A general method for interconversion of boronic acid protecting groups: Trifluoroborates as common intermediates

Article abstract of DOI:10.1021/acs.joc.5b00182

We have developed a general protocol for the interconversion of diverse protected boronic acids, via intermediate organotrifluoroborates. N-Methyliminodiacetyl boronates, which have been hitherto resistant to direct conversion to trifluoroborates, have been shown to undergo fluorolysis at elevated temperatures. Subsequent solvolysis of organotrifluoroborates in the presence of trimethylsilyl chloride and a wide range of bis-nucleophiles enables the generation of a variety of protected boronic acids.

Full text of DOI:10.1021/acs.joc.5b00182

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Article
A General Method for Interconversion of Boronic Acid
Protecting Groups: Trifluoroborates as Common Intermediates
Quentin Isaac Churches, Joel F Hooper, and Craig A Hutton
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.5b00182 • Publication Date (Web): 20 May 2015
Downloaded from http://pubs.acs.org on May 21, 2015
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The Journal of Organic Chemistry
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A General Method for Interconversion of
Boronic Acid Protecting Groups:
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Trifluoroborates as Common Intermediates
Quentin I. Churches,†,‡ Joel F. Hooper† and Craig A. Hutton*,†
† School of Chemistry and Bio21 Molecular Science and Biotechnology Institute, University of
Melbourne, Parkville, VIC 3010, Australia
‡ CSIRO Materials Science and Engineering, Clayton, VIC 3168, Australia
chutton@unimelb.edu.au
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ABSTRACT: We have developed a general protocol for the interconversion of diverse protected
boronic acids, via intermediate organotrifluoroborates. N-Methyliminodiacetyl boronates, which have
been hitherto resistant to direct conversion to trifluoroborates, have been shown to undergo fluorolysis
at elevated temperatures. Subsequent solvolysis of organotrifluoroborates in the presence of
trimethylsilyl chloride and a wide range of bisꢀnucleophiles enables the generation of a variety of
protected boronic acids.
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INTRODUCTION
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The utility of organoboron compounds in organic synthesis has flourished in recent years,1
particularly through developments in the Suzuki–Miyaura coupling. Boronic acids are also extremely
valuable substrates for other metalꢀcatalyzed reactions,2 the Petasis reaction,3 and the iterative synthesis
of polyene natural products.4,5a
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Free boronic acids are often unstable, difficult to handle, or are prone to dehydration to give the
corresponding boroxine.1,4d Bulky boronate esters such as pinacolyl and hexyleneglycolyl boronate
esters have been used extensively as ‘blocking’ groups that reduce the reactivity of the organoboronate
through steric effects.5b More recently, true ‘protecting’ groups for boronic acids have been introduced
that modulate reactivity through both steric and electronic effects. Suginome developed the
diaminonaphthalenyl (dan) group that diminishes the reactivity of an organoboron compound toward
metalꢀcatalyzed crossꢀcouplings.6 Burke developed the N-methyliminodiacetyl (mida) group as a
boronic acid protecting group that similarly renders the organoboron unreactive toward crossꢀcoupling
reactions.7 Both the B(dan) and B(mida) protecting groups require removal to regenerate the boronic
acid to reꢀestablish reactivity of the organoboron in crossꢀcoupling reactions.
To function as useful protecting groups, the B(dan), B(mida) and other boronate systems must be able
to be introduced and removed under mild conditions, preferably in an orthogonal manner. The B(mida)
and B(dan) protecting groups employ orthogonal deprotection strategies – mild base4e and acid,6c
respectively. However, problems frequently arise in the introduction of these protecting groups, such as
the requirement of high temperatures and long reaction times,4c the generation of highly reactive
dibromoborane intermediates,4e and low yields or conversion efficiency.
Organotrifluoroborate salts, extensively investigated by Molander,7 have also been used as protecting
groups for boronic acids. Organotrifluoroborates are generally easily handled, stable crystalline solids.
In the absence of protic solvents they have limited reactivity, though they are readily hydrolyzed to the
reactive boronic acid.8
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We have previously shown that trifluoroborates can be hydrolyzed to the corresponding boronic acid
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with TMSꢀCl in the presence of water.9 We envisaged that elaboration of the TMSꢀCl promoted
solvolysis of trifluoroborates, incorporating various bisꢀnucleophiles in place of water, would enable the
generation of a variety of protected boronic acids. Together with a general method for the generation of
trifluoroborates from protected boronic acid derivatives, this process would enable the facile
interconversion of virtually any combination of boronic acid protecting groups. Such a process would
allow the protection of boronic acids under a wide range of reaction conditions such that they could be
carried through multiꢀstep processes, a frequent limitation of the use of organoborons in synthesis.4g
To establish the generality of this interconversion process, we needed to establish the generality of
both the preparation of trifluoroborates from protected boronic acids, and subsequently that of the
reverse process; the preparation of protected boronic acids from trifluoroborates.
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RESULTS AND DISCUSSION
R–BXY to R–BF3K
Organotrifluoroborates are readily prepared from boronic acids by treatment with KHF2,10 or
KF/tartaric acid.11 The use of KHF2 also enables the conversion of boronate esters,9,12,13 boroxines,7d
trisalkoxyboronates,14 and diaminoboranes15 to trifluoroborates, and thus appears quite general.
However, no examples of the direct conversion of N-based boron protecting groups, such as the
important B(mida) or B(dan) groups, to trifluoroborates have been reported. B(mida) groups have been
converted to trifluoroborates indirectly, via the corresponding boronic acid intermediates.16 Several
studies of compounds possessing a B(dan) group and a second boronate group suggest the B(dan) group
is resistant to conversion to trifluoroborates under these conditions,17 but this has not been extensively
investigated. Accordingly, we sought to determine whether the KHF2 method was suitable for the
preparation of trifluoroborates from B(mida), B(dan) and other N-based boronate derivatives.
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Phenyl diethanolamineꢀboronate 1b was converted to PhBF3K 1a in good yield under standard
conditions (Table 1, entry 1). However, the other N-based boronate derivatives were considerably less
reactive. The anthranilamide (aam) boronate18 1e was only partially converted to the corresponding
trifluoroborate at room temperature in 1 hour. Nevertheless, heating the reaction to 70 °C did result in
complete conversion of PhB(aam) 1e to the trifluoroborate 1a (Table 1, entry 4, 72% isolated yield).
PhB(mida) 1c was unreactive to KHF2 at room temperature, but similarly underwent complete
conversion to 1a at elevated temperature (Table 1, entry 2).
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Table 1. Conversion of B–N boronates to trifluoroborates a
entry
Ph–BXY
yield of 1a
(%)
85a,b
1
2
0a
90c
0a
0c
3
4
29a
72c
a
b
Conditions; 4.5 M KHF2, MeOH, rt, 1 h. Product
contaminated with diethanolamine•HF. c Isolated yield
upon heating in MeOH at 70 °C.
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PhB(dan) 1d was unreactive to KHF2 treatment, even at elevated temperature (Table 1, entry 3).
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These results are in accordance with the hydrolytic stabilities determined for protected boronic acid
derivatives determined by Suginome,18 and indicate that the conversion of boronate derivatives to
trifluoroborates with KHF2 appears general for all systems except Bꢀ(sp2)N,ꢀ(sp2)N systems.
With the finding that PhB(mida) 1c could be converted to the corresponding trifluoroborate 1a with
KHF2 at elevated temperature, the transformation of a range of aryl, vinyl and alkynylꢀB(mida)
compounds to the corresponding trifluoroborates was investigated. In general, conversion of the
B(mida) compounds to the corresponding trifluoroborates proceeded in good yield, with minimal steric
and electronic effects evident (see Table 2). The conversion of the pyridylꢀ2ꢀB(mida) 7c to the pyridylꢀ
2ꢀtrifluoroborate 7a was achieved at room temperature, and avoided decomposition of the product that
occurred at 70 °C (Table 2, entry 6).
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Table 2. Conversion of B(mida) boronates to trifluoroborates a
entry R–B(mida)
1
R–BF3K
yield (%)
74
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~80b
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99
78
8
9
Me
N
BF3K
B
62
O
O
10
O
O
11a
11c
a standard conditions; 4.5 M KHF2, MeOH, 70 °C, 1–1.5 h. b rt,
16 h, approx. yield (contaminated with MIDA, see ref. 7e).
R–BF3K to R–BXY
With the conversion of most common boronate protecting groups to trifluoroborates shown to be
readily achievable, we next sought to investigate the generality of the complementary conversion of
trifluoroborates to a range of boronate derivatives.
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We have previously developed a method for the hydrolysis of trifluoroborates to boronic acids by
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treatment with TMSCl/H2O.9 Molander reported a closely related method employing silica gel as the
fluorophile.19 While the conversion of trifluoroborates to organoboron derivatives via the boronic acid
derivatives20 and dichloroboranes21 has been demonstrated, the direct conversion of trifluoroborates to
organoborons other than the boronic acid is rare, and is limited to elaboration of the TMSꢀCl method
with bisꢀsilyl ethers22 or esters.23 Molander has used silica gel in the presence of an alcohol to generate
boronate esters from trifluoroborates, though this transformation possibly proceeds via hydrolysis to
boronic acid followed by in situ boronate ester formation.19
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Table 3. Optimization of diol boronate formation
entry
equiv of diol
TMSꢀCl
equivs.
base
equiv of base
yield (%)
1
1.1
1.1
1.1
1.1
2
1.1
1.1
1.1
1.1
1.1
1.1
1
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
K2CO3
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2
3
4
3
3
3
3
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3
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65
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75
88
93
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1.2
1.2
1.2
1.2
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Our initial investigations focused on the direct conversion of trifluoroborates to boronate esters.
Conversion of potassium phenyltrifluoroborate 1a to the corresponding hexyleneglycol boronate 1f was
initially optimized, with variation of the amount of diol and TMSꢀCl and the type of base employed.
Reasonable yields were obtained under a variety of conditions, with optimum yields for this
transformation obtained with a slight excess of diol and 3 equiv of both TMSꢀCl and K2CO3 (Table 3).
This direct method of conversion of trifluoroborates to boronate esters with a diol and TMSꢀCl is
simpler than the related reaction of diol bisꢀsilyl ethers as prior preparation of the diol bisꢀsilyl ether is
not required. The use of pinacol or pinanediol under similar conditions (2 equiv TMSꢀCl and K2CO3)
gave the corresponding boronate esters 1g and 1h in excellent yields (Table 4, entries 1,2).
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Table 4. Conversion of trifluoroborates to other boronate derivatives a
entry Ph–BXY
yield (%)
97
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96
99
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86b, 63c
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82b
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a
9
Conditions; nucleophile (1.1 equiv), K2CO3 (2
b
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equiv), TMSꢀCl (2 equiv), CH3CN, rt, 2 h. DMF, rt,
c
18 h.
MIDA disodium salt used in place of
MIDA/K2CO3.
The conversion of phenyltrifluoroborate 1a to the N-based boronate derivatives was next investigated.
Incorporating diaminonaphthalene as the bisꢀnucleophile generated the PhB(dan) derivative 1d in
excellent yield (Table 4, entry 3). The use of anthranilamide, MIDA and diethanolamine also proceeded
in good yield to generate the corresponding boronate derivatives 1e, 1c and 1b (Table 4, entries 4–6).
The use of MIDA with base (K2CO3) provided the PhB(mida) derivative 1c in greater yield than
employing the MIDA disodium salt (Table 4, entry 5).
To probe further the scope of the boronate interconversion a variety of alkenylꢀ and alkylꢀboron
species were investigated. The parent vinyltrifluoroborate 12a was converted to the corresponding
diethanolamine boronate 12b and B(mida) derivative 12c in reasonable yield (Table 5, entries 1,2).
Although a number of protected vinylboronic acid derivatives are commercially available,
vinyltrifluoroborate 12a is considerably less expensive per mole than all others.24 Similarly, propenyl
trifluoroborate 13a was converted to the corresponding B(mida) derivative 13c in excellent yield (entry
3). Styrenyltrifluoroborate 9a was converted to the corresponding B(pin) and B(dan) derivatives, 9g and
9d, in excellent yield (entries 4,5). Hexyl and cyclopropyl trifluoroborates 15a and 16a were converted
to the corresponding volatile B(pin) derivatives 15g and 16g (entries 6,7). Allyl and benzofuranꢀ2ꢀyl
trifluoroborates 11a and 17a, were converted to the corresponding B(mida) compounds 11c and 17c in
reasonable to excellent yields (entries 8,9).
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Table 5. Scope of boronate interconversions
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9
entry R–BF3K
R–BXY
yield (%)
74
HN
B
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BF3K
12a
O
1
2
O
12b
Me
N
B
O
O
O
O
65
94
91
12c
Me
N
B
BF3K
13a
O
O
O
O
3
13c
O
4
BF3K
B
O
Ph
Ph
Ph
9a
9g
HN
B
5
76
N
H
9d
O
B
6
7
8
63b
33b
75
BF3K
O
5
5
15a
15g
O
B
BF3K
16a
O
16g
Me
N
B
O
O
O
O
BF3K
11a
11c
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Me
N
1
2
3
BF3K
B
O
O
O
O
9
89
O
O
4
5
17c
17a
6
7
a
Conditions; nucleophile (1–1.2 equiv), K2CO3 (1–1.2
b
8
9
equiv), TMSꢀCl (3 equiv), CH3CN, rt, 2–18 h. quantitative
conversion by TLC; some loss on isolation due to volatility.25
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Orthogonal interconversion of bisboronates
Given the differential reactivity of the B(mida), B(aam) and B(dan) groups, we next sought to exploit
these differences in the orthogonal interconversion of differentiallyꢀprotected bisboronate systems.
Accordingly, treatment of the vinylꢀB(pin)/B(mida) bisboronate 18 with KHF2 underwent selective
fluorolysis of the pinacol boronate to give the vinylꢀBF3K/B(mida) bisboronate 19 in 63% yield
(Scheme 1), highlighting the stability of the B(mida) group and the orthogonal nature of the B(pin) and
B(mida) protecting groups under standard KHF2 conditions. Further, addition of Bu4NOH enabled
isolation of the more soluble tetrabutylammonium trifluoroborate salt. Similarly, the arylꢀ
B(pin)/B(mida) and arylꢀB(pin)/B(dan) bisꢀboronates, 24 and 20, respectively, also underwent selective
conversion of the B(pin) group to generate the corresponding monotrifluoroborates 23 and 21 with
retention of the B(mida) and B(dan) protecting groups, respectively (Scheme 1). Pleasingly, conditions
were also found that enabled selective fluorolysis of the B(aam) group in the B(aam)/B(mida) bisꢀ
boronate 22, generating the arylꢀBF3K/B(mida) bisboronate 23 in reasonable yield, despite the B(aam)
group being only slightly more reactive under fluorolysis conditions than the B(mida) group. Lastly,
following conversion of the arylꢀB(pin)/B(mida) bisꢀboronate 24 to the corresponding
monotrifluoroborate 23, subsequent treatment with TMSꢀCl in the presence of anthranilamide gave the
corresponding B(aam)/B(mida) bisꢀboronate 22 in excellent yield, exemplifying the use of this
procedure for the orthogonal interconversion of boronate protecting groups.
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Scheme 1. Orthogonal interconversion of bisboronates
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CONCLUSIONS
In summary, we have developed a general protocol for the interconversion of diverse protected
boronic acids, via intermediate organotrifluoroborates. B–N containing boronate derivatives are resistant
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to direct conversion to trifluoroborates, though B(aam) and B(mida) compounds undergo the conversion
at elevated temperatures. Importantly, this allows the selective conversion of differentially protected
bisboronates to monoꢀtrifluoroborates. Subsequent solvolysis of organotrifluoroborates in the presence
of TMSꢀCl and a wide range of bisꢀnucleophiles allows the general conversion to a wide variety of
protected boronic acids.
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EXPERIMENTAL SECTION
1
General Information. H NMR spectra were recorded at 400 or 500 MHz. Residual solvent peaks
were used as internal references: chloroform (δ 7.26 ppm), d3ꢀmethanol (δ 3.31 ppm), d5ꢀacetone (δ 2.05
13
ppm) and d5ꢀDMSO (δ 2.50 ppm). C NMR spectra were recorded at 100 or 125 MHz, with sovent
used as an internal reference: dꢀchloroform (δ 77.00), d6ꢀacetone (δ 30.83 ppm), d4ꢀmethanol (δ 49.00
ppm) and d6ꢀDMSO (δ 39.52). IR spectra were obtained as thin films. Mass spectra were recorded on an
FTꢀICR mass spectrometer by electrospray ionisation in the negative mode, unless otherwise noted.
General Procedure 1; Conversion of B(mida) boronates to trifluoroborates. To a stirred solution
of the B(mida) derivative in methanol (40 mL/mmol) was added aq. KHF2 solution (3–4 equiv., 4.5 M
solution) and the mixture was stirred at 70 °C (or at room temperature, see tables) for 1–2 h. The solvent
was removed under reduced pressure and the crude residue was thoroughly dried under high vacuum.
The solid was extracted with hot acetone, filtered and the solvent evaporated. The crude product was
recrystallized (acetone/hexanes) yielding the corresponding potassium trifluoroborate derivative.
Potassium phenyltrifluoroborate (1a). Prepared from PhB(mida) derivative 1c (70 mg, 0.3 mmol) and
aq. KHF2 solution (0.2 mL, 0.9 mmol, 4.5 M) at 70°C for 1 h, according to General Procedure 1. The
1
product was purified by recrystallization (acetone/hexanes) yielding a white solid (50.1 mg, 90 %). H
NMR (400 MHz, acetoneꢀd6) δ 7.45 (d, J = 7.2 Hz, 2H), 7.17–6.71 (m, 3H).11
Potassium 2-tolyltrifluoroborate (2a). Prepared from oꢀtolylꢀB(mida) derivative 2c (74 mg, 0.3 mmol)
and aq. KHF2 solution (0.2 mL, 0.9 mmol, 4.5 M) according to General Procedure 1 at 70 °C for 90
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minutes. The crude residue was extracted with hot acetone and the mixture was filtered, the filtrate was
concentrated in vacuo and the crude residue recrystallised from a minimal amount of hot acetone and
1
2
3
4
5
6
1
pet. spirits, to afford the potassium trifluoroborate as a white powder (42 mg, 71 %). H NMR (400
7
8
13
MHz, acetoneꢀd6) δ 7.48 (d, J = 6.7 Hz, 1H), 7.05–6.75 (m, 3H), 2.40 (s, 3H). C NMR (100 MHz,
9
acetoneꢀd6) δ 141.8, 132.8, 129.1, 126.2, 124.2, 22.1 (carbon bearing boron substituent not observed).26
Potassium 4-iodophenyltrifluoroborate (3a). Prepared from 4ꢀiodophenylꢀB(mida) derivative 3c (108
mg, 0.3 mmol) and aq. KHF2 solution (0.2 mL, 0.9 mmol, 4.5 M) according to General Procedure 1 at
70 °C for 1 h. The product was purified by recrystallization (acetone/hexanes) yielding a white solid (82
mg, 88 %). 1H NMR (400 MHz, acetoneꢀd6) δ 7.34 (ddd, J = 7.9, 1.2, 0.7 Hz, 2H), 7.14 (d, J = 7.9 Hz,
2H). 13C NMR (100 MHz, acetoneꢀd6) δ 135.3, 134.1, 90.9 (carbon bearing boron substituent not
observed).27
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11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
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46
47
48
49
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53
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55
56
57
58
59
60
Potassium 4-methoxyphenyltrifluoroborate (4a). Prepared from 4ꢀmethoxyphenylꢀB(mida) derivative
4c (79 mg, 0.3 mmol) and aq. KHF2 solution (0.2 mL, 0.9 mmol, 4.5 M) according to General Procedure
1 at 70 °C for 60 minutes. The crude residue was extracted with hot acetone and the mixture was
filtered, the filtrate was concentrated in vacuo and the crude residue recrystallised from a minimal
1
amount of hot acetone and pet spirits, to afford a white solid (56 mg, 87 %). H NMR (400 MHz,
13
acetoneꢀd6) δ 7.57–7.21 (m, 2H), 6.78–6.49 (m, 2H), 3.72 (s, 3H). C NMR (100 MHz, acetoneꢀd6) δ
158.8, 133.4, 112.7, 55.0 (carbon bearing boron substituent not observed).26
Potassium 3-cyanophenyltrifluoroborate (5a). Prepared from 3ꢀcyanophenylꢀB(mida) derivative 5c
(77 mg, 0.3 mmol) and aq. KHF2 solution (0.2 mL, 0.9 mmol, 4.5 M) according to General Procedure 1
at 70 °C for 60 minutes. The crude residue was extracted with hot acetone and the mixture was filtered,
the filtrate was concentrated in vacuo and the crude residue recrystallised from a minimal amount of hot
1
acetone and pet. spirits, to afford a white solid (56 mg, 87 %). H NMR (400 MHz, acetoneꢀd6) δ 7.72
13
(dt, J = 4.4, 2.3 Hz, 2H), 7.39 (dt, J = 7.6, 1.6 Hz, 1H), 7.33–7.16 (m, 1H). C NMR (100 MHz,
acetoneꢀd6) δ 137.0 (q, J = 1.8 Hz), 136.0 (q, J = 1.9 Hz), 129.6 , 128.00 , 121.0 , 111.0.7c
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Potassium 5-benzofurazantrifluoroborate (6a). Prepared from 5ꢀbenzofurazanꢀB(mida) derivative 6c
(82.5 mg, 0.3 mmol) and aq. KHF2 solution (0.2 mL, 0.9 mmol, 4.5 M) according to General Procedure
1 at 70 °C for 60 minutes. The crude residue was extracted with hot acetone and the mixture was
filtered, the filtrate was concentrated in vacuo and the crude residue recrystallised from a minimal
1
2
3
4
5
6
7
8
9
1
amount of hot acetone and pet. spirits, to afford a white powder (50 mg, 80 %). H NMR (400 MHz,
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16
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18
19
20
21
22
23
24
25
26
27
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29
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59
60
acetoneꢀd6) δ 7.72 (dt, J = 4.4, 2.3 Hz, 2H), 7.39 (dt, J = 7.6, 1.6 Hz, 1H), 7.25 (m, 1H). 13C NMR (100
MHz, acetoneꢀd6) δ 150.5 , 150.1, 138.7 , 115.9 (q, J = 2.8 Hz), 113.2. Spectral data matched that of an
authentic commercial sample.
Potassium pyridine-2-trifluoroborate (7a). To a stirred solution of pyridylꢀ2ꢀB(mida) derivative 7c
(140 mg, 0.6 mmol) in methanol (6 mL) was added aq. KHF2 solution (0.4 mL, 1.8 mmol, 4.5 M) and
the mixture was stirred at room temperature for 16 h. The solvent was removed under reduced pressure
and the crude residue was thoroughly dried under high vacuum. The crude solid was extracted with
methanol, filtered and the solvent evaporated. The residue was extracted with a small amount of
methanol which was evaporated to afford the corresponding potassium pyridineꢀ2ꢀtrifluoroborate 7a and
1
methyliminodiacetic acid as a 1:1 mixture (79.8 mg, 80 %). H NMR (400 MHz, methanolꢀd4) δ 8.58
(br d, J = 6.0 Hz, 1H), 8.36 (td, J = 7.7, 1.2 Hz, 1H), 8.01 (br d, J = 7.8 Hz, 1H), 7.81 (ddd, J = 7.6, 6.0,
13
1.5 Hz, 1H); MIDA peaks appear at δ 3.71 (s, 4H), 2.95 (s, 3H). C NMR (100 MHz, methanolꢀd4) δ
144.8, 140.5, 131.3, 125.6.7e
Potassium N-(phenylsulfonyl)indolyl-2-trifluoroborate (8a). Prepared from Nꢀphenylsulfonylindolylꢀ
B(mida) derivative 8c (124 mg, 0.3 mmol) and aq. KHF2 solution (0.2 mL, 0.9 mmol, 4.5 M) according
to General Procedure 1 at 70 °C for 90 minutes. The crude residue was extracted with hot acetone and
the mixture was filtered, the filtrate was concentrated in vacuo and the crude residue recrystallised from
1
a minimal amount of hot acetone and pet. spirits, to afford a white powder (110 mg, 100%). H NMR
(400 MHz, acetoneꢀd6) δ 8.20–8.10 (m, 2H), 8.10–8.03 (m, 1H), 7.56–7.48 (m, 1H), 7.48–7.38 (m, 3H),
7.22–7.05 (m, 2H), 6.78 (d, J = 0.9 Hz, 1H). 13C NMR (100 MHz, acetoneꢀd6) δ 140.4, 138.7, 133.9,
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132.7, 129.5, 127.9 (q, J = 1.8 Hz), 123.4 , 123.3 , 120.9, 116.2 (q, J = 3.1 Hz), 115.1. HRMS: m/z
S ([M–K+]–), 324.0477; found m/z 324.0474.
1
2
3
calculated forC14
H10BF
3
NO
2
4
5
6
7
8
Potassium trans-styrenyltrifluoroborate (9a). Prepared from transꢀstyrenylꢀB(mida) derivative 9c
(78 mg, 0.3 mmol) and aq. KHF2 solution (0.2 mL, 0.9 mmol, 4.5 M) according to General Procedure 1
at 70 °C for 1 h. The product was purified by recrystallization (acetone/hexanes) yielding a white solid
(45 mg, 71%). 1H NMR (400 MHz, acetoneꢀd6) δ 7.37–7.32 (m, 2H), 7.22 (app dd, J = 8.4, 6.9 Hz, 2H),
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
13
7.09 (app tt, J = 7.3, 3.5 Hz, 1H), 6.67 (d, J = 18.2 Hz, 1H), 6.32 (dq, J = 18.3, 3.7 Hz, 1H). C NMR
(100 MHz, acetoneꢀd6) δ 140.9, 133.9, 128.1, 125.7, 125.6 (carbon bearing boron substituent not
observed).11
Potassium 2-(4-tolyl)-ethynyltrifluoroborate (10a). Prepared from 4ꢀmethylphenylethynylꢀB(mida)
derivative 10c (49 mg, 0.18 mmol) and aq. KHF2 solution (0.2 mL, 0.9 mmol, 4.5 M) according to
General Procedure 1 at 70 °C for 60 minutes. The crude residue was extracted with hot acetone and the
mixture was filtered, the filtrate was concentrated in vacuo and the crude residue recrystallised from a
1
minimal amount of hot acetone and pet. spirits, to afford a white powder (31 mg, 78%). H NMR (400
MHz, acetoneꢀd6) δ 7.29–7.13 (m, 2H), 7.11–6.91 (m, 2H), 2.27 (s, 3H). 13C NMR (100 MHz, acetoneꢀ
d6) δ 136.0, 131.1, 128.6, 123.5, 89.1, 20.4.28
Potassium allyltrifluoroborate (11a). Prepared from allylꢀB(mida) derivative 11c (100 mg, 0.57
mmol) ) and aq. KHF2 solution (0.33 mL, 4.5M, 1.52 mmol) at room temperature for 1 h according to
General Procedure 1. The product was collected by filtration to yield the product 11a (52 mg, 62%) as a
white crystalline solid. 1H NMR (400 MHz, acetoneꢀd6) δ 5.94 (dq, J = 10.0, 7.9 Hz, 1H), 4.64 (m, 2H),
13
1.14 (br s, 2H); C NMR (100 MHz, acetoneꢀd6) δ 142.1, 109.2 (carbon bearing boron substituent not
observed).
Potassium trans-2-trifluoroboryl-1-vinyl-B(mida) (19). To a solution of transꢀ2ꢀB(pin)ꢀvinylꢀB(mida)
derivative 18 (105 mg, 0.34 mmol) in methanol (1 mL) was added aqueous KHF2 solution (0.43 mL,
4.5 M, 1.93 mmol). The resulting white slurry was stirred at room temperature for 1 h and then
concentrated in vacuo. The crude solid obtained was extracted exhaustively with anhydrous hot acetone
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(product poorly soluble). The extract was filtered and the filtrate concentrated in vacuo. The crude
residue obtained was recrystallised from a minimal amount of hot acetone and ether (5 mL), to afford an
amorphous solid (62 mg, 63%). 1H NMR (400 MHz, DMSOꢀd6) δ 6.09 (dq, J = 20.5, 3.4 Hz, 1H), 5.63
1
2
3
4
5
6
7
8
13
(d, J = 20.6 Hz, 1H), 4.09 (d, J = 17.1 Hz, 2H), 3.86 (d, J = 17.0 Hz, 2H), 2.67 (s, 3H). C NMR (100
9
11
MHz, DMSOꢀd6) δ 169.5, 61.0, 46.5 (carbon bearing boron substituents not observed). B NMR (128
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11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
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32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
MHz, DMSOꢀd6) δ 12.3, 2.51. HRMS: m/z calculated for C
7
H
9
B
2
F
3
NO
([M–K+]–), 250.0670; found m/z
4
250.0669. IR (neat, cmꢀ1) 526, 545, 556, 567, 592, 607, 633, 648, 675, 737, 763, 823, 850, 892, 1004,
1025, 1050, 1073, 1221, 1294, 1333, 1374, 1460, 1633, 1727, 3265, 3324, 3348.
Tetrabutylammonium trans-2-trifluoroboryl-1-vinyl-B(mida). To a solution of transꢀ2ꢀB(pin)ꢀvinylꢀ
B(mida) derivative 18 (0.50 g, 1.62 mmol) in methanol (1 mL) was added aqueous KHF2 (1.05 mL,4.5
M, 4.6 mmol, 3.1 eq). After stirring for 30 min, Bu4N+OHꢀ (1.10 mL, 40 % aq. solution, 1.62 mmol) was
added dropwise and the mixture stirred for 1 h. The reaction was diluted with DCM (10 mL) and the
aqueous layer was extracted with DCM (5 × 10 mL). The combined organic layers were dried (MgSO4),
filtered and the solvent evaporated to afford the tetrabutylammonium trifluoroborate salt (0.560 g, 70%,
~90% purity) contaminated with tetrabutyl ammonium species. Careful washing with minimal
1
tetrahydrofuran was able to remove excess tetrabutylammonium salts with minor loss of product. H
NMR (400 MHz, DMSOꢀd6) δ 6.09 (dq, J = 20.5, 3.4 Hz, 1H), 5.63 (d, J = 20.6 Hz, 1H), 4.09 (d, J =
13
17.1 Hz, 2H), 3.86 (d, J = 17.0 Hz, 2H), 2.67 (s, 3H). C NMR (100 MHz, DMSOꢀd6) δ 169.9, 61.8,
59.1, 46.9, 46.8, 24.3, 20.2, 13.8 (carbon bearing boron substituents not observed). 11B NMR (128 MHz,
DMSOꢀd6) δ 12.3, 2.51. HRMS: m/z calculated for C
7
H9
B2
F
3
NO
([M–Bu4N+]–), 250.0670; found m/z
4
250.0669.
Potassium 4-trifluoroborylphenyl-B(dan) (21). To a solution of B(pin)/B(dan) 1,4ꢀbenzenebisboronate
20 (51.7 mg, 0.139 mmol) in methanol (1 mL) was added aqueous KHF2 (0.43 mL, 4.5 M, 1.93 mmol).
The resulting white slurry was stirred at room temperature for 15 min, the solvent was then removed
under reduced pressure and the residue extracted with hot acetone. The mixture was filtered, the filtrate
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was concentrated in vacuo and the residue recrystallised from a minimal amount of hot acetone and pet.
1
2
3
spirits, to afford a light purple powder (39 mg, 80 %). 1H NMR (400 MHz, acetoneꢀd6) δ 7.63 (d, J = 7.4
4
5
6
13
Hz, 2H), 7.54 (d, J = 7.4 Hz, 2H), 7.50 (s, 2H), 7.08 (t, J = 7.8 Hz, 2H), 6.62 (d, J = 7.6 Hz, 2H). C
7
8
NMR (100 MHz, acetoneꢀd6) δ 143.7, 137.6, 132.4, 131.2, 128.5, 121.0, 117.5, 106.6 (carbon bearing
9
11
boron substituents not observed). B NMR (128 MHz, acetoneꢀd6) δ 32.04, 5.59. IR (neat, cmꢀ1) 629,
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11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
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31
32
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40
41
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53
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59
60
701, 755, 824, 890, 965, 1005, 1024, 1047, 1092, 1179, 1196, 1234, 1272, 1304, 1389, 1449, 1513,
1585, 1596, 1613, 1649, 2854, 2925, 2966, 3219, 3294, 3308, 3327, 3334. HRMS: m/z calculated for
C
16
H12
B2
F3
N
([M–K+]–), 311.1139; found m/z 311.1142.
2
Potassium 4-trifluoroborylphenyl-B(mida) (23).
from 4-B(pin)-phenyl-B(mida) 24; Prepared from B(pin)/B(mida) 1,4ꢀbenzenebisboronate 24 (500mg,
1.46 mmol) and aq. KHF2 solution (0.92 mL, 4.5M, 4.14 mmol) at room temperature for 1 h according
to General Procedure 1. The precipitate was collected by filtration to yield the product 23 (415 mg,
83%) as a white solid. 1H NMR (400 MHz, acetoneꢀd6) δ 7.30 (s, 2H), 7.15 (s, 2H), 4.25 (d, J = 17 Hz,
13
2H) 4.03 (d, J = 17 Hz, 2H), 2.42 (s, 3H); C NMR (100 MHz, acetoneꢀd6) δ 170.0, 131.3, 130.8,
11
130.1, 62.0, 47.9 (carbon bearing boron substituent not observed); B NMR (128 MHz, acetoneꢀd6) δ
16.6, 7.9; IR (neat, cmꢀ1) 3302, 1741, 1657, 1620, 1338, 1302, 1231, 1035, 983, 817; HRMS: m/z
calculated for C11H11B2F3NO4 ([M–K+]–), 300.0826; found m/z 300.0856.
from 4-B(aam)-phenyl-B(mida) 22; To a solution of B(aam)/B(mida) 1,4ꢀbenzenebisboronate 22
(41mg, 0.11 mmol) in a mixture of 2ꢀpropanol and DMF (1:1) was added aq. KHF2 solution (73 ꢁL,
4.5M, 0.33 mmol). The mixture was heated to 50 ˚C for 1 h and cooled to room temperature. The
solution was cooled to –20 ˚C overnight, and the product was collected by filtration to yield 23 (18 mg,
48%) as a white solid.
General Procedure 2; conversion of trifluoroborates to boronate ester derivatives. To a mixture
of the potassium trifluoroborate, potassium carbonate (2 equiv.), and diol (1 equiv.) in acetonitrile was
added TMSꢀCl (1.5 eq) and the reaction stirred 2 h. The reaction was diluted with ether, filtered and the
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solvent evaporated under reduced pressure. The crude residue obtained was purified by passage through
a silica plug eluting with ethyl acetate/hexane yielding the corresponding boronate ester.
1
2
3
4
5
6
7
8
Hexyleneglycolyl phenylboronate (1f). Prepared from phenyltrifluoroborate 1a (138 mg, 0.747 mmol)
and 2,4ꢀhexylene glycol (90 mg, 0.76 mmol) according to General Procedure 2. The boronate ester was
9
1
isolated by column chromatography on silica gel (hexanes/ethyl acetate = 1:4). H NMR (400 MHz,
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
CDCl3) δ 7.81 (dd, J = 8.0, 1.5 Hz, 2H), 7.50–7.24 (m, 3H), 4.32 (dqd, J = 11.5, 6.2, 3.0 Hz, 1H), 1.83
(dd, J = 13.9, 3.0 Hz, 1H), 1.69–1.48 (m, 1H), 1.36 (s, 3H), 1.34 (s, 3H), 1.33 (d, J = 6.2 Hz, 3H).29
Pinacolyl phenylboronate (1g). Prepared from phenyltrifluoroborate 1a (138 mg, 0.747 mmol) and
pinacol (90 mg, 0.76 mmol) according to General Procedure 2. The boronate ester was isolated by
column chromatography on silica gel (hexanes/ethyl acetate = 1:4) yielding a white solid (148 mg, 97
%).30
Pinanediolyl phenylboronate (1h). Prepared from phenyltrifluoroborate 1a (138 mg, 0.747 mmol) and
(1R,2R,3S,5R)ꢀ(–)ꢀpinanediol (129 mg, 0.760 mmol) according to General Procedure 2. The boronate
ester was isolated by column chromatography on silica gel (hexanes/ethyl acetate = 1:4) yielding a
viscous oil which solidified upon standing (183 mg, 96%). 1H NMR (200 MHz, CDCl3) δ 7.82 (dd, J =
7.9, 1.6 Hz, 3H), 7.57–7.28 (m, 3H), 4.46 (dd, J = 8.7, 1.9 Hz, 1H), 2.59–2.09 (m, 3H), 2.08–1.85 (m,
13
2H), 1.49 (s, 3H), 1.32 (s, 3H), 1.23 (d, J = 10.5 Hz, 1H), 0.90 (s, 3H). C NMR (50 MHz, CDCl3) δ
134.9, 131.3, 127.9, 86.4, 51.6, 39.7, 38.4, 35.8, 28.9, 27.3, 26.7, 24.2.31
Pinacolyl trans-styrenylboronate (9g). Prepared from potassium transꢀstyrenyltrifluoroborate 9a (102
mg, 0.747 mmol) and pinacol (90 mg, 0.760 mmol) according to General Procedure 2. The product was
isolated by column chromatography on silica gel (hexanes/ethyl acetate = 1:4) yielding a colourless
liquid (156 mg, 91% yield). 1H NMR (400 MHz, CDCl3) δ 7.54–7.21 (m, 6H), 6.15 (d, J = 18.5 Hz,
1H), 1.30 (s, 12H). 13C NMR (100 MHz, CDCl3) δ 149.7, 137.6, 129.0, 128.7, 127.2, 83.5, 25.0 (carbon
bearing boron substituent not observed).32
Pinacolyl 1-hexylboronate (15g). Prepared from potassium n-hexaneꢀ1ꢀtrifluoroborate 15a (143 mg,
0.747 mmol) and pinacol (90 mg, 0.760 mmol) according to General Procedure 2. The product was
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isolated by column chromatography on silica gel (hexanes/ethyl acetate = 9:1) yielding a clear liquid
(100 mg, 63%). 1H NMR (400 MHz, CDCl3) δ 1.44–1.30 (m, 2H), 1.31–1.13 (m, 18H), 0.82 (t, J = 6.9
Hz, 3H), 0.72 (t, J = 7.8 Hz, 2H). 13C NMR (101 MHz, CDCl3) δ 82.9, 32.2, 31.7, 24.9, 24.0, 22.7, 14.2
(carbon bearing boron substituent not observed).30
1
2
3
4
5
6
7
8
9
Pinacolyl cyclopropylboronate (16g). Prepared from potassium cyclopropyltrifluoroborate 16a (111
mg, 0.747 mmol) and pinacol (90 mg, 0.760 mmol) according to General Procedure 2. The product was
isolated by column chromatography on silica gel (hexanes/ethyl acetate = 1:4) as a colourless liquid (42
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
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32
33
34
35
36
37
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39
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41
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43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
mg, 33%; unoptimized, compound lost due to volatility). H NMR (400 MHz, CDCl3) δ 1.21 (s, 12H),
0.60 (ddd, J = 9.2, 6.1, 3.3 Hz, 2H), 0.49 (td, J = 6.0, 3.3 Hz, 2H), –0.20 (tt, J = 9.3, 6.1 Hz, 1H).33
General Procedure 3; conversion of trifluoroborates to B(aam) or B(dan) derivatives. To a
mixture of potassium carbonate (103 mg, 0.747 mmol), potassium phenyltrifluoroborate (69 mg, 0.374
mmol) in acetonitrile (7.5 mL) was added TMSꢀCl (132 µL, 1.12 mmol, 3 eq) and anthranilamide or
1,8ꢀdiaminonaphthalene (0.380 mmol) and the reaction stirred for 30 min. The reaction was diluted with
ethyl acetate (20 mL), filtered and the solvent evaporated under reduced pressure. The crude residue
obtained was purified by column chromatography (silica, eluting with ethyl acetate/petroleum spirits)
yielding the product as a whiteꢀpurple solid.
PhB(dan) (1d). Prepared from phenyltrifluoroborate 1a (69 mg, 0.374 mmol) and 1,8ꢀ
diaminonaphthalene (59 mg, 0.374 mmol) according to General Procedure 3. The product was isolated
by column chromatography on silica gel (eluting with 1:4 hexanes/ethyl acetate then 1:1 hexanes/ethyl
1
acetate) yielding 1d (91 mg, 100 %) as a whiteꢀpurple solid. H NMR (200 MHz, CDCl3) δ 7.72–7.60
(m, 2H), 7.55–7.41 (m, 3H), 7.26–7.00 (m, 4H), 6.43 (dd, J = 7.0, 1.3 Hz, 2H), 6.03 (br s, 2H). 13C
NMR (50 MHz, CDCl3) δ 141.2 (2C), 136.5, 131.5 (2C), 130.4, 128.4 (2C), 127.7 (2C) 120.1, 118.0
(2C), 106.2 (2C) (carbon bearing boron substituent not observed).34
PhB(aam) (1e). Prepared from phenyltrifluoroborate 1a (138 mg, 0.747 mmol) and anthranilamide
(104 mg, 0.760 mmol) according to General Procedure 3. The product was isolated by column
chromatography on silica gel (hexanes/ethyl acetate = 1:1) yielding 1e (130 mg, 78%) as a whiteꢀpurple
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solid. 1H NMR (400 MHz, CDCl3) δ 8.26 (dd, J = 8.0, 1.6 Hz, 1H), 7.76–7.66 (m, 2H), 7.66–7.44 (m,
5H), 7.18 (ddd, J = 8.1, 7.2, 1.1 Hz, 1H), 7.12 (dd, J = 8.1, 0.5 Hz, 1H), 6.82 (br s, 1H). 13C NMR (100
MHz, CDCl3) δ 166.6, 144.2, 133.9, 131.8 (2C), 131.1, 129.2, 128.6 (2C), 121.9, 117.6, 115.0 (carbon
bearing boron substituent not observed).18
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trans-styrenyl-B(dan) (9d). Prepared from potassium transꢀstyrenyl trifluoroborate 9a (101.0 mg,
0.747 mmol) and 1,8ꢀdiaminonaphthalene (60 mg, 0.380 mmol) according to General Procedure 3. The
product was isolated by column chromatography on silica gel (hexanes/ethyl acetate = 3:1) yielding the
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product 9d (77 mg, 76 %) as a whiteꢀpurple solid. H NMR (500 MHz, CDCl3) δ 7.55–7.51 (m, 2H),
7.43–7.37 (m, 2H), 7.34 (d, J = 7.5 Hz, 1H), 7.20–7.10 (m, 3H), 7.05 (dd, J = 8.4, 0.9 Hz, 2H), 6.38 (dd,
13
J = 7.3, 1.0 Hz, 2H), 6.34 (d, J = 18.6 Hz, 1H), 5.86 (br s, 2H). C NMR (125 MHz, CDCl3) δ 143.9,
141.4, 137.8, 136.6, 128.94, 128.92, 127.8, 127.0, 120.1, 117.9, 106.0 (carbon bearing boron substituent
not observed).6a
General Procedure 4; conversion of trifluoroborates to B(mida) derivatives. To a stirred solution
of trifluoroborate salt (0.374 mmol), Nꢀmethyliminodiacetic acid (59 mg, 0.4 mmol) and potassium
carbonate (104 mg, 0.747 mmol) in dimethylformamide (5 mL) under an atmosphere of nitrogen was
added TMSꢀCl (132 µL, 1.12 mmol) and the mixture was stirred overnight. Ethyl acetate (20 mL) was
added and the solution was filtered, and the flask rinsed with a small amount of ethyl acetate. For small
scale reactions, the solvent was removed under reduced pressure. For large scale reactions an aqueous
workup was employed to remove the DMF; the solution was washed with brine (20 mL), water (20 mL
x 2), brine (20 mL) and then dried (MgSO4). The solvent was removed under reduced pressure and the
product was purified by recrystallization (ethyl acetate–petroleum spirits) to give the product.
PhB(mida) (1c). Prepared from phenyltrifluoroborate 1a (68.8 mg, 0.374 mmol) and Nꢀ
methyliminodiacetic acid (58.9 mg, 0.4 mmol) according to General Procedure 4. The crude residue was
recrystallised (ethyl acetate/petroleum spirits) yielding 1c (75 mg 86 %) as a white solid. 1H NMR (400
MHz, DMSOꢀd6) δ 7.45–7.36 (m, 2H), 7.36–7.26 (m, 3H), 4.29 (d, J = 17.2 Hz, 2H), 4.07 (d, J = 17.2
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Hz, 2H), 2.45 (s, 3H). 13C NMR (101 MHz, DMSOꢀd6) δ 169.4, 132.3, 128.9, 127.7, 61.8, 47.6 (carbon
bearing boron substituent not observed). 11B NMR (128 MHz, DMSOꢀd6) δ 13.44.35
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Allyl-B(mida) (11c). Prepared from potassium allyltrifluoroborate 11a (55 mg, 0.374 mmol) and Nꢀ
methyliminodiacetic acid (59 mg, 0.4 mmol) according to General Procedure 4. The crude residue was
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1
recrystallised (ethyl acetate/petroleum spirits) yielding 11 (110 mg, 75%) as a white solid. H NMR
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(400 MHz, acetoneꢀd6) δ 5.89 (ddt, J = 17.0, 10.2, 7.6 Hz, 1H), 5.00 (ddt, J = 17.1, 2.5, 1.6 Hz, 1H),
4.90 (ddt, J = 10.2, 2.5, 1.2 Hz, 1H), 4.21 (d, J = 16.9 Hz, 2H), 3.99 (d, J = 16.8 Hz, 2H), 2.54 (s, 3H),
13
11
1.66 (d, J = 7.6 Hz, 2H). C NMR (100 MHz, acetoneꢀd6) δ 168.7, 137.0, 115.3, 62.9, 46.3, 41.3. B
NMR (128 MHz, acetoneꢀd6) δ 12.16. Spectral data matched that of an authentic commercial sample.
Vinyl-B(mida) (12c). Prepared from potassium vinyltrifluoroborate 12a (51 mg, 0.374 mmol) and Nꢀ
methyliminodiacetic acid (59 mg, 0.4 mmol) according to General Procedure 4. The crude residue was
1
recrystallised (ethyl acetate/petroleum spirits) yielding 12c (45 mg, 65%) as a white solid. H NMR
(500 MHz, CDCl3) δ 6.11–5.94 (m, 2H), 5.92–5.77 (m, 1H), 3.84 (d, J = 16.3 Hz, 2H), 3.69 (d, J = 16.3
Hz, 2H), 2.85 (s, 3H).4e
2-Propenyl-B(mida) (13c). Prepared from potassium isopropenyltrifluoroborate 13a (55 mg, 0.374
mmol) and Nꢀmethyliminodiacetic acid (118 mg, 0.8 mmol) according to General Procedure 4. The
crude residue was recrystallised (ethyl acetate/petroleum spirits) yielding 13c (55 mg, 94 %) as a white
solid. 1H NMR (400 MHz, acetoneꢀd6) δ 5.45 (br s, 1H), 5.31 (d, J = 2.5 Hz, 1H), 4.23 (d, J = 17.0 Hz,
2H), 4.04 (d, J = 17.0 Hz, 2H), 3.00 (s, 3H), 1.78 (br s, 3H).4c
2-Benzofuranyl-B(mida) (17c). Prepared from potassium 2ꢀbenzofuranyltrifluoroborate 17a (84 mg,
0.374 mmol) and Nꢀmethyliminodiacetic acid (59 mg, 0.4 mmol) according to General Procedure 4. The
crude residue was recrystallised (ethyl acetate/petroleum spirits) yielding 17c (91 mg, 89%) as a white
solid. 1H NMR (400 MHz, DMSOꢀd6) δ 7.64 (app d, J = 7.1 Hz, 1H), 7.57 (app d, J = 8.2 Hz, 1H), 7.29
(td, J = 8.3, 7.7, 1.4 Hz, 1H), 7.22 (td, J = 7.5, 1.0 Hz, 1H), 7.07 (br s, 1H), 4.41 (d, J = 17.2 Hz, 2H),
4.17 (d, J = 17.2 Hz, 2H), 2.69 (s, 3H).4d
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4-B(aam)-phenyl-B(mida) (22). Prepared from potassium trifluoroborate salt 23 (50mg, 0.147 mmol)
and anthranilamide (21 mg, 0.147 mmol) according to General Procedure 4. The crude product was
purified by flash chromatography (10% MeOH/EtOAc), to give 24 (51mg, 92%) as a colourless solid.
1H NMR (400 MHz, acetoneꢀd6) 9.66 (s, 1H), 9.31 (s, 1H), 8.04–7.99 (m, 3H), 7.75 (d, J = 8 Hz, 1H),
7.55 (m, 1H), 7.50 (d, J = 8 Hz, 2H), 7.40 (d, J = 8 Hz, 2H), 7.09 (t, J = 11 Hz, 1H); 4.34 (d, J = 17
Hz, 2H), 4.13 (d, J = 17 Hz, 2H), 2.52 (s, 3H); 13C NMR (100 MHz, acetoneꢀd6) δ 169.8, 166.7, 145.9,
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133.6, 131.8, 128.4, 121.2, 119.2, 118.6 (carbon bearing boron substituent not observed); B NMR
(128 MHz, acetoneꢀd6) δ 15.8; IR (neat, cmꢀ1) 3303, 1743, 1658, 1620, 1538, 1339, 1303, 1241, 1037,
988, 716, 725; HRMS: m/z calculated for C18H18B2N3O5 ([M+H]+), 378.1433; found m/z 378.1446.
General Procedure 5; conversion of trifluoroborates to diethanolamine boronate derivatives. To
a stirred solution of potassium phenyltrifluoroborate (69 mg, 0.374 mmol), diethanolamine (42 mg, 0.4
mmol) and K2CO3 (56 mg, 0.4 mmol) in acetonitrile (5 mL) under an atmosphere of nitrogen was added
TMSꢀCl (132 µL, 1.12 mmol) and the mixture was stirred for 2 h. Dichloromethane (20 mL) was added
to the reaction and the solution was filtered, and the precipitate was washed with dichloromethane (3 x
20 mL). The filtrate was evaporated under reduced pressure and the crude solid residue was purified by
recrystallization (dichloromethane/ether).
Diethanolaminyl phenylboronate (1b). Prepared from phenyltrifluoroborate 1a (69 mg, 0.374 mmol)
and diethanolamine (42 mg, 0.4 mmol) according to General Procedure 5. The product was
recrystallised (dichloromethane/ether) yielding 1b (59 mg, 82%) as a white solid. 1H NMR (400 MHz,
DMSOꢀd6) δ 7.44 (dd, J = 7.8, 1.7 Hz, 2H), 7.24–7.14 (m, 3H), 6.88 (br s, 1H), 3.87 (td, J = 9.2, 5.4 Hz,
2H), 3.78 (ddd, J = 9.6, 6.6, 3.5 Hz, 2H), 3.13–3.01 (m, 2H), 2.87–2.76 (m, 2H).36
Diethanolaminyl vinylboronate (12b). Prepared from potassium vinyltrifluoroborate 12a (50 mg,
0.374 mmol) and diethanolamine (40 mg, 0.38 mmol) according to General Procedure 5. The product
1
was recrystallised (dichloromethane/ether) yielding 12b (39 mg, 74%) as a white solid. H NMR (400
MHz, DMSOꢀd6) δ 6.72 (s, 1H), 5.83 (dd, J = 19.5, 13.2 Hz, 1H), 5.50–5.24 (m, 2H), 4.01–3.66 (m,
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2H), 3.65–3.46 (m, 2H), 3.16–2.84 (m, 2H), 2.84–2.59 (m, 2H). C NMR (100 MHz, DMSOꢀd6) δ
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2
3
122.5, 62.3, 50.4 (carbon bearing boron substituent not observed).37
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8
4-B(pin)-phenyl-B(mida) 24. To a solution of 4ꢀbromophenylꢀB(mida) boronate (1.11g, 3.53 mmol),
KOAc (690 mg, 7.1 mmol) and bispinacolatodiboron (3.58 g, 14.1 mmol) in DMF (40 mL) was added
Pd(dppf)Cl2 (128 mg, 0.17 mmol). The solution was heated to 90 °C overnight and cooled to room
temperature. The solvent was removed in vacuo, and the residue was purified by flash chromatography
(20–50% EtOAc/pet. spirits), followed by recrystallisation (CH2Cl2/pet. spirits) to give 24 (950 mg,
75%) as a colourless crystalline solid. 1H NMR (400 MHz, acetoneꢀd6) δ 7.75 (d, J = 7.5 Hz, 2H), 7.56
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(d, J = 7.5 Hz, 2H), 4.36 (d, J = 17 Hz, 2H), 4.15 (d, J = 17 Hz, 2H), 2.68 (s, 3H), 1.30 (s, 12H). C
NMR (100 MHz, acetoneꢀd6) δ 168.4, 133.9, 131.8, 83.6, 61.9, 47.4, 24.3 (carbon bearing boron
11
substituent not observed). B NMR (128 MHz, acetoneꢀd6) δ 30.5, 11.6. IR (neat, cmꢀ1) 1745, 1657,
1615, 1391, 1363, 1337, 1300, 1220, 1036, 990, 961, 816. HRMS: m/z calculated for C17H24B2NO6
([M+H]+), 360.1790; found m/z 360.1799.
ASSOCIATED CONTENT
Supporting Information
13
1H and C NMR spectra for all compounds. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Email: chutton@unimelb.edu.au (C.A.H.).
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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