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Massive Steric Hindrance in Two ‘Thiocarbonyl Ylides': Cycloadditions with Tetra‐Acceptor‐Substituted Ethylenes via Zwitterionic Intermediates
Author(s) -
Giera Henry,
Huisgen Rolf,
Langhals Elke,
Polborn Kurt
Publication year - 2002
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(200206)85:6<1523::aid-hlca1523>3.0.co;2-o
Subject(s) - chemistry , steric effects , isomerization , ketene , medicinal chemistry , stereochemistry , acceptor , solvent , photochemistry , organic chemistry , catalysis , physics , condensed matter physics
The switch from a concerted to a two‐step pathway of 1,3‐dipolar cycloadditions was recently established for the reactions of sterically hindered ‘thiocarbonyl ylides' with acceptor ethylenes. This mechanism via zwitterionic intermediates is studied here for 1,3‐dipoles 5A and 5B , which are derived from 2,2,5,5‐tetramethylcyclopentanethione and 1,1,3,3‐tetramethylindan‐2‐thione, respectively, and contain a highly screened reaction center. In the reactions of 8A and 8B (the precursors of 5A and 5B ) with dimethyl 2,3‐dicyanofumarate ( 15 ) and 2,3‐dicyanomaleate ( 16 ), virtually identical ratios of cis ‐ and trans ‐thiolanes were observed ( 17 / 18 93 : 7 for 5a and 94 : 6 for 5B ). Thus, full equilibration of rotameric zwitterions precedes cyclization; an anteceding disturbing isomerization 15 ⇌ 16 had to be circumvented. The cis , trans assignment of the cycloadducts rests on three X‐ray analyses. The kinetically favored cis ‐thiolanes 17 isomerize at >80° to 18 ( trans ), and irreversible cleavage leads to thione 7 and trans , cis isomeric dimethyl 1,2‐dicyanocyclopropane‐1,2‐dicarboxylates ( 27 and 28 , resp.). Furthermore, the zwitterionic intermediates equilibrate with the cyclic seven‐membered ketene imine 21 , which was intercepted under conditions where the solvent contained 2 vol‐% of H 2 O or MeOH. Lactams 22 were obtained with H 2 O in high yields, and the primary products of capturing by MeOH were the cyclic ketene O , N ‐acetals 23 , which subsequently tautomerized to the lactim methyl ethers 24 . When 5B was reacted with ethenetetracarbonitrile in CDCl 3 /MeOH (98 : 2 vol‐%), the analogous cyclic ketene imine 13B was trapped to the extent of 93%.