Substituent and Steric Effects on Periselectivity

A surprising steric sensitivity is frequently observed for these radical cation reactions. As was shown previously during the discussion of chemoselectivity, the variable positioning of bulky substituents has effects on periselectivity as well. DFT calculations on the influence of diene substitutions for the neutral reaction have demonstrated a behavior similar to that of the radical cation reaction. Although 

SCHEME 4.11 Acyclic diene substituent effects on periselectivity. 

Diene 41 was used to further test this hypothesis. By adding an electron-withdrawing group to the critical C-l-position, one would expect a decrease in [4 + 2] additions on account of destabihzation of a positive charge at C-1. A methyl group was added to C-3, depicted as diene 42, to stabilize intermediate 50 and favors [2 - - 2] cyclization. The [4 -f 2] additions were observed exclusively on account of sterically induced isomerization to the E isomer. Finally, 43 added to 36e yielded only the [2-1-2] product due to steric bulk at the C-1 -position. The (Z) methyl substituent impedes rotation to the s-cis diene that would be conducive to [4 + 2] product formation. 

SCHEME 4.13 Singly linked intermediates of ET catalyzed reaction between 18 and 19. 

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