Allenes as Dipolarophiles and 1,3-Dipole Precursors: Synthesis of Carbocyclic and Heterocyclic Compounds

The use of allene derivatives in the 1,3-dipolar cycloaddition either as dipolarophiles or 1,3-dipole precursors is discussed. Allenes participate in cycloadditions with a variety of 1,3-dipoles namely nitrile oxides, nitrones, carbonyl ylides, nitrile imines, azides and diazocompounds. They also react as the 2??-component in other annulation reactions such as the cycloaddition of ambident 1,3-dipolar species generated from benzimidazole carbenes and isothiocyanates, palladium- catalyzed overall intramolecular [3+2] cycloaddition with alkylidenecyclopropanes and rhodium(I)-catalyzed formal [3+2] annulation reaction of chromium alkenyl(methoxy)carbene complexes. The reactivity of allenes as 1,3-dipole precursors and the subsequent annulation reactions are also covered. Allenes can be used to generate nitrones and other dipolar species can be obtained from allenylazines, via π-allylPd intermediates generated from allenes and from the reaction of allenes with phosphines.