5-Exo-Cyclizations of Pentenyliminyl Radicals: Inversion of the gem-Dimethyl Effect

Resumen

This paper describes how the rates of 5-exo-ring closures of unsaturated iminyl radicals to pyrrolomethyl radicals respond to substituents in the pentenyl chain and at the C=N bond. Benzyl- and acyl oxime esters, as well as dioxime oxalates, were identified as suitable iminyl radical sources for electron paramagnetic resonance (EPR) spectroscopy. Pentenyliminyl radicals with aryl substituents at their C=N bonds, and one with an alkyl substituent at its C=N bond, were studied in solution by steady-state continuous wave EPR spectroscopy. AU the pentenyliminyls selectively ring closed in the 5-exo-mode rather than the 6-endo-mode. EPR monitoring of the decay of the 2,2-dimethyl-l-phenylpent-4-enyliminyl radical showed that it underwent bimolecular combination at about the diffusion controlled limit (2kt ∼3 × 108 M-1 s-1 at 245 K). The rate constant for 5-exo-ring closure of phenylpentenyliminyl (8.8 × 10 3 s-1 at 300 K) was a factor of 25 smaller than the rate constant for hex-5-enyl radical cyclization. The rate of cyclization was slower for an iminyl having a Me group at the site of 5-e*o-cyclization but faster for an iminyl with an Et substituent at the terminus of the C=C double bond. Surprisingly, the 2,2-dimethyl-1-phenylpent-4-enyliminyl radical, with a bismethyl group in its pentenyl chain, ring closed more slowly than the unsubstituted analogue. DFT computations were in accord with this inverse gem-dimethyl effect and suggested it resulted from steric interaction of the Ph and bis-Me groups which forced the aromatic ring out of the plane of the imine moiety. To check on the role of the Ph substituent, pentenyliminyls lacking this group were sought. A pentenyliminyl radical with an alkyl group in place of the Ph group, and a single Me group in its pentenyl chain, was generated by means of an unsymmetrical dioxime oxalate precursor. The kc for this species was a factor of 2.5 larger than kc for the original pentenyliminyl, suggesting that the normal positive gem-dimethyl effect does operate for pentenyliminyls lacking the aromatic substituent at the C=N bond. DFT computations also successfully reproduced this trend for model iminyls. It appears that for alkenyliminyl radicals positive or negative gemdimethyl effects on the cyclization can be induced by appropriate choice of the second substituent on the C=N bond. © 2009 American Chemical Society.