CL-141027
Received: November 11, 2014 | Accepted: December 19, 2014 | Web Released: December 27, 2014
Intramolecular Stabilization of the Phosphine Radical Cation by the Second Phosphorus Atom
31
during the Photooxidation of Diphosphines: P NMR Spectroscopic Analysis
Shinro Yasui*1 and Shoko Yamazaki2
1Faculty of Contemporary Human Life Science, Tezukayama University, Gakuen-Minami, Nara 631-8585
2Department of Chemistry, Nara University of Education, Takabatake-cho, Nara 630-8528
(E-mail: yasui@tezukayama-u.ac.jp)
Ph
Diphosphines, Ph2P(CH2)nPPh2 1 (n = 1, 2, 3, 4, and 6),
were photolyzed by a xenon lamp in air. The 31P NMR
spectroscopic analysis of the reaction showed that 1 is oxidized,
according to first-order kinetics, to the monoxide, which is
further oxidized to the dioxide. The dependence of the rate
constants for the first oxidation on the chain-length n in 1 is
interpreted in terms of the orientation of the p-orbitals on the two
phosphorus atoms in the intermediate, the diphosphine radical
cation.
Ph
Ph
Ph
Ph
Ph
Ph
Xe-lamp (>310 nm)
CDCl3 / air
P CH P Ph
P CH
2 n
P
2 n
O
2
1a; n = 1
1b; n = 2
1c; n = 3
1d; n = 4
1e; n = 6
Ph
P Ph
Ph
Ph P CH
2 n
O
O
3
Scheme 1. Photoreaction of diphosphine 1.
The trivalent phosphorus radical cation, Z3P•+, which is
readily generated upon treatment of trivalent phosphorus
compounds, Z3P, with electron-deficient compounds, behaves
as either a radical or a cation.1 Sometimes, the parent Z3P acts as
a nucleophile to promote the cationic character of Z3P•+. For
example, electron impact on triarylphosphines, Ar3P, during ion
cyclotron resonance mass spectrometry (ICR-MS) affords the
dimeric radical cation, [Ar3P-PAr3]•+,2 which certainly results
from the nucleophilic attack of the parent phosphine, Ar3P, on
the initially generated radical cation, Ar3P•+. This occurs
because Ar3P exists in large excess under the stated conditions,
but even a small amount of Ar3P may assist in the formation of
Ar3P•+ through a nucleophilic interaction. To disclose how a
neutral phosphine interacts with the phosphine radical cation,
diphosphine, which has two phosphine moieties in a single
molecule, may be useful. Thus, the radical cation generated on a
phosphine moiety could be intramolecularly stabilized by the
other phosphine moiety. The difference in the structure of the
spacer between the two phosphine moieties would result in the
difference in stabilization due to the different spatial arrange-
ments of the two phosphorus atoms.
We have found that the steady-state photolysis of triaryl-
phosphines, Ar3P, under aerobic conditions generates the radical
cation, Ar3P•+, very likely through electron transfer from Ar3P in
the excited state to oxygen.3,4 The radical cation is then trapped
by oxygen to eventually form the phosphine oxide, Ar3P=O.
In this system, no sensitizer is required and no significant by-
products are produced. Based on these features, this system is
a convenient way to generate Ar3P•+. In the present study, we
carried out the steady-state photolysis of bis(diphenylphos-
phino)alkanes, Ph2P(CH2)nPPh2 1 (n = 1, 2, 3, 4, and 6), as
listed in Scheme 1, using a xenon lamp in air. By analyzing the
reaction mixture periodically, by 31P NMR spectroscopy, it was
found that 1 disappears according to first-order kinetics with the
appearance of diphosphine monoxide 2 as well as diphosphine
dioxide 3. The first-order rate constants of the disappearance of
1 were variable, depending on the length of the methylene-chain
spacer. This observation is interpreted in terms of the difference
in the geometry of the phosphine radical cation intermediate.
The diphosphines 1 were purchased (Aldrich) and purified
by recrystallization from ethanol, except for 1c. Recrystalliza-
tion of 1c was not feasible due to its rather low melting point
(mp: 63-65 °C) compared to those of the others (ranging from
118 to 142 °C). The 1c sample was used without purification
because its solution gave no appreciable signal other than that
from 1c on the 31P NMR spectrum. The deuterochloroform
(CDCl3) solution of 1 (20 mM) was prepared in a quartz NMR
tube. The solution was irradiated with light from a xenon lamp
(an Ushio xenon short arc UXL-500D-0 lamp), in which light
of wavelengths shorter than 310 nm was cutoff by a glass filter
(a Toshiba sharp-cut glass filter UV-31). The solution was
periodically analyzed by 31P NMR spectroscopy using a Varian
INOVA400 operating at 161.9 MHz. The external 85% phospho-
ric acid was used as the reference standard (0.0 ppm). The
intensity of each signal was quantitatively evaluated using
triethyl phosphate as the internal standard.5
The time-course of the spectral change for the photooxida-
tion of 1d, as a representative example, is shown in Figure 1. The
signal (¹14.2 ppm) from the phosphorus of the starting diphos-
phine 1d gradually disappeared with the concomitant appearance
of two new signals. One of them (¹14.2 ppm) resonated at only
a slightly higher field than the signal of 1d, and another appeared
at a low field (34.1 ppm).6 Consulting the literature data,7 these
signals are judged to result from diphosphine monooxide 2d.
These signals disappeared, being accompanied by the appearance
of another new signal at a low field (34.1 ppm), which is
assignable to diphosphine dioxide 3d.7c These observed spectrum
changes correspond to the consecutive oxidation of two phos-
phorus atoms in the starting material; namely, the oxidation of 1
to 2 and further to 3 (Scheme 1).8 Several unidentified signals
also appeared in the 31P NMR spectra, e.g., the signals at 23.5 and
43.4 ppm during the photolysis of 1b. A possibility is the ab-
straction of chlorine atom(s) from the solvent CDCl3 by a radical
intermediate to afford the chlorinated products. We have not
tried to identify these products at this stage of the present study.
Table 1 summarizes the chemical shifts ¤ of 1-3. The
downfield ¤ shifts of 1 were observed as the reaction proceeded.
© 2015 The Chemical Society of Japan