Ketenimmonium salt

Ketenimmonium salt A compound with the general structure R2C=C=NR2+... 

Reaction of the fluoroborate salt of (319) with acyclic dienes affords 3-vinylcyclo-butanone derivatives, but with cis-fixed dienes the ketenimmonium salt behaves as a dienophile and gives only [2 H- 4] adducts. Thus (319) shows a duality of behaviour, resembling both a keten and an allene depending on its cycloaddition partner. ... 

Cyclobutenylideneammonium salts, and hence cyclobutenones, are formed by the [2 + 2] addition of ketenimmonium salts to acetylenes. The reaction of dimethylketenimmonium tetrafluoroborate with the acetylenes gives the salts (205) in good yield, but the reaction is not highly regioselective. The salts are, themselves, excellent dienophiles, and may be used directly in reactions with dienes. The salt (206) gives the adducts (207) and (208) with butadiene or cyclopentadiene. Dimethyl-cyclobutenone (209) has also been prepared by the adcfition of dimethylketen to methyl vinyl sulphide, followed by methylation at sulphur and elimination of dimethyl sulphide. The ketone itself is a useful dienophile, but is less reactive than its imminium salt. The cyclobutenone (209) is functionally equivalent to terelactone which is an extremely reluctant dienophile. 

Closely related to cycloadditions of olefins with ketens are the stereospecific and evidently concerted cycloadditions of olefins with ketenimmonium salts. As shown in Scheme 28, the initial adducts and the subsequently obtained cyclobutanones are produced in high yield. With 1,3-dienes the cyclobutanones were uncontaminated with Diels-Alder products. 

Cyclic amidinium salts (71) are obtained by [2 + 2] cycloaddition of ketenimmonium compounds (69) to Schiff bases (70 R = H, Ph, MeOaC) or thioimidates (70 R = SBn) their thiolysis provides an elegant access to 3-thiolactams (72 equation 32). ... 

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