Phosphine triggered [3+2] allenoate–acrylate annulation: a mechanistic enlightenment

Phosphine‐Catalyzed [4+2] Annulations of 2‐Alkylallenoates and Olefins: Synthesis of Multisubstituted Cyclohexenes

“…Use of a stoichiometric amount of HMPT provided a similar result (Table 2, entry 14). Although there are reports of nucleophilic phosphine catalyzed reactions of allenes being facilitated by water, [15–17] we found that the presence of water had a detrimental effect on cyclohexene formation.…”

Phosphine‐Catalyzed [3 + 2] Annulation: Synthesis of Ethyl 5‐(tert‐Butyl)‐2‐Phenyl‐1‐Tosyl‐3‐Pyrroline‐3‐Carboxylate

“…4 In the first examples of such reactions, reported by Xiyan Lu, activated allenes were reacted with electron-deficient alkenes and imines to produce cyclopentenes and pyrrolines, respectively. 5 Since Lu’s original report of such [3 + 2] annulations, many enantioselective variants of the procedure have been described, as well as several applications in the realm of natural product synthesis. 6,7 Our laboratory discovered that α-alkyl allenoates behave as 1,4-dipole–like species in novel [4 + 2] annulations with imines to form tetrahydropyridines.…”

“…Note that product 2c (Table 1, entry 3) was formed in a similar reaction employing the 2-butynoate, but in only 63% yield. 5b …”

PPh3‐Catalyzed Reactions of Alkyl Propiolates with N‐Tosylimines: A Facile Synthesis of Alkyl 2‐[aryl(tosylimino)methyl]acrylate and an Insight into the Reaction Mechanism

“…These results clearly demonstrate that, in the presence of D 2 O, an intermolecular pathway for the hydrogentransfer process exists, along with intramolecular pathway. [11] At present, we do not have convincing evidence to clarify the reaction mechanism, but the outcome of the reactions in Schemes 2, 4, 5 and 6 tempts us to discuss it (Scheme 7). Following the same reaction pathway as that in Scheme 3, the interaction between 1 a, 2 a and PPh 3 gives intermediate C1.…”