Procedure:  250 mL 1-neck flask, stirbar, septum, N₂ inlet

                  Dissolved 18.6 g of I in 200 mL of dry CH₂Cl₂.  Stirred at 0 C.  Added 19.20 g of 4 A sieves followed by careful addition of 20.30 g of PCC (Caution:  addition of PCC to CH₂Cl₂ is exothermic).  The reaction mixture was allowed to slowly warm to rt.  After 16 h, the solvent was removed by rotary evaporation and the resulting brown solid was triturated with Et₂O and filtered through a plug of silica gel.  The filtrate was concentrated by rotary evaporation.  The product was isolated by Kugelrohr distillation (1.0 mm, 65 C) as a clear, colorless oil. 

 

¹H NMR (CDCl₃, 400 MHz) d 9.58 (d, J = 8.1 Hz, CHO), 6.87 (dt, J = 15.4, 3.3 Hz, H3), 6.38 (ddt, J = 15.4, 8.1, 2.2 Hz, H2), 4.43 (dd, J = 3.2, 2.2 Hz, H4), 0.90 (s, SiC(CH₃)₃), 0.07 (s, Si(CH₃)₂). 

 

notes

 

Pyridinium chlorochromate was first described as an efficient reagent for the oxidation of alcohols by Corey and Suggs (see Corey, E.J.; Suggs, J.W.  Tetrahedron Lett.  1975, 2647).  The reagent is slightly acidic in nature but reactions can be buffered with NaOAc when acid labile functionality is a concern. 

 

A postulated mechanism (see Banerji, K.K.;  J. Chem. Soc.,  Perkin Trans. 2  1978, 639) is...

 

The addition of 4 A molecular sieves has been shown to expedite the oxidation process with PCC (see Herscovici, J.; Antonakis, K.  J. Chem. Soc., Chem. Commun.  1980, 561).  Nucleosides and sugars that give incomplete reaction without sieves are satisfactorily oxidized with them.  The increase in reactivity can be related to the size of the molecular sieves in the order 3 A > 4 A > 10 A > 5 A.  The optimized conditions for the reaction are reported to be 0.5-1.0 g of molecular sieves per mmol of substrate and 2-3 equivalents of PCC (see Herscovici, J.; Egron, M-J.; Antonakis, K.  J. Chem. Soc. Perkin Trans. I  1982, 1967).  The catalysis appears to be a surface phenomenon.