Polar Effects in Free-Radical Reactions
J. Am. Chem. Soc., Vol. 121, No. 34, 1999 7761
poor alkenes by nucleophilic radicals generated by Pb(IV)
oxidation of carboxylic acids. These new synthetic achievements
were made possible by evaluation of the involved polar effects
and by knowing the absolute rate constants of some key
elementary steps of the reactions.
Results and Discussion
Homolytic Iodination of Alkanes by Perfluoroalkyl Iodides
and Reduction of Alkyl Iodides by t-BuOOH. In a preliminary
communication, we8 have reported a new direct homolytic
iodination of alkanes by perfluoroalkyl iodides, RfI, in a chain
process (eqs 6 and 7) initiated by t-BuOOH in acetic acid. The
interest of this new method is related to the fact that the direct
free-radical iodination of alkanes by iodine, unlike the other
halogens, is not feasible owing to the large unfavorable enthalpy
for hydrogen abstraction (eq 8).
Figure 1. Iodination of cyclohexane (5 mL of cyclohexane, 5 mL of
AcOH, 1 mmol of C4F9-I, and 2 mmol of t-BuOOH at 80 °C). Yields
based on C4F9-I.
The initiation of the radical chain of eqs 6-7 occurs through
the equilibrium of eq 9, favored by acid catalysis. The
equilibrium is continuously shifted at right by the irreversible
thermolysis of the perester (eq 10), which acts as effective source
of methyl radical (â-scission is the main behavior of tert-butoxyl
radical in refluxing acetic acid).9 t-BuOOH is not an efficient
thermal initiator for free-radical chain processes, but it becomes
efficient in AcOH solution through eqs 9 and 10: the rate
constants for the unimolecular decomposition of CH3-
COOOBu-t and t-BuOOH are respectively10 2 × 10-5 (120 °C)
and 1 × 10-5 s-1 (150 °C). Methyl radical initiates the chain
process by abstracting iodine atom from perfluoroalkyl iodide
(eq 11).
Figure 2. Iodination of cyclohexane (5 mL of cyclohexane, 5 mL of
AcOH, 1 mmol of C4F9-I, and variable amounts of t-BuOOH at 80
°C). Yields and percentage of t-BuOOH based on C4F9-I.
stronger than CH3-H (BDE ) 104.8 kcal mol-1 13
and the
)
CF3-CF2-H bond (BDE ) 102.7 kcal mol-1) is stronger than
CH3-CH2-H (BDE ) 101.1 kcal mol-1).14 However, we
observed that the yields of alkane iodination in acetic acid were
directly related to the amount of t-BuOOH. The yields of
cyclohexane iodination related to the concentration of cyclo-
hexane and t-BuOOH, to reaction time, and to temperature are
reported in Figures 1-3; initially the yields rapidly increase at
higher temperature (∼100 °C) (Figure 3), but after a maximum
value they slowly decrease. Our previous results,15,16 concerning
the generation of alkyl radicals from alkyl iodides and methyl
radical, suggested that this behavior should be due to the further
reaction of alkyl iodides with the methyl radical. Actually, the
results obtained with 1-iodododecane and iodocyclohexane
indicate that alkyl iodides react with t-BuOOH under the same
conditions, leading, paradoxically, mainly to the corresponding
alkanes and for a minor extent to the alkenes. The equilibrium
constants for iodine abstraction from primary, secondary, and
Iodine abstraction according to eqs 7 and 11 is very effective
for both polar and enthalpic reasons (the R-I bond is expected
to be stronger than the Rf-I bond). On the other hand, hydrogen
abstractions from C-H bonds by Rf• radicals (eq 6) are >103
times faster11 than by the corresponding alkyl radicals R•, always
for both polar and enthalpic effects (the Rf-H bond is stronger
than the R-H bond). This makes the chain propagation (eqs 6
and 7) particularly effective. Actually, calculations12 would
indicate that the BDE of an Rf-H bond is not significantly
different from that of an R-H bond, while experimental values
indicate that the CF3-H bond (BDE ) 106.7 kcal mol-1) is
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