C-C coupling by thermolysis of alkynyl phosphonium borates

Article abstract of DOI:10.1002/chem.201001575

When the heat is on, alkynyl phosphonium borates, tBu₂PHC≡ CBH(C₆F₅)₂ and (tBu₂PC≡ CB(C₆F₅)₂)(BuCH₂CH)(tBu ₂PC≡CB(C₆F₅)₂), undergo C-C coupling reactions leading to a C₄ alkene derivative, (tBu ₂PCHBF(C₆F₄)(C₆F₅)CH) ₂, and a cumulene derivative, (tBu₂P)C₆F ₄BF(C⁶F₅)C₄B(C₆F ₅)₂(BuCH₂CH)(PtBu₂), respectively (see figure). (Figure Presented)

Full text of DOI:10.1002/chem.201001575

DOI: 10.1002/chem.201001575
À
C C Coupling by Thermolysis of Alkynyl Phosphonium Borates
Xiaoxi Zhao,[a] Thomas M. Gilbert,[b] and Douglas W. Stephan*[a]
Dedicated to the memory of Professor Pascal LeFloch
Compounds containing both phosphine and borane frag-
ments have attracted increasing attention over the past sev-
eral years. Perhaps the most widely studied use of such mol-
ecules has been as ligands for transition metals. Indeed sys-
tems that incorporate both P and B centers have been
shown to be quite versatile for the coordination chemistry[1]
of transition metals. Moreover the resulting complexes give
rise to unusual reactivity[2] and have furnished new organo-
metallic catalysts.[3] While the impact on the reactivity of
such ancillary P/B ligands is interesting, materials containing
P and B centers also exhibit interesting properties. For ex-
ample, phosphorus ylide–borane adducts have been em-
ployed as latent catalysts for thermosetting resins.[4] In addi-
tion, more elaborate molecules containing P and B centers
have been prepared to exploit the combination of such elec-
tron-donor and -acceptor capabilities. Together with highly
conjugated organic p systems, such materials exhibit intrigu-
ing electronic and optical properties and have found appli-
cations as fluorescent and nonlinear optical[5] materials.[6] In
another application, Gabbai et al have pioneered combina-
tions of the Lewis acidity of boranes with the electron-with-
drawing abilities of phosphonium groups to effect enhanced
Lewis acidity at the B center for use as fluoride sensors.[7]
In a somewhat more fundamental context, we have re-
cently described “frustrated Lewis pairs” comprising P and
B centers in which steric demands preclude classical Lewis
acid–base adduct formation. Such systems retain Lewis acid-
ity and basicity that can be exploited to activate a variety of
small molecules including olefins,[8] dienes,[9] THF, BH
bonds,[10] terminal alkynes,[11] CO2,[12] and N2O.[13] Most nota-
bly these systems effect the heterolytic activation of H2 and
can be subsequently employed in metal-free hydrogenation
catalysis of imines, aziridines, nitriles, enamines, and silyl-
AHCTUNGTRENNUNG
enolethers.[10,14,19] More recently, Bercaw et al[15] have used
related phosphine-borane systems to effect H2 activation
and reduce metal-bound carbonyl fragments, while Bouris-
sou and co-workers have exploited donor/acceptor proper-
ties of phosphine-boranes to effectively trap reactive inter-
mediates such as singlet O2.[16]
The nature of the organic links between the P and B cen-
ters in the above systems are frequently arene-based[17] al-
though two-carbon alkyl[18] or alkenyl[19] chains have also
been examined. Alkynyl-bridged systems bearing P and B
centers were prepared some years ago by Marder[20] and
others.[21] and shown to exhibit nonlinear optical proper-
ties.[20,22] More recently, Yamaguchi and co-workers have de-
scribed an ingenious synthetic pathway for phosphonium
borate linked stilbene derivatives formed by intramolecular
ꢀ
nucleophilic addition of phosphine and borane across a C C
unit in efforts to develop new organic materials.[23] Despite
the interest in the electronic properties, little attention has
focused on the reactivity of such alkynyl-bridged P/B sys-
tems. In a recent report, we described the unconventional
ꢀ
binding mode of the alkynyl phosphine-borane, tBu2PC CB-
A
(C6F5)2, to Ni0.[24] Herein, the thermal reactivity of alkynyl
phosphonium borates is examined. In two such systems the
strongly polarized alkynyl group is shown to react via group
À
migration and C C coupling to give rise to unusual C4 ole-
finic and cumulenic fragments. The mechanism of these un-
usual rearrangements is probed computationally.
[a] X. Zhao, Prof. Dr. D. W. Stephan
ꢀ
Thermolysis of the phosphonium borate tBu2PHC CBH-
Department of Chemistry, University of Toronto
80 St. George St., Toronto, ON, M5S 3H6 (Canada)
E-mail: dstephan@chem.utoronto.ca
N
for 12 h. The solution became yellow and subsequently
orange after about 5 min. Following completion of the ther-
molysis, the solution was allowed to cool and a product (2)
crystallized from the solution (Scheme 1). The crystals were
isolated and washed with benzene affording 2 in 40% yield.
Homepage: http://www.chem.utoronto.ca/staff/DSTEPHAN
[b] Prof. Dr. T. M. Gilbert
Department of Chemistry & Biochemistry
Northern Illinois University
DeKalb, IL 60115 (USA)
1
The H NMR spectrum of 2 showed broad multiplets at d=
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001575.
5.90 and 3.01 ppm as well as doublets at d=1.46 and
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COMMUNICATION
In targeting related thermolysis reactions, we prepared a
second phosphonium borate in the following fashion. Com-
pound 1 was treated with one equivalent each of BACHTUNGTRENNUNG
and PtBu3 generating the salt [HPtBu3][HBCAHTUNGTRENNNUG
ꢀ
tBu2PC CB
(C6F5)2 via proton and hydride transfer. Al-
though this latter species was not isolated, subsequent addi-
tion of one equivalent of 1-hexene afforded the new species
3, which was isolated as a fluffy white solid in 74% yield.
The H and 13C{1H} NMR spectra were consistent with the
1
incorporation of hexene and the alkyne-linked phosphine-
ꢀ
borane in a 1:2 ratio and formulation of 3 as (tBu2PC CB-
A
U
with inequivalent quaternary P and B centers. The 19F spec-
trum showed broad signals at room temperature and 20 res-
onances at À508C, suggesting inhibited rotation about the
Scheme 1. Synthesis of 2–4.
À
B C bonds. These data collectively are consistent with addi-
1.18 ppm. The latter resonances correspond to inequivalent
tertiary butyl groups. The broad 11B {1H} signal was ob-
served at d=3.09 ppm, while the 19F NMR spectrum exhibit-
ed a signal at d=À185.99 ppm consistent with the presence
of BF fragments. In addition the 19F NMR resonance pat-
terns were consistent with the presence of C6F5 and ortho-
substituted C6F4 fragments. A singlet was observed in the
31P NMR spectrum at d=75.24 ppm. EI-mass spectral data
revealed a molecular ion with an m/z ratio of 1032.2. While
this mass corresponds to dimerization of the phosphonium
borate, the spectral data were inconsistent with a simple di-
merization process, suggesting a chemical transformation.
Unambiguous confirmation of the nature of 2 was obtained
by X-ray crystallography (Figure 1).[25] This species is a
tion of P and B to hexene generating phosphonium and
borate centers. The presence of the ions enhances the Lewis
acidity and basicity of the pendant P and B centers and pro-
motes dative bond formation. The steric congestion resulting
from this coordination presumably generates the observed
rotational restriction. This reaction is analogous to the addi-
tion of simpler frustrated Lewis pairs to olefins that we have
previously reported.[8]
Thermolysis of 3 in [D8]toluene at 808C for 10 h convert-
ed the initially white suspension into a yellow solution. Fol-
lowing workup, a new air- and moisture-stable compound
(4), consisting of two diastereomers in a 1:1.6 ratio, was iso-
lated in 52% yield. Although it was impossible to resolve
the diastereomers in quantitative yields, single crystals of
both diastereomers 4a and 4b were obtained on a few crys-
tallization attempts. The 31P{1H} and 11B{1H} NMR data for
4a were consistent with the presence of two types of phos-
phonium and borate fragments, while the 19F NMR spectrum
À
revealed the presence of a B F unit as well as an o-substi-
tuted C6F4 fragment and three inequivalent C6F5 rings. Al-
though it was not possible to unambiguously determine the
backbone structure of 4a from the spectroscopic data, X-ray
methods confirmed its formulation as (tBu2P)C6F4BF-
(PtBu2) (Figure 2),[25] which is
AHCNUTTGER(NGNUN C6F5)C4BACHUTGTNRENNUG(C6F5)2ACHTUNGERTNN(GUN BuCH2CH)CAHTNUGTRENNGUN
a di-zwitterion with two discrete phosphonium-borate frag-
ments linked through a chain of four carbons forming a bu-
Figure 1. POV-ray drawing of 2, H atoms have been deleted for clarity.
trans-disubstituted olefin in which the two equivalent sub-
stituents are five-membered rings derived from boron and
phosphorus atoms linked by a fluoroarene ring and a me-
thine carbon. The methine carbon atoms are bound to the
olefin-linking fragment. Each of the P centers are cationic
as they are also bound to two tert-butyl groups, whereas the
B centers are anionic being quaternized by an additional
C6F5 ring and a fluoride ion. The metric parameters are un-
À
exceptional, with B F distance of 1.758(6) ꢁ and the olefin-
ic C=C bond length of 1.331(4) ꢁ. It is noteworthy that the
molecule sits on a crystallographically imposed center of
symmetry and thus is the meso isomer for the chiral carbon
and boron centers.
Figure 2. POV-ray drawing of 4a, All H atoms have been deleted for
clarity.
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D. W. Stephan et al.
tatriene fragment. It is noteworthy that the overall geometry
is one in which the two B centers and the two P centers
adopt trans dispositions. The molecule contains two chiral
centers, one on the carbon derived from hexene and the
second on the B center which is chiral as a result of o-substi-
tution of one of the fluoroarene rings. The relative chirality
of these two centers is RR, however, the molecule is isolated
as a racemic mixture of the RR and SS enantiomers, consis-
tent with the crystallization of the species in a centrosym-
metric space group. The metric parameters within 4 are not
exceptional. The cumulene linkage is essentially linear with
Scans of the potential energy surfaces associated with
these peripheral group processes suggest that the surfaces
are rather flat, and that formation of the PC6F4B “caps” pro-
ceeds with barriers below that required in the initial step.
These latter steps are not rate-determining, and so have not
been studied in detail. This proposed pathway (Figure 3) is
reasonable as cyclic structures related to boratacyclopro-
pene 1a have been reported in literature.[26] The inability to
observe the formation of 1a spectroscopically is consistent
with the slow first step. As this boratacycle is expected to be
a powerful reductant,[26] the presence of a reducible moiety
within the molecule presumably prompts the facile dimeri-
zation. It is also noteworthy that other possible mechanisms
are not supported by experimental evidence. Radical path-
ways are eliminated as both a radical scavenger (TEMPO)
and a radical initiator (AIBN) did not substantially affect
the reaction. A process involving rapid H2 evolution and re-
uptake is dismissed as there is no deuterium incorporation
in the presence of D2.
À
C-C-C angles of 172.0(2)8 and 178.3(3)8 and C C distances
of 1.323(3), 1.265(3), and 1.330(3) ꢁ. 1H, 11B{1H}, 19F, and
31P{1H} NMR and IR data of 4a and 4b revealed these spe-
cies to be structurally similar. Indeed, X-ray data confirmed
4b is the RS/SR diastereomer of 4.
The reactions that form 2 and 4 have some common fea-
tures. To probe the mechanism for the formation of 2, par-
À
ticularly the C C coupling reaction that leads to the C4
À
chain, DFT calculations were performed. These suggest that
the rate-determining step involves the nearly isoenergetic
rearrangement of 1 to boratacyclopropene 1a through the
transition state 1aTS akin to that for hydroboration of the
alkyne fragment, with a barrier of about 40 kcalmolÀ1
(Figure 3). Dimerization of the resulting boratacyclopropene
In the case of the thermolysis of 3, the absence of B H
fragments precludes boracyclopropene intermediates. How-
ever, the strong polarization of the alkyne fragment result-
ing from the phosphonium, phosphine, borane, and borate
fragments in 3 suggests the possibility of thermally induced
intramolecular attack and rearrangements to give the result-
ing cumulene unit. Similar P attack at the ortho positions of
one of the fluoroarene rings with concurrent F transfer to B
completes the transformation to 4 (see Scheme 2).
Figure 3. Energy diagram of proposed mechanism of formation of 2
based on DFT calculations; energies [M06-2x/6-311GACTHUNRTGNEUNG(d,p)//oniMPW1K/
6-31+G(d)] are given in kcalmolÀ1
.
À
ring by C C coupling involves passage through transition
state 1bTS and its smaller barrier of about 30 kcalmolÀ1.
The product of bond rearrangement and dimerization is a
symmetrical trans-alkene with a borataalkene substituent on
each side (1b) that lies about 65 kcalmolÀ1 below 1. Com-
pound 1b undergoes a slightly endothermic proton migra-
tion from P to the adjacent C to form monoene 1c. Subse-
quent P attack at ortho positions of the fluoroarene rings
with concurrent F transfer to B are collectively exothermic
by nearly 200 kcalmolÀ1 (Figure 3).
Scheme 2. One proposed mechanism for formation of 4.
In the generation of both 2 and 4, a major driving force is
believed to be the formation of five-membered cycles via
ortho-F activation of C6F5 groups by the “frustrated” P and
B bound to the same carbon atom. As indicated in Figure 3,
one cyclization provides about 65 kcalmolÀ1 of thermody-
namic stabilization. In agreement with the facile nature of
this cyclization process, an attempt to synthesize a CH2-
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À
ꢀ
C C Coupling in PC CB Species
COMMUNICATION
X-ray diffraction were obtained from a reaction mixture. 1H NMR: d=
5.90 (m, 2H, =CH), 3.01 (m, 2H, P(B)CH), 1.46 (d, 18H, 3JHP =16.0 Hz,
[27]
bridged phosphine-borane from ClB
(C6F5)2
and
tBu2PCH2Li[28] led to the isolation of the cyclic product 5
(Scheme 3). Compound 5 was isolated as a mixture of the
3
tBu), 1.18 ppm (d, 18H, JHP =15.6 Hz, tBu); 11B {1H} NMR: d=3.09 ppm
(br); 31P {1H} NMR: d=75.24 ppm (s).
ꢀ
ꢀ
(tBu2PC CB
ACTHGNUTRNE(NUGN C6F5)2)ACHTUNEGTRGN(NNU BuCH2CH)(tBu2PC CBACHTNGUTERN(NUGN C6F5)2) (3): A mixture of
BACTHUNGTR(UNENNG C6F5)3 (86 mg, 0.17 mmol) and PtBu3 (34 mg, 0.17 mmol) dissolved in
toluene (1.0 mL) was added to a solution of 1 (88 mg, 0.17 mmol) in tolu-
ene (1.5 mL). The reaction mixture immediately turned orange-yellow,
and became cloudy upon stirring for 1 h. Hexanes (8 mL) were added,
and the reaction was left standing at À358C for 2 h. The mixture was
then filtered through Celite and washed with cold hexanes. After addi-
tion of 1-hexene (2.3 mL) to the combined filtrate, the yellow solution
was left standing at 258C without stirring until a white fluffy solid pre-
cipitated out (typically 1–2 days). The product was washed with pentane
Scheme 3. Synthesis of 5.
and dried in vacuo. Yield: 70 mg, 74%. 1H NMR: d=2.71 (m, 1H,
3
PCH), 2.61 (m, 2H, PCHCH2), 1.83 (m, 2H, BCH2), 1.60 (d, 9H, JHP
=
15.4 Hz, tBu), 1.56 (m, 2H, PCHCH2CH2) 1.49 (d, 9H, 3JHP =14.9 Hz,
tBu), 1.43 (d, 9H, 3JHP =14.7 Hz, tBu), 1.07 (d, 9H, 3JHP =14.9 Hz, tBu),
1.02 (m, 2H, CH2CH3), 0.68 ppm (t, 3H, 3JHH =7.3 Hz, CH2CH3);1B{1H}
NMR: d=À16.50 (br s), À18.02 ppm (s); 31P{1H} NMR: d=41.47 (s),
ꢀ
fluoro- and chloroborate compounds and X-ray data con-
firmed the structure to be (C6F5)ACHTUNGRTNEN(NUG C6F4)XBCH2PtBu2 (X=F
5a/Cl 5b, ratio: 50:50) (Figure 4).
25.45 ppm (br s). IR (thin film from CH2Cl2, cmÀ1): n˜ =2124 (n
ACHTUNGTRENNUNG(C C)).
ACHNUTTGRNEGU(N tBu2P)C6F4BFACHTUNTGERU(NNNG C6F5)C4BACHTNUTRGENNG(UN C6F5)2AHCTNUGTREN(NUNG BuCH2CH)ACHTUNTGNRUEGN(PtBu2) (4): A suspension of
(3: 21 mg, 0.02 mmol) in [D8]toluene (0.5 mL) was heated at 808C in a J-
Young NMR tube for 10 h, during which time the reaction mixture
turned yellow. After confirming the full conversion of the product by
NMR spectroscopy, the solvent was pumped off completely from the
mixture. The product was then purified on a flash column of neutral alu-
mina (initial wash with hexane followed by elution of the yellow band
with hexane/ethyl acetate=9:1). The eluent was dried under vacuum to
afford a yellow product. The product was further recrystallized from
CH2Cl2/diethyl ether/hexane when necessary. Yield: 11 mg, 52%. Slow
evaporation of toluene from a product solution gave single crystals of 4a,
and another setup afforded single crystals of 4b, both of which were suit-
1
able for X-ray diffraction. 4a: H NMR: d=3.01 (m, 1H, PCH), 2.42 (m,
2H, BCH2), 2.07 (m, 1H, PCHCH2), 1.86 (m, 1H, PCHCH2), 1.80 (m,
3
Figure 4. POV-ray drawing of 5a. H atoms have been deleted for clarity.
2H, PCHCH2CH2), 1.51 (d, 9H, 3JHP =14.6 Hz, tBu), 1.48 (d, 9H, JHP
=
15.0 Hz, tBu), 1.46 (m, 2H, CH2CH3), 1.44 (d, 9H, 3JHP =14.6 Hz, tBu),
0.97 (d, 9H, 3JHP =14.4 Hz, tBu), 0.96 ppm (t, 3H, 3JHH =7 Hz, CH3);
11B{1H}: d=3.50 (br), À9.76 ppm (br). 31P{1H}: d=59.10 (m), 55.56 ppm
(dm, 5JPP =29 Hz). 4b: 1H NMR: d=3.08 (m, 1H, PCH), 2.42 (m, 2H,
BCH2), 2.07 (m, 1H, PCHCH2), 1.84 (m, 1H, PCHCH2), 1.80 (m, 2H,
PCHCH2CH2), 1.58 (d, 9H, 3JHP =16.7 Hz, tBu), 1.46 (m, 2H, CH2CH3),
1.31 (d, 9H, 3JHP =16.5 Hz, tBu), 1.29 (d, 9H, 3JHP =14.7 Hz, tBu), 1.17
(d, 9H, 3JHP =14.5 Hz, tBu), 0.99 ppm (t, 3H, 3JHH =7 Hz, CH3); 11B{1H}:
d=3.50 (br), À9.76 ppm (br); 31P{1H}: d=59.13 (m), 56.70 ppm (dm,
In conclusion, herein we have shown that alkynyl phos-
phonium borates undergo unusual thermolysis to afford C4
derivatives, providing a new strategy to extended and conju-
gated systems with electron-rich and -deficient centers. Fur-
ther studies of the reactivity, properties and utility of alky-
nylphosphonium borate species are the subjects of current
investigations.
5JPP =32 Hz); IR (thin film from CH2Cl2, cmÀ1): n˜ =1734 (u
ACHTUNGTRENNUNG
ACHUTNGREN(NUG C6F5)ACHTNUGTRENNNUG
AHCTUNGTRENNUNG
sion of tBu2PCH2Li (182 mg, 1.10 mmol) in hexanes (8 mL) at room tem-
perature. The reaction was stirred overnight and the solvents were
pumped off completely. The resulting solid residue was redissolved in
benzene (10 mL) and the insoluble lithium salts were filtered out using
Celite. The filtrate was passed through a column of neutral alumina to
eliminate the brown-colored impurities. Benzene was pumped down from
the filtrate, and the concentrated solution (0.5 mL) was layered with hex-
anes (10 mL). After two days of standing at 258C, a colorless microcrys-
talline solid was isolated, washed with pentane, and dried in vacuo.
Yield: 196 mg, 38%. Single crystals suitable for X-ray diffraction were
grown by layering a CH2Cl2 solution of the product with pentane at
258C. 5a: 1H NMR: d=1.53 (m, 1H, CH2), 1.44 (d, 9H, 3JHP =16.0 Hz,
tBu), 1.38 (d, 9H, 3JHP =15.6 Hz, tBu), 1.29 ppm (m, 1H, CH2); 11B
NMR: d=4.63 ppm (d, 2JBF =74 Hz); 31P{1H} NMR: d=87.13 ppm (m).
5b: 1H NMR: d=1.99 (dd, 1H, 2JHH =16.5 Hz, 2JHP =12.8 Hz, CH2), 1.65
(dd, 1H, 2JHH =16.5 Hz, 2JHP =8.8 Hz, CH2), 1.50 (d, 9H, 3JHP =16.0 Hz,
tBu), 1.25 ppm (d, 9H, 3JHP =15.6 Hz, tBu); 11B NMR: d=À2.02 ppm
(br); 31P{1H} NMR: d=88.76 ppm (m).
Experimental Section
General considerations: NMR spectra were obtained on
a Bruker
Avance 400 MHz spectrometer and spectra were referenced to residual
solvent (1H, 13C) or externally (11B; BF3OEt2, 19F; CFCl3, 31P; 85%
H3PO4). IR spectra were recorded on a Perkin–Elmer Spectrum One FT-
IR spectrometer. Combustion analysis was performed in house on a
Perkin–Elmer CHN Analyzer. (C6F5)2HBCCPHtBu2 (1) was prepared as
previously reported.
(tBu2PCHBFACHTUNGTRENNUNG(C6F4)ACHTUNGTRENNUNG(C6F5)CH)2 (2): AJ-Young NMR tube charged with 1
(84 mg, 0.163 mmol) dissolved in [D5]bromobenzene (1 mL) was heated
at 1508C for 12 h. The color of the solution started turning yellow after a
few minutes, and then started turning orange after about 5 min. After the
heat source was removed, the product was allowed to precipitate out of
the orange solution by layering benzene (1 mL) overnight. The crystalline
solid was filtered out and washed with benzene, and was dried in vacuo
to give a yellow powder. Yield: 34 mg, 40%. Single crystals suitable for
Chem. Eur. J. 2010, 16, 10304 – 10308
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10307

D. W. Stephan et al.
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Acknowledgements
[10] D. W. Stephan, Dalton Trans. 2009, 3129–3136.
[11] M. A. Dureen, D. W. Stephan, J. Am. Chem. Soc. 2009, 131, 8396–
8397.
[12] C. M. Mçmming, E. Otten, G. Kehr, R. Frçhlich, S. Grimme, D. W.
Stephan, G. Erker, Angew. Chem. 2009, 121, 6770–6773; Angew.
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D.W.S. gratefully acknowledges financial support of NSERC of Canada.
D.W.S. is grateful for the support of a Canada Research Chair and a
Killam Research Fellowship from the Killam Foundation. X.Z. thanks
the Province of Ontario and Digital Specialty Chemicals for an Ontario
Graduate Scholarship (OGSST).
[13] E. Otten, R. C. Neu, D. W. Stephan, J. Am. Chem. Soc. 2009, 131,
9918–9918.
À
Keywords: alkyne complexes · boron · C C coupling ·
frustrated Lewis pairs · phosphorus
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Received: June 5, 2010
Published online: July 21, 2010
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