Russian Journal of General Chemistry, Vol. 73, No. 5, 2003, pp. 825 826. Translated from Zhurnal Obshchei Khimii, Vol. 73, No. 5, 2003,
pp. 871 872.
Original Russian Text Copyright
2003 by Borovinskii, Krivchun, Ionin.
LETTERS
TO THE EDITOR
Photochemical Phosphination of Bromotoluenes
K. I. Borovinskii, M. N. Krivchun, and B. I. Ionin
St. Petersburg State Institute of Technology, St. Petersburg, Russia
Received January 29, 2003
Side-chain radical halogenation of alkylarenes is a
known reaction. The mechanism of this reaction has
been firmly established, and the reaction has been
widely used in organic synthesis [1]. Photochemical
phosphination with phosphorus trihalides has been
found to involve initial photoinitiated dissociation of
the PHlg3 molecule [2]. As a result, a halogen radical
is formed, which makes possible a reaction similar to
halogenation. According to [3], photoinitiated reaction
of phosphorus tribromide with various alkylarenes
leads mainly to benzyldibromophosphines.
24 g, was added dropwise with stirring at 45 C over
the course of 1 h under UV irradiation with a low-
pressure immersion mercury lamp to 329.7 g of PCl3.
The mixture was irradiated for 4 h, after which un-
reacted PCl3 and 4-bromotoluene were distilled off,
and the residue was distilled in vacuo to obtain 4.91 g
of a fraction boiling at 80 110 C (1 mm). It contained,
according to 31P NMR data, dichloro(p-tolyl)phos-
phine (Ia) and ca. 1% of (4-bromobenzyl)dichloro-
phosphine (IIa). The yield of compound Ia was 4.5 g
1
(15% per bromotoluene ), bp 87 C (1 mm). H NMR
3
spectrum (CDCl3), , ppm: 7.83 t (2I, JHP 8 Hz),
We found that an analogous reaction of phosphorus
trichloride with bromtoluenes takes another pathway
involving preferential bromine substitution in the
aromatic ring to form corresponding dichloro(tolyl)-
phosphines.
7.34 d (2H), 2.46 s (3H). 13C NMR spectrum (CDCl3),
1
C, ppm: 142.01 s (Cp), 136.81 d (Ci, JCP 52.84 Hz),
2
3
131.09 d (Co, JCP 28.69 Hz), 129.21 d (Ni, JCP
9.56 Hz), 21.27 s (CH3). 31P NMR spectrum (CDCl3),
P, ppm: 161.87 s.
The reaction was conducted using an immersion
mercury lamp. The total yield of phosphorus-contain-
ing products was up to 16%. They comprised a major
product formed by bromine substitution in the ring
along with, probably, isomeric dichloro(tolyl)phos-
phines (compounds I), as well as minor products
formed by substitution of methyl hydrogen by a di-
chlorophosphino group (compounds II).
(4-Bromobenzyl)dichlorophosphine (IIa). 31P
NMR spectrum (CDCl3), , ppm: 177.15 t (2JHP
13.62 Hz).
Dichloro(o-tolyl)phosphine (Ib). Yield 5.2 g
1
(16% per bromotoluene), bp 80 C (1 mm). H NMR
spectrum (CDCl3), , ppm: 7.1 8.2 m (4H), 2.64 d
4
(3H, JHP 4.01 Hz). 13N NMR spectrum (CDCl3),
,
ppm: 137.25 d (Ci, 1JCP 52.71 Hz), 21.06 d (CH3, 3JCCP
5.4 Hz). 31P NMR spectrum (CDCl3), P, ppm:
163.86 s.
PCl2
CH3
CH2
CH3
h
(2-Bromobenzyl)dichlorophosphine (IIb). 31P
NMR spectrum (CDCl3), P, ppm: 181.19 t (2JHP
14.21 Hz).
+
+ PCl3
PCl2
R
R
I
II
2Dichloro(m-tolyl)phosphine (Ic). Yield 2.1 g
1
R = 4-Br (a), 2-Br (b), 3-Br (c). I : II = 19 : 1.
(9.7% per bromotoluene), bp 84 C (1 mm). H NMR
spectrum (CDCl3), , ppm: 7.2 7.8 m (4H), 2.45 s
The same reaction with bromobenzene failed.
(3H). 13C NMR spectrum (CDCl3), C, ppm: 139.70 d
1
3
(Ci, JCP 51.55 Hz), 138.38 d (C3, JCP 8.14 Hz),
In the absence of UV irradiation the reaction did
not occur. The structure of the resulting compounds
was confirmed by 1H, 13C, and 31P NMR spectroscopy.
2
133.23 d (C4), 130.06 d (C2, JCP 31.20 Hz), 128.43 d
3
2
(C5, JCP 8.14 Hz), 126.83 d (C6, JCP 31.20 Hz),
21.00 s (CH3). 31P NMR spectrum (CDCl3), P, ppm:
161.88 s.
Dichloro(p-tolyl)phosphine (Ia). 4-Bromotoluene,
1070-3632/03/7305-0825$25.00 2003 MAIK Nauka/Interperiodica
Borovinskii
