Free radical, arylation bromination

B-2,6-Dimethylarylborazines have not been obtained in a direct borazine synthesis, though they can be prepared by Grignard arylation of l,3,5-trimethyl-2,4,6-trichloroborazine 01.92), These compounds show high hydrolytic stability, even in acid and basic medium, and they can be subjected to substitution reactions at the aromatic sites 91>. For example, free radical bromination 93> or Friedel-Crafts acetylation 94> has been accomplished. 

Replacement of aliphatic hydrogens with bromine can be done under free radical substitution conditions, but reaction at aromatic carbons is unfavorable because of the very high energy of the aryl radical. Benzylic substitution is usually the only product observed. 

For synthetic purposes cyclized radicals are preferentially trapped by chlorine [16], bromine [60], or iodine atom donors [54] to provide y9-functionalized tetrahy-drofurans, for instance halides 35-37 (Scheme 9), which serve as building blocks for subsequent ionic or free-radical reactions. This radical version of the classical halogen cyclization (Bartlett reaction [61]) is particularly useful if functionalized tetrahydrofurans can be obtained from terminal alkyl- or aryl-substituted alkenols. If these compounds are reacted for example with iodine or with A -bromosuccin-imide, tetrahydropyrans are formed from ionic cyclofunctionalizations [62], If, however, the corresponding alkenols are converted into a thiohydroxamic acid... 

Most oxy or halo free radicals readily oxidize phenjienediamines to radical cations called Wurster salts (12), which are stable in water-ethanol solutions at pH 3. On the other hand, carbon and sulfur radicals generally do not produce Wurster salts. The intense color of Wurster salts can be used as a quick spot test for variously substituted phenylenediamines when oxidized with bromine in carbon tetrachloride solution. For example, Al,AT-diaIkyl substitution gives red, N,N,N N gives blue, and N-alkyl-TST-aryl gives light blue or green. 

While generation of aryl substituted a-azoxy radicals like 79 is not difficult, " almost nothing is known about their aliphatic counterparts. One reaction that presumably gives such a radical is P-scission of 60 but in this case we were unable to isolate any product attributable to 62. Although we have found no published ESR spectra of purely aliphatic hydrazonyl oxides, such radicals must exist because free radical bromination of azoxyalkanes (e g. 20) gives the expected substitution products. ... 

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