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

摘要

This paper describes how the rates of 5-exo-ring closures of unsaturated iminyl radicals to pyrrolomethylradicals 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 paramagneticresonance (EPR) spectroscopy. Pentenyliminyl radicals with aryl substituents at their C=N bonds, and onewith an alkyl substituent at its C=Nbond, were studied in solution by steady-state continuous wave EPRspectroscopy. All 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- 1-phenylpent-4-enyliminyl radical showed that it underwent bimolecular combination at about the diffusion controlled limit (2kt～3 × 10~8M~(-s~(-1) at 245 K). The rateconstant for 5-exo-ring closure of pltenylpentenyliminyl (8.8 × 10~3s~(-1)at 300 K) was a factor of 25 smallerthan the rate constant for hex-5-enyl radical cyclization. The rate of cyclization was slower for an iminylhaving a Me group at the site of 5-exo-cyclization but faster for an iminyl with an Et substituent at theterminus of the C=C double bond. Surprisingly, the 2,2-dimethyl- 1 -phenylpent-4-enyliminyl radical, with abismethyl group in its pentenyl chain, ring closed more slowly than the unsubstituted analogue. DFTcomputations were in accord with this inverse gem-dimethyl effect and suggested it resulted from stericinteraction 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 pentenyliminylradical with an alkyl group in place of the Ph group, and a single Me group in its pentenyl chain, was generatedby means of an unsymmetrical dioxime oxalate precursor. The k_c for this species was a factor of 2.5 largerthan k_cfor the original pentenyliminyl, suggesting that the normal positive gem-dimethyl effect does operatefor pentenyliminyls lacking the aromatic substituent at the C=N bond. DFT computations also successfullyreproduced this trend for model iminyls. It appears that for alkenyliminyl radicals positive or negative gem-dimethyl effects on the cyclization can be induced by appropriate choice of the second substituent on theC=N bond.