The 1,1-carboboration of bis(alkynyl)phosphanes as a route to phosphole compounds

Article abstract of DOI:10.1002/anie.201107398

Neat and tidy: B(C₆F₅)₃ efficiently converts a series of bis(alkynyl)phosphanes into highly substituted 3-borylphospholes through a twofold 1,1-carboboration reaction sequence. The boron substituted phospholes were also used as substrates in Suzuki-Miyaura type cross-coupling reactions (see scheme).

Full text of DOI:10.1002/anie.201107398

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Angewandte
Communications
DOI: 10.1002/anie.201107398
Phosphorus Heterocycles
The 1,1-Carboboration of Bis(alkynyl)phosphanes as a Route to
Phosphole Compounds**
Juri Mçbus, Quentin Bonnin, Kirika Ueda, Roland Frçhlich, Kenichiro Itami, Gerald Kehr, and
Gerhard Erker*
Dedicated to Professor Dieter Hoppe on the occasion of his 70th birthday
Phospholes have been receiving increasing interest as a class
of compounds. They possess a planar five-membered hetero-
cyclic framework that has the heavy Group 15 element
incorporated in a markedly nonplanar coordination geometry.
Consequently, in contrast to the other first-row five-mem-
bered heteroarenes, the phospholes are only weakly aro-
matic.[1] There are far fewer established synthetic pathways to
phospholes compared to their first-row-element relatives. The
classical elimination pathway described by Mathey et al.[2] has
been augmented by some Group 4 metallocene alkyne
coupling/phosphorylation pathways and related syntheses.[3]
Scheme 1. Formation of silols by 1,1-carboboration.
Nevertheless, the phosphole framework is increasingly being
used, for example, in materials science,[4] especially in
conjunction with boron-based acceptor substituents in con-
jugation with the P donor.[5] Also rigid conjugated zwitter-
ionic phosphonium/borate frameworks have recently been
studied and found considerable interest.[6] We have now found
a new very simple synthesis that directly makes 3-boryl
substituted phospholes available from suitable bis-
(alkynyl)phosphanes in a one-pot reaction. Several represen-
tative examples are described and characterized herein and
their synthetic utilization explored.
reagents LiC C-R (R = SiMe3 (a), n-C3H7 (b), Ph (c)). The
ꢀ
ꢀ
mesitylP(C C-SiMe3)2 starting material 6a was prepared
analogously. The bis(alkynyl)phosphane 5a was treated with
one equivalent of B(C6F5)3 in toluene solution at 708C (2 h) to
give the phosphole 7a in 79% yield (Scheme 2). Compound
We have recently shown that bis(alkynyl)silanes
1
undergo a special variant of the “Wrackmeyer 1,1-carbobo-
ration reaction” sequence[7] under very mild reaction con-
ditions upon treatment with the strong Lewis acid tris(penta-
fluorophenyl)borane (2) to give the respective borylsiloles
3.[8,9] The reaction probably involves a sequence as depicted in
Scheme 1, involving a typical series of 1,1-carboboration
reactions of activated alkynes.[10]
The arylbis(alkynyl)phosphanes 5a–c were prepared by
treatment of the (tipp)PX2 compounds 4 (X = Cl, Br; tipp =
2,4,6-triisopropylphenyl) with the respective alkynyl lithium
Scheme 2. Preparation of phospholes by 1,1-carboboration.
7a shows the typical broad 11B NMR resonance of a trivalent
R-B(C6F5)2 boron Lewis acid at d = 63 ppm, a 31P NMR signal
at d = 59.1 ppm and a pair of 29Si NMR resonances at d = À7.8
and À8.2 ppm. The X-ray crystal-structure analysis of 7a
(Figure 1) shows the planar central five-membered ring.
[*] J. Mçbus, Q. Bonnin, K. Ueda, Dr. R. Frçhlich, Dr. G. Kehr, G. Erker
Organisch-Chemisches Institut der Universitꢀt Mꢁnster
Corrensstrasse 40, 48149 Mꢁnster (Germany)
À
À
Typically, the phosphorus–carbon distances P1 C1 and P1
C4 inside the weakly aromatic heterocycle are markedly
E-mail: erker@uni-muenster.de
À
shorter than the exocyclic P C(aryl) bond (Table 1), but the
K. Ueda, Prof. K. Itami
Department of Chemistry, Graduate School of Science
Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602 (Japan)
coordination geometry at phosphorus is markedly nonplanar.
The coordination plane of the trigonal-planar boron moiety is
rotated markedly from the central ring plane (dihedral angle
C1-C2-B1-C51: À52.0(6)8).
[**] Financial support from the Deutsche Forschungsgemeinschaft is
gratefully acknowledged. J.M. thanks the NRW Research School
(University Mꢁnster) for a stipend. K.U. thanks JSPS and the
International Research Training Group Mꢁnster/Nagoya for fellow-
ship and support.
The reaction of 5b with B(C6F5)3 proceeded analogously
at 808C (1 h) to give 7b in near to quantitative yield (97%,
NMR: d = 62 (11B), d = 16.5 ppm (31P))[11a] and we prepared
the phosphole 7c similarly from 5c and B(C6F5)3. Both
phospholes were also characterized by X-ray crystal structure
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201107398.
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Chemie
Figure 1. A view of the crystal structure of the phosphole 7a. See
Table 1 for selected bond lengths and angles.
Table 1: Selected structural data of the phospholes 7 and 10.[a]
Parameter
7a
7b
10c
Figure 2. The reaction course of the 5b/B(C6F5)3 system monitored by
P1–C1
P1–C4
P1–C21
C1–C2
C3–C4
C2–C3
C2–B1
Angles
ꢀ at P
ꢀ at B
1.772(4)
1.769(4)
1.821(4)
1.391(6)
1.380(5)
1.462(6)
1.550(6)
1.774(2)
1.779(3)
1.823(2)
1.387(3)
1.362(3)
1.464(3)
1.541(4)
1.788(2)
1.788(2)
1.836(2)
1.369(2)
1.369(3)
1.457(2)
–
31P NMR spectroscopy ([D6]benzene, 81 MHz, 318 K to 343 K).
328.1
359.9
321.5
360.0
314.7
–
[a] Bond lengths [ꢂ], angles [8].
Scheme 3. Formation of phosphirenium borate zwitterion 8b.
analysis (for details see Table 1 and the Supporting Informa-
tion).
In each case the reaction course was monitored by
31P NMR spectroscopy. The reaction of 5b with B(C6F5)3 is
a
representative example. After 1 min at 318 K in
[D6]benzene solution the 5b/B(C6F5)3 pair had converted
quantitatively into a single new product (8b) characterized by
a typical 31P NMR signal at d = À166.2 ppm.[12]
Upon standing of the product solution at 318 K, two
products were formed from 8b, namely the phosphole 7b and
a new product (9b, 31P NMR: d = À35.5 ppm, t, JPF ꢁ 41 Hz).
Both the secondary products 7b and 9b are initially formed at
about the same rate, but after some time 9b is also consumed
to eventually yield the stable phosphole 7b (Figure 2). The
reaction systems 5a/B(C6F5)3 and 5c/B(C6F5)3 show a similar
behavior.
We were able to isolate compound 8b by performing the
reaction of 5b with B(C6F5)3 under strict kinetic control
(Scheme 3).[11b] The 5b/B(C6F5)3 reaction mixture was stirred
in pentane solution for 30 min at room temperature and the
product crystallized at À358C. The product was isolated in
78% yield (31P NMR: d = À166.2 ppm, 11B NMR: d =
À17.7 ppm) and identified as the phosphirenium borate
zwitterion 8b. The X-ray crystal structure analysis
(Figure 3) revealed the formation of a central three-mem-
Figure 3. A view of the crystal structure of the zwitterionic phosphi-
renium borate 8b. Selected bond lengths [ꢂ] and angles [8]: P–C1
1.771(2), P–C2 1.734(2), P1–C11 1.726(2), P1–C21 1.793(2), C1–C2
1.323(3), C1–B1 1.623(3); C1-P1-C2 44.3(1), C11-P1-C21 110.4(1).
and the B(C6F5)3 substituent is attached at carbon atom C1 of
the central three-membered ring.
The follow-up product 9b was not isolated directly but it
was identified by comparison with an analogous product in a
closely related mesityl–phosphorus system. Treatment of
À
bered phosphorus containing heterocycle. It contains P C1
À
ꢀ
and P C2 s-bonds. The phosphorus atom is tetracoordinate
mesitylP(C C-SiMe3)2 (6a) with B(C6F5)3 for 5 min in
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Angewandte
Communications
Scheme 4. Formation of product 9b.
toluene at room temperature and crystallization (from n-
pentane) gave 9d in 87% yield (Scheme 4). The X-ray crystal-
structure analysis revealed that this product was formed by
1,1-carboboration at one of the P-bound alkynyl groups
(Figure 4). The resulting phosphinoalkene moiety is Z-
configured and the other alkynyl unit at the phosphorus
atom has remained intact.
Scheme 5. Proposed reaction mechanism of the 1,1-carboboration
reaction sequence leading to the phospholes 7.
typical Suzuki–Miyaura conditions (10 mol% [Pd(PPh3)4]
catalyst, NaOH base) resulted in the formation of the cross-
coupling product 10a (Scheme 6). The analogous treatment
Scheme 6. Cross-coupling reactions of borylphospholes.
of the phospholes 7b and 7c gave the products 10b and 10c,
respectively. Both the compounds 10a and 10c were charac-
terized by X-ray diffraction (for details see the Supporting
Information). The phosphole derivative 10c (Figure 5) shows
a fivefold substituted central phosphole framework with the
phenyl groups at the carbon atoms C1 and C4 originating
from the alkynyl moieties of the starting material (5c), the
pentafluorophenyl group at C3 having been introduced by the
B(C6F5)3 reagent during the 1,1-carboboration reaction, and
the phenyl group at carbon atom C2 having been attached in
the final palladium-catalyzed cross-coupling reaction (for
details see Figure 4 and Table 1).
Figure 4. A view of the crystal structure of compound 9d. Selected
bond lengths [ꢂ] and angles [8]: P1–C1 1.838(3), P1–C11 1.762(3), C1–
C2 1.364(4), C11–C12 1.206(5), C2–B1 1.545(4), C1–Si3 1.931(3); P1-
C1-C2 122.7(2), C1-C2-B1 128.6(3); sum of bond angles at P1: 313.8.
Product 9d shows a typical 11B NMR resonance at d =
59 ppm. It shows a set of 19F NMR features of the B(C6F5)2
functional group at d = À126.3 (o), À146.1 (p), À161.2 ppm
(m) and a separate set of signals of the C6F5 group at carbon
C2 (d = À136.5 (o), À155.6 (p), À162.6 ppm (m)). Most
significantly, compound 9d exhibits a 31P{1H} NMR signal at
d = À33.5 ppm[13] (t, JPF ꢁ 34 Hz), that is characteristic for this
class of compounds. Heating of 9d in [D6]benzene (908C,
1.5 h) gave the corresponding phosphole 7d (for details see
the Supporting Information).
These observations indicated to us that phosphanyl groups
may serve as “activating” migrating groups in 1,1-carbobora-
tion reactions (Scheme 5). This may be due to stabilization of
the phosphirenium subunit in the zwitterionic intermediates
(8) by their alleged “s*-aromaticity” features as has been
proposed for their analogous cationic parent compounds.[14]
The phospholes (7) that are formed so readily from the
arylbis(alkynyl)phosphanes 5 and B(C6F5)3 are not only
interesting P/B systems, but they are also reagents for
metal-catalyzed cross-coupling reactions.[9] Treatment of the
borylphosphole 7a with iodobenzene at 658C in THF under
Figure 5. Molecular structure of the phosphole 10c. See Table 1 for
selected bond lengths and angles.
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[7] B. Wrackmeyer, Coord. Chem. Rev. 1995, 145, 125 – 156.
Our study shows that our new variant of the 1,1-
carboboration sequence[9] can be applied to the synthesis of
new phosphole derivatives. This result indicates that the 1,1-
carboboration reaction may have an even greater potential
for preparing useful olefinic and heterocyclic borane deriv-
atives than previously thought. We are actively pursuing the
extension of this development.
[8] a) J. Ugolotti, G. Kehr, R. Frçhlich, G. Erker, Chem. Commun.
2010, 46, 3016 – 3018; b) G. Dierker, J. Ugolotti, G. Kehr, R.
Frçhlich, G. Erker, Adv. Synth. Catal. 2009, 351, 1080 – 1088.
[9] a) C. Chen, T. Voss, R. Frçhlich, G. Kehr, G. Erker, Org. Lett.
2011, 13, 62 – 65; b) C. Chen, G. Kehr, R. Frçhlich, G. Erker,
J. Am. Chem. Soc. 2010, 132, 13594 – 13595; c) C. Chen, R.
Frçhlich, G. Kehr, G. Erker, Chem. Commun. 2010, 46, 3580 –
3582; d) C. Chen, F. Eweiner, B. Wibbeling, R. Frçhlich, S.
Senda, Y. Ohki, K. Tatsumi, S. Grimme, G. Kehr, G. Erker,
Chem. Asian J. 2010, 5, 2199 – 2208.
Received: October 20, 2011
Published online: January 13, 2012
[10] B. Wrackmeyer, Heteroat. Chem. 2006, 17, 188 – 208.
[11] a) Preparation of 7b: Compound 6b (0.52 mmol, 1 equiv) and
B(C6F5)3 (0.52 mmol, 1 equiv) were mixed in toluene (25 mL)
and the solution heated to 808C for 1 h. Evaporation of the
volatiles, redissolving in pentane (10 mL), removal of solids by
filtration, and removal of solvents in vacuo yielded 7b
(0.50 mmol, 97%) as orange oil which crystallized when left
standing. Crystal data of C43H37BF15P, Mr = 880.51, monoclinic,
P21/c (No. 14), a = 13.4531(3), b = 32.5343(7), c = 9.3758(2) ꢂ,
Keywords: boranes · carboboration · cross-coupling ·
phospholes
.
[1] a) D. B. Chesnut, L. D. Quin, Heteroat. Chem. 2007, 18, 754 –
758; b) E. Mattmann, F. Mathey, A. Sevin, G. Frison, J. Org.
Chem. 2002, 67, 1208 – 1213; c) D. Delaere, A. Dransfeld, M. T.
Nguyen, L. G. Vanquickenborne, J. Org. Chem. 2000, 65, 2631 –
2636.
b = 90.246(1)8, V= 4103.63(15) ꢂ3, 1calcd = 1.425 gcmÀ3
, m =
0.166 mmÀ1, F(000) = 1800, Z = 4, l = 0.71073 ꢂ, T= 223(2) K,
33333 reflections collected (Æ h, Æ k, Æ l), [(sinq)/l] = 0.66 ꢂÀ1
,
[2] a) F. Mathey, Chem. Rev. 1988, 88, 429 – 453; See also: b) A.
Marinetti, F. Mathey, Tetrahedron Lett. 1987, 28, 5021 – 5024.
[3] a) Y. Matano, M. Nakashima, H. Imahori, Angew. Chem. 2009,
121, 4062 – 4065; Angew. Chem. Int. Ed. 2009, 48, 4002 – 4005;
b) Y. Matano, M. Nakashima, A. Saito, H. Imahori, Org. Lett.
2009, 11, 3338 – 3341; c) Y. Matano, T. Miyajima, H. Imahori, Y.
Kimura, J. Org. Chem. 2007, 72, 6200 – 6205; d) Y. Matano, T.
Miyajima, T. Nakabuchi, Y. Matsutani, H. Imahori, J. Org.
Chem. 2006, 71, 5792 – 5795; e) P. J. Fagan, W. A. Nugent, J. C.
Calabrese, J. Am. Chem. Soc. 1994, 116, 1880 – 1889; f) P. J.
Fagan, W. A. Nugent, Org. Synth. 1992, 70, 272 – 275; g) P. J.
Fagan, W. A. Nugent, J. Am. Chem. Soc. 1988, 110, 2310 – 2312.
[4] a) Y. Matano, H. Imahori, Org. Biomol. Chem. 2009, 7, 1258 –
1271; b) Y. Matano, A. Saito, T. Fukushima, Y. Takudome, F.
Suzuki, D. Sakamaki, H. Kaji, A. Ito, K. Tanaka, H. Imahori,
Angew. Chem. 2011, 123, 8166 – 8170; Angew. Chem. Int. Ed.
2011, 50, 8016 – 8020; c) A. Fukazawa, Y. Ichihashi, S. Yamagu-
chi, New J. Chem. 2010, 34, 1537 – 1540, and references cited
therein.
9543 independent (Rint = 0.036), and 6764 observed reflections
[I ꢂ 2s(I)], 557 refined parameters, R = 0.062, wR2 = 0.158,
GOF = 1.054. b) Preparation of 8b: Compound 6b (1.2 mmol,
1 equiv) and B(C6F5)3 (1.2 mmol, 1 equiv) were mixed in
pentane (8 mL), the solution stirred for 30 min at room temper-
ature and precipitated at À358C. Removal of the supernatant
solution, washing of the solid with cold pentane (5 mL) and
drying in vacuo gave 8b (1.0 mmol, 78%) as pale yellow solid.
ꢁ
Crystal data for C43H37BF15P, Mr = 880.51, triclinic, P1 (No. 2),
a = 10.7736(7), b = 11.1932(5), c = 19.2748(4) ꢂ, a = 74.564(2),
b = 80.781(4), g = 69.127(4)8, V= 2087.96(17) ꢂ3, 1calcd
=
1.401 gcmÀ3
,
m = 1.464 mmÀ1
,
F(000) = 900, Z = 2, l =
1.54178 ꢂ, T= 223(2) K, 25680 reflections collected (Æ h, Æ k,
Æ l), [(sinq)/l] = 0.60 ꢂÀ1, 7119 independent (Rint = 0.044), and
6360 observed reflections [I ꢂ 2s(I)], 549 refined parameters,
R = 0.044, wR2 = 0.121, GOF = 1.021.
[12] a) O. Ekkert, G. Kehr, R. Frçhlich, G. Erker, Chem. Commun.
2011, 47, 10482 – 10484; See also: b) B. Wrackmeyer, G. Kehr, S.
Willbold, J. Organomet. Chem. 1999, 590, 93 – 103.
[13] B. Wrackmeyer, S. Kundler, A. Ariza-Castolo, Phosphorus
Sulfur Silicon Relat. Elem. 1994, 91, 229 – 236.
[5] a) O. Ekkert, G. Kehr, R. Frçhlich, G. Erker, J. Am. Chem. Soc.
2011, 133, 4610 – 4616; b) J. Grobe, K. Lꢀtke-Brochtrup, B.
Krebs, M. Lꢁge, H.-H. Niemeyer, E.-U. Wꢀrthwein, Z. Natur-
forsch. B 2006, 61, 882 – 895; c) Z. Yuan, N. J. Taylor, Y. Sun,
T. B. Marder, I. D. Williams, L.-T. Cheng, J. Organomet. Chem.
1993, 449, 27 – 37.
[14] A. Gçller, H. Heydt, T. Clark, J. Org. Chem. 1996, 61, 5840 –
5846.
[6] A. Fukazawa, E. Yamaguchi, E. Ito, H. Yamada, J. Wang, S. Irle,
S. Yamaguchi, Organometallics 2011, 30, 3870 – 3879.
Angew. Chem. Int. Ed. 2012, 51, 1954 –1957
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1957