Ring bromination of aromatic compounds using 1,3-dibromo-5,5-dimethylhydantoin in dichloromethane is promoted by the addition of strong acids. Both organic and inorganic acids whose pKa values are lower than -2 showed the promoting effect. This acid-catalyzed bromination is both practical and effective, even for aromatics having electron-withdrawing substituents.

The S(N)i mechanism, which was previously proposed for the isomerization of 1-acylaziridines to the oxazolines, was reexamined theoretically by performing molecular orbital (MO) calculations of 1-formylaziridine and its derivatives as model compounds and experimentally by using 1(R)-[alpha-methoxy-alpha-(trifluoromethyl)phenylacetyl]-2(S)-methylaziridine (5). At the MP2/6-31G//RHF/6-31G level, the activation energy was estimated to be 38.9 kcal mol(-1) for the S(N)i mechanism in which N-protonated 1-formylaziridine 8a(NH(+)) isomerizes to the N-protonated oxazoline 9a(NH(+)). Intrinsic reaction coordinate calculations showed that this reaction proceeds with retention of the ring carbon configuration. Methyl substitution in the aziridine ring reduces the activation energy by 10 kcal mol(-1). The ring closure of N-(2-chloroethyl)formamide (10a) to the oxazoline, which is a model reaction of the rate-determining step for the addition-elimination mechanism, was estimated to have an activation energy of 45.4 kcal mol(-1). The results of these MO calculations are consistent with the observation that the isomerization of the acylaziridine 5 to the oxazoline 6 is facilitated in the presence of weak nucleophiles such as with BF(3).OEt(2) while the formation of 6 is very slow in the presence of stronger nucleophiles such as p-toluenesulfonate. Both theoretical and experimental results suggest that the S(N)i mechanism explains well the isomerization of (R,S)-5 to the oxazoline with BF(3).OEt(2) in refluxing benzene.

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Many metallic sulphates and hydrogen sulphates supported on silica gel efficiently catalyse dehydration of secondary and tertiary alcohols under mild conditions. The sulphates of CeiV, TiiV, and Fell1 were most active. Silica gel was essential for the efficient dehydration in each case. The dehydration catalysed b y Fe,(SO,),, CuSO,, and NaHSO, o n silica gel was studied in detail. The order of the reactivity of alcohols was tertiary > secondary > primary. The dehydration activity of methanesulphonic acid was also enhanced (ca. 14 times at 90 "C) b y the presence of silica gel. Some mechanistic investigations were carried out.The use of reagents or reactants on insoluble inorganic supports for the dehydration of alcohols is increasingly popular.' Such heterogeneous reactions compared with their homogeneous counterparts, often have the advantage of ease of set-up and work-up, mild reaction conditions, increased yields, and greater selectivity., We have already reported in preliminary form results for the dehydration of alcohols catalysed by CuSO, supported on silica gel (abbreviation; CUSO,-S~O,).~ Here we describe the corresponding results for other metallic sulphates and hydrogen sulphates. Results and DiscussionSince FeCl, supported on silica gel is reported to catalyse the transformation of cholesterol to a chlorinated compound and an ether with evolution of hydrogen chloride on heating: we chose to work with metallic sulphates because of their thermal stability and the low nucleophilicity of the sulphate ion. The catalysts were prepared by mixing sulphate solutions with chromatographic silica gel and evaporating the water under reduced pressure with heating. There seems to be an optimal temperature for drying the catalysts, CuS0,-SiO, dried at 240 "C for 1 h, or at 200 "C for 3 h, being the most active and that dried at 300°C the least: most were dried at 240 "C for 1 h. Catalysts containing less thermally stable sulphates and hydrogen sulphates, were dried under milder conditions. (See Experimental section). The importance of the amount of water in supported reagents has been mentioned in several reports6The dehydrations were straightforward and after completion, the olefins, isolated from the catalyst by filtration, were generally obtained pure. Increased yields of product were obtained by washing the catalysts with a polar solvent (acetone): small quantities of alcohols were also recovered. Catalytic Activity.-The catalytic activity of sulphates and hydrogen sulphates (assumed to be thermally stable at the reaction temperatures) was examined in the case of cyclododecanol. The results are summarized in Table 1. The catalytic activity of the supported sulphates, decreased in the following order:

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