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Photoreactions of oximes have been known for 120 years. The earliest recognized reaction was geometric isomerization, which is still the most prominent oxime photoreaction; it proceeds via a ππ* triplet state. More than sixty years later the intriguing photo-Beckmann rearrangement came to light, a C–C=N–O to O=C–N–C conversion, generating, for example, ɛ-caprolactam from cyclohexanone ξιμɛ. The rearrangement proceeds via a ππ* singlet state forming an oxazirane intermediate; the oxazirane ring usually opens in stereospecific fashion. Detailed insight into the stereo- and regiochemistry of the rearrangement was gained from steroidal oximes, the most thoroughly investigated class of oximes. Additional photoreactions of oximes or of their ethers and esters include: net loss of hydroxylamine generating carbonyl compounds; loss of water from aldoximes forming nitriles; conversion of ketoximes to nitriles by more complex mechanisms; photochemical O–H cleavage generating iminoxyl radicals, an interesting family of free radicals; N–O cleavage giving rise to iminyl radials, which undergo radical cyclizations. β,γ-Unsaturated oximes undergo aza-di-π-methane rearrangements, cyclize to oxazolines, and undergo further reorganizations. The palette of oxime reactions has been further enriched by triplet energy transfer or electron transfer sensitization; oximes serve as electron donors or acceptors. Keywords: Light-induced chemistry of oximes and derivatives; HEINZ D. ROTH; E, Z isomerization; photo-Beckmann rearrangement; oxazirane formation; O–H cleavage; N–O cleavage; aza-di-π-methane rearrangement; isoxazoline formation; iminoxyl radical; iminyl radial; steroidal oximes; oxo-oximes

Photoinduced electron transfer has attracted much attention because of its central role in the chemistry of life and as a method for generating radical ion pairs in solution. Radical cations so generated can be characterized by many spectroscopic techniques, and their reactions can be studied conveniently. Photoinduced electron transfer is compared with alternative methods of radical ion generation; the application of these methods is discussed in conjunction with suitable techniques of observation.

We have studied Jahn-Teller (JT) type distortions in a series of methyl-substituted cyclopropane cations with ab initio molecular orbital techniques. Two sets of cyclopropane cation structures are considered for the parent (1a) and the 1-methyl-substituted (1b), 1,1-dimethyl-substituted (1c), 2,3-dimethyl-substituted (1d (trans), 1e (cis)), and 2,2,3,3-tetra-methyl-substituted (1T) species

Incorporation of acetyl or benzoyl oximes into the channels of a pentasil zeolite (ZSM-5) gives rise to iminoxyl radicals which show characteristic solid state EPR powder patterns.

The electron transfer photo-sensitized reaction of quadricyclanone (1) with methanol produces a single product, 7- syn -methoxybicyclo[2.2.1]hept-5-en-2-one (2) via nucleophilic attack of methanol on the radical cation of 1. The structure of the product and the stereochemistry of the methoxy group reveal that the radical cation is captured by backside attack of methanol on one cyclopropane function followed by a specific cyclopropylcarbinyl to butenyl rearrangement.

The photosensitized oxidation of two cyclic azoalkane derivatives (Azo-Q, Azo-N) of quadricyclane (Q) and norbomadiene (N) has been investigated using steady-state and laser flash photolysis techniques as well as chemically induced dynamic nuclear polarization (CIDNP). Irradiation of acetonitrile solutions of 9,lO-dicyanoan- thracene (DCA), 2,6,9,1O-tetracyanoanthracene (TCA), or chloranil (Chl) in the presence of Azo-Q results in rapid and efficient deazatization to afford N. Similar irradiation of DCA, TCA, and Chl in the presence of Azo-N yields N and Q. CIDNP experiments reveal that polarized N is formed as a cage product in the Chl sensitized photolysis of Azo-Q while polarized N and Q are both formed as cage products from Azo-N. The results are consistent with competitive fragmentation of Azo-"+ to N'+ and Q+, and selective deazatization of Azo-@+ to N'+. Irradiation of 1-cyanonaphthalene in the presence of Azo-Q or Azo-N affords Q andor N in chemical yields similar to those obtained from direct and naphthalene sensitized irradiations, indicative of singlet energy transfer quenching. Bimolecular rate constants for quenching by Azo-N and Azo-Q were determined by steady-state fluorescence methods for singlet sensitizers, or by laser flash photolysis (LFP) for 3Chl*.

Organic radical cations, generated by electron transfer from bi-or trifunctional donor terpenes to photo-excited acceptors, undergo a variety of intra- and intermolecular reactions, including nucleophilic capture and sigmatropic shifts, deprotonation, intramolecular nucleophilic capture, and various cyclizations.

Incorporation of 2-phenyl-1,3-dithiane 1 into ZSM-5 generated 1,2-dithiolane radical cation 2·+, the EPR spectrum of which shows an orthorhombic powder pattern (g1 = 2.0293, g2 = 2.0193, g3 = 2.0030); each g component showed a 1:4:6:4:1 pattern (a1 = 10.5, a2 = 9.3 and a3 = 9.2 G) due to coupling to four equivalent 1H nuclei, suggesting that the pseudo-axial and –equatorial α-protons undergo conformational equilibration whereas the sulfur centers are held stationary on the EPR time scale.

Radical ion pairs generated by photo-induced electron transfer or formed upon encounter of independently generated radical ions may undergo back electron pairs of triplet multiplicity. Triplet recombination has been probed by luminescence time-resolved optical spectroscopy (in combination with magnetic field effects), CIDNP spectroscopy, and opto-acoustic calorimetry. The free energy of back electron transfer and the topologies of the potential surfaces of parent molecule, radical ion and the accessible triplet state or biradical play significant roles in this recombination. The free energy governs the rate of back electron transfer and determines whether this process is competitive. The topologies of the intermediates may influence the course of the overall reaction. The topologies of the potential energy surfaces of ground state, radical ion, and triplet related in three different ways. First, aromatic donors/acceptors typically undergo only minor geometry upon oxidation, reduction or excitation to the triplet state. Therefore, rearrangements are unlikely. Alternatively, the connectivities of some radical ions resemble those molecule, whereas the corresponding triplet states (biradicals) have structures of different connectivities. In these cases, back electron transfer will populate a triplet (biradical) with cleaved bonds (“dissociative” electron return) or one with newly formed bonds (“associative” electron return) resulting in structural reorganizations. Third, the radical ions of some strained ring compounds may rearrange to ring-opened structures of lower energy. Here, triplet states or biradicals with geometries related to the rearrangement radical ions may be populated. Finally, triplet radical ion pairs may form CC, CN, or CO bonds between radical anion and cation (charge recombination), generating “bimolecular” states. Following intersystem crossing, the biradicals may fragment to regenerate the reagents, generate adducts by forming a second bond (cyclization) or transfer of an atom or group between the two functions. The different intra-pair reactions of triplet radical are illustrated with appropriate examples.

Le radical cationique du methylene-9 bicyclo [4.2.1] nonatriene-2,4,7 subit une cyclisation intramoleculaire rapide pour generer un radical cationique derivant d'une structure tetracyclique

Les radicaux cationiques du bicyclo [6.1.0] nonatriene et de ses derives sont generes par transfert d'electrons photoinduit, en solution

The radical cations of various cyclopentadiene dimers can be generated by photoinduced electron transfer to strong electron acceptors. Nuclear spin polarization effects observed during these reactions provide insight into the radical cation structures. Three of the systems studied, endo-[4 + 2]-(1), anti-[2+2]-(2), and exo-(4+2]-dicyclopentadiene (5) give rise to unusual singly linked, delocalized radical cations, whereas the anti-[4 + 4]-dimer (3) and 1,3-bishomocubane (4) give rise to more conventional radical cations. The reactions of spiroheptadiene (9) and di(spiroheptadiene) (10) provide evidence for two different dimer radical cations, a doubly linked (D) and a singly linked (S) species. This finding supports a stepwise mechanism for the radical cation Diels-Alder reaction of 9.