Scheme 3 (a) n = 0, 1 and 2; 1 NaH, DMSO, vacuum (30 Torr), 25 uC,
1 h; (b) n = 3, 1.5 NaNH2, DMSO, 25 uC, 1 h.
first equilibrium acidity corresponds to a DMSOpKa of 18.0 ¡ 0.5
(5c–H+), the second to a DMSOpKa of 19.8 ¡ 0.5 (8c–H+), and the
last to a DMSOpKa of 23.6 ¡ 0.5 (10–H+). It is worthy of note that
these high pKa values are slightly weaker than those reported by
Schwesinger for the corresponding bases, probably owing to the
different nature of the phosphorus substituents.
In conclusion, with these preliminary, results we have developed
from Ph3PLNLi6 a new, efficient and simple method for the
synthesis of various strong non-ionic bases. Work is now in
progress to extend this new method to the synthesis of still
stronger bases with the use of other azayldiides and phosphorus
halides, R3PLNLi and R9xPCly (R, R9 = NMe2 or tert-butyl),
respectively.
Scheme 2 (a) 2 n-BuLi, THF, 220 uC, 30 min; (b) 0.5 PhPCl2, THF,
220 uC to RT; (c) 1.1 C2Cl6, RT, 2 h; (d) 4 t-BuNH2, reflux, 2 h; (e) NaI
(5%), H2O; (f) 0.25 PCl5, 265 uC, 1 h and 25 uC, 12 h; (g) BnNH2, 100 uC,
4 h; (h) NaCl (10%), H2O.
1P), 214.88 (s br, 1P)}. This was then trapped with a slight excess
of chlorodiphenylphosphine to generate almost quantitatively after
few minutes, the linear P3 phosphazene 7 {31P NMR, THF: three
peaks (relative area 1 : 1 : 1): d (ppm) 37.84 (d, 2Jpp = 114.4 Hz),
11.91 (d, 2Jpp = 1.6 Hz), 10.89 (dd, 2Jpp = 114.4 Hz, 2Jpp = 1.6 Hz,
N–PLN)}. Finally, after a treatment with C2Cl6, followed by an
amine, the desired P3–H+ salts (8a–c–H+), precursors of the
expected linear P3 bases, were isolated in 67–79% yields.
Notes and references
1 (a) R. Schwesinger, Chimia, 1985, 39, 269; (b) R. Schwesinger and
H. Schlemper, Angew. Chem., Int. Ed. Engl., 1987, 26, 1167; (c)
R. Schwesinger, J. Willaredt, H. Schempler, M. Keller, D. Schmitt and
H. Fritz, Chem. Ber., 1994, 127, 2435; (d) R. Schwesinger, H. Schlemper,
C. Hasenfratz, J. Willaredt, T. Dambacher, T. Breuer, C. Ottaway,
M. Fletschinger, J. Boele, H. Fritz, D. Putzas, H. W. Rotter,
F. G. Bordwell, A. V. Satish, G.-Z. Ji, E.-M. Peters, K. Peters,
H. G. von Schnering and L. Walz, Liebigs Ann., 1996, 1055.
2 (a) H. Schmidt, C. Lensik, S. K. Xi and J. G. Verkade, Z. Anorg. Allg.
Chem., 1989, 578, 75; (b) C. Lensik, S. K. Xi, L. M. Daniels and
J. G. Verkade, J. Am. Chem. Soc., 1989, 111, 3478; (c) J. G. Verkade
and P. B. Kisanga, Tetrahedron, 2003, 59, 7819; (d) J. G. Verkade, Top.
Curr. Chem., 2003, 223, 1.
3 (a) A. Costa, C. Na´jera and J. Sansano, J. Org. Chem., 2002, 67, 5216;
(b) G. A. Kraus, N. Zhang, J. G. Verkade and R. Schwesinger, Org.
Lett., 2000, 2, 2409.
4 (a) M. Taillefer, H.-J. Cristau, A. Fruchier and V. Vicente, J. Organomet.
Chem., 2001, 624, 307; (b) M. Taillefer and H.-J. Cristau, Top. Curr.
Chem., 2003, 229, 41.
5 (a) M. Taillefer, N. Inguimbert, L. Ja¨ger, K. Merzweiler and
H.-J. Cristau, Chem. Commun., 1999, 565; (b) H.-J. Cristau,
M. Taillefer and I. Jouanin, Synthesis, 2001, 69; (c) N. Inguimbert,
M. Taillefer, L. Ja¨ger and H.-J. Cristau, Eur. J. Org. Chem., 2004, 4870.
6 H.-J. Cristau, M. Taillefer and N. Rahier, J. Organomet. Chem., 2002,
646, 94.
7 (a) E. J. Corey and J. Kang, J. Am. Chem. Soc., 1982, 104, 4724; (b)
M. Schlosser, H. Ba Tuaong, J. Respondek and B. Schaub, Chimia,
1983, 37, 10.
We were also interested in the synthesis of branched bases P3
(iso structure of 8) and P4, potentially more basic than the linear P2
and P3 phosphazenes.1d In order to obtain the corresponding
protonated bases, the quenching of Ph3PLNLi with half an
equivalent of dichlorophenylphosphine was first performed.
Following the procedure used for P2 and linear P3 structures we
could thus synthesise without any difficulty the expected precursor
P3–H+ salt (10–H+, yield 81%) (Scheme 2).9
Following another method, we also observed that Ph3PLNLi
could substitute three chlorine atoms from PCl5 (0.25 equiv.) to
form the branched P4–H+ salt 12–H+ (Scheme 2). The addition of
electrophile was performed at very low temperature (265 uC) in
order to minimize the formation of by-products, the precipitate 11
being recovered by filtration {31P, DMSO-d6: d (ppm) 12.71 (s,
3P), 23.28 (s, 1P)} and subsequently treated without any other
purification with benzylamine to give 12–H+ (isolated yield 58%).
Finally, the corresponding bases were prepared in situ in
dimethyl sulfoxide (or DMSO-d6) by deprotonation of the
corresponding salts with sodium hydride for the P2 and P3 series
and sodium amide for the P4 structure (Scheme 3). All the bases
8 I. N. Zhmurova, A. P. Martynyuk, A. S. Shtepanek, V. A. Zasorina and
V. P. Kukhar, Zh. Obshch. Khim., 1974, 44, 79.
9 9 is proposed as an intermediate; 31P NMR {THF–DMSO-d6: d 51.49
1
(dd, JP–+P = 236, 98 Hz, 1P); 21.67 (dd, J = 98.1, 75.2 Hz, 1P); 17.56
(ddt, 1JP–P+ = 236.5, 75.2, 34.3 Hz, 1P); 11.16 (d, J = 34.3 Hz, 1P); 10.85
(d, J = 34.8 Hz, 1P). For such a 1JP–+P value see: R. W. Alder, D. D. Ellis,
R. Gleiter, C. J. Harris, H. Lange, A. G. Orpen, D. Read and
P. N. Taylor, J. Chem. Soc., Perkin Trans. 1, 1998, 1657. Another
indication of the existence of 9 is the presence in the mixture of the
expected corresponding unreacted 0.25 equivalent of Ph3PLNLi.
10 F. G. Bordwell, Acc. Chem. Res., 1988, 21, 456.
1
were quantitatively obtained and analyzed by H, 13C and 31P
NMR.
In other respects, determination in DMSO of the acid–base
equilibria was performed with couples 5c/5c–H+, 8c/8c–H+ and 10/
10–H+, on the basis of an overlapping indicator method.10 The
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