Trifluoromethyl inversion barriers

Carbanions a to the sulfonyl group can be easily prepared due to their high acidity in comparison with a-Aio carbanions. Although a-sulfonyl carbanions have a more sp -hybridized character than a-thio carbanions [24,56], a-sulfonyl carbanions have been shown by Gais and coworker to have a somewhat higher inversion barrier to racemization than a-thio carbanions [35]. For example, when an enantiomerically pure trifluoromethyl sulfone was first deprotonated with n-BuLi and subsequently protonated with trifluoroacetic acid, the sulfone was recovered with 90% ee [Eq. (12)]. 

Other recent PRDDO studies on transition metal complexes include an examination of inversion barriers in metal-phosp-hido complexes. such a.s Cp2ClTiP4)2, and substituent effects in trifluoromethyl[tris(pyrazoly)J borate complexes. ... 

Radicals with very polar substituents e.g. trifluoromethyl radical 2), and radicals that arc part of strained ring systems (e.g. cydopropyl radical 3) arc ct-radicals. They have a pyramidal structure and are depicted with the free spin resident in an spJ hybrid orbital. nr-Radicals with appropriate substitution are potentially chiral, however, barriers to inversion are typically low with respect to the activation energy for reaction. 

As expected, fluorine substitution has some consequences on structure and stability of the radicals, which are different from the hydrocarbon counterparts. a-F radicals prefer the pyramidal structure because of minimizing 1 repulsion. The trifluoromethyl radical F3C is essentially tetrahedral and has a significant barrier to inversion of about 25 kcal mol - .39 In contrast, the methyl radical H3C itself is planar. Fluorine /J to the radical site is of minor structural consequence. Thus, the pcrfluoro-/er/-butyl radical exhibits a more planar geometry. 

Fluorine substituents have a dramatic impact upon the structure of alkyl radicals. The methyl radical itself is planar UV, IR, PES and ESR spectroscopy, as well as the highest level of theoretical analysis, all indicate that its conformational properties are best defined as deriving from a single minimum [2], Fluoromethyl radicals, on the other hand, are increasingly pyramidal [3], with the trifluoromethyl radical being essentially tetrahedral [3 - 7], with a significant barrier to inversion [8,9]. 

Carbanions are trivalent spedes with sp hybridization. The lone pair of electrons occupies one of the sp orbitals, and the geometry is thus tetrahedral. The tetrahedron can undergo inversion or retain its stereochemistry depending upon the attached substituents. However, the geometry of a carbanion stabiUzed by conjugation with substituents is different. A methyl carbanion has a barrier to inversion of about 2 kcal mol , whereas the trifluoromethyl carbanion has a... 

The barriers to nitrogen inversion in the 2,2-bis(trifluoromethyl)aziridines (1) have been studied. The barrier is least in the Af-methyl compound Ed = 7-0 2 kcal mol 0, substantial for the bromo-compound (22-7 ... 

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