Chloro- and phenoxy-phosphines in frustrated Lewis pair additions to alkynes

Article abstract of DOI:10.1039/c1dt11196e

The reaction of tBu(C₆H₄O₂)P, with the borane B(C₆F₅)₃ gives rise to NMR data consistent with the formation of the classical Lewis acid-base adduct tBu(C ₆H₄O₂)P(B(C₆F₅) ₃) (1). In contrast, the NMR data for the corresponding reactions of tBu(C₂₀H₁₂O₂)P and Cl(C₂₀H ₁₂O₂)P with B(C₆F₅)₃ were consistent with the presence of equilibria between free phosphine and borane and the corresponding adducts. Nonetheless, in each case, the adducts tBu(C ₂₀H₁₂O₂)P(B(C₆F₅) ₃) (2) and Cl(C₂₀H₁₂O₂)P(B(C ₆F₅)₃) (3) were isolable. The species 1 reacts with PhCCH to give the new species tBu(C₆H₄O ₂)P(Ph)CCHB(C₆F₅)₃ (4) in near quantitative yield. In an analogous fashion, the addition of PhCCH to solutions of the phosphines tBu(C₂₀H₁₂O₂)P, tBuPCl ₂ and (C₆H₃(2,4-tBu₂)O)₃P each with an equivalent of B(C₆F₅)₃ gave rise to L(Ph)CCHB(C₆F₅)₃ (L = tBu(C ₂₀H₁₂O₂)P 5, tBuPCl₂6 and (C ₆H₃(2,4-tBu₂)O)₃P 7). X-Ray data for 1, 2, 6 and 7 are presented. The implications of these findings are considered. The Royal Society of Chemistry.

Full text of DOI:10.1039/c1dt11196e

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An international journal of inorganic chemistry
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Volume 41 | Number 1 | 7 January 2012 | Pages 1–308
ISSN 1477-9226
COVER ARTICLE
Stephan et al.
Chloro- and phenoxy-phosphines in frustrated Lewis pair additions to alkynes
1477-9226(2012)41:1;1-J

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PAPER
Chloro- and phenoxy-phosphines in frustrated Lewis pair additions to alkynes†
Christopher B. Caputo, Stephen J. Geier, Eva Y. Ouyang, Christoph Kreitner and Douglas W. Stephan*
Received 23rd June 2011, Accepted 16th August 2011
DOI: 10.1039/c1dt11196e
The reaction of tBu(C6H4O2)P, with the borane B(C6F5)3 gives rise to NMR data consistent with the
formation of the classical Lewis acid–base adduct tBu(C6H4O2)P(B(C6F5)3) (1). In contrast, the NMR
data for the corresponding reactions of tBu(C20H12O2)P and Cl(C20H12O2)P with B(C6F5)3 were
consistent with the presence of equilibria between free phosphine and borane and the corresponding
adducts. Nonetheless, in each case, the adducts tBu(C20H12O2)P(B(C6F5)3) (2) and Cl(C20H12O2)P-
(B(C6F5)3) (3) were isolable. The species 1 reacts with PhCCH to give the new species tBu(C6H4O2)-
P(Ph)C CHB(C6F5)3 (4) in near quantitative yield. In an analogous fashion, the addition of PhCCH
to solutions of the phosphines tBu(C20H12O2)P, tBuPCl2 and (C6H3(2,4-tBu2)O)3P each with an
equivalent of B(C6F5)3 gave rise to L(Ph)C CHB(C6F5)3 (L = tBu(C20H12O2)P 5, tBuPCl2 6 and
(C6H3(2,4-tBu2)O)3P 7). X-Ray data for 1, 2, 6 and 7 are presented. The implications of these findings
are considered.
Introduction
“Frustrated Lewis pairs” (FLPs) were first derived from the
combination of a sterically hindered Lewis base and a sterically
bulky Lewis acid.1–4 These combinations of molecules have been
shown to have remarkable reactivity as a result of the availability
of unquenched Lewis acidity and basicity. In what is considered
the most dramatic demonstration of this chemistry, FLPs have
been shown to effect the heterolytic cleavage of H2.5–13 However
these systems also effect a wide range of other new reactivity in-
cluding the activation of alkynes,14–17 dienes,18 olefins,19 boranes,20
disulfides,21 N2O,22–24 CO2 25–30 as well as effecting the ring opening
of THF and ethers and lactones.31–34
The variability of the Lewis acid and bases has been ex-
plored to some extent. While much of the initial demonstra-
Scheme 1 Examples of FLP alkyne addition reactions.
tion of this chemistry was done employing bulky phosphines and
B(C6F5)3, the analogous chemistry using bulky amines quickly
although this proved accessible only with the bulky phos-
phines.
followed. Subsequent worked showed that sterically demanding
carbenes,35,36 pyridines32,37 and N-heterocycles38 were effective in
H2 splitting by FLPs, while N-donors, sulfides and C-donors
derived from pyrroles effected FLP reactions with terminal
alkynes (Scheme 1).14–16 Variation in the Lewis acid has received
has received lesser attention although the electrophilic alane
In the known FLP chemistry in which phosphines have been
employed, the reactive systems have largely been limited to
combinations of trialkyl- or triarylphosphines and B(C6F5)3. We
have also reported hydrogen activation with bulky phosphinites
tBu2POR (R = Ph, tBu) and tBu2PCl in combination with
B(C6F5)3.40 Recently we have reported that the sterically demand-
ing secondary phosphine (C6H2Me3)2PH and the primary phos-
phine (C6H2tBu3)PH2 are also capable of effecting FLP addition to
alkynes (Scheme 1).15 In the present work, we explore the utility of
phosphines that incorporate halides and phenoxide substituents,
demonstrating that electron deficient and electron-rich donors are
capable of FLP additions to alkynes. The implications of these
findings are considered.
39
Al(C6F5)3,15,16 boranes of the form RB(C6F5)2 and B(C6F4H)3
have been utilized. More recently FLP chemistry of CO2 has
been demonstrated with the Al-halides AlX3 (X = Cl, Br, I),25,26
Department of Chemistry, 80 St. George St., University of Toronto, Toronto,
Ontario, Canada, M5S 3H6. E-mail: dstephan@chem.utoronto.ca
† CCDC reference numbers 831913–831916. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/c1dt11196e
This journal is
The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 237–242 | 237
©

120.2 (d, JPC = 3 Hz), 43.1 (d, JPC = 15 Hz), 26.6 (d, JPC = 5 Hz).
Anal. Calcd. for C42H21BF15O2P (%) C: 57.04, H: 2.39; found C:
53.27, H: 2.75.
Experimental
General procedures
1
3
(3) Yield: 165 mg (98%). H NMR (CDCl3) d: 8.08 (d, JhH
=
All preparations were done under an atmosphere of dry, O2-free
N2 by usage of an Innovative Technology glovebox and a Schlenk
vacuum line. Solvents were purified with a Grubbs-type column
system manufactured by Innovative Technology and dispensed
into thick-walled Schlenk glass bombs equipped with Young-type
Teflon valve stopcocks (pentanes, hexanes, toluene, CH2Cl2) or
were dried over the appropriate agents and distilled into the same
kind of Young bombs (CHCl3). All solvents were thoroughly
degassed after purification (repeated freeze-pump-thaw cycles).
Deuterated solvents were dried over the appropriate agents (CaH2
for CD2Cl2, CDCl3), vacuum-transferred into Young bombs, and
degassed accordingly. All NMR spectra were recorded on Bruker
9 Hz, 1H), 8.01 (m, 2H), 7.98 (d, 3JHH = 9 Hz, 1H), 7.55 (m, 2H),
7.50 (d, 3JHH = 9 Hz, 1H), 7.35 (m, 4H), 7.15 (d, 3JHH = 9 Hz, 1H).
11B NMR (CDCl3) d: 29.5 (br. s). 19F NMR (CDCl3) d: -128.0
3
(d, JFF = 19 Hz, 6F, o-CF), -147.9 (br. s 3F, p-CF), -161.2 (t,
1
3JFF = 20 Hz, 6F, m-CF). 31P{ H} NMR (CDCl3) d: 162.5 (s). 13
C
NMR (partial, CDCl3) d: 112.7, 120.0 (d, JPC = 3 Hz), 121.0 (d,
JPC = 2 Hz), 122.5 (d, JPC = 4 Hz), 122.7 (d, JPC = 3 Hz), 126.1 (d,
JPC = 5 Hz), 127.0 (d, JPC = 13 Hz), 127.1, 128.6 (d, JPC = 10 Hz),
130.8, 131.2, 132.0, 132.6, 137.6 (dm, JFC = 252 Hz), 143.4 (d,
JFC = 252 Hz), 147.2 (d, JPC = 9 Hz), 148.3, 148.4, 148,5 (dm, JFC
=
248 Hz). Anal. Calcd. for C38H12BF15O2PCl (%) C: 52.90, H: 1.40;
found C: 52.69, H: 1.68.
1
Avance-300 or 400 spectrometers. All chemical shifts of H and
13C spectra are given relative to SiMe4 and referenced to the
residual solvent signal. Those of 11B, 19F and 31P NMR spectra
are given relative to an external standard (11B, (Et2O)BF3; 19F,
CFCl3; 31P, 85% H3PO4). In some instances, signal and/or coupling
assignment was derived from two-dimensional NMR experiments.
Chemical shifts are reported in ppm and coupling constants as
scalar values in Hz. Combustion analyses were performed in
house by employing a Perkin-Elmer CHN analyzer.41 Chemicals
were obtained from Strem Chemicals, Inc. (USA) and from
Sigma Aldrich. tBu(C6H4O2)P and tBu(C20H12O2)P were prepared
following literature preparations.42
Synthesis of L(Ph)C CHB(C6F5)3 (L = tBu(C6H4O2)P 4,
tBu(C20H12O2)P 5, tBuPCl2 6, (C6H3(2,4-tBu2)O)3P 7)
These compounds were synthesized in a similar fashion, thus
only one preparation will be detailed. To a solution of tert-
butyl(catechol)phosphine (38 mg, 0.195 mmol) and B(C6F5)3
(100 mg, 0.195 mmol) in dichloromethane (5 mL) was added
phenyl acetylene (30 mg, 0.29 mmol). The solution was allowed
to stir for 4 h. All volatiles were removed in vacuo and the residue
was washed with hexanes (2 ¥ 2 mL) and again dried in vacuo.
1
3
(4) Yield: 157 mg (99%). H NMR (CDCl3) d: 9.10 (d, JPH
=
=
=
37 Hz, 1H, PC = CH), 7.27 (m, 2H), 7.20 (dd, 3JHH = 6 Hz, 4JHH
Preparation of tBu(C6H4O2)P(B(C6F5)3) (1), tBu(C20H12O2)P(B-
(C6F5)3) (2) and Cl(C20H12O2)P(B(C6F5)3) (3)
3
3
4 Hz, 2H), 7.10 (t, JHH = 7 Hz, 2H, m-CH), 6.96 (d, JHH
7 Hz, 2H, o-CH), 1.38 (d, 3JPH = 19 Hz, 9H, C(CH3)3). 11B NMR
These compounds were synthesized in a similar fashion, thus only
one preparation will be detailed. To a solution of tBu(C20H12O2)P
(72 mg, 0.195 mmol) in CH2Cl2 (5 mL) was added B(C6F5)3
(100 mg, 0.195 mmol). The solution was allowed to stir for 4
h. All volatiles were removed in vacuo and the residue was washed
(CDCl3) d: -16.2 (d, 3JPB = 17 Hz). 19F NMR (CDCl3) d: -132.1
3
3
(d, JFF = 22 Hz, 6F, o-CF), -161.4 (t, JFF = 20 Hz, 3F, p-CF),
-166.1 (td, 3JFF = 21 Hz, 4JFF = 8 Hz, 6F, m-CF). 31P{ H} NMR
1
(CDCl3) d: 107.7 (q, 3JPB = 17 Hz). 13C NMR (partial, CDCl3) d:
144.5 (d, JPC = 3 Hz), 132.3, 129.4 (d, JPC = 6 Hz), 129.2 (d, JPC
=
with hexanes (2 ¥ 2 mL) and again dried in vacuo.
3 Hz), 129.0, 128.6 (d, JPC = 2 Hz), 128.5, 114.0 (d, J = 9 Hz), 37.6
(d, JPC = 57 Hz), 23.9. Anal. Calcd. for C36H19BF15O2P (%) C:
3
1
(1) Yield: 135 mg (98%).1H NMR (CDCl3) d: 7.07(dd, JHH
=
6 Hz, 4JHH = 4 Hz, 2H), 6.99 (dd, 3JHH = 6 Hz, 4JHH = 4 Hz, 2H),
53.36, H: 2.36; found C: 54.02, H: 2.82.
1
3
1.12 (d, 3JPH = 15 Hz, 9H). 11B NMR (CDCl3) d: -8.7 (br. s). 19
F
(5) Yield: 186 mg (97%). H NMR (CDCl3) d: 8.96 (d, JPH =
3
3
NMR (CDCl3) d: -128.2 (d, JFF = 18 Hz, 6F, o-CF), -158.9 (t,
35 Hz, 1H, PC = CH), 8.03 (m, 3H), 7.96 (d, JHH = 8 Hz, 1H),
7.57 (q, 3JHH = 8 Hz, 1H), 7.35 (ov m, 4H), 7.20 (d, J = 9 Hz, 1H),
7.07 (d, 3JHH = 9 Hz, 1H), 7.02 (td, 3JHH = 7 Hz, 4JHH = 1 Hz, 1H,
3JFF = 20 Hz, 3F, p-CF), -163.3 (td, 3JFF = 22 Hz, 4JFF = 22 Hz, 6F,
1
m-CF). 31P{ H} NMR (CDCl3) d: 178.8 (s). 13C NMR (partial,
3
CDCl3) d: 25.2 (d, JPC = 6 Hz), 88.0 (d, JPC = 4 Hz), 112.6 (d, JPC
=
p-CH), 6.85 (t, JHH = 8 Hz, 2H, m-CH), 6.42 (br s, 2H, o-CH),
6 Hz), 128.1, 137.2 (dm, JFC = 253 Hz), 141.1 (dm, JFC = 258 Hz),
146.5 (d, JFC = 6 Hz), 148.2 (dm, JFC = 245 Hz). Anal. Calcd. for
C28H13BF15O2P (%) C: 47.49, H: 1.85; found C: 47.57, H: 2.15.
(2) Yield: 165 mg (96%). 1H NMR (CD2Cl2) d: 8.02 (d, 3JHH
1.43 (d, 3JPH = 18 Hz, 9H, C(CH3)3). 11B NMR (CDCl3) d: -16.3
(d, 3JPB = 16 Hz). 19F NMR (CDCl3) d: -131.1 (d, 3JFF = 22 Hz, 6F,
o-CF), -161.8 (t, 3JFF = 22 Hz, 3F, p-CF), -166.3 (td, 3JFF = 22 Hz,
1
=
4JFF = 7 Hz, 6F, m-CF). 31P{ H} NMR (CDCl3) d: 91.1 (q, 3JPB
=
3
3
9 Hz, 1H), 7.94 (d, JHH = 8 Hz, 1H), 7.88 (d, JHH = 8 Hz, 1H),
16 Hz). 13C NMR (partial, CDCl3) d: 147.1 (d, JPC = 10 Hz), 145.3
7.71 (d, 3JHH = 9 Hz, 1H), 7.59 (d, 3JHH = 9 Hz, 1H), 7.35 (m, 4H),
(d, JPC = 10 Hz), 132.9, 132.7, 132.5, 132.4, 132.1, 132.0 (d, JPC
=
7.46 (q, 3JHH = 8 Hz, 2H), 7.35 (d, 3JHH = 9 Hz, 1H), 7.22 (t, 3JHH
=
1 Hz), 129.3 (d, JPC = 5 Hz), 129.0 (d, JPC = 21 Hz), 129.0, 128.5,
128.0 (ov m), 127.7 (d, JPC = 20 Hz), 127.1 (d, JPC = 3 Hz), 126.9,
38.0 (d, 1JPC = 67 Hz), 25.1. Anal. Calcd. for C50H27BF15O2P (%)
C: 60.87, H: 2.76; found C: 60.44, H: 2.48.
7 Hz, 2H), 7.06 (d, 3JHH = 9 Hz, 2H), 1.06 (d, 3JPH = 15 Hz, 9H).
11B NMR (CD2Cl2) d: 12.2 (br. s). 19F NMR (CD2Cl2) d: -126.3
(br. s, 6F, o-CF), -151.7 (br. s 3F, p-CF), -162.2 (br. s, 6F, m-CF).
1
1
3
31P{ H} NMR (CD2Cl2) d: 167.4 (s). 13C NMR (partial, CD2Cl2)
(6) Yield: 146 mg (97%). H NMR (CDCl3) d: 9.16 (d, JPB
=
=
=
3
3
d: 149.0 (d, JPC = 15 Hz), 148.5 (dm, JFC = 241 Hz), 148.4 (d, JPC
=
49 Hz, 1H, PC = CH), 7.29 (t, JHH = 7 Hz 2H), 7.17 (t, JHH
7 Hz, 2H), 6.95 (dd, 3JHH = 7 Hz, 4JHH = 2 Hz, 2H), 1.54 (d, 3JPH
8 Hz), 141.5 (d, JFC = 254 Hz), 137.5 (dm, JFC = 249 Hz), 133.3,
132.8, 131.9 (d, JPC = 16 Hz), 131.3, 129.9, 128.1, 127.5, 127.1,
126.5, 126.1, 121.7 (d, JPC = 3 Hz), 121.0, 120.4 (d, JPC = 3 Hz),
25 Hz, 9H, C(CH3)3) 11B NMR (CDCl3) d: -15.9 (d, 3JBP = 21 Hz).
19F NMR (CDCl3) d: -130.7 (d, 3JFF = 23 Hz, 6F, o-CF), -159.7
238 | Dalton Trans., 2012, 41, 237–242
This journal is
The Royal Society of Chemistry 2012
©

Table 1 Crystallographic data
1
2
6
7·C6H14
Formula
wt
C28H13BF15O2P
708.16
Monoclinic
P21/n
10.8146(14)
12.6680(15)
19.986(2)
90
93.540(4)
90
2732.9(6)
4
1.721
0.0622
0.232
6288
3819
C42H21BF15O2P
884.37
Monoclinic
P21
15.6025(6)
11.5308(4)
21.2356(7)
90
103.904(2)
90
3708.5(2)
4
1.584
0.0467
0.189
33228
13881
C30H15BCl2F15P
773.10
Monoclinic
P21/c
13.4163(4)
18.3323(5)
14.0090(5)
90
118.077(1)
90
3040.05
4
1.689
0.0342
0.382
6934
4546
C68H69BF15O3P
1261.01
Monoclinic
P21/n
15.2876(6)
17.8123(7)
26.9165(12)
90
104.511(2)
90
7095.7(5)
4
1.180
0.0870
0.119
16272
8842
Cryst. syst.
Space grp
˚
a/A
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Z
d(calc)/g cm-3
R(int)
m/mm-1
Total data
2
>2s(Fo
)
Variables
R (>2s)
Rw
424
1099
442
787
0.0495
0.1196
0.996
0.0437
0.0949
0.988
0.0394
0.0915
1.001
0.0708
0.2030
1.051
GOF
Flack Param.
0.05(8)
3
3
(t, JFF = 21 Hz, 3F, p-CF), -164.5 (br.t, JFF = 22 Hz, 6F, m-
routine.44–46 The remaining non-hydrogen atoms were located from
successive difference Fourier map calculations. The refinements
were carried out by using full-matrix least-squares techniques on F,
minimizing the function w (Fo - Fc)2 where the weight w is defined
as 4Fo2/2s (Fo2) and Fo and Fc are the observed and calculated
structure factor amplitudes, respectively. In the final cycles of each
refinement, all non-hydrogen atoms were assigned anisotropic
temperature factors in the absence of disorder or insufficient data.
In the latter cases atoms were treated isotropically. C–H atom
positions were calculated and allowed to ride on the carbon to
CF). 31P{ H} NMR (CDCl3) d: 119.5 (q, 3JPB = 21 Hz). 13C NMR
1
(partial, CDCl3) d: 148.0 (dm, JFC = 240 Hz), 138.8 (dm, JFC
=
221 Hz), 136.6 (dm, JFC = 246 Hz), 131.4 (d, JPC = 19 Hz), 130.0
(d, JPC = 7 Hz), 129.4 (d, JPC = 8 Hz), 128.5 (d, JPC = 3 Hz), 46.8 (d,
JPC = 29 Hz), 24.5 (d, JPC = 2 Hz). Anal. Calcd. for C30H15BCl2F15P
(%) C: 46.61, H: 1.96; found C: 47.37, H: 2.38.
1
3
(7) Yield: 130 mg (53%). H NMR (CDCl3) d: 9.45 (d, JPH
=
44 Hz, 1H, PC = CH), 7.44 (t, J = 2 Hz, 3H), 7.34 (d, 3JHH = 9 Hz,
3H), 7.22 (dd, 3JHH = 9 Hz, J = 2 Hz, 3H), 7.10 (td, 3JHH = 8 Hz,
4JHH = 2 Hz, 1H, p-CH), 6.90 (t, JHH = 8 Hz, 2 H, m-CH), 6.51
which they are bonded assuming a C–H bond length of 0.95 A. H-
3
˚
(d, 3JHH = 7 Hz, 2H, o-CH). 11B NMR (CDCl3) d: -16.4 (d, 3JPB
=
atom temperature factors were fixed at 1.10 times the isotropic
temperature factor of the C-atom to which they are bonded.
The H-atom contributions were calculated, but not refined. The
locations of the largest peaks in the final difference Fourier map
calculation as well as the magnitude of residual electron densities
in each case were of no chemical significance (Table 1). In the
case of 7, the severe disorder of the included solvent precluded
a chemically reasonable model and thus the data were squeezed
using PLATON to obtain the final solution.
20 Hz). 19F NMR (CDCl3) d: -130.9 (d, 3JFF = 23 Hz, 6F, o-CF),
-161.9 (t, 3JFF = 20 Hz, 3F, p-CF), -166.4 (t, 3JFF = 22 Hz, 6F, m-
CF). 31P{ H} NMR (CDCl3) d: 15.0 (q, 3JPB = 20 Hz). 13C NMR
1
(partial, CDCl3) d: 150.1, 147.2 (d, JPC = 9 Hz), 138.4 (d, JPC
9 Hz), 129.5 (d, JPC = 6 Hz), 129.0 (d, JPC = 4 Hz), 128.4 (d, JPC
=
=
3 Hz), 125.2, 118.3 (d, JPC = 3 Hz), 35.1, 35.0, 31.4, 30.2. Anal.
Calcd. for C68H69BF15O3P ∑ 1 eq. hexane (as per crystal structure)
(%) C: 65.97, H: 6.21; found C: 66.19, H: 6.57.
X-Ray data collection and reduction
Results and discussion
Crystals were coated in Paratone-N oil in the glovebox, mounted
on a MiTegen Micromount and placed under an N2 stream, thus
maintaining a dry, O2-free environment for each crystal. The data
were collected on a Bruker Apex II diffractometer. The data were
collected at 150( 2) K for all crystals. The frames were integrated
with the Bruker SAINT software package using a narrow-frame
algorithm. Data were corrected for absorption effects using the
empirical multiscan method (SADABS).
Lewis acid–base adducts
The reaction of tBu(C6H4O2)P, with the borane B(C6F5)3 gives
rise to NMR data consistent with the formation of a classical
Lewis acid–base adduct. For example, a single broad resonance
in the 11B NMR spectrum at -8.7 ppm is attributable to
tBu(C6H4O2)P(B(C6F5)3) (1). This species also gives rise to a gap
between the 19F NMR signals attributable to the meta- and para-
fluorines of 4.4 ppm as well as a 31P{ H} NMR resonance at
1
Structure solution and refinement
178.8 ppm, consistent with adduct formation. This assignment
was subsequently confirmed crystallographically (Fig. 1). In this
Non-hydrogen atomic scattering factors were taken from the
literature tabulations.43 The heavy atom positions were determined
using direct methods employing the SHELXTL direct methods
˚
case, the P–B bond distance was found to be 2.069(3) A, while the
remaining metric parameters were unexceptional.
This journal is
The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 237–242 | 239
©

Fig. 1 ORTEP drawing of 1, 50% thermal ellipsoids are shown, hydrogen
atoms are omitted for clarity.
In contrast, the NMR data for the corresponding reactions
of tBu(C20H12O2)P and Cl(C20H12O2)P with B(C6F5)3 gave rise
to 11B NMR signals at 12.2 and 29.5 ppm and gaps be-
tween the meta- and para-fluorines in the 19F NMR spectra
Scheme 2 Examples of Lewis Acid–base equilibria from adduct to FLP.
the impact of steric demand on the stability of the adduct
(Scheme 2(b)). A similar situation may be ascribed to the observa-
tion of the equilibrium for 3, although this could also be attributed
in part to the diminished basicity of Cl(C20H12O2)P as a result
of the presence of the electron withdrawing chloride substituent.
In exploring this ambiguity, we noted that the combination of
(C6H3(2,4-tBu2)O)3P or tBuPCl2 with B(C6F5)3 resulted in NMR
data identical with the constituents of the mixtures, unperturbed
by the presence of the other reagent. Thus these mixtures appear
to be exclusively FLPs (Scheme 2(c)). In the case of (C6H3(2,4-
tBu2)O)3P, the inability to form a Lewis acid–base adduct is clearly
attributable to steric demands of the bulky phenoxide substituents.
This view is consistent with our initial notions of FLPs. However
the latter case of tBuPCl2 is different as the inability to form an
adduct in this case can be attributed to the electron withdrawing
substituents on the P atom. In this case, one could argue that the
“frustration” is derived from an electronic effect rather than steric
factors.
1
10.5 and 13.3 ppm, respectively. The corresponding 31P{ H}
NMR resonances are seen at 167.4 and 162.5 ppm. These
data are consistent with the presence of equilibria between
free phosphine and borane and the corresponding adducts
(Scheme 2). Nonetheless, in each of these cases, removal of
the solvent afforded isolation of the adducts formulated as
tBu(C6H4O2)P(B(C6F5)3) (1), tBu(C20H12O2)P(B(C6F5)3) (2) and
Cl(C20H12O2)P(B(C6F5)3) (3) in high yields. The presence of the
equilibria in the latter cases suggests that steric demands of the
phosphine substituents plays a critical role. X-ray crystallography
was used to characterize 2 (Fig. 2). This species is as expected
˚
with quaternized B centers and B–P bond length of 2.072(4) A,
respectively.
FLP Reactivity with PhCCH
Despite the fact that 1 is an isolable adduct of tBu(C6H4O2)P and
B(C6F5)3, subsequent addition of PhCCH results in the formation
of a new species 4 in near quantitative yield. The 1H NMR
spectrum of 4 exhibits a distinctive signal at 9.10 ppm with a
3JPH coupling of 37 Hz. The 11B NMR signal at -16.2 ppm is char-
acteristic of an anionic B center. Consistent with this is the small
gap of 4.7 ppm between the 19F NMR resonances arising from the
meta- and para-fluorines. The 31P{ H} NMR spectrum of 4 shows
1
a quartet at 107.7 ppm with a P–B coupling of 18 Hz. This small
coupling, also observed in the B spectrum, is consistent with a trans
substitution of B and P on an olefinic fragment, prompting the for-
mulation of 4 as tBu(C6H4O2)P(Ph)C CHB(C6F5)3 (Scheme 3).
In an analogous fashion, the addition of PhCCH to solutions of
the phosphines tBu(C20H12O2)P, tBuPCl2 and (C6H3(2,4-tBu2)O)3
Fig. 2 ORTEP drawing of one of the molecules of 2 in the asymmetric
unit, 50% thermal ellipsoids are shown, hydrogen atoms are omitted for
clarity.
The exclusive formation of 1 in solution (Scheme 2(a)), while
2 exists in equilibrium at room temperature clearly points to
240 | Dalton Trans., 2012, 41, 237–242
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The Royal Society of Chemistry 2012
©

Scheme 3 Synthesis of the FLP addition products 4–7.
P each with an equivalent of B(C6F5)3 gave rise to new products
5–7, respectively (Scheme 3). These products were isolated in
yields ranging from 53–97%. The lower yield of 7 is attributed
to the hexane solubility resulting from the tBu substituted aryl
rings on the phosphine. Similar to 4, each of these products
Fig. 4 ORTEP drawing of 7, 50% thermal ellipsoids are shown, hydrogen
atoms are omitted for clarity. Only one conformation of the disordered
tBu groups are shown.
FLP chemistry. Firstly, the formation of 4, despite the formation
of an apparent “tight” adduct (compound 1) demonstrates that
even if not evident by spectroscopy, an indiscernible equilibrium
may allow the reactions of seemingly classical Lewis acid–base
adduct. This notion is further confirmed by a previously report
of the reactivity of the seemingly robust adduct Ph3P(B(C6F5)3)
with alkyne.15 A second teaching from the present observations
relates to the formation of 5–7. While the steric demands of
tBu(C20H12O2)P and (C6H3(2,4-tBu2)O)3P allow for the generation
of an FLP with B(C6F5)3 and subsequent reactivity to give 5 and
7, the formation of 6 is a rare example where electronic factors
“frustrates” adduct formation, permitting the FLP addition to
alkyne to occur.
1
showed a distinctive resonance in the H NMR spectra at 8.96,
9.16 and 9.45 ppm, attributable to an olefinic proton. The 11B
NMR spectra of 5–7 were consistent with the presence of the
anionic borate fragment with 11B signals at -16.3, -15.8 and -16.4,
respectively. Similarly, the 19F spectral data supported this aspect
1
of the formulation. 31P{ H} NMR spectra were consistent with the
quaternization of P with resonances at 91.1, 119.5 and 15.0 ppm
respectively. Each of these signals shows P–B coupling on the
order of 17–20 Hz, similar to that seen for 4. Collectively these
data support the formulation of 5–7 as L(Ph)C CHB(C6F5)3 (L =
tBu(C20H12O2)P 5, tBuPCl2 6 and (C6H3(2,4-tBu2)O)3P 7).
The nature of 4–7 was further confirmed via crystallographic
characterization of 6 and 7 (Fig. 3, 4). These data confirm the
trans-addition of phosphine and borane to PhCCH. The resulting
Conclusion
˚
newly formed P–C and B–C bonds were found to 1.794(2) A and
˚
˚
˚
1.642(3) A and 1.755(4) A and 1.642(6) A, respectively in 6 and 7.
In this work we broadened the range of phosphines that effect
FLP addition reactions to alkynes. While FLPs derived from
phosphines with phenoxide substituents and B(C6F5)3 are shown
to span the range from strong adduct to non-interacting FLPs,
all of these systems react with alkynes. Similarly, phosphines with
electron withdrawing chloride substituents do not form classical
adducts with borane, yet subsequent addition of the FLP to alkyne
proceeds. The factors relating FLP formation and subsequent
reactivity continue be the subject of study in our laboratories.
˚
The C C double bonds were typical being 1.345(3) A in both 6
and 7.
Notes and references
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2 D. W. Stephan, Dalton Trans., 2009, 3129–3136.
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5 P. A. Chase, T. Jurca and D. W. Stephan, Chem. Commun., 2008, 1701–
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6 P. A. Chase, G. C. Welch, T. Jurca and D. W. Stephan, Angew. Chem.,
Int. Ed., 2007, 46, 8050–8053.
Fig. 3 ORTEP drawing of 6, 50% thermal ellipsoids are shown, hydrogen
atoms are omitted for clarity.
7 D. W. Stephan, S. Greenberg, T. W. Graham, P. Chase, J. J. Hastie, S. J.
Geier, J. M. Farrell, C. C. Brown, Z. M. Heiden, G. C. Welch and M.
Ullrich, Inorg. Chem., 2011, DOI: 10.1021/ic200663v.
8 G. C. Welch, R. R. S. Juan, J. D. Masuda and D. W. Stephan, Science,
2006, 314, 1124–1126.
The nature of the species 4–7 is similar to those previously
reported for the trimolecular reaction of FLPs and alkynes.
Nonetheless, these examples illustrate several important aspects of
9 G. C. Welch and D. W. Stephan, J. Am. Chem. Soc., 2007, 129, 1880–
1881.
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The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 237–242 | 241
©

10 D. P. Huber, G. Kehr, K. Bergander, R. Fro¨hlich, G. Erker, S. Tanino,
Y. Ohki and K. Tatsumi, Organometallics, 2008, 27, 5279–5284.
11 P. Spies, G. Erker, G. Kehr, K. Bergander, R. Fro¨hlich, S. Grimme and
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12 P. Spies, S. Schwendemann, S. Lange, G. Kehr, R. Fro¨hlich and G.
Erker, Angew. Chem., Int. Ed., 2008, 47, 7543–7546.
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2, 170–176.
41 Repeated attempts to obtain elemental analysis on crystallize samples
results in analyses where the C determined was low while the H was
expected. This situation has been seen before for fluorinated borane
derivatives and is attributed to the formation of B-Carbides during
combustion.
42 M. Wieber and W. R. tIoos, Monatsh. Chem., 1970, 101, 776–781.
43 D. T. Cromer and J. T. Waber, Int. Tables X-Ray Crystallogr., 1974, 4,
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48, 9709–9712.
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25 G. Me´nard and D. W. Stephan, J. Am. Chem. Soc., 2010, 132, 1796–
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45 G. M. Sheldrick, Bruker AXS Inc Madison, WI, 2000.
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242 | Dalton Trans., 2012, 41, 237–242
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The Royal Society of Chemistry 2012
©