Interaction of NO2 with Alkenes, Dienes and Polyenes

NO reacts with alkenes at room temperature by a free-radical mechanism [4,59-62], Gas-phase and liquid-phase studies revealed the formation of two types of products addition products and ally lie substitution products [62], The change in concentrations of NO leads to variation of the addition/substitution ratio. As the NO concentration is decreased, the relative amounts of substitution products increase. These observations were made with cyclohexene as substrate [61]. In principle, NO or its dimer may be involved in these reactions. One pathway is represented by the following scheme [4, 63-65]  

The NO addition to double bonds results in geometrical olefin isomerisation by bond rotation in nitroalkyl radicals [59, 60]. For the reaction  

If NO3 can abstract the allylic hydrogen of alkyl-substituted alkenes, the following scheme has been proposed [59, 61, 62]  

The isomerisation of cis- or trans-2-o t n to 1-olefin in reactions with NO has not been detected by the chromatographic analysis [59]. 

The kinetics of the formation of nitration products in reactions of various alkenes with NO + in CCl, CH Cl and hexane were investigated by stopped-flow spectroscopy [62]. The rates of reactions of 2,3-dimethyl-2-butene, cyclohexene, and 1-hexene were measured over a wide range of NO concentrations, from 0.1 mM to 760 mM. For all of these substrates, the order in NO is 2 at high NO and decreases to 1 as the concentration of NO decreases. These data indicate the presence of at least two reaction pathways. One involves the diamagnetic dimer in an addition mechanism (Equation 5.117), and the other involves monomer NO (Equation 5.116). The reaction of the monomer NO proceeds also through abstraction of allylic hydrogen atoms. 

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