Palladium-Mediated CO2 Extrusion Followed by Insertion of Allenes: Translating Mechanistic Studies to Develop a One-Pot Method for the Synthesis of Alkenes

Abstract

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 isoelectronic 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.