In conclusion, we have developed a nucleophilic catalyst based
upon a polyether imidazole where the reduction in rate imposed by
the additional steric bulk of a polyether side-chain may be
2
overcome to some degree by addition of Mg(OTf) . Although
direct identification of the postulated intermediate 2 was not
possible, compelling evidence infers such a species. Further work
is in progress to conclusively identify any supramolecular
reactive intermediates that are formed in this reaction and use
this as a basis for the preparation of chiral bifunctional
catalysts.
Notes and references
{
1-Phenylethanol was used in these studies instead of cyclohexanol to help
develop analytical methods to determine enantiomeric excess at a later date.
Similar trends were observed with cyclohexanol.
3
1
Fig. 3 Intermediates observed in the P NMR spectrum.
1
(a) R. Wegler, Liebigs Ann. Chem., 1932, 498, 62; (b) W. Steglich and
G. Hofle, Angew. Chem., Int. Ed. Engl., 1969, 8, 981; (c) S. K.
Chaudhary and O. Hernandez, Tetrahedron Lett., 1979, 20, 99; (d)
U. Ragnarsson and L. Grehn, Acc. Chem. Res., 1998, 31, 494.
For reviews, see: (a) A. C. Spivey, A. Maddaford and A. J. Redgrave,
Org. Prep. Proceed. Int., 2000, 32, 331; (b) D. E. J. E. Robinson and
S. D. Bull, Tetrahedron: Asymmetry, 2003, 14, 1407.
For examples, see: (a) J. C. Ruble, H. A. Latham and G. C. Fu, J. Am.
Chem. Soc., 1997, 119, 1492; (b) B. R. Sculimbrene, A. J. Morgan and
S. J. Miller, J. Am. Chem. Soc., 2002, 124, 11653; (c) E. Vedejs,
O. Daugulis, L. A. Harper, J. A. MacKay and D. R. Powell, J. Org.
Chem., 2003, 68, 5020; (d) A. C. Spivey, T. Fekner, S. E. Spey and
H. Adams, J. Org. Chem., 1999, 64, 9430.
all cases except Cu(OTf)
longest chain polyether 8 in combination with Mg(OTf)
Further evidence for the intermediacy of a coordinated
2
, the best combination being with the
2
.
2
3
3
1
intermediate was provided from P NMR data. An equimolar
ratio of catalyst 8 and (PhO) P(O)Cl gave three signals at
2
d 225.27 ppm, 211.78 ppm and 24.89 ppm corresponding to
9
the pyrophosphate 10, the imidazolium phosphate 11 and
diphenylchlorophosphate, respectively (Fig. 3). The presence of
the pyrophosphate comes from the partial hydrolysis of the
chlorophosphate by advantageous water followed by reaction of
the diphenylphosphate with more chlorophosphate and is a
4
E. Vedejs and X. Chen, J. Am. Chem. Soc., 1996, 118, 1809.
5 (a) G. T. Copeland and S. J. Miller, J. Am. Chem. Soc., 2001, 123, 6496;
(b) B. R. Sculimbrene and S. J. Miller, J. Am. Chem. Soc., 2001, 123,
10
common phenomenon in these systems. Upon addition of
Mg(OTf) , the signal at 211.78 ppm completely disappeared and a
10125.
2
6
For an overview see: M. Shibasaki, M. Kanai and K. Funabashi, Chem.
Commun., 2002, 1989.
7 M. Pascaly, M. Duda, A. Rompel, B. H. Sift, W. Meyer-Klaucke and
new signal at 216.32 ppm appeared which was assigned as the co-
ordinated catalyst 12. Addition of MgBr , MgCl or Mg(ClO ) all
2
2
4 2
led to a reduction in the signal at 211.78 ppm and appearance of
the same signal, ruling out the possibility of counter-ion exchange
effects. Furthermore there is a good correlation between the level
of cooperative rate enhancement and the stability of the species
corresponding to this signal. For example, in the case of catalyst 1
B. Krebs, Inorg. Chim. Acta, 1999, 291, 289.
(a) S. Jones and D. Selitsianos, Org. Lett., 2002, 4, 3671; (b) S. Jones,
D. Selitsianos, K. J. Thompson and S. M. Toms, J. Org. Chem., 2003,
8
68, 5211.
9 The reported chemical shift for the adduct of N-methylimidazole and an
alkyl-aryl chlorophosphate is in the range of d 213.38 A 214.41 ppm.
See W. Dabkowski, F. Cramer and K. Michalski, Tetrahedron Lett.,
6
and KPF , only a small rate enhancement was observed (see
1
987, 28, 3561.
0 J. Nilsson, A. Kraszewski and J. Stawinski, J. Chem. Soc., Perkin Trans.
2, 2001, 2263.
31
P
Table 2), and consequently no additional signal was seen in the
NMR spectrum.
1
3
834 | Chem. Commun., 2005, 3832–3834
This journal is ß The Royal Society of Chemistry 2005