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Cycloadditions of `Thiocarbonyl Ylides' with Tetracyanoethylene (=Ethenetetracarbonitrile): Interception of Intermediates
Author(s) -
Huisgen Rolf,
Mloston Grzegorz,
Langhals Elke
Publication year - 2001
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/1522-2675(20010613)84:6<1805::aid-hlca1805>3.0.co;2-t
Subject(s) - chemistry , tetracyanoethylene , zwitterion , intramolecular force , medicinal chemistry , steric effects , ylide , nucleophile , cyclopropane , cycloaddition , regioselectivity , nucleophilic substitution , stereochemistry , ring (chemistry) , organic chemistry , molecule , catalysis
Thiocarbonyl ylides (=sulfonium ylides) belong to the most nucleophilic 1,3‐dipoles (high HO energy). In their reactions with tetracyanoethylene (TCNE=ethenetetracarbonitrile; low LU energy), a borderline crossing from the concerted mechanism to a two‐step pathway via a 1,5‐zwitterion was observed. Steric hindrance at one or both termini of the 1,3‐dipole is an additional requirement. The ylides 3 and 13 , set free by N 2 elimination of dihydro‐1,3,4‐thiadiazoles, underwent electrocyclization or 1,4‐H shift. Ylides 3 and 13 are bases and afforded MeOH adducts of different regiochemistry. Whereas 3 and TCNE in abs. THF at 45° furnished the (3+2) cycloadduct 20 , a MeOH content of 0.5 – 5 vol‐% in THF gave rise to a seven‐membered lactim ether 22 and thiolane 20 in a 65: 35 ratio ( Scheme 4 ). Water (0.5 – 1 vol‐%) in THF led to lactam 24 and adduct 20 in the same ratio. The zwitterion 26 , assumed to be the first intermediate, enters competing reactions: the irreversible ring closure to thiolane 20 and the reversible formation of a strained, cyclic seven‐membered `ketene imine' 28 , which is intercepted by MeOH or H 2 O. The gauche ‐conformation 32 of an analogous zwitterion, produced from the tetrasubstituted `thiocarbonyl ylide' 13 with TCNE ( Scheme 5 ), led to the thiolane derivative 35 , while the anti ‐conformation 33 afforded the thioxo compound 5 and cyclopropane derivative 36 by intramolecular nucleophilic substitution.