Chemistry of tetra-alkoxyethenes. Part VII. Thermal [2 + 2] cyclo-additions with 1-cyanobutadienes

Abstract

Acyclic 1 -cyanobutadienes (2) having a second electron-withdrawing substituent at C-1 behave towards tetraalkoxyethenes (1) like equally substituted simple olefins. The normal product is a [ 2 + 21, not a [4 + 21 cycloaddition product. The correspondence in reactivity of these electron-poor olefins and butadienes towards the nucleophilic tetra-alkoxyethenes is further exemplified by the similarity of substituent effects on the cycloadditions. A possible explanation is given.WE have shown previously2j3 that under thermal conditions tetra-alkoxyethenes (1) give [2 + 21 cycloaddition products with electron-poor olefins having two electron-withdrawing groups at one of the olefinic carbon atoms. However, when one of the electron-withdrawing substituents in the electrophilic olefin is an acyl residue, as in a-cyano-ap-unsaturated carbonyl compounds, only [4 + 21 cycloaddition products arise.In connection with these results we have now investigated the behaviour of tetra-alkoxyethenes towards

Properties and reactions of ylidenemalononitriles

“…A possible mechanism involves the initial formation of a zwitterion (222) which is reversibly deprotonated by a second amine molecule (eq 23). A subsequent anilinium ion assisted expulsion Although a crystalline adduct was not obtained with hydrazine and 106, unsymmetrical hydrazines (e.g., 1,1-dimethylhydrazine) and benzidine, via involvement of both amino groups, gave similar crystalline dyes (221 ). 289 Other interesting reactions of 106 and amines have been reported.…”

“…127 Electrophilic ylidenemalononitriles undergo [2 + 2] cycloadditions with electron-rich (nucleophilic) alkenes to give substituted cyclobutanes (eq 16),218-223 which may be converted to cyanocyclobutene or 1,2-cyclobutenedione derivatives. 219,221 The conjugated ylidenemalonontriles (131) undergo [2 + 2] cycloaddition with 128, instead of [4 + 2] cycloaddition, to give 132, which are easily converted to 13 3220,221 (eq 16). Bulky or electron-releasing substituents at the 2-position in ylidenemalononitriles retard cyclobutane formation.…”

KeteneO,O-acetals

“…The expected five-membered heterocycles are obtained; thus nitrilimines (127) are converted t o pyrazoles' 6 9 (128), nitriloxides (129) and nitrones to isoxazolines' 7 0 (130) or isoxazolidines' l , azides (131) t o triazoles' 7 2 (132) and diazo ketones (133) to dihydro furans' 73 (134) …”