Selective halogenation using molecular sieves

Abstract

Furthermore, no 1 : 2-adduct was formed by the reaction of trimethyl phosphite with p-nitrobenzaldehyde in the presence of an excess of 2-methyl-2-nitrosopropane in CH,CI, at room temperature during 40 hours. This means, that radical reactions are stopped by the scavenger, because no alternative reaction path is available. ExperimentalESR spectra were recorded at room temperature on a Varian E-4 spectrometer. The dimer of 2-methyl-2-nitrosopropane was prepared as described by Stowel16 from t-butylamine, Na,WO,. 2H,O and hydrogen peroxide. 3-Benzylidene-acetylacetone', 2,7-dinitrophenanthrenequinone8and tris(dimethylamino)phosphine9 were prepared by standard methods. Trimethyl phosphite and triethyl phosphite were commercially available (BDH) and were distilled from sodium before use. Methyl vinyl ketone (Schuchardt, Miinchen) was distilled before use and 9,lO-phenanthrenequinone (Fluka AG) was crystallized from benzene.J . C. Stowell, J. Org. Chem. 36, 3055 (1971).' M . E. McEntee and A . R. Pinder, J. Chem. SOC. 4426 (1957). ' J . Schmidt, Ber. 35, 3 122 (1 902).R. Burgada, Ann. Chim. (France) 8, 348 (1963). IntroductionSelective halogenation using molecular sieves J. van Abstract. On treatment of aromatic compounds, entirely adsorbed in type X molecular sieves, with bromine or chlorine a selective halogenation is found to take place. Alkylbenzenes give mainly -and halogenobenzenes almost exclusively -the para isomers. The bromination of alkylbenzenes takes place only in the ring. Phenyl ethers undergo a strong deactivation of the aromatic ring. In halogenations of toluene and halogenobenzenes, the use of catalytic amounts of CaX in which in most cases part of the cation is exchanged by Fe+++, result also in a significantly higher paralortho ratio, than in reactions homogeneously catalyzed by FeCI,.Although the molecular sieves (synthetic zeolites) have found extensive use in separations and as high temperature catalysts' of isomerization, cracking or alkylation reactions, only a few applications in preparative organic synthesis have been reported. Examples include the use of molecular sieves as selective adsorbents in the preparation of acetals and enamines, as described by Roelofson and van Bekkum'. We have now studied the use of molecular sieves in the halogenation of aromatic compounds, especially the halogenation of bromobcnzene, chlorobenzene, several alkylbenzenes and methoxybenzene. The halogenation of benzene with a zeolite containing Fe is described by Levina et aL3, but without any mention of the quantitative composition of the reaction products. We considered it possible that molecules adsorbed in the welldefined, relatively narrow cavities of the zeolite crystals might be restricted in their orientations. Under this influence or on account of the electrostatic forces within the crystal lattice the substitution pattern might be changed in comparison with reactions in solution. A related example of this, published during our investigation, is found in the selective (para) halogenation of anisole cn...

Halogenation Reactions

“…The chlorination of aromatic compounds using solid zeolite catalysts has been studied extensively [7,[16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][134][135][136][137][138][139]. Yet, zeolites have not been shown to provide any advantages over other high-surface-area materials (such as silica) in those halogenations which proceed via a free-radical mechanism, including the chlorination of side chains in aromatic and aliphatic compounds.…”