Azoalkanes, bicyclic

Use of mild conditions was crucial and the development of diimide reduction of singlet oxygenates, silver-salt-assisted displacement of halide by peroxide nucleophiles, peroxymercuration and demercuration, peroxide transfer from organotin to alkyl triflates, and oxygen trapping of azoalkane-derived diradicals have all played a part in providing the rich harvest of new bicyclic peroxides described herein. 

The Diels-Alder reaction of isopyrazoles 365 with MTAD gives azoalkanes 366. Direct as well as triplet-sensitized (benzophenone) photolysis of these compounds leads to the corresponding housanes (bicyclo[2.1.0]pentanes) 367. Under acidic conditions, the housanes rearrange to the corresponding bicyclic products 368 <1995JOC308,... 

Virtually all bicyclic azoalkanes are made via this route from cyclic 1,3-dienes. (For a recent review on azoalkanes, see Ref. 191.) Azodicarboxylic... 

Although cyclic azoalkanes are well known as biradical precursors [159] they have been used as 1,2- and 1,3-radical cation precursors only recently [160-164]. Apart from the rearrangement products bicyclopentane 161 and cyclopentene 163, the PET-oxidation of bicyclic azoalkane 158 yields mostly unsaturated spirocyclic products [165]. Common sensitizers are triphenyl-pyrylium tetrafluoroborate and 9,10-dicyanoanthracene with biphenyl as a cosensitizer. The ethers 164 and 165 represent trapping products of the proposed 1,2-radical cation 162. Comparison of the PET chemistry of the azoalkane 158 and the corresponding bicyclopentane 161 additionally supports the notion that the non-rearranged diazenyl radical cation 159 is involved (Scheme 31). 

The reactions outlined in Schemes 8-11 indicate that 1,4-diradicals can trap triplet oxygen to give 6-membered cyclic peroxides in variable yields. For example, even under 10 atm oxygen pressure, Ph2CO-sensitized photolyses of the azoalkanes 42 afford the dienes 43 and bicydo[2.1.1]hexanes 44 as the major products while the bicyclic peroxides 45 are formed in very poor yields 1.5%) (Scheme Failure in oxygen... 

Zhang, X., Gramlich, G., Wang, X. and Nau, W.M. (2002) A joint structural, kinetic, and thermodynamic investigation of substituent effects on host-guest complexation of bicyclic azoalkanes by -cyclodextrin./. Am. Chem. Soc., 124 (2), 254-263. 

W. Adam and A. V. Trofimov, Photomechanistic aspects of bicyclic azoalkanes triplet states, photoreduction, and double inversion. Chapter 93 in ref. 22. 

A review of the phototochemistry of A-oxides has appeared and this includes a section on the photo-induced deoxygenation of heterocyclic A-oxides. A second review deals with the mechanistic aspects of the photochemistry of bicyclic azoalkanes and considers their photo-reduction by hydrogen donors, a process for which hydrogen-atom transfer and CT mechanisms have been suggested. 

Localized bicyclic 7.3.2.1 Alkyl-aryl [C16H20, X = H] Photolysis of azoalkane ... 

Charge Transfer and Intermolecular Hydrogen Transfer Mediated by a Conical Intersection Quenching the Fluorescence of Bicyclic Azoalkanes... 

Energy Transfer between Ketones and Bicyclic Azoalkanes... 

Photomechanistic Aspects of Bicyclic Azoalkanes Triplet States, Photoreduction, and Double Inversion... 

Direct Generation of Triplet-Excited Bicyclic Azoalkanes and Their Photophysical Behavior Triplet-State Reactions of Bicyclic Azoalkanes... 

The bicyclic derivatives 2,3-diazabicyclo[2.2.1]hept-2-ene, DBH (1), and 2,3-diazabicyclo[2.2.2]oct-2-ene, DBO (2), serve as instructive probes for the photochemical and photophysical properties of azoalkanes in view of their contrasting behavior. While derivatives of DBH (1) readily extrude molecular nitrogen from their excited states with quantum yield close to unity, - DBO (2) and most of its derivatives are photoreluctant, that is, they exhibit very low decomposition yields. - With a few exceptions, substituents in the DBH and DBO molecules do not significantly alter the photochemical properties of the parent molecules. Despite the contrasting photochemical reactivity of DBH (1) and DBO (2), their photophysical characteristics are similar. For both 1 and 2, the intersystem crossing is inefficient and triplet lifetimes are very short consequently, these azoalkanes do not phosphoresce, either in solution or even in matrix at 77K. ... 

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