The reaction of diphenyl and dialkyl phosphorochloridates with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). Formation of phosphonate diesters via N→C phosphorus migration

Journal of the Chemical Society. Perkin Transactions 2

Volumn , Issue 10, 1999, Pages 2071-2075

  Kers, Annika ^a   Kers, Inger ^a   Stawinski, Jacek ^a  

^a STOCKHOLM UNIVERSITY   (Sweden)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 0003687626     PISSN: 03009580     EISSN: None     Source Type: Journal    
DOI: 10.1039/a904502c     Document Type: Article

References (40)

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20. In all reactions of diphenyl phosphorochloridate 1 and DBU. a small amount of a side product (<3%), resonating at 29.9 ppm, was formed.
21. 31P NMR spectra assigned to the initial adducts of chlorophosphates and DBU (intermediates of type 3) were, most likely, due to secondary intermediates (type 4 in this paper) having the same atoms bound to the phosphorus, but differing in a distal part of the DBU moiety.
22. Considering the substrates used for the reaction and the observed chemical shifts, it was possible that both intermediates might contain a P-N bond.
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27. The signal was shifted ca. 0.6 ppm towards higher field probably due to the solvent effect. The intermediate, which gave rise to this signal, reacted further to produce 6. No signal due to 3 could be detected under the reaction conditions.
28. One can speculate that a plausible mechanism for such phosphorus migration could involve (i) a tbp intermediate with the four-membered ring in the preferred apical-equatorial posture, (ii) a pseudorotation to another low energy tbp with the phosphetidine nitrogen in an apical and the carbon in an equatorial position, and finally (iii) a ring opening with the P-N bond cleavage.
29. In this instance, however, the formation of amidine adduct 10 was probably slower (due to steric hindrance in 8) than its deprotonation to form alkene aminal 11, which allowed the former intermediate to be detected only at later stages of the reaction, when the concentration of DBU became low.
30. 31P NMR chemical shift of the isolated compound was identical to that observed in the reaction mixture (ca. 22 ppm).
31. P vs. DBU concentration was linear up to 0.6 equiv. (for 12·HCl, 4 points, R = 0.999, slope 9.50) and 1 equiv. (for 6·HCl, 6 points, R = 0.999, slope 5.7) of the added DBU.
32. obs = 20.62 ppm.
33. 31P NMR data, is unclear.
34. Multiplicity of the aromatic carbon signals was due to the diastereotopic character of the phenyl groups and the presence of phosphorus carbon couplings.
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36. Formation of some unidentified side-products (as was apparent from the presence of several resonances of low intensity, especially in the reactions involving 9a) was also observed.
37. Formation of complicated mixtures of products can be due to competing dealkylation of 13 and 14, and some subsequent transformations of the produced reactive species.
38. A. Kers, I. Kers and J. Stawinski, unpublished results.
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