Umsetzungen von Bis‐[1.3‐diaryl‐imidazolidinylidenen‐(2)] mit Iso‐, Isothio‐ bzw. Isoselenocyanaten

Abstract

Reactions of Bis-[l,3-diaryl-imidazolidinyEidenes-(2)1 with Iso-, Isothio-or Isoselenocyanates1)Bis-[1,3-diaryl-imidazolidinylidenes-(2)] (la-c) react with isocyanates in a ratio of 1 : 4 to give 2,4-dioxo-1,3,6,9-tetraaza-spiro[4,4]nonanes (3a-i). As shown in the case of 3f and g, acyl-azides may also be used for this reaction because they rearrange to isocyanates under the reaction conditions; 2-acylimino-l,3-diphenyl-imidazolidines (6a and b) are formed then as byproducts. The reaction of the electron-rich olefine l a with aryl-azides leads to the triazenes l l a or b without loss of nitrogen. Isothio-or isoselenocyanates also undergo a 4 : 1-reaction with l a to give the spiroheterocycles 13a-j. A dipole intermediate 16 could be isolated from 1 a and p-nitrophenyl isocyanate. Analogous 2 : 1-reactions (21 a and b) are found with carbamoyl-isothiocyanates.

Betaine Adducts of N‐Heterocyclic Carbenes: Synthesis, Properties, and Reactivity

“…The intermediate NHC·RNCO betaines undergo a further [3+2] dipolar cycloaddition with residual isocyanate species present in the reaction media to afford the corresponding spirocyclic hydantoins. Indeed, a wide range of 1:2 cycloadducts have been isolated starting from bis(imidazolidin-2-ylidene) [86,87] or bis(benzimidazolidin-2-ylidene) [29,30] species [see Scheme 15 and Equation (52), respectively]. A similar clean reaction has been observed between Ender's stable triazolin-5-ylidene and phenyl isocyanate [Equation (53)].…”

“…Scheme 15), although no evidence for the isolation of NHC·RNCSe inner salts was reported. [87] ( 57) Initial synthetic efforts to isolate NHC·RNCNR betaines by treating thiazolin-2-ylidene species generated in situ with di-p-tolylcarbodiimide led to the corresponding hydrogen halides instead (Scheme 19). Various attempts to deprotonate these products failed, and a different reaction path was evidenced when aqueous sodium carbonate was used as a base.…”

“…[26,27,87] Conversely, protonation occurs on the nitrogen end of the thiocarbamoyl function when imidazolinium-2-acylthiocarbamoyl betaines are treated with perchloric acid diluted in ethanol or n-butanol [Equation (47)]. [27] Under more forcing hydrolysis conditions, the resulting benzamido moiety is cleaved off and the corresponding NHC·CO 2 adduct ultimately obtained (cf.…”

“…(38) (39) In many cases, however, the reaction of enetetramines with isothiocyanates bearing electron-donating alkyl or aryl substituents does not afford NHC·RNCS betaines as these adducts react further with the heteroallene via 1,3-cycloaddition (see below). Conversely, introduction of electronwithdrawing acyl [27] or carbamoyl [87] moieties onto the isothiocyanate function effectively stabilizes the corresponding NHC zwitterionic adducts and allows their isolation in satisfactory yields [Equation (40)]. (40) The deprotonation of azolium salts with a strong base, followed by in situ trapping of the free carbenes with suitable isothiocyanates, has provided a convenient alternative access to NHC·RNCS betaines.…”

“…[26] This procedure was subsequently extended to a few other starting materials bearing aryl groups on their nitrogen atoms [Equation (38)], [27,86,87] and to symmetrical (R = RЈ = Me) [29] or unsymmetrical (R = CH 2 CH 2 Ph, RЈ = Me, Et) [30] benzimidazole derivatives [Equation (39)]. (38) (39) In many cases, however, the reaction of enetetramines with isothiocyanates bearing electron-donating alkyl or aryl substituents does not afford NHC·RNCS betaines as these adducts react further with the heteroallene via 1,3-cycloaddition (see below).…”