Boronic, diboronic and boric acid esters of 1,8-naphthalenediol-synthesis, structure and formation of boronium salts

Article abstract of DOI:10.1039/d0dt00745e

The 1,8-naphthalenediolate [1,8-O₂C₁₀H₈] supported boronic and boric acid esters of general formula X-B(1,8-O₂C₁₀H₈), where X = C₆H₅ (1a), C₆F₅ (2a), 3,4,5-F₃-C₆H₂ (3a), 2,4,6-F₃-C₆H₂ (4a), 2,6-F₂-C₆H₃ (5a), 2,6-Cl₂-C₆H₃ (6a), 2,4,6-Me₃-C₆H₂ (7a), 2,6-(MeO)₃-C₆H₃ (8a), Buⁿ (9a), MeO (10a), OH (11a) and Cl (13a), were synthesized, NMR spectroscopically characterized, and the solid-state structures of 1a-5a, 8a and 10a determined by X-ray crystallography. The acceptor numbers of 1a-7a and 13a were determined and found to be similar to their catecholate analogues, R-Bcat, indicating similar Lewis acidities of these two classes of boronic acid esters. The reaction of B₂(NMe₂)₄ with 1,8-naphthalenediol, followed by addition of HCl furnished the diboronic acid ester B₂(1,8-O₂C₁₀H₈)₄ (16a) in ca. 70% yield. Cl-B(1,8-O₂C₁₀H₈) (13a) was shown to react with OPEt₃, DMAP, 1,10-phenanthroline and 2,2′-bipyridine, resp., to give the boronium salts [(Et₃PO)₂B(1,8-O₂C₁₀H₈)]Cl (18a), [(DMAP)₂B(1,8-O₂C₁₀H₈)]Cl (22a), [(2,2′-bipyridine)B(1,8-O₂C₁₀H₈)]Cl (23a) and [(1,10-phenanthroline)B(1,8-O₂C₁₀H₈)]Cl (24a), which were characterized by NMR spectroscopy and X-ray crystallography.

Full text of DOI:10.1039/d0dt00745e

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14 March 2017
Pages 3073-3412
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DOI: 10.1039/D0DT00745E
ARTICLE
Boronic, Diboronic and Boric Acid Esters of 1,8-Naphthalenediol –
Synthesis, Structure and Formation of Boronium Salts
Chamila P. Manankandayalage, Daniel K. Unruh and Clemens Krempner *
Received 00th January 20xx,
Accepted 00th January 20xx
The 1,8-naphthalenediolate [1,8-O2C10H8] supported boronic and boric acid esters of general formula X-B(1,8-O2C10H8),
where X = C6H5 (1a), C6F5 (2a), 3,4,5-F3-C6H2 (3a), 2,4,6-F3-C6H2 (4a), 2,6-F2-C6H3 (5a), 2,6-Cl2-C6H2 (6a), 2,4,6-Me3-C6H2 (7a),
2,6-(MeO)2-C6H2 (8a), Bun (9a), MeO (10a), OH (11a) and Cl (13a), were synthesized, NMR spectroscopically characterized,
and the solid-state structures of 1a-5a, 8a and 10a determined by X-ray crystallography. The acceptor numbers of 1a-7a and
13a were determined and found to be similar to their catecholate analogues, R-BCat, indicating similar Lewis acidities of
these two classes of boronic acid esters. The reaction of B2(NMe2)4 with 1,8-naphthalenediol, followed by addtion of HCl
furnished the diboronic acid ester B2(1,8-O2C10H8)4 (16a) in ca. 70% yield. Cl-B(1,8-O2C10H8) (13a) was shown to smoothly
react with O=PEt3, DMAP, 1,10-phenanthroline and 2,2’-bipyridine, resp., to give the boronium salts [(Et3P=O)2B(1,8-
O2C10H8)]Cl (21a), [(DMAP)2B(1,8-O2C10H8)]Cl (22a), [(2,2’-bipyridine)B(1,8-O2C10H8)]Cl (23a) and [(1,10-
phenanthroline)B(1,8-O2C10H8)]Cl (24a), which were characterized by NMR spectroscopy and X-ray crystallography.
DOI: 10.1039/x0xx00000x
base adducts for the formation of supramolecular assemblies 10
or to function as active Lewis acid catalysts in organic
transformations.11 Catechol boronic acid esters (II), on the other
Introduction
The last three decades have witnessed the “evolution” of
boronic acid ester from ordinary organo element compounds to
working horses in organic and organometallic chemistry and
catalysis.1 The seemingly never-ending interest in these types of
compounds appears to be fuelled by the high efficiency and
broad applicability of the Suzuki-Miyaura coupling 2, specifically
in the synthesis of pharmaceutical and agrochemical
intermediates and natural products.3-7 Arguably, pinacol
boronic acid esters (III), 1,8-diaminonaphthyl boronic acid
hand, are relatively strong Lewis acids owing to decreased
oxygen to boron p-donation resulting from ring strain and
competing conjugation with the phenyl ring.1b For similar
reasons, however, they are more prone to nucleophilic attack
at the central boron for example by water, and therefore rapidly
hydrolyse when used in combination with “wet” solvents.
We envisioned that boronic acid esters derived from the 1.8-
naphthalenediolate scaffold (I) should be of similar Lewis acidity
as their catechol analogues (II) but more thermally robust and
less sensitive to hydrolysis due to the larger size of the six-
membered dioxaborinane ring. However, apart from Noeth’s 12
disclosure of the synthesis of hydroborane (I) (R = H), which
underwent rhodium catalysed hydroboration of cyclopentene
ca. 25 times faster than catechol borane (II) (R = H), less is
known about these boron compounds.12-15 Therefore, we wish
to report the synthesis, structures and chemical properties of a
range of 1,8-naphthalenediolate supported boronic and boric
acid esters as well as boronium salts.
8
9
amides (IV) and MIDA-boronates (V) (MIDA = N-methyl-
iminodiacetic acid) are the most frequently employed reagents
in coupling chemistry primarily due to their thermal robustness
and hydrolytic stability allowing them to be purified by column
chromatography under benchtop conditions (Scheme 1).
H
R
O
O
N
N
O
O
N
B
O
O
O
R
B
R
B
R
B
H
R
B
O
R
O
O
I
II
V
III
IV
Scheme 1. Selected organoboron esters and amides.
Results and Discussion
However, these undoubtedly favourable properties are at the We initially studied the acid-ester equilibrium of phenyl boronic
expense of Lewis acidity of the electron-deficient boron centre acid with 1,8-naphthalenediol (1:1 molar ratio) at room
limiting the ability of these species to form stable Lewis acid- temperature in CDCl3 as solvent, which was not pre-dried
1
(Scheme 2). H NMR spectroscopic analyses of the reaction
mixture revealed rapid and quantitative formation of the
boronic acid ester 1a, confirming the relative stability of 1a
a. Department of Chemistry and Biochemistry, Texas Tech University, Memorial Dr.
& Boston, Lubbock, TX, 79409, USA. Fax: 806-742-1289; Tel: 806-834-3507; E-
mail: clemens.krempner@ttu.edu
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
towards hydrolysis in the presence of small amounts of water.
To compare the thermodynamic stability of 1a with those of the
information available should be included here]. See DOI: 10.1039/x0xx00000x
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phenyl boronic acid esters 1b-d, ligand exchange experiments
were undertaken.
View Article Online
DOI: 10.1039/D0DT00745E
OH
OH
HO
HO
O
O
CDCl3
+
B
+ 2 H2O
B
1a
Scheme 2. Reaction of 1,8-naphthalenediol with phenyl boronic acid.
Equimolar amounts of 1,8-naphthalenediol were treated with
1b-d, resp., in CDCl3 as solvent (Figure 1A), and the resulting
mixtures analysed by 1H NMR spectroscopy after reaching
equilibrium. Upon adding 1,8-naphthalenediol, the catechol
ester 1b within minutes quantitatively converted into 1a at
room temperature confirming our hypothesis that owing to its
lower ring strain 1a is thermodynamically significantly more
stable than 1b. In case of the bulkier pinacol ester 1c, heating at
50°C overnight was required to reach equilibrium resulting in
the formation of 50% 1c and 50% 1a. In contrast, several days
of heating 50°C were required to equilibrate a CDCl3 solution of
1d and 1,8-naphthalenediol to a mixture of 1a and 1d in a 30/70
ratio. Note that treating ester 1a with the corresponding diols
b-d, resp., in CDCl3 as solvent gave the same equilibrium
mixtures, which confirms the validity of the approach and the
following order in stability with respect to ligand exchange: 1b
<< 1c ≈ 1a < 1d.
The results from the ligand exchange experiments are in good
agreement with the obtained hydrolysis data of 1a-d, showing the
hydrolytic stability but also the rate of hydrolysis in DMSO-D6/water
(10 vol% H2O) to be in the following order 1b >> 1c > 1a >> 1d (Figure
1B). Thus, while 1,8-diaminonaphthalene compound 1d did not
hydrolyse at all even after 12 days under the conditions applied,
catechol ester 1b quantitatively hydrolysed within 5 minutes.
Astonishingly, also ester 1a proved to be significantly more stable
than 1b; after ca. 5 min. only 15% and after 18 hrs. ca 70% of 1a
converted into 1,8-naphthalenediol, reaching equilibrium after ca. 3
days with a 1,8-naphthalenediol/1a ratio of ca. 73/27. Pinacol ester
1c hydrolysed at a much lower rate than 1a, reaching equilibrium
after ca. 9 days with a pinacol/1c ratio of ca. 65/35.
Figure 1. Ligand exchange experiments (A) and hydrolysis (B) of 1a-d in DMSO-D6/H2O
(10 vol% H2O).
Encouraged by the high stability and ease of formation of 1a,
various 1,8-naphthalenediol derived aryl boronic acid esters
were prepared (Scheme 3). Thus, the reaction of 1,8-
naphthalenediol with the respective aryl boronic acids in
acetonitrile at room temperature gave crystalline precipitates
of the boronic esters 1a, 3a-5a and 8a in good to excellent
isolated yields. The esters 2a, 6a and 7a were synthesized by a
slightly modified procedure (see supporting information for
details) as they did not crystallize from acetonitrile. Isolated 1a-
8a were fully characterized by multi nuclear NMR spectroscopy
and combustion analysis.
HO
HO
O
O
ArB(OH)2
- 2 H2O
Ar
B
1a-8a
1a
3a
5a
7a
2a
4a
6a
8a
Ar = C6H6
(84%)
Ar = C6F5
Ar = 2,4,6-F3-C6H2
Ar = 2,6-Cl2-C6H3
(60%)
(92%)
(84%)
Ar = 3,4,5-F3-C6H2 (95%)
Ar = 2,6-F2-C6H3 (85%)
Ar = 2,4,6-Me3-C6H2 (63%)
Ar = 2,6-(MeO)2-C6H3 (75%)
Scheme 3. Synthesis of the aryl boronic acid esters 1a-8a.
A) Ligand exchange equilibrium
Further confirmation of the structural connectivity of 1a-5a and 8a
was obtained from single-crystal X-ray analyses; the results are
shown in Figures 2-7. Owing to extensive π-stacking the aromatic ring
systems in 1a-4a are co-planar with respect to each other, similar to
what is seen for the analogous catechol esters 1b-5b.16,17 Only
compounds 5a and 8a show in the solid state twisted structures with
a C1-C2-B1-O2 dihedral angle of 60° for 5a and a C16-C11-B1-O2
dihedral angle of 73° for 8a. For comparison, selected bond
parameter of 1a-5a and the analogous catechol esters 1b-5b are
summarized in table 1. As expected, due to their larger ring size, 1a-
5a have larger O-B-O angles (122°-123°) then the respective five-
membered ring catechol derivatives 1b-5b with angles of ca. 112°.
While the C-O distances are fairly similar for both classes of
compounds ranging from 1.385 to 1.395 Å, the B-O distances of 1a-
5a are somewhat shorter and the B-C distances are slightly longer
(both by only 0.01-0.02 Å) than those of 1b-5b.
O
O
HO
HO
HX
X
X
CDCl3
+
B
B
+
HX
b-d
1b-d
1a
a
ratio: 1a 1b
1a 1c
/
1a 1d
/
=
99/1
=
50/50
/
=
30/70
HX
HX
HO
HO
HO
HO
H2N
H2N
HO
HO
=
c
b
a
d
B) Hydrolysis equilibrium
OH
OH
HX
HX
X
X
DMSO-D6
+
B
B
+ H2O
(10 vol% H2O)
a-d
1a-d
2 | J. Name., 2012, 00, 1-3
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View Article Online
DOI: 10.1039/D0DT00745E
Figure 2. Solid-state structure of C6H5-BNad (1a).
Figure 6. Solid-state structure of 2,6-F2-C6H3-BNad (5a) (green = fluorine).
Figure 3. Solid-state structure of C6F5-BNad (2a) (green = fluorine).
Figure 7. Solid-state structure of 2,6-(MeO)2-C6H3-BNad (8a).
This becomes particularly evident by comparing individual pairs
of boronic acid esters with identical X-aryl groups and can be
attributed to a slightly increased -donating ability of the
oxygen’s to boron in the six-membered ring structures as
opposed to the smaller five-membered rings present in 1b-5b.
However, caution should be taken as the C-O, B-O and B-C
distances for both classes of boronic acid ester are generally
very similar.
Table 1. Selected bond lengths [Å] and angles [°] of 1a-5a and 1b-5b.
-
B-C
B-O c
C-O c
O-B-O
Figure 4. Solid-state structure of 3,4,5-F3-C6H2-BNad (3a) (green = fluorine).
C6H5Bnad (1a)
C6H5Bcat (1b)a
C6F5Bnad (2a)
C6F5Bcat (2b)b
1.561(2) 1.375(2) 1.384(1) 122.0(1)
1.537(5) 1.394(3) 1.395(4) 109.8d
1.578(2) 1.367(1) 1.389(1) 122.9(1)
1.558(2) 1.382(2) 1.389(2) 112.4(1)
3,4,5-F3-C6H2Bnad (3a) 1.558(3) 1.370(3) 1.395(2) 122.7(2)
3,4,5-F3-C6H2Bcat (3b)b 1.546(2) 1.381(2) 1.386(2) 112.2(1)
2,4,6-F3-C6H2Bnad (4a) 1.573(2) 1.370(1) 1.385(1) 122.3(1)
2,4,6-F3-C6H2Bcat (4b)b 1.553(2) 1.383(2) 1.385(2) 112.1(1)
2,6-F2-C6H3Bnad (5a)
2,6-F2-C6H3Bcat (5b)b
1.574(2) 1.366(1) 1.388(1) 123.2(1)
1.553(2) 1.386(2) 1.385(2) 111.7(1)
nad = 1,8-naphthalenediolate; a ref. 16; b ref. 17; c average distances; d
standard deviation unavailable.
Figure 5. Solid-state structure of 2,4,6-F3-C6H2-BNad (4a) (green = fluorine).
We next investigated the synthetic potential of 1,8-
naphthalenediol as a supporting ligand for boron with various
electronic and coordination environments (Scheme 4). For
example, refluxing a equimolar amounts of BunB(OH)2 and 1,8-
naphthalenediol in acetonitrile gave after vacuum distillation
the aliphatic boronic acid ester 9a in almost quantitative yields
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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as an air and moisture-stable, colourless liquid (Scheme 5).
Heating a hexanes solution of 1,8-naphthalenediol in the
presence of a 3-fold excess of B(OMe)3 gives the boric ester 10a
as a colourless solid in excellent isolated yields; its solid-state
structure is depicted in Figure 8. The corresponding hydroxy
borate 11a was synthesized in 93% yield from refluxing 1,8-
naphthalenediol with a twofold excess of boric acid in
acetonitrile. To our surprise, 11a proved to be remarkably
thermally robust as elimination of water and condensation
reactions did not occur at 100°C under vacuum. The reaction of
stochiometric amounts of 1,8-naphthalenediol with B(NMe2)3
did not give the targeted amino borate Me2N-BNad; rather the
spirocyclic ammonium borate 12a was rapidly formed as the
sole product. Even the use of an excess of B(NMe2)3 at low
temperatures did not produce Me2N-BNad. However, almost
quantitative yields of 12a were obtained when two equivalents
of 1,8-naphthalenediol were treated with one equivalent of
B(NMe2)3 at room temperature.
View Article Online
DOI: 10.1039/D0DT00745E
Figure 8. Solid-state structure of 10a. (Only one of the three independent molecules in
the unit cell is shown for clarity). Selected bond lengths [Å] and angles [°]: O1 B1 1.380(1),
O1 C1 1.381(1), O2 B1 1.375(1), O2 C3 1.379(1), O3 B1 1.349(1), O3 C11 1.439(1), O3 B1
O2 116.34(10), O3 B1 O1 121.40(10), O2 B1 O1 122.26(10), B1 O1 C1 119.87(9).
O
O
Cl
B
Bu
B
O
O
13a
(95%)
BunB(OH)2
BCl3
9a
(93%)
O
O
HO
HO
B(OMe)3
B(OH)3
MeO
B
Figure 9. Solid-state structure of 14a. Selected bond lengths [Å] and angles [°]: O1 B1
1.370(1), O1 C1 1.388(1), O2 B1 1.365(2), O2 C3 1.381(1), O3 B2 1.374(1), O3 C11
1.382(1), O4 B2 1.370(1), O4 C13 1.379(1), O5 B2 1.352(1), O5 B1 1.364(1), B2 O5 B1
133.03(9), O5 B1 O2 116.09(9), O5 B1 O1 120.13(10), O2 B1 O1 123.75(10), O5 B2 O4
116.60(10), O5 B2 O3 120.12(10), O4 B2 O3 123.19(10).
10a
(98%)
(93%)
B(NMe2)3
O
O
O
O
HO
B
B
H2NMe2
O
O
Diboronic esters such as B2Pin2 and B2Cat2 have been utilized
extensively as borylating reagents in organic reactions involving
the formation of C-B bonds.18 Initial attempts to synthesize the
corresponding diborane 16a directly via the reaction of 1,8-
naphthalenediol with (HO)2B-B(OH)2 failed; instead the
protonated spirocyclic borate 15a was formed as the major
product along with B(OH)3 according to NMR spectroscopic
studies (Scheme 5). Following an alternative protocol reported
by Marder and co-workers19, 1,8-naphthalenediol was reacted
with (Me2N)2B-B(NMe2)2 to give a white precipitate of the
diborane dimethylamine adduct 17a. Subsequent treatment of
17a with ethereal HCl gave rise to the formation to the donor
free diboronic ester 16a in overall yields of ca. 70%. Compound
16a is a thermally stable colourless solid that is insoluble in
aliphatic and aromatic hydrocarbons and diethyl ether,
sparingly soluble in THF but shows good solubility in DMSO.
11a
12a
(96%)
Scheme 4. Synthesis of 9a-13a.
The addition of an two-fold excess of BCl3 to a CH2Cl2 solution
of 1,8-naphthalenediol quantitatively generated after removal
of solvent under vacuum chloro borate 13a in 95% yield
(Scheme 4). In contrast to the above described compounds, 13a
is an extremely air and moisture sensitive colourless solid that
can be stored under nitrogen for a few weeks without showing
notable signs of decomposition. However, when solutions of
13a in dry C6D6 were exposed air and moisture, crystals slowly
formed over time, which by NMR spectroscopy and X-ray
analysis were identified as a mixture of boroxane 14a (Figure 9)
and hydroxy borate 11a.
4 | J. Name., 2012, 00, 1-3
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All borate esters have been characterized by multiVnieuwcAlertiaclreNOnMlinRe
11
spectroscopy and combustion analysis. FoDrOcI:o1m0.1p0a3r9i/sDo0nD,Tt0h0e745BE
HO
HO
O
O
O
O
B2(OH)4
NMR chemical shifts are listed in Table 2. The 11B NMR chemical
shifts of 1a-11a and 14a are in the typical range for tri-
coordinate boron compounds with central OBO2 and CBO2
units. The 11B NMR signals of 1a-9a appear within the relatively
narrow range of 27-32 ppm, and are slightly shifted to higher
field relative to their catechol ester analogues 1b-9b. Likewise,
the tri-coordinate borates 10a, 11a and 14a display 31B NMR
signals within a narrow range (17-18 ppm), and are high-field
shifted relative to their catechol analogues 10b, 11b and 14b.
B
H
15a
B2(NMe2)4
(not isolated)
NHMe2
O
O
O
O
O
O
O
2 HCl
B
B
B
B
O
Me2HN
17a
16a
(~70%)
Table 2. 11B NMR chemical shifts [ in ppm] of R-BNad (1a-17a) and RBCat
(1b-17b).a
Scheme 5. Synthesis of 16a.
The solid-state structures of 16a (Figure 10) and 17a (Figure 11)
were determined by X-ray crystallography; suitable single-
crystals were grown from THF, resp. For 16a, each Bnad moiety
is virtually planar and, the two of them in each molecule are
coplanar (O1-B1-B1-O1, 0°). As expected, the B-B distance 16a
with 1.699(3) Å is significantly shorter than that of 17a with
1.739(2) Å, where both boron centres are tetra-coordinated.
Note also that the B-B distances of 16a is comparable to those
of other oxy-functionalized diboranes such as B2(OH)4 [1.715
R-BNad
11B
R-BCat
11B
C6H5-Bnad (1a)
27.9 C6H5-Bcat (1b)
30.7c
29.1c
30.8c
29.7c
29.9c
30.7
C6F5-Bnad (2a)
27.9 C6F5-Bcat (2b)
3,4,5-F3-C6H2-Bnad (3a)
2,4,6-F3-C6H2-Bnad (4a)
2,6-F2-C6H3-Bnad (5a)
2,6-Cl2-C6H3-Bnad (6a)
2,4,6-Me3-C6H2-Bnad (7a) 29.8 2,4,6-Me3-C6H2-Bcat (7b) 32.9d
Bun-Bnad (9a)
27.0 3,4,5-F3-C6H2-Bcat (3b)
27.2 2,4,6-F3-C6H2-Bcat (4b)
27.2 2,6-F2-C6H3-Bcat (5b)
27.5 2,6-Cl2-C6H3-Bcat (6b)
32.0 Bun-Bcat (9b)
17.8 MeO-Bcat (10b)
18.4 HO-Bcat (11b)
-
Å]20, B2neop2 [1.712 Å]21, B2pin2 [1.704 Å]22 and B2cat2 [1.678 Å].
MeO-Bnad (10a)
HO-Bnad (11a)
23.4e
22.9f
14.0g
28.8h
22.4i
31.6j
-
23
[H2NMe2][B(nad)2] (12a)
Cl-Bnad (13a)
0.5b
[H2NMe2][B(cat)2] (12b)
24.8 Cl-Bcat (13b)
17.0 catB-O-Bcat (14b)
5.4b
1.4b
nadB-O-Bnad (14a)
nadB-BNad (16a)
[Bnad(HNMe2)]2 (17a)
catB-Bcat (16b)
[Bcat(HNMe2)]2 (17b)
a
nad = 1,8-naphthalenediolate; measured in CDCl3 if not otherwise
stated; b DMSO-D6; c ref. 24; d ref. 25; e ref. 26, C6D6; f ref. 27, C6D6; g ref.
19; C6D6/CH3CN; h CD2Cl2; i ref. 26, C6D6; j ref. 19, CD2Cl2;
We next set out to determine the relative Lewis acid strength of
the newly prepared esters 1a-7a via the classical Gutmann-
Beckett method.24, 28-30 In this method, O=PEt3 is combined with
Figure 10. Solid-state structure of 16a (for clarity positional disorder not shown).
Selected bond lengths [Å] and angles [°]: B1 B1 1.699(3), O1 B1 1.373(2), O2 B1 1.376(2),
O1 C1 1.385(2), O2 C3 1.386(2), C1 C2 1.415(2), C2 C3 1.416(2), B1 O1 C1 120.43(12), B1
O2 C3 120.19(12), O1 C1 C2 118.55(13), O1 B1 O2 121.69(13), O1 B1 B1 119.20(16), O2
B1 B1 119.11(16).
the corresponding Lewis acid in benzene and the change in 31
P
NMR chemical shift for O=PEt3 is measured and the acceptor
numbers (ANs) are then calculated according to the formula
shown in Table 3. Generally, higher ANs indicate higher Lewis
acid strength. For comparison, the results for 1a-7a as well as
for the respective catechol derivatives 1b-7b are listed in Table
3 for C6D6 as solvent (for more details see Tables S1 and S2).
Both classes of compounds display similar ANs, suggesting
similar electronic contributions from both the catecholate and
the 1,8-naphthalenediolate ligand toward the central electron
deficient boron. Furthermore, the Lewis acidity appears to
correlate reasonably well with the electronic properties of the
X-aryl group, e. g. electron-withdrawing X groups increase the
AN, while electron donating groups lower it. As expected, and
as is the case for both classes of boronic acid esters the AN
increases with increasing the number of electron-withdrawing
fluorine atoms attached to the aryl substituent. However, some
discrepancies were also noted demonstrating the limitation of
Figure 11. Solid-state structure of 17a. Selected bond lengths [Å] and angles [°]: B1 B1
1.739(2), O1 C1 1.352(1), O1 B1 1.482(1), O2 C3 1.352(1), O2 B1 1.470(1), N1 C11
1.482(2), N1 C12 1.487(2), N1 B1 1.680(2), O2 B1 B1 112.61(11), O1 B1 B1 112.03(11),
N1 B1 B1 112.05(11), O2 B1 O1 112.74(9), O2 B1 N1 103.21(8), O1 B1 N1 103.51(8).
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using ANs as a means of quantifying the Lewis acidity of chemical shift is fairly close to that of the in situ generated
View Article Online
boranes.
boronium salts [F2B(O=PEt3)2]OTf33 (84.D8OpI:p10m.10i3n9/CDD0DCTl30)07a4n5dE
[CatB(O=PEt3)2]OTf27 (83.9 ppm in CH2Cl2) suggesting similar
Lewis acidities of the cationic boron centres in all three species.
On the other hand, the 31P NMR signal of O=PBu3 in [9-
Table 3. Acceptor numbers (ANs)a of selected boronic acid esters.
34
BBN(O=PBu3)2]NTf2 with 71.9 ppm (CH2Cl2) is markedly up-
R-BNad
AN
R-BCat
AN
field shifted relative to 18a. This may be attributed to the more
sterically crowded BBN framework resulting in weaker donor-
acceptor interactions of two the O=PEt3 units with the boron
cation. On the other hand, the 31P NMR signal of O=PEt3 in [8-
C6H5-Bnad (1a)
C6F5-Bnad (2a)
3,4,5-F3-C6H2-Bnad (3a)
2,4,6-F3-C6H2-Bnad (4a)
2,6-F2-C6H3-Bnad (5a)
2,6-Cl2-C6H3-Bnad (6a)
2,4,6-Me3-C6H2-Bnad (7a) 11.8 2,4,6-Me3-C6H2-Bcat (7b) 12.2
Cl-Bnad (13a)
nad = 1,8-naphthalenediolate; The 31P NMR chemical shifts were
measured in C6D6. The respective acceptor numbers were calculated as
follows: AN = (δ31P − 41.0) × (100/(86.1 − 41.0).29
59.6 C6H5-Bcat (1b)
80.9 C6F5-Bcat (2b)
72.7 3,4,5-F3-C6H2-Bcat (3b)
73.8 2,4,6-F3-C6H2-Bcat (4b)
72.3 2,6-F2-C6H3-Bcat (5b)
53.7 2,6-Cl2-C6H3-Bcat (6b)
62.5
78.9
75.2
73.4
73.0
64.8
35
Ph2P-C10H6B(O=PEt3)2][NTf2]2 with 90.8 ppm (CD2Cl2) is
significantly down-field shifted relative to 18a, which can be
well understood in terms of the “dicationic” nature of the boron
centre. The results of the X-ray analysis of 18a (Figure 12)
further corroborated the structural assignment of a tetrahedral
boronium cation with a fully charge separated chloride anion.
Nonetheless, despite the formally positive charge at boron, the
B-O ring distances with 1.447(2) and 1.443(2) Å are longer than
those of the esters 2a-5a with trigonal planar coordination
environment for boron. Only in the tetra-coordinated diborane
17a, are the B-O ring distances [1.482(1) and 1.470(1) Å]
somewhat longer.
89.7 Cl-Bcat (13b)
87.5
a
For example, the 2,6-dichlorophenyl substituted esters 6a and
6b show significantly lower ANs than the 2,6-difluorophenyl
derivatives 5a and 5b. Note also that 5a (AN = 72.3) and 5b
(73.0) have almost identical ANs, while the ANs for 6a (AN =
53.7) and 6b (AN = 64.8) are markedly different. The reason for
these inconsistencies appears to be steric in nature, and it may
very well be concluded that the 1,8-naphthalenediolate ligand
requires somewhat more space than its catecholate
counterpart.
Apparently, due to the lack of appreciable levels of steric
hindrance, fairly similar ANs were found for the chloro borates
13a (AN = 89.7) and 13b (AN = 87.5). Note also that both values
are significantly higher than those of the strong and relatively
“soft” Lewis acid B(C6F5)3 (AN = 77.6)30,31 but similar to that of
the “hard” Lewis acid B(OC6F5)3 (AN = 86.5)32 and markedly
lower than Ingleson’s adduct [CatBOTf(O=PEt3)] with a 31P NMR
chemical shift of 31P = 85.4 ppm (CH2Cl2) (AN = 98).27 We further
noticed that upon adding two equivalents of O=PEt3 to a C6D6
solution of 13a, a crystalline precipitate formed, which by multi-
nuclear NMR spectroscopy and X-ray analysis was identified as
the boronium salt 18a (Scheme 6). Comparison of the integrals
of the aromatic signals with those of the ethyl signals of O=PEt3
in the 1H NMR spectrum of 18a indeed confirmed the presence
of two molecules of O=PEt3 per 1,8-naphthalenediolate unit.
Figure 12. Solid-state structure of 18a (chloride anion omitted for clarity). Selected bond
lengths [Å] and angles [°]: O2 B1 1.447(2), O3 B1 1.471(2), O4 B1 1.499(2), O1 B1
1.443(2), P1 O3 1.551(1), P2 O4 1.547(1), O1 C1 1.365(2), O2 C3 1.364(2), B1 O3 P1
136.14(10), B1 O4 P2 134.79(11), O1 B1 O2 115.53(14), O1 B1 O3 109.61(13), O2 B1 O3
107.70(14), O1 B1 O4 106.75(13), O2 B1 O4 109.31(12), O3 B1 O4 107.70(13).
Cl
B
Et3P
O
O
O
PEt3
Cl
B
O
O
O
+ 2 Et3P=O
The rapid formation of the boronium salt prompted us to
investigate the Lewis acid-base behaviour of 13a (Scheme 7).
Thus, upon adding one equivalent of dry pyridine to 13a, the
expected Lewis acid-base adduct 19a was obtained as a
colourless solid material in almost quantitative yields. Addition
of a second equivalent of pyridine gave rise to a change of the
NMR spectroscopic features of 19a indicating the formation of
the boronium species 20a being in equilibrium with 19a. Adding
a large excess of pyridine fully shifted the equilibrium towards
the boronium salt 20a, which could not be isolated but was
characterized in solution by NMR spectroscopy. Astonishingly,
18a
13a
Scheme 6. Formation of boronium chloride 18a.
The observed 31P and 11B NMR chemical shifts of 83.9 ppm and
0.1 ppm (CD2Cl2), respectively, suggest tetrahedral
coordination environment for the central boron cation with two
O=PEt3 coordinating. It is worthwhile noting that its 31P NMR
a
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ARTICLE
reacting two equiv. of the stronger donor DMAP with 13a,
enabled the formation and isolation of boronium salt 22a as a
colourless solid in good yields. However, 22a is thermally and
moisture sensitive and susceptible to partial loss of DMAP with
formation of 21a upon drying under vacuum for extensive
periods of time. Hoping to circumvent the loss of donor from
the boronium cation, 13a was reacted with equimolar amounts
of 2,2’-bipyridine and 1,10-phenanthroline, resp., in toluene as
solvent. In both reactions reddish-orange crystals precipitated,
which by multi-nuclear NMR spectroscopy were identified as
the boronium salts 23a and 24a. As in the previous case, both
compounds are heat and moisture sensitive but can be stored
at room temperature under nitrogen over several weeks
without decomposition.
View Article Online
DOI: 10.1039/D0DT00745E
X
X
X
Figure 13. Solid-state structure of 23a (chloride anions and disordered 11a omitted for
clarity). Selected bond lengths [Å] and angles [°]: O1 B1 1.418(2), O2 B1 1.422(2), O3 B2
1.423(2), O4 B2 1.416(2), N1 B1 1.610(2), N2 B1 1.585(2), N3 B2 1.582(2), N4 B2 1.615(2),
O1 C1 1.371(2), O2 C3 1.365(2), O3 C21 1.364(2), O4 C23 1.368(2), O5 B3 O6 117.7(4),
O5 B3 O7 121.0(4), O6 B3 O7 121.3(5), O1 B1 O2 118.62(14), O1 B1 N2 110.67(15), O2
B1 N2 108.76(14), O1 B1 N1 110.04(14), O2 B1 N1 111.25(15), N2 B1 N1 94.95(11), O4
B2 O3 117.86(14), O4 B2 N3 111.44(15), O3 B2 N3 108.35(14), O4 B2 N4 109.07(14), O3
B2 N4 112.42(15), N3 B2 N4 95.45(12).
Cl
B
N
N
N
Cl
O
B
B
Cl
O
O
O
O
O
X-pyridine
X-pyridine
19a
X = H (70%)
20a
22a
13a
X = H (detected)
X = NMe2 (85%)
21a
X = NMe2 (detected)
phenanthroline
2,2-bipyridine
Conclusions
Inspired by the design of robust Lewis acids for potential
applications in Lewis acid catalysed transformations, the 1,8-
naphthalenediolate-supported boronic and boric acid esters 1a-
11a and 13a as well as diboronic ester 16a were prepared from
reactions of commercially available boron compounds with 1,8-
naphthalendiol. In addition, a series of highly sensitive but room
temperature stable boronium salts were synthesized via
treatment of chloro borate 13a with the neutral donors O=PEt3,
N
N
N
N
B
B
Cl
Cl
O
O
O
O
23a
24a
(90%)
(86%)
Scheme 7. Lewis acid base reactions of 13a with nitrogen based donor molecules.
DMAP, 1,10-phenanthroline and 2,2’-bipyridine, resp.
A
comparison of the hydrolytic stability of boronic acid ester 1a
with its catechol counterpart 1b and ligand exchange
experiments suggest the 1,8-naphthalenediolate esters to have
greater thermodynamic and hydrolytic stabilities than their
catecholate analogues, most likely owing to differences in ring
sizes resulting in different ring strain. The lower ring strain in
the 1,8-naphthalenediolate esters does not seem to negatively
affect their Lewis acidities, as the acceptor numbers determined
by the Gutman-Beckett method showed similar values for both
classes of boronic acid esters. Applications of these new boron-
containing Lewis acids in catalytic organic transformations are
currently undergoing in our lab.
Surprisingly few examples of well-characterized boronium
salts36 supported by chelating diolate ligands are documented
in the literature. Among those, most notable are
[CatB(bipy)]ClO437, [PinB(DMAP)2]Br38, [PinB(py)2]OTf39, and the
boroxine-derived salt [(py)4B3O3][AlBr4]2[Al2Br7]40, of which only
the latter two compounds have been structurally characterized
by X-ray crystallography. Attempts to grow crystals of the
boronium salts 22a-24a suitable for X-ray analysis were
complicated by their extreme moisture sensitivity and tendency
to form microcrystalline materials or powders. Only for 23a was
it possible to determine its solid state structure by X-ray
crystallography (Figure 13); suitable crystals were grown from
benzene. Due to partial hydrolysis during crystal growth, the
unit cell contains two molecules of 23a and one molecule of
highly disordered 11a. Nonetheless, the results of the X-ray
analysis clearly confirmed chelation of the 2,2’-bipyridine unit
to the boron cation, and the chloride anion being fully
dissociated. This leads to a distorted tetrahedral coordination
environment for boron with an average B-0 ring distance of ca.
1.42 Å being slightly shorter than that of the boronium salt 18a
with ca. 1.46 Å.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The majority of this research was supported by the U.S.
Department of Energy, Office of Science, Office of Basic Energy
Sciences, Catalysis Science Program, under Award DE-
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17 I. D. Madura, K. Czerwinska, M. Jakubczyk, A. Pawelko, A.
SC0019094. Parts were also funded by the NSF (grant no.
1407681; Project SusChEM: IUPAC) as part of the IUPAC
International Funding Call on “Novel Molecular and
Supramolecular Theory and Synthesis Approaches for
Sustainable Catalysis”
View Article Online
Adamczyk-Wozniak and A. SporzynsDkOi,I:C10ry.1s0t3a9l/DG0rDoTw00th745&E
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Kleeberg, U. Radius and T. B. Marder, Dalton Trans., 2017, 46,
3661-3680.
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DOI: 10.1039/D0DT00745E
TOC
New 1,8-Naphthalenediolate supported Boronic, Diboronic and Boric Acid Esters and
Boronium salts have been synthesized and structurally characterized.