The bromination reaction of saturated hydrocarbons under GoAggn* conditions (FeC13.6H20, picolinic acid, Ha&, in pyridine/acetic acid) and under radical chain conditions (dibenzoyl peroxide in pytidine/acetic acid or initiation by UV light) are compared. Differences in the selectivity and kinetic behavior for a series of polyhaloallcanes are in agreement with a non-radical mechanism for GoAggm bromination.
Unusual chemical versatility is shown in the process of selective functionalixation of saturated hydrocarbons by Gif-type systems.' The "natural" chemistry displayed by these systems is the lcetonization of non-activated methylene groups, with quantitative yields at conversions ranging from 15 to 30%. However, if a suitable reagent is added to the reaction mixture, the formation of ketone is diverted toward the corresponding monosubstituted alkyl derivative. For instance, addition of PPh3 affords the cotresponding alcohol; with P(OMe)3 the alkyl dimethyl phosphate is form&, Ph$% produces a quantitative (in Se) yield of the allcyl phenyl selenide; sodium sulfide yields diallcyl oligosulfides.2 Gif chemistry is also able to convert saturated hydrocarbons into alltyl halides. In this case the reagent to be added to the GoAgg" reaction mixture is a polyhaloalkane. such as CBrC13, CBr4, or CCle3
Another interesting aspect of Gif chemistry is the mechanism of the hydrocarbon activation process. Extensive work points against a radical reaction pathway. A mechanism involving a reaction intermediate bearing a carbon-iron bond has been postulated (Figure 1). 2 We felt that additional evidence regarding this point could be collected by comparing different aspects of the well known radical pathway with the characteristics of the halogenation reaction under GoAggtn conditions.
Radical chain halogenation of non-activated C-H bonds is a well investigated process in Organic Chemistry. The selectivity of this reaction, with a variety of substrates and halogenating reagents has been studied. In particular, we were interested in the efficiency of diffennt polyhaloalkanes as carbon radical traps. To our best knowledge, data on relative reaction rates of chain radical halogenation reactions are scarce and questionable because the possibility of side reactions between the competing halogenating reagents was not taken into account.4 Thus, we carried out the competitive experiments required to establish a kinetic order for polyhaloalkanes as radical chain halogenating agents. To avoid the above mentioned problems associated with cross-reactions between the two polyhaloalkanes taking part in the competition, they were competed against a third species, thiophenol. The latter reacts by hydrogen atom transfer to carbon radicals, affording alkanes.5 Then, from the ratios alkyl halidelalkane the relative reactivity nttios wcrt determined. The results obtained with a series of brominating agents are shown in Table 1. Thus, in agreement with partial reported data, the order of decreasing reaction rate for radical reactions is CBr4 > CBrC13 (> Ccl, 3.