M. Stratakis et al. / Tetrahedron Letters 44 (2003) 349–351
351
Acknowledgements
This work was supported by the Greek Secretariat of
Research and Technology and by the EPEAEK pro-
gram. We thank Emeritus Professor G. J. Karabatsos
for valuable comments.
References
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2
3
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Scheme 6. Proposed concerted mechanism for the oxygen
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Thus, attack from the oxygen atom of bis(p-
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ate I), in agreement with the data of Scheme 4, would
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As a reasonable mechanistic rationalization for the
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6. In the transition state for the deoxygenation reaction
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a partial p bond with phosphorus. Therefore, the P
atom bears a partial negative charge (l−), and the Se a
partial positive charge (l+). In the case of bis(p-
methoxyphenyl) selenoxide, we propose that the transi-
tion state is more ‘late’ in nature. The positive charge
on the Se atom makes the diaryl selenium moiety a
better leaving group. Therefore, in the transition state,
the Se–O bond is almost completely broken, the p bond
has developed significantly between P and O, and the
phosphorous atom bears a partial positive charge (l+).
These ‘early’ and ‘late’ transition state arguments for a
concerted mechanism are in agreement with the Ham-
mett kinetics, and the fact that bis(p-methoxyphenyl)
selenoxide is more reactive compared to diphenyl
selenoxide. A mechanism involving nucleophilic attack
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12. Triaryl phosphites were prepared by reaction of 4
equiv. of the phenols with 1 equiv. of PBr3 in anhy-
drous diethyl ether (room temperature, 2 h) containing
5 equiv. of pyridine.
31
13. The P NMR data (in ppm) of the triaryl phosphites
(p-X-C H O) P and the corresponding phosphates (p-
6
4
3
X-C H O) P=O are as follows. Phosphites: X=H,
6
4
3
128.63; X=F, 128.20; X=Me, 129.18; X=I, 127.16;
X=Cl, 127.55; X=OMe, 129.55. Phosphates: X=H,
−17.03; X=F, −16.31; X=Me, −16.33; X=I, −17.66;
X=Cl, −17.18; X=OMe, −15.33.
5
of P to Se, as proposed earlier, is rigorously ruled out.
1
4. Diphenyl selenoxide was prepared by oxidation of
diphenyl selenide with N-chlorosuccinimide followed by
alkaline hydrolysis as described in the literature: Detty,
M. R. J. Org. Chem. 1980, 45, 274–279.
To the best of our knowledge, this work represents the
first example in the literature where the sign of the z
value in a Hammett plot is reversed without a signifi-
cant change in the reaction mechanism (for both
selenoxides, the reaction arises from nucleophilic attack
of the oxygen atom to the phosphorus). Further work
to examine the mechanistic details of similar oxygen
atom transfer reactions are currently in progress.
15. Bis(p-methoxyphenyl) selenoxide is commercially avail-
able from TCI (Japan).
16. For a previous example of nucleophilic oxygen atom
transfer to phosphites, see: Stratakis, M.; Orfanopoulos,
M.; Foote, C. S. Tetrahedron Lett. 1991, 32, 863–866.