Anionic 5-endo-dig cyclizations: an experimental investigation of in-plane aromaticity involving a non-enolate carbanion nucleophile

Cyclitive additions of aliphatic carbanions to non-electrophilic carbon-carbon triple bonds under mild, transition-metal-free conditions are described for the first time. These results confirm theoretical models that invoke in-plane aromaticity to pre-dict the favorability of 5-endo-dig reactions in these systems. In contrast to related Conia-ene cyclizations (5-enolexo-endo-dig), our results generally led to cyclic and allene products in near parity ratios across a broad range of substrates, suggesting that cyclization may proceed via an early ambimodal transition state. Experimental results are presented with a view to re-fining existing mechanistic models for this growing class of alkyne reactions.