Tert-Butyl group large size

The perfluoroalkyl group CF3 has a relatively large van der Waals volume -larger than methyl, mono-fluoromethyl and di-fluoromethyl and between those of the iso-propyl and the tert-butyl groups [35]. The latter is comparable in size to the perfluoro-iso-propyl group. 

This decrease of the stereoselectivity shows the influence of A -strain effects. If the size of the group R at the radical center becomes smaller, the repulsion between R and the large substituent L at the stereogenic center decreases. As a consequence, the dihedral angle between these groups is reduced and the shielding of the syn face becomes less effective. This will be demonstrated by radicals 7 and 20. In tertiary radical 7A the (1-tert-butyl substituent and the jff-H atom are on opposite faces of the... 

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