σ- versus π-activation of alkynyl benzoates using B(C6F5)3

Article abstract of DOI:10.3390/molecules20034530

We have prepared a range of alkynyl benzoates in high yields and have investigated their reactivities with the strong Lewis acid B(C₆F₅)₃. In such molecules both σ-activation of the carbonyl and π-activation of the alkyne are possible. In contrast to the reactivity of propargyl esters with B(C₆F₅)₃ which proceed via 1,2-addition of the ester and B(C₆F₅)₃ across the alkyne, the inclusion of an additional CH₂ spacer switches off the intramolecular cyclization and selective σ-activation of the carbonyl group is observed through adduct formation. This change in reactivity appears due to the instability of the species which would be formed through B(C₆F₅)₃ activation of the alkyne.

Full text of DOI:10.3390/molecules20034530

Molecules 2015, 20, 4530-4547; doi:10.3390/molecules20034530
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Communication
σ- versus π-Activation of Alkynyl Benzoates Using B(C
6 5 3
F )
Alexander Bähr, Lewis C. Wilkins, Kevin Ollegott, Benson M. Kariuki and Rebecca L. Melen *
School of Chemistry, Main Building, Cardiff University, Cardiff CF10 3AT, Cymru/Wales, UK;
E-Mails: alexander.baehr@rub.de (A.B.); WilkinsLC@cardiff.ac.uk (L.C.W.);
kevin.ollegott@rub.de (K.O.); KariukiB@cardiff.ac.uk (B.M.K.)
*
Author to whom correspondence should be addressed; E-Mail: MelenR@cardiff.ac.uk;
Tel.: +44-2920-879667.
Academic Editor: John Spencer
Received: 8 February 2015 / Accepted: 28 February 2015 / Published: 12 March 2015
Abstract: We have prepared a range of alkynyl benzoates in high yields and have
investigated their reactivities with the strong Lewis acid B(C
σ-activation of the carbonyl and π-activation of the alkyne are possible. In contrast to the
reactivity of propargyl esters with B(C which proceed via 1,2-addition of the ester and
B(C across the alkyne, the inclusion of an additional CH spacer switches off the
6
F
5
)
3
. In such molecules both
6
F )
5 3
6
F
5
)
3
2
intramolecular cyclization and selective σ-activation of the carbonyl group is observed
through adduct formation. This change in reactivity appears due to the instability of the
species which would be formed through B(C
6
F
5
)
3
activation of the alkyne.
Keywords: boron; trispentafluorophenyl borane; B(C
6
F )
5 3
; Lewis acid; alkyne
1
. Introduction
The intrinsic Lewis acidity of Group 13 compounds has led to their broad application in Lewis
acid- catalyzed reactions in synthetic chemistry. In particular, B(C , which was first reported in the
960s [1,2] has been widely employed in an assortment of transformations in both organic and
organometallic chemistry [3–12]. Owing to its highly electrophilic but sterically protected nature,
B(C has been commonly used as the Lewis acid component in Frustrated Lewis Pair (FLP)
6
F )
5 3
1
6
F )
5 3
chemistry [13–20]. Combinations of a Lewis acid and a Lewis base that do not form an adduct
constitute FLPs and have been widely used in a range of small molecule activation reactions and in
metal free catalysis [13–20]. In our research we have focused on the activation of alkynes by

Molecules 2015, 20
4531
electrophilic boron reagents towards nucleophilic attack. This reactivity is well-precedented in FLP
chemistry and many examples have been reported in which the FLP adds in a 1,2-manner across the
alkyne. Lewis bases in these reactions include bulky amines [21,22] phosphines [23–26] and pyrroles [27].
In the case of terminal alkynes deprotonation may occur, particularly in the case of more basic
t
phosphines, e.g., P Bu
3
, to yield phosphonium borate salts [R
3
PH][R′-C≡C-B(C
6
F
5
3
) ] [26,28–32].
B(C
6
F
5
3
) may also react with an alkyne in the absence of a nucleophile in a 1,1-carboboration
reaction [33–38]. Such reactions have been used to generate intramolecular FLPs and complex boron
containing compounds [39].
Previously we have probed how B(C
6
F
5
3
) can mimic established precious metal π-Lewis acid
catalysts in intramolecular alkyne activation for the generation of oxazoles from propargyl amides [40]
and formation of versatile boron allylation reagents from propargyl esters (Scheme 1) [41]. In all cases
these intramolecular cyclization reactions involve the 1,2-addition of the carbonyl oxygen atom from
the ester or amide and the borane across the alkyne [40–42]. Unlike the reactions of FLPs with
alkynes, in these reactions the Lewis basic carbonyl oxygen atom is not sterically protected and
thus coordination of the oxygen lone pairs to the borane is possible. This competitive activation
process between the carbonyl and the alkyne is reflected in the rates of these cyclization reactions.
For example, amide carbonyl groups coordinate better when compared to ester groups leading to
slower cyclization as a result of poorer alkyne activation [43]. Conversely, propargyl esters undergo
faster π-alkyne activation and hence faster 1,2-addition.
Scheme 1. Cyclization pathways of propargyl amides and esters with the Lewis acid B(C
6
F
5
3
) .
In this study we describe the synthesis of a range of alkynyl benzoates which include an additional
methylene spacer between ester and alkynyl functionalities. We investigate their reactivity with
B(C
6
F
5
3
) potentially affording access to ring-expanded derivatives of the established chemistry
outlined in Scheme 1. Interestingly, π-activation appears to be entirely suppressed in favor of σ-adduct
formation between the carbonyl group and the Lewis acid. Such differences in reactivity between these
alkynyl benzoate substrates and the related propargyl esters and amides are discussed.

Molecules 2015, 20
. Results and Discussion
A series of alkynyl benzoates 1a–c were synthesized in moderate to high yields (72%–83%) from
4532
2
the room temperature reactions of hex-3-yn-1-ol with the corresponding benzoyl chloride derivatives
in the presence of triethylamine as a weak base (Scheme 2). These compounds were fully characterized
by multinuclear NMR, IR and mass spectroscopies.
Scheme 2. Synthesis of alkynyl benzoates.
Addition of the Lewis acid B(C
6
F
5
) to 1 at ambient temperature resulted in adduct formation between
3
1
1
the ester oxygen atom and the vacant orbital at boron, evidenced by B-NMR data which displayed a
1
1
19
broad peak consistent with other carbonyl adducts of B(C
6
F
5
)
3
[43]. The B- and F-NMR spectra are
to alkynyl
the F and B spectra correspond closely to that of free
dependent upon both the concentration of the reaction and on the mole ratio of B(C
6
F )
5 3
19
11
benzoate. With a large excess of B(C
6
F
5
)
3
1
9
11
B(C . Conversely with a large excess of ester, the peaks in both the F- and B-NMR spectra shift
6
F )
5 3
11
to high field. In the B-NMR spectrum the signal is broad and its chemical shift is consistent with
adduct formation. These observations are consistent with an equilibrium whose dynamics are rapid on
the NMR timescale. These are supported by concentration dependent measurements which show an
1
1
upfield shift in the B-NMR spectrum with increasing concentration whose chemical shifts are close
to that with excess alkynyl benzoate. At a low concentration (0.04 M) the positions correspond closely
to the reactions with a ten-fold excess of B(C
6
F
5
)
3
and that of free B(C
6
F
5
3
) (Figure 1).
1
9
Figure 1. F stacked spectra of the reactions of 1b with B(C
6
F
5
3
) .

Molecules 2015, 20
4533
The 1:1 stoichiometric reactions of alkynyl benzoates with B(C
6
F
5
)
3
on a 0.2 mmol scale followed
by recrystallization resulted in the formation of the ester-B(C
6
F
5
3
) adducts 2a–c (Scheme 3) which
were characterized by X-ray diffraction (vide infra). The IR spectra of the adducts 2 all show a
−1
red-shift in the carbonyl stretching frequency relative to the alkynyl benzoates 1 of ca. 70 cm upon
coordination to boron (Table 1).
Scheme 3. Formation of adducts from the reactions of alkynyl benzoates with B(C
6
F
5
3
) .
Table 1. IR stretching frequencies for free and coordinated alkynyl benzoates.
−
1
IR Carbonyl Stretching Frequency (cm )
X
ν
CO (1)
ν
CO (2)
ΔνCO
70
H (a)
Me (b)
OMe (c)
1717
1717
1713
1647
1647
1645
70
68
2
.1. Crystallographic Studies
Large colorless crystals of 2a–c suitable for X-ray diffraction could be obtained by cooling a very
concentrated hot toluene/petroleum ether solution. The solvent could then be decanted off and the
crystals washed to give analytically pure 2a and 2b in 36%–38% recovered yield, whilst 2c was
recovered in 26% yield. The adducts 2a–c all crystallized in the triclinic P-1 space group with one
molecule in the asymmetric unit (Figure 2). Compounds 2a and 2b show very similar B(1)-O(1) bond
lengths [1.589(2) Å and 1.585(2)Å (2a and 2b respectively)] and are identical within error (Table 2).
However, the B(1)-O(1) bond length in 2c is shorter at 1.565(2) Å. These distances are all similar to
B-O bond lengths observed previously; a search of the CSD (2013) revealed 2822 structures
containing a B-O bond between 4-coordinate boron and 2-coordinate oxygen with a mean B-O
distance of 1.48(4) Å. The C=O bond lengths in 2 are 1.247(2) Å (2a), 1.255(2) Å (2b), 1.255(2) Å
(
2c) and are just slightly longer than the mean C=O bond distances for conventional ester compounds
reported on the CSD (52047 structures, 2013) at 1.20 Å. In all three cases, the adducts adopt a bent
2
geometry from the donation of the HOMO of the carbonyl (one of the lone pairs in an sp orbital on
the oxygen atom) with the borane coordinated in a formally cis conformation to the aryl group with
respect to the C=O. The C(1)-O(1)-B(1) angles in 2a–c are 135.5(1)°, 135.8(1)° and 138.4(1)°

Molecules 2015, 20
4534
respectively. The C(2)-C(1)-O(1)-B(1) dihedral angle in 2a–c are 23.18°, 29.62° and 33.80° respectively
with the boron atom lying out of the C(2)-C(1)-O(1) plane. This presumably arises due to steric
interactions between the aryl group on the alkynyl benzoates and the perfluoroaryl groups on boron
since a dihedral angle of 0° would be expected to be the most favorable energetically [43]. In all cases
the aryl ring on the alkynyl benzoates is rotated slightly such that there is reduced conjugation with the
carbonyl group with C(3)-C(2)-C(1)-O(1) dihedral angles of 32.54° (2a), 31.12° (2b) and 13.02° (2c).
The distortions are very similar for 2a and 2b although this distortion for 2c is much less suggesting a
greater extent of conjugation presumably brought about by the electron donating ability of the para-oxygen
atom. This is also reflected in a slightly shorter C(2)-C(1) bond between the aryl ring and the carbonyl
group of 1.451(2) Å (2c) in relation to the same C(2)-C(1) bond length in 2a [1.474(2) Å].
Figure 2. Crystal structure of 2a (top), 2b (middle) and 2c (bottom). C: grey, O: red, H:
white, B: yellow-green, F: pink.

Molecules 2015, 20
4535
Table 2. Structural properties of 2a–c.
Compound
2b
2
a
2c
B(1)-O(1)/Å
C(1)-O(1)/Å
1.589(2) 1.585(2) 1.565(2)
1.247(2) 1.255(2) 1.255(2)
1.474(2) 1.462(2) 1.451(2)
135.5(1) 135.8(1) 138.4(1)
C(2)-C(1) bond length/Å
C(1)-O(1)-B(1) angle/°
C(2)-C(1)-O(1)-B(1) dihedral angle/°
C(3)-C(2)-C(1)-O(1) dihedral angle/°
23.18
32.54
29.62
31.12
33.80
13.02
2.2. Computational Studies
The electron-donating abilities of the aryl ring increase in the order Ph < p-MeC
6
H
4
< p-MeOC
6
H
4
based on their Hammett parameters (0.000, −0.170 and −0.268 respectively) [44]. These appear in
agreement with the B-O bond lengths which show a shortening with increasing donor ability.
However, the variation in the C-O bond lengths and particularly the change in νCO are more ambiguous
and prompted us to undertake theoretical calculations to probe this behavior. DFT studies were
undertaken to determine the optimized structures (B3LYP/6-31G*) and thermodynamic calculations
were determined using the higher level triple zeta 6-311G* basis set. Calculations were undertaken on
the esters 1, B(C
6
F
5
3
) and the corresponding adducts 2. The B-O and C-O bond lengths in the
geometry-optimized structures and the energetics of adduct formation (corrected for ZPE) are
presented in Table 3. These clearly support the general geometric changes reflected in the
crystallographic and IR data that adduct formation occurs with concomitant weakening of the C=O
bond with the computed energetics correlating well with those expected based on the Hammett
parameter. The apparent anomalous behavior in the IR spectra of 2c is not manifested in these
calculations and may arise as a feature of the solid state packing (in relation to gas phase
−1
computations). The slightly smaller shift in ΔνCO (2 cm ) corresponds to just 0.02 kJ/mol and some
slight weakening of this interaction could easily be absorbed to accommodate crystal packing forces.
In this context it is notable that the torsion associated with the aryl-carboxyl fragment is substantially
smaller for 2c than 2a and 2b.
Table 3. B3LYP/6-31G* geometry-optimized B-O and C-O bond lengths determined for 1
and 2 along with energetics of adduct formation based on B3LYP/6-311G* calculations.
Ester C=O/Å Adduct C-O/Å B-O/Å ∆Hadduct/kJ·mol−1
1
a
1.21
1.21
1.21
2a
2b
2c
1.25
1.25
1.25
1.61
1.60
1.60
−8
1
b
−14
−17
1
c

Molecules 2015, 20
4536
The enthalpy of adduct formation in all cases is small when compared to a classical B-O covalent
bond (ca. 530 kJ/mol) [45] but is consistent with the significant steric demands of the B(C group.
6
F )
5 3
These enthalpy changes indicate that this is likely a reversible process as is experimentally observed
for adducts of propargyl amides and esters [40–42]. Indeed the Gibbs free energy changes for adduct
formation are all positive, in agreement with such a supposition.
2
.3. Effect of Temperature
We subsequently investigated if these compounds would undergo 1,2-addition to form the zwitterionic
,2-addition products similar to those seen previously with the reactions of propargyl esters and amides
1
with B(C
6
F
5
) [40–42]. In both those cases the initial adduct could be driven to dissociate and, at
3
elevated temperatures, undergo 1,2-addition at the alkyne. In contrast to the propargyl esters and amides,
even after extended heating these reactions showed no significant sign of 1,2-addition products.
11
Although, in the in situ B-NMR reactions of 1 with B(C
6
F
5
) a sharp signal of extremely low intensity
3
at −17.0 ppm (1a) and at −17.1 ppm (1c) could be observed after 4 days at 45 °C. This sharp signal is
typical for four coordinate borate species indicating possible B−C bond formation. This chemical shift
is similar to that observed for the cyclization of propargyl esters with B(C
chemical shift at −17.1 ppm [41]. At elevated temperatures there is no doubt that the initial adduct will
be in equilibrium with the free acid B(C and ester in solution, the lack of reactivity of the alkyne
6
F
5
)
3
which gave rise to a
6
F )
5 3
is therefore unexpected and presumably arises from some instability in the initial six-membered ring
product formed by cyclization. In this context we considered the mechanistic process in more detail
(
Scheme 4). Previously it was suggested that sterically demanding propargyl amides may undergo
reversible 1,2-addition [40].We therefore attribute the lack of significant amounts of 1,2-addition product
to the instability of the carbocation in the zwitterionic product (I). In previous studies, propargyl
amides undergo 1,2-addition to afford stable zwitterionic 5-alkylidene-4,5-dihydrooxazolium borate
compounds [40] (II) in which the positive charge is localized predominantly on the amide nitrogen atom
which exhibits better stabilization of positive charge over oxygen. In the case of the isolobal propargyl
esters the 1,2-addition product was also observed to give III. However, this was found to be unstable
and to rearrange rapidly with ring opening in solution to give allyl boron compounds (Scheme 5) [41].
This rapid rearrangement was attributed to the instability of the carbocation formed in the 1,2-addition
product and also supports the instability of the 1,2-addition product, I. In addition formation of
6
-membered rings is somewhat less favorable than 5-membered rings and so the additional methylene
group in 1 compared to the propargyl esters also mitigates the propensity ring closure.
Scheme 4. Adduct formation versus 1,2-addition.

Molecules 2015, 20
4537
Scheme 5. 1,2-Addition products.
3. Experimental Section
3.1. General Information
With the exception of the synthesis of starting materials, all reactions including storage of the starting
materials, room temperature reactions, product recovery and sample preparation for analysis were carried
out under a dry, O -free atmosphere using a nitrogen-filled glove box (MBRAUN, Garching, Germany).
2
Molecular sieves (4 Å) were dried at 150 °C for 48 h prior to use. Toluene and DCM solvents were
dried by employing a Grubbs-type column system (MBRAUN), degassed and stored over molecular
sieves under a nitrogen atmosphere. Petroleum ether (bp. 40–60 °C) was distilled and stored over
molecular sieves. Deuterated CDCl
3
was dried over molecular sieves before use. Chemicals were
1
13
11
purchased from commercial suppliers and used as received. H, C and B and spectra were recorded
1
9
on Avance DPX-500 or 400 spectrometers (Bruker, Billerica, MA, USA). F-NMR were recorded on
a JEOL Eclipse 300 spectrometer (Peabody, MA, USA). Chemical shifts are expressed as parts per
million (ppm, δ) downfield of tetramethylsilane (TMS) (δ = 0 ppm) and are referenced to CDCl
3
as
1
9
(11B). All
internal standards. NMR spectra were referenced to CFCl
3
( F) and BF
3
•Et
2
O/CDCl
3
coupling constants are absolute values and J values are expressed in Hertz (Hz). Mass spectral data
were performed in house employing electrospray ionization techniques in positive ion mode. Infrared
−1
spectra were recorded on an IRAffinity FT-IR spectrometer (Shimadzu, Kyot, Japan). Infrared data
−1
are quoted in wavenumbers (cm ). Elemental analysis results were determined by Mr. Stephen Boyer
using the elemental analysis service at London Metropolitan University, U.K.
3.2. Synthesis of Starting Materials
3
.2.1. Synthesis of Hex-3-yn-1-yl benzoate (1a)
To DCM (100 mL), triethylamine (TEA, 14 mL, 100 mmol) and benzyl chloride (5.8 mL, 50 mol)
were added at 273 K. 3-Hexyn-1-ol (5.5 mL, 50 mmol,) was then added slowly to this solution. The

Molecules 2015, 20
4538
reaction was stirred overnight at 298 K. The resulting solution was then washed with water and brine
and the solvent was removed to give a dark yellow oil. The oil was cooled to −50 °C to give a solid
which was then washed with cold hexane to give pure 1a. Yield: 8.27 g, 41 mmol, 82%. IR
−
1
(
wavenumbers in cm ): 2978, 2938, 2361, 1717, 1603, 1584, 1452, 1385, 1316, 1267, 1109, 1069,
1
, 298 K): 8.03 (m, 2H, o-ArH), 7.52 (tt, 1H, 3JHH = 7.8 Hz,
1
026, 708. H-NMR (500 MHz, CDCl
3
4JHH = 1.1 Hz, p-ArH), 7.40 (t, 2H, 3JHH = 7.9 Hz, m-ArH), 4.35 (t, 2H, 3JHH = 7.0 Hz, -CH
O(CO)-),
2
-C≡), 1.08 (t, 3H, 3JHH = 7.4 Hz, -CH
). C-NMR
13
2
.60 (m, 2H, -C≡CCH
125 MHz, CDCl , 298 K): 166.3, 132.9, 130.2, 129.6, 128.3, 83.5, 74.9, 63.3, 19.4, 14.1, 12.4. MS
ES , m/z): 202.10 (M+), 123.03, 105.69, 80.05, 77.02, 76.03, 65.04.
2
-CH
2
-), 2.13 (m, 2H, Me-CH
2
3
(
(
3
+
3
.2.2. Synthesis of Hex-3-yn-1-yl 4-methylbenzoate (1b)
To DCM (100 mL), TEA (14 mL, 100 mmol) and p-tolyl chloride (8.0 mL, 60 mmol) were added at
73 K. 3-Hexyn-1-ol (5.5 mL, 50 mmol,) was then slowly added. The reaction was stirred overnight at
98 K. The solution was then washed with water and brine and the solvent removed to give a yellow
2
2
oil. The oil was purified by column chromatography using a mixture of hexane and ethyl acetate
−
1
(
80/20 vol. %) to give pure 1b. Yield: 7.82 g, 36 mmol, 72%. IR (wavenumbers in cm ): 2972, 2940,
1
2
367, 1717, 1613, 1578, 1508, 1454, 1385, 1310, 1270, 1177, 1105, 1020, 752. H-NMR (400 MHz,
CDCl
3JHH = 7.1 Hz, -CH
CH
), 2.15 (qt, 2H, 3JHH = 7.5 Hz, 4JHH = 2.4 Hz, Me-CH
3
, 298 K): 7.94 (d, 2H, 3JHH = 8.4 Hz, o-ArH), 7.23 (d, 2H, 3JHH = 8.4 Hz, m-ArH), 4.36 (t, 2H,
O(CO)-), 2.61 (tt, 2H, 3JHH = 7.1 Hz, 4JHH = 2.4 Hz, -C≡CCH
-CH -), 2.40 (s, 3H,
-C≡), 1.11 (t, 3H, 3JHH = 7.5 Hz, -CH
).
2
2
2
-
3
2
3
13C-NMR (101 MHz, CDCl
, 298 K): 166.5, 143.7, 129.8, 129.1, 127.5, 83.6, 80.0, 63.2, 21.8, 19.5,
3
+
+
1
4.2, 12.5. MS (ES , m/z): 216.115 (M ), 137.04, 119.75, 91.04, 79.04, 77.04, 65.03.
3
.2.3. Synthesis of Hex-3-yn-1-yl 4-methoxybenzoate (1c)
To DCM (100 mL), TEA (14 mL, 100 mmol) and 6.8 mL 4-methoxybenzylchloride (50 mmol)
were added at 273 K. 3-Hexyn-1-ol (5.5 mL, 50 mmol) was then added slowly. The reaction was
subsequently stirred overnight at 298 K. The resulting solution was washed with water and brine and
the solvent removed to give a brown solid which was washed with cold hexane to give pure 1c. Yield:
−
1
9
1
8
.62 g, 41 mmol, 83%. Melting point: 42 °C. IR (wavenumbers in cm ): 2974, 2363, 1780, 1713,
1
605, 1578, 1512, 1387, 1317, 1256, 1167, 1101, 1028, 843, 768. H-NMR (500 MHz, CDCl
3
, 298 K):
.00 (d, 2H, 3JHH = 8.9 Hz, o-ArH), 6.91 (d, 2H, 3JHH = 8.9 Hz, m-ArH), 4.34 (t, 2H, 3JHH = 7.1 Hz,
O(CO)-), 3.85 (s, 3H, -OCH -CH -), 2.15
), 2.60 (tt, 2H, 3JHH = 7.1 Hz, 4JHH = 2.3 Hz, -C≡CCH
qt, 2H, 3JHH = 7.5 Hz, 4JHH = 2.3 Hz, Me-CH -C≡), 1.11 (t, 3H, 3JHH = 7.5 Hz, -CH ). 13C-NMR
-
(
(
CH
2
3
2
2
2
3
125 MHz, CDCl
3
, 298 K): 166.2, 163.5, 131.8, 122.8, 113.7, 83.6, 75.1, 63.2, 55.6, 19.6, 14.3, 12.5.
+
+
MS (ES , m/z): 232.11 (M ), 152.02, 135.02, 107.05, 92.02, 80.06, 79.05, 77.03, 65.04, 64.03.
3
.2.4. Synthesis of Trispentafluorophenylborane, B(C
6
F )
5 3
Trispentafluorophenylborane [B(C
previously [46]. Magnesium turnings (7.2 g, 0.3 mol) were suspended in ether (ca. 600 mL) and a
small amount of iodine added followed by the addition of a little BrC (74.1 g, 0.3 mol) dropwise
F
6 5
3
) ] was synthesized in a manner similar to that reported
F
6 5

Molecules 2015, 20
4539
was
resulting in a turbid grey mixture. Once the Grignard reaction had initiated, the remaining BrC
F
6 5
added slowly whilst making sure the solution does not reflux by cooling the reaction on an ice bath
when necessary. Once the addition of BrC was complete, the resulting mixture was stirred for 1h at
room temperature giving a dark brown/black solution. The solution was then cooled to 0 °C and
transferred to a cooled solution of BF ·OEt (14.19 g, 0.1 mol) in toluene (ca. 200 mL). The resulting
F
6 5
3
2
solution was allowed to warm to room temperature and the majority of the ether solvent was removed
in vacuo. The resulting solution was then heated to 95 °C for 1h and the remaining solvent removed to
give a brown solid. The solid was extracted with hot petroleum ether (500 mL) and the solution cooled
to −80 °C to result in crystallization of B(C
6
F ) . The solid was extracted three further times using the
5
3
same solvent from the recrystallization mixture. The solvent was then filtered off from the B(C
6
F
5 3
)
1
9
and the product dried under vacuum. F-NMR (376 MHz, CDCl
143.32 (br. s, 1F, p-F), −159.91 (m, 2F, m-F).
3
, 298 K): −127.89 (br. s, 2F, o-F),
−
3
.3. Synthesis of Adducts
3
.3.1. Synthesis of 2a
Compound 1a (40 mg, 0.2 mmol) was dissolved in toluene (5 mL) and was added to B(C
5
F )
6 3
(
105 mg, 0.2 mmol). The solution was left overnight and the solvent was removed and the remaining
brown oil was recrystallized from a concentrated solution of pet. ether (40–60) and DCM. The crystals
were washed with pet. ether (3 × 2 mL) to afford the pure product 2a. Yield: 51 mg, 0.07 mmol, 36%.
−1
Melting point: 119 °C. IR (wavenumbers in cm ): 3420, 3171, 2336, 1647, 1591, 1570, 1458, 1285,
1
1
235, 1103, 980, 719. H-NMR (500 MHz, CDCl
3
, 298 K) crystals: 8.03 (m, 2H, o-ArH), 7.57 (tt, 1H,
3JHH = 7.4 Hz, 4JHH = 1.3 Hz, p-ArH), 7.44 (br. t, 2H, 3JHH = 7.8 Hz, m-ArH), 4.37 (t, 2H, 3JHH = 7.1 Hz,
-
CH
4JHH = 2.5 Hz, Me-CH
98 K, 0.2 M): 170.4 (s), 148.0 (m, C
s), 128.5 (s), 115.8 (m, C ), 84.3 (s), 74.7 (s), 66.1 (s), 19.4 (s), 14.1 (s), 12.5 (s). F-NMR (376
MHz, CDCl , 298 K) crystals: −128.26 (br. s, 2F, o-F), −143.71 (br. s, 1F, p-F), −160.01 (m, 2F, m-F).
Elemental analysis calcd (%) for C31 ·2DCM: C 44.83, H 2.05; Obs. C 44.73, H 1.78.
2
O(CO)-), 2.62 (tt, 2H, 3JHH = 7.1 Hz, 4JHH = 2.4 Hz, -C≡CCH
-C≡), 1.11 (t, 3H, 3JHH = 7.5 Hz, -CH
), 141.8 (m, C ), 137.4 (m, C
2
-CH
2
-), 2.16 (qt, 2H, 3JHH = 7.5 Hz,
13
2
3
). In situ C-NMR (125 MHz, CDCl
3
,
2
6
F
5
6
F
5
6
F ), 134.2 (s), 130.2 (s), 128.7
5
19
(
F
6 5
3
H14BF15
O
2
3
.3.2. Synthesis of 2b
Compound 1b (43 mg, 0.2 mmol) was dissolved in toluene (5 mL) and was added to B(C
F
5 6
)
3
(
105 mg, 0.2 mmol). The solution was left overnight and the solvent was removed and the remaining
brown oil was recrystallized from a concentrated solution of pet. ether (40–60) and DCM. The crystals
were washed with pet. ether (3 × 2 mL) to afford the pure product 2b. Yield: 55 mg, 0.08 mmol, 38%.
−1
Melting point: 130 °C. IR (wavenumbers in cm ): 3217, 2916, 2346, 1647, 1587, 1559, 1518, 1464,
1
, 298 K) crystals: 7.86 (d, 2H, 3JHH = 8.1 Hz,
1
285, 1105, 1086, 970. H-NMR (500 MHz, CDCl
3
o-ArH), 7.21 (d, 2H, 3JHH = 8.1 Hz, m-ArH), 4.44 (t, 2H, 3JHH = 6.9 Hz, -CH
O(CO)-), 2.64 (tt, 2H,
2
3JHH = 7.0 Hz, 4JHH = 2.4 Hz, -C≡CCH
4JHH = 2.4 Hz, Me-CH -C≡), 1.11 (t, 3H, 3JHH = 7.5 Hz, -CH
98 K, 0.2 M): 171.1 (s), 147.9 (m, C
-CH
-), 2.40 (s, 3H, -CH
), 2.15 (qt, 2H, 3JHH = 7.5 Hz,
2
2
3
13
2
3
). In situ C-NMR (125 MHz, CDCl
3
,
2
6
F
5
), 145.9 (s), 145.5 (s), 141.0 (m, C
6
F
5
), 137.3 (m, C
6
5
F ), 130.5

Molecules 2015, 20
4540
19
(
s), 129.2 (s), 116.5 (m, C
376 MHz, CDCl , 298 K) crystals: −130.23 (br. s, 2F, o-F), −147.73 (br. s, 1F, p-F), −160.06 (m, 2F,
: C 52.78, H 1.98; Obs. C 52.65, H 1.73.
6
F
5
), 84.3 (s), 74.0 (s), 66.4 (s), 21.8 (s), 19.4 (s), 14.1 (s), 12.4 (s). F-NMR
(
3
m-F). Elemental analysis calcd (%) for C32
H16BF15
O
2
3
.3.3. Synthesis of 2c
Compound 1c (46 mg, 0.2 mmol) was dissolved in toluene (5 mL) and was added to B(C
5
F )
6 3
(
105 mg, 0.2 mmol). The solution was left overnight and the solvent was removed and the remaining
brown oil was recrystallized from a concentrated solution of pet. ether (40–60) and DCM. The crystals
were washed with pet. ether (3 × 2 mL) to afford the pure product 2c. Yield: 39 mg, 0.05 mmol, 26%.
−1
Melting point: 118 °C. IR (wavenumbers in cm ): 3165, 2955, 2355, 1645, 1605, 1557, 1516, 1454,
1
1
379, 1277, 1177, 1099, 1026, 974, 770. H-NMR (500 MHz, CDCl
3
, 298 K) crystals: 7.95 (d, 2H,
O(CO)-),
3JHH = 9.0 Hz, o-ArH), 6.89 (d, 2H, 3JHH = 9.0 Hz, m-ArH), 4.41 (t, 2H, 3JHH = 7.0 Hz, -CH
2
3
.86 (s, 3H, -OCH
3JHH = 7.6 Hz, 4JHH = 2.3 Hz, Me-CH
MHz, CDCl , 298 K, 0.2 M): 171.6 (s), 165.2 (s), 147.8 (m, C
33.2 (s), 119.1 (s), 116.1 (m, C ), 113.7 (s), 84.3 (s), 73.6 (s), 66.9 (s), 66.1 (s), 55.6 (s), 19.2 (s),
3.9 (s), 12.2 (s). F-NMR (376 MHz, CDCl , 298 K) crystals: −129.89 (br. s, 2F, o-F), 147.07 (br. s,
F, p-F), −160.89 (m, 2F, m-F). Elemental analysis calcd (%) for C32 : C 51.64, H 2.17; Obs.
3
), 2.63 (tt, 2H, 3JHH = 7.0 Hz, 4JHH = 2.3 Hz, -C≡CCH
-C≡), 1.11 (t, 3H, 3JHH = 7.6 Hz, -CH
), 141.2 (m, C
2
-CH
2
-), 2.16 (qt, 2H,
13
2
3
). In situ C-NMR (125
3
6
F
5
6
F
5
), 137.1 (m, C
6
F ),
5
1
1
1
F
6 5
1
9
3
H16BF15
O
3
C 51.54, H 2.07.
3.4. In Situ NMR Studies of Varying Concentration
3
.4.1. NMR Experiments of 2a
Samples of 0.4, 0.2, 0.1 and 0.04 M concentration were prepared by dissolving the required amount
1
of 2a and B(C
6
F
5
)
3
in a 1:1 ratio in CDCl
3
(0.5 mL). H-NMR (500 MHz, CDCl
3
, 298 K, 0.4 M): 7.87
(
dd, 2H, 3JHH = 8.7 Hz, 4JHH = 1.2 Hz, o-ArH), 7.59 (br. tt, 1H, 3JHH = 7.5, 4JHH = 1.2 Hz, p-ArH), 7.40
br. t, 2H, 3JHH = 7.9, m-ArH), 4.58 (t, 2H, 3JHH = 7.0 Hz, -CH O(CO)-), 2.68 (tt, 2H, 3JHH = 7.0 Hz,
(
2
4JHH = 2.5 Hz, -C≡CCH
-CH
-), 2.18 (qt, 2H, 3JHH = 7.6 Hz, 4JHH = 2.3 Hz, Me-CH
-C≡), 1.12 (t, 3H,
2
2
2
3JHH = 7.6 Hz, -CH
CDCl
500 MHz, CDCl
3JHH = 7.5, 4JHH = 1.2 Hz, p-ArH), 7.40 (br. t, 2H, 3JHH = 8.1, m-ArH), 4.53 (t, 2H, 3JHH = 6.7 Hz,
CH -CH
O(CO)-), 2.67 (tt, 2H, 3JHH = 7.0 Hz, 4JHH = 2.5 Hz, -C≡CCH -), 2.17 (qt, 2H, 3JHH = 7.6 Hz,
4JHH = 2.3 Hz, Me-CH
). B-NMR (160 MHz, CDCl , 298 K,
, 298 K, 0.2 M): −131.73 (s, 2F, o-F), −150.78 (s,
F, p-F), −161.93 (m, 2F, m-F). H-NMR (500 MHz, CDCl , 298 K, 0.1 M): 8.0 (dd, 2H,
). B-NMR (160 MHz, CDCl
11
, 298 K, 0.4 M): 40.10, 28.51. F-NMR (376 MHz,
19
3
3
1
3
, 298 K, 0.4 M): −132.07 (s, 2F, o-F), −151.36 (s, 1F, p-F), −162.18 (m, 2F, m-F). H-NMR
(
3
, 298 K, 0.2 M): 7.91 (dd, 2H, 3JHH = 8.6 Hz, 4JHH = 1.1 Hz, o-ArH), 7.59 (br. tt, 1H,
-
2
2
2
-C≡), 1.12 (t, 3H, 3JHH = 7.5 Hz, -CH
11
2
3
3
1
9
0
.2 M): 40.20, 30.27. F-NMR (376 MHz, CDCl
3
1
1
3
3JHH = 8.2 Hz, 4JHH = 1.4 Hz, o-ArH), 7.58 (br. tt, 1H, 3JHH = 7.5, 4JHH = 1.3 Hz, p-ArH), 7.41 (br. t,
2
H, 3JHH = 8.0, m-ArH), 4.49 (t, 2H, 3JHH = 6.9 Hz, -CH
4JHH = 2.4 Hz, -C≡CCH -), 2.17 (qt, 2H, 3JHH = 7.6 Hz, 4JHH = 2.4 Hz, Me-CH
-CH
3JHH = 7.5 Hz, -CH ). 11B-NMR (160 MHz, CDCl
CDCl
2
O(CO)-), 2.66 (tt, 2H, 3JHH = 7.0 Hz,
2
2
2
-C≡), 1.12 (t, 3H,
, 298 K, 0.1 M): 39.51. 19F-NMR (376 MHz,
3
3
1
3
, 298 K, 0.1 M): −130.79 (s, 2F, o-F), −148.84 (s, 1F, p-F), −161.40 (m, 2F, m-F). H-NMR

Molecules 2015, 20
4541
(
500 MHz, CDCl
3JHH = 7.5, 4JHH = 1.3 Hz, p-ArH), 7.43 (br. t, 2H, 3JHH = 7.9, m-ArH), 4.45 (t, 2H, 3JHH = 7.0 Hz,
CH -CH
O(CO)-), 2.64 (tt, 2H, 3JHH = 7.0 Hz, 4JHH = 2.4 Hz, -C≡CCH -), 2.17 (qt, 2H, 3JHH = 7.6 Hz,
4JHH = 2.3 Hz, Me-CH
). B-NMR (160 MHz, CDCl , 298 K,
, 298 K, 0.04 M): −129.60 (s, 2F, o-F), −146.36 (s, 1F,
3
, 298 K, 0.04 M): 8.0 (dd, 2H, 3JHH = 8.5 Hz, 4JHH = 1.1 Hz, o-ArH), 7.57 (br. tt, 1H,
-
2
2
2
-C≡), 1.12 (t, 3H, 3JHH = 7.5 Hz, -CH
.04 M): 46.88. F-NMR (376 MHz, CDCl
p-F), −160.73 (m, 2F, m-F).
11
2
3
3
1
9
0
3
3
.4.2. NMR Experiments of 2b
Samples of 0.4, 0.2, 0.1 and 0.04 M concentration were prepared by dissolving the required amount
1
of a 2b and B(C
6
F
5
)
3
in a 1:1 ratio in CDCl
3
(0.5 mL). H-NMR (500 MHz, CDCl
3
, 298 K, 0.4 M):
7
.75 (d, 2H, 3JHH = 8.3 Hz, o-ArH), 7.18 (d, 2H, 3JHH = 8.0 Hz, m-ArH), 4.58 (t, 2H, 3JHH = 6.9 Hz,
CH -CH -), 2.39 (s, 3H, Ar-CH ), 2.17
O(CO)-), 2.68 (tt, 2H, 3JHH = 6.9 Hz, 4JHH = 2.4 Hz, -C≡CCH
qt, 2H, 3JHH = 7.5 Hz, 4JHH = 2.3 Hz, Me-CH -C≡), 1.12 (t, 3H, 3JHH = 7.5 Hz, -CH ). 11B-NMR
160 MHz, CDCl , 298 K, 0.4 M):
, 298 K, 0.4 M): 18.09, −16.63. 19F-NMR (376 MHz, CDCl
-
(
(
2
2
2
3
2
3
3
3
−
133.11 (d, 2F, 3JFF = 19.7 Hz, o-F), −153.76 (t, 1F, 3JFF = 20.1 Hz, p-F), −162.85 (td, 2F, 3JFF = 20.9 Hz,
4JFF = 6.9 Hz, m-F). H-NMR (500 MHz, CDCl
1
, 298 K, 0.2 M): 7.76 (d, 2H, 3JHH = 8.2 Hz,
O(CO)-), 2.67 (tt, 2H,
), 2.17 (qt, 2H, 3JHH = 7.6 Hz,
, 298 K,
3
o-ArH), 7.19 (d, 2H, 3JHH = 8.2 Hz, m-ArH), 4.55 (t, 2H, 3JHH = 6.9 Hz, -CH
3JHH = 6.9 Hz, 4JHH = 2.3 Hz, -C≡CCH
4JHH = 2.4 Hz, Me-CH
2
2
-CH
2
-), 2.39 (s, 3H, Ar-CH
3
-C≡), 1.12 (t, 3H, 3JHH = 7.5 Hz, -CH
). B-NMR (160 MHz, CDCl
11
2
3
3
0
.2 M): 23.07. 19F-NMR (376 MHz, CDCl
152.60 (t, 1F, 3JFF = 19.8 Hz, p-F), −162.46 (td, 2F, 3JFF = 20.7 Hz, 4JFF = 6.9 Hz, m-F). H-NMR
3
, 298 K, 0.2 M): −132.58 (d, 2F, 3JFF = 18.4 Hz, o-F),
1
−
(
500 MHz, CDCl
3
, 298 K, 0.1 M): 7.80 (br. d, 2H, 3JHH = 8.4 Hz, o-ArH), 7.20 (d, 2H, 3JHH = 8.0 Hz,
O(CO)-), 2.65 (tt, 2H, 3JHH = 7.0 Hz, 4JHH = 2.4 Hz,
), 2.17 (qt, 2H, 3JHH = 7.6 Hz, 4JHH = 2.4 Hz, Me-CH
-C≡), 1.11
, 298 K, 0.1 M): 27.34. F-NMR (376 MHz,
m-ArH), 4.50 (t, 2H, 3JHH = 7.0 Hz, -CH
2
-
(
C≡CCH
2
-CH
2
-), 2.40 (s, 3H, Ar-CH
3
2
t, 3H, 3JHH = 7.5 Hz, -CH
). B-NMR (160 MHz, CDCl
11
19
3
3
CDCl
3
, 298 K, 0.1 M): −132.13 (d, 2F, 3JFF = 19.7 Hz, o-F), −152.60 (br. s, 1F, p-F), −162.16 (td, 2F,
3JFF = 21.8 Hz, 4JFF = 6.8 Hz, m-F). H-NMR (500 MHz, CDCl
1
, 298 K, 0.04 M): 7.84 (br. d, 2H,
O(CO)-),
), 2.17 (qt, 2H,
3
3JHH = 8.4 Hz, o-ArH), 7.21 (d, 2H, 3JHH = 8.1 Hz, m-ArH), 4.46 (t, 2H, 3JHH = 6.9 Hz, -CH
2
2
.64 (tt, 2H, 3JHH = 6.9 Hz, 4JHH = 2.3 Hz, -C≡CCH
3JHH = 7.6 Hz, 4JHH = 2.4 Hz, Me-CH
CDCl
, 298 K, 0.04 M): 43.58. 19F-NMR (376 MHz, CDCl
3JFF = 16.4 Hz, o-F), −147.64 (br. s, 1F, p-F), −161.08 (br. t, 2F, 3JFF = 17.4 Hz, m-F).
2
-CH
2
-), 2.40 (s, 3H, Ar-CH
3
-C≡), 1.11 (t, 3H, 3JHH = 7.5 Hz, -CH
). B-NMR (160 MHz,
11
2
3
3
3
, 298 K, 0.04 M): −130.20 (d, 2F,
3
.4.3. NMR Experiments of 2c
Samples of 0.4, 0.2, 0.1 and 0.04 M concentration were prepared by dissolving the required amount
1
of 2c and B(C
6
F
5
)
3
in a 1:1 ratio in CDCl
3
(0.5 mL). H-NMR (500 MHz, CDCl
3
, 298 K, 0.4 M): 7.86
(
-
d, 2H, 3JHH = 8.9 Hz, o-ArH), 6.84 (d, 2H, 3JHH = 9.0 Hz, m-ArH), 4.55 (t, 2H, 3JHH = 6.9 Hz,
CH O(CO)-), 3.85 (s, 3H, -OCH -CH -),
), 2.67 (br. tt, 2H, 3JHH = 6.9 Hz, 4JHH = 2.2 Hz, -C≡CCH
.17 (qt, 2H, 3JHH = 7.5 Hz, 4JHH = 2.3 Hz, Me-CH
). B-NMR
2
3
2
2
2
2
-C≡), 1.11 (t, 3H, 3JHH = 7.5 Hz, -CH
3
11
1
9
(160 MHz, CDCl
3
, 298 K, 0.4 M): 40.16, 10.33. F-NMR (376 MHz, CDCl
3
, 298 K, 0.4 M): −133.70
d, 2F, 3JFF = 19.2 Hz, o-F), 154.94 (br. s, 1F, p-F), −163.16 (t, 2F, 3JFF = 17.5 Hz, m-F). H-NMR
1
(

Molecules 2015, 20
4542
(
500 MHz, CDCl
3
, 298 K, 0.2 M): 7.87 (d, 2H, 3JHH = 9.03 Hz, o-ArH), 6.84 (d, 2H, 3JHH = 8.9 Hz,
O(CO)-), 3.85 (s, 3H, -OCH
), 2.66 (br. tt, 2H, 3JHH = 6.9 Hz,
m-ArH), 4.51 (t, 2H, 3JHH = 6.9 Hz, -CH
2
3
-CH
2
-), 2.17 (qt, 2H, 3JHH = 7.4 Hz, 4JHH = 2.3 Hz, Me-CH
2
-C≡), 1.11 (t, 3H,
2
11
19
3
, 298 K, 0.2 M): 39.88, 12.71. F-NMR (376 MHz,
CDCl
3
, 298 K, 0.2 M): −133.45 (d, 2F, 3JFF = 19.2 Hz, o-F), 154.36 (t, 1F, 3JFF = 19.4 Hz, p-F),
162.91 (td, 2F, 3JFF = 21.9, 4JFF = 6.5 Hz, m-F). H-NMR (500 MHz, CDCl
1
, 298 K, 0.1 M): 7.88 (br.
−
3
d, 2H, 3JHH = 8.9 Hz, o-ArH), 6.85 (br. d, 2H, 3JHH = 8.9 Hz, m-ArH), 4.49 (t, 2H, 3JHH = 6.9 Hz,
CH O(CO)-), 3.86 (s, 3H, -OCH -CH -),
), 2.65 (br. tt, 2H, 3JHH = 7.0 Hz, 4JHH = 2.4 Hz, -C≡CCH
.16 (qt, 2H, 3JHH = 7.5 Hz, 4JHH = 2.4 Hz, Me-CH
). B-NMR
, 298 K, 0.1 M): −132.83
-
2
3
2
2
2
2
-C≡), 1.11 (t, 3H, 3JHH = 7.5 Hz, -CH
3
11
1
9
(160 MHz, CDCl
3
, 298 K, 0.1 M): 40.27, 19.46. F-NMR (376 MHz, CDCl
3
(
d, 2F, 3JFF = 19.2 Hz, o-F), 153.08 (br. s, 1F, p-F), −162.52 (td, 2F, 3JFF = 21.9 Hz, 4JFF = 7.1 Hz,
1
, 298 K, 0.04 M): 7.91 (br. d, 2H, 3JHH = 8.9 Hz, o-ArH), 6.87 (br. d,
O(CO)-), 3.86 (s, 3H, -OCH ), 2.64 (tt,
-), 2.17 (qt, 2H, 3JHH = 7.5 Hz, 4JHH = 2.4 Hz,
m-F). H-NMR (500 MHz, CDCl
3
2
2
H, 3JHH = 9.0 Hz, m-ArH), 4.46 (t, 2H, 3JHH = 6.9 Hz, -CH
H, 3JHH = 7.1 Hz, 4JHH = 2.4 Hz, -C≡CCH
-C≡), 1.11 (t, 3H, 3JHH = 7.5 Hz, -CH
2
3
2
-CH
2
11
Me-CH
2
3
). B-NMR (160 MHz, CDCl
3
, 298 K, 0.04 M): 39.45,
7.72. F-NMR (376 MHz, CDCl
, 298 K, 0.04 M): −132.00 (d, 2F, 3JFF = 19.2 Hz, o-F), 151.37 (s,
1
9
2
1
3
F, p-F), −162.04 (td, 2F, 3JFF = 20.1 Hz, 4JFF = 8.1 Hz, m-F).
3
.5. In Situ NMR Studies of Varying Stoichiometry
Two separate samples were made up with a 10:1 excess of 2b (10 mg, 0.05 mmol) over B(C
6
F )
5 3
(
(
256 mg, 0.5 mmol) in one, then conversely a 10:1 excess of B(C
6
F
5
3
) (50 mg, 0.1 mmol) over 2b
216 mg, 1 mmol) all in in all the other, all in CDCl (0.5 mL).
3
3
.5.1. Excess 2b
1H-NMR (500 MHz, CDCl
3JHH = 8.0 Hz, m-ArH), 4.65 (t, 2H, 3JHH = 6.8 Hz, -CH
, 298 K): 7.70 (d, 2H, 3JHH = 8.4 Hz, o-ArH), 7.18 (d, 2H,
O(CO)-), 2.71 (tt, 2H, 3JHH = 6.8 Hz,
3
2
4JHH = 2.4 Hz, -C≡CCH
-CH
-), 2.40 (s, 3H, Ar-CH
), 2.18 (qt, 2H, 3JHH = 7.5 Hz, 4JHH = 2.4 Hz,
2
2
3
-C≡), 1.13 (t, 3H, 3JHH = 7.5 Hz, -CH
11
19
Me-CH
2
3
). B-NMR (160 MHz, CDCl
3
, 298 K): 53.05. F-NMR
(
376 MHz, CDCl
3
, 298 K): −128.62 (d, 2F, 3JFF = 20.7 Hz, o-F), −144.31 (br. s, 1F, p-F), −160.60 (td,
F, 3JFF = 20.5 Hz, 4JFF = 7.5 Hz, m-F).
2
3
.5.2. Excess B(C
6
F )
5 3
1H-NMR (500 MHz, CDCl
3JHH = 8.1 Hz, m-ArH), 4.36 (t, 2H, 3JHH = 7.0 Hz, -CH
, 298 K): 7.90 (d, 2H, 3JHH = 8.3 Hz, o-ArH), 7.18 (d, 2H,
O(CO)-), 2.59 (tt, 2H, 3JHH = 7.1 Hz,
3
2
4JHH = 2.4 Hz, -C≡CCH
Me-CH
-CH
-), 2.35 (s, 3H, Ar-CH
), 2.13 (qt, 2H, 3JHH = 7.5 Hz, 4JHH = 2.4 Hz,
2
2
3
-C≡), 1.08 (t, 3H, 3JHH = 7.5 Hz, -CH
). B-NMR (160 MHz, CDCl
11
, 298 K): 3.32. F-NMR
19
2
3
3
(
376 MHz, CDCl
3
, 298 K): −134.27 (d, 2F, 3JFF = 19.8 Hz, o-F), −156.03 (t, 1F, 3JFF = 20.7 Hz, p-F),
163.26 (t, 2F, 3JFF = 20.4 Hz, m-F).
−

Molecules 2015, 20
.6. Crystallographic Studies
Single crystals of 2a–c were grown under an inert atmosphere and protected from atmospheric air
4543
3
and moisture using an inert per-fluorinated polyether oil. Single crystals of 2a–c were mounted in a
cryoloop and crystallographic data collected on an Agilent Dual SuperNova diffractometer using
monochromatic Cu-Kα radiation (1.54184 Å) and a CCD area detector. Data were collected at 150(2)
K (2a,c) or 200(2) K (2b). Data collection and processing implemented CrysalisPro [47] and a
gaussian absorption correction applied within the CrysalisPro suite. The structures were solved by
2
direct methods and refined against F using the SHELXTL package [48]. In the case of 2a, a region of
diffuse electron density was treated with SQUEEZE incorporated within the PLATON package [49]
with both the void volume and electron count corresponding to one toluene molecule per unit cell. The
structures have been deposited with the Cambridge Crystallographic Data Centre under CCDC
deposition numbers 1046813-1046815. Crystallographic data for 2a–2c are presented in Table 4.
Table 4. Crystallographic data for compounds 2a–2c.
Compound
2a
2b
2c
Formula
C
31
H14BF15
O
2
*
C
32
H16BF15
O
2
C
32
H16BF15
O
3
M
714.23
728.26
744.26
Crystal System
Triclinic
Triclinic
Triclinic
Space Group
P-1
P-1
P-1
a
b
c
α
β
10.9048(8)
11.4463(6)
13.7289(6)
84.827(4)
74.262(5)
63.545(6)
1475.75(16)
150(2)
10.5395(4)
11.3370(4)
13.2062(4)
102.149(3)
97.888(3)
94.962(3)
1517.15(9)
200(2)
11.1660(5)
12.5550(5)
12.6939(4)
78.409(3)
68.678(4)
65.281(4)
1503.32(13)
150(2)
γ
V
T/K
Z
2
2
2
D
c
1.607
1.594
1.644
θmin, θmax
4.264–73.720
3.468–74.078
3.744–73.995
Crystal size
μ/mm
F(000)
Total Reflections
Independent Reflections
Rint
0.26 × 0.08 × 0.07 0.33 × 0.28 × 0.23 0.46 × 0.34 × 0.12
−
1
1.479
712
1.451
728
1.505
744
9959
10254
10549
5788
5926
5876
0.0211
0.0349
0.1043
1.020
0.0164
0.0439
0.1377
1.045
0.0164
0.0323
0.0945
1.013
R (I > 2s(I))
1
wR
2
(all data)
S
−
3
Min/max e /Å
+0.28/−0.27
+0.78/−0.31
+0.30/−0.24
*
Excludes 0.5 DCM solvent of crystallization estimated from SQUEEZE.
3.7. Computational Studies
DFT calculations were undertaken within Jaguar [50]. Initial geometry optimizations implemented
the B3LYP functional [51] and Pople double zeta 6-31G* basis set [52]. Additional triple zeta

Molecules 2015, 20
4544
calculations were performed using the 6-311G* basis set for the thermodynamic calculations and zero
point energy corrections made.
4
. Conclusions
In conclusion, we have synthesized a range of alkynyl benzoates bearing both ester and alkyne
functionalities and have investigated their reactivities with the strong Lewis acid B(C
6
F
5
3
) . Since both
σ-activation of the carbonyl and π-activation of the alkyne are possible, we have observed that in these
cases σ-activation dominates and adduct formation occurs. In these cases no 1,2-addition product from
the addition of the ester and the boron across the alkyne could be isolated since the carbocation that
would be formed is not very stable. This is different to the reactions seen previously with propargyl
amides which do undergo 1,2-addition of the amide and B(C
6
F
5
3
) across the alkyne. In this case the
positive charge is more stable since nitrogen is better able to stabilize a positive charge than oxygen. In
the case of propargyl esters 1,2-addition does occur and the cyclized 1,2-addition product can be
isolated however, this species undergoes rapid rearrangement to afford a more thermodynamically
stable product. The investigation of the reactivities of alkynyl benzoates bearing terminal or internal
alkynes in 1,2-addition reactions with external nucleophiles are currently being undertaken and will be
the focus of our future studies. In addition, the reactions of these compounds (and their derivatives) in
1
,1-carboboration reactions will also be investigated.
Supplementary Materials
Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/20/03/4530/s1.
Acknowledgments
We would like to thank Fluorochem for the kind donation of B(C
6
F
5
3
) . A.B. and K.O. would like to
thank the ERAMUS+ for funding.
Author Contributions
The experimental work was principally carried out by Alexander Bähr, Lewis C. Wilkins and Kevin
Ollegott, Benson M. Kariuki collected the X-ray data. The paper was written by Rebecca L. Melen.
Conflicts of Interest
The authors declare no conflict of interest.
References
1
. Massey, A.G.; Park, A.J.; Stone, F.G.A. Tris(pentafluorophenyl)boron. Proc. Chem. Soc.
963, 212.
. Massey, A.G.; Park, A.J. Tris(pentafluorophenyl)boron.
1
2
J. Organomet. Chem. 1964, 2, 245–250.

Molecules 2015, 20
4545
3
.
Yang, X.; Stern, C.L.; Marks, T.J. Cationic Zirconocene Olefin Polymerization Catalysts Based
on the Organo-Lewis Acid Tris(pentafluorophenyl)borane. A Synthetic, Structural, Solution
Dynamic, and Polymerization Catalytic Study. J. Am. Chem. Soc. 1994, 116, 10015–10031.
Parks, D.J.; Piers, W.E. Tris(pentafluorophenyl)boron-Catalyzed Hydrosilation of Aromatic
Aldehydes, Ketones, and Esters. J. Am. Chem. Soc. 1996, 118, 9440–9441.
4.
5.
6.
7.
8.
Piers, W.E.; Chivers, T. Pentafluorophenylboranes: From obscurity to applications. Chem. Soc. Rev.
1
997, 26, 345–354.
Blackwell, J.M.; Foster, K.L.; Beck, V.H.; Piers, W.E. B(C
A Mild, General Method for Synthesis of Silyl Ethers. J. Org. Chem. 1999, 64, 4887–4892.
Blackwell, J.M.; Sonmor, E.R.; Scoccitti, T.; Piers, W.E. B(C -Catalyzed Hydrosilation of
Imines via Silyliminium Intermediates Org. Lett. 2000, 2, 3921–3923.
6
F
5
3
) -Catalyzed Silation of Alcohols:ꢀ
6
F )
5 3
Roesler, R.; Har, B.J.N.; Piers, W.E. Synthesis and Characterization of (Perfluoroaryl)borane-
Functionalized Carbosilane Dendrimers and Their Use as Lewis Acid Catalysts for the
Hydrosilation of Acetophenone. Organometallics 2002, 21, 4300–4302.
9
1
.
Piers, W.E. The chemistry of perfluoroaryl boranes. Adv. Organomet. Chem. 2004, 52, 1–76.
0. Erker, G. Tris(pentafluorophenyl)borane: A special boron Lewis acid for special reactions.
Dalton Trans. 2005, 11, 1883–1890.
1
1. Rendler, S.; Oestreich, M. Conclusive Evidence for an S
N
2-Si Mechanism in the B(C
6
F ) -Catalyzed
5
3
Hydrosilylation of Carbonyl Compounds: Implications for the Related Hydrogenation. Angew. Chem.
Int. Ed. 2008, 47, 5997–6000.
1
1
1
1
1
2. Piers, W.E.; Marwitz, A.J.V.; Mercier, L.G. Mechanistic Aspects of Bond Activation with
Perfluoroarylboranes. Inorg. Chem. 2011, 50, 12252–12262.
3. Stephan, D.W. “Frustrated Lewis pairs”: A concept for new reactivity and catalysis.
Org. Biomol. Chem. 2008, 6, 1535–1539.
4. Stephan, D.W. Frustrated Lewis pairs: A new strategy to small molecule activation and
hydrogenation catalysis. Dalton Trans. 2009, 3129–3136.
5. Stephan, D.W. Activation of dihydrogen by non-metal systems. Chem. Commun. 2010, 46,
8
526–8533.
6. Stephan, D.W.; Erker, G. Frustrated Lewis Pairs: Metal-free Hydrogen Activation and More.
Angew. Chem. Int. Ed. 2010, 49, 46–76.
1
7. Stephan, D.W. “Frustrated Lewis pair” hydrogenations. Org. Biomol. Chem. 2012, 10, 5740–5746.
8. Stephan, D.W. Frustrated Lewis Pairs: From Concept to Catalysis. Acc. Chem. Res. 2015, 48,
1
3
06–316.
1
2
2
9. Topics in Current Chemistry: Frustrated Lewis Pairs I; Stephan, D.W., Erker, G., Eds.; Springer
Press: New York, NY, USA, 2013.
0. Topics in Current Chemistry: Frustrated Lewis Pairs II; Stephan, D.W., Erker, G., Eds.; Springer
Press: New York, NY, USA, 2013.
1. Voss, T.; Chen, C.; Kehr, G.; Nauha, E.; Erker, G.; Stephan, D.W. Cyclizations via Frustrated
Lewis Pairs: Lewis Acid Induced Intramolecular Additions of Amines to Olefins and Alkynes.
Chem. Eur. J. 2010, 16, 3005–3008.
2
2. Voss, T.; Mahdi, T.; Otten, E.; Fröhlich, R.; Kehr, G.; Stephan, D.W.; Erker, G. Frustrated Lewis
Pair Behavior of Intermolecular Amine/B(C
6
F
5
) Pairs. Organometallics 2012, 31, 2367–2378.
3

Molecules 2015, 20
4546
2
3. McCahill, J.S.J.; Welch, G.C.; Stephan, D.W. Reactivity of “Frustrated Lewis Pairs”:
Three-Component Reactions of Phosphines, a Borane, and Olefins. Angew. Chem. Int. Ed. 2007,
6, 4968–4971.
4
2
4. Chen, C.; Fröhlich, R.; Kehr, G.; Erker, G. Remarkably variable reaction modes of frustrated
Lewis pairs with non-conjugated terminal diacetylenes. Chem. Commun. 2010, 46, 3580–3582.
5. Liedtke, R.; Fröhlich, R.; Kehr, G.; Erker, G. Frustrated Lewis Pair Reactions with
Bis-Acetylenic Substrates: Exploring the Narrow Gap Separating Very Different Competing
Reaction Pathways. Organometallics 2011, 30, 5222–5232.
2
2
2
2
2
6. Dureen, M.A.; Stephan, D.W. Terminal Alkyne Activation by Frustrated and Classical Lewis
Acid/Phosphine Pairs. J. Am. Chem. Soc. 2009, 131, 8396–8397.
7. Dureen, M.A.; Brown, C.C.; Stephan, D.W. Addition of Enamines or Pyrroles and B(C
Frustrated Lewis Pairs” to Alkynes. Organometallics 2010, 29, 6422–6432.
6
F )
5 3
“
8. Jiang, C.; Blacque, O.; Berke, H. Activation of Terminal Alkynes by Frustrated Lewis Pairs.
Organometallics 2010, 29, 125–133.
9. Mömming, C.M.; Kehr, G.; Wibbeling, B.; Fröhlich, R.; Schirmer, B.; Grimme, S.; Erker, G.
Formation of Cyclic Allenes and Cumulenes by Cooperative Addition of Frustrated Lewis Pairs to
Conjugated Enynes and Diynes. Angew. Chem. Int. Ed. 2010, 49, 2414–2427.
3
0. Chen, C.; Eweiner, F.; Wibbeling, B.; Fröhlich, R.; Senda, S.; Ohki, Y.; Tatsumi, K.; Grimme, S.;
Kehr, G.; Erker, G. Exploring the Limits of Frustrated Lewis Pair Chemistry with Alkynes: Detection
of a System that Favors 1,1-Carboboration over Cooperative 1,2-P/B-Addition. Chem. Asian J.
2
010, 5, 2199–2208.
3
3
3
3
1. Dureen, M.A.; Brown, C.C.; Stephan, D.W. Deprotonation and Addition Reactions of Frustrated
Lewis Pairs with Alkynes. Organometallics 2010, 29, 6594–6607.
2. Winkelhaus, D.; Neumann, B.; Stammler, H.G.; Mitzel, N.W. Intramolecular Lewis acid–base
pairs based on 4-ethynyl-2,6-lutidine. Dalton Trans. 2012, 41, 9143–9150.
3. Chen, C. Kehr, G. Fröhlich, R. Erker, G. Carbon−Carbon Bond Activation by 1,1-Carboboration
of Internal Alkynes. J. Am. Chem. Soc. 2010, 132, 13594–13595.
4. Ekkert, O.; Kehr, G.; Fröhlich, R.; Erker, G. P−C Bond Activation Chemistry: Evidence for
1,1-Carboboration Reactions Proceeding with Phosphorus−Carbon Bond Cleavage. J. Am. Chem. Soc.
2
011, 133, 4610–4616.
3
5. Chen, C.; Voss, T.; Fröhlich, R.; Kehr, G.; Erker, G. 1,1-Carboboration of 1-Alkynes: A Conceptual
Alternative to the Hydroboration Reaction. Org. Lett. 2011, 13, 62–65.
3
6. Möbus, J.; Bonnin, Q.; Ueda, K.; Fröhlich, R.; Itami, K.; Kehr, G.; Erker, G. The 1,1-Carboboration
of Bis(alkynyl)phosphanes as a Route to Phosphole Compounds. Angew. Chem. Int. Ed. 2012, 51,
1
954–1957.
3
7. Kehr, G.; Erker, G. 1,1-Carboboration. Chem. Commun. 2012, 48, 1839–1850, and references therein.
8. Eller, C.; Kehr, G.; Daniliuc, C.G.; Fröhlich, R.; Erker, G. Facile 1,1-Carboboration Reactions of
Acetylenic Thioethers. Organometallics 2013, 32, 384–386.
3
3
9. Melen, R.L. Applications of pentafluorophenyl boron reagents in the synthesis of heterocyclic and
aromatic compounds. Chem. Commun. 2014, 50, 1161–1174, and references therein.

Molecules 2015, 20
4547
4
4
4
4
0. Melen, R.L.; Hansmann, M.M.; Lough, A.J.; Hashmi, A.S.K.; Stephan, D.W. Cyclisation versus
1
1
,1-Carboboration: Reactions of B(C
6
F
5
3
) with Propargyl Amides. Chem. Eur. J. 2013, 19,
1928–11938.
1. Hansmann, M.M.; Melen, R.L.; Rominger, F.; Hashmi, A.S.K.; Stephan, D.W. Activation of
Alkynes with B(C –Boron Allylation Reagents Derived from Propargyl Esters. J. Am. Chem. Soc.
014, 136, 777–782.
2. Hansmann, M.M.; Melen, R.L.; Rominger, F.; Hashmi, A.S.K.; Stephan, D.W. B(C
6 5 3
F )
2
6
F
5
3
) promoted
cyclisation of internal propargyl esters: Structural characterisation of 1,3-dioxolium compounds.
Chem. Commun. 2014, 50, 7243–7245.
3. Parks, D.J.; Piers, W.E.; Parvez, M.; Atencio, R.; Zaworotko, M.J. Synthesis and Solution and
Solid-State Structures of Tris(pentafluorophenyl)borane Adducts of PhC(O)X (X = H, Me, OEt,
NPri
). Organometallics 1998, 17, 1369–1377.
2
4
4
4. Hammett, L.P. The Effect of Structure upon the Reactions of Organic Compounds. Benzene
Derivatives. J. Am. Chem. Soc. 1937, 59, 96–103.
5. Sana, M.; Leroy, G.; Wilante, C. Enthalpies of formation and bond energies in lithium, beryllium,
and boron derivatives. 2. Dative, single, and triple bonds. Organometallics 1992, 11, 781–787 and
references therein.
4
6. Lancaster, S. Alkylation of boron trifluoride with pentafluorophenyl Grignard reagent;
Tris(pentafluorophenyl)boron; borane. ChemSpider SyntheticPages 2003, doi:10.1039/SP215.
7. CrysAlisPro, Version 1.171.37.33; Agilent Technologies: Santa Clara, CA, USA, 2014.
8. SHELXTL; Bruker AXS: Madison, WI, USA, 2008.
4
4
4
5
5
9. Spek, A.L. PLATON. Acta Crystallogr. 2009, D65, 148–155.
0. Jaguar v.8.5; Schrödinger LLC: New York, NY, USA, 2014.
1. Lee, C.; Yang, W.; Parr, R.G. Development of the Colle-Salvetti correlation-energy formula into a
functional of the electron density. Phys. Rev. B 1988, 37, 785–789.
5
2. Raghavachari, K.; Binkley, J.S.; Seeger R.; Pople, J.A. Self-consistent molecular orbital methods.
XX. A basis set for correlated wave functions. J. Chem. Phys. 1980, 72, 650–654.
Sample Availability: Samples of alkynyl benzoates 1 are available from the authors.
©
2015 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution license
http://creativecommons.org/licenses/by/4.0/).
(