✍ Scribed by Adam, Waldemar ;Pasquato, Lucia ;Will, Bernd ;Lucchi, Ottorino De
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The reaction of PTAD with 2-trimethylsilyl-, 2-chloro-, 2-cyano-, and 2-methoxycarbonylnorbornadienes 1 ad, respectively, was investigated. In all cases homo Diels-Alder adducts were obtained, for 1 a the unexpected regioisomer, the 1-trimethylsilyl derivative 2a, and for other norbornadienes 1 b-d the usual cyclopropanesubstituted products 2b-d. Except for the trimethylsilyl system la, for which also the dicarboximides 4a and 4'a, respectively, (rearrangement urazoles) were obtained, the other norbornadienes 1 b-d afforded the insertion products Sb-d. With increasing electron-withdrawal by the 2-substituents the insertion products Sb-d increased at the expense of the homo Diels-Alder adducts 2b-d. These results are mechanistically rationalized in terms of stepwise cycloaddition via 1,s-dipolar intermediates. In the case of 2-chloronorbornadiene (1 b), besides the homo Diels-Alder 2 b, the rearrangement urazole 4b and the insertion product Sb, also the [2 + 23 cycloadduct 3 b was formed.
The usual cycloaddition of dienophiles with norbornadienes is the homo Diels-Alder reaction". In the case of carbenes and singlet oxygen, such electrophilic species lead besides homo Diels-Alder products also to [2 + 21 cycloadducts? In fact, with difluorocnrbene [2 + 23 cycloaddition predominates; but with increasing electron-withdrawing substituents at the 2-position of the norbornadiene, the proportion of homo Diels-Alder reaction decreases. This interesting reactivity pattern has been rationalized" in terms of LUMO carbene -HOMO diene interaction and polarization of the n bonds by the 2-substituents, suggesting that other electrophilic dienophiles should portray this cycloaddition behavior.
Previously we reported ' ) our preliminary results on the reaction of PTAD with 2-chloronorbornadiene, which exhibits most unusual cycloaddition behavior (Eq. 1; X = CI). While precedents Tor homo Diels-Alder, [2 + 23, and rearrangement products 2,3, and 4 (X = CI). respectively, are documented4). the unusual insertion product 5 (X = CI) was new.
Since in the difluorocarbene reaction 2-substituents in the norbornadiene markedly influence the cycloaddition mode (homo Diels-Alder versus [2 + 13 reactivity)*', it was our interest to explore such substituent e&cts on the product distribution for the PTAD reaction. The results are sum-Cycloadditionsverhalten von 2-substituierten Norbornadienen mit 4-Phenyl-4H-l,2,4-triazol-3,5-dion (PTAD): Homo-Diels-Alder-Reaktivitat gegeniiber Einschiebung, Umlagerung und [2 + 21-Cycloaddition Die Reaktion von PTAD mit den 2-Trimethylsilyl-, 2-Chlor-, 2-Cyan-oder 2-Methoxycarbonylnorbornadienen 1 ad wurde 'H-NMR decoupling experiments, permitted unequivocal characterization of the cycloadducts Zb, 3b, and 4b (cf. Experimental), an X-ray analysis was essential in determining the structure of the unprecedented insertion product 5b". The structures of the remaining insertion products 5c and 5d, as well as the other cycloadducts, were arrived at by
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Cycloaddition of PTAD to 7-methylenenorbornene (1 a) afforded the rearrangement urazole 2a, but not with 7-(pheny1methylene)norbornene (1 b) and 7-(chloromethylene)norbornene (lc). Instead, besides the [2 + 23 cycloadducts 3b and 3c, respectively, l b gave the double [4 + 23 adduct 4b and l c the no
## Abstract An (__E__)/(__Z)__ mixture (3 : 2) of 7‐benzylidenecycloocta‐1,3,5‐triene (**5**) is obtained when 1‐benzylcycloocta‐1,3,5,7‐tetraene (**7**), prepared by an improved procedure, is treated with __t__‐BuOK in THF. Alternatively, a __ca.__ 9 : 1 mixture (__E__)/(__Z__)‐**5** can be prepar
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