A novel pattern of dimerization for 1,4-diphospha-1,3-butadienes

Article abstract of DOI:10.1002/hc.10123

A pattern for the dimerization of 1,4-diphospha-1,3-butadienes was discussed. The molecular structures of the monomer and the dimer were determined by x-ray analysis. The bond length of the 1,4-diphospha-1,3-butadiene moiety of 1,4-bis(2,4,6-tri-tert-butylphenyl)-1,4-diphospha-1,3-butadiene contained the essential information about conjugation and the participation for the phosphorus atoms in the process.

Full text of DOI:10.1002/hc.10123

Heteroatom Chemistry
Volume 14, Number 3, 2003
A Novel Pattern of Dimerization
for 1,4-Diphospha-1,3-butadienes
Alex S. Ionkin and William J. Marshall
DuPont Central Research & Development, Experimental Station, Wilmington,
Delaware, 19880-0328
Received 22 July 2002; revised 23 September 2002
1-cyclobutene ligands [11]. Two examples of
ABSTRACT: A new example of [2+4] cycloaddition
of the low-coordinated diphospha-analog of butadi-
ene protected with a 2,4-di-tert-butyl-6-methylphenyl
group has been described. X-ray structure investiga-
tions of monomer 4b and dimer 5a have been re-
1,4-diphosha-1,3-butadienes with more flexible
backbones were described [12,13]. 2,3-Alkoxy-
substituted 1,4-diphospha-1,3-butadienes undergo
[2+2]-cycloaddition, leading to correspondingly
substituted 3,4-diphosphacyclobutenes [13], unless
sterically protected by a super-mesityl group to
retard the self-dimerization by a [2+2]-motive [12].
The 2,4,6-tri-tert-butylphenyl group (super-mesityl)
has been shown to be one of the best in supporting
low coordination in phosphorus chemistry. However,
it seems to be too sterically excessive in providing
polymers with high molecular weight when com-
pared with the α-diimines [7,8]. In this report, we de-
scribe a novel pattern of dimerization of moderately
carbo-substituted 1,4-diphospha-1,3-butadienes.
ꢀ
C
ported for the first time.
2003 Wiley Periodicals, Inc.
Heteroatom Chem 14:231–234, 2003; Published online
in Wiley InterScience (www.interscience.wiley.com). DOI
10.1002/hc.10123
INTRODUCTION
The P C ꢀ-bond in phosphaalkenes is practically
apolar [1–3]. As a matter of fact the polarity of this
bond could be reversed by dialkylamino-substituents
on the carbon side [4] .The regioselectivity by the
addition of a polar reagent at the P C ꢀ-bond can
also be controlled by the type of substituents at the
bond. Examples of proton addition to the different
sides of P C bonds, as well as hydride addition to
the different sides upon variation of the substituent,
are known [5,6]. This kind of tunability is of great
interest in catalytic chemistry, where retaining the
same metal with a variation of sterical or electronic
factors around the ligand could result in the appear-
ance of new technologies [7]. The phosphaalkenes
as a part of the chelating ligands have been in use for
ethylene polymerization [8], hydroamination [9],
and silylation [10]. The above-mentioned catalysts
were based on the rigid 3,4-bis(phosphamethylene)-
RESULTS AND DISCUSSION
R. Appel’s scheme [12] was used for generating
1,4-diphospha-1,3-butadienes. The 2,4-di-tert-butyl-
6-methylphenyl group, which allows stabilization
of diphosphenes [14], phosphaalkenes [15], dithio-
phosphorane [16], and arsaalkenes [17], as well
as the 1,4-diphospha-1,3-butadienes with a rigid
cyclobutene backbone [18], was employed as a pro-
tection group. 2,4-Di-tert-butyl-6-methylphenyllithi-
um (1a) was reacted with 1,2-bis(dichlorophos-
phino)ethane (2) in a molar ratio of 2:1, followed
by the addition of 1,5-diazabicyclo[4.3.0]non-5-ene
(DBN) to synthesize 1,4-diphospha-1,3-butadienes
4a,b (Scheme 1). The intermediate 1,2-bis(2,4-di-
tert-butyl-6-methylphenylchlorophosphino)ethane
(3a) has two chemical shifts in the 31P NMR at
δ = 95.09 and 94.71 ppm, because of the existence
of two isomers. The 1,4-diphospha-1,3-butadiene 4a
Correspondence to: Alex S. Ionkin; e-mail: alex.s.ionkin@usa.
dupont.com.
Contract grant sponsor: DuPont.
2003 Wiley Periodicals, Inc.
c
ꢀ
231

232 Ionkin and Marshall
group enhanced the dienophile properties of the
1,4-diphospha-1,3-butadiene 4a as well as made the
phosphaalkene part of 4a more accessible. The com-
pound 4a does not contain any substituents with a
lone pair of electrons capable of altering the polar-
ity of P C bond. This could also contribute to the
[2+4] cyclization pattern of the dimerization of 1,4-
diphospha-1,3-butadiene.
Although 1,4-diphospha-1,3-butadiene 4b has
been prepared [12], its X-ray structure has not been
reported yet. The bond lengths of the 1,4-diphospha-
1,3-butadiene moiety of 4b contain the essential in-
formation about conjugation and the participation
of the phosphorus atoms in the process. The small
crystals of 4b, which were suitable for X-ray analy-
sis using synchrotron radiation, were grown from a
toluene solution. The structure is shown in Fig. 2.
For example, the bond length between C(1) and
C(1a) is 1.419(6) A, which is shorter than 1.49(2) A
as reported for the 1,4-diphospha-1,3-butadiene with
phenylthio-substituents in the 2,3-positions [20].
This could indicate a stronger conjugation in 2,3-
unsubstituted 1,4-diphospha-1,3-butadienes.
In conclusion, we successfully demonstrated a
novel pattern of dimerization for 1,4-diphospha-1,3-
butadienes without rigid backbones. The molecular
structures of the monomer 4b and the dimer 5a
were determined by X-ray analysis. Their catalytic
properties are now under investigation and will be
reported in due time.
SCHEME 1 Syntheses of 1,4-diphospha-1,3-butadienes
4a,b and the dimerization of 4a.
˚
˚
has been found to be unstable at room temperature
and it underwent [2+4] cycloaddition to form the
dimer 5a. Elemental analysis, NMR spectroscopy,
and X-ray analysis (Fig. 1) proved the structure of
5a. For example, there is a downfield signal in the
13C NMR spectrum at δ = 173.40 ppm (doublet,
1 JPC = 40.1 Hz, phosphaalkene sp2-carbon).
The Diels–Alder reactions of 4b with only norbor-
nadienes were published [19]. In our case, reducing
the sterical bulk near the phosphorus atom by re-
placing one ortho-tert-butyl group with a methyl
CCDC contains the supplementary crystallo-
graphic data for this paper. These data can
be obtained free of charge via www.ccdc.cam.
ac.uk/conts/retrieving.html (or from the Cambridge
Crystallographic Data Centre, 12, Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033;
e-mail: deposit@ccdc.cam.ac.uk). The deposition
numbers are CCDC 192384 & 192385 for 4b and 5a,
respectively.
EXPERIMENTAL
1,4-Bis(2,4,6-tri-tert-butylphenyl)-1,4-
diphospha-1,3-butadiene (4b)
The compound 4b was prepared by the dehydrohalo-
genation procedure with DBN (1,5-diazabicyclo-
[4.3.0]non-5-ene) as described [12]. 1,2-Bis(2,4,6-
tri-tert-butylphenylchlorophosphano)ethane (4.8 g,
0.0074 mol) was used for the preparation of 4b. The
yield of 4b as a yellow solid was 1.49 g (35%), with
mp 261.4◦C. 1H, 13C, and 31P NMR data were practi-
cally identical with the reported values; for example,
31P (500 MHz, CDCl3) δ = 270.91 against 269.5 ppm
for the major isomer.
FIGURE
1
ORTEP
drawing
of
the
tetrahydro-
[1,2,4]triphosphinine 5a. [Disordered positions and most
hydrogen atoms were omitted for clarity. Thermal ellipsoids
are drawn to the 50% probability level.]

A Novel Pattern of Dimerization for 1,4-Diphospha-1,3-butadienes 233
FIGURE 2 ORTEP drawing of the 1,4-diphospha-1,3-butadiene 4b. [Thermal ellipsoids are drawn to the 50% probability level.]
merce and the Board of Higher Education Grant
IBHE HECA NWU 96. Use of the Advanced Pho-
ton Source was supported by the U.S. Department
of Energy, Basic Energy Sciences, Office of Energy
Research under Contract No. W-31-102-Eng-38.
The authors thank Zdzislaw Wawrzak of DND-
CAT for expert assistance with synchrotron experi-
ments.
1,2,4-Tris(2,4-di-tert-butyl-6-methyl-phenyl)-3-
(2,4-di-tert-butyl-6-methyl-phenylphosphany-
lidenemethyl)-1,2,3,4- tetrahydro-
[1,2,4]triphosphinine (5a)
The compound 5a was prepared by the same
dehydrohalogenation procedure with DBN (1,5-
diazabicyclo[4.3.0]non-5-ene) as described [12].
2-Bromo-1,5-di-tert-butyl-3-methyl-benzene (5.37 g,
0.019
mol)
and
1,2-bis(dichlorophosphino)-
methane (2.0 g, 0.0086 mol) were used for the
REFERENCES
preparation of 5a. The yield of 5a as slightly yellow
1
solid was 1.15 g (27%), with mp 124.8◦C. H NMR
[1] Dillon, K. D.; Mathey, F.; Nixon, J. F. Phosphorus: The
Carbon Copy; Wiley: Chichester, 1998; pp. 1–14.
[2] Quin, L. D. A Guide to Organophosphorus Chemistry;
Wiley: New York, 2000; pp. 307–345.
[3] Appel, R. In Multiple Bonds and Low Coordina-
tion in Phosphorus Chemistry; Regitz, M.; Scherer,
O. J. (Eds.); Georg Thieme Verlag: Stuttgart, 1990;
pp. 157–219.
[4] Patsanovskii, I. I.; Ishmaeva, E. A. Zh Obshch Khim
1993, 63, 2552–2568.
[5] Meriem, A.; Majoral, J.-P.; Revel, M.; Navech, J Tetra-
hedron Lett 1983, 24, 1975–1978.
(CD2Cl2) δ = 1.06 (s, broad, 36H, t-Bu), 1.62 (s,
broad, 36H, t-Bu), 2.20 (s, 3H, Me), 2.25 (s, 3H,
Me), 2.30 (s, 3H, Me), 2.45 (s, 3H, Me), 6.1–7.95
(m, 11H, aromatic and C H protons). 31P NMR
1
3
(CDCl3) δ = −34.74 (dd, J(PP) = 197.5 Hz, J(PP)
= 37.3 Hz; (P-1)), δ = −16.97 (d,d,d, 1 J(PP) =
197.5 Hz, 2 J(PP) = 41.4 Hz, 3 J(PP) = 34.6 Hz, (P-2)),
2
3
δ = −9.04 (d,d J(PP) = 41.4 Hz, J(PP) = 37.3 Hz,
(P-3)), δ = 256.16 (d, 3 J(PP) = 34.6 Hz, (P C)). Anal.
Calcd (%) for C64H96P4: C, 77.70; H, 9.78; P, 12.52.
Found: C, 77.51; H, 9.51; P, 12.38.
[6] Ermolaeva, L. V.; Ionkin, A. S. THEOCHEM 1992, 95,
25–33.
[7] Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem Rev
2000, 100, 169–1203.
[8] Ikeda, S.; Ohhate. F.; Miyoshi, M.; Tanaka, R.;
Minami, T.; Ozawa, F.; Yoshifuji, M. Angew Chem,
Int Ed 2000, 39, 4512–4513.
[9] Minami, T.; Okamoto, H.; Ikeda, S.; Tanaka, R.;
Ozawa, F.; Yoshifuji, M. Angew Chem, Int Ed 2001,
40, 4501–4503.
[10] Ozawa, F.; Yamamoto, S.; Kawagishi, S.; Hiraoka, M.;
Ikeda, S.; Minami, T.; Ito, S.; Yoshifuji, M. Chem Lett
2001, 10, 972–973.
[11] Appel, R.; Winkhaus, V.; Knoch, F. Chem Ber 1987,
120, 243.
ACKNOWLEDGMENT
Portions of this work were performed at the
DuPont-Northwestern Dow Collaborative Access
Team (DND-CAT) Synchrotron Research Center lo-
cated at Sector 5 of the Advanced Photon Source
at Argonne National Laboratory. DND-CAT is sup-
ported by the E.I. DuPont de Nemours & Co., the
Dow Chemical Company, the U.S. National Science
Foundation through Grant DMR-9304725 and the
State of Illinois through the Department of Com-
[12] Appel, R.; Hu¨nerbein, J.; Siabalis, N. Angew Chem,
Int Ed 1987, 26, 779–780.

234 Ionkin and Marshall
[13] Appel, R.; Barth, V.; Knoch, F. Chem Ber 1983, 116,
938–950.
[14] Yoshifuji, M.; Shibayama, K.; Inamoto, N.;
Matsushita, T.; Nishimoto, K. J Am Chem Soc
1983, 105, 2495–2497.
[15] Baudler, M.; Joachim, S. Chem Ber 1988, 121, 281–
285.
Akad Nauk SSSR, Ser Khim 1991, 7, 1654–1656.
[17] Ionkin, A. S.; Nikolaeva, N. V.; Musin, R. Z.; Efremov,
Y. Y.; Arbuzov, B. A. Phosphorus, Sulfur Silicon Relat
Elem 1992, 66, 257–260.
[18] Yoshifuji, M.; Ichikawa, Y.; Toyota, K.; Kasashima, E.;
Okamoto, Y. Chem Lett 1997, 1, 87–88.
[19] Siabalis, N. Dissertation, Bonn, 1989.
[16] Ionkin, A. S.; Nekhoroshkov, V. M.; Efremov, Y. Y. Izv
[20] Ito, S.; Yoshifuji, M. Chem Lett 1998, 5, 651–652.