Bromine photochemical addition

One example of selective photochemistry with lasers is the gasphase photochemical addition of bromine to olefin molecules induced by the monochromatic light near 6940 A from a pulsed, tunable ruby laser, as studied by Tiffany... 

In the absence of catalysts and in the dark, pure benzene does not react with bromine, as is the case also with chlorine. Photochemical addition of bromine, like that of chlorine, is a radical chain reaction.130 131 Bromine has a more powerful substituting action than chlorine, and its rate of addition is slower.132 At low temperatures light and addition of peroxides favor addition of bromine. To date, two isomeric hexabromocyclohexanes have been isolated, by very slow addition of bromine to benzene irradiated at 0° 1% sodium hydroxide solution must be placed under the benzene and frequently renewed even so, yields are poor.126b 133... 

The regioselectivity of addition of HBr to alkenes under normal (electrophilic addi tion) conditions is controlled by the tendency of a proton to add to the double bond so as to produce the more stable carbocatwn Under free radical conditions the regioselec tivity IS governed by addition of a bromine atom to give the more stable alkyl radical Free radical addition of hydrogen bromide to the double bond can also be initiated photochemically either with or without added peroxides... 

A cross-conjugated perfluorotnene is unreactive toward bromine, however, photochemical chlorination yields the 1,4 addition product [I2 (equation 5)... 

If handled responsibly, PFCs can be excellent choices to replace ozone-depleting compounds in many demanding, high-performance applications. Perfluorinated liquids are colorless, odorless, essentially nontoxic, and nonflammable. In addition, since they are not precursors to photochemical smog, PFCs are exempt from the U.S. EPAs volatile organic compounds (VOC) definition. Most importantly, these materials do not contain the carbon-bound chlorine or bromine, which can cause ozone depletion. 

Bromination of allylic positions cannot be achieved specifically by using elementary bromine, unless electrophilic addition to the rc-bond (Eq. 18) is unfavorable because the substituents have a high negative inductive effect. Efficiency of electrophilic addition of Br2 may also be diminished by steric effects. In addition to these secondary thermal reactions, the specificity of photochemical brominations of allylic positions using Br2 will also suffer from the competing (radical) addition of Br to the double bond (Eq. 19) [31]. 

Rather specific thermal or photochemical bromination of allylic positions is, however, possible by using N-bromosuccinimide as brominating agent. Both procedures produce, however, variable quantities of Br2 depending on reaction conditions [32]. In thermal procedures, the concentration of intermediate Br2, and, hence, the importance of secondary addition products, can be controlled by the relative quantity of radical initiator (e.g., A1BN) and by reaction temperature. The appearance of addition products in a photochemical procedure would be evidence for the mechanism proposed by Adam et al. [2, 3, 33] which includes the intermediate production of Br2 (Eq. 21). 

In the photochemical procedure, addition product can be minimized in keeping the relative NBS concentration as small as possible. In addition, substrate concentrations should be optimized with regard to the exitance of the chosen light source to avoid secondary recombination reactions. Under these conditions 4-bromomethyl-5-methyl-l,3-dioxol-2-one can be prepared with only minor impurities (bromine addition and multiple allylic bromination reactions (Eq. 23)) [34]. 

The allyl ethers of phenols may be converted via a Claisen rearrangement to o-allyl derivatives and thence to the benzo[6]furans. The products of addition of bromine to the o-allylphenols give good yields of the heterocycle on prolonged heating with alkali, but the photochemical rearrangement of O-allylphenol is not so effective. 

The addition of radicals follows a different pattern which is determined by access to the alkene. The addition of hydrogen bromide under photochemical conditions, or in the presence of peroxides, involves the initial attack of a bromine radical on the alkene. The consequences are illustrated by the reaction of hydrogen bromide with allyl bromide (Scheme 3.4). 

The preparative interest in the halogen addition to cycloproparenes is very limited, but it has been used occasionally as a diagnostic test. When the reaction of benzocyclopropene with iodine was carried out in carbon tetrachloride at 25 C, rather than under photochemical conditions, 1,6-diidocyclohepta-l, 3,5-triene was formed in only 6% yield, while cleavage to 2-iodo-benzyl iodide (la, 85%) predominated. With bromine benzocyclopropene reacted exclusively to give 2-bromobenzyl bromide (lb, 98%). The same type of reaction prevailed with iodine and 1,1-difluorobenzocyclopropene in refluxing carbon tetrachloride for 48 hours giving l-(difluoroiodomethyl)-2-iodobenzene in poor yield (9%), which was only partially characterized. The reaction between 1,1-difluorobenzocyclopropene and bromine is discussed in Section 2.1. 

The rate constants for the reaction of hydroxy radicals with a variety of bromo-fluoroalkenes have been measured. The photobromination of the benzonorbornadienes (50) in CCI4 results in the formation of the bromine adducts (51) and (52). No rearrangement of the structure was observed during the additions. The photochemical thioacetylation of (— )-A-cadinene affords the... 

---