incorporation of the borinic acid
group into a 6p electron system was expected to decrease its
Lewis acidity,18 potentially generating a more nucleophilic tet-
racoordinate borinate adduct. Literature data also suggested
that members of this class of compounds would be resistant
to oxidation.19–21 Heteraborinines have been the subject of
numerous studies aimed at probing their structure and
bonding, and their potential applications as components of
chemical sensors, organic electronics, ligands and hydrogen
storage materials have generated interest in recent years.22–24
Compounds 2a–c were prepared by cyclization of the bis-ortho-
lithiated precursors with tributyl borate, followed by hydrolysis
with aqueous acid, in analogy to reported methods
(Scheme 2).20,25
Recrystallization yielded the dehydrated, dimeric borinates,
which showed no evidence of decomposition upon storage for
months in closed vials under ambient conditions. The solid-
state structures of the dimeric forms of 2b and 2c (Fig. 1)
indicate that the central oxa- and azaborinine rings are planar,
with trigonal planar geometries for the boron atoms (Sangles,2b
¼ 360.0ꢀ; Sangles,2c ¼ 360.0ꢀ) and, in the case of 2c, the nitrogen
atoms (Sangles ¼ 360.0ꢀ). Trends in bond length are consistent
with a degree of cyclic delocalization in both the 6p azaborinine
and, to a lesser extent, oxaborinine ring systems. The average
˚
˚
B–Cipso bond distances are 1.527 A and 1.538 A for the dibor-
oxanes derived from 2c and 2b, respectively; these are both
shorter than the corresponding average B–C bond distances in
(Ph2B)2O (1.571 A).26 The B–O bond distances show the reverse
˚
Scheme 2 Preparation of 9-hetero-10-boraanthracene-derived borinic acids. (i)
n-BuLi (2.2 equiv.), THF, –30 ꢀC / 23 ꢀC, 18 h; (ii) B(OBu)3 (1.1 equiv.), reflux, 2 h;
and (iii) 4 N HCl, 15 min, 23 ꢀC.
Fig. 1 X-Ray structures of the dimeric borinates derived from (a) 2b and (b) 2c.
ꢀ
˚
Selected bond lengths (A) and angles ( ): (a) O(1)–B(1) 1.358(3), B(1)–C(1)
However, the reversible covalent interactions between heter- 1.540(3), C(1)–C(6) 1.392(3), C(6)–O(2) 1.377(2), O(2)–C(7) 1.380(2), C(7)–C(12)
1.386(3), C(12)–B(1) 1.540(3), C(1)–B(1)–C(12) 115.85(18), C(12)–B(1)–O(1)
aborinine-derived borinic acids and diols have not been studied
quantitatively, and it was not clear whether compounds of this
type would be useful diol activation catalysts. Herein, we
119.9(2), O(1)–B(1)–C(1) 124.15(19), B(1)–O(1)–B(2) 145.8(2); and (b) O(1)–B(1)
1.376(2), B(1)–C(1) 1.528(2), C(1)–C(6) 1.419(2), C(6)–N(1) 1.393(2), N(1)–C(7)
1.402(2), C(7)–C(12) 1.414(2), B(1)–O(1)–B(2) 136.15(14), C(1)–B(1)–C(12)
describe experiments indicating that 9-oxa-, thia- and aza-10-
115.79(15), C(12)–B(1)–O(1) 123.53(16), O(1)–B(1)–C(1) 120.63(16), C(6)–N(1)–
boraanthracene-derived borinic acids, while interacting weakly C(7) 122.87(13), C(7)–N(1)–C(13) 118.23(14), C(13)–N(1)–C(6) 118.85(14).
This journal is ª The Royal Society of Chemistry 2013
Chem. Sci., 2013, 4, 3298–3303 | 3299

View Article Online
Chemical Science
Edge Article
trend, with the diboroxanes of 2c and 2b having average B–O borinates of Ph2BOH and 2a–c in the gas phase) were carried
˚
distances of 1.376 and 1.368 A, both of which are longer than out using density functional theory (DFT) with the B3LYP
that of tetraphenyldiboroxane (1.346 A). This latter trend is functional and 6-311+G(d,p) basis set (Fig. 3).27 The calculated
˚
consistent with reduced B–O bond orders due to delocalization methoxide affinities of the heteraboraanthracene derivatives
in the aza- and oxaborinane rings. Computational modelling were lower than that of Ph2BOMe, and decreased in the order 2a
(B3LYP/6-31+G(d,p), gas phase) revealed shorter C–B and longer > 2b > 2c. This pattern is in line with the expected trend of
B–O bond distances for the diboroxane derived from 2c relative increasing delocalization in the thia-, oxa- and azaborinane
to that from 2b, consistent with the solid-state structural data rings, respectively.
(see the ESI†).
Experimental determinations of the association constants
A comparison of the rates of oxidation of 2b and Ph2BOH was between 2a–c and catechol were carried out by UV-vis titrations
carried out by nuclear magnetic resonance (NMR) spectroscopic in a pH 7.4 HEPES buffer (3 : 1 methanol–water). Addition of
analysis of solutions in air-saturated CD3OD/D2O (3 : 1 v/v) catechol was accompanied by a decrease in absorbance at the
(Fig. 2). Whereas oxidative fragmentation of Ph2BOH to phenol long-wavelength, anthracene-type band, consistent with inter-
and phenylboronic acid was evident within 24 h, the spectrum ruption of conjugation upon binding (Fig. 4). Values of Ka were
of 2b showed no detectable change aer 140 h. These results are
consistent with qualitative observations of Maitlis concerning a
similar 9-aza-10-boraanthracene-derived borinic acid.20 Elec-
tronic effects arising from incorporation of the boron substit-
uent into a 6p electron system, as well as structural constraints
of the fused-ring architecture that suppress Lewis base coordi-
nation or C–B bond migration,21 may contribute to the resis-
Fig. 3 Calculated gas-phase energies of interaction between methoxide and the
tance of 2b towards oxidation.
methylborinates derived from Ph2BOH and 2a–c (B3LYP/6-311+G(d,p)).
Reversible covalent interactions with diols
While previous literature suggested that 2a–c should display
attenuated Lewis acidities relative to diphenylborinic acid,18
quantitative data regarding their reversible covalent interac-
tions with diols were not available. Computational studies of
a model system (complexation of methoxide by the methyl
Fig. 4 (a) Overlay of absorption spectra of 2a (0.15 mM) upon addition of
catechol in a 10 mM HEPES (3 : 1 CH3OH : H2O), pH 7.4 solution. (b) Data from (a)
Fig. 2 Relative oxidative stability of (a) Ph2BOH and (b) 2b in CD3OD–D2O (3 : 1
v/v), as evaluated by 1H NMR over 140 h.
fitted to a 1 : 1 binding isotherm. (c) Average Ka of duplicate trials (ꢁ20%) for
borinic acids 2a–c.
3300 | Chem. Sci., 2013, 4, 3298–3303
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Edge Article
Chemical Science
determined by tting the concentration dependence of the
absorbance data to 1 : 1 binding isotherms by standard
methods.28 Each measurement was conducted in duplicate with
an estimated uncertainty of 20%. In agreement with the calcu-
lations described above, the trend in catechol affinities of the
cyclic borinic acids was 2a > 2b > 2c, each of which was lower
than that of Ph2BOH.29 The magnitude of this variation is
signicant, with the association constant between 2c and cate-
Fig. 6 Calculated average Mulliken atomic charges on O for borinate esters of
ethylene glycol (B3LYP/6-311+G(d,p)).
chol being at least three orders of magnitude lower than that have found to be signicant for this class of catalytic trans-
of Ph2BOH.
formations). Nonetheless, our previous studies have revealed
several instances in which calculated trends are consistent with
observed regiochemical outcomes and catalyst structure–
activity relationships.9,11
Catalytic activities for diol activation
The catalytic activities of 2a–c were evaluated for sulfonylation
of 1-phenyl-1,2-ethanediol with para-toluenesulfonyl chloride
(TsCl) at 0.1 mol% loading, using 1H NMR spectroscopy to
monitor reaction progress (Fig. 5).30 Despite its markedly
reduced affinity for diols, oxaboraanthracene 2b displayed
superior catalytic activity to 1a under these conditions, while
maintaining a high level of selectivity for formation of the
mono-sulfonylated product. This observation suggests that the
2b–diol adduct is signicantly more nucleophilic than that
derived from Ph2BOH. Indeed, calculated Mulliken charges at
the oxygen atoms of tetracoordinate borinate esters derived
from 2a–c are greater than those of the corresponding diphe-
nylborinate (Fig. 6). Caveats regarding the interpretation of
these computational results are numerous: Mulliken charges
are not directly related to nucleophilicity, and the gas-phase
calculations ignore counterion and solvent effects (which we
A rationale for the relative activities of 2a, 2b and 2c is not
evident in the absence of kinetic data for each catalyst. (For
example, it appears that catalyst 2c undergoes deactivation as
the reaction progresses. Catalyst oxidation is unlikely given the
resistance of 2c to this pathway (see below), and we found no
evidence to suggest that 2c is unstable in the presence of TsCl
and i-Pr2NEt. We also note that the activity of 2c as a sulfony-
lation catalyst is not inhibited by product 3a or by
i-Pr2NEt$HCl.) However, preliminary kinetics experiments
employing 2b as catalyst were consistent with the hypothesis of
enhanced rate of catalyst turnover at the expense of substrate
binding: in the presence of excess TsCl and i-Pr2NEt,
consumption of cis-1,2-cyclohexanediol followed rst-order
kinetics. This behavior stands in contrast to the apparent zero-
order (saturation) kinetics observed using catalyst 1a under
these conditions. The contrasting kinetic proles of the sulfo-
nylation reactions catalysed by 1a and 2b are depicted in Fig. 7.
Under optimized conditions, diol monotosylates 3a and 3b were
isolated in 90% and 95% yields, respectively, using catalyst 2b at
0.1 mol% loading.31
The utility of the heteraborinane catalysts extends to reac-
tions of more complex, carbohydrate-derived triol substrates.
Fig. 5 (a) Monosulfonylation of 1-phenyl-1,2-ethanediol with catalysts 1a and
2a–c; (b) Yield of product 3a versus reaction time as a function of catalyst
structure. Yields were determined by 1H NMR spectroscopy with 1,3,5-trime-
thoxybenzene as internal standard. Lines connecting the points are not fits to
kinetic expressions, and are provided only as guides for the eye. (c) Preparation of
tosylates 3a and 3b using catalyst 2b (0.1 mol%).
Fig. 7 Plots of diol concentration versus time for sulfonylation of cis-1,2-cyclo-
hexanediol using catalysts 1a and 2b. Conditions: 1 mol% 1a, 0.5 mmol diol, 5
equiv. TsCl and i-Pr2NEt, 10 mL CH3CN; 3 mol% 2b, 0.2 mmol diol, 5 equiv. TsCl
and i-Pr2NEt, 10 mL CH3CN. Conversions were determined by 1H NMR with 1,3,5-
trimethoxybenzene as internal standard. Different x-axis scales are used for the
two catalytic reactions to emphasize the difference in the shapes of the graphs.
This journal is ª The Royal Society of Chemistry 2013
Chem. Sci., 2013, 4, 3298–3303 | 3301

View Article Online
Chemical Science
Edge Article
Using regioselective monobenzylation of a-L-rhamnopyranoside
as a test reaction, the optimal catalyst was again found to be 2b
(see the ESI†). Employing the latter enabled a reduction in both
the catalyst loading (from 10 mol% to 5 mol%) and the reaction
time (from 48 h to 24 h) in comparison to our previously reported
conditions using 1a.10 This observation was found to hold for
regioselective benzylation of rhamno-, manno-, arabino- and
fucopyranoside substrates (Scheme 3). In each case, the identity
of the major regioisomer was consistent with that obtained using
1a as precatalyst. The levels of regiocontrol exerted by 1a and 2b
were also found to be similar. The products of manno congu-
ration (4a and 4b) were formed along with the C2-OBn
regioisomers: the ratios of 3-OBn : 2-OBn isomers determined by
1H NMR analysis of crude reaction mixtures using catalyst 2b
were 7.4 : 1 for 4a and 5.0 : 1 for 4b. With precatalyst 1a, the
corresponding ratios were 5.1 : 1 and 5.3 : 1, respectively.
Finally, 2a–c were investigated as catalysts for regioselective
activation of glycosyl acceptors. For this transformation, it was
azaborinane 2c that provided the highest yields, despite dis-
playing the lowest catechol affinity of the three heteraborinanes
(Scheme 4). This result highlights the utility of tuning the
electronic properties at boron by varying the identity of the
heteroatom in 2a–c. Control experiments suggest that the utility
of 2c as a glycosylation catalyst may stem from its low rate of
oxidation in the presence of Ag2O, relative to those of 2a–b and
Ph2BOH (see the ESI†). While Ag2O is employed as a halide
abstracting reagent and base in the glycosylation reactions, it is
also known to be a mild oxidant. Consistent with the idea that
2c resists oxidation by Ag2O, it was recovered intact from
glycosylation reaction mixtures (70% average yield of recovered
catalyst for the glycosylations shown in Scheme 4).
Conclusions
The improved stability towards oxidation and enhanced activity
for diol activation that characterizes the heteraborinine-derived
borinic acids constitutes a useful set of properties, and one that
has previously been elusive. Opportunities exist to explore other
members of this interesting class of arene analogs, to further
ne-tune their catalytic activity by heteroatom and substituent
variation, and to develop chiral variants.
Acknowledgements
This work was funded by NSERC (Discovery Grants Program,
Graduate Scholarships to E. D.), the Canadian Foundation for
Innovation and the Province of Ontario. M. S. T. is a Fellow of
the A. P. Sloan Foundation. Kimia Moozeh (University of
Toronto) is acknowledged for contributions to early experi-
ments. Dr Alan J. Lough (University of Toronto) is acknowledged
for solving the X-ray structures.
Notes and references
1 E. Colby Davie, S. M. Mennen, Y. Xu and S. J. Miller, Chem.
Rev., 2007, 107, 5759–5812; N. A. Afagh and A. K. Yudin,
Angew. Chem., Int. Ed., 2010, 49, 262–310.
2 P. G. M. Wuts and T. W. Greene, Greene's Protective Groups in
Organic Synthesis, John Wiley & Sons, Inc., Hoboken, New
Jersey, 4th edn, 2007.
3 For a review: D. Lee and M. S. Taylor, Synthesis, 2012, 44,
3421–3431.
4 B. R. Sculimbrene, A. J. Morgan and S. J. Miller, J. Am. Chem.
Soc., 2002, 124, 11653–11656; B. R. Sculimbrene, A. J. Morgan
and S. J. Miller, Chem. Commun., 2003, 1781–1785;
B. R. Sculimbrene, Y. Xu and S. J. Miller, J. Am. Chem. Soc.,
2004, 126, 13182–13183; A. J. Morgan, Y. K. Wang,
M. F. Roberts and S. J. Miller, J. Am. Chem. Soc., 2004, 126,
15370–15371; Y. Xu, B. R. Sculimbrene and S. J. Miller, J.
Org. Chem., 2006, 71, 4919–4928; A. J. Morgan, S. Komiya,
Y. Xu and S. J. Miller, J. Org. Chem., 2006, 71, 6923–6931;
Y. K. Wang, W. Chen, D. Blair, M. Bu, Y. Xu, S. J. Miller,
A. G Redeld, T. C. Chiles and M. F. Roberts, J. Am. Chem.
Soc., 2008, 130, 7746–7755; P. A. Jordan, K. J. Kayser-
Bricker and S. J. Miller, Proc. Natl. Acad. Sci. U. S. A., 2010,
107, 20620–20624.
Scheme 3 Selective monobenzylation of pyranoside substrates catalyzed by 2b.
5 C. A. Lewis and S. J. Miller, Angew. Chem., Int. Ed., 2006, 45,
5616–5619; C. A. Lewis, J. Merkel and S. J. Miller, Bioorg.
Med. Chem. Lett., 2008, 18, 6007–6011; C. A. Lewis,
K. E. Longcore, S. J. Miller and P. A. Wender, J. Nat. Prod.,
2009, 72, 1864–1869; K. Yoshida, T. Furuta and T. Kawabata,
Tetrahedron Lett., 2010, 51, 4830–4832; B. S. Fowler,
K. M. Laemmerhold and S. J. Miller, J. Am. Chem. Soc., 2012,
134, 9755–9761; Y. Ueda, K. Mishiro, K. Yoshida, T. Furuta
and T. Kawabata, J. Org. Chem., 2012, 77, 7850–7857;
B. C. Wilcock, B. E. Eno, G. L. Bromann, M. J. Clark,
T. M. Anderson and M. D. Burke, Nat. Chem., 2012, 996–1003.
6 See ref. 5 and: K. S. Griswold and S. J. Miller, Tetrahedron,
2003, 59, 8869–8875; K. W. Fiori, A. L. A. Puchlopek and
Scheme 4 Regioselective glycosylations catalysed by 2c. aYields from ref. 12.
3302 | Chem. Sci., 2013, 4, 3298–3303
This journal is ª The Royal Society of Chemistry 2013

View Article Online
Edge Article
Chemical Science
S. J. Miller, Nat. Chem., 2009, 1, 630–634; T. Kawabata, 22 Reviews on azaborines: M. J. D. Bosdet and W. E. Piers, Can.
W. Muramatsu, T. Nishio, T. Shibata and H. Schedel, J. Am.
Chem. Soc., 2007, 129, 12890–12895; A. D. Worthy, X. Sun
and K. L. Tan, J. Am. Chem. Soc., 2012, 134, 7321–7324.
7 M. J. Martinelli, R. Vaidyanathan, J. M. Pawlak, N. K. Nayyar,
U. P. Dhokte, C. W. Doecke, L. M. H. Zollars, E. D. Moher,
J. Chem., 2009, 87, 8–29; P. G. Campbell, A. J. V. Marwitz and
S.-Y. Liu, Angew. Chem., Int. Ed., 2012, 51, 6074–6092.
23 Oxaborines: M. J. S. Dewar and R. Dietz, J. Chem. Soc., 1960,
1344–1347; J. Chen, Z. Bajko, J. W. Kampf and A. J. Ashe III,
Organometallics, 2007, 26, 1563–1564.
V. Van Khau and B. Kosmrjl, J. Am. Chem. Soc., 2002, 124, 24 Thiaborines: F. A. Davis and M. J. S. Dewar, J. Am. Chem. Soc.,
3578–3585; Y. Demizu, Y. Kubo, H. Miyoshi, T. Maki,
Y. Matsumura, N. Moriyama and O. Onomura, Org. Lett.,
2008, 10, 5075–5077.
1968, 90, 3511–3515; M. B. Ryzhikov, A. N. Rodionov,
V. V. Neksarov and D. N. Shigorin, Zh. Fiz. Khim., 1988, 62,
2491; A. J. Ashe III, J. W. Kampf and J. R. Waas,
´
Organometallics, 1995, 14, 3141–3142; S. Sole and
8 P. A. Clarke, N. E. Kayaleh, M. A. Smith, J. R. Baker, S. J. Bird
and C. Chan, J. Org. Chem., 2002, 67, 5226–5231;
R. S. Dhiman and R. Kluger, Org. Biomol. Chem., 2010, 8,
2006–2008; E. V. Evtushenko, J. Carbohydr. Chem., 2010, 29,
369–378.
¨
F. P. Gabbaı, Chem. Commun., 2004, 1284–1285; T. Agou,
J. Kobayashi and T. Kawashima, Chem.–Eur. J., 2007, 13,
8051–8060.
25 J. M. Davidson and C. M. French, J. Chem. Soc., 1960, 191–
9 D. Lee and M. S. Taylor, J. Am. Chem. Soc., 2011, 133, 3724–
3727.
195.
¨
26 T. Lange, U. Bohme and G. Roewer, Inorg. Chem. Commun.,
10 L. Chan and M. S. Taylor, Org. Lett., 2011, 13, 3090–3093.
2002, 5, 377–379.
11 D. Lee, C. L. Williamson, L. Chan and M. S. Taylor, J. Am. 27 DFT calculations were carried out with Gaussian 09. See the
Chem. Soc., 2012, 134, 8260–8267.
ESI† for details and the full reference to the Gaussian 09
12 C. Gouliaras, D. Lee, L. Chan and M. S. Taylor, J. Am. Chem.
Soc., 2011, 133, 13926–13929.
13 T. M. Beale and M. S. Taylor, Org. Lett., 2013, 15, 1358–1361.
soware package.
28 K. J. Hirose, J. Inclusion Phenom. Macrocyclic Chem., 2001, 39,
193–209.
14 For
a
review on organoboron acids as catalysts: 29 The association constant between diphenylborinic acid and
´
E. Dimitrijevic and M. S. Taylor, ACS Catal., 2013, 3, 945–962.
15 R. Nishiyabu, Y. Kubo, T. D. James and J. S. Fossey, Chem.
Commun., 2012, 47, 1106–1123.
catechol in H2O (0.1 M sodium phosphate buffer, pH 7.0) is
2.0 ꢂ 104 Mꢃ1: M. G. Chudzinski, Y. Chi and M. S. Taylor,
Aust. J. Chem., 2011, 64, 1466–1469. In the methanol/HEPES
buffer system used here, the association between alizarin
red S (the indicator dye) and Ph2BOH is too high to be
accurately determined by UV-vis spectroscopy, and we were
unable to implement the indicator displacement assay.
16 R. L. Letsinger and I. Skoog, J. Am. Chem. Soc., 1955, 77,
2491–2492.
17 R. L. Letsinger and J. R. Nazy, J. Org. Chem., 1958, 23, 914–
915; H. C. Brown and J. V. N. V. Prasad, J. Org. Chem.,
1986, 51, 4526–4530; J. Simpelkamp and J. B. Jones, Bioorg. 30 As described in an earlier section, 2b and 2c exist as their
Med. Chem. Lett., 1992, 2, 1391–1394.
dehydrated, dimeric forms in the solid state. It is likely
that the same is true of 2a, although crystals suitable for
X-ray diffraction could not be obtained in this case. The
diboroxanes were used as the precatalysts for the
experiments described here. Reaction monitoring showed
no evidence of induction periods, indicating rapid entry
into the catalytic cycle in each case.
18 T. Agou, M. Sekine, J. Kobayashi and T. Kawashima, Chem.–
Eur. J., 2009, 15, 5056–5062; T. Agou, M. Sekine, J. Kobayashi
and T. Kawashima, Chem. Commun., 2009, 1894–1896;
M. Lepeltier, O. Lukoyanova, A. Jacobson, S. Jeeva and
D. F. Perepichka, Chem. Commun., 2010, 46, 825–829.
19 M. J. S. Dewar, R. Dietz, V. P. Kubba and A. R. Lepley, J. Am.
Chem. Soc., 1961, 83, 1754–1756.
31 For diol monosulfonylations catalysed by low loadings of
20 P. M. Maitlis, J. Chem. Soc., 1961, 425–429.
Bu2SnO
or
its
1,3,2-dioxastannolane
derivative:
21 Z. Zhou, A. Wakamiya, T. Kushida and S. Yamaguchi, J. Am.
Chem. Soc., 2012, 134, 4529–4532.
M. Guillaume and Y. Lang, Tetrahedron Lett., 2010, 51,
579–582.
This journal is ª The Royal Society of Chemistry 2013
Chem. Sci., 2013, 4, 3298–3303 | 3303