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 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 = 2,6dimethoxyphenyl) 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.

Automated summary

A palladium-mediated one-pot synthesis of alkenes from aromatic carboxylic acids and allene substrates is reported. Evidence shows that the key aryl-palladium intermediate reacts to form a stable allyl-palladium product via insertion of the allene at the C2 position, enabling the synthesis of alkenes. Various experiments and computational data were used to support this finding.