Bromination cyclic

In addition reactions of bromine to alkenes, Risbood and Ruthven154 have shown that 5A zeolite preadsorbed with bromine can selectively brominate styrene in presence of cyclohexene. By simple alteration of the order of adsorption of reactants to Pentasil zeolites, Smith and Fry155 have been able to add selectively bromine to the double bond of linear alkenes in presence of cyclic and branched alkenes, or to brominate cyclic or branched alkenes in presence of linear ones. An example is shown in equation 9. 

Hydroxyethyl)cyclopent-l-ene and benzenetellurinyl acetate formed an addition product containing a cyclie-ether functionality. This intermediate was reduced to the telluride, the telluride converted to the tellurium dibromide, and the dibromide heated in dimethylformamide with sodium bromide to give the brominated cyclic ether5. 

The study of the food web between Aplysia dactylomela and species of Laurencia has yielded from the mollusk brominated cyclic macroethers characteristic of certain species of this genus, as well as of the genus Chondria (see Chapter 13). Lauroxonane and its two bicyclic isomers are closely related to obtusenyne and compounds isolated from Laurencia obtusa (King et al, 1979) (see also Chapter 13). 

Such a carbocation however has been demonstrated to be less stable than an alterna tive structure called a cyclic bromonium ion, m which the positive charge resides on bromine not carbon... 

We present here examples of this condensation with an aromatic aldehyde and a cyclic ketone. Both of these examples are useful because, although other methods are available for their preparation, problems often attend these syntheses. In the synthesis of cyclohexy11deneaceton1tr11e, for example, the standard method results exclusively In the g.y-lsomer and none of the a,g-Isomer. In Part A of this procedure, cyclohexanone Is condensed with acetonitrile to give predominantly the conjugated Isomer (80-83%) whicfi is then separated from the nonconjugated isomer by selective bromination. 

The stereochemistry of both chlorination and bromination of several cyclic and acyclic dienes has been determined. The results show that bromination is often stereo-specifically anti for the 1,2-addition process, whereas syn addition is preferred for 1,4-addition. Comparable results for chlorination show much less stereospeciftcity. It appears that chlorination proceeds primarily through ion-pair intermediates, whereas in bromina-hon a stereospecific anfi-l,2-addition may compete with a process involving a carbocation mtermediate. The latter can presumably give syn or anti product. 

As with both terminal and cyclic olefins containing vinylic chlorine or bromine, acyclic counterparts partly lose the halogen first and then undergo dihydrogenation to the tetrahydro dcnvative [54] (equation 44) When the reachon IS earned out in the hquid phase in the presence of base, yields of tetrahydro product are enhanced [55] (equation 44)... 

Addition of halogens (Sections 6.14-6.16) Bromine and chlorine add to alkenes to form vicinal dihalides. A cyclic halonium ion is an intermediate. Stereospecific anti addition is observed. 

A piridazine ring forms the nucleus for a rather unusual nontricyclic antidepre.ssant. Condensation of the keto ester 136 with hydrazine leads to the cyclic hydrazide 137. Oxidation, for example with bromine, gives the corresponding pyridazone 138. The oxygen is then replaced by chlorine by reaction with phosphorus oxychloride. Displacement of the halogen in 139 with N-ethylami-nomorpholine affords minaprine 140 [30]. 

Acid-catalyzed epoxide opening takes place by protonation of the epoxide to increase its reactivity, followed by nucleophilic addition of water. This nucleophilic addition is analogous to the final step of alkene bromination, in which a cyclic bromonium ion is opened by a nucleophile (Section 7.2). That is,... 

Braverman and Reisman111 have found that addition of a carbon tetrachloride solution of bromine to bis-y,y-dimethylallenyl sulfone 20 at room temperature unexpectedly resulted in spontaneous and quantitative fragmentation of the sulfone, with formation of the cyclic a, /3-unsaturated sulfmate (y-sultine) 43a and the tribromo products 44 and 45 (equation 38). Analogously, treatment of the same sulfone with trifluoroacetic acid gives rise to y-sultine 43b. It is interesting to note that from a synthetic point of view it is not even necessary to prepare the diallenyl sulfone 20, since one can use its sulfinate precursor (equation 24) to obtain exactly the same results, under the same conditions. The authors suggested that the fragmentation-cyclization of sulfone 20 may take place by the mechanism depicted in equation 39. 

At present, this rule fails only when functional neighboring substituents, capable of anchimeric assistance and in a convenient position with respect to the developing positive charge, can compete with bromine in the charge stabilization of the cationic intermediate (ref. 15). For example, the reaction of some unsaturated alcohols (ref. 16) goes through five- or six-membered cyclic oxonium ions, rather than through bromonium ions. 

Among the peculiar features of 2-bromoamides there are the following i) possibility of substitution at the tertiary C-Br (RCO2H, RR NH, or a saccharide, as the nucleophiles) ii) chiral stability and stereochemical control at the secondary C-Br atom (RR NH, ROH or a saccharide as the nucleophiles) iii) the presence of bromine allows cyclic voltammetry and electroreduction at controlled potential both of starting compounds and relevant intermediates iv) the Ca polarity can be reversed upon electroreduction, and the resulting Ca enolate forms a C-C bond (CO2 as the electrophile). 

Temperature programmed GC (Fig. 2) separates these components as well as a cyclic formal. The mono, di and tri brominated products of 1 require higher temperatures to elute in a reasonable time more than the column can withstand. TMS derivatives do not require temperatures quite so high (Fig. 3). Using this technique for quantitation, however, is complicated by the decreasing sensitivity of the FID to increasing bromine content. 

---