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On the mechanism of diels‐alder reactions of nitroso alkenes: exo/endo selectivity, stereospecificity, E/Z selectivity, and relative reactivity of various olefins
Author(s) -
Reißig HansUlrich,
Hippeli Claudia,
Arnold Thomas
Publication year - 1990
Publication title -
chemische berichte
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 0009-2940
DOI - 10.1002/cber.19901231224
Subject(s) - chemistry , silylation , enol , steric effects , cycloaddition , stereospecificity , reactivity (psychology) , nitroso , selectivity , bicyclic molecule , enol ether , alkene , stereochemistry , nitroso compounds , medicinal chemistry , organic chemistry , catalysis , medicine , alternative medicine , pathology
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 l a as dienophile. On the other hand, the siloxycyclopentene 1 c gives a mixture of cycloadducts 3 c 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 1 b and 1 d. Experiments with enol ethers 1 f and 1 g also occur stereospecifically. α‐Nitrosostyrene 2b reveals surprisingly high k E/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.