Allenyl exchange reactions

Iodine-magnesium exchange reaction of allenyl iodides occurs by the action of PrMgBr in Et2O. Subsequent reaction with aldehydes or ketones provides homopropargyl alcohols with high regioselectivity (Scheme 3.96). 

A fundamentally different approach to the synthesis of 3-pyrrolines is evidenced in the annulation in Eq. 13.50 [58]. Ethyl 2,3-butadienoate 150 reacts with N-sulfony-limine 151 in the presence of triphenylphosphine under very mild conditions to give JV-protected 3-pyrroline 152 in 90% yield. The mechanism that has been postulated is related to that of the Baylis-Hillman reaction. Michael addition of triphenylphosphine to the allenyl ester generates a zwitterion that combines with the imine to give 153 in a non-concerted process. This is followed by ring closure, proton exchange and expulsion of triphenylphosphine to give 152. This annulation is successful only for aromatic or cinnamyl imines [59]. 

Allenyl tellurides can be submitted to Te/Li exchange and subsequent reaction with aldehydes, giving homopropargylic alcohols. ... 

Metal exchange. Allenyltributylstannane is transformed into propargylmetal halides by reaction at low temperatures with MX, where M = P, As, Sb, Ge, B. The allenyl isomers are favored at higher temperatures. 

The reaction was anticipated to proceed through an initial ligand exchange between the lithium allenyl alkoxide and the diisopropoxytitanium alkene complex, followed by stereoselective carhometallation (Scheme 10.154). 

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