Structural and Kinetic Relations Holding in the Halogen Abstraction from Organohalogen Compounds by Alkyl Radicals

Abstract

The relative rate constants were determined, and the absolute rate constants were estimated, for halogen transfer from N,N-dihaloarenesulfonamides, (dichloroiodo)arenes, benzyl bromides, and arenesulfonyl chlorides to cyclohexyl radical and from N,N-dihaloarenesulfonamides to benzyl radical. Polar effect of the substituent was found to be the main factor determining the rate of halogen transfer from benzyl bromides and arenesulfonyl chlorides; it increases with rise in the electrophilicity of the organohalogen substrate due mainly to charge distribution in the transition state.While considering the relations between the structure of organic molecules and the rate of radical transfer of hydrogen, the main factor is the heat effect of the hydrogen transfer stage, and its rate increases in parallel with the exothermic effect. In keeping with the Bell EvansPolanyi principle, in most cases the rate of transfer of an atom is linearly related to the heat effect. Exceptions are possible when a radical to which an atom is transferred possesses pronounced electrophilic or nucleophilic properties. In terms of the model based on the Marcus equation, increase in the electrophilicity or nucleophilicity of a radical leads to change of the internal reaction barrier as a result of charge separation in the transition state [1]. For example, the rate of abstraction of a hydrogen atom by chlorine and some other electrophilic radical species depends on polar effects of substituents in the substrate to a greater extent than on the heat effect. On the other hand, general relations holding in halogen transfer to alkyl radicals have been studied poorly. Therefore, we examined the effect of the structure of organic halogen compounds on the rate of halogen transfer therefrom to alkyl radicals.We determined the relative rate constants for abstraction of chlorine and bromine atoms from halogenating agents and arenesulfonyl chlorides by cyclohexyl and benzyl radical, following the procedure developed previDeceased.ously [2]. This procedure is based on competing abstraction of halogen from reagents XHlg 1 and YHlg 2 by alkyl radicals generated by radical chain halogenation of the corresponding alkane under conditions excluding formation of molecular halogen. Using the absolute rate constant for bromine transfer from N-bromosuccinimide (NBS) to benzyl radical, k(NBS) = (5.0 ± 0.3) 1 0 5 l mol 1 s 1 [3], we estimated the absolute rate constants of halogen transfer in the examined reactions. The relative and absolute rate constants are given in Table 1.

An Efficient Radical Procedure for the Halogenation and Chalcogenation of B‐Alkylcatecholboranes

“…[24] This assumption is confirmed by the recent report that PhSO 2 Cl reacts with the cyclohexyl radical at a rate constant k = 1.2 10 6 M À1 s À1 at 18 8C. [25] These rates fit well with the observation that the reaction of (À)-2-carene 13 affords exclusively the ring-opened product 14 in 56% yield [Eq. (2)].…”

Employing exchange reactions involving hypervalent iodine compounds for the direct synthesis of azide-containing linear and branched polymers

“…The transfer of a chlorine atom from ArICl 2 to cyclohexyl radicals is rather efficient and is characterized by a rate constant of 7.6 Â 10 7 , 8.0 Â 10 7 , or 8.3 Â 10 7 M À1 s À1 for Ar ¼ 4-O 2 NC 6 H 4 , Ph, and 4-H 3 CC 6 H 4 , respectively. 95 In analogy with the ArICl 2 compounds, ArI(N 3 )(O 2 CCH 3 ) and ArI(N 3 ) 2 may also be able to transfer azide radicals to the propagating chains. Although the precise mechanism of the reaction of carbon-centered radicals with PhI(N 3 )(O 2 CCH 3 ) and PhI(N 3 ) 2 will need to be elucidated further, it is plausible to assume that transfer reactions take place.…”

Employing Heterocyclic Hypervalent Iodine Compounds with ICl Bonds as Initiators and Chain Transfer Agents in the Synthesis of Branched Polymers

“…Dichloroiodoarenes ArICl 2 can easily transfer Cl atoms to carbon‐centered radicals and some rate coefficients have been reported . In this work, the ability of compounds 3a – d , which also contain a HV iodine(III) center and the labile ICl bond, to participate in chlorine transfer reactions with propagating radicals in polymerization reactions was examined.…”

“…Dichloroiodoarenes ArICl 2 can easily transfer Cl atoms to carbon-centered radicals and some rate coefficients have been reported. [37] In this work, the ability of compounds 3a-d, which also contain a HV iodine(III) center and the labile ICl bond, to participate in chlorine transfer reactions with propagating radicals in polymerization reactions was examined. Our initial efforts were focused on determining chain transfer coefficients (C CTA ) and transfer rate coefficients (k tr ) of the newly synthesized HV iodine(III) compounds in the polymerization of Sty.…”