Stereochemical Consequences of Recoil Halogen Atom Substitution

Abstract

Hot atom chemistry / Halogen atom substitutions / Stereochemistry SummaryThe stereochemical consequences of recoil ^®Cl-for-X (X = Br, Cl, F) atom substitutions in (2S, 3R)-dl-and (2S, 3S)-dlbromofluorobutanes (BFB's) and in (2S, 3R)-dl-and (2S, 3S)dl-chlorofluorobutanes (CFB's) were investigated in the gas phase as a function of kinetic energy moderator concentration. Results from exothermic ^®Cl-for-Br substitutions in the BPH stereoisomers revealed that the reaction stereochemistry changed from predominant retention of configuration to predominant Inversion of configuration with increasing kinetic energy moderator concentration. An inspection of individual yields revealed that the Channel for Inversion product formation was insensitive to moderator concentration, while the Channel for retention product formation decreased in importance as the moderator concentration was increased. In contrast, results from thermoneutral '®Cl-for-Cl and endothermic ^®Cl-for-F substitutions in the CFB stereoisomers revealed that reaction stereochemistry for both cases was insensitive to changes in the moderator concentration. An inspection of the individual product yields from both reactions revealed that the Channels for retention and Inversion product formation exhibited the same dependence on moderator concentration. Both Channels decreased in importance as the moderator concentration was increased.

Stereochemical Consequences of Recoil Halogen Atom Substitution IV : Effect of Kinetic Energy Moderator Concentration on the 18F-for-X (X=Br, F) Reactions in Gaseous Diastereomeric 2,3-Bromofluorobutanes

“…Several consistencies should be noted between the present work and results from parallel studies involving recoil Cl-for-X substitution [10,11] as well as recoil 18 F-for-X substitution [12] with DFB, DCB, FCB, and FBB stereoisomers. Regardless of the nature of the hot atom, the stereochemical consequences of exothermic substitution reactions are such that predominant retention of configuration always prevails at high kinetic energies, while inversion of configuration is seen to increase in importance as the energy of the nucleogenic species is lowered.…”

“…A detailed description of the synthesis of (2S,3R)-dZand (2S,3S)-i//-BFB's can be found in the second paper of this series [11]. In the general method [18,19], addition of bromide ion to trans-and cis-2-butenes was carried out in the presence of a fluoride ion source to yield net BrF addition across the double bond of each alkene.…”

“…Several features of these reactions have come to light, as well as old concepts reinforced, from systematic studies in the gas phase that have been carried out on these types of molecules [10][11][12][13][14]. The present work represents paper 4 of a six paper series that we have published on the subject.…”

“…Parallel studies on 18 F-for-X (X = F, CI) substitution in the same dihalobutane systems [11] revealed two additional features of reaction. First, the overall reaction efficacy was sensitive to the size and mass of the leaving group, but only when the hot atom itself was relatively large.…”

“…Additional support for this contention was seen from work published in this series [11] where a detailed study of three reactions possessing different thermochemical properties was carried out in order to ascertain whether systematic trends in the reaction stereochemistry could be correlated with kinetic energy. The reactions investigated included exothermic 38 Cl-for-Br substitution in the BFB's isomers, thermoneutral 38 Cl-for-Cl substitution in the CFB's isomers, and endothermic 38 Cl-for-F substitution in the CFB isomers.…”