Gold-catalyzed [4C+2C] cycloadditions of allenedienes, including an enantioselective version with new phosphoramidite-based catalysts: mechanistic aspects of the divergence between [4C+3C] and [4C+2C] pathways.
Gold(I) complexes featuring electron acceptor ligands such as phosphites and phosphoramidites catalyze the [4C+2C] intramolecular cycloaddition of allenedienes. The reaction is chemo- and stereoselective, and provides trans-fused bicyclic cycloadducts in good yields. Moreover, using novel chiral phosphoramidite-based gold catalysts it is possible to perform the reaction with excellent enantioselectivity. Experimental and theoretical data dismiss a cationic mechanism involving intermediate II and suggest that the formation of the [4C+2C] cycloadducts might arise from a 1,2-alkyl migration (ring contraction) in a cycloheptenyl Au-carbene intermediate (IV), itself arising from a [4C+3C] concerted cycloaddition of the allenediene. Therefore, these [4C+2C] allenediene cycloadditions and the previously reported [4C+3C] counterparts most likely share such cycloaddition step, differing in the final 1,2-migration step.
Alonso
Isaac
I
Departamento de Química Orgánica, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain.
Trillo
Beatriz
B
López
Fernando
F
Montserrat
Sergi
S
Ujaque
Gregori
G
Castedo
Luis
L
Lledós
Agustí
A
Mascareñas
Jose L
JL