Photoreactions of Nitroso Compounds in Solution. XXII. Stereochemical Consequences of the Photoaddition of N-Nitrosopiperidine to Conjugated Acyclic Dienes: a Nuclear Magnetic Resonance Study of Enone Oximes

In the presence of an acid, photoexcited N-nitrosopiperidine adds to conjugated acyclic 1,3-dienes to give the 1,4-adducts as the major product in addition to 1,2-adducts. In the photoaddition to 1,3-pentadienes the reverse 4,1-adduct and 4,3-adducts are also identified. These results suggest that a singlet photoexcited N-nitrosopiperidine–acid complex rapidly decays to piperidinium radical which irreversibly attacks the dienes as the first step, followed by either a short propagation step or a termination step to give C-nitroso compounds. The C-nitroso compounds are rapidly removed by irreversible tautomerization to the final oximes. The reactivity of the piperidinium radical toward conjugated double bonds decreases in the order of RCH=CH2 > cis-RCH=CHR > trans-RCH=CHR. This mechanism is in accord with the following stereochemical results. Firstly, the newly-formed double bond in the 1,4-adduct (or the adduct of the reverse orientation) is exclusively trans-oriented. Secondly, the remaining double bonds in the 1,2-adducts maintain their original stereochemistry.The stereochemistry of the photoadducts, enone oximes 1–11, was determined by means of n.m.r. spectroscopy. From the correlation of the n.m.r. chemical shifts of these enone oximes it has been shown that (i) the α-olefinic proton generally resonates at a lower field than the β-olefinic proton (with one exception) and (ii) in changing from CDCl3 to an aromatic solvent the α-olefinic proton and the α′-alkyl protons are de-shielded by 0.3–0.05 p.p.m. while the β-olefinic proton is shifted only slightly either to a higher or a lower field. It is observed that the order of the chemical shifts of the olefinic protons and the aromatic solvent effects on the chemical shifts in these enone oximes are both opposite to those observed in enone systems.