Phosphirenium-borate zwitterion: Formation in the 1,1-carboboration reaction of phosphinylalkynes

Article abstract of DOI:10.1039/c1cc13008k

Reaction of the acetylene Mes₂P-C≡C-Ar with B(C ₆F₅)₃ at rt gives a zwitterionic phosphirenium product, which reacts further at >100 °C to complete the 1,1-carboboration reaction.

Full text of DOI:10.1039/c1cc13008k

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COMMUNICATION
Phosphirenium-borate zwitterion: formation in the 1,1-carboboration
reaction of phosphinylalkyneswz
¨
Olga Ekkert, Gerald Kehr, Roland Frohlichy and Gerhard Erker*
Received 23rd May 2011, Accepted 4th August 2011
DOI: 10.1039/c1cc13008k
R
Reaction of the acetylene Mes2P–C C–Ar with B(C6F5)3 at rt
gives a zwitterionic phosphirenium product, which reacts further
prepared by treating p-tolylacetylene with n-butyl lithium
followed by Mes2PCl. The product (3c) was isolated in 40%
yield. It features a 31P NMR resonance at d ꢀ55.7 and
13C NMR signals of the central acetylene unit at d 107.6
(2JPC = 8.4 Hz) and d 87.8 (1JPC = 6.2 Hz). The X-ray crystal
structure analysis of 3c shows the presence of a linear central
framework (P1–C1: 1.766(2) A, C1–C2: 1.204(3) A, C2–C3:
1.433(3) A, angles P1–C1–C2: 165.0(2)1, C1–C2–C3: 176.1(2)1;
see Fig. 1).
at 4100 8C to complete the 1,1-carboboration reaction.
1,1-Carboboration1 of alkynes is increasingly becoming
synthetically useful. It was recently shown that using strongly
electrophilic RB(C6F5)2 type boranes has allowed carrying out
facile 1,1-carboboration reactions of simple 1-alkynes.2–4 In
these cases the boron bonded alkyl groups R migrate prefer-
entially (Chart 1). The resulting alkenylboranes (1) are interesting
strong Lewis acids in themselves and they have served as
useful substrates in palladium catalyzed cross-coupling
reactions. Thus, such 1,1-carboboration reactions of terminal
acetylenes are increasingly becoming useful conceptual
alternatives to the ubiquitous 1,2-hydroboration of alkynes.3
Recently it was shown that the 1,1-carboboration reaction can
even be used for carbon–carbon s-bond activation. We found
that the internal acetylene 4-octyne reacted with B(C6F5)3 at
elevated temperatures to cleanly give the 1,1-carboboration
product (2), formed by cleavage of a strong C(sp3)–C(sp)
s-bond and 1,2-migration of the n-propyl substituent. Several
additional related examples were reported.5,6 Carbon–phosphorus
s-bonds can be cleaved analogously. We recently described the
1,1-carboboration reaction of phosphinylalkynes7 at elevated
temperature as a synthetic alternative to the previously described
nucleophilic routes8 to 1,2-phosphinyl/boryl-substituted alkenes
(4), compounds that are of considerable interest e.g. due to
their photophysical properties.9 We have now found that the
reaction of B(C6F5)3 with a phosphinylalkyne proceeds
rapidly even at low temperature, but to a different outcome.
A first example of an interesting novel class of compounds has
become readily available in this way.
We had previously shown that dimesitylphosphinylalkynes
undergo 1,1-carboboration with B(C6F5)3 at temperatures
4100 1C. Consequently, the Mes2P-substituted acetylene
(3c) reacted cleanly with B(C6F5)3 at 105 1C for 6 h to give
Chart 1
Starting material for our study was the 1-dimesityl-
phosphinyl-2-p-tolylacetylene 3c.10 This compound was
Organisch-Chemisches Institut, Universitat Munster, Corrensstraße
¨
¨
40, D-48149 Munster, Germany. E-mail: erker@uni-munster.de;
¨
¨
Fax: +49 251-83-36503
w Dedicated to Professor Heinz Berke on the occasion of his 65th
birthday.
z Electronic supplementary information (ESI) available: Additional
experimental and spectroscopic data. CCDC 826907–826909. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1cc13008k
y X-Ray structure analyses.
Fig. 1 A view of the molecular geometry of compound 3c z.
c
10482 Chem. Commun., 2011, 47, 10482–10484
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Scheme 1
Fig. 3 Molecular structure of the zwitterionic compound 5 z.
extra-stabilisation of the unsaturated three-membered ring
system e.g. by ‘‘s*-conjugation’’ as it had been discussed for
some 1H-phosphirenium cations in the literature.11 The bond
angles inside the central phosphirenium ring of compound 5
amount to 66.6(3)1 [65.8(3)1] (C2–C1–P1), 69.6(3)1 [70.1(3)1]
(C1–C2–P1) and 43.8(2)1 [44.0(2)1] (C1–P1–C2), respectively. The
bulky (C6F5)3B-substituent is attached at carbon atom C1 of
the central core of compound 5 (C1–B: 1.647(7) A [1.629(7) A]).
The p-tolyl substituent at the ring carbon atom C2 (C2–C3:
1.464(7) A [1.477(6) A]) is rotated markedly out of the plane of
the unsaturated phosphirenium system (Y C1–C2–C3–C4:
123.5(7)1 [35.2(9)1]).
Fig. 2 A view of the molecular structure of compound 4c z.
Compound 5 features a 11B NMR signal at d ꢀ16.5 and
19F NMR signals of the three symmetry-equivalent adjacent
C6F5 groups in a typical borate range [d ꢀ130.3(o), ꢀ159.5(p),
ꢀ164.5(m)]. The 31P NMR signal at d ꢀ137.8 corresponds well
with the phosphirenium character12 of the central unit of the
zwitterion 5. The adjacent phosphirenium carbon atoms show
13C NMR resonances at d 151.6 ([B]CQ) and d 136.7
(QC[tolyl]), respectively.
Upon heating to 105 1C compound 5 rearranges to the
1,1-carboboration product 4c. A crossover experiment revealed
that the formation of the phosphirenium borate zwitterion 5 is
at least partly reversible. Heating of 5 for prolonged time at
70 1C in the presence of Ph2P–CRC–Ph (3a) (5 : 3a E 1 : 1.3)
gave the intramolecular 1,1-carboboration product 4c admixed
with a small amount of the crossover product 4a and the free
Mes2P–CRC–p-tol acetylene derivative (3c). In a typical
experiment these three products and remaining 3a were
obtained in a ratio of 4c, 4a, 3c and 3a of ca. 5.8 : 1 : 1 : 3.4.
These results show that treatment of a phosphinylacetylene
with B(C6F5)3 results in the migration of the diorganyl-
phosphinyl group at ambient conditions. The reaction proceeds
to a phosphirenium type product that provides sufficient
internal stabilisation for the system to serve as a resting stage
of this cooperative rearrangement process. It shows that the
modern variants of the 1,1-carboboration reactions have
synthetic power making interesting organic main group element
compounds readily available. The observation that the title
the product 4c that was isolated in 65% yield. Compound 4c
features a 11B NMR resonance at d 0 and a 31P NMR signal at
+14.6. It shows sets of each three 19F NMR signals of the
C6F5 substituent at carbon [d ꢀ136.7 (o), ꢀ156.3 (p), ꢀ163.7
(m)] and the pair of C6F5 substituents at boron [d ꢀ127.3 (o),
ꢀ158.0 (p), ꢀ165.2 (m)]. The X-ray crystal structure analysis
of compound 4c shows a central four-membered heterocyclic
framework [C1–C2: 1.353(2) A, B–C1: 1.634(2) A, P1–C2:
1.827(2) A, angles C2–C1–B: 107.9(1)1, C1–C2–P1: 97.0(1)1].
The Bꢁ ꢁ ꢁP bond is long at 2.094(2) A (see Scheme 1 and
Fig. 2).
At room temperature the reaction of the Mes2P-substituted
alkyne 3c with the boron Lewis acid B(C6F5)3 stops at the
intermediate 5. Stirring of 3c with B(C6F5)3 in pentane at
ambient temperature for 30 min gave a white precipitate of the
new product 5 that was isolated in 76% yield.8 Compound 5
was characterized spectroscopically, by C,H elemental analysis
and by X-ray diffraction (see Scheme 1 and Fig. 3).
It revealed the formation of a central three-membered
heterocyclic phosphirenium ring system, with pertinent bonding
parameters of the pair of crystallographically independent
molecules of C1–C2: 1.325(6)
A [1.330(6) A], C1–P1:
1.793(5) A [1.799(5) A] and C2–P1: 1.755(5) A [1.746(4) A].
The latter bond is markedly shorter than the C(sp2)–P bonds
to the mesityl substituents [P1–C11: 1.802(5) A [1.809(4) A],
P1–C21: 1.809(5) A [1.807(5) A], which might indicate some
c
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compounds of this study, the phosphirenium-borate zwitter-
ions, are easily obtained even at room temperature lets us hope
that a specific chemistry of these neutral derivatives of the
phosphirenium cations will be developed.
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Notes and references
%
z Crystal data: 3c: C27H29P, M = 384.47, triclinic, P1, (No. 2),
a = 10.1751(5), b = 10.6783(4), c = 11.6493(7) A, a = 96.462(2)1,
b = 107.570(4)1, g = 109.165(4)1, V = 1107.87(10) A3, Dc = 1.153 g cmꢀ3
,
m = 1.142 mmꢀ1, F(000) = 412, Z = 2, l = 1.54178 A, T = 223(2) K,
15 949 reflections collected (ꢂh, ꢂk, ꢂl), [(sin y)/l] = 0.60 Aꢀ1, 3784
independent (Rint = 0.048), and 3388 observed reflections [I Z 2s(I)], 260
refined parameters, R = 0.044, wR2 = 0.123, GoF = 1.039. CCDC
826907.
%
4c: C45H29BF15P, M = 896.46, triclinic, P1, (No. 2), a = 12.4342(5),
b = 13.4995(6), c = 14.2303(6) A, a = 82.805(2)1, b = 66.678(2)1, g =
64.037(2)1, V = 1968.75(14) A3, Dc = 1.512 g cmꢀ3, m = 1.570 mmꢀ1
,
F(000) = 908, Z = 2, l = 1.54178 A, T = 223(2) K, 28 030 reflections
collected (ꢂh, ꢂk, ꢂl), [(sin y)/l] = 0.60 Aꢀ1, 6739 independent
(Rint = 0.038), and 6261 observed reflections [I Z 2s(I)], 566 refined
parameters, R = 0.037, wR2 = 0.109, GoF = 1.034. CCDC 826908.
5: C48H36BClF15P, M = 975.00, orthorhombic, Pca21 (No. 29),
a = 21.7497(6), b = 22.5390(4), c = 17.9477(6) A, V = 8798.3(4) A3,
Dc = 1.472 g cmꢀ3, m = 1.998 mmꢀ1, F(000) = 3968, Z = 8, l =
1.54178 A, T = 223(2) K, 47 800 reflections collected (ꢂh, ꢂk, ꢂl),
[(sin y)/l] = 0.60 Aꢀ1, 14 469 independent (Rint = 0.074), and 12 099
observed reflections [I Z 2s(I)], 1206 refined parameters, R = 0.059,
wR2 = 0.157, GoF = 1.033. CCDC 826909.
8 Preparation of compound 5: B(C6F5)3 (0.400 g, 0.780 mmol) and 3c
(0.300 g, 0.780 mmol) were suspended in pentane (15 ml) and stirred
for 30 minutes at room temperature. After filtration 5 (0.534 g, 0.596
mmol, 76%) was obtained as a white solid. Crystals suitable for X-ray
crystal structure analysis were grown by slow diffusion of pentane into
a solution of 5 in dichloromethane at ꢀ36 1C. [Elemental analysis,
found: C, 60.37; H, 3.51%; calcd. for C45H29BF15P: C, 60.29; H,
3.26%; 13C NMR d = 151.6 (br, BC=), 136.7 (TolC=); 19F NMR
d = ꢀ130.3 (o), ꢀ159.5 (p), ꢀ164.5 (m); 11B NMR d = ꢀ16.5;
31P NMR d = ꢀ137.8.]
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10484 Chem. Commun., 2011, 47, 10482–10484
This journal is The Royal Society of Chemistry 2011