The enthalpy for the reaction of di-ferr-butyl peroxide with phenol to give zerf-butyl alcohol and phenoxyl radical (i.e. í-BuOOBu-í + 2 PhOH -2 f-BuOH + 2 PhO*) has been determined in a number of solvents using photoacoustic calorimetry. The effect of the solvent on the thermochemistry of this process is remarkably large with the reaction being ca. 10 kcal mol-1 more exothermic in acetonitrile or ethyl acetate than in isooctane. The relationship between the observed enthalpy changes and the PhO-H bond energy is discussed in detail. It is shown that in order to extract the bond energy from the experimental (apparent) enthalpy change, it is necessary to account for a number of processes, viz., the volume change for the overall reaction, the solvent effect associated with the conversion of 1 mol of di-ferf-butyl peroxide to 2 mol of ferf-butyl alcohol, and the differences in solvation of phenol and the phenoxyl radical. The contributions from each of these processes to the observed reaction enthalpy were derived from a separate set of experiments with 1,4-cyclohexadiene instead of phenol or from data available in the literature. These data allow one to determine solution bond energies, i.e., the enthalpy of homolysis for which the standard state is the solvated reactant and products, and to quantify the solvent effect on these values. Thus, PhO-H bond energies in isooctane (88 kcal mol-1), benzene (89 kcal mol-1), carbon tetrachloride (90 kcal mol-1), ethyl acetate (95 kcal mol-1), and acetonitrile (95 kcal mol-1) have been obtained. Most of the differences between these values can be accounted for from the known hydrogen bonding equilibrium between the solvents and the phenol. A number of purported determinations of the PhO-H "gas-phase bond energy" which utilized electrochemical (EC) measurements and, of necessity, highly polar solvents are shown to be seriously in error. Similar errors must be present in many other EC "gas-phase" bond energies which also were determined in polar solvents.Bond dissociation energies (BDE) are of fundamental importance because they permit chemists to decide whether or not a particular reaction will be enthalpically favored. Such decisions are usually based on bond dissociation energy differences (ABDEs). For example, for the hydrogen atom abstraction X' + RH -XH + R"(1) the reaction enthalpy, AHr, is determined from the difference in BDEs for XH (eq 2) and RH (eq 3), ABDE = BDE(X-H) -BDE(R-H). Unfortunately, the vast majority of BDEs in the XH -X' + H* (2) RH -R' + H* (3) literature refer to gas-phase reactions, while most of the chemistry to which they are applied occurs in solution. However, even for homolytic reactions involving a neutral radical and substrate, it is not certain that the quantity (AflD£)gas will be equal to (ABDE)ml. In addition, many BDEs of interest
The influence of the anomeric effect on radical cyclization has been examined by determining the stereochemical outcome of the ring closure of 10 suitably substituted radicals. The stereochemistry of the products formed from the acyclic precursors 4a-f indicates that for suitably constituted radicals anomeric interactions stabilize pseudoaxially substituted transition structures 14 thus affording products with stereochemistry the reverse of that normally observed.In recent times, the use of free radical reactions for chemical synthesis has grown steadily in popularity as the processes which govern and control their outcomes have become better understood. 1 Intramolecular addition reactions of suitably constituted alkenyl radicals and related species are particularly interesting since they readily afford a variety of ring systems commonly found in many natural products. 1 The advantages offered by such reactions generally include high regioselectivity, modest to good stereoselectivity, 1-5 and high chemoselectivity. [1][2][3][4][5] The highly regioselective exo-cyclization of the hex-5-enyl radical has been intensively studied. The accurate determination of its kinetics 6 has underpinned its widespread use as a mechanistic probe and as a radical clock. 7 Most monosubstituted hex-5-enyl radicals also undergo regioselective exo-cyclization while conforming to the general stereochemical guideline that 1-or 3-substituted radicals afford predominantly cis-disubstituted products, while 2-or 4-substituted radicals give mainly transdisubstituted products. 5 It is now generally accepted that the cyclization behavior of simple hex-5-enyl systems reflects the stereoelectronic demands of the transition structure. [2][3][4][5] The hypothesis that the strain energy engendered in attaining the optimum orbital overlap for 1,6-ring closure outweighs the steric and thermodynamic factors which would otherwise favor six-membered ring formation 2-5 has received support from theoretical analyses 8 and the modeling of transition structures by a combination of molecular orbital and molecular mechanics calculations. 8 These calculations confirm the original hypothesis that the transition structure for 1,5-exo-cyclization of hex-5-enyl radicals and related species resembles cyclohexane in its chair form. 2-5 Hence the stereoselectivity exhibited in the ring closure of monosubstituted hexenyl radicals and related species is seen to arise from the propensity of cyclizations to proceed through the more stable pseudoequatorially substituted cyclohexane-like conformation of the transition structure rather than through its less stable pseudoaxially substituted conformer. 8 Since calculations of the relative selectivities of the cyclized products tend to be overestimated when only axially substituted chairlike structures are considered, it appears that boatlike transition structures might also be involved in some reactions. 4,8 Cyclizations of hex-5-enyl type radicals containing heteroatoms have also been studied. 1,9,10 In general such spec...
From intermolecular and intramolecular competition experiments, it has been established that, by comparison with an N-methyl substituent, an N-acetyl group deactivates glycine residues in piperazine-2,5-diones towards free-radical bromination. Combined with the ease of introduction and removal of N-acetyl substituents, the deactivating effect provides a method for regiocontrolled functionalization of these compounds.
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