Palladium-mediated CO2 extrusion followed by insertion of allenes: translating mechanistic studies to develop a one-pot method for the synthesis of alkenes

A palladium-mediated one-pot synthesis of alkenes from aromatic carboxylic acids and allene substrates (R¹R²C═C═CH₂, where either: R¹ = Ph, R² = H; R¹ = Ph, R² = Me or R¹ = nPe, R² = H) is reported. This is an iso-electronic variant of CO₂ extrusion–insertion (ExIn) reactions previously developed using heterocumulenes for the synthesis of thioamides (from isothiocyanates, RNCS), amidines (from carbodiimides, RNCNR), and amides (from isocyanates, RNCO). Evidence that the key aryl-palladium intermediate reacts to form a stable allyl-palladium product via insertion of the allene at the C2 position that enables the synthesis of alkenes was provided by a combination of gas-phase multistage mass spectrometry (MSⁿ) experiments and condensed-phase monitoring using ¹H NMR spectroscopy as well as theoretical computational data. Isolation and X-ray crystallographic analysis of the salt [(phen)Pd(CH₂CH(Ar)CHPh)]⁺(CF₃CO₂^–) (where Ar = 2,6-dimethoxyphenyl) confirmed insertion of the allene at the C2 position and revealed a syn relationship between the two aryl groups. A survey of different hydrogen sources to promote the removal of palladium from the alkene product revealed that NaBH₄ provides the highest yield and with the Z-alkene as the major isomer.