Studies on stable 1,3-diphosphacyclobutane-2,4-diyls

Article abstract of DOI:10.1080/10426500701734588

Various kinds of stable 1,3-diphosphacyclobutane-2,4-diyls, sterically protected by bulky Mes* (2,4,6-tri-t-butylphenyl) groups, were prepared from Mes*-phosphaethyne. The structures and physical and chemical characters were studied. They are discussed in

Full text of DOI:10.1080/10426500701734588

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Phosphorus, Sulfur, and Silicon
and the Related Elements
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Studies on Stable 1,3-
Diphosphacyclobutane-2,4-
diyls
Masaaki Yoshifuji a , Anthony J. Arduengo III a &
Shigekazu Ito b
a Department of Chemistry , The University of
Alabama , Tuscaloosa, AL, USA
b Department of Chemistry, Graduate School of
Science , Tohoku University , Aoba, Sendai, Japan
Published online: 20 Jun 2008.
To cite this article: Masaaki Yoshifuji , Anthony J. Arduengo III & Shigekazu Ito (2008)
Studies on Stable 1,3-Diphosphacyclobutane-2,4-diyls, Phosphorus, Sulfur, and Silicon
and the Related Elements, 183:2-3, 335-339, DOI: 10.1080/10426500701734588
To link to this article: http://dx.doi.org/10.1080/10426500701734588
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Phosphorus, Sulfur, and Silicon, 183:335–339, 2008
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500701734588
Studies on Stable 1,3-Diphosphacyclobutane-2,4-diyls
Masaaki Yoshifuji,1 Anthony J. Arduengo, III,1
and Shigekazu Ito2
1Department of Chemistry, The University of Alabama, Tuscaloosa,
AL, USA
2Department of Chemistry, Graduate School of Science,
Tohoku University, Aoba, Sendai, Japan
Various kinds of stable 1,3-diphosphacyclobutane-2,4-diyls, sterically protected
by bulky Mes* (2,4,6-tri-t-butylphenyl) groups, were prepared from Mes*-
phosphaethyne. The structures and physical and chemical characters were stud-
ied. They are discussed in terms of ground-state singlet biradicals and have been
confirmed by spectroscopic analyses, X-ray crystallography and theoretical calcula-
tions. EPR studies were also carried out to validate the paramagnetic species related
to the 1,3-diphosphacyclobutane-2,4-diyls. The reactions of the biradical species are
described including oxidation and sulfurization.
Keywords Biradical; radicals; phosphorus heterocycles; steric protection
INTRODUCTION
Radicals are highly reactive but can be stabilized by bulky sub-
stituents and/or by electronic factors. Biradicals are expected
more reactive and are considered to be much more difficult to
isolate. By use of a bulky Mes* group (2,4,6-tri-t-butylphenyl),
Received April 15, 2007; accepted September 17, 2007.
This work was supported in part by Grants-in-Aid for Scientific Research (No.
13304049 and 14044012) from the Ministry of Education, Culture, Sports, Science and
Technology, Japan; the National Science Foundation (CHE-0413521, CHE-0342921 and
CHE-0079498), U.S.A.; U.S. Department of Energy Grant (DE-FG02-86ER13465); and
the Department of Energy, Office of Energy Efficiency and Renewable Energy under the
Hydrogen Storage Grand Challenge (DE-PS36-03GO93013).
The authors thank Dr. Hiroki Sugiyama and Mr. Manabu Kikuchi at To-
hoku University for the synthetic and analytical studies, as well as Prof. Low-
ell D. Kispert and Dr. Tatyana Konovalova at the University of Alabama for EPR
studies.
Address correspondence to Masaaki Yoshifuji, Department of Chemistry, The Univer-
sity of Alabama, L243C Shelby Hall, Box 870336, Tuscaloosa, AL 35487, USA. E-mail:
myoshifuji@bama.ua.edu
335

336
M. Yoshifuji et al.
FIGURE 1 Cyclic voltammograms of 8. (a): Oxidation waves. (b) Reduc-
tion waves. Conditions: 1 mM in dichloromethane; supporting electrolyte:
0.1 M tetrabutylammonium perchlorate (TBAP); working electrode: glassy car-
bon; counter electrode: platinum wire; reference electrode: Ag/AgCl (E1/2 (fer-
rocene/ferricinium) = +0.60 V) at 20◦C; scan rate: 50 mV s−1
.
however, unusually stable biradicals, 2,4-bis(2,4,6-tri-t-butylphenyl)-
1,3-diphosphacyclobutane-2,4-diyls can be isolated and characterized.
RESULTS
Reactions of Mes*-Phosphaethyne and Isolation
of Stable Biradicals
As depicted in Scheme 1, asterically protected phosphaalkyne 1 reacted
with 0.5 equiv of t-butyllithium to give a 4-membered ring anion 3,
SCHEME 1

1,3-Diphosphacyclobutane-2,4-diyls
337
SCHEME 2
via intermediate 2. When 3, which was characterized by 31P-NMR
spectroscopy (δP = 268.5, 87.6; 2 J = 86.8 Hz), was quenched with
PP
methanol as a proton source, 1,3-diphosphacyclobutene 4 (δP = 308.8,
89.2; 2 J = 94.1 Hz)1 was obtained and its structure was unambigu-
PP
ously confirmed by X-ray crystallography.2
When anion 3 was quenched with methyl iodide, a stable dark blue
biradical species 5 was obtained (δP = 55.9, –11.3; 2 J = 362.8 Hz).3
PP
The structure of 5 was determined by X-ray analysis indicating that
two Mes* groups efficiently protect two carbon atoms. Compound 5,
1,3-diphosphacyclobutane-2,4-diyl, is a member of a new class of stable
biradicals, on which Niecke 4 and Bertrand5 recently reported.
Changing the identities of the nucleophile (from t–BuLi) and the
electrophile (from MeI), various derivatized biradicals 7 (X = Me, n-
Bu, t-Bu, Ph, and i-Pr2N; Y = Me, Et, n-Bu, t-Bu, PhCH2, n-C18H37,
n-C3F7, PhC(O), MeOCH2, and MeOCOCH2) were formed via anions 6
(Scheme).6−8 NMR, UV, and CV data for 7 were studied and several of
these biradicals were characterized by X-ray crystallography.
Oxidation of 3 with 0.5 equiv of iodine resulted in the formation of a
pink neutral radical 8. Radical 8 is a stable crystalline compound and
its structure was analyzed by X-ray diffraction and EPR spectroscopy.9
Electrochemistry of 8 was examined and from the voltammograms,
the compound showed three oxidation waves and one reduction wave
(Figure 1). It is plausible that the first (lowest potential) oxidation oc-
curs at the radical center and the following two waves correspond to the
oxidation of phosphorus lone pairs. A relatively low reduction potential
SCHEME 3

338
M. Yoshifuji et al.
SCHEME 4
indicates that the radical can be easily reduced to its radical anion.
These phenomena are supported by calculations of a model compound.
Furthermore, 5 reacts with tris(4-bromophenyl)ammoniumyl hex-
achloroantimonate as an oxidant to give the corresponding cation rad-
ical 9,10 the structure of which was established by analysis of its EPR
spectrum (Scheme 3).
Oxidation and Sulfurization of a Biradical
A biradical 10 (= 7, where X = i-Pr2N, Y = Me) was allowed to react with
TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) on photo-irradiation to
give the corresponding oxide 11 (δP = 121.8, 25.4; 2 J = 183.2 Hz),6
PP
and the reaction with elemental sulfur gave the corresponding sulfide
12 (δP = 101.7, 45.7; 2 J = 194.2 Hz).8 Other chemical studies of 7 are
PP
in progress (Scheme 4).
DISCUSSION
Structures of Biradicals and Radicals
From the X-ray analysis of biradical species 5 and 7, two carbon atoms in
the 4-membered ring have sp2 configurations, while the two phosphorus
atoms exhibit sp3 configurations. In addition, the interatomic distance
˚
between the two carbon centers (2.50 A) is too long to suggest a high
degree of direct interaction between them. These considerations lead
us to the conclusion that the structures of 5 and 7 should be drawn as
biradicals as depicted in Schemes 1 and 2 .
The EPR studies of neutral radical 8 and cation radical 9 reveal the
spin densities are localized on the carbons in the 4-membered ring show-
ing spectra of typical carbon centered radicals. Computational model-
ing also explicates the hyper-fine coupling constants. Some important
resonance contributors of 8 are shown in Scheme 5.9

1,3-Diphosphacyclobutane-2,4-diyls
339
SCHEME 5
REFERENCES AND NOTES
[1] S. Ito, H. Sugiyama, and M. Yoshifuji, Chem. Commun., 1744 (2002).
[2] The structure of 4 was misreported as a linear molecule. A.M. Arif, A.R. Barron,
A.H. Cowley and S.W. Hall, J. Chem. Soc.,Chem. Commun., 171 (1988).
[3] H. Sugiyama, S. Ito, and M. Yoshifuji, Angew. Chem. Int. Ed., 42, 3802 (2003).
[4] E. Niecke, A. Fuchs, F. Baumeister, M. Nieger, and W.W. Schoeller, Angew. Chem.,
Int. Ed. Engl., 34, 555 (1995).
[5] D. Scheschkewitz, H. Amii, G. Gornitzka, W.W. Schoeller, D. Bourissou, and G.
Bertrand, Science, 295, 1880 (2002).
[6] H. Sugiyama, S. Ito, and M. Yoshifuji, Chem. Eur. J., 10, 2700 (2004).
[7] M. Yoshifuji, H. Sugiyama, and S. Ito, J. Organomet. Chem., 690, 2515 (2005).
[8] S. Ito, M. Kikuchi, H. Sugiyama, and M. Yoshifuji, J. Organomet. Chem. 692, 2761
(2007). DOI: 10.1016/j.jorganchem.2006.10.066.
[9] S. Ito, M. Kikuchi, M. Yoshifuji, A. J. Arduengo III, T .A. Konovalova, and L. D.
Kispert, Angew. Chem. Int. Ed., 45, 4341 (2006).
[10] M. Yoshifuji, A. J. Arduengo III, T. A. Konovalova, L. D. Kispert, M. Kikuchi, and S.
Ito, Chem. Lett., 35, 1136 (2006).