128388-82-9Relevant academic research and scientific papers
On the Mechanism of Diels-Alder Reactions of Nitroso Alkenes: exo/endo Selectivity, Stereospecificity, E/Z Selectivity, and Relative Reactivity of Various Olefins
Reissig, Hans-Ulrich,Hippeli, Claudia,Arnold, Thomas
, p. 2403 - 2411 (2007/10/02)
The mechanism of the hetero Diels-Alder reaction of nitroso alkenes 2 with silyl enol ethers and other olefins has been investigated.Using the bicyclic nitroso compound 2a a study of the exo/endo selectivity has demonstrated that the exo approach is preferred with the siloxyethene 1a as dienophile.On the other hand, the siloxycyclopentene 1c gives a mixture of cycloadducts 3c with an excess of endo product (endo:exo = 82:18).The stereospecificity of the nitroso alkene cycloaddition could be demonstrated with the stereochemically homogeneous silyl enol ethers 1b and 1d.Experiments with enol ethers 1f and 1g also occur stereospecifically. α-Nitrosostyrene 2b reveals surprisingly high kE/Z values when E/Z-isomeric olefins are compared in competition experiments.Also, a detailed reactivity scale of 2b including various structurally different silyl enol ethers and other typical dienophiles shows that a complex interplay of electronic and steric effects is operating.The large influence of steric effects is taken as evidence for a highly ordered transition state in the cycloaddition.All mechanistic details for the Diels-Alder reactions of nitroso alkenes 2 with (silyl) enol ethers are in strong accord with a concerted mechanism and exclude the involvement of zwitterions or diradicals as intermediates.
