W. Levason et al. / Journal of Organometallic Chemistry 688 (2003) 280Á
/282
281
sample (b) the initial free diphosphine resonance de-
creased over time and a resonance at d ꢁ31 grew in.
This is assigned to o-C H (P(O)Ph ) [5] which, after 1
3. Experimental
/
Dichloromethane was dried by distillation from CaH
under nitrogen, and the phosphine ligands dried in
vacuo. The SnI (BDH), Ph P, Ph PO, Ph As and
Ph AsO (Aldrich) were used as received. The dipho-
sphines Ph PCH CH PPh , o-C (PPh and their
dioxides were made by literature methods [5,9,10].
6
4
2 2
2
day, was the only phosphorus species present. The
reaction of o-C H (PPh ) with dry air in CH Cl in
6
4
2 2
2
2
4
3
3
3
the presence of a small crystal of SnI4 also gave
complete conversion to o-C H (P(O)Ph ) in 2d.
3
H
)
2 2
6
4
2 2
2
2
2
2
6
4
3
1
Dichloromethane solutions of Ph P [d( P) ꢀ
/
6], and
13] containing a small
crystal of SnI were similarly cleanly converted into
3
1
8
3
1
O2 was obtained from BOC. All reactions were
conducted in flame-dried apparatus under nitrogen or
in a glove box unless indicated otherwise.
Ph PCH CH PPh [d( P) ꢀ
/
2
2
2
2
4
3
1
ꢀ1
Ph PO [d( P)
3
ꢁ
/
26, n(PO)ꢂ
Ph P(O)CH CH P(O)Ph [d( P)ꢁ36, n(PO)ꢂ
/
1195 cm
]
and
1177
3
1
/
/
2
ꢀ
2
2
2
1
3.1. NMR experiments
cm ], over a few days, the products being identified
3
by comparison of their P{ H}-NMR and IR spectra
1
1
3
1
1
(General procedure) A 10 mm OD NMR tube was
charged with the phosphine (0.1 g), dry CH Cl (4 ml)
with those of genuine samples. The P{ H}-NMR
spectra showed no other phosphorus-containing pro-
ducts were formed. The reaction can be converted into a
synthesis for the phosphine oxides using either dry air or
dry dioxygen. However, in marked contrast, a CH Cl
2
2
and an appropriate amount of SnI (see text). The tube
4
3
1
1
was purged with dry nitrogen, sealed, and the P{ H}-
NMR spectrum recorded immediately, and at set
intervals over several days. In other experiments the
nitrogen was replaced by dry air or dioxygen.
2
2
4
solution of Ph As containing a small crystal of SnI
3
showed no reaction after 1 week, the IR spectrum of the
recovered solid showing only Ph As and no evidence for
3
3.2. Preparations
Ph AsO.
3
The mechanism of the oxidation reaction is not clear.
t
Very basic phosphines such as Bu P react [6] with SnCl
3
(General procedure) A three necked 250 cm quickfit
3
4
flask fitted with two Young’s taps (gas inlet and outlet),
a Suba-seal and a teflon coated stirrer bar, was purged
with dry nitrogen. It was then charged with dry CH Cl
or GeCl
t
to form chlorophosphonium salts,
ꢀ
4
ꢁ
[
Bu PCl] [Sn(Ge)Cl ] , which could hydrolyse to
3 3
2
2
phosphine oxides, but similar chemistry has not been
observed with arylphosphines. Moreover, the reactions
described here occur under anhydrous conditions. In
(50 ml) containing the phosphine ligand (2 mmol) and a
crystal of SnI and stirred. The gas atmosphere was
4
replaced by dry dioxygen by flushing. Experiments in
which dry air was used as oxidant were performed by
replacing the suba-seal with a drying tube packed with
order to provide further insight, the reaction with Ph P
3
1
8
was conducted using O , and the product was shown
2
1
8
to be exclusively Ph P O by IR spectroscopy [n(PO)ꢂ
/
3
anhydrous CaCl . After the reaction was judged com-
2
ꢀ
1
157 cm ]. The simple diatomic oscillator model
1
predicts n(P O) as 1148 cm
plete, the solvent was removed on a vacuum line. The
residue was treated with aqueous brine to destroy the tin
1
8
ꢀ1
and coupling with the
appropriate n(PÃC) at lower frequency accounts for the
/
iodide, and extracted with CH Cl (3ꢃ50 ml), the
/
2
2
slightly higher observed value. The EI mass spectrum
shows a base peak at m/z 279 which corresponds to
organic extracts dried over MgSO , the solvent was
then removed, and the residue recrystallised from EtOH.
The yield was essentially quantitative.
4
1
8
16
[
Ph P O Ã
/
H]; the base peak in Ph P O is at m/z 277
3
3
1
6
corresponding to [Ph P O Ã
/
H] [7]. This establishes air/
3
dioxygen rather than water as the source of the oxygen
in the phosphine oxides, but does not clarify the
mechanism any further. It seems likely that the weak
Acknowledgements
Lewis acidity of the SnI promotes the oxidation, whilst
4
18
We thank Dr. J.S. Ogden for assistance with the O2
experiments.
the extensive dissociation of the tin iodide adducts
formed allows the reaction to cycle, making the use of
catalytic amounts of SnI possible. For other tin(IV)
4
halides, the oxidation occurs, but as stronger Lewis
acids these tend to precipitate the complexed oxidised
ligand, hence removing the tin from the reaction. The
reaction offers a convenient alternative route to the
synthesis of arylphosphine oxides from the correspond-
ing phosphines. It avoids the potential hazards of using
References
[1] T.S. Lobana,F.R. Hartley (Eds.),The Chemistry of Organopho-
sphorus Compounds, vol. 2 (Chapter 8), Wiley, NY, 1992.
[
2] A.K. Bhattacharya, N.K. Roy,F.R. Hartley (Eds.), The Chem-
istry of Organophosphorus Compounds, vol. 2 (Chapter 6),
Wiley, NY, 1992.
H O [8] and is cleaner than the halogen oxidation
2
2
[3] J. Halpern, B.L. Goodall, G.P. Khare, H.S. Lim, J.J. Plath,J. Am.
Chem. Soc., vol. 97.
followed by hydrolysis route [5].
Levason, William
