A steric scale of 20 recurrent groups was established from comparison of rotational barriers on N-(o-substituted aryl)thiazoline-2-thione atropisomers. The resulting energy of activation ΔG reflects the spatial requirement of the ortho substituent borne by the aryl moiety, electronic aspects and external parameters (temperature and solvent) generating negligible contributions. Concerning divergent rankings reported in the literature, the great sensitivity of this model allowed us to show unambiguously that a methyl appears bigger than a chlorine and gave the following order in size: CN > OMe > OH. For the very bulky CF and iPr groups, constraints in the ground state decreased the expected ΔG values resulting in a minimization of their apparent sizes.

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Cocatalysis by pivalic acid or copper bromide allows a very fast, clean, and high-yielding palladium-catalyzed coupling of a large array of aryl, thienyl, and pyridyl halides with cyclic nitrones, including DMPO. The study of the reaction conditions, scope, and mechanism is presented. Applied to the chiral nitrone MiPNO, this transformation provides a straightforward access to enantiopure α-methyl α-arylglycine esters.