Substituent effects on the Lewis acidity of 4,6-di-tert-butylchatechol boronate esters

Article abstract of DOI:10.1016/j.tet.2018.11.033

In studying the factors which contribute to the Lewis acidity of organoboron compounds we investigated approaches to the design of robust, novel Lewis acids purposed for metal-free catalysis. Based on a sterically encumbered catechol motif, a series of boronate esters are shown to demonstrate modest Lewis acidities for the conventional Gutmann-Beckett test as an inquisitive investigation.

Full text of DOI:10.1016/j.tet.2018.11.033

Accepted Manuscript
Substituent effects on the Lewis acidity of 4,6-di-tert-butylchatechol boronate esters
Jordan N. Bentley, Christopher B. Caputo
PII:
S0040-4020(18)31373-5
https://doi.org/10.1016/j.tet.2018.11.033
TET 29944
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 20 October 2018
Revised Date: 12 November 2018
Accepted Date: 14 November 2018
Please cite this article as: Bentley JN, Caputo CB, Substituent effects on the Lewis acidity of 4,6-
di-tert-butylchatechol boronate esters, Tetrahedron (2018), doi: https://doi.org/10.1016/j.tet.2018.11.033.
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Substituent Effects on the Lewis Acidity of
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4,6-di-tert-butylcatechol Boronate Esters.
Jordan N. Bentley, and Christopher B. Caputo
Department of Chemistry, York University, 4700 Keele Street, Toronto, Ontario, M3J1P3, Canada.

1
ACCEPTED MANUSCRIPT
Tetrahedron
journal homepage: www.elsevier.com
Substituent Effects on the Lewis Acidity of 4,6-Di-tert-butylchatechol Boronate
Esters
Jordan N. Bentleya, and Christopher B. Caputoa
a Department of Chemistry, York University, 4700 Keele Street, Toronto, Ontario, M3J1P3, Canada.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
In studying the factors which contribute to the Lewis acidity of organoboron compounds we
investigated approaches to the design of robust, novel Lewis acids purposed for metal-free
catalysis. Based on a sterically encumbered catechol motif, a series of boronate esters are shown
to demonstrate modest Lewis acidities for the conventional Gutmann-Beckett test as an
inquisitive investigation.
Available online
2018 Elsevier Ltd. All rights reserved
.
Keywords:
Lewis Acid
Boron
Fluoride Ion Affinity
Gutmann-Beckett Method
Catalysis
———
Corresponding author. e-mail: caputo@yorku.ca

2
Tetrahedron
A general goal is not to necessarily synthesize the strongest
ACCEPTED MANUSCRIPT
Lewis acid, rather it is to determine the appropriate Lewis acid
that is most selective for a specific substrate or chemical
transformation. Contrary to Brønsted acids (which have a well-
defined, universal pKa scale), a discreet and direct method to
quantify Lewis acidity remains a topic of discussion. One such
method is the Childs Method, which indicates the strength of a
Lewis acid by measuring the change in chemical shift of the β-
1. Introduction
Main-group Lewis acids are widely applied in modern
synthetic chemistry to facilitate organic transformations.1 Cheap,
commercially-available group 13 trihalides such as BF3 and
AlCl3, as well as simple organoboranes are widely utilized as
activators and catalysts. These species typically serve to enhance
electrophilicity of reactive sites on the substrate by drawing
electron density from Lewis basic moieties or unsaturation.
Recently, more complex main group Lewis acid species have
drawn attention for their ability to mediate a wider range of
transformations. Credited to relatively recent advancements in
frustrated Lewis pair (FLP) chemistry,2 low-valent main-group,3
and hypervalent species such as phosphonium and stibonium
cations,4,5 p-block Lewis acid development has seen exponential
growth in the 21st century. These new compounds demonstrate
unprecedented reactivities, primarily owed to the Lewis acidity of
the active center, and have found significant application in metal-
free transformations. Our group and many others have
investigated the application of borane Lewis acids in catalysis
and a select few are highlighted in Scheme 1.5-9
1
proton of crotonaldehyde in the H NMR spectrum before and
after binding a Lewis acid.10 The Gutmann-Beckett method is
another spectroscopic technique, which uses Et3PO as a probe
molecule and investigates the chemical shift change in the 31P
NMR spectrum upon coordination to a Lewis acid; reported as
unitless scalar values defined as an acceptor number (AN).11,12
Finally, a measure of fluoride ion affinity (FIA) can be performed
in silico or by comparing the isodesmic product of a fluoride
adduct with the Lewis acid in question to provide a relative
measure of affinity.13
The most commonly employed Lewis acids in main-group
catalysis are from the family of sterically hindered,
perhalogenated boranes, such as B(C6F5)3. Strides have been
made to improve the ambient stability of these Lewis acids.
Work from the Ashley lab has shown that combination of
dioxane and B(C6F5)3 can tolerate water in hydrogenation
catalysis.14 Soos and co-workers also discovered that a water-
tolerant borane, B(p-C6F4H)2(o-C6Cl2H3), could be synthesized
and applied as a catalyst by altering the halogen substitution
pattern on the aromatic rings.15 Nevertheless, most Lewis acid
catalysts are still notoriously sensitive towards air and moisture,
requiring handling under inert atmospheres. Substitution of an
aryl-group with a heteroatom substituent (such as O or N) will
diminish the Lewis acidity of the boron center through lone-pair
donation, but can increase the relative stability. To that end, the
use of catechol substituents on main-group Lewis acids has
recently garnered interest. Derivatives of catechol boronate esters
have been extensively utilized in Suzuki-Miyaura cross-coupling
reactions due to their advantageous cost, stability and ease of
preparation.16 Greb and co-workers have shown that
perchlorinated catechol substituents on a silicon center generate
the neutral Lewis super acid Si(O2C6Cl4)2, capable of activating
Sb–F bonds in the SbF6 anion.17 We were inspired by the bulky
3,5-di-tert-butylcatechol ligands used by Westcott and co-
workers in their investigations into the synthesis and applications
of arylspiroborates.18,19 Therefore, to modulate the Lewis acid
strength, while keeping a sterically bulky ligand, we endeavoured
to explore the use of 3,5-di-tert-butylcatechol as a ligand for the
synthesis of a series of boronate esters (Scheme 2). Herein we
describe the impact that the third substituent has on the Lewis
acidity of these boronate esters.
Scheme 1 – A selection of Lewis acid catalyzed transformations.
Strong Lewis acids are more widely applicable in catalysis
and are generally achieved by the introduction of electron
withdrawing
excessively strong Lewis acids can lead to issues of
chemoselectivity or diminish turnover numbers.
Chemoselectivity issues often result from indiscriminate
interactions with donor atoms on the substrate, while an
irreversibly strong association of the Lewis acid center to the
substrate can have deleterious effects on catalytic turnover. The
latter issue can be mitigated to an extent by using sterically
hindered substrates (as in FLP catalysis). Furthermore, Lewis
acids inherently bear empty, low-lying acceptor orbitals, making
them often susceptible to detrimental association with water or
coordinating solvents, which limits their practical application.
Therefore, there is significant interest in synthesizing Lewis acids
that are stable towards ambient conditions, yet acidic enough to
promote chemical reactivity.
2. Results and Discussion
pentafluorophenyl
substituents.
However,
To explore the effects of an aryl substituent on the Lewis acidity
of bulky boronate esters, we synthesized a series of 4,6-di-tert-
butylcatechol boronate esters using standard procedures via a
dehydration reaction from the corresponding boronic acid and
diol.20 The aryl substituents chosen for this study were phenyl, 4-
methoxyphenyl, 4-(trifluoromethyl)phenyl, 4-nitrophenyl, and
pentafluorophenyl groups. These represent
a
variety of
substituents containing both electron-donating and electron-
withdrawing functionalities. Starting from the corresponding
arylboronic acid, a condensation reaction was performed with
3,5-di-tert-butylcatechol to produce the corresponding boronate
esters (1-5) in excellent yields (60-99%) (Scheme 2).

3
trifluoromethyl substituent has a greater impact on the Lewis
ACCEPTED MANUSCRIPT
acidity than a nitro substituent. Finally, 5 was found to be the
strongest Lewis acid of the series as it bears a pentafluorophenyl
substituent. The 31P NMR chemical shift of the Et3PO adduct was
66.7 ppm (AN=56.7). In this case, the bulkier C6F5 substituent
hinders the free rotation of the Et3PO adduct, thus resulting in a
slightly broader resonance. These data are compiled in Table 1.
Scheme 2 – Synthesis of boronate ester series.
Multinuclear NMR spectroscopy was used to characterize the
1
products. The H NMR spectrum of compound 1 showed two
distinct t-butyl resonances at 1.55 ppm and 1.40 ppm in addition
to the phenyl-resonances from the boronic acid. The 11B NMR
spectrum of 1 exhibited a broad resonance at 32.8 ppm,
characteristic of tricoordinate boronate esters. Similarly,
compounds 2-5 also displayed inequivalent t-butyl resonances in
1
the corresponding H NMR spectra between 1.30 – 1.57 ppm.
Evidence of the condensation products was apparent for
compounds 2-5 in the 11B NMR with broad resonances observed
between 29 – 31 ppm. These results show the aryl substituent on
the boronate ester does not have a significant impact on the 11B
NMR resonance. Compounds 2-4 all indicated that a para-
substituted aromatic ring was present in the products with two
apparent doublets observed in the aromatic region in the 1H NMR
spectra. Finally, the 19F NMR spectrum for compound 5 showed
three resonances for the ortho-, para-, and meta-fluorine atoms
of the pentafluorophenyl ring at -128.15, -146.85, and -160.81
ppm, respectively. The large ꢀ δpara-meta indicates that a three-
coordinate boron center is present and implies that there could be
a high Lewis acidity. Further evidence for the formation of 1-5
was provided by elemental analyses consistent with the expected
products in each case. All five compounds were found to be
relatively air and moisture stable in the solid state; even over a
period of months. This was encouraging, and we sought to
explore how Lewis acidic these compounds were.
Figure 1 – 31P NMR Stack plot of Gutmann-Beckett tests with 1-5.
The Gutmann-Beckett acceptor numbers align well with other
reported values in the literature.21 According to this method,
these Lewis acids are weaker than those which have been used in
Lewis acid catalysis. For example, B(C6F5)3 has an AN of 78.1,
which is significantly higher than any of those in the reported
series. Interestingly, 2, as having a p-OMe substituent on the
aromatic ring, shows similar Lewis acidity to trivalent alkyl
borates (B(OR)3).21 The remaining boronate esters show similar
Lewis acidity to those of other reported aryl boronate esters.21
The effects of the two t-butyl substituents are clear, the Gutmann-
Beckett AN has been reported for the catechol analogue,
C6F5B(O2C6H4) and was found to be 65.2, which is significantly
more acidic than the 56.7 we found for 5.22 Thus, the electron
donating abilities of the two t-butyl groups clearly have an
impact on the resultant Lewis acidity of the boron center.
GB 31P NMR δ (ppm)(AN) TBAF 19F NMR δ (ppm)
1
2
3
4
5
51.1 (22.2)
47.3 (13.9)
61.9 (46.2)
60.3 (42.6)
66.7 (56.7)
-133.6
-132.8
-134.5
-135.6
-123.9
Scheme 3 – Gutmann-Beckett method with 1-5.
We initially investigated the Lewis acidity of these species by
using the Gutmann-Beckett method. Free Et3PO has a chemical
shift of 41.0 ppm in the 31P NMR spectrum in CDCl3, and upon
reaction with 1-5 we see a significant change in the 31P NMR
resonance for the Et3PO probe (Figure 1). We utilized an excess
of the phosphine oxide (1.5 eq.) to favour adduct formation as
depicted in Scheme 3. The Et3PO adduct with 1 resulted in a 31P
NMR resonance at 51.1 ppm which correlates to an acceptor
number (AN) of 22.2. The acceptor number is calculated using a
scale where Et3PO has an AN=0 and SbCl5 has an AN=100 (δ
31P: 86.1).21 The addition of donor substituents (-OMe) to the aryl
substituent in 2 appears to decrease the Lewis acidity, as the
corresponding 31P NMR chemical shift of the adduct was found
to be 47.3 ppm (AN=13.9). The introduction of electron
withdrawing substituents in the para-position of the third
substituent (-CF3, 3; -NO2, 4) resulted in an increase in Lewis
acidity according to the Gutmann-Beckett method. The 31P NMR
chemical shift of the Et3PO adduct of 3 was 61.9 ppm (AN=46.2)
and 4 was 60.3 ppm (AN=42.6). These results imply that a
Table 1 – Lewis acidity measurements of 1-5.
In order to further investigate the Lewis acidity of these
boronate esters, we treated 1-5 with one equivalent of
tetrabutylammonium fluoride (TBAF) to observe if the boronate
esters were Lewis acidic enough to bind a fluoride anion.
Analyzing the reactions by 19F NMR spectroscopy indicated that
in all cases fluoride binding was occurring, forming the
corresponding ammonium fluoroborate salts (See Supporting
Information). Free TBAF appears at -112 ppm in the 19F NMR
spectrum and in all cases an upfield shift is observed upon
reaction with 1-5 (Table 1). We were able to isolate the fluoride
adduct of the most Lewis acidic species (5), as the
tetrabutylammonium fluoroborate salt, 6. The product was
evident through multinuclear NMR analysis, with an observed

4
Tetrahedron
ACCEPTED MANUSCRIPT
doublet resonance in the 11B NMR spectrum at 9.11 ppm (1JBF
=
In conclusion, we have synthesized a series of boronate esters
52.6 Hz), indicative of the formation of a tetracoordinate
fluoroborate anion. Furthermore, the 19F NMR spectrum shows
that the meta-para fluorine gap decreases, indicating the
formation of a 4-coordinate species (ꢀ13.96 ppm to ꢀ4.46 ppm).
Also observed in the 19F NMR spectrum is the B–F resonance at -
123.86 ppm. Finally, 6 was unambiguously identified by X-ray
crystallography as the tetrabutylammonium fluoroborate salt
(Figure 2). In the solid state the B–F bond length was found to be
1.416(2) Å, which is typical for a fluoroborate salt.23 The bond
angles around the boron atom sum to 328.3°, indicating a near
perfect tetrahedral geometry around the boron center.
using 3,5-di-tert-butylcatechol as a substituent and analyzed how
various functional groups on the second aromatic ring influence
the relative Lewis acidity. The trend shows that substituting
electron donating or withdrawing groups in the para position of
the aromatic ring has a significant influence on the relative Lewis
acidity, and through the Gutmann-Beckett method, found that the
Lewis acidity trend follows 5 > 3 > 4 > 1 > 2. We were able to
isolate and crystalize the fluoride adduct of the most Lewis acidic
species, bearing a pentafluorophenyl substituent (6). Research is
underway in our laboratory to explore the ability of these air
stable Lewis acids to act as catalysts.
3. Experimental section
All manipulations were performed using either an MBraun
LABstar Glove Box Workstation under N2 atmosphere or in a
fume hood. All glassware was dried in an oven at 110 °C before
being transferred into the glovebox. Solvents were prepared from
an MBraun MB-SPS 800 solvent drying system under N2
atmosphere using oven-heated glassware. Commercially
available reagents were purchased from either Sigma-Aldrich,
TCI Chemicals or Oakwood Chemicals and employed without
further purification; unless otherwise stated. Chloroform-d and
benzene-d6 were transferred to Strauss flasks and dried over
activated molecular sieves, then degassed with cycles of freeze-
pump-thawing, following transfer to the glovebox. Adduct
formation reactions were done in 20 mL scintillation vials with
appropriately sized, oven dried, Teflon stir bars. Experiments
monitored by NMR spectroscopy over time, were conducted in
NMR tubes (8” x 5 mm) sealed with standard plastic caps and
Figure 2 – Molecular structure of 6 (the tetrabutylammonium cation
and hydrogens are excluded for clarity). C: Black, O: Red, B: Pink,
F: Yellow/Green. Thermal ellipsoids at 50% probability.
1
wrapped with parafilm or J-Young NMR tubes (8” x 5 mm). H,
11B{1H}, 13C{1H}, and 19F NMR spectra were acquired at 25 °C
on either a Bruker 700 MHz Spectrometer, Bruker DRX 600
MHz Spectrometer, Bruker ARX 400 MHz Spectrometer
equipped with a variable temperature probe, or Bruker ARX 300
MHz Spectrometer. Chemical shifts are given relative to SiMe4
and referenced to the residual solvent signal (1H, 13C{1H}) either
to CDCl3 (δ 7.26, 77.16 ppm) respectively. 11B{1H} and 19F{1H}
NMR spectra were referenced relative to either 15% BF3-Et2O or
an internal reference such as starting material. NMR spectra were
analyzed using either TopSpin 3.2 or MestReNova 6.0.2-5475
software. Chemical shifts are reported in ppm and coupling
constants as scalar values in Hz. The conventional abbreviations
were used as follows: s (singlet), d (doublet), t (triplet), q
(quartet), dd (doublet of doublets), m (multiplet), br (broad).
Elemental Analysis was carried out on an ElementarVarioELcube
using VarioELcube software (V4.0.13), samples were run in
triplicate. Crystallographic data for the structure 6 has been
deposited with the Cambridge Crystallographic Data Center as
supplementary publication no. 1872813. Copies of the data can
be obtained free of charge on application to CCSC, 12 Union
Road, Cambridge, CB2 1EZ, UK (fax: (+44) 1223-336-033;
email: deposit@ccdc.cam.ac.uk)
Empirical Formula
Formula wt. (g mol-1)
C36H56BF6NO2
659.62
Monoclinic
P21/n
Crystal System
Space Group
a (Å)
14.1568(6)
15.1202(6)
17.6289(7)
90
b (Å)
c (Å)
α (°)
β (°)
99.166(2)
90
γ (°)
V (Å3)
3725.3
4
Z
d (mg m-3)
1.176
µ (mm-1)
0.761
General Procedure for the synthesis of 4,6-di-tert-
butylcatechol boronate esters (1-5)
Total Data
67834
6632
Unique Data
I > 2σ (I2)
5785
To a 250 mL round-bottom flask, 3,5-di-tert-butylcatechol
(1.6 mmol) and corresponding boronic acid (1.6 mmol) in
CH2Cl2 (100 mL), were stirred under ambient conditions for 12-
16 hrs. The reaction mixture was dried over MgSO4 and filtered.
Solvent was removed in vacuo yielding a crude oil. The crude
material was dissolved in minimal acetonitrile and crystallized in
Variables
425
R1
0.0462
0.1133
1.017
wR2
GOF
o
a freezer (-25 C). The precipitate was vacuum filtered to yield
the corresponding boronate ester. NMR spectra were obtained
Table 2 – Crystallographic data for 6.

5
from samples which have been stored on the bench for extended
periods of time.
MHz, CDCl3) δ 29.24; 19F NMR (376 MHz, CDCl3) δ -128.15,
ACCEPTED MANUSCRIPT
-146.85, -160.81; 13C NMR (101 MHz, CDCl3) δ 150.27, 147.79,
146.89, 144.14, 143.42, 137.75, 135.72, 117.49, 108.18, 35.26,
34.59, 31.91, 29.83. EA for C20H20O2BF5: Expected 60.33% C,
5.06% H, 0% N. Found 62.69% C, 4.72% H, 0.11% N.
(1) 4,6-di-tert-butyl-2-phenylbenzo[d][1,3,2]dioxaborole:
Following the general procedures, 3,5-di-tert-butylcatechol
(0.3604 g, 1.6 mmol) and phenylboronic acid (0.1954 g, 1.6
mmol) was stirred for 12 hrs and the product isolated in 84.9 %
(6) Tetrabutylammonium 4,6-di-tert-butyl-2-fluoro-2-
(perfluorophenyl)benzo[d][1,3,2]dioxaborol-2-uide
1
3
(0.4186 g) yield. H NMR (400 MHz; CDCl3): δ 8.13 (d, JHH
=
3
3
7.3 Hz, 2H), 7.59 (t, JHH = 7.0 Hz, 1H), 7.52 (t, JHH = 7.2 Hz,
2H), 7.28 (s, 1H), 7.13 (s, 1H), 1.55 (s, 9H), 1.40 (s, 9H); 11B
NMR (128 MHz, CDCl3) δ 32.79; 13C NMR (101 MHz, CDCl3) δ
148.60, 145.88, 144.26, 135.12, 135.04, 132.24, 128.32, 116.66,
107.78, 35.15, 34.62, 31.96, 29.96. EA for C20H25O2B: Expected
77.94% C, 8.18% H, 0% N. Found 80.11% C, 8.05% H, 0.16%
N.
Compound 5 (47.8 mg, 0.12 mmol) was added to TBAF (31.4
mg, 0.12 mmol) in benzene (1 mL) and CHCl3 (0.2 mL). After
stirring for 1 hr, the solution was triturated with pentanes. The
precipitated was vacuum filtered, yielding a pale-yellow powder
in 34.4% (0.0272 g) yield. 1H NMR (400 MHz, CDCl3) δ 7.28 (s,
1H), 6.60 (s, 1H), 6.59 (m, 8H), 3.17 (m, 8H), 1.56 (m, 8H), 1.43
(s, 9H), 1.36 (m, 8H), 1.27 (s, 9H), 0.96 (m, 12H); 11B NMR (128
1
MHz, CDCl3) δ 9.11 (d, JBF = 52.6 Hz) 19F NMR (376 MHz,
(2):4,6-di-tert-butyl-2-(4-methoxyphenyl)benzo[d][1,3,2]-
dioxaborole:
CDCl3) δ -123.86, -133.75, -161.23, -165.69; 13C NMR (101
MHz, CDCl3) δ 151.61, 148.08, 147.67, 140.34, 138.89, 136.71,
129.90, 111.51, 104.25, 58.51, 34.42, 34.11, 32.09, 29.77, 23.86,
19.58, 13.59. EA for C36H56O2NBF6: Expected 65.55% C, 8.56%
H, 2.12% N. Found 68.53% C, 8.86% H, 2.93% N.
Following the general procedures, 3,5-di-tert-butylcatechol
(0.3604 g, 1.6 mmol) and 4-methoxyphenylboronic acid (0.2434
g, 1.6 mmol) was stirred for 12 hrs and the product isolated in
1
95.0 % (0.5141 g) yield. H NMR (400 MHz; CDCl3): δ 8.10 (d,
General Procedure for the probing of the Lewis acidity of 1-5
using the Gutmann-Beckett method
4
4
3JHH = 8.5 Hz, 2H), 7.28 (d, JHH = 1.7 Hz, 1H), 7.13 (d, JHH
=
3
1.7 Hz, 1H), 7.05 (d, JHH = 8.1 Hz, 2H), 3.90 (s, 3H), 1.57 (s,
9H), 1.41 (s, 9H); 11B NMR (128 MHz, CDCl3) δ 31.33; 13C
NMR (101 MHz, CDCl3) δ 162.95, 148.69, 145.65, 144.34,
136.87, 134.93, 116.47, 113.96, 107.69, 107.61, 55.26, 35.11,
34.74, 31.98, 29.97. EA for C21H27 O3B: Expected 74.57% C,
8.05% H, 0% N. Found 76.97% C, 7.89% H, 0.24% N.
Under N2 atmosphere, Et3PO (1.5 eq.) was added to the
corresponding boronate ester 1-5 (0.2 mmol). The mixture was
warmed, melting the Et3PO, followed by the addition of C6D6 and
sealed under N2 in an NMR tube for NMR spectroscopy.
General Procedure for the probing of the Lewis acidity of 1-5
using Tetrabutylammonium fluoride
(3):4,6-di-tert-butyl-2-(4-(trifluoromethyl)phenyl)benzo[d][1,3,2]
dioxaborole:
Under N2 atmosphere, TBAF (1.5 eq.) was added to the
corresponding boronate ester 1-5 (0.2 mmol) and dissolved in
C6D6, and an internal standard of α,α,α-trifluoromethyltoluene
was added. The mixture was then sealed under N2 in an NMR
tube for NMR spectroscopy.
Following the general procedures, 3,5-di-tert-butylcatechol
(0.3604 g, 1.6 mmol) and 4-trifluoromethylphenylboronic acid
(0.3041 g, 1.6 mmol) was stirred for 12 hrs and the product
isolated in 90.9 % (0.5472 g) yield. 1H NMR (400 MHz; CDCl3):
δ 8.27 (d, 3JHH = 7.9 Hz, 2H), 7.79 (d, 3JHH = 7.9 Hz, 2H), 7.34 (s,
1H), 7.22 (s, 1H), 1.60 (s, 9H), 1.45 (s, 9H);z 19F NMR (376
MHz, CDCl3) δ -63.03; 11B NMR (128 MHz, CDCl3) δ 31.05;
13C NMR (101 MHz, CDCl3) δ 148.54, 146.41, 144.19, 135.44,
X-ray Data Collection, Reduction, Solution and Refinement
Single crystals were coated in Paratone-N oil in the glove-box,
mounted on a MiTegen Micromount and placed under an N2
stream. The data were collected on a Bruker Apex II
diffractometer. The data were collected at 150(±2) K for all
crystals. Data reduction was performed using the SAINT
software package, and an absorption correction was applied using
SADABS. The structures were solved by direct methods using
XS and refined by full-matrix least squares on F2 using XL as
implemented in the SHELXTL suite of programs. All
nonhydrogen atoms were refined anisotropically. Carbon-bound
hydrogen atoms were placed in calculated positions using an
appropriate riding model and coupled isotropic temperature
factors.
2
135.32, 133.84 (q, JCF = 32.6 Hz), 130.54 (m), 125.05, 124.21
(q, 1JCF = 270.8 Hz), 120.15, 117.10, 108.00, 35.22, 34.70, 31.96,
30.03. EA for C21H24O2BF3: Expected 67.04% C, 6.43% H.
Found 68.93% C, 6.01% H, 0.16% N.
(4):4,6-di-tert-butyl-2-(4-nitrophenyl)benzo[d][ 1,3,2 ]dioxaboro-
le:
Following the general procedures, 3,5-di-tert-butylcatechol
(0.3604 g, 1.6 mmol) and 4-nitrophenylboronic acid (0.2673 g,
1.6 mmol) was stirred for 12 hrs and the product isolated in 59.5
1
3
% (0.5651 g) yield. H NMR (400 MHz; CDCl3): δ 8.31 (d, JHH
3
= 8.4 Hz, 2H), 8.26 (d, JHH = 8.1 Hz, 2H), 7.28 (s, 1H), 7.17 (s,
1H), 1.55 (s, 9H), 1.40 (s, 9H); 11B NMR (128 MHz, CDCl3) δ
31.38; 13C NMR (101 MHz, CDCl3) δ 150.34, 148.31, 146.58,
143.96, 135.85, 135.46, 123.04, 117.25, 107.97, 35.16, 34.62,
31.88, 29.95. EA for C20H24O4B: Expected 68.01% C, 6.85% H,
3.97% N. Found 69.45% C, 6.58% H, 4.16% N.
Acknowledgments
The authors gratefully acknowledge financial support from
NSERC of Canada and the Canadian Foundation for
Innovation. JNB is appreciative of a Dalton Pharma Services
Award in Organic Chemistry. CBC is grateful for the award
of a Canada Research Chair. The authors thank Dr. Alan
Lough for his assistance with X-Ray Crystallography.
(5):4,6-di-tert-butyl-2-(perfluorophenyl)benzo[d][1,3,2]-
dioxaborole:
Following the general procedures, 3,5-di-tert-butylcatechol
(0.3604 g, 1.6 mmol) and pentafluorophenylboronic acid (0.3392
g, 1.6 mmol) was stirred for 12 hrs and the product isolated in
References and notes
1
98.7% (0.6288 g) yield. H NMR (400 MHz, CDCl3) δ 7.31 (s,
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6
Tetrahedron
ACCEPTED MANUSCRIPT
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Supplementary Material
All NMR spectra and X-Ray crystallographic data are
provided in the Supplementary Materials documents.