金属催化的计量硝化反应

近年来由于对环境保护越来越重视，一类新的硝化方法发展了起来。这种方法用镧系金属 lanthanide(III)或 IV 族金属的三氟磺酸盐 [Ln(OTf)₃, Ln=La-Lu; M(OTf) ₄, M= Hf, Zr] 以及 tris(trifluoromethanesulphonyl)methides('triflides') [M(CTf₃) ₃; M=Yb, Sc] 作催化剂【a. Waller, F. J.;Barrett, A. G. M.; Braddock, D. C.; Ramprasad, D. Chem. Commun. 1997,613. b. For an interesting report utilizing La(NO3)3/HCl/NaNO3 for thenitration of phenols see: Ouertani, M.; Girard, P.; Kagan, H. B. TetrahedronLett. 1982, 23, 4315. c. Waller, F. J.; Barrett, A. G. M.;Braddock, D. C.; Ramprasad, D. Tetrahedron Lett. 1998, 39,1641.】，催化等当量的硝酸（69％）硝化苯环，这种方法对一般的苯环及一些钝化的苯环都能得到高产率。由于只用等当量的硝酸，所以副产物仅仅是水，没有废酸产生，而且催化剂可以回收，并经过简单处理后反复使用。因此这种方法是较为环保的一种方法。

Nitric acid (69%; 274 mg, 3.0 mmol) was added to a stirredsuspension of ytterbium(III) triflate (186 mg, 0.30 mmol) in 1,2-dichloroethane(5 mL). The suspension dissolved to give a two phase system in which theaqueous phase was the more dense.  Toluene(270 mg, 3.0 mmol) was added and the stirredmixture was heated at reflux for 12 h.  Duringthe reaction a white solid precipitated and the organic phase became yellow,and after 12 h no phase boundary was apparent.  The solution was allowed to cool and dilutedwith water. The yellow organic phase was dried (MgSO₄) and evaporated to give nitrotoluene (390 mg, 95%). Thecolourless aqueous phase was evaporated to give ytterbium(III) triflate as a whitefree-flowing solid (183 mg, 98%).