Yuan L. Chow

Reduction of aromatic nitro-compounds to amines with sodium borohydride–copper(II) acetylacetonate

The reduction of aromatic nitro-compounds by NaBH4 in ethanol in the presence of various transition metal cetylacetonates has been investigated. Copper(II) acetylacetonate was found to be the most effective catalyst for reduction to the corresponding amines. This reaction appears to proceed through a hydride metal complex.

[2 + 2] Photocycloadditions of dichloromaleimide and dichloromaleic anhydride to cyclic olefins

The photocycloaddition of dichloromaleimide to cyclohexadiene, cyclopentadiene, and cyclopentene on direct photolysis gave [2 + 2] addition products with the cis,endo- and cis,exo-configurations as the major products. However, a similar photocycloaddition to cyclohexene yielded the trans-fused cycloaddition product (between the four- and six-membered rings) as the major product in addition to the cis,endo- and cis,exo-isomers. The photocycloadditions of dichloromaleic anhydride to cyclopentadiene and cyclohexadiene were reinvestigated; two major products were obtained which were identified as the cis,endo- and cis,exo-addition products, while the reported trans-fused isomers were not isolated. The structures of these photoaddition products were investigated by 400 MHz n.m.r. spectroscopy to ascertain the configuration of the ring fusion. An X-ray crystallographic analysis established the stereochemistry of one product.

Synthesis of azapolycyclic compounds by the aminium radical route: rearrangements of C-nitroso compounds and their stereoelectronic requirements

The aminium radicals generated from the photolysis of N-nitroso-endo-2-methylamino-methylbicyclo[2.2.1]hept-5-en and N-nitroso-endo-2-methylaminomethylbicyclo[2.2.2]oct-5-ene cyclize at the C-6 position, since the relevant reacting centers can achieve excellent orbital overlap with minimum molecular motion in comparison to the alternative cyclization path at the C-5 position. The photolysis of the former compound gave a C-nitroso compound which underwent a series of unusual intramolecular cleavages, deoximations, and redox reactions during the work-up. Stereoelectronic requirements for these intramolecular reactions are discussed. Thermolysis of N-chloro-endo-2-methylaminomethylbicyclo[2.2.2]oct-5-en in methanol also gave the product with azaisotwistane skeleton.

Syntheses of azapolycyclic compounds by aminium radical routes: trapping of the radical intermediates

Photolysis of three alkenyl nitrosamines in the presence of oxygen or bromotrichloromethane resulted in the interception of the intermediate C-radicals by these radical trapping agents and the reaction pathways were cleanly diverted leading to the formation of the nitrate esters or halides with pyrrolidine rings as the primary products. The exo-nitrates in the oxidative photolyses decomposed by secondary ionic pathways; these reactions were hydrolysis, nitrous acid elimination and a cleavage reaction (promoted by a β-amino group), among others. The efficiency of the cleavage reaction is controlled by a stereoelectronic factor that requires the participating bonds and the lone-pair nitrogen orbital be oriented in an antiperiplanar conformation. When such a conformation exists in a rigid or semiflexible framework, cleavage occurs extensively. However, in freely rotating acyclic systems, cleavage does not occur even when the required conformation can be attained. Only halides resistant to intramolecular nucl...

Nitrosamine photoaddition to norbornene and the mechanism of nitrosoalkane cleavage

Evidence is presented that the facility of the functional groups to assume cis-coplanarity governs the cleavage of a 2-ammonio-1-nitrosoalkane which is obtained from nitrosamine photoaddition to an olefin.

Addition reactions of aminium radicals: oxidative and non-oxidative photoaddition of nitrosoamines to non-conjugated polyenes

The oxidative photoaddition of N-nitrosodimethylamine to various olefins gives products similar to those obtained from N-nitrodimethylamine despite certain dis-similarity in the mechanisms. The oxidative photoaddition to trans,trans,trans-cyclododeca-1,5,9-triene in the presence of an excess of N-nitrosodimethylamine apparently gave good yields of the expected amino-nitrates. These were reduced by lithium aluminium hydride to afford predominantly the open-chain α,ω-substituted amino-alcohol. The non-oxidative photoaddition of N-nitrosodimethylamine to hepta-1,6-diene gave small amounts of cyclized pentane derivatives whereas similar addition of bulkier N-nitrosopiperidine gave higher yields of the cyclized products. While photoadditions of N-nitrosodimethylamine to trans,trans,trans-cyclododeca-1,5,9-triene failed to give a cyclization product, similar photoadditions to cis,trans-cyclodeca-1,5-diene gave a pair of epimeric alcohols derived from a stereospecifically cyclized perhydroazulenoid skeleton under oxidative conditions, and the corresponding oximes under non-oxidative conditions. The structures of these azulenoid compounds have been elucidated and the remarkable stereospecificity of addition–cyclization process is discussed.

Addition reactions of aminium radicals: synthetic studies of oxidative photoaddition of N-nitrodimethylamine (NNOD)

Photoadditions of NNOD to various olefins under oxygen lead to 2-dimethylamino-1-nitrate esters in good yields as primary products. The amino-nitrates are stable in acidic solution but their stability towards bases varies over a wide range depending on configuration. In most cases, the crude photoaddition products were reduced with lithium aluminium hydride to give good yields of the alcohols. The nitrates derived from cyclododeca-1,5,9-trienes were stable and reduced by lithium aluminium hydride to give, quite unexpectedly, the acyclic α,ω-dimethylamino-alcohols. The oxidative photoaddition to cis,trans,trans-cyclododecatriene occurred preferentially at the trans-double bond and that to dicyclopentadiene at the norbornene double bond. In both cases, good regioselectivity was observed. The precursors to the nitrates were suggested to be the corresponding peroxynitrites which might also undergo various ionic or light-induced reactions.

SYNTHESIS OF AZAPOLYCYCLIC COMPOUNDS BY THE AMINIUM RADICAL ROUTE: REARRANGEMENTS OF C-NITROSO COMPOUNDS AND THEIR STEREOELECTRONIC REQUIREMENTS

The aminium radicals generated from the photolysis of N-nitroso-endo-2-methylamino-methylbicyclo[2.2.1]hept-5-en and N-nitroso-endo-2-methylaminomethylbicyclo[2.2.2]oct-5-ene cyclize at the C-6 position, since the relevant reacting centers can achieve excellent orbital overlap with minimum molecular motion in comparison to the alternative cyclization path at the C-5 position. The photolysis of the former compound gave a C-nitroso compound which underwent a series of unusual intramolecular cleavages, deoximations, and redox reactions during the work-up. Stereoelectronic requirements for these intramolecular reactions are discussed. Thermolysis of N-chloro-endo-2-methylaminomethylbicyclo[2.2.2]oct-5-en in methanol also gave the product with azaisotwistane skeleton.