Dynamic covalent self-assembled macrocycles prepared from 2-formyl-aryl-boronic acids and 1,2-amino alcohols

Article abstract of DOI:10.1039/b815138e

Reaction of 2-formyl-aryl-boronic acids with 1,2-amino alcohols results in dynamic covalent self assembly to quantitatively afford tetracyclic macrocyclic Schiff base boracycles containing bridging boron-oxygen-boron functionality. The Royal Society of Ch

Full text of DOI:10.1039/b815138e

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Dynamic covalent self-assembled macrocycles prepared from 2-formyl-
aryl-boronic acids and 1,2-amino alcoholsw
¨
Ewan Galbraith, Andrew M. Kelly, John S. Fossey, Gabriele Kociok-Kohn,
Matthew G. Davidson, Steven D. Bull* and Tony D. James*
Received (in Durham, UK) 2nd September 2008, Accepted 18th September 2008
First published as an Advance Article on the web 31st October 2008
DOI: 10.1039/b815138e
Reaction of 2-formyl-aryl-boronic acids with 1,2-amino alcohols results in dynamic covalent self
assembly to quantitatively afford tetracyclic macrocyclic Schiff base boracycles containing
bridging boron–oxygen–boron functionality.
Introduction
Results and discussion
The development of boronic acid based saccharide sensors
that rely on the dynamic covalent interaction of boronic acids
with diols has been widely investigated.1–9 Boronic ester
formation with diols has also been used for the construction
of discrete macrocycles and cages.10 The reversible nature
of boronic acid complexation with diols makes this type of
interaction highly suitable for the reversible self-assembly of
multicomponent systems. With these types of reversible
systems any errors that occur during the assembly process may
be corrected because equilibration of the reactive species
We have recently reported the development of versatile three-
component derivatization protocols for determining the
enantiomeric excess of chiral primary amines, diols or
diamines.29–35 For the case of amines, this approach involves
derivatization of a chiral amine 1 with 2-formyl-phenyl-
boronic acid 2 and enantiopure BINOL (S)-3 in CDCl3 to
quantitatively afford a mixture of diastereoisomeric imino-
boronate esters (S,S)-4 and (S,R)-5. The diastereoisomeric
ratio of (S,S)-4:(S,R)-5 is then determined by 1H NMR
spectroscopic analysis, and since no kinetic resolution occurs
this value is an accurate reflection of the enantiomeric excess
of the parent amine (Scheme 1).
results in formation of
a thermodynamically favoured
product. A number of boracycles have been prepared
that employ a combination of facile imine formation and
boronic acid esterification to afford multicomponent macro-
cycles.11–24 For example, Severin has prepared a series
of self assembled macrocycles/cages by combining 3- or 4-for-
myl-phenyl-boronic acids with bis or tris primary amines
and pentaerythritol (tetraol).25,26 Nitschke has also prepared
a macrocycle derived from pentaerythritol, 2-formyl-phenyl-
boronic acid and para-diaminobenzene, as well as a cage
compound arising from self assembly of cyclotricatechylene,
meta-xylylenediamine and 2-formyl-phenyl-boronic acid.27
Farfan has prepared boracycles from boric acid, 4-diethyla-
mino salicylaldehyde and (R)-phenylglycinol 6d. This
complex was formed in two steps involving reaction of
4-diethylamino salicylaldehyde and (R)-6d to produce an
imine, followed by reflux with boric acid in toluene under
Dean–Stark conditions for 18 h to produce the observed
complex.28
We reasoned that this type of three-component derivatiza-
tion protocol might also be useful for analyzing the enantio-
purity of chiral 1,2-amino alcohols. Therefore, (S)-leucinol 6b
was treated with 2-formyl-phenyl-boronic acid 2 and (S)-
BINOL 3 in CDCl3 and its 1H NMR spectrum acquired after
ten minutes. The resultant 1H NMR spectra revealed the
presence of a complicated mixture of interconverting products
We now report herein that simple room temperature mixing
of 2-formyl-aryl-boronic acids with 1,2-amino alcohols results
in dynamic covalent self assembly to afford stable tetracyclic
macrocyclic Schiff base complexes that contain a rigid brid-
ging boron–oxygen–boron functionality.
Department of Chemistry, University of Bath, Bath, UK BA2 7AY.
E-mail: S.D.Bull@bath.ac.uk. E-mail: T.D.James@bath.ac.uk;
Tel: +44 1225 383810
w CCDC reference numbers 694358–694361 [(S,2R,4S)-7, 8a, 8f and
8h]. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b815138e
Scheme 1 Three-component protocol for determining the enantio-
1
meric purity of chiral amines by H NMR spectroscopic analysis.
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Scheme 2 Formation and X-ray crystal structure of boronic ester
(S,2R,4S)-7.
Scheme 3 Condensation of 2-formyl-phenylboronic acid 2 with chiral
amino alcohols 6a–f and achiral amino alcohols 6g, h affords four-
component boracycles 8a–h.
that was clearly unsuited for carrying out ee determination.
However, on standing overnight, the crude reaction product
fractionally crystallised to afford the expected oxazolidine-
boronate ester (S,2R,4S)-7, whose structure was subsequently
confirmed by X-ray crystallographic analysis (Scheme 2).
In order to investigate this complexation reaction further, it
was decided to determine what products would be formed
when 2-formyl-phenyl-boronic acid 2 was individually reacted
with either (S)-BINOL 3 or (R)-valinol 6a. Two-component
mixing of 2-formyl-phenyl-boronic acid 2 with (S)-BINOL 3
in CDCl3 resulted in no reaction occurring. However, reaction
of 2 with (R)-valinol 6a at room temperature in chloroform
resulted in exclusive formation of a new boracycle (R,R)-8a in
quantitative yield (Scheme 3). The structure of symmetrical
boracycle (R,R)-8a was confirmed by X-ray crystallographic
analysis (Fig. 1), which revealed it to be the condensation
product of two equivalents of 2-formyl-phenyl-boronic acid
2 with two equivalents of (R)-valinol 6a, with concomitant
elimination of five molecules of water. This complexation
reaction results in formation of the densely packed central
core of boracycle (R,R)-8a which comprises two fused seven
membered rings formed from two tetrahedral sp3-boron
atoms, two imino alcohol fragments, and a central oxygen
atom that bridges both boron atoms. This architecture results
in its central fused bicyclic ring structure being further
appended by two five-membered rings formed from two
imino-boronate ester linkages that confer sp3 character on
the boron atoms. The scope and limitation of this four-
component condensation reaction was then investigated via
treatment of a series of five chiral amino alcohols 6b–f with
2-formyl-phenyl-boronic acid 2, which resulted in clean for-
mation of their respective boracycles 8b–f in 84–96% isolated
yield (Scheme 3).
Fig. 1 Crystal structure of macrocycle 8a. (a) Viewed along the
boron–boron axis. (b) Viewed perpendicular to the boron–boron axis.
Mass spectrometry indicated that this solution now contained
a mixture of three macrocycles, (S,S)-8a (M + H 431 m/z),
(S,S)-8b (M + H 445 m/z) and a mixed macrocycle derived
from (S)-6a and (S)-6b (M + H 417 m/z) in a statistical
1:1:2 ratio.
Norman and coworkers have previously reported the synth-
esis of achiral boracycle 8g derived from condensation of
2-aminophenol with 2-formyl-phenyl-boronic acid
2 in
ethanol at reflux.36 Attempts to repeat this condensation
reaction using our mild complexation conditions at room
temperature resulted in no reaction occurring. However,
heating 2-aminophenol 6g (or 4-methyl-2-aminophenol 6h)
with 2-formyl-phenyl-boronic acid 2 at reflux in 95:5 ethanol:
benzene under Dean–Stark conditions did result in quantita-
tive formation of the boracycles 8g (or 8h). Comparison of the
X-ray crystal structures of boracycle (S)-8a with that of
boracycle 8h (Fig. 2) revealed that whilst they belong to the
The reversible nature of macrocycle formation of these
boracycles 8a–f was confirmed by adding one equivalent of
amino alcohol (S)-6a to macrocycle (S,S)-8b in chloroform.
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Conclusions
In conclusion, a range of covalent self-assembled macrocycles
8 and 10 containing bridging O–B–O–B–O have been prepared
and fully characterised. Their ease of preparation suggests that
this class of boracycle is well suited for the reversible self-
assembly of multicomponent systems, and we are currently
investigating the recognition properties of this structurally
diverse class of macrocycle.
Experimental
Fig. 2 Crystal structure of macrocycle 8h. (a) Viewed along the
boron–boron axis. (b) Viewed perpendicular to the boron–boron axis.
General synthetic methods
same class of bridging boracycle, their three dimensional
architectures are very different. In the case of boracycle 8a,
the central bridging oxygen atom lies on the opposite side to
the other two oxygen atoms about the plane bisected by the
two boron atoms. This results in the alkyl side-chains of their
amino alcohol fragments adopting a conformation that creates
the walls of a potential binding cavity centred around its
bridging oxygen atom, with its aryl rings acting as buttressing
elements to contribute structural rigidity. Conversely, for the
case of macrocycle 8h, the presence of the more rigid amino-
phenol fragments results in the three oxygen atoms now being
presented on the same face of the plane bisected by the boron
atoms. This, in turn, results in the aryl rings of the boronic
acid fragment forming the walls of a cavity centred around the
bridging oxygen atom, with its aminophenol derived frag-
ments now adopting the role of buttressing substituents to
confer structural rigidity.
The solvents and reagents were reagent grade unless otherwise
stated and were purchased from Acros Organics, Alfa Aesar,
Fisher Scientific UK, Frontier Scientific Europe Ltd., TCI
Europe or Sigma-Aldrich Company Ltd., and were used
without further purification. Infra-red spectra were recorded
on a Perkin Elmer Spectrum RX spectrometer between 4400 cmÀ1
and 450 cmÀ1. Samples were evaporated from CHCl3 on
to a NaCl disc (film). Nuclear magnetic resonance spectra
were run in either chloroform-d. A Bruker AVANCE 300 was
1
used to acquire H-NMR spectra and recorded at 300 MHz,
11B-NMR spectra at 100 MHz and 13C{1H} NMR spectra at
75 MHz. Chemical shifts (d) are expressed in parts per million
and are reported relative to the residual solvent peak or to
1
tetramethylsilane as an internal standard in H and 13C{1H}
NMR spectra. Boron trifluoride diethyl etherate was used as
an external standard in 11B NMR spectra. Mass spectra were
acquired with
a micrOTOFQ electrospray time-of-flight
We have also varied the nature of the boronic acid template
used for supramolecular assembly, demonstrating that com-
plexation of 2-formyl-furanyl-boronic acid 9 with chiral
aminoalcohols 6a–e in chloroform quantitatively affords their
corresponding four-component boracycles 10a–e in 85–92%
isolated yield (Scheme 4). 11B NMR spectroscopic analysis of
these macrocycles reveals that the boron atoms of the furan
derived boracycles 10a–e (d 4.6–5.4 ppm) have more tetra-
hedral character than their corresponding phenyl derived
boracycles 8a–f (d 10.5–11.5 ppm). This increased tetrahedral
character may be a consequence of the need to incorporate a
more geometrically constrained five-membered furan ring into
these complexes. It may also explain why reaction of achiral
amino alcohols 6g–h with 2-formyl-furanyl-boronic acid 9 did
not result in clean formation of their corresponding four
component boracycles, which may be precluded by the oppos-
ing steric demands of incorporating tetrahedral sp3 boron
atoms and vicinal sp2 aryl carbon atoms into the central
boracyclic core of the macrocyclic ring system.
(ESI-TOF) mass spectrometer (Bruker Daltonik GmbH).
General procedure for the preparation of boracycles 8a-f and
10a–e
2-Formyl-phenyl-boronic acid 2 (60 mg, 0.4 mmol) or 3-formyl-
furanyl-2-boronic acid 9 (56 mg, 0.4 mmol) was stirred with a
chiral 1,2-amino alcohol 6a–f or 6a–e (0.4 mmol) in chloro-
form (5 mL) for 10 min. The solvent was then removed
under reduced pressure to afford boracycles 8a–f or 10a–e in
84–96% yield.
(R,R)-8a. Yellow oil (70 mg, 84%); [a]2D0 +22.0 (c 1.0,
CH2Cl2); vmax (film) 1628 (CQN); dH (300 MHz; CDCl3)
8.08 (2H, s, CHQN), 7.51 (2H, d, J 7.4, ArH), 7.35–7.27
(4H, m, ArH), 7.11 (2H, dt, J 7.4 and 1.1, ArH), 4.26 (2H, dd,
J 12.2 and 1.3, CHAHB(O)), 3.98 (2H, dd, J 12.2 and 1.3,
CHAHB(O)), 3.13 (2H, m, CH(iPr)–N), 2.96–2.83 (2H, m,
CH(CH3)2), 1.02 (6H, d, J 6.8, C(CH3)(CH3)) and 0.88
(6H, d, J 6.8, C(CH3)(CH3)); dC (75 MHz; CDCl3) 167.3
(CQN), 136.8, 133.6, 129.4, 127.2, 127.0, 126.2, 76.0, 60.9,
27.0, 21.1 and 19.4; dB (100 MHz; CDCl3) 10.7; m/z LRMS
(ESI+) 418 [(M + H)+, 13%], 283.2 (100), 200.1 (2); HRMS
(ESI+) found 417.2531 ([M + H]+ C24H30B2N2O3 requires
417.2515).
(S,S)-8b. Yellow solid (79 mg, 89%); m.p. 206–210 1C (dec);
[a]2D0 À26.1 (c 1.0, CH2Cl2); vmax (film) 1628 (CQN);
dH (300 MHz; CDCl3) 8.16 (2H, s, CHQN), 7.51 (2H, d,
J 7.4, ArH), 7.36–7.27 (4H, m, ArH), 7.11 (2H, dt, J 7.4 and 1.1,
Scheme 4 Preparation of boracycles 10a–e.
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ArH), 4.35 (2H, dd, J 11.9 and 1.7, CHAHB(O)), 3.81–3.73
(4H, m, CHAHB(O) and CH–N), 2.16–2.06 (2H, m,
CHAHBC(N)), 2.02–1.92 (2H, m, CHAHBC(N)), 1.72–1.63
(2H, m, CH(CH3)2) and 0.90 (12H, app t, J 6.8, C(CH3)2);
8g. 2-Aminophenol 6g (200 mg, 1.83 mmol) and 2-formyl-
phenylboronic acid 2 (275 mg, 1.83 mmol) were dissolved in
95:5 ethanol–benzene (25 mL) in a round bottom flask fitted
with a Dean–Stark condenser and stirred at reflux for 4 h. The
reaction mixture was cooled and the solvent removed under
reduced pressure. Washing with a little cold methanol afforded
8g as a yellow powder (709 mg, 90%): m.p. 182–183 1C (dec.)
(Lit. 180 1C (dec.)36); dH (300 MHz, CDCl3) 8.65 (s, 2H), 7.49
(2H, d, J = 7.5 Hz), 7.40–7.37 (2H, m, Ar), 7.29–7.12 (8H, m),
6.92 (4H, m); dC (75 MHz, CDCl3) d = 160.9, 155.5, 135.1,
134.2, 134.13, 133.1, 132.9, 131.5, 127.9, 118.7, 115.8, 113.7; dB
(96.3 MHz, CDCl3) 9.6; m/z HRMS (ESI+) found 429.1571.
([M + H]+ C26H19B2N2O3 (M + H+) requires 429.1582).
d
C (75 MHz; CDCl3) 166.9 (CQN), 136.9, 133.5, 133.1, 129.4,
127.2, 126.1, 66.7, 62.5, 40.4, 24.8, 23.0 and 22.9; dB (100 MHz;
CDCl3) 11.5; m/z LRMS (ESI+) 445 [(M + H)+, 14%], 412.4
(100), 292.2 (66), 227.2 (15); HRMS (ESI+) found 445.2873
([M+H]+ C26H34B2N2O3 requires 445.2828).
(R,R)-8c. Yellow oil (69 mg, 88%); [a]2D0 +14.7 (c 1.0,
CH2Cl2); vmax (film) 1628 (CQN); dH (300 MHz; CDCl3)
8.13 (2H, s, CHQN), 7.50 (2H, t, J 7.4, ArH), 7.3–7.26
(4H, m, ArH), 7.12–7.07 (2H, m, ArH), 4.33 (2H, dd, J 12.0 and
1.7, CHAHB(O)), 3.78 (2H, dd, J 12.0 and 1.7, CHAHB(O)),
3.51 (2H, m, CH(Et)–N), 2.20–2.09 (4H, m, CHAHBMe) and
0.93 (6H, t, J 7.6, CH3); dC (75 MHz; CDCl3) 167.3 (CQN),
136.9, 133.5, 133.3, 129.3, 127.2, 126.2, 70.7, 62.4, 24.8
and 11.5; dB (100 MHz; CDCl3) 11.2; m/z LRMS (CI+) 389
[(M + H)+, 6%], 188.2 (50), 106.0ꢁ(46), 72.0 (100); HRMS
(EI+) found 388.2126 (2 Â 11B) (M+ C22H26B2N2O3 requires
388.2124).
8h. 2-Hydroxy-5-methylaniline 6h (123 mg, 1.0 mmol) and
2-formyl-phenyl-boronic acid 1 (150 mg, 1.0 mmol) were
dissolved in 95:5 ethanol–benzene (20 mL) in a round bottom
flask fitted with a Dean–Stark condenser and stirred at reflux
for 4 h. The reaction mixture was cooled and the solvent
removed under reduced pressure. Washing with a little cold
methanol afforded 8h as a orange powder (374 mg, 82%): m.p.
231–232 1C (dec.); dH (300 MHz, CDCl3) 8.64 (2H, s,
CHQN), 7.42 (2H, m, ArH), 7.36 (2H, s, ArH), 7.29–7.16
(6H, m, ArH), 7.11 (2H, d, J 8.4 ArH), 6.85 (2H, d, J 8.4), 2.35
(6H, s, CH3); dC (75 MHz, CDCl3) 158.24, 154.9, 148.8, 134.9,
134.1, 133.9, 132.9, 131.2, 128.2, 127.8, 115.3, 113.8, 21.5; dB
(100 MHz, CDCl3) 8.9; m/z HRMS (ESI+) found 457.2011.
([M + H]+ C28H23B2N2O3 (M + H+) requires 457.1889).
(R,R)-8d. Yellow oil (88 mg, 91%); [a]2D0 +21.1 (c 1.0,
CH2Cl2); vmax (film) 1627 (CQN); dH (300 MHz; CDCl3)
7.66–7.63 (4H, m, CHQN and ArH), 7.43–7.30 (12H, m,
ArH), 7.20 (2H, br t, J 7.4, ArH), 7.09 (2H, dt, J 7.4 and
0.8, ArH), 5.25 (2H, m, CHAHB(O)), 4.65 (2H, dd, J 11.9 and
10.4, CH(Ph)-N), 3.95 (2H, m, CHAHB(O)); dC (75 MHz;
CDCl3) 166.1 (CQN), 137.0, 135.7, 133.9, 133.5, 132.3, 130.0,
129.9, 129.7, 129.5, 127.2, 126.8, 126.7, 71.4, and 69.1; dB (100
MHz; CDCl3) 11.3; m/z LRMS (ESI+) 485 [(M + H)+, 9%],
368.2 (10), 312.1 (100), 278.2 (16); HRMS (ESI+) found
485.2230 ([M + H]+ C30H26B2N2O3 requires 485.2202).
(S,S)-10a. Dark brown solid (74 mg, 93%); m.p. 131–140 1C
(dec); [a]2D0 À36.8 (c 1.0, CH2Cl2); vmax (film) 1649 (CQN);
dH (300 MHz; CDCl3) 8.13 (2H, d, J 3.0 CHQN), 7.35 (2H, d,
J 1.9, ArH), 6.33 (2H, d, J 1.9, ArH), 4.39 (2H, dd, J 9.4 and
6.2, CHAHB(O)), 4.15 (2H, dd, J 9.4 and 4.0, CHAHB(O)),
3.90–3.84 (2H, m, CHQN), 2.28–2.17 (2H, m, CH(CH3)2) and
1.06 (12H, app dd, J 6.8 and 7.2, C(CH3)2); dC (75 MHz;
CDCl3) 157.2, 144.5, 132.6, 123.3, 110.2, 69.3, 63.5, 32.6, 20.0
and 17.4; dB (100 MHz; CDCl3) 4.7; m/z LRMS (ESI+) 397
[(M + H)+, 9%], 345.2 (100), 283.2 (36); HRMS (ESI+)
found 397.2122 ([M + H]+C20H26B2N2O5 requires 397.2100).
(R,R)-8e. Yellow solid (140 mg, 95%); m.p. 125–129 1C
(dec); [a]2D0 +19.4 (c 1.0, CH2Cl2); vmax (film) 1635 (CQN); dH
(300 MHz; CDCl3) 8.25 (2H, s, CHQN), 7.64 (2H, d, J 7.0,
ArH), 7.48–7.14 (16H, m, ArH), 5.46 (2H, br d, J 9.8,
CHAHB(N)), 4.50–4.42 (2H, m, CH(Ph)(O)) and 4.03 (2H,
br d, J 9.8, CHAHB(N)); dC (75 MHz; CDCl3) 166.0, 137.0,
135.6, 133.9, 133.5, 132.3, 130.0, 129.9, 129.7, 129.4, 128.0,
127.3, 126.8, 71.3 and 65.8; dB (100 MHz; CDCl3) 10.5; m/z
LRMS (ESI+) 485 [(M + H)+, 100%], 312.1 (99); HRMS
(ESI+) found 485.2219 ([M + H]+C30H26B2N2O3 requires
485.2202).
(S,S)-10b. Red oil (76 mg, 90%); [a]2D0 À34.0 (c 1.0, CH2Cl2);
v
max (film) 1649 (CQN); dH (300 MHz; CDCl3) 8.13 (2H, d, J
3.0, CHQN), 7.36 (2H, d, J 1.9, ArH), 6.32 (2H, d, J 1.9,
ArH), 4.39 (2H, dd, J 8.9 and 6.0, CHAHB(O)), 4.24–4.15
(2H, m, CH–N), 3.99 (2H, dd, J 8.9 and 7.4, CHAHB(O)),
1.77–1.65 (6H, m, CHAHBCH(CH3)2) and 1.00 (12H, app t, J
5.5, C(CH3)2); dC (75 MHz; CDCl3) 157.2, 144.7, 132.7, 122.8,
110.0, 71.9, 52.2, 32.6, 20.5, 8.9 and 8.1; dB (100 MHz; CDCl3)
4.6; m/z LRMS (ESI+) 425 [(M + H)+, 32%], 412.4 (27),
389.3 (35), 375.2 (100); HRMS (ESI+) found 425.2452
([M + H]+ C22H30B2N2O5 requires 425.2419).
(rac)-8f. Yellow solid (85 mg, 96%); m.p. 142–144 1C (dec);
v
max (film) 1625 (CQN); dH (300 MHz; CDCl3) 8.20 (2H, d, J
3.0, CHQN), 7.47 (2H, d, J 6.8, ArH), 7.35 (2H, d, J 7.4,
ArH), 7.28 (2H, app dt, J 7.5 and 1.1, ArH), 7.10 (2H, app dt,
J 7.5 and 1.1, ArH), 3.96–3.88 (2H, m, CH(N)), 3.79–3.70 (2H,
m, CH(O)), 2.26 (2H, br d, J 12.0, CHAHBC–O), 1.88–1.81
(4H, m, CHAHBC–O and CHAHBC–N), 1.71–1.66 (2H, m,
CHAHBC–N) and 1.50–1.12 (8H, m, 2Â(CH2)2); dC (75 MHz;
CDCl3) 164.0 (CQN), 137.2, 133.2, 129.9, 129.1, 126.9, 126.3,
65.6, 36.3, 29.8, 27.3, 24.9 and 24.8; dB (100 MHz; CDCl3)
10.8; m/z LRMS (ESI+) 440 [(M + H)+, 100%], 290.2 (15);
HRMS (ESI+) found 441.2549 ([M + H]+ C26H30B2N2O3
requires 441.2515).
(R,R)-10c. Red oil (134 mg, 91%); [a]2D0 +33.0 (c 1.0,
CH2Cl2); vmax (film) 1656 (CQN); dH (300 MHz; CDCl3)
8.13 (2H, s, CHQN), 7.34 (2H, d, J 1.9, ArH), 6.31 (2H, d,
J 1.9, ArH), 4.42–4.39 (2H, m, CHAHB(O)), 4.09–3.99 (4H, m,
CHAHB(O) and CH–N), 2.05–1.90 (4H, m, CH2Me) and 1.07
(6H, t, J 7.0, CH3); dC (75 MHz; CDCl3) 156.3, 144.5, 132.6,
123.3 110.2, 67.7, 65.7, 26.4 and 10.4; dB (100 MHz; CDCl3)
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4.8; m/z LRMS (ESI+) 369 [(M+H)+, 65%], 288.2 (100),
201.1 (34); HRMS (ESI+) found 369.1791 ([M+H]+
C18H22B2N2O5 requires 369.1785).
9 L. I. Bosch, T. M. Fyles and T. D. James, Tetrahedron, 2004, 60,
11175–11190.
10 N. Fujita, S. Shinkai and T. D. James, Chem.–Asian J., 2008, 3,
1076–1091.
11 H. Hopfl, Struct. Bonding, 2002, 103, 1–56.
(R,R)-10d. Red solid (79 mg, 85%); m.p. 115–118 1C (dec);
[a]2D0 +39.6 (c 1.0, CH2Cl2); vmax (film) 1657 (CQN); dH
(300 MHz; CDCl3) 7.83 (2H, d, J 3.0, CHQN), 7.53–7.44
(10H, m, ArH), 7.42 (2H, d, J 1.9, ArH), 6.24 (2H, d, J 1.9,
ArH), 5.35–5.28 (2H, m, CH(Ph)–N) and 4.57–4.45 (4H, m,
CHAHB(O)); dC (75 MHz; CDCl3) 158.3, 144.9, 136.6, 131.4,
129.8, 129.60, 129.59, 129.55, 129.50, 124.6, 110.3, 71.3
and 70.4; dB (100 MHz; CDCl3) 5.4; m/z LRMS (ESI+) 465
[(M + H)+, 100%], 415.2 (33), 335.2 (36), 292.1 (32), 215.1 (10);
HRMS (ESI+) found 465.1833 ([M + H]+C26H22B2N2O5
requires 465.1787).
12 N. Farfan, H. Hopfl, V. Barba, M. E. Ochoa, R. Santillan,
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70–81.
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15 V. Barba, H. Hopfl, N. Farfan, R. Santillan, H. I. Beltran and
L. S. Zamudio-Rivera, Chem. Commun., 2004, 2834–2835.
16 V. Barba, R. Villamil, R. Luna, C. Godoy-Alcantar, H. Hopfl,
H. I. Beltran, L. S. Zamudio-Rivera, R. Santillan and N. Farfan,
Inorg. Chem., 2006, 45, 2553–2561.
17 V. Barba and I. Betanzos, J. Organomet. Chem., 2007, 692,
4903–4908.
18 H. Hopfl, M. Sanchez, V. Barba, N. Farfan, S. Rojas and
R. Santillan, Inorg. Chem., 1998, 37, 1679–1692.
19 H. Hopfl and N. Farfan, J. Organomet. Chem., 1997, 547, 71–77.
20 A. Mitra, L. J. Depue, J. E. Struss, B. P. Patel, S. Parkin and
D. A. Atwood, Inorg. Chem., 2006, 45, 9213–9224.
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N. Farfan, R. Santillan and E. Gomez, Inorg. Chem., 2004, 43,
8490–8500.
22 M. Sanchez, H. Hopfl, M. E. Ochoa, N. Farfan, R. Santillan and
S. Rojas, Inorg. Chem., 2001, 40, 6405–6412.
23 M. Sanchez, T. S. Keizer, S. Parkin, H. Hopfl and D. A. Atwood,
J. Organomet. Chem., 2002, 654, 36–43.
(R,R)-10e. Dark brown solid (85 mg, 92%); m.p. 124–126 1C
(dec); [a]2D0 +18.5 (c 1.0, CH2Cl2); vmax (film) 1662 (CQN); dH
(300 MHz; CDCl3) 8.26 (2H, br s, CHQN), 7.56 (4H, br d,
ArH), 7.45 (2H, d, J 1.9, ArH), 7.39–7.26 (6H, m, ArH), 6.34
(2H, d, J 1.9, ArH), 5.56–5.51 (2H, m, CHAHB(N)) and
4.11–4.08 (4H, m, CHAHB(N)) and CH(Ph)(O)); dC
(75 MHz; CDCl3) 157.7, 145.1, 142.1, 129.7, 128.8, 128.6,
128.1, 126.7, 124.5, 110.4, 76.2 and 63.7; dB (100 MHz; CDCl3)
5.0; m/z LRMS (ESI+) 465 [(M + H)+, 65%], 415.2 (100),
323.2 (83); HRMS (ESI+) found 465.1841 ([M + H]+
C26H22B2N2O5 requires 465.1787).
24 N. Yalcin, A. Kenar, C. Arici, O. Atakol and M. Tastekin, Main
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25 N. Christinat, R. Scopelliti and K. Severin, J. Org. Chem., 2007,
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Acknowledgements
26 N. Christinat, R. Scopelliti and K. Severin, Angew. Chem., Int. Ed.,
2008, 47, 1848–1852.
We would like to acknowledge the EPSRC, Royal Society, the
Leverhulme Trust, Beckman-Coulter and University of Bath
for funding.
27 M. Hutin, G. Bernardinelli and J. R. Nitschke, Chem.–Eur. J.,
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28 H. Reyes, J. M. Rivera, N. Farfan, R. Santillan, P. G. Lacroix,
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