Preparation and reaction of sterically crowded N-(2,4-di-t-butylphenyl)-N-methylaminodichlorophosphine

Article abstract of DOI:10.1016/S0040-4039(01)02096-2

One of the o-t-butyl groups was eliminated from 2,4,6-tri-t-butyl-N-methylaniline by reaction with phosphorus trichloride in DME to afford sterically crowded N-(2,4-di-t-butylphenyl)-N-methylaminodichlorophosphine, which was utilized for preparation of a novel unsymmetrical diphosphene.

Full text of DOI:10.1016/S0040-4039(01)02096-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 119–121
Preparation and reaction of sterically crowded
N-(2,4-di-t-butylphenyl)-N-methylaminodichlorophosphine
Franc¸ois Rivie`re,† Shigekazu Ito and Masaaki Yoshifuji*
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578, Japan
Received 29 August 2001; revised 1 November 2001; accepted 2 November 2001
Abstract—One of the o-t-butyl groups was eliminated from 2,4,6-tri-t-butyl-N-methylaniline by reaction with phosphorus
trichloride in DME to afford sterically crowded N-(2,4-di-t-butylphenyl)-N-methylaminodichlorophosphine, which was utilized
for preparation of a novel unsymmetrical diphosphene. © 2001 Elsevier Science Ltd. All rights reserved.
Since the first example of kinetically stabilized dipho-
sphene 1 (Fig. 1) was reported,1 many efforts have been
made to synthesize low-coordinated phosphorus com-
pounds with bulky substituents.2–4 The 2,4,6-tri-t-
butylphenyl (hereafter abbreviated to the Mes*) group
is a most useful protecting substituent for various low-
coordinated organophosphorus compounds. On the
other hand, thermodynamic stabilization by electronic
effects has also been applied for stabilization of unusual
phosphorus compounds.2,3 In the last decade, we have
investigated such stabilizing groups as to have both
kinetic and thermodynamic stabilizing abilities, and
they have been utilized for stabilization of unusual
and/or unstable compounds bearing such as phospho-
rus–chalcogen double bonds.5,6 In the course of these
investigations, the Mes* framework has acted as a
prototype of novel protecting groups.7 The 2,4,6-tri-t-
butylphenoxy group is one substituent providing both
kinetic and thermodynamic stabilizations, and has been
utilized for diphosphene 28 (Fig. 1) as well as many
inorganic compounds as a catalyst.9 On the other hand,
the 2,4,6-tri-t-butylanilino moiety has hardly been uti-
lized for low-coordinated organophosphorus com-
pounds until now, although it has been applied for
some monomeric organogallium compounds.10 In this
paper we report preparation of a bulky amino-
dichlorophosphine starting from N-methyl-2,4,6-tri-t-
butylaniline, and its application for preparation of a
diphosphene. In the course of this study, we observed
an interesting elimination reaction of one of the o-t-
butyl groups from the Mes*N(Me) moiety.
N-Methyl-N-(2,4,6-tri-t-butylphenyl)aminodichloro-
phosphine (3) was selected as the target compound. At
first, N-methylaniline 4 (Mes*NHMe)11 was lithiated to
derive Mes*NMeLi, and was allowed to react with
PCl3. However, the desired compound 3 was not
formed and Mes*NHMe was recovered almost quanti-
tatively, while Mes*NHLi reacted with PCl3 to afford
N - (2,4,6 - tri - t - butylphenyl)aminodichlorophosphine
(Mes*NHPCl2).12 Steric bulkiness within the Mes*N-
(Me) moiety might weaken the nucleophilicity, and
indeed the reaction of Mes*NHMe with PCl3 did not
proceed in the presence of a base such as triethylamine
or potassium t-butoxide. Next, we investigated the
reaction of 4 with PCl3 in DME at 100°C as described
in Scheme 1, and observed an aminodichlorophosphine
in the reaction mixture by monitoring 31P NMR spec-
troscopy. The major product was not the target product
3, but N - (2,4 - di - t - butylphenyl) - N - methylaminodi-
chlorophosphine (5) was obtained in 80% yield.13
Compound 5 might be formed by elimination of
a t-butyl group. A possible intermediate would be
6, indicating the Lewis acid character of PCl3. Al-
though we did not confirm the generation of 2
-methylpropene (isobutene), we found that the reac-
Mes*
Mes*
P P
P P
O Mes*
Mes*
1
2
Figure 1.
Keywords: phosphorus compounds; anilides; elimination reaction;
steric effects.
* Corresponding author. Fax: (81) 22 217 6562; e-mail: yoshifj
@mail.cc.tohoku.ac.jp
† Postdoctoral Fellowships for Foreign Researchers of the Japan
Society for the Promotion of Science.
0040-4039/02/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)02096-2

120
F. Ri6ie`re et al. / Tetrahedron Letters 43 (2002) 119–121
Scheme 1.
tion of deuterated Mes*NDMe with PCl3 gave N-(2,4-
di-t-butyl-6-deuteriophenyl)-N-methylaminodichloro-
phosphine of ca. 50% D/H ratio determined by the H
Figure 2. An ORTEP drawing of 9 with 50% probability
ellipsoids. Hydrogen atoms are omitted for clarity.
1
NMR spectroscopy indicating that intramolecular rear-
rangement might be involved to some extent. Alterna-
tively, an electron transfer mechanism might operate
during the reaction to give 5. A similar de-t-butylation
of Mes*NHMe 4 was reported in the reaction with
iodomethane under high pressure affording 2,4-di-t-
of diphosphene 11, an AB signal was observed, and the
chemical shifts indicated the trans configuration.20–22
The phosphorus atom PA shows higher chemical shift
(lP 308) due to the resonance effect by the nitrogen
atom [>NꢀPꢁPꢀl>N+ꢁPꢀP−ꢀ], which is similar to the
case of 2.8 Although diphosphene 11 slowly decom-
posed at room temperature, suggesting that 11 is
slightly less stable than 2, it is obvious that the N-(2,4-
di-t-butylphenyl)-N-methylamino group is applicable to
construct the PꢁP skeleton.
butyl-N,N-dimethylanilinium iodide.14 Although
a
Lewis acid would play an important role in giving the
de-t-butylated compound, the reaction of 4 with PCl3
in the presence of aluminum trichloride did not give
5.15,16
Compound 11 was irradiated with a medium-pressure
mercury lamp through a Pyrex filter in dichloromethane
for 5 h to afford a mixture of E/Z isomers in a ratio of
3:1.23 The 31P chemical shift of 12 was observed in a
We investigated some reactions of aminodichlorophos-
phine 5 as shown in Scheme 2. Methanolysis of 5
yielded the phosphoric acid derivative 7 (lP 14), which
was easily hydrolyzed to afford N-methyl-2,4-di-t-butyl-
aniline (8). Aniline 8 was allowed to react with HCl to
afford the corresponding ammonium chloride 9, which
was analyzed by X-ray crystallography to reveal elimi-
nation of a t-butyl group, but the quality of the analyt-
ical result was not satisfactory (Fig. 2).17 The reaction
of 5 with lithium 2,4,6-tri-t-butylphenylphosphide (10)
and DBU gave the corresponding diphosphene 11 in
47% yield after purification by column chromatography
(SiO2, hexane) and GPC.18,19 In the 31P NMR spectrum
1
higher field (lP 351, 187, JPP 539 Hz) than that of 11
1
(lP 458, 308, JPP 543 Hz). Complex formation of 11
with W(CO)5(thf) afforded the corresponding product
13 in 23% yield. The 31P chemical shift indicated the
coordination of tungsten at PB with Z-configuration [lP
1
300 (1JPW 203 Hz), 160, JPP 550 Hz] (Scheme 3),
suggesting that the E/Z isomerization took place during
the reaction.24 Further investigation on 11 is in
progress.
Scheme 2.

F. Ri6ie`re et al. / Tetrahedron Letters 43 (2002) 119–121
121
Me
N Ar
P P
Me
hν
Mes*
N Ar
W(CO)5(thf)
Mes*
11
P P
W(CO)5
Ar = 2,4-t-Bu2C6H3
12
13
Scheme 3.
3
Acknowledgements
1.25 (9H, o-t-Bu), 2.91 (3H, d, JPH 7 Hz, NMe), 6.98
(1H, ddd, JPH 0.8 Hz, JHH 8 Hz, JHH 2 Hz, H6), 7.13
5
3
5
(1H, dd, JHH 8 Hz, JHH 2 Hz, H5), 7.48 (1H, dd, JHH
3
4
4
5
This work was supported in part by a Scientific Grant-
in-Aid (No. 13304049) from the Ministry of Education,
Culture, Sports, Science and Technology of Japan.
2 Hz, JHH 2 Hz, H3); 13C{1H} NMR (50 MHz, CD2Cl2):
l 30.1 (p-CMe3), 31.4 (o-CMe3), 33.7 (p-CMe3), 34.9
(o-CMe3), 35.9 (NMe), 124.8 (C6), 125.4 (C5), 130.9 (C3),
140.2 (d, JPC 26 Hz, ipso), 147.8 (C2), 151.5 (C4).
1
14. Okamoto, Y.; Shimizu, H. J. Am. Chem. Soc. 1968, 90,
6145.
15. Yoshifuji, M.; Fujishima, I.; Okazaki, R.; Inamoto, N.
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