The unexpected chemistry of a bis(phospholyl)acetylene

Article abstract of DOI:10.1002/hc.20457

The reaction of lithium 3,4-dimethyl-phospholide with tetrachloroethylene gives the corresponding 1,2-bis(phospholyl)acetylene that has been characterized by X-ray crystal structure analysis of its bis(pentacarbonylmolybdenum) complex. The reaction with sulfur leads to the corresponding disulfide that spontaneously undergoes a self-condensation via a Diels-Alder cycloaddition between the Q=C triple bond and the phosphole dienic system giving, after aromatization by loss of the phosphorus bridge, the corresponding 1,2-bis(phospholyl)benzene.

Full text of DOI:10.1002/hc.20457

Heteroatom Chemistry
Volume 19, Number 5, 2008
The Unexpected Chemistry of a
Bis(phospholyl)acetylene
Matthew P. Duffy, Bruno Donnadieu, and Franc¸ois Mathey
Department of Chemistry, UCR–CNRS Joint Research Laboratory, University of California
Riverside, Riverside, CA 92521-0403
Received 20 February 2008; revised 27 March 2008
whether it was possible to use the [1,5]-sigmatropic
ABSTRACT: The reaction of lithium 3,4-dimethyl-
shifts of R-groups from P to Cα that are character-
phospholide with tetrachloroethylene gives the corre-
istic of phosphole chemistry [6] to install an alkynyl
sponding 1,2-bis(phospholyl)acetylene that has been
group between the α-positions of two phospholes.
characterized by X-ray crystal structure analysis of
In a preliminary work, we demonstrated that such
its bis(pentacarbonylmolybdenum) complex. The re-
a shift is possible on one phosphole, although poly-
action with sulfur leads to the corresponding disul-
merization by self-condensation between the C C
fide that spontaneously undergoes a self-condensation
triple bond and the 2H-phosphole is difficult to avoid
via a Diels–Alder cycloaddition between the C C
[7]. We report hereafter on the unexpected outcome
triple bond and the phosphole dienic system giv-
of our study.
ing, after aromatization by loss of the phosphorus
bridge, the corresponding 1,2-bis(phospholyl)benzene.
ꢀ
C
2008 Wiley Periodicals, Inc. Heteroatom Chem 19:537–
RESULTS AND DISCUSSION
541, 2008; Published online in Wiley InterScience
Our primary target was the bis(3,4-dimethyl-
phospholyl)acetylene (1). It must be noted
here that the bis(2,3,4,5-tetraethylphospholyl)-
acetylene has already been prepared by the group
of Yoshifuji using the chemistry of zircona-
cyclopentadienes [8]. Our synthetic approach is
entirely different. We first decided to investigate the
reaction of the 1-cyanophosphole (2) [9] with the
readily available dilithioacetylene [10] (Eq. (1)).
(www.interscience.wiley.com). DOI 10.1002/hc.20457
INTRODUCTION
Conjugated oligomers incorporating phosphorus
heterocycles as modulating units are now studied
in some depth in view of their potential applications
in the manufacture of organic light-emitting diodes
(OLED’s) and organic transistors [1,2]. In particu-
lar, the group of Chujo has described the synthe-
sis of several oligomers including phosphole and
alkyne units [3,4]. On our side, we have prepared
several α-substituted alkynylphospholes from a 2-
lithiophosphole [5], but the yields were relatively low
and the overall scheme was somewhat cumbersome.
Our initial aim in the present study was to check
Whatever the conditions, the reaction always yielded
the monosubstituted acetylene (3), which was char-
acterized by its 31P resonance at −45 ppm and by
X-ray analysis of its P-sulfide.
We then decided to follow a procedure re-
ported in the literature for the synthesis of
bis(diphenylphosphino)acetylene [11]. Accordingly,
Correspondence to: Franc¸ois Mathey; e-mail: francois.mathey@
ucr.edu.
2008 Wiley Periodicals, Inc.
c
ꢀ
537

538 Duffy et al.
we reacted the lithium 3,4-dimethylphospholide
with tetrachloroethylene (Eq. (2)).
deg. This cannot be due to the repulsion between
the two bulky molybdenum units since the bending
We reproducibly obtained a 1:1 mixture of the de-
sired bis(phospholyl)acetylene (1) (δ31P −45) and the
1,1ꢁ-biphospholyl (4) (δ31P −29). The formation of
the biphospholyl suggests that dichloroacetylene is
intermediately formed in the reaction. The postu-
lated mechanism is shown in Eq. (3).
is directed toward the metals. A similar bending of
the C C triple bond has been observed previously in
several bis(diphenylphosphino)acetylene complexes
[12].
The preliminary C C shift experiments at high
temperature were negative, but, while studying the
The yield of (1) is 20% versus a maximum the-
oretical yield of 50%. The highly reactive biphos-
pholyl was destroyed by the reaction with sulfur at
room temperature. Bis(phospholyl)acetylene is in-
ert under such conditions. A quick chromatography
allowed to get (1) in the pure state. It was char-
acterized by mass spectrometry and 1H, 13C, and
31P NMR spectroscopy. A further characterization
was performed by X-ray crystal structure analysis
of its bis(pentacarbonylmolybdenum) complex (5)
(Fig. 1). The structure shows a very short P C(sp)
sulfurization of (1) at 60◦C, we made a surprising
observation. The initial disulfide (6) (δ31P 11 ppm)
rapidly evolves to give a new sulfide (7) at δ31P
41 ppm. The 13C spectrum of (7) shows two types
of methyl groups and two types of sp2-C directly
linked to P at 126.23 (1 JC-P = 86.1 Hz) (CH) and
2
133.53 (1 JC-P = 89.9 Hz, JC-P = 8.2 Hz) (C). The
mass spectrum indicates that one C6H8 unit has been
added to (1) (m/z = 391 (M + H)). The X-ray crystal
structure analysis (Fig. 2) shows that this product
results from the self-condensation of (6), involving
the Diels–Alder condensation of the C C triple bond
with the phosphole dienic unit, leading, after arom-
atization by loss of the P(S)R bridge, to the ortho-
diphospholylbenzene (7) (Eq. (4)).
˚
bond at 1.782(4) A, significantly shorter than the en-
˚
docyclic P C bonds (1.807(4) and 1.813(4) A), but
the most characteristic phenomenon is the nonlin-
earity of the C C triple bond: P C C 170.49(15)
Heteroatom Chemistry DOI 10.1002/hc

Unexpected Chemistry of a Bis(phospholyl)acetylene 539
FIGURE 2 X-ray crystal structure of disulfide (7). Ellip-
soids are scaled to enclose 50% of the electronic density.
˚
FIGURE 1 X-ray crystal structure of molybdenum complex
(5). Ellipsoids are scaled to enclose 50% of the electronic den-
sity. Main bond lengths (A) and angles (deg.): P C1 1.782(4),
C1–C1 1.216(9), P C2 1.807(4), P C5 1.813(4), P–Mo
2.5177(18); P C1–C1 170.49(15), Mo–P C1 110.64(11),
C2–P C5 91.4(2).
Main bond lengths (A) and angles (deg.): P1–S1 1.9507(9),
P2–S2 1.9508(10), P1–C1 1.826(3), P2–C6 1.833(3), P1–
C7 1.791(3), P1–C10 1.791(3), P2–C11 1.784(3), P2–
C14 1.799(3), C1–C6 1.410(4); P1–C1–C6 126.24(19),
P2–C6–C1 126.6(2), C7–P1–C10 91.97(13), C11–P2–C14
91.48(13), C1–P1–S1 113.88(9), C6–P2–S2 114.67 (9).
˚
Bis(3,4-dimethylphospholyl)acetylene (1)
The molecule is sterically congested as shown by
the P1–C1–C6 and P2–C6–C1 angles larger than
120◦. (126.24(19) and 126.6(2)) and by the relatively
To a Schlenk flask containing tetrachloroethy-
lene (1.09 mL, 10.6 mmol) in THF (80 mL) was
added dropwise via cannula, at room tempera-
ture under nitrogen a solution of lithium 3,4-
dimethylphospholide (42.5 mmol) in THF (240 mL).
After stirring for 30 min, the reaction was monitored
by 31P NMR. To destroy the biphospholyl by-product,
sulfur (0.68 g, 21.3 mmol) was added and the solu-
tion was stirred for 1 h and monitored by 31P NMR.
Once all the biphospholyl was destroyed, the sol-
vent was removed under vacuum, the residue was
dissolved in a minimum amount of methylene chlo-
ride and chromatographed on silica. The compound
was eluted with an 80:20 hexane:methylene chloride
mixture. Solvent was removed under vacuum, and
the product was checked by 31P NMR. Yield 1.07 g
(4.3 mmol, 20%) of an orange-yellow solid.
˚
long C1–C6 bond at 1.410(4) A versus 1.387(4)–
˚
1.400(4) A for the other C C bonds of the benzene
ring. It must be recalled here that monomeric 3,4-
dimethylphosphole sulfides are known to be quite
reactive dienes in the Diels–Alder reactions and that
the [4+2] cycloadducts thus obtained often lose
their phosphorus bridges [13]. Besides, the reac-
tion of lithium 3,4-dimethylphospholide with 1,2-
dibromobenzene in the presence of a nickel catalyst
essentially stops at the monosubstitution [14] and
only one ortho-diphospholylbenzene has been ob-
tained previously by a rather complex sequence of
reactions [15]. Finally, the bis(phospholyl)acetylene
disulfide prepared by the group of Yoshifuji [8] does
not undergo this kind of chemistry, probably be-
cause the phosphole dienic reactivity is quenched
by the tetraethyl substitution.
31P NMR (CDCl3) δ: −38.1 ppm; 1H NMR (CDCl3)
δ: 2.09 (d, 4 J(P-H) = 3.3 Hz, 12H, CH3), 6.30 (d, 2 J(P-
H) = 39.2 Hz, 4H, CH); 13C NMR (CDCl3) δ: 17.92
1
(s, CH3), 100.44 (d, J(P-C) = 26.0 Hz, C), 124.92
EXPERIMENTAL
(s, CH), 150.45 (m, CMe) ppm. Mass spectrum
(FAB+): 247 (MH+)
Nuclear magnetic resonance spectra were obtained
on a Bruker-Avance 300 and Varian-Inova spectrom-
eter operating at 300.13 MHz for 1H, 75.45 MHz for
13C, and 121.496 MHz for 31P. Chemical shifts are
expressed in parts per million downfield from ex-
ternal TMS (1H and 13C) and external 85% H3PO4
(31P). Mass spectra were obtained on VG 7070 and
Hewlett–Packard 5989A GC/MS spectrometers.
Decacarbonyldimolybdenum Complex (5)
To a Schlenk flask with bis(phospholyl)acetylene (1)
(0.752 g, 3.05 mmol) in acetonitrile (10 mL) was
added dropwise under nitrogen a solution of freshly
made Mo(CO)5(MeCN) (6.1 mmol) in acetonitrile
Heteroatom Chemistry DOI 10.1002/hc

540 Duffy et al.
(20 mL). The reaction mixture was stirred overnight
and monitored by 31P NMR. Solvent was removed
under vacuum, and the residue was purified by col-
umn chromatography on silica with 97:3 petroleum
ether:ethyl acetate as the eluent, yielding 0.50 g (0.7
mmol) of a pale yellow solid (23.0%).
ing a narrow-frame integration algorithm. The inte-
grated frames yielded for.
5: A total of 6677 reflections at a maximum 2θ
angle of 46.52◦ of which 2062 were independent re-
flections (R = 0.0488, R = 0.0465, completeness
int
sig
= 99.9%) and 1807 (87.63%) reflections were greater
than 2σ(I). A monoclinic cell space group C2/c was
found; the unit cell parameters were a = 25.368(19)
31P NMR (CDCl3) δ: −4.0 ppm; 1H NMR (CDCl3)
δ: 2.14 (s, 12H, CH3), 6.26 (d, 2 J(P-H) = 38.8 Hz, 4H,
3
CH); 13C NMR (CDCl3) δ 17.71 (d, J(P-C) = 12.0
◦
˚
˚
˚
A, b = 9.006(7) A, c = 13.883(11) A, α = 90.0 , β =
1
◦
◦
3
˚
Hz, CH3), 99.11 (d, J(P-C) = 44.9 Hz, C), 125.85
115.486(8) , γ = 90.0 , V = 2863(4) A , Z = 8, calcu-
lated density Dc = 1.666 Mg/m3.
(d, 1 J(P-C) = 44.9 Hz, CH), 152.48 (d, 2 J(P-C) =
11.4 Hz, CMe), 204.58 (d, 2 J(P-C) = 8.2 Hz, Mo C O
eq), 209.40 (d, 2 J(P-C) = 20.3 Hz, Mo C O ax) ppm.
Mass spectrum: 719 (MH+).
7: A total of 26,584 reflections at a maximum
2θ angle of 51.76◦, of which 3916 were independent
reflections (R = 0.0645, R = 0.0450, complete-
int
sig
ness = 99.9%) and 2700 (68.95%) reflections were
greater than 2σ(I). A orthorhombic cell space group
Pbca was found, the unit cell parameters were a =
4,5-Dimethyl-1,2-bis(3,4-
dimethylphospholyl)benzene
Disulfide (7)
˚
˚
˚
15.8034(15) A, b = 15.3256(15) A, c = 16.6866(16) A,
◦
3
˚
α = β = γ = 90.0 , V = 4041.4(7) A , Z = 8, calcu-
lated density Dc = 1.283 Mg/m3.
To a Schlenk flask with bis(phospholyl)acetylene (1)
(0.13 g, 0.53 mmol) in THF (5 mL) under nitro-
gen, sulfur (0.037 g, 1.2 m mol) at room tempera-
ture was added. The mixture was stirred overnight
and monitored by 31P NMR to confirm the formation
of the bis(phospholyl)acetylene disulfide (6). Then,
the reaction mixture was heated at 60◦C for 3 h and
monitored by 31P NMR till there was no longer any
bis(phospholyl)acetylene disulfide left, leaving only
the final product (7). After the solvent was removed
under vacuum, the residue was purified by column
chromatography on silica with hexanes and methy-
lene chloride. The product was finally eluted with
diethyl ether to yield 0.04 g (0.10 mmol) of a light
yellow solid (18.9%).
Absorption corrections were applied for data us-
ing the SADABS [19] program. The program SIR92
[20] was used for phase determination and struc-
ture solution, followed by some subsequent differ-
ence Fourier maps. From the primary electron den-
sity map got most of the non-hydrogen atoms were
located, and with the aid of subsequent isotropic re-
finement all of the non-hydrogen atoms were identi-
fied. Atomic coordinates, isotropic and anisotropic
displacement parameters of all the non-hydrogen
atoms were refined by means of a full matrix least-
squares procedure on F2. The H-atoms were in-
cluded in the refinement in calculated positions rid-
ing on the C atoms to which they were attached. The
refinement converged for.
31P NMR (CDCl3) δ: +40.7 ppm; 1H NMR (CDCl3)
5 at R = 0.0391, wR = 0.0998, with intensity,
1
2
δ: 2.09 (s, 12H, CH3), 2.24 (s, 6H, CH3), 6.80 (d, 2 J(P-
I > 2σ(I), 7 at R = 0.0400, wR2 = 0.0915 with in-
1
4
H) = 28.7 Hz, 4H, CH), 7.57 (m, 3 J(P-H) + J(P-H)
tensity, I > 2σ(I). Drawings of molecules were per-
formed using of ORTEP32 [21]. (Further details on
the crystal structure investigation are available on re-
quest from the Director of the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, GB-Cambridge
CB21EZ UK, on quoting the full journal citation.
= 19.3 Hz, 2H, H benzene); 13C NMR (CDCl3) δ: 17.68
(m, CH3), 19.98 (s, Ph-CH3) 126.23 (d, 1 J(P-C) = 86.1
1
2
Hz, CH), 133.5 (dd, J(P-C) = 89.9 Hz, J(P-C) =
8.2 Hz, P-C = C-P), 134.4 (m, Ph), 140.8 (m, Ph),
152.4 (m, CMe) ppm; Mass spectrum (FAB+): 391
(MH+).
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Heteroatom Chemistry DOI 10.1002/hc