Synthesis, characterisation and reactions of phosphine-substituted alkynylboronates and alkynyltrifluoroborate salts

Article abstract of DOI:10.3390/molecules191221324

The synthesis and structural characterisation of phosphine-substituted alkynylboronates is reported. A P(III)-centred alkynylboronate (2) was prepared that showed little evidence for the conjugation of the P-lone pair to the boron via the alkyne π-system,

Full text of DOI:10.3390/molecules191221324

Molecules 2014, 19, 21324-21334; doi:10.3390/molecules191221324
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Synthesis, Characterisation and Reactions of
Phosphine-Substituted Alkynylboronates and
Alkynyltrifluoroborate Salts
Jérôme F. Vivat, Sylvestre P. J. T. Bachollet, Harry Adams and Joseph P. A. Harrity *
Department of Chemistry, University of Sheffield, Sheffield S3 7HF, UK;
E-Mails: vivat.jerome@gmail.com (J.F.V.); s.bachollet@sheffield.ac.uk (S.P.J.T.B.);
h.adams@shef.ac.uk (H.A.)
* Author to whom correspondence should be addressed; E-Mail: j.harrity@sheffield.ac.uk;
Tel.: +44-(0)114-22-29496; Fax: +44-(0)114-22-29436.
External Editor: John Spencer
Received: 13 November 2014; in revised form: 5 December 2014 / Accepted: 11 December 2014 /
Published: 18 December 2014
Abstract: The synthesis and structural characterisation of phosphine-substituted
alkynylboronates is reported. A P(III)-centred alkynylboronate (2) was prepared that
showed little evidence for the conjugation of the P-lone pair to the boron via the alkyne
π-system, as judged by X-ray crystallography studies of 2 and a related P(V) compound, 3.
In addition, corresponding alkynyltrifluoroborate salts were prepared that showed
improved stability by comparison to their boronic ester counterparts. These salts undergo
Pd-catalysed cross-coupling reactions with aryl halides.
Keywords: alkynylboronate; trifluoroborate salt; conjugation; phosphine; cross-coupling
1. Introduction
Since their discovery by Matteson in 1960 [1], alkynylboronates have proven to be a versatile and
popular class of synthetic intermediates in organic synthesis. They combine the traditional
nucleophilic properties of alkyne-based organometallics and, as such, undergo metal catalysed
cross-coupling [2–4], as well as addition to aldehydes and imines [5]. Moreover, these reagents are

Molecules 2014, 19
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subject to chemistry at the triple bond. For example, hydrogenation [6], metallation [7,8],
metathesis [9] and various cycloaddition reactions [10–20] have all been reported.
Whilst a range of reactions of alkynylboronates are now documented, there is relatively little in the
literature regarding heteroatom-substituted alkynylboronates of the type shown in Figure 1. Indeed,
an early example of an ynol ether-derived alkynylboronate [21] suggested that these compounds were
unstable, and this precluded extensive studies into their synthetic chemistry and their structural
characterisation. We wanted to explore the chemistry of this type of alkyne and, in particular, the
potential for the conjugation of the heteroatom to the boron centre (Figure 1). Evidence for related
mesomeric B-C π-bonding in alkynylboronates has been documented by Yamamoto [22], and it
seemed feasible that this would be enhanced by the inclusion of an appropriate Lewis basic donor.
We report herein our studies on the synthesis, characterisation and reactions of phosphine-substituted
alkynylboronates and their closely-related trifluoroborate salts.
Figure 1. Heteroatom-substituted alkynylboronates.
OR2
OR2
OR2
OR2
R1
R1
X
X
B
B
2. Results and Discussion
The most convenient method of generating alkynylboronates is via the appropriate
alkynyllithium reagent, following the procedure of Brown [23]. Indeed, we were pleased to find that
ethynyldiphenylphosphine 1 could be smoothly transformed into the desired alkynylboronate (2) in
good overall yield when this approach was employed (Scheme 1). Alkyne 2 was found to be highly
sensitive to protodeboronation induced by exposure to air and moisture; however, we were able to
grow crystals of 2 suitable for single-crystal X-ray diffraction analysis; the structure is shown in
Figure 2, and selected bond lengths and bond angles are given in Table 1.
Scheme 1. Synthesis of a phosphine substituted alkynylboronate.
O
1. BuLi
Ph2P
Ph2P
B
2. PinBOPri
3. HCl, ether
67%
O
1
2
Figure 2. ORTEP diagram of alkyne 2. H-atoms have been omitted for clarity.

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Table 1. Selected bond lengths (Ǻ) and angles (°) for 2.
2
C(13A)-C(14A)
B(1A)-C(14A)
P(1A)-C(13A)
1.203(4)
1.531(4)
1.761(3)
177.8(3)
175.5(2)
114.8(2)
122.0(2)
123.2(2)
102.19(12)
100.56(13)
100.46(12)
B(1A)-C(14A)-C(13A)
P(1A)-C(13A)-C(14A)
O(1A)-B(1A)-O(2A)
O(1A)-B(1A)-C(14A)
O(2A)-B(1A)-C(14A)
C(7A)-P(1A)-C(1A)
C(1A)-P(1A)-C(13A)
C(7A)-P(1A)-C(13A)
The X-ray crystal structure showed that 2 exhibits typical C-C and C-B bond lengths for
an alkynylboronate [24]. Moreover, the C-P bond length is marginally longer than those found in
related alkynylphophonium salts, where conjugation is not possible [25]. Furthermore, inspection of
the selected bond angles shows that the alkyne is close to linear and that the boron atom is contained
within a trigonal structure (sum of internal bond angles: ~360°). In contrast, the sum of the internal
bond angles around phosphorus is ~303°, and it therefore adopts a trigonal pyramidal geometry.
Taking this data into account, while the P-C-C-B array in 2 is linear and the structure adopts a
conformation that provides the potential for conjugation, it appears that delocalisation of the P-lone
pair is only weak in this system. These observations mirror those of Marder and co-workers in their
studies of alkynylboranes [26].
We next decided to explore some reactions at the P-centre of 2 and attempted to generate the
corresponding phosphine oxide by oxidation. Unfortunately, this led to rapid decomposition, and only
ethynyldiphenylphosphine oxide could be recovered. In contrast, treatment of 2 with elemental sulphur
in anhydrous THF provided the corresponding phosphine sulphide 3 in an acceptable yield
(Scheme 2). Alkyne 3 was also found to be sensitive to protodeboronation, once again; however, we
were able to grow crystals of 3, and the X-ray structure is shown in Figure 3; selected bond lengths
and bond angles are given in Table 2.
Table 2. Selected bond lengths (Ǻ) and angles (°) for 3.
3
C(1)-C(2)
1.197(4)
1.544(5)
1.770(3)
179.3(4)
176.7(3)
114.7(4)
122.1(4)
123.0(4)
103.82(16)
104.20(16)
106.05(14)
B(1)-C(1)
P(1)-C(2)
B(1)-C(1)-C(2)
P(1)-C(2)-C(1)
O(1)-B(1)-O(2)
O(1)-B(1)-C(1)
O(2)-B(1)-C(1)
C(9)-P(1)-C(2)
C(3)-P(1)-C(2)
C(3)-P(1)-C(9)

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Scheme 2. Synthesis of phosphine sulphide.
S
O
O
O
S8
Ph2P
B
Ph2P
B
THF, rt
55%
O
2
3
Figure 3. ORTEP diagram of alkyne 3. H-atoms have been omitted for clarity.
O1-O2-C15-C16 were found to be disordered and refined to an occupancy of 49%–51%.
The X-ray crystal structure shows that 3 exhibits similar C-C, C-B and Calkyne-P bond lengths to
those measured in 2, and again, the alkyne is relatively linear. The boron atom remains within a
trigonal structure (sum of internal bond angles: ~360°), whilst the sum of the internal bond angles of
the three hydrocarbon groups around phosphorus has expanded to ~314°, and the structure adopts a
distorted tetrahedral geometry. This data shows that the oxidation of P(III) in 2 has little effect on the
bond lengths and angles in the alkynylboronate motif, providing further evidence for minimal
conjugation of the P-lone pair with the boronate moiety in 2.
The air and moisture sensitivity of compounds 2 and 3 prompted us to examine the chemistry of
the analogous trifluoroborate salts. Aromatic trifluoroborates have recently emerged as practical
alternatives to boronic acid derivatives for cross-coupling and functionalisation reactions [27,28].
These compounds are typically air and moisture stable crystalline solids and usually generated from
the corresponding boronic acid derivatives after treatment by KHF2. In this event, we were able to
transform ethynyldiphenylphosphine 1 to the corresponding potassium tetrafluoroborate 4, albeit in
modest yield. Moreover, we were able to efficiently generate the more soluble tetraethylammonium
derivative 5 by counterion exchange (Scheme 3).
We were pleased to find that compounds 4 and 5 showed improved stabilities over boronic ester
analogue 2 and set out to investigate the oxidation chemistry of the P-centre in this molecule. Indeed,
we found that we were able to prepare both phosphine sulphide 6 and phosphine oxide 7 from this
compound in good yields (Scheme 4).

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Scheme 3. Alkynyltrifluoroborate synthesis.
1. BuLi
Ph2P
Ph2P
BF3K
2. B(OMe)3
3. KHF2(aq)
39%
1
4
4
Et4NOH(aq)
92%
Ph2P
BF3NEt4
5
Scheme 4. Alkynyltrifluoroborate oxidation chemistry.
Our final objective was to confirm the potential of these substrates for further organic synthesis,
and we chose to investigate the chemistry of trifluoroborate salt 4. Preliminary studies towards Lewis
acid promoted and directed cycloadditions of 2-pyrones and 1,2,4-triazines [29–31] were
disappointing, returning protodeborylated alkyne in all cases attempted. However, we were more
successful in performing cross-coupling reactions. As shown in Scheme 5, Pd-catalysed cross-coupling
with electron-deficient aryl bromides proceeded in a modest yield to provide the corresponding
phosphine oxide after work-up with hydrogen peroxide [32,33].
Scheme 5. Alkynyltrifluoroborate cross-coupling chemistry.
O
1. 9 mol% PdCl2(dppf)
Br
+ Ph2P
PPh2
3 eq. Cs2CO3
THF-H2O (20:1)
reflux, 16 h
2. H2O2
BF3K
R
4
R
8a;
R = CN;
51%
THF-H2O (1:1)
rt, 7 h
R = NO2; 8b; 40%
3. Experimental Section
3.1. General Remarks
All reactions were conducted in an oven or flame-dried glassware under an inert atmosphere of dry
nitrogen. Flash chromatography was performed on silica gel (Fluorochem Davisil silica gel 43–60).
The solvent system used was a gradient of petroleum ether (40–60), increasing in polarity to ethyl
acetate. Thin layer chromatography (TLC) was performed on aluminium-backed plates pre-coated

Molecules 2014, 19
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with silica (0.2 mm, Merck DC-alufolien Kieselgel 60 F254), which were developed using standard
visualizing agents: ultraviolet light or potassium permanganate.
1H-NMR spectra were recorded on a Bruker AV-250 (250 MHz) and AV-400 (400 MHz). Chemical
shifts are reported in ppm with the solvent resonance as the internal standard (CHCl3: δ 7.26 ppm).
13C-NMR spectra were recorded on a Bruker AC-250 (62.9 MHz) and AMX-400 (100.5 MHz) with
complete proton decoupling. Chemical shifts are reported in ppm with the solvent resonance as the
internal standard (CDCl3: δ 77.0 ppm).
Infrared (FTIR) spectra were recorded on a Perkin Elmer Paragon 100 FTIR spectrophotometer,
νmax in cm−1. Bands are characterized as broad (br), strong (s), medium (m) and weak (w). Samples were
recorded as thin films using sodium chloride plates.
Low resolution mass spectra were recorded on Micromass Autospec, operating in E.I., C.I. or FAB
mode; or a Perkin-Elmer Turbomass Benchtop GC-MS operating in either E.I. or C.I mode.
High-resolution mass spectroscopy (HRMS), recorded for accurate mass analysis, were performed on
either a MicroMass LCT operating in Electrospray mode (TOF ES+) or a MicroMass Prospec
operating in either FAB (FAB+), EI (EI+) or CI (CI+) mode.
Melting points were recorded on a Gallenkamp melting point apparatus and are uncorrected.
3.2. Synthesis of Diphenyl((4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)ethynyl)phosphine 2
To a solution of ethynyldiphenylphosphine [34] 1 (1.00 g, 4.76 mmol) in diethyl ether (20 mL)
at 0 °C was added a 2 M solution of n-butyllithium in hexane (2.38 mL, 4.76 mmol). The reaction was
stirred at 0 °C for 30 min before 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (0.97 mL,
4.76 mmol) was added. The reaction was then stirred for 2 h before an anhydrous solution of hydrogen
chloride in diethyl ether (1.3 eq, 1 M) was added and the reaction stirred for a further 30 min. The salts
were removed by filtration under nitrogen and washed with diethyl ether. The volatiles were removed
in vacuo. Diethyl ether was added to the residue, and crystallisation was induced by slow evaporation
1
of the solvent using a nitrogen flow (1.07 g, 67%). M.p. 54–56 °C. (dec.). H-NMR (250 MHz,
CDCl3): δ 1.31 (12H, s, CH3), 7.33–7.38 (6H, m, CH), 7.57–7.66 (4H, m, CH). 13C-NMR (62.9 MHz,
CDCl3): δ 24.7, 47.4, 84.6, 128.6 (d, J = 8.0 Hz), 129.1, 133.0 (d, J = 21.0 Hz), 135.0. 31P-NMR
(101 MHz, CDCl3): δ −32.5. FTIR: 3055 (m), 2979 (s), 2930 (m), 2139 (s), 1436 (s), 1324 (s) cm−1.
HRMS calcd. for C20H22BO2P: 336.1450. Found: 336.1465.
3.3. Synthesis of Diphenyl((4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)ethynyl)thiophosphine 3
Diphenyl((4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)ethynyl)phosphine 2 (500 mg, 1.48 mmol)
was dissolved in THF, and sulphur (142 mg, 4.44 mmol) was added. The reaction was stirred at room
temperature for 48 h. THF was removed by cannula filtration. After evaporation of the solvent, the
residue was dissolved in anhydrous diethyl ether and the product crystallised by slow evaporation of
the solvent using a nitrogen flow (300 mg, 55%). M.p. 75–77 °C (dec.). 1H-NMR (250 MHz, CDCl3):
13
δ 1.30 (12H, s, CH3), 7.39–7.51 (6H, m, CH), 7.88–7.98 (4H, m, CH). C-NMR (62.9 MHz, CDCl3)
31
δ 24.6, 85.3, 94.5, 128.6 (d, J = 15.0 Hz), 131.0 (d, J = 11.0 Hz), 132.0, 133.7. P-NMR (101 MHz,
CDCl3): δ 20.3. FTIR: 3055 (w), 2979 (m), 2925 (m), 2055 (m), 1438 (s), 1348 (s), 1308 (s) cm−1.
HRMS calcd. for C20H22BO2PS: 368.1171. Found: 368.1187.

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3.4. Synthesis of Potassium ((Diphenylphosphino)ethynyl)trifluoroborate 4
To a solution of ethynyldiphenylphosphine 1 (1.0 g, 4.76 mmol) in THF (20 mL) at 0 °C was added
a 2.2 M solution of n-butyllithium in hexane (2.16 mL, 4.76 mmol). After 30 min, trimethyl borate
(0.8 mL, 7.14 mmol) was added and the reaction mixture stirred at this temperature for 1 h, then a
saturated aqueous solution of KHF2 (2.23 g, 28.56 mmol) was added at −10 °C. After 1 h, the reaction
was allowed to warm to room temperature and stirred for a further hour. The organic phase was
separated, dried over MgSO4 and concentrated in vacuo. The residue was purified by flash
chromatography on silica gel (CH3CN/DCM: 1/1) to give potassium ((diphenylphosphino)ethynyl)-
1
trifluoroborate (0.589 g, 39%) as a colourless solid. M.p. 80–82 °C (dec.). H-NMR (250 MHz,
13
d6-DMSO): δ 7.28–7.40 (6H, m, Ar-H), 7.53–7.64 (4H, m, Ar-H). C-NMR (62.9 MHz, d6-DMSO):
19
δ 128.8, 129.0, 129.1, 132.1, 132.4. 31P-NMR (101.1 MHz, d6-DMSO) δ −33.2. F-NMR (376.5 MHz,
CDCl3) δ −136.5. FTIR: 3056 (w), 1478 (m), 1434 (m), 986 (s) cm−1. HRMS calcd. for C14H10BF3K2
+
[Ph2CCBF3K2 ]: 354.9845 Found: 354.9839.
3.5. Synthesis of Tetraethylammonium ((Diphenylphosphino)-ethynyl)trifluoroborate 5
A 25% by volume solution of tetraethylammonium hydroxide in water (0.3 mL, 0.53 mmol) was
added to a suspension of potassium ((diphenylphosphino)-ethynyl)trifluoroborate 4 (155 mg, 0.49 mmol)
in a 4:1 mixture of DCM and water (2.5 mL). The biphasic mixture was stirred for 30 min, and by this
time, all of the starting material had dissolved. The organic layer was separated and the aqueous layer
washed with DCM (2 × 5 mL). The combined organic fractions were dried over MgSO4 and the
solvent evaporated to give tetraethylammonium ((diphenylphosphino)-ethynyl)trifluoroborate (183 mg,
1
92%) as a colourless solid. M.p. 59–68 °C (dec.). H-NMR (250 MHz, CDCl3): δ 1.14 (12H, app. tt,
J = 7.0 Hz, J = 2.0 Hz, CH3), 3.09 (8H, q, J = 7.0 Hz, CH2), 7.24–7.34 (6H, m, Ar-H), 7.59–7.70 (4H,
13
m, Ar-H). C-NMR (62.9 MHz, CDCl3): δ 7.3, 52.4, 128.3–128.6 (m, 2C), 130.8 (d, J = 11.5 Hz),
132.5 (d, J = 20.0 Hz), 137.6 (d, J = 6.5 Hz). 31P-NMR (101.1 MHz, CDCl3) δ −33.8. FTIR: 2991 (w),
−
1480 (m), 1440 (m), 1014 (w), 1484 (m) cm−1. HRMS calcd. for C14H10BF3P [Ph2PCCBF3 ]:
277.0557. Found: 277.0565.
3.6. Synthesis of Tetraethylammonium ((Diphenylphosphorothioyl)-ethynyl)trifluoroborate 6
Sulphur (85 mg, 2.66 mmol) was added to
a
suspension of potassium
((diphenylphosphino)ethynyl)trifluoroborate 4 in THF (1 mL), and the mixture was stirred at room
temperature for 24 h. The solution was filtered via a filter cannula and the solvent evaporated. The
residue was then suspended in a 4:1 mixture of DCM and water (3 mL), and a 25% by volume solution
of tetraethylammonium hydroxide in water (0.36 mL, 0.73 mmol) was added to the reaction mixture.
The biphasic mixture was stirred for 30 min. The organic layer was separated and the aqueous layer
washed with DCM (2 × 5 mL). The combined organic fractions were dried over MgSO4 and the
solvent evaporated to give tetraethylammonium ((diphenylphosphorothioyl)ethynyl)-trifluoroborate
1
(120 mg, 70%) as a yellow solid. M.p. 97–99 °C. H-NMR (250 MHz, CDCl3): δ 1.14 (12H, app. tt,
J = 7.0 Hz, J = 1.5 Hz, CH3), 3.12 (8H, q, J = 7.0 Hz, CH2), 7.35–7.48 (6H, m, Ar-H), 7.92–8.02 (4H,
m, Ar-H). 13C-NMR (62.9 MHz, CDCl3): δ 7.3, 52.4, 80.0, 128.5 (d, J = 14.3 Hz), 130.7 (d, J = 12.5 Hz),
131.5, 134.7 (d, J = 97.0 Hz). 31P-NMR (101.1 MHz, CDCl3): δ 17.9. FTIR: 3627 (w), 3055 (w), 2991

Molecules 2014, 19
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(m), 2360 (m), 2141 (m), 1480 (s), 1438 (s), 1395 (m), 1174 (s), 1101 (s), 1034 (s) cm−1. HRMS calcd.
−
for C14H10BSF3P [Ph2PSCCBF3 ]: 309.0278 Found: 309.0286.
3.7. Synthesis of Tetraethylammonium ((Diphenylphosphoryl)ethynyl)-trifluoroborate 7
A 30% solution of H2O2 (0.122 mL) was added to a suspension of potassium
[(diphenylphosphino)ethynyl]trifluoroborate 4 (530 mg, 1.67 mmol) in a 1:1 mixture of THF and
water (1 mL). The reaction was stirred at room temperature for 16 hours. Then, 2 mL of DCM and a
25% by volume solution of tetraethylammonium hydroxide in water (1.02 mL, 1.80 mmol) were added
to the reaction mixture. The biphasic mixture was stirred for 30 min. The organic layer was separated
and the aqueous layer washed with DCM (2 × 5 mL). The combined organic fractions were dried over
MgSO4, and the solvent evaporated to give tetraethylammonium [(diphenylphosphoryl)ethynyl]-
1
trifluoroborate (602 mg, 85%) as a colourless oil. H-NMR (250 MHz, CDCl3): δ 1.20 (12H, app. tt,
J = 7.0 Hz, J = 1.5 Hz, CH3), 3.19 (8H, q, J = 7.0 Hz, CH2), 7.36–7.54 (6H, m, Ar), 7.80–7.92 (4H, m,
Ar). 13C-NMR (62.8 MHz, CDCl3): δ 7.3, 52.4, 128.4 (d, J = 13.0 Hz), 128.8, 130.8 (d, J = 11.0 Hz),
131.7 (d, J = 2.5 Hz), 134.2 (d, J = 120.0 Hz). 31P-NMR (101.1 MHz, CDCl3) δ 7.4. FTIR: 3,619 (w),
3436 (m), 2992 (m), 2144 (m), 1485 (m), 1439 (s), 1396 (m), 1184 (s), 1121 (s), 1035 (s), 975 (s) cm−1.
−
HRMS calcd. for C14H10BOF3P [Ph2POCCBF3 ]: 293.0514. Found: 293.0514.
3.8. Synthesis of 4-((Diphenylphosphoryl)ethynyl)benzonitrile 8a
Potassium [(diphenylphosphino)ethynyl]trifluoroborate 4 (158 mg, 0.50 mmol, 1 equiv),
4-bromobenzonitrile (91 mg, 0.50 mmol, 1 equiv), PdCl2(dppf).CH2Cl2 (37 mg, 0.045 mmol, 9 mol %),
and caesium carbonate (489 mg, 1.50 mmol, 3 equiv) were mixed with dry THF (5.0 mL) and degassed
water (0.25 mL) under argon (20:1 THF to water ratio). The solution was heated at reflux overnight.
After cooling, 10 mL of water were added, and the resulting solution was extracted with ethyl acetate.
The combined organic extracts were washed with brine, dried over MgSO4 and the solvent evaporated
under reduced pressure. The crude mixture was redissolved in a 1:1 mixture of THF and water, and a
30% solution of H2O2 (0.06 mL, 0.5 mmol, 1 equiv) was added. After 7 h, the organic layer was
extracted, washed with brine and dried over MgSO4, and the solvent was evaporated under reduced
pressure. The resulting oil was purified by flash chromatography on silica gel (starting with petrol
ether, ending with ethyl acetate) to afford 4-[(diphenylphosphoryl)ethynyl]benzonitrile (84 mg, 51%)
as a brown solid. M.p. 144–146 °C. 1H-NMR (400 MHz, CDCl3): δ 7.47–7.51 (4H, m, Ar), 7.54–7.58
(2H, m, Ar), 7.65–7.66 (4H, m, Ar), 7.83–7.89 (4H, m, Ar). 13C-NMR (100.6 MHz, CDCl3): δ 86.5 (d,
J = 161.5 Hz), 102.4 (d, J = 28.0 Hz), 114.0, 117.8, 124.6 (d, J = 4.0 Hz), 128.8 (d, J = 13.0 Hz), 131.0
31
(d, J = 11.0 Hz), 131.6, 132.2, 132.6 (d, J = 3.0 Hz), 133.0 (d, J = 1.5 Hz). P-NMR (162.0 MHz,
CDCl3) δ 8.5. FTIR: 3452 (w), 3058 (m), 2229 (m), 2180 (s), 1603 (w), 1499 (m), 1438 (s), 1204 (s),
1122 (s), 852 (s), 725 (s) cm−1. HRMS calcd. for C21H15NOP: 328.0891. Found: 328.0886.
3.9. Synthesis of ((4-Nitrophenyl)ethynyl)diphenylphosphine Oxide 8b
Potassium ((diphenylphosphino)ethynyl)trifluoroborate 4 (158 mg, 0.50 mmol, 1 equiv), 1-bromo-
4-nitrobenzene (101 mg, 0.50 mmol, 1 equiv), PdCl2(dppf).CH2Cl2 (37 mg, 0.045 mmol, 9 mol %) and
caesium carbonate (489 mg, 1.50 mmol, 3 equiv) were mixed with dry THF (5.0 mL) and degassed water

Molecules 2014, 19
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(0.25 mL) under argon (20:1 THF to water ratio). The solution was heated at reflux overnight. After
cooling, 10 mL of water were added, and the resulting solution was extracted with ethyl acetate. The
combined organic extracts were washed with brine, dried over MgSO4 and the solvent evaporated under
reduced pressure. The crude mixture was redissolved in a 1:1 mixture of THF and water, and a 30%
solution of H2O2 (0.06 mL, 0.5 mmol, 1 equiv) was added. After 7 h, the organic layer was extracted,
washed with brine and dried over MgSO4, and the solvent was evaporated under reduced pressure. The
resulting oil was purified by flash chromatography on silica gel (starting with petrol ether, ending with
ethyl acetate) to afford ((4-nitrophenyl)ethynyl)diphenylphosphine oxide (70 mg, 40%) as a brown solid.
1
M.p. 158–160 °C. H-NMR (400 MHz, CDCl3): δ 7.48–7.53 (4H, m, Ar), 7.55–7.59 (2H, m, Ar),
7.73–7.76 (2H, m, Ar), 7.84–7.90 (4H, m, Ar), 8.20–8.24 (2H, m, Ar). 13C-NMR (125.8 MHz, CDCl3):
δ 87.8 (d, J = 161.0 Hz), 102.0 (d, J = 27.5 Hz), 123.8, 126.5 (d, J = 4.0 Hz), 128.9 (d, J = 14.0 Hz),
131.0 (d, J = 11.5 Hz), 131.7, 132.7 (d, J = 4.0 Hz), 133.5 (d, J = 1.5 Hz), 148.5. 31P-NMR
(162.0 MHz, CDCl3) δ 8.5. FTIR: 3661 (w), 3059 (m), 2181 (s), 1594 (m), 1521 (s), 1438 (m), 1345 (s),
1204 (s), 1122 (m), 865 (s), 708 (s) cm−1. HRMS calcd. for C20H15NO3P: 348.0790. Found: 348.0797.
4. Conclusions
In summary, we report the synthesis and structural characterisation of a novel class of
phosphine-substituted alkynylboronates. Moreover, we have shown that this chemistry can be readily
extended to the corresponding trifluoroborates, these latter reagents being more stable than the
corresponding boronic esters.
Acknowledgments
This work was supported by the EPSRC (EP/D03213X/1) and the FP7 Marie Curie Actions of the
European Commission via the ITN ECHONET Network (MCITN-2012-316379).
Author Contributions
J.F. Vivat and S.P.J.T Bachollet performed the research, H. Adams carried out X-ray
crystallographic analysis and J.P.A. Harrity wrote the manuscript. All authors have approved the final
content of the manuscript.
Conflicts of Interest
The authors declare no conflict of interest.
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