Frustrated Lewis pair addition to conjugated diynes: Formation of zwitterionic 1,2,3-butatriene derivatives

Article abstract of DOI:10.1039/c2dt30321c

The frustrated Lewis pair B(C₆F₅)₃/P(o- tolyl)₃ (4a) reacts with 4,6-decadiyne to give the trans-1,2-addition product 5. In contrast, the B(C₆F₅)₃/P ^tBu₃ FLP (4b) reacts with this substrate to give the trans-1,4-adduct trans-6. The cumulene trans-6 undergoes trans-/cis- isomerization upon photolysis to give a ca. 1:1 trans-6/cis-6 mixture. The FLP 4b reacts with 2,6-hexadiyne at r.t. to yield a ca. 4:1 mixture of their trans-1,2- and trans-1,4-addition products (7,8). DFT calculations showed that the zwitterionic 1,4-addition products are favored under thermodynamic control. Thermolysis of the kinetic trans-1,2-addition product (7) (80 °C, bromobenzene) does not lead to the thermodynamically favored 1,4-isomer (8), but instead elimination of isobutylene occurs to the formal trans-1,2-adduct (9) of the B(C₆F₅)₃/PH^tBu₂ pair. Compounds 5, 6, 7, 8, 9 were analyzed by X-ray diffraction.

Full text of DOI:10.1039/c2dt30321c

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Frustrated Lewis Pairs
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Konstantin Chernichenko, Martin Nieger, Markku Leskelä and Timo Repo
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Frustrated Lewis pair addition to conjugated diynes: Formation of zwitterionic 1,2,3-butatriene
derivatives
Philipp Feldhaus, Birgitta Schirmer, Birgit Wibbeling, Constantin G. Daniliuc, Roland Fröhlich,
Stefan Grimme, Gerald Kehr and Gerhard Erker
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An international journal of inorganic chemistry
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Volume 41 | Number 30 | 14 August 2012 | Pages 8999–9242
ISSN 1477-9226
COVER ARTICLE
Erker et al.
Frustrated Lewis pair addition to conjugated diynes: Formation of
zwitterionic 1,2,3-butatriene derivatives

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Cite this: Dalton Trans., 2012, 41, 9135
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PAPER
Frustrated Lewis pair addition to conjugated diynes: Formation of
zwitterionic 1,2,3-butatriene derivatives†
Philipp Feldhaus,a Birgitta Schirmer,a Birgit Wibbeling,a Constantin G. Daniliuc,a Roland Fröhlich,a
Stefan Grimme,b Gerald Kehra and Gerhard Erker*a
Received 13th February 2012, Accepted 20th April 2012
DOI: 10.1039/c2dt30321c
The frustrated Lewis pair B(C6F5)3/P(o-tolyl)3 (4a) reacts with 4,6-decadiyne to give the trans-1,2-
addition product 5. In contrast, the B(C6F5)3/PtBu3 FLP (4b) reacts with this substrate to give the
trans-1,4-adduct trans-6. The cumulene trans-6 undergoes trans-/cis-isomerization upon photolysis to
give a ca. 1 : 1 trans-6/cis-6 mixture. The FLP 4b reacts with 2,6-hexadiyne at r.t. to yield a ca.
4 : 1 mixture of their trans-1,2- and trans-1,4-addition products (7,8). DFT calculations showed that the
zwitterionic 1,4-addition products are favored under thermodynamic control. Thermolysis of the kinetic
trans-1,2-addition product (7) (80 °C, bromobenzene) does not lead to the thermodynamically favored
1,4-isomer (8), but instead elimination of isobutylene occurs to the formal trans-1,2-adduct (9) of the
B(C6F5)3/PHtBu2 pair. Compounds 5, 6, 7, 8, 9 were analyzed by X-ray diffraction.
Introduction
Results and discussion
Frustrated Lewis pairs (FLPs) add to a variety of unsaturated
organic molecules,1 e.g., 1,2-addition reactions of intra- as well
as intermolecular FLPs to carbonyl compounds,2 including
carbon dioxide,3 alkenes4 and alkynes were described.5 FLPs
can undergo selective 1,4-addition reactions to conjugated
dienes.4d,6 We had recently shown that the highly reactive intra-
molecular P/B FLP 1 undergoes a remarkable 1,4-addition reac-
tion to a variety of conjugated diynes to yield the heterocyclic
eight-membered zwitterionic addition products 2 (Chart 1).7
This posed the question of the behavior of related intermolecular
phosphane/borane frustrated Lewis pairs toward conjugated
diynes. Would we observe preferred 1,4-addition of the com-
ponents of the FLP to the di-acetylenes in these acyclic cases as
well, making the respective zwitterionic butatriene derivatives
readily available or would other pathways of product formation
be more favorable in these open chain systems and thus
prevail? We have carried out a small series of representative
example reactions of such systems which we describe in this
account.
FLP addition reactions to conjugated diynes
We first reacted the substrate 4,6-decadiyne (3a) with the B
(C6F5)3/P(o-tolyl)3 FLP (4a). The system behaved in a compli-
cated manner when it was kept homogenous by using e.g. deut-
erated benzene or dichloromethane as a solvent; however, when
we performed the reaction in pentane at room temperature for a
prolonged time (here: 4 days) a precipitate of colorless crystals
gradually formed. These were isolated (18% yield) and identified
as the trans-1,2-addition product 5 to a single CuC triple bond
by spectroscopy and by X-ray diffraction (see Scheme 1 and
Fig. 2)
Product 5 shows an 11B NMR resonance (δ ¬14.0) with a
typical borate chemical shift and a corresponding phosphonium
Chart 1 FLP addition to diynes.
aOrganisch-Chemisches Institut der Westfälischen Wilhelms-Universität
Münster, Corrensstrasse 40, 48149 Münster, Germany.
E-mail: erker@uni-muenster.de; Fax: (+49)251-83-36503
bMulliken Center for Theoretical Chemistry, Institut für Physikalische
und Theoretische Chemie, Universität Bonn, Beringstrasse 4,
53115 Bonn, Germany
†Electronic supplementary information (ESI) available: Experimental
and analytical details as well as detailed results for DFT calculations.
CCDC reference numbers 866994–866999. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c2dt30321c
Scheme 1 Reaction of the FLP 4a with the diyne 3a.
Dalton Trans., 2012, 41, 9135–9142 | 9135
This journal is © The Royal Society of Chemistry 2012

Fig. 1 19F-NMR spectrum of compound 5 (470 MHz, 233 K, d2-
dichloromethane).
Scheme 2 Reaction of the FLP 4b with the diyne 3a.
Fig. 2 A view of the molecular structure of the 1,2-FLP addition
product 5.
31P NMR signal at δ 21.2. At 233 K the borate section appar-
ently features a –B(C6F5)3 unit in a “frozen” chiral propeller con-
formation on the 19F NMR time scale. It shows six well
separated o-F signals and three different p-F resonances and also
partly resolved m-F signals of the three –C6F5 groups (see
Fig. 1).
Similarly, the three o-tolyl groups at phosphorus are also dia-
stereotopic in 5. We observed three methyl singlets in d2-dichloro-
methane at 233 K. In addition, we measured the typical H/13C
1
Fig. 3 Molecular structure of the zwitterionic cumulene products
trans-6 (top) and cis-6 (bottom).
NMR signals of a pair of chemically differentiated n-propyl sub-
stituents (at the ends of the central C4-framework) and 13C
NMR signals of the central conjugated enyne moiety at δ 197.3
(vCB), 107.2 (1JPC ≈ 77 Hz, vCP), 78.7 and 99.6 (CuC).
Compound 5 was characterized by X-ray diffraction (see
Fig. 2). It shows the bulky –B(C6F5)3 (C4–B1 = 1.671(3) Å) and
–P(o-tolyl)3 groups (C5–P1 = 1.832(2) Å) trans-1,2-bonded
to the central CvC bond (C4–C5 = 1.363(3) Å) of the carbon
framework. Both of these substituents feature chiral propeller-
like conformations as it was expected from the NMR behavior in
solution (see above). The remaining CuC triple bond (C6–C7 =
1.194(3) Å) of the conjugated diyne starting material was kept
unchanged upon the formation of the zwitterionic 1,2-P/B FLP
addition product 5.
but the polar zwitterionic products crystallize or precipitate.
After stirring the 4b/4,6-decadiyne reaction mixture for 6 days at
ambient temperature a close to quantitative amount of a colorless
precipitate was formed. It was isolated by evaporation of the
solvent in vacuo.
The product (trans-6) was obtained pure in 42% yield upon
crystallization from dichloromethane–pentane. Compound 6 (see
Scheme 2) was characterized by C,H elemental analysis, by
spectroscopy and by X-ray diffraction (see Fig. 3). In the crystal
compound trans-6 exhibits an almost linear central 1,2,3-
butatriene derived framework (C1–C2 = 1.335(3) Å, C2–C3 =
1.259(3) Å, C3–C4 = 1.328(3) Å, angles C1–C2–C3 =
171.9(2)°, C2–C3–C4 = 176.6(2)°) to which the remaining
pair of n-propyl substituents from the 4,6-decadiyne starting
material are 1,4-trans bonded (dihedral angle C5–C1⋯C4–
C8 = 169.97°). Consequently the bulky –B(C6F5)3 (C4–B1 =
We then treated the 4,6-decadiyne starting material with the
B(C6F5)3/PtBu3 FLP (4b) which contains a markedly more
nucleophilic phosphane Lewis base component. This gave a
different result. The reaction was again carried out in pentane,
a solvent in which the starting materials are well dissolved at r.t.
9136 | Dalton Trans., 2012, 41, 9135–9142
This journal is © The Royal Society of Chemistry 2012

1.640(3) Å) and –PtBu3 (C1–P1 = 1.821(2) Å) substituents were
found in the remaining trans-1,4-positions.
The trans-1,4-P/B FLP addition product to 4,6-decadiyne
features a 11B{1H} NMR signal at δ −12.7 and a 31P NMR
resonance at δ 45.6, both as expected for an anionic borate and a
cationic phosphonium moiety, respectively. The linear central
1
cumulene unit shows 13C NMR signals at δ 84.6 (d, JPC
=
71.4 Hz, C1), 187.5 (C2), 153.7 (C3) and 170.2 (br, C4).
Photolysis of the pure trans-6 isomer in d2-dichloromethane
(HPK 125, Pyrex filter, r.t.) resulted in a rapid trans-/cis-isomeri-
zation of the cumulene product to give a close to equimolar
photostationary equilibrium of the trans-6 and cis-6 isomers.
The typical NMR data of cis-6 were obtained from the mixture
[cis-6: 31P{1H} δ 45.7, 11B{1H} δ −13.5, 19F δ −130.9 (o–F),
−162.6 (p–F), −166.3 (m–F)].
Single crystals suited for the X-ray crystal structure analysis of
the isomer cis-6 were obtained from dichloromethane–pentane
solution. The structure shows the central CvCvCvC cumu-
lene unit, to which a pair of n-propyl groups is cis-1,4-bonded
and to which the –B(C6F5)3 and –PtBu3 groups originating from
the P/B FLP 4b are also cis-1,4-attached. Although these bulky
substituents are separated by a total of five bonds [B1–C4 =
1.640(5) Å, C4–C3 = 1.330(5) Å, C3–C2 = 1.268(5) Å, C2–C1
= 1.335(5) Å, C1–P1 = 1.816(3) Å] we still notice some remain-
ing steric interaction between these groups that leads to a slightly
convex shaped central cumulene unit [angles C1–C2–C3 =
170.1(3)°, C2–C3–C4 = 173.7(3)°] (see Fig. 3).
We used 2,4-hexadiyne as a second conjugated diyne for this
study. Treatment of this substrate with the B(C6F5)3/PtBu3 fru-
strated Lewis pair under our typical reaction conditions (pentane
solution, r.t., several days reaction time) gave a crystalline pre-
cipitate in a close to 50% yield that contained the 1,2- and
1,4-addition products 7 and 8 in a ca. 4 : 1 ratio after crystalliza-
tion from dichloromethane–pentane solution.
Fig. 4 A comparison of the molecular structures of the 1,2- and
1,4-addition products 7 (top) and 8 (bottom) of the B(C6F5)3/PtBu3 FLP
(4b) to 2,4-hexadiyne.
The compounds were spectroscopically characterized from the
mixture, but we were able to obtain single crystals of each of the
isomers that allowed us their characterization by X-ray diffrac-
tion (see Fig. 4). The structure of the major product 7 features
the –B(C6F5)3 and PtBu3 groups trans-1,2-attached at the central
CvC double bond (C2–B1 = 1.680(6) Å, C3–P1 = 1.877(4) Å,
C2–C3 = 1.364(6) Å, angles B1–C2–C3 = 121.7(4)°, C2–C3–
P1 = 128.0(3)°) and the remaining CuC bond in conjugation
with the olefinic C2–C3 moiety (C4–C5 = 1.196(6) Å, C3–C4 =
1.429(6) Å, angles C2–C3–C4 = 117.7(4)°, C3–C4–C5 =
178.2(5)°). In solution, compound 7 features typical borate
NMR data (11B{1H}: δ −12.2, and a phosphonium 31P NMR
resonance at δ 59.3 [tBu: δ 1.67 (1H), 44.3, 33.0 (13C)].
The X-ray crystal structure analysis of the minor product
8 established it originating from 1,4-FLP addition of the
B(C6F5)3/PtBu3 FLP (4b) to the 2,4-hexadiyne structure. In the
crystal we find a central tetrasubstituted 1,2,3-butatriene unit
(C2–C3 = 1.323(3) Å, C3–C4 = 1.262(3) Å, C4–C5 = 1.320(3)
Å) to which a pair of methyl groups is trans attached at its
terminal carbon atoms. The bulky –B(C6F5)3 and –PtBu3 sub-
stituents, consequently, are also trans-1,4-bonded to the central
C4-cumulene unit (B1–C5 = 1.641(3) Å, P1–C2 = 1.809(2) Å).
In solution, compound 8 features a set of very typical cumu-
lene 13C NMR resonances at δ 79.1 (C2), 187.5 (C3), 152.9
(C4), (C5 adjacent to boron not observed) as opposed to the
Scheme 3 1,4-Addition of B(C6F5)3/PtBu3 FLP (4b) to the 2,4-
hexadiyne.
typical conjugated enyne 13C NMR features that we observed for
the isomer 7 (see above) [7: 13C δ 96.6, 83.7 (CuC), 105.8
1
(vCP), 199.4 (br 1 : 1 : 1 : 1 q, JCB ≈ 55 Hz, vCB)]. Com-
1
pound 8 shows H NMR methyl signals at δ 1.96 (B-side) and
δ 1.97 (P-side) and heteroatom signals at δ −12.8 (11B{1H}),
δ −131.1, −162.7, −166.8 (19F) and δ 45.8 (31P), respectively.
Theoretical considerations
To gain further insight into the nature of these reactions we also
investigated the reactions from Schemes 2 and 3 by means of
quantum chemistry. We used X-ray structures as starting points
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9135–9142 | 9137

Table 1 Reaction energies for the formation of addition products as
calculated with B2PLYP-D3 and TPSS-D3. All energies in kcal mol−1
for further details, see the text
;
Gas phase
Dichloromethane
B2PLYP-D3a
B2PLYP-D3a
TPSS-D3
TPSS-D3
cis-6
trans-6
8
7
−38.4
−36.9
−32.7
−26.9
−39.4
−38.3
−34.5
−25.5
−44.1
−43.5
−41.1
−30.4
−44.9
−44.7
−41.8
−35.6
Scheme 4 Thermolysis of the FLP-trans-1,2-addition product 7.
a TPSS-D3/def2-TZVP optimized structures.
for unconstrained geometry optimizations with the meta-GGA
functional TPSS8 and the large Gaussian-AO basis set def2-
TZVP.9 Subsequent single point calculations with the double
hybrid density functional B2PLYP10 and the same basis set
yielded the reaction energies given in Table 1. For both types of
calculations the resolution of the identity approximation (RI)11
and the recently developed D3-dispersion correction12a in
the version employing the damping function of Becke and
Johnson12b,c have been applied. The inclusion of dispersion
corrections to standard density functionals is crucial for these
sterically crowded systems in which also intramolecular non-
covalent interactions can make substantial contributions to the
total energy. The best theoretical level can be abbreviated
in detail as B2PLYP-D3(BJ)/def2-TZVP//TPSS-D3(BJ)/def2-
TZVP. Solvent effects have been taken into account by additional
calculations (optimizations and single point calculations) apply-
ing the COSMO13 solvation model for dichloromethane (ε =
9.0). The atomic radii for B and P have been set to 1.0530 Å and
2.1060 Å, respectively, as suggested by the COSMO imple-
mentation in the ORCA program package.14 All calculations in
this work have been carried out with the TURBOMOLE 6.3
suite of programs.15
As can be seen from Table 1 the formation of cis-6 is about
1 kcal mol−1 more favored than the formation of trans-6 which
is within the range of accuracy of the method. Since we exper-
imentally only find trans-6 to be formed in the thermally
induced FLP addition reaction, this seems to be kinetically
controlled.
For the second reaction we found that the 1,4-addition product
8 is about 6 kcal mol−1 more stable than the 1,2-addition
product 7 in the gas phase. (The same trends can be assumed for
an unpolar solvent like pentane.) In the polar solvent dichloro-
methane this gap even widens to 11 kcal mol−1. These results
strongly contradict the experimental findings of a ratio of 4 : 1
for 7 : 8 and again point towards a reaction pathway proceeding
under kinetic control.
Fig. 5 Aview of the molecular structure of compound 9.
cumulene isomers. For that purpose we chose to thermolyse the
FLP-trans-1,2-addition product 7 (Scheme 4).
We thermolyzed the 1,2-addition product 7 in d5-bromoben-
zene solution for 5 h at 80 °C. This resulted in a >90% conver-
sion to the new product 9. Compound 9 was formed from 7 by
elimination of isobutylene. We obtained single crystals of 9 from
the d5-bromobenzene solution that were suited for the structure
determination by X-ray diffraction (see Fig. 5).
In the crystal compound 9 exhibits a molecular framework as
we had already observed it in the starting material (7). It is
characterized by a central conjugated enyne framework (C2–C3
= 1.356(3) Å, C3–C4 = 1.437(3) Å, C4–C5 = 1.191(3) Å,
angles C2–C3–C = 127.7(2)°, C3–C4–C5 = 170.7(2)°) to which
the bulky –B(C6F5)3 group is bonded (C2–B1 = 1.645(3) Å).
This group shows a chiral propeller geometry. The newly formed
–PHtBu2 substituent is bonded to the olefinic carbon atom
C3 (P1–C3 = 1.808(2) Å, angles P1–C3–C2 = 123.8(2)°,
P1–C3–C4 = 108.5(1)°). The pseudotetrahedral phosphorus
centre shows a C11–P1–C21 angle of 119.9(1)°. In solution
1
compound 9 shows the characteristic PH H NMR resonance at
δ 5.86 with a 1JPH coupling constant of 431 Hz. The correspond-
1
ing 31P NMR signal (with the same JPH) was monitored at δ
21.4. The bulky –B(C6F5)3 unit shows hindered rotation in the
19F NMR spectra at r.t. Due to the chiral overall geometry of the
–B(C6F5)3 substituent in 9 (see Fig. 5) we observed a total of
15 separate signals of this group in the 19F NMR spectrum of
compound 9 at 273 K (see Fig. 6). Due to the presence of the
persistant chiral –B(C6F5)3 conformation at 273 K the adjacent
Thermolysis of the 1,2-adduct 7
From the DFT analysis of the system it was evident that the
FLP-1,4-addition products to the conjugated diyne substrates
were the thermodynamically favored products. Therefore, it was
tempting to see, whether the respective FLP-trans-1,2-addition
product that we had in some cases obtained by carrying out the
reaction under kinetic control, would be converted to the favored
1
–PHtBu2 substituent features the H NMR signals of a pair of
diastereotopic tert-butyl groups at phosphorus [δ 1.57, (d, 3JPH
16.9 Hz), 1.32 (d, 3JPH = 16.9 Hz)].
=
These results indicate that the 1,2-addition reaction that gives
product 7 by FLP addition under kinetic control does not
9138 | Dalton Trans., 2012, 41, 9135–9142
This journal is © The Royal Society of Chemistry 2012

(1H, 300 MHz, 13C, 76 MHz, 31P, 122 MHz, 11B, 96 MHz, 19F,
282 MHz). Assignments of the resonances were supported by
2D experiments. X-Ray diffraction: Data sets were collected
with a Nonius KappaCCD diffractometer. Programs used: data
collection, COLLECT;19 data reduction Denzo-SMN;20 absorp-
tion correction, Denzo;21 structure solution SHELXS-97;22 struc-
ture refinement SHELXL-9723 and graphics, XP (Bruker AXS,
2000). Thermals ellipsoids are shown with 50% probability,
R-values are given for observed reflections, and wR2 values are
given for all reflections.
Fig. 6 19F NMR spectrum of 9 at 273 K (in d2-dichloromethane,
564 MHz).
Preparation of 5
B(C6F5)3 (256 mg, 0.500 mmol, 1.00 eq) was dissolved in
n-pentane (10 ml). In a small Schlenk tube tri-ortho-tolylphos-
phane (152 mg, 0.500 mmol, 1.00 eq) was dissolved in
n-pentane (5 ml) and 4,6-decadiyne (67.1 mg, 81.8 μl,
0.500 mmol, 1 eq) was added. Afterwards the solution was
added to the borane solution via syringe. The yellowish mixture
was stirred for four days at room temperature. The product grew
in small crystals on the glass bottom. The overlaying solution
was decanted, the obtained residue washed with n-pentane (once
6 ml) and dried in vacuo (yield 86.3 mg, 18%). Crystals suitable
for X-ray crystal structure analysis were obtained by diffusion
method of a dichloromethane–pentane solution at −40 °C. Anal.
Calc for C49H35BF15P: C 61.91, H 3.71; found: C 61.45, H
become reversible, even at 80 °C. We observe a different reaction
instead. Kinetically controlled FLP-addition seems to be prevail-
ing in the systems described in this account.
Conclusions
Our study featured some remarkable results. The addition of the
FLPs 4a and 4b, containing bulky trisarylphosphane or tris-
alkylphosphane Lewis base components, were probably carried
out under kinetic control at room temperature. The outcome
depended strongly on the nucleophilicity of the Lewis base and
to some extent on its steric bulk. Thus, the B(C6F5)3/PtBu3 pair
reacted cleanly with 4,6-decadiyne to yield the respective trans-
1,4-addition product, whereas we only isolated the 1,2-addition
product (albeit in low yield) from the reaction of this substrate
with the B(C6F5)3/P(o-tolyl)3 FLP. DFT calculations showed that
the 1,4-addition products were thermodynamically favored. The
thermolysis of the kinetic trans-1,2-addition product (7) of 4b to
2,4-hexadiyne gives a remarkable result. Instead of forming the
thermodynamic product 8 the system 7 loses an equivalent of
isobutylene to give 9. This reaction probably takes place in an
intermolecular 1,2-elimination reaction. This observation brings
us increasing evidence for the special role the tert-butyl phos-
phane Lewis pair seems to play in FLP chemistry. Piers et al.
have recently observed the first specific example of a related
1
3.65. M.p. 180 °C. H NMR (500 MHz, [D2]-dichloromethane,
233 K): δ = 8.39 (m, 1H, o′-Tola), 7.69 (m, 1H, p-Tola), 7.67 (m,
1H, m′-Tola) 7.64 (m, 1H, p-Tolc), 7.56 (m, 1H, m-Tolc), 7.46
(m, 1H, p-Tolb), 7.45 (m, 1H, o′-Tolc), 7.33 (m, 3H, m-Tola,
o′-Tolb, m′-Tolc), 7.21 (m, 2H, m-Tolb, m′-Tolb), 2.58 (m, 1H,
vCH2[B]), 2.30 (m, 1H, vCH2′[B]), 2.21 (s, 3H, o-TolCH3c), 1.87
(s, 3H, o-TolCH3b), 1.68 (m, 1H, uCH2[P]), 1.61 (s, 3H,
o-TolCH3a), 1.54 (m, 1H, uCH2′[P]), 0.82 (m, 1H, CH2[P]), 0.68
3
(m, 1H, CH2′[P]), 0.46 (m, 1H, CH2′[B]), 0.39 (t, JHH = 7.0 Hz,
3H, CH3[P]), −0.04 (t, JHH = 7.0 Hz, 3H, CH3[B]), −0.79 (m,
3
1H, CH2[B]). 13C{1H} NMR (126 MHz, [D2]-dichloromethane,
1
233 K): δ = 197.3 (m, vCB), 145.0 (d, JPC = 9.4 Hz, o-Tolc),
1
1
144.6 (d, JPC = 7.4 Hz, o-Tolb), 143.3 (d, JPC = 7.5 Hz,
t
isobutylene elimination pathway16 that takes place in the Bu3P–
o-Tola), 137.4 (m, o′-Tola), 134.4 (d, JCH = 11.1 Hz, o′-Tolb),
1
C6F4–BF(C6F5)2 zwitterion.17 Our present study seems to indi-
cate that new elimination pathways may be more frequently
encountered and play a more significant role then previously
thought. The straightforward 7 to 9 conversion moreover indi-
cates that this reaction may become synthetically helpful in FLP
product chemistry.
4
1
134.2 (d, JPC = 2.7 Hz, p-Tola), 134.1 (d, JCH = 11.7 Hz,
o′-Tolc), 133.9 (d, JPC = 2.7 Hz, p-Tolc), 133.5 (d, JPC
=
4
4
2.7 Hz, p-Tolb), 133.1 (d, JPC ≈ 11 Hz, m-Tolc), 133.0 (d,
3
3JPC ≈ 11 Hz, m-Tola), 132.6 (d, JPC = 10.9 Hz, m-Tolb), 127.4
3
(d, JPC = 12.5 Hz, m′-Tolc), 127.1 (d, JPC = 13.2 Hz, m′-Tola),
3
3
3
1
125.6 (d, JPC = 12.7 Hz, m′-Tolb), 119.9 (d, JPC ≈ 88 Hz,
i-Tolb), 119.2 (d, JPC ≈ 80 Hz, i-Tolc), 118.2 (d, JPC ≈ 80 Hz,
1
1
i-Tola), 107.2 (dm, JPC = 77.1 Hz, vCP), 99.6 (d, J = 10.2 Hz,
1
uCCH2), 78.7 (d, J = 18.0 Hz, uC), 46.9 (m, vCH2[B]), 23.1
Experimental
(m, o-TolCH3b), 22.9 (d, JPC = 3.3 Hz, o-TolCH3a), 22.8 (d, JPC
= 4.0 Hz, o-TolCH3c), 21.5 (d, J = 17.2 Hz, CH2[B]), 20.9
(uCH2[P]), 20.9 (CH2[P]), 15.1 (d, J = 4.0 Hz, CH3[B]), 12.7
(CH3[P]), [C6F5 not listed]. 31P{1H} NMR (202 MHz, [D2]-
dichloromethane, 233 K): δ = 21.2 (m). 11B{1H} NMR
(160 MHz, [D2]-dichloromethane, 233 K): δ = −14.0 (ν1/2 ≈ 40
Hz). 19F NMR (470 MHz, [D2]-dichloromethane, 233 K): δ =
−126.8 (o), −130.6 (o′), −164.0 (p), −169.8 (m), −170.2 (m′),
4
4
General information
All reactions were carried out under argon atmosphere with
Schlenk-type glassware. Solvents were dried18 and stored under
argon prior to use. The following instruments were used for
physical characterization of the compounds. Elemental analyses:
Foss–Heraeus CHN-O-Rapid. NMR: Varian Inova 500 (1H,
500 MHz; 13C, 126 MHz, 31P, 202 MHz, 11B, 160 MHz, 19F,
470 MHz), Varian UnityPlus 600 (1H, 600 MHz; 13C, 151 MHz,
31P, 242 MHz, 11B, 192 MHz, 19F, 564 MHz), Bruker AV300
a
(each m, each 1F, C6F5 ), [Δδ19Fm,p = 5.8, 6.2]; −128.1 (o),
−136.2 (o′), −162.9 (p), −166.8 (m′), −167.5 (m), (each m, each
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9135–9142 | 9139

1F, C6F5 ), [Δδ19Fm,p = 3.9, 4.6]; −132.2 (o), −135.7 (o′),
empirical absorption correction (0.664 ≤ T ≤ 0.755), Z = 2, tri-
b
c
ˉ
−164.9 (p), −169.1 (m′), −169.2 (m), (each m, each 1F, C6F5 ),
clinic, space group P1 (No. 2), λ = 1.54178 Å, T = 223(2) K, ω
[Δδ19Fm,p = 4.2, 4.3].
and φ scans, 24 463 reflections collected ( h, k, l), [(sin θ)/λ]
= 0.60 Å−1, 7099 independent (Rint = 0.042) and 6401 observed
reflections [I > 2σ(I)], 539 refined parameters, R = 0.043, wR2 =
X-ray crystal structure analysis of 5
0.121, max. (min.) residual electron density 0.32 (−0.26) e Å−3
hydrogen atoms calculated and refined as riding atoms.
,
Formula C49H35BF15P, M = 950.55, colourless crystal, 0.30 ×
0.15 × 0.10 mm, a = 24.8125(7), b = 19.0369(6), c = 20.6797(5)
Å, β = 82.784(2)°, V = 8774.90(4) Å3, ρcalc = 1.439 g cm−3, μ =
1.442 mm−1, empirical absorption correction (0.671 ≤ T ≤
0.869), Z = 8, monoclinic, space group C2/c (No. 15), λ =
1.54178 Å, T = 223(2) K, ω and φ scans, 65 464 reflections col-
lected ( h, k, l), [(sin θ)/λ] = 0.60 Å−1, 7808 independent
(Rint = 0.057) and 6738 observed reflections [I > 2σ(I)], 600
refined parameters, R = 0.047, wR2 = 0.120, max. (min.) residual
electron density 0.37 (−0.48) e Å−3, hydrogen atoms calculated
and refined as riding atoms.
Preparation of cis-6
The cumulene trans-6 (10 mg, 0.01 mmol) was dissolved in
deuterated dichloromethane and irradiated with UV-light (Philips
HPK 125 lamp) for 20 min to give a photostationary equilibrium
of cis- and trans-isomers in a ratio of 0.9 : 1 (1H NMR). Crystals
suitable for X-ray crystal structure analysis were obtained by dif-
fusion method of the reaction mixture with dichloromethane–
1
pentane at −40 °C. H NMR (500 MHz, [D2]-dichloromethane,
298 K): δ = 2.45 (br, 2H, vCH2[B]), 2.37 (m, 2H, vCH2[P]),
3
t
1.69 (m, CH2[P]), 1.54 (d, JPH = 13.9 Hz, 27H, Bu), 1.17 (br,
Preparation of trans-6
2H, CH2[B]), 0.95 (t, 3JHH = 7.3 Hz, 3H, CH3[P]), 0.74 (t, 3JHH
=
7.4 Hz, 3H, CH3[B]), [[P] and [B] tentative assigned]. 13C{1H}
B(C6F5)3 (332.8 mg, 0.650 mmol, 1.3 eq) and tri-tert-butylphos-
phane (131.5 mg, 0.650 mmol, 1.3 eq) were dissolved in
n-pentane (15 ml). In a second flask 4,6-decadiyne (174.4 mg,
212.7 μl, 1.300 mmol, 2 eq, excess) was dissolved in n-pentane
(4 ml). The FLP solution was added to the diyne solution and
stirred for six days. Then the reaction mixture was dried in vacuo
to get a white solid (388 mg). This crude product was a mixture
of trans-6 and a second species which could not be identified yet
[trans-6: unknown species = 3 : 1 (NMR)]. Subsequently the
crude product (336 mg) was purified by crystallization from
dichloromethane–pentane (r.t.) to yield trans-6 as white crystals
(235 mg, 42%). Crystals suitable for X-ray crystal structure
analysis were obtained by diffusion method of a dichloro-
methane–pentane solution at −40 °C. Anal. Calc for
C40H41BF15P: C 56.62, H 4.87; found: C 55.18, H 4.55.
M.p. 227 °C. HRMS calc for C40H41BF15PNa+: 871.2699;
found: 871.2684. 1H NMR (600 MHz, [D2]-dichloromethane,
298 K): δ = 2.47 (m, 2H, vCH2[P]), 2.14 (m, 2H, vCH2[B]),
NMR (150 MHz, [D2]-dichloromethane, 298 K): δ = 42.8 (br m,
vCH2[B]), 41.6 (d, JPC = 29.7 Hz, Bu), 40.5 (d, JPC = 8.6 Hz,
2
t
3
vCH2[P]), 31.0 (tBu), 25.3 (br, CH2[B]), 24.7 (d, JPC = 2.8 Hz,
3
CH2[P]), 14.3 (CH3[B]), 13.7 (CH3[P]), n.o. (vCB, vCv, vCP),
[C6F5 not listed]. 31P{1H} NMR (202 MHz, [D2]-dichloro-
methane, 298 K): δ = 45.7 (m). 11B{1H} NMR (160 MHz,
[D2]-dichloromethane, 298 K): δ = −13.5 (ν1/2 ≈ 20 Hz). 19F
NMR (470 MHz, [D2]-dichloromethane, 298 K): δ = −130.9 (br,
2F, o-C6F5), −162.6 (m, 1F, p-C6F5), −166.3 (br, 2F, m-C6F5),
[Δδ19Fm,p = 3.7].
X-ray crystal structure analysis of cis-6
Formula C40H41BF15P × 1.5 CH2Cl2, M = 975.90 colourless
crystal, 0.30 × 0.28 × 0.13 mm, a = 12.9180(3), b = 14.0824(8),
c = 24.6152(13) Å, α = 84.704(3), β = 82.226(2), γ = 82.855
(3)°, V = 4389.3(4) Å3, ρcalc = 1.477 g cm−3, μ = 3.087 mm−1
,
1.67 (d, JPH = 13.8 Hz, 27H, Bu), 1.09 (m, 2H, CH2[B]), 0.89
3
t
3
(m, 2H, CH2[P]), 0.69 (t, JHH = 7.3 Hz, 3H, CH3[P]), 0.68
empirical absorption correction (0.458 ≤ T ≤ 0.690), Z = 4, tri-
ˉ
3
(t, JHH = 7.3 Hz, 3H, CH3[B]). 13C{1H} NMR (150 MHz,
clinic, space group P1 (No. 2), λ = 1.54178 Å, T = 223(2) K, ω
[D2]-dichloromethane, 298 K): δ = 187.5 (br m, vCvP), 170.2
and φ scans, 57 825 reflections collected ( h, k, l), [(sin θ)/λ]
= 0.60 Å−1, 15 193 independent (Rint = 0.062) and 12 199
observed reflections [I > 2σ(I)], 1131 refined parameters, R =
0.066, wR2 = 0.187, max. (min.) residual electron density 1.08
(−0.68) e Å−3, hydrogen atoms calculated and refined as riding
atoms.
1
(br , vCB), 153.7 (dm, J = 15.1 Hz, vCvB), 84.6 (d, JPC
=
1
t
71.4 Hz, vCP), 41.9 (d, JPC = 29.6 Hz, Bu), 40.7 (dm, J =
5.1 Hz, vCH2[P]), 38.5 (d, JPC = 7.7 Hz, vCH2[B]), 31.3 (tBu),
2
26.5 (m, CH2[P]), 23.8 (d, 3JPC = 3.1 Hz, CH2[B]), 14.7 (CH3[P]),
13.5 (d, 4JPC = 0.5 Hz, CH3[B]), [C6F5 not listed]. 31P{1H} NMR
(242 MHz, [D2]-dichloromethane, 298 K): δ = 45.6 (m). 11B
{1H} NMR (96 MHz, [D2]-dichloromethane, 298 K): δ = −12.7
(ν1/2 ≈ 20 Hz). 19F NMR (564 MHz, [D2]-dichloromethane,
295 K): δ = −130.8 (m, 2F, o-C6F5), −162.6 (m, 1F, p-C6F5),
−166.7 (m, 2F, m-C6F5), [Δδ19Fm,p = 4.1].
Preparation of 7/8
B(C6F5)3 (256 mg, 0.500 mmol, 1 eq) and tri-tert-butyl-
phosphane (101.1 mg, 0.500 mmol, 1 eq) were dissolved in
n-pentane (10 ml). In a second flask 2,4-hexadiyne (39.1 mg,
0.500 mmol, 1 eq) was dissolved in n-pentane (6 ml). The
obtained FLP-solution was added to the diyne solution and the
obtained reaction mixture was stirred at room temperature for
three days. After that time small crystals could obtained from
that solution. The supernatant was decanted and the light orange
solid (156 mg) was dried in vacuo. After additional five days at
X-ray crystal structure analysis of trans-6
Formula C40H41BF15P, M = 848.51, colourless crystal, 0.30 ×
0.30 × 0.20 mm, a = 12.9072(5), b = 13.7445(4), c = 14.2563(6)
Å, α = 66.140(2), β = 65.660(2), γ = 66.373(2)°, V =
2022.53(13) Å3, ρcalc = 1.393 g cm−3, μ = 1.485 mm−1
,
9140 | Dalton Trans., 2012, 41, 9135–9142
This journal is © The Royal Society of Chemistry 2012

room temperature small crystals were observed in the super-
natant. The solution was again decanted and the solid was dried
in vacuo to yield a light orange solid (112 mg). Both solids were
mixtures of 7 and 8 (1,2- and 1,4-addition product) in a ratio of
4 : 1(31P NMR). The combined crude products (268 mg) were
purified by crystallization (dichloromethane–pentane, −40 °C) to
get a white solid (48%, 194 mg). Crystals suitable for X-ray
crystal structure analyses of both products were obtained by
diffusion method of a dichloromethane–pentane solution at
−40 °C. HRMS calc for C36H33BF15PNa+: 815.2072; found:
815.2097. 7: 1H NMR (500 MHz, [D2]-dichloromethane,
Å, β = 102.704(3)°, V = 3577.4(2) Å3, ρcalc = 1.471 g cm−3
,
μ = 1.638 mm−1, empirical absorption correction (0.791 ≤ T ≤
0.894), Z = 4, monoclinic, space group P21/c (No. 14), λ =
1.54178 Å, T = 223(2) K, ω and φ scans, 25 708 reflections col-
lected ( h, k, l), [(sin θ)/λ] = 0.60 Å−1, 6202 independent
(Rint = 0.042) and 5201 observed reflections [I > 2σ(I)], 489
refined parameters, R = 0.045, wR2 = 0.118, max. (min.) residual
electron density 0.27 (−0.24) e Å−3, hydrogen atoms calculated
and refined as riding atoms.
3
298 K): δ = 2.47 (m, 3H, vCH3), 1.67 (d, JPH = 14.2 Hz, 27H,
Preparation of 9
tBu), 1.52 (d, J = 3.8 Hz, 3H, uCH3). 13C{1H} NMR
(126 MHz, [D2]-dichloromethane, 298 K): δ = 199.4 (br
Compound 8 (20 mg) was dissolved in deuterated bromobenzene
and filled in an NMR tube. The NMR tube was sealed and the
clear reaction solution was heated for 5 h at 80 °C. During
heating small crystals grew at the wall of the NMR tube [yield
>90% (1H NMR)]. A few of these crystals were collected and
used for X-ray crystal structure analysis. The rest of the solution
mixture was dried in vacuo to yield a white solid. M.p. 227 °C.
HMRS: calc for C32H25BF15PNa+: 759.1445; found: 759.1445.
1H NMR (600 MHz, [D2]-dichloromethane, 273 K): δ = 5.86 (d,
1JPH = 431.5 Hz, PH), 2.02 (s, 3H, vCH3), 1.64 (d, J = 4.8 Hz,
1
1
1 : 1 : 1 : 1 q, JCB ≈ 55 Hz , vCB), 105.8 (dm, JPC = 52.1 Hz,
vCP), 96.6 (d, JPC = 9.9 Hz, uCCH3), 83.7 (br d, JPC
≈
3
2
17 Hz, uC), 44.3 (d, JPC = 23.7 Hz, Bu), 33.0 (tBu) 32.3
(m, vCH3), 4.9 (d, J = 7.3 Hz, uCH3), [C6F5 not listed]. 31P{H}
NMR (202 MHz, [D2]-dichloromethane, 298 K): δ = 59.3 (m).
11B{1H} NMR (160 MHz, [D2]-dichloromethane, 298 K): δ =
1
t
−12.2 (ν1/2 ≈ 30 Hz). 19F NMR (470 MHz, [D2]-dichloro-
3
methane, 298 K): δ = −127.1 (o), −134.1 (o′), −162.1 (t, JFF
=
3
t
3
3H, uCH3), 1.57 (d, JPH = 16.9 Hz, 9H, Bu′), 1.32 (d, JPH
=
20.5 Hz, 1F, p), −165.8 (m′), −166.5 (m) (each m, each 1F,
a
C6F5 ), [Δδ19Fm,p = 3.7; 4.4]; −130.2 (o), −132.9 (o′), −163.8
16.9 Hz, 9H, Bu). 13C{1H} NMR (150 MHz, 273 K, [D2]-
t
b
dichloromethane): δ = 97.1 (dm, JPC ≈ 69 Hz, vCP), 96.4 (d,
1
(p), −168.1 (m), −168.8 (m′) (each m, each 1F, C6F5 ),
[Δδ19Fm,p = 4.2; 5.0]; −127.3 (o), −130.9 (o′), −163.5 (p),
3JPC = 8.7 Hz, uCCH3), 77.3 (m, uC), 35.7 (d, 1JPC = 35.3 Hz,
−168.2 (m′), −168.8 (m) (each m, each 1F, C6F5 ), [Δδ19Fm,p
=
c
tBu′), 35.5 (d, JPC = 35.5 Hz, Bu), 28.0 (CH3tBu′), 27.9
(CH3tBu), 24.8 (br, vCH3), 4.1 (d, J = 2.0 Hz, uCH3), n.o.
(vCB), [C6F5 not listed]. 31P NMR (242 MHz, [D2]-dichloro-
1
t
4.6; 5.3]. 8: 1H NMR (500 MHz, [D2]-dichloromethane, 298 K):
δ = 1.97 (d, JPH = 9.6 Hz, 3H, CH3[P]), 1.96 (br, 3H, CH3[B]),
3
1.66 (d, JPH = 13.9 Hz, 27H, Bu). 13C{1H} NMR (150 MHz,
3
t
methane, 273 K): δ = 21.4 (br d, JPH ≈ 431 Hz). 11B{1H}
1
298 K, [D2]-dichloromethane): δ = 187.5 (vCvP)1, 152.9
NMR (192 MHz, [D2]-dichloromethane, 273 K): δ = −13.2 (ν1/2
(vCvB)1, 79.1 (d, JPC ∼ 80 Hz, vCP)1, 41.9 (d, JPC
=
1
1
≈ 30 Hz). 19F NMR (564 MHz, 273 K, [D2]-dichloromethane):
29.8 Hz, Bu), 31.2 (tBu), 26.0 (m, JPC = 8.8 Hz, CH3[P]), 22.1
(m, CH3[B]), n.o. (vCB), [C6F5 not listed.1 from ghmbc exper-
iment]. 31P{1H} NMR (202 MHz, [D2]-dichloromethane,
298 K): δ = 45.8 (m). 11B{1H} NMR (160 MHz, [D2]-dichloro-
methane, 298 K): δ = −12.8 (ν1/2 ≈ 20 Hz). 19F NMR
(470 MHz, [D2]-dichloromethane, 298 K): δ = −131.1 (m, 2F,
t
2
3
δ = −128.4 (o), −131.9 (o′), −162.3 (t, JFF = 20.6 Hz, 1F, p),
−166.8 (m′), −167.4 (m) (each m, each 1F, C6F5 ), [Δδ19Fm,p
=
a
4.5; 5.1]; −130.9 (o), −132.3 (o′), −162.1 (t, 3JFF = 21.1 Hz, 1F,
b
p), −166.0 (m′), −166.2 (m) (each m, each 1F, C6F5 ), [Δδ19Fm,p
3
= 3.9; 4.0]; −130.7 (o), −132.4 (o′), −163.3 (t, JFF = 20.6 Hz,
1F, p), −167.0 (m′), −168.3 (m) (each m, each 1F, C6F5 ),
c
3
[Δδ19Fm,p = 3.8; 5.0].
o-C6F5), −162.7 (t, JFF = 20.4 Hz, 1F, p-C6F5), −166.8 (m, 2F,
m-C6F5) [Δ19Fm,p = 4.1].
X-ray crystal structure analysis of 9
X-ray crystal structure analysis of 7
Formula C32H25BF15P, M = 736.30, colourless crystal, 0.50 ×
0.25 × 0.02 mm, a = 13.6661(5), b = 15.4254(4), c = 15.4744(4)
Formula C36H33BF15P, M = 792.40, colourless crystal, 0.23 ×
0.05 × 0.03 mm, a = 17.5937(4), b = 13.1421(3), c = 15.8360(3)
Å, β = 105.939(3)°, V = 3136.67(16) Å3, ρcalc = 1.559 g cm−3
,
Å, β = 109.675(1)°, V = 3447.80(13) Å3, ρcalc = 1.527 g cm−3
,
μ = 1.459 mm−1, empirical absorption correction (0.463 ≤ T ≤
0.964), Z = 4, monoclinic, space group P21/n (No. 14), λ =
1.54178 Å, T = 223(2) K, ω and φ scans, 24 861 reflections col-
lected ( h, k, l), [(sin θ)/λ] = 0.60 Å−1, 5435 independent
(Rint = 0.050) and 4795 observed reflections, space group
[I > 2σ(I)], 453 refined parameters, R = 0.040, wR2 = 0.107,
μ = 0.188 mm−1, empirical absorption correction (0.958 ≤ T ≤
0.994), Z = 4, monoclinic, space group P21/c (No. 14), λ =
0.71073 Å, T = 223(2) K, ω and φ scans, 10 075 reflections col-
lected ( h, k, l), [(sin θ)/λ] = 0.59 Å−1, 5964 independent
(Rint = 0.043) and 4637 observed reflections [I > 2σ(I)], 460
refined parameters, R = 0.077, wR2 = 0.177, max. (min.) residual
electron density 0.37 (−0.26) e Å−3, hydrogen atoms calculated
and refined as riding atoms.
max. (min.) residual electron density 0.22 (−0.27) e Å−3
hydrogen atoms calculated and refined as riding atoms.
,
X-ray crystal structure analysis of 8
Acknowledgements
Formula C36H33BF15P, M = 792.40, colourless crystal, 0.15 ×
0.12 × 0.07 mm, a = 14.0349(5), b = 13.4667(4), c = 19.4024(6)
Financial support from the Deutsche Forschungsgemeinschaft is
gratefully acknowledged.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 9135–9142 | 9141

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