Bis-3-oxo-λ5-phosphole: Isolation, structural analyses, and synthesis of phosphorus-ylide containing conjugated heterocycle

Article abstract of DOI:10.1021/jo1004235

Figure presented A new class of phosphorus-ylide containing conjugate heterocycle was isolated from a mixture of colored products of the reaction of a silylphosphine and dimethyl acetylenedicarboxylate. The structure was determined and the selective synthesis was developed. The indigo-like bis-phosphole structure appears a green to blue color, which is derived from the low energy-gap of the phosphole.

Full text of DOI:10.1021/jo1004235

pubs.acs.org/joc
Bis-3-oxo-λ5-phosphole: Isolation, Structural
Analyses, and Synthesis of Phosphorus-ylide
Containing Conjugated Heterocycle
heterocycles have received much attention. Thus, increasing
numbers of reports have recently appeared concerning
phospholes,2,3 most of which are trivalent 1H-phospholes,
or their oxides and sulfides. In contrast, phospholes with an
ylide in a conjugate system have been rarely reported,
although some examples of their synthesis had been reported
in early 1970s.4 Herein, we wish to report the first examples
of a new class of conjugate heterocycles containing a phos-
phorus-ylide, 3-oxo-λ5-phosphole (Figure 1a), isolated as a
symmetrical bis-oxophosphole (Figure 1b).
Yasunobu Nishimura, Yuka Kawamura,
Yutaka Watanabe, and Minoru Hayashi*
Department of Materials Science and Biotechnology,
Graduate School of Science and Engineering, Ehime
University, 3 Bunkyo-cho, Matsuyama 790-8577, Japan
On the course of our research using t-BuMe2SiPPh2 1,5 we
faced unexpected formation of a complex mixture of colored
products in the reaction with dimethyl acetylenedicarboxy-
late (DMAD), when the reaction was conducted under rt.6,7
Careful separation of the products by column chromatogra-
phy afforded a number of colored materials, one of which
was isolated as a green solid.8 Conventional spectral analyses
of the green compound gave only limited information about
its detailed structure.9 Careful assignments of 13C signals
with the aid of the 2D NMR spectra suggested the existence
of one conjugate carbonyl carbon (166 ppm) and two alkenyl
carbons (120 and 158 ppm) in addition to one ylidic carbon
(90 ppm).9 Characteristic multiple couplings of these carbon
signals indicate that there are two phosphorus atoms with
different connections around these carbons, in symmetrical
position.9 We speculated that the green compound might
have a symmetrically connected bis-3-oxo-λ5-phosphole 2a,
as depicted in Figure 1b.
hayashi@eng.ehime-u.ac.jp
Received March 6, 2010
A new class of phosphorus-ylide containing conjugate
heterocycle was isolated from a mixture of colored products
of the reaction of a silylphosphine and dimethyl acetylene-
dicarboxylate. The structure was determined and the selec-
tive synthesis was developed. The indigo-like bis-phosphole
structure appears a green to blue color, which is derived
from the low energy-gap of the phosphole.
Fortunately, recrystallization from ClCH2CH2Cl/MeOH
gave a single crystal of 2a 2ClCH2CH2Cl suitable for an
3
X-ray diffraction study (Figure 2).10 To the best of our
knowledge, it is the first isolation of the phosphole ring with
a 3-oxo-λ5-phosphole structure.
The molecule has crystallographic Ci symmetry and com-
prises two 3-oxo-σ4λ5-phosphole ring fragments directly
connected at each 2-position. It is in accordance with the
presumption from the spectral analyses. Each phosphorus
atom has a distorted tetrahedral environment: (C(1)-P-
C(4) = 93.7(2)°, C(1)-P-C(15) = 119.3(2)°). Lengths of
Conjugate heterocycles have been quite attractive because
of their wide utilities in both electronic and optical functional
materials.1 Among them, phosphorus-containing conjugate
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(6) Selective 1:1 addition of 1 to DMAD proceeded at -78 °C.5g Silylpho-
sphines also reacted with propiolates and alkynyl ketones to give the
corresponding 1:1 adducts.5g,7
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separated colored materials still remained unidentified.
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DOI: 10.1021/jo1004235 Published on Web 05/07/2010
r
J. Org. Chem. 2010, 75, 3875–3877 3875
2010 American Chemical Society

Nishimura et al.
JOCNote
FIGURE 1. (a) 3-Oxo-λ5-phosphole. (b) Bis-3-oxo-λ5-phosphole
2a (green material).
FIGURE 3. UV-vis spectrum of bis-oxophosphole (curve, obser-
ved for 2a in CHCl3 (9.3 ꢀ 10-5 M); perpendicular bar, calculated
for 2d.9
clearly shows the extension of conjugation through two rings.
The broad absorption around the range of 500-1100 nm
(λmax=807 nm, ε=4.9ꢀ103 dm3 mol-1 cm-1) also indicated
the extended conjugation (Figure 3). A computational study
using density functional theory methods (B3LYP/6-311Gþ
(d,p)) gave an optimized structure for 2d (R=R0=Me) that
was in excellent agreement with that obtained by the X-ray
crystallography for 2a.9 The computed absorption maxima
(λmax = 811 nm) calculated by using TD-DFT methods
correspond well with those determined experimentally
(Figure 3). This result indicates that 2a retains a planar
structure in solution even though the central C-C single
bond can rotate. A considerably small HOMO-LUMO
gap (1.81 eV) is consistent with the π-conjugation extending
across two oxophosphole moieties as expected from the
X-ray structure. Low-lying HOMO and LUMO (-4.58
and -2.77 eV, respectively) are also characteristic properties
of this new class of phospholes.9
FIGURE 2. X-ray molecular structure of the bis-oxophosphole 2a
(primed atoms are generated by an inversion center).10 H atoms are
omitted for clarity. Thermal ellipsoids are drawn at the 30%
˚
probability level. Selected bond lengths [A] and angles [deg]: C(1)-
C(1)0 1.439(7), P-C(1) 1.744(5), P-C(4) 1.821(6), C(1)-C(2) 1.396(6);
C(1)-P-C(4) 93.7(2), C(1)-P-C(15) 119.3(2).10
(C(4)-P), respectively, which clearly indicate an ylide struc-
ture of the C(1)-P bond. The shorter single bond (C(1)-
C(2), 1.396(6) A) shows a conjugation to the carbonyl group.
This complicated condensation of five molecules into one
most likely started from a nucleophilic attack of silylpho-
sphane 1 to DMAD to give the 1:1 adduct A,5g plausibly
followed by the multiple cascade reactions as proposed in
Scheme 1.
˚
Two oxophosphole rings are almost planar (torsion angles
0
˚
within (1.1°). TheslightlyshorterC(1)-C(1) bond (1.439(7) A)
(9) Conventional spectral analyses (1H, 13C, 31P NMR, IR, and FABMS)
of 2a gave only limited information about its detailed structure. However, its
symmetrical structure was confirmed in addition to its composition (two
phosphane parts and three alkynes). FABMS spectrum of 2a showed the
molecular ion peak at m/z=734, which was a little (may be two MeO parts,
vide infra) smaller than the sum of two phosphine parts (PPh2) and three
DMAD parts. However, 31P NMR spectrum showed one singlet signal at
27 ppm, which suggested that the molecule has a symmetrical structure in
which two phosphines are placed under magnetical equivalence. That only
two methyl signals appeared in the 1H NMR spectrum also supported a
symmetric structure. No TBDMS group was observed in the 1H NMR
spectrum, which is in accordance with the result of the FABMS spectrum.
The ratio of integrals of the methyl esters and the aromatic parts was 3:5,
which should be 12:20 on the basis of FABMS. It is also in accordance with
four methyl esters in the molecule, although three DMAD were embedded
(two MeO groups were eliminated). Further information from careful
assignments of 13C signals with the aid of the 2D NMR spectra (HMQC,
HMBC) suggested the existence of one conjugate carbonyl carbon (166 ppm)
and two alkenyl carbons (120 and 158 ppm) in addition to one ylidic carbon
(90 ppm). The carbon signals at 163 and 160 ppm gave cross peaks with Me
ester singlets in the HMBC spectrum, which showed that these signals are the
ester carbonyl carbons. The proton signal at 8.0 ppm (ortho-H) gave a cross
peak with 166 ppm (conjugate ketone), 120 ppm (alkenyl), 118 ppm (Ph-ipso)
and 90 ppm (ylide-C), which suggested a cyclic conjugate ylide structure. The
carbon signals with characteristic multiple couplings, especially the ABX
spin system of the signals at 90 and 120 ppm, indicate that there are two
phosphorus atoms with different connections around those carbons, in a
symmetrical position. The IR absorption (1693 cm-1), assigned as conjugate
carbonyls, also supported the above NMR analysis.This speculation was
strongly supported by spectral analyses of the C4-C5 reduced 3-oxo-4,5-
dihydro-σ4λ5-phosphole 3 obtained by hydride reduction of 2 as a pair of
diastereomers.
The trivalent phosphorus atom of the initially formed 1:1
adduct A attacked the alkynyl carbon of DMAD to give the
zwitter ionic intermediate B, followed by a nucleophilic acyl
substitution at the ester group resulting in the formation of
phosphonium intermediate C. This was then attacked by
another 1 followed by desilylation to give the half ylide inter-
mediate D. A similar cyclization process was then repeated
with another DMAD to afford bis-oxophosphole 2a. Since a
bulky silyl group would interfere in each reaction (such as the
addition of 1 to C), the use of a less hindered secondary
phosphine gave the bis-oxophosphole 2 in relatively selective
manner (Scheme 2).11
In summary, we have isolated a new phosphorus-ylide
containing conjugate heterocycle, 3-oxo-λ5-phosphole, as
bis-oxophosphole. A selective approach by a five-molecule
condensation was also established. Since oxophospholes
(10) Crystal data. 2a 2ClCH2CH2Cl: C40H32O10P2 2(C2H4Cl2); MW=
932.50; monoclinic; space group P21/c (No. 14); a=13.15(3), b=12.75(-31),
3
3
3
˚
c=14.29(3); b=114.524(15); V=2180(8) A ; Z=2; m(Mo KR)=0.402 mm
;
T=150 K; 16357 reflections collected; Rint=0.0582; R(F)=0.0847 for 3659
data with I > 2σ(I), wR(F2)=0.2438 for all 4966 independent data. CCDC-
759535 contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge Crystal-
lographic Date Centre via www.ccdc.cam.ac.uk/data_request/cif.
3876 J. Org. Chem. Vol. 75, No. 11, 2010

Nishimura et al.
JOCNote
SCHEME 1. Plausible Formation Mechanism of 2a
isolated by column chromatography on silica gel (CHCl3/
AcOEt=3:1 to 1:1) despite low solubility of the compounds re-
quiring a large amount of eluent. 1H NMR (CDCl3/CF3CO2D)
δ 3.59 (s, 6 H, CH3ꢀ2), 3.79 (s, 6 H, CH3ꢀ2), 7.54-7.61 (m, 8 H,
Ph-meta), 7.65-7.73 (m, 4 H, Ph-para), 7.92-7.99 (m, 8 H, Ph-
ortho); 13C NMR δ 52.8 (s, CH3ꢀ2), 53.1 (s, CH3ꢀ2), 90.1 (ABX,
ylide-Cꢀ2), 118.1 (ABX, Ph-ipso-Cꢀ4), 119.8 (ABX, alkenyl-
Cꢀ2), 129.5 (t, J=7.0 Hz, Ph-meta-Cꢀ4), 133.3 (t, J=5.6 Hz,
Ph-ortho-Cꢀ4), 134.1 (s, Ph-para-Cꢀ2), 157.6 (t, J=4.4 Hz,
alkenyl-Cꢀ2), 160.3 (t, J=6.0 Hz, ester-CdOꢀ2), 163.1 (t, J=
8.7 Hz, ester-CdOꢀ2), 165.6 (t, J=17.6 Hz, 3-CdOꢀ2); 31P
NMR δ 25.8; IR 1743, 1693 cm-1; FABMS 734 [M]þ; HRMS
calcd for C40H32O10P2 734.1470, found 734.1443. Mp 210 °C (dec).
Bis[4,5-bis(ethoxycarbonyl)-3-oxo-1,1-diphenyl-σ4λ5-phosphole]
2b. By the same procedure for 2a, 2b was obtained as a green
solid in 33% (813 mg) from Ph2PH (4.4 mmol). 1H NMR (C6D6/
CF3CO2D) δ 0.54 (t, J=7.1 Hz, 6 H, CH3ꢀ2), 1.07 (t, J=7.1 Hz,
6 H, CH3ꢀ2), 3.55 (q, J=7.1 Hz, 4 H, CH2ꢀ2), 4.26 (q, J=
7.1 Hz, 4 H, CH2 ꢀ 2), 7.18-7.30 (m, 12 H, Ph-meta,para),
8.14-8.22 (m, 8 H, Ph-ortho); 13C NMR δ 13.1 (s, CH3ꢀ2), 13.2
(s, CH3ꢀ2), 62.2 (s, CH2ꢀ2), 63.6 (s, CH2ꢀ2), 93.1 (ABX, ylide-
Cꢀ2), 116.9 (ABX, Ph-ipso-Cꢀ4), 121.3 (ABX, alkenyl-Cꢀ2),
129.7 (t, J=7.1 Hz, Ph-meta-Cꢀ4), 133.4 (t, J=5.9 Hz, Ph-ortho-
Cꢀ4), 134.5 (s, Ph-para-Cꢀ2), 156.7 (t, J=4.7 Hz, alkenyl-Cꢀ2),
159.5 (t, J=6.0 Hz, ester-CdOꢀ2), 162.8 (t, J=8.6 Hz, ester-
CdOꢀ2), 165.1 (t, J=18.0 Hz, 3-CdOꢀ2); 31P NMR δ 27.6;
IR 1732, 1686, 1531, 1269, 1249, 1110, 1041 cm-1; FABMS 791
[MþH]þ;HRMScalcdfor C44H41O10P2 791.2175, found 791.2156.
Mp 245 °C (dec.).
SCHEME 2. Selective Synthesis of Bis-oxophosphole 2
Bis[4,5-bis(methoxycarbonyl)-3-oxo-1,1-dicyclohexyl-σ4λ5-
phosphole] 2c. By the same procedure for 2a, 2c was obtained as a
deep-blue solid in 14% (27 mg) from c-Hex2PH (0.51 mmol) by a
column chromatography on silica gel (CHCl3/AcOEt=3:1). 1H
NMR (CDCl3) δ 1.07-2.11 (m, 40 H, CH2ꢀ20), 3.25-3.39 (br,
4 H, CHꢀ4), 3.77 (s, 6 H, CH3ꢀ2), 3.92 (s, 6 H, CH3ꢀ2); 13
C
NMR δ 25.7 (s, CH2ꢀ4), 26.5 (t, J=6.5 Hz, CH2ꢀ4), 26.5 (s,
CH2ꢀ4), 26.7 (t, J=7.2 Hz, CH2ꢀ4), 28.2 (s, CH2ꢀ4), 34.6 (t, J=
22.4 Hz, P-CHꢀ4), 52.4 (s, CH3ꢀ2), 52.8 (s, CH3ꢀ2), 83.0 (ABX,
ylide-Cꢀ2), 111.9 (ABX, alkenyl-Cꢀ2), 162.8 (br, ester-CdOꢀ
2), 164.5 (br, alkenyl-Cꢀ2), 165.6 (t, J=7.3 Hz, ester-CdOꢀ2),
168.0 (t, J=14.8 Hz, 3-CdOꢀ2); 31P NMR δ 47.2; FABMS 758
[M]þ; HRMS calcd for C40H56O10P2 758.3348, found 758.3365.
Mp 232-234 °C.
having other substituents could not be accessed by this app-
roach, generally applicable synthetic methods are now being
developed in our lab and will appear elsewhere.
Experimental Section
Bis[4,5-bis(methoxycarbonyl)-3-oxo-1,1-diphenyl-σ4λ5-phosphole]
2a. A THF solution (3 mL) of DMAD (280 µL, 2.3 mmol, 4.0
equiv.) was added diphenylphosphine (107 mg, 0.57 mmol) at rt
under N2. After stirring for 3 h, a green precipitate was formed.
After the mixture was concentrated to dryness, a small amount
of MeOH was added to the residue. Pure 2a was obtained by a
simple filtration (54%). Alternatively, the product could be
Acknowledgment. This work was partly supported by
Grant-in-Aid for Scientific Research (C) (20550046) and
Ehime University COE incubation program. We also thank
Venture Business Laboratory (VBL) and Institute for Co-
operative Science (INCS) of Ehime University for the mea-
surements of NMR spectra and X-ray crystallographical
analyses.
(11) A reaction of Ph2PH with DMAD was reported to give 2,3-dipho-
sphinosuccinate in ether,12 but we could not reproduce the result; 2a was
formed instead as the main product in CH2Cl2, THF, and other solvents at rt
in various ratios of Ph2PH and DMAD. Although the reactions in Scheme 2
were conducted under optimized conditions, they also gave a number of
colored materials probably because of the multiple mode of additions which
occurred simultaneously in these reactions. Therefore, we could not identi-
fied the intermediates C, D, and others in the reaction mixture even when
Ph2PH and DMAD were reacted in a 1:2 ratio.
Supporting Information Available: Experimental details and
1
characterization data for 2a-2c; H, 13C, 31P NMR and 2D
NMR spectra of 2a-2c; CIF file of 2a; and computational
details of 2d. This material is available free of charge via the
Internet at http://pubs.acs.org.
(12) Shaw, M. A.; Tebby, J. C. J. Chem. Soc. C 1970, 5–9.
J. Org. Chem. Vol. 75, No. 11, 2010 3877