Carbaborane-based alkynylphosphanes and phospholes

Article abstract of DOI:10.1039/c4cc08449g

The reaction of 1,2-bis(N,N-dimethylaminochloro-phosphanyl)-1,2-dicarba-closo-dodecaborane(12) and lithiated phenylacetylene gives a five-membered dihydrophosphole derivative with an exocyclic phosphanyl group. This unexpected reaction opens new possibilities for the synthesis of carbaborane-containing phosphorus heterocycles. P,P′-alkynylated 1,2-bis(phosphanyl)-1,2-dicarba-closo-dodecaborane(12)s are obtained from alkynylchlorophosphanes and dilithiated carbaborane. This new class of alkynes can be used for future applications in cyclizations and polymerizations. This journal is

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Carbaborane-based alkynylphosphanes and
phospholes†
Cite this:DOI: 10.1039/c4cc08449g
Anika Kreienbrink,a Menyhart B. Sarosi,ab Robert Kuhnert,a Peter Wonneberger,a
´
´
Anna I. Arkhypchuk,c Peter Lonnecke,a Sascha Ottc and Evamarie Hey-Hawkins*a
Received 26th October 2014,
Accepted 11th November 2014
¨
DOI: 10.1039/c4cc08449g
www.rsc.org/chemcomm
The reaction of 1,2-bis(N,N-dimethylaminochloro-phosphanyl)-1,2- alkynylphosphanes with ortho-carbaborane would be an inter-
dicarba-closo-dodecaborane(12) and lithiated phenylacetylene gives esting addition to this field, as such structures would merge the
a five-membered dihydrophosphole derivative with an exocyclic electron-withdrawing and -delocalizing properties of an ortho-
phosphanyl group. This unexpected reaction opens new possibilities carbaborane6 with the electronic properties and reactivity of the
for the synthesis of carbaborane-containing phosphorus hetero- alkynylphosphane moiety. Such combinations of carbaboranes
cycles. P,P0-alkynylated 1,2-bis(phosphanyl)-1,2-dicarba-closo- with alkynylphosphanes would make fascinating building
dodecaborane(12)s are obtained from alkynylchlorophosphanes blocks for new polymeric or heterocyclic systems.
and dilithiated carbaborane. This new class of alkynes can be used
for future applications in cyclizations and polymerizations.
There are several synthetic approaches for alkynylphosphanes,
the most widely applied of which is the nucleophilic attack of
an acetylide at halophosphanes.7 Therefore, in a first attempt to
Alkynylphosphanes [4P–CRC–] have attracted considerable obtain P,P0-alkynylated 1,2-bis(phosphanyl)-1,2-dicarba-closo-
attention, not only due to their unique electronic properties dodecaborane(12)s, 1,2-bis(N,N-dimethylaminochlorophosphanyl)-
that arise from interactions of phosphorus-based orbitals with 1,2-dicarba-closo-dodecacaborane (1)8 was treated with two
the p system of the alkyne unit, but also because of their equivalents of lithiated phenylacetylene (Scheme 1).9
interesting spectroscopic and thermodynamic properties.1
The 31P NMR spectrum of the reaction mixture showed the
Furthermore, owing to the well-established coupling chemistry formation of the proposed product (6a, see Scheme 3) as well as
of the acetylene moiety, they are appealing building blocks for many other products. Attempts to isolate 6a failed; however,
more elaborate molecular architectures such as phospha- yellow crystals of the highly substituted five-membered hetero-
pericyclynes and oligomers.2 In addition, the acetylenes can cycle 2 were isolated in 20% yield (Scheme 1). Compound 2 is a
be engaged in subsequent chemistry, and allow, for example, highly functionalized dihydrophosphole to which a carbaborane
the formation of a variety of hetero- and carbocycles, such as unit is fused via two carbon centers, with a dimethylamino
phospholes3a and diphosphinidenecyclobutenes.3b,c
The combination of acetylenes with electron-deficient moieties phosphanyl substituent.
group at the phosphole P atom as well as an exocyclic alkynyl-
offers the possibility to prepare monodisperse oligomers or poly-
The 31P{1H} NMR spectrum of 2 (in C6D6) exhibits two
mers with high electron-acceptor capacities. Acetylenic scaffolds doublets at 35.3 (P2, see Fig. 1) and 87.6 ppm (P1, 2JPP = 20.7 Hz).
with fullerene units4 and cyanoethynylethenes5 are probably the The exocyclic alkynyl group is observed in the 13C{1H} NMR
best studied compounds in this context. The combination of spectrum as two doublets at 78.1 (1JCP = 90.0 Hz) and 108.3 ppm
(3JCP = 4.2 Hz), and the signals for C3 and C4 (Fig. 1) of the
a
¨
¨
Institut fur Anorganische Chemie, Universitat Leipzig, Johannisallee 29,
D-04103 Leipzig, Germany. E-mail: hey@uni-leipzig.de; Fax: (+49)-341-9739319
b Faculty of Chemistry and Chemical Engineering, Babes--Bolyai University,
´
Arany Janos 11, RO-400028, Cluj-Napoca, Romania.
E-mail: msarosi@chem.ubbcluj.ro; Fax: (+40)264-590818
c
¨
Department of Chemistry, Ångstrom Laboratory, Uppsala University, Box 523,
75120 Uppsala, Sweden. E-mail: sascha.ott@kemi.uu.se; Fax: (+46)-018-4717340
† Electronic supplementary information (ESI) available: All experimental proce-
dures and data, details of computational studies. CCDC 976541. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4cc08449g
Scheme 1 Synthesis of carbaborane-based alkynylphosphane-substituted
phosphole 2.
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Scheme 2 Proposed mechanism for the formation of 2 and the observed
byproduct 4.
Fig. 1 Molecular structure of 2. Selected bond lengths (Å) and angles (1):
C1–C2 1.624(3), C2–C3 1.506(3), C3–C4 1.342(2), C1–P1 1.878(2),
P1–C4 1.842(2), C4–P2 1.836(2), P2–C13 1.776(3), C13–C14 1.198(3),
B–B 1.749–1.785, B–C 1.706–1.733, C1–C2–C3 118.1(2), C2–C3–C4
116.1(2), C3–C4–P1 116.8(2), C4–P1–C1 89.58(9), P1–C1–C2 108.9(1),
P1–C4–P2 118.3(1) (hydrogen atoms have been omitted for clarity; ellip-
soids are shown at 50% probability). Only the SP, SP isomer is shown.
(Scheme 2), alkynylphosphanes 3 and 4 were synthesized
independently and fully characterized. On the basis of their
31P NMR shifts, both species could be identified in the reaction
solution, and the formation of 4 by reaction of 1 and lithiated
phenylacetylene was thus verified. Compound 4 polymerizes
very fast above 0 1C and seems to be responsible for the dark
color of the reaction solution. Polymerization of 4 is presumably
also responsible for the moderate yield of 2.
While the unexpected reactivity of 1 towards lithium acetylides
gives rise to the unusual phosphole 2, the initially desired
bis(alkynylphosphanyl)carbaboranes remain elusive by this syn-
thetic strategy. They can, however, be prepared in a different
procedure from alkynylchlorophosphanes 5a–5d and lithiated
ortho-carbaborane (Scheme 3) in a twofold nucleophilic substitu-
tion reaction. The bis(alkynylphosphanyl)carbaboranes 6a–6d are
stable under ambient conditions and were obtained in moderate
to good yields.
Compounds 1, 2, and 6d were investigated by cyclic voltam-
metry (CV) to elucidate the electronic communication between
the different P-substituents in the three compounds and the
carbaborane unit. Phosphole 2 features one irreversible oxida-
tion at Ep,a = 0.85 V vs. Fc+/0, and thus at a potential that is very
similar to that found for 6d (Ep,a = 1.00 V). It is thus tempting to
assign this process to an oxidation of mainly the P(NMe2)(CCR)
substituent, which is present in both compounds. Supporting
this assignment is the notion that the oxidation potential of
the P(NMe2)(CCR) group in 2 can be expected to be somewhat
lower than that of 6d due to the presence of the conjugated
endocyclic double bond in the former. Neither CV of the two
compounds shows a reductive feature within the solvent window
down to ꢀ2.3 V vs. Fc+/0, in analogy to the reported reduction
endocyclic double bond are observed as two doublets of doub-
lets at 149.2 (1JCP = 43.1, 43.3 Hz) and 150.0 ppm (2JCP = 5.1, 5.6
Hz). Furthermore two doublets are observed for the carbon
cluster atoms of the heterocycle at 84.6 (1JCP = 20.5 Hz) and 88.0
ppm (2JCP = 6.8 Hz). The 11B{1H} NMR spectrum shows five
signals in the ratio 1 : 1 : 2 : 2 : 4.
An X-ray structure analysis was carried out for 2 (Fig. 1).10
Yellow crystals of 2 were obtained at ꢀ20 1C from n-hexane. The
endocyclic C2–C3, C3–C4, C1–P1, and P1–C4 bond lengths
(1.506(3), 1.342(2), 1.878(2), and 1.842(2) Å) in 1-diethylamido-
2,3-dihydro-1H-phosphole 2 compare well with the corresponding
bonds found in 1H-phosphindole-1-sulfides11 or pentaphenyl-2,3-
dihydro-1H-phosphole.12 The C1–C2 bond (1.624(3) Å) is longer
than in these heterocycles (1.391(4), 1.523(3) Å),9,10 but similar to
those of carbaborane-substituted 1,2-diphosphetanes (1.645(4),
1.631(5) Å).13 The P2–C13 and C13–C14 bond lengths (1.776(3),
1.198(3) Å) are in the same range as found in [8-(dimethylamino-
kN)-1-naphthalenyl-kC]-diphenylethynylphosphorus (1.786(9),
1.184(9) Å).14 The phosphorus atoms P1 and P2 in the isolated
rac isomer of compound 2 have a pyramidal, and the nitrogen
atoms N1 and N2 a trigonal-planar environment.
A mechanism for the formation of 2 was proposed on the basis
of computational and 31P NMR spectroscopic studies. The first step
of the sequence is the cleavage of one P–C bond in 1 by lithiated
phenylacetylene, resulting in intermediates A and 3 (Scheme 2
and ESI†). Such exo-polyhedral C–E bond (E = P, C) cleavage in
mono- and disubstituted dicarba-closo-dodecaborane(12)s has been
reported earlier.15 The second equivalent of lithiated phenyl-
acetylene reacts with 3 to give the dialkynyl(amido)phosphane 4
(Scheme 2). Subsequently, the Li–Ccluster bond of A inserts into
one of the carbon–carbon triple bonds of 4, and 2 is formed
after LiCl elimination. The addition of organolithium compounds
to carbon–carbon triple bonds has been described in the
literature.16 For the investigation of the proposed mechanism
Scheme 3 Synthesis of P,P0-alkynylated 1,2-bis(N,N-dialkylaminophos-
phanyl)-1,2-dicarba-closo-dodecaborane(12)s 6a–6d.
Chem. Commun.
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in Modern Acetylene Chemistry, ed. P. J. Stang and F. Diederich,
Wiley-VCH, Weinheim, 2007, pp. 321–351.
3 (a) M. Yoshifuji, Y. Ichikawa, N. Yamada and K. Toyota, Chem. Commun.,
1998, 27–28; (b) M. Yoshifuji, Y. Ichikawa, K. Toyota, E. Kasashima and
potential of the unsubstituted, parent dicarba-closo-dode-
caborane(12).17 The only cathodic process that is observed for
the three compounds is an irreversible reduction of rac/meso-1
at Ep,c = 2.20 V (see ESI†) which is assigned to the electron-
deficient phosphane units, rather than to processes that are
localized on the carbaborane core.17
¨
Y. Okamoto, Chem. Lett., 1997, 87–88; (c) J. Mobus, Q. Bonnin, K. Ueda,
¨
R. Frohlich, K. Itami, G. Kehr and G. Erker, Angew. Chem., 2012, 124,
1990–1993 (Angew. Chem., Int. Ed., 2012, 51, 1954–1957).
4 L. Isaacs, P. Seiler and F. Diederich, Angew. Chem., 1995, 107,
1636–1639 (Angew. Chem., Int. Ed. Engl., 1995, 34, 1466–1469).
5 M. Kivala and F. Diederich, Acc. Chem. Res., 2009, 42, 235–248.
6 (a) R. N. Grimes, Carboranes, Academic Press, New York, 1970;
(b) V. I. Bregadze, Chem. Rev., 1992, 92, 209–223; (c) H. Beal,
in Boron Hydride Chemistry, ed. E. Muetterties, Academic Press,
New York, 1975.
In conclusion, the reaction of 1,2-bis(N,N-dimethyl-amino-
chlorophosphanyl)-1,2-dicarba-closo-dodecaborane(12) (1) and
lithiated phenylacetylene gave an unusual five-membered
heterocycle 2 containing an exocyclic alkynylphosphane unit
and a dimethylamino group at the phosphole P atom. The high
degree of functionalization should offer multiple possibilities
for the synthesis of future carbaborane-containing phos-
phorus heterocycles. 1,2-Bis(phosphanyl)-1,2-dicarba-closo-
dodecaborane(12)s 6a–6d are accessible from the reaction of
alkynylchlorophosphanes and dilithiated ortho-carbaborane.
Comparative cyclic voltammetric studies show that the HOMO of
2 is localized mainly on the P(NMe2)(CCR) group. The dihydro-
phosphole unit is thus rather inert to oxidation, an effect that
could point towards strong electronic coupling to the electron-
deficient carbaborane that is fused onto the phosphole ring.
Further studies on the reactivity of this new class of alkynyl-
phosphanes are underway.
7 E. Bernoud, R. Veillard, C. Alayrac and A.-C. Gaumont, Molecules,
2012, 17, 14573–14587.
¨
8 S. Stadlbauer, R. Frank, I. Maulana, P. Lonnecke, B. Kirchner and
E. Hey-Hawkins, Inorg. Chem., 2009, 48, 6072–6082.
9 Y. Wang, W.-X. Zhang, Z. Wang and Z. Xi, Angew. Chem., Int. Ed.,
2011, 50, 8122–8126.
10 Structural data of 2: C22H32B10P2N2, M = 494.54, monoclinic, space
group P21/c, a = 14.5505(3) Å, b = 15.3833(3) Å, c = 12.6324(3) Å,
b = 92.549(2)1, V = 2824.8(1) Å3, Z = 4, rcalcd = 1.163 Mg mꢀ3
,
m = 0.170 mmꢀ1, ymax. = 26.371, R(all data) = 0.0830, wR(all data) = 0.1411,
T = 293(2) K, 5771 independent reflections, 409 parameters, no
restraints, maximal residual electron density 0.400 e Åꢀ3. The
structure determination was carried out at room temperature
because the sample was twinned at lower temperatures, e.g. 130 K,
structure solution with SIR92 (A. Altomare, G. Cascarano,
C. Giacovazzo, A. Guagliardi, M. C. Burla, G. Polidori and
M. Camalli, J. Appl. Crystallogr., 1994, 27, 435). Anisotropic refine-
ment of all non-hydrogen atoms with SHELXL-97 (G. M. Sheldrick,
Acta Crystallogr., 2008, A64, 112–122). With the exception of the
methyl groups, all H atoms were located on difference Fourier maps
calculated at the final stage of the structure refinement. CCDC
976541 (2).
Financial support of the Konrad-Adenauer Stiftung (doctoral
grant for A.K.), the Graduate School Leipzig School of Natural
Sciences – Building with Molecules and Nano-objects (BuildMoNa),
COST Action CM1302 SIPs and COST Action CM0802 PhoSciNet
(STSM for A.K. in Uppsala), the German Academic Exchange
11 Y. Miquel, A. Igau, B. Donnadieu, J.-P. Majoral, L. Dupuis, N. Pirio
and P. Meunier, Chem. Commun., 1997, 279–280.
¨
Service (DAAD) (short-term research stay at Universitat Leipzig
¨
12 K. Issberner, E. Niecke, E. Wittchow, K. H. Dotz and M. Nieger,
for M.B.S.), and the Swedish Research Council is gratefully
acknowledged. We thank Chemetall GmbH for a generous
donation of butyllithium.
Organometallics, 1997, 16, 2370–2376.
´
¨
13 A. Kreienbrink, M. B. Sarosi, E. G. Rys, P. Lonnecke and E. Hey-
Hawkins, Angew. Chem., 2011, 123, 4798–4800 (Angew. Chem., Int.
Ed., 2011, 50, 4701–4703).
´
14 S. B. Bushuk, F. H. Carre, D. M. H. Guy, W. E. Douglas,
Y. A. Kalvinkovskya, L. G. Klapshina, A. N. Rubinov, A. P. Stupak
and B. A. Bushuk, Polyhedron, 2004, 23, 2615–2623.
Notes and references
1 (a) L. T. Scott and M. Unno, J. Am. Chem. Soc., 1990, 112, 7823–7825; 15 (a) A. V. Kasantsev, M. N. Zhubekova and L. I. Zakharkin, Zh.
(b) D. E. Cabelli, A. H. Cowley and M. J. S. Dewar, J. Am. Chem. Soc.,
1981, 103, 3290–3292; (c) I. K. Ahmad, H. Ozeki and S. Sato, J. Chem.
Phys., 1997, 107, 1301–1307; (d) P. Bharathi and M. Periasamy,
Organometallics, 2000, 19, 5511–5513.
Obshch. Khim., 1971, 41, 2027–2033; (b) A. Kreienbrink,
¨
P. Lonnecke, M. Findeisen and E. Hey-Hawkins, Chem. Commun.,
2012, 48, 9385–9387; (c) L. I. Zakharkin and A. I. L’vov, J. Organomet.
¨
Chem., 1966, 5, 313–319; (d) W. Neumann, M. Hiller, P. Lonnecke
2 (a) S. G. A. van Assema, P. B. Kraikivskii, S. N. Zelinskii, V. V. Saraev,
and E. Hey-Hawkins, Dalton Trans., 2014, 43, 4935–4937.
G. Bas de Jong, F. J. J. de Kanter, M. Schakel, J. C. Slootweg and 16 (a) S. C. Cohen, A. J. Tomlinson, M. R. Wiles and A. G. Massey,
K. Lammertsma, J. Organomet. Chem., 2007, 692, 2314–2323;
(b) R. Shiozawa and K. Sakamoto, Chem. Lett., 2003, 32,
J. Organomet. Chem., 1968, 11, 385–392; (b) A. Krebs, W. Born,
B. Kaletta, W.-U. Nickel and W. Rueger, Tetrahedron Lett., 1983,
24, 4821–4824; (c) A. I. Meyers and P. D. Pansegrau, Tetrahedron
Lett., 1983, 24, 4935–4938; (d) W. Bauer, M. Feigel, G. Mueller and
P. von Rague Schleyer, J. Am. Chem. Soc., 1988, 110, 6033–6046.
¨
1024–1025; (c) G. Markl, T. Zollitsch, P. Kreitmeier, M. Prinzhorn,
S. Reitinger and E. Eibler, Chem. – Eur. J., 2000, 6, 3806–3820;
(d) S. G. A. van Assema, G. B. de Jong, A. W. Ehlers, F. J. J. de
Kanter, M. Schakel, A. L. Spek, M. Lutz and K. Lammertsma, 17 K. Hosoi, S. Inagi, T. Kubo and T. Fuchigami, Chem. Commun., 2011,
Eur. J. Org. Chem., 2007, 2405–2412; (e) L. T. Scott and M. J. Cooney,
47, 8632–8634.
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