Halogen atoms abstraction

The efficiency of the halomethane addition process depends on the relative rate of halogen-atom abstraction versus that of addition to the alkene ... 

Tri-rc-butylstannane is able to reductively replace halogen by hydrogen. Mechanistic studies indicate a free radical chain mechanism.199 The order of reactivity for the halides is RI > RBr > RC1 > RF, which reflects the relative ease of the halogen atom abstraction.200... 

Radicals for addition reactions can be generated by halogen atom abstraction by stannyl radicals. The chain mechanism for alkylation of alkyl halides by reaction with a substituted alkene is outlined below. There are three reactions in the propagation cycle of this chain mechanism addition, hydrogen atom abstraction, and halogen atom transfer. 

Alkoxy radical fragmentation is also involved in ring expansion of 3- and 4-haloalkyl cyclohexanones. The radical formed by halogen atom abstraction adds to the carbonyl group, after which fragmentation to the carboethoxy-stabilized radical... 

There are two other mechanistic possibilities, halogen atom abstraction (HAA) and halonium ion abstraction (EL), represented in Schemes 4.4 and 4.5, respectively, so as to display the stereochemistry of the reaction. Both reactions are expected to be faster than outer-sphere electron transfer, owing to stabilizing interactions in the transition state. They are also anticipated to both exhibit antiperiplanar preference, owing to partial delocalization over the C—C—Br framework of the unpaired electron in the HAA case or the electron pair in the EL case. Both mechanisms are compatible with the fact that the activation entropies are about the same as with outer-sphere electron donors (here, aromatic anion radicals). The bromine atom indeed bears three electron pairs located in two orthogonal 4p orbitals, perpendicular to the C—Br bond and in one s orbital. Bonded interactions in the transition... 

One can set up to do this using the competition between dimerization and halogen atom abstraction from RX to form the rhodium(III) halide complex. As a function of [RX], the product ratio is quite easily evaluated. From that, one can get the rate constant ratio but, knowing independently the rate constant for dimerization, it is possible to extract from those data rate constants for reactions of the rhodium(II) complex with these organic halides. The rate constants obtained are listed in Table I. 

If one takes out of this list the polyhalomethanes and tries to understand what the chemistry is about, certainly a simple-minded view is that this is simply a halogen atom abstraction reaction such as one finds in a gas phase reaction, like sodium atom with organic halides. There is a linear free energy correlation in that the logarithms of the rate constants bear a linear relation to one another. 

Cp2Fe2(CO)4] promotes photosubstitution of CO by RNC in [CpFe (CO)2I].120 The propagation step in these reactions is halogen atom abstraction from reactant by the 17-electron radicals [CpMo(CO)2PR3] and [CpFe (CO)2(CNR)], respectively. A radical chain pathway has also been proposed for the [Cp2Fe2(CO)4]-promoted photosubstitution in Eq. (27).121 In... 

Halogen atom abstraction. The metal radicals react with alkyl and aralkyl halides (RX, X = Cl, Br, I) to yield CpM(CO)3X and R. The metal halide product is a species known independently, and its formation in these reactions was verified by IR. The organic free radicals were not detected directly, but were inferred from the final organic products determined by GC these products were consistent with those known to be formed when R undergoes dimerization and disproportionation reactions. 

Halogen atom abstraction occurs also with metal halides, both (NH3)5Co -X and X-Rh (dmgH)2PPh3 having been studied. For the latter, 1 1 = x 0 Br mol s for the reactions of... 

A halogen atom abstracts hydrogen from the alkane (RH) to form an alkyl radical (R ). The radical in turn abstracts a halogen atom from a halogen molecule to yield the alkyl halide (RX). 

The physical aspects underlying the reactivity of P-centred radicals are essential in order to benefit from the full potential offered by phosphorus-containing compounds. Hydrogen atom transfers (HATs), halogen atom abstractions, cyclisations and additions are the tool box of the widely applied free radical chemistry to date. Several authors have determined key rate constants through various physical methods, including time-resolved ESR spectroscopy and flash photolysis.3... 

These radicals are reactive towards halogen atom abstraction from CX4. (X-Cl, Br) to give mononuclear pseudo-square planar [MX(CNMe),] (M-Pd, Pt). 25 photolysis of irans-(Pd(PPrS) (NCS) ]... 

The issue of the role of bridged radicals in the stereochemistry of halogenation has recently been examined computationally and a new interpretation offered. The structure, rotational barriers, and for halogen atom abstraction for P-haloethyl radicals were studied. For the reactions where X=C1 or Br, the halogen atom abstraction reaction shows a preference for a trans TS. 

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