Dissociation or Cyclization: Options for a Triad of Radicals Released from Oxime Carbamates

A set of oxime carbamates having N-alkyl and N,N-dialkyl substituents were prepared via carbonyldiimidazole intermediates. It was shown by EPR spectroscopy that they underwent clean homolysis of their N–O bonds upon UV photolysis. During photolysis of acetophenone O-allylcarbamoyl oxime, the corresponding oxazolidin-2-onylmethyl radical was detected by EPR spectroscopy, providing the first evidence that N-monosubstituted carbamoyloxyl radicals can hold their structural integrity. N,N-Disubstituted carbamoyloxyl radicals dissociated rapidly at the lowest accessible temperatures. Above room temperature, both types of oxime carbamate acted as selective new precursors for aminyl and iminyl radicals. Rate parameters were measured for 5-exo cyclization of N-benzyl-N-pent-4-enylaminyl radicals; the rate constant was smaller than for C-centered and O-centered analogues. Oxime carbamates derived from the volatile diethylamine afforded aryliminyl radicals that proved convenient for phenanthridine preparations.


2.
With norbornene: The NMR spectrum and GC-MS showed a complex set of compounds including Et 2 NH, PhC(=O)O), ImNEt 2 , Im-Im and many other unidentified S10 components. The adducts of Im and Et 2 N with norbornene could not be observed with certainty.
The photolysis was repeated without the MAP and gave 3-methoxy-6-methylphenanthridine 10 (61%). In both cases the spectra showed the presence of some unidentified by-products.

EPR Spectroscopy
EPR spectra were obtained at 9.5 GHz with 100 kHz modulation employing a Bruker EMX

Derivation of Kinetic Equations (1 & 2) for Cyclization of Aminyl Radicals.
For a general Aminyl radical, let RN(  )Pe be N, the iminyl radical, Im, and the cyclized radical C. In the temperature region where decarboxylation is fast the mechanism may be represented by: k -c (backwards) Im + Im  non-radical products Im + N  non-radical products Im + C  non-radical products N + N  non-radical products N + C  non-radical products C + C  non-radical products Assuming all the terminations are fast and diffusion controlled with the same rate constant 2k t then, making the steady-state approximation: Since equi-molar amounts of N and Im are formed in the initial photochemical bond fission then: If reverse cyclization is negligible {k -c /2k t << 2[Im]} this simplifies to: (1) in main text]. S16

Computational Methods
Radical ground-state calculations were carried out using the Gaussian 09 program package. 3 Becke's three-parameter hybrid exchange potential (B3) 4 was used with the LYP correlation functional, B3LYP. This method has previously described the chemistry of iminyl radicals accurately. The correlation consistent polarized triple zeta cc-pvtz basis set was employed.
Geometries were fully optimized for all model compounds. Optimized structures were characterized as minima or saddle points by frequency calculations. The experimental kinetic and spectroscopic data was all obtained in the non-polar hydrocarbon solvents tertbutylbenzene or cyclopropane. Solvent effects, particularly differences in solvation between the neutral reactants and neutral transition states, are therefore expected to be minimal. In view of this, no attempt was made to computationally model the effect of the solvent. S18