Ligandless, anionic, arylpalladium halide intermediates in the Heck reaction

Area of Science:

• Organometallic Chemistry
• Catalysis
• Organic Synthesis

Background:

• The Heck-Mizoroki reaction is a cornerstone of modern organic synthesis for C-C bond formation.
• Understanding catalytic intermediates is crucial for optimizing reaction efficiency and selectivity.
• Ligandless palladium catalysis offers practical advantages but often lacks mechanistic clarity.

Purpose of the Study:

• To isolate and characterize novel anionic arylpalladium complexes.
• To investigate the role of these anionic complexes as intermediates in Heck-Mizoroki reactions.
• To compare the reactivity of anionic versus neutral palladium complexes in olefin insertion.

Main Methods:

• Synthesis of anionic arylpalladium complexes via reaction of neutral precursors with bromide.
• Kinetic studies to determine rates of olefin insertion for anionic and neutral complexes.
• Regioselectivity studies using aryl halides and butyl vinyl ether with different catalysts.

Main Results:

• Isolation of anionic arylpalladium complexes with the general structure [Pd(Ar)Br(2)](2)(2-).
• Anionic complexes exhibit significantly faster olefin insertion rates at room temperature compared to neutral complexes.
• Halide addition accelerates reactions, supporting in situ formation of anionic intermediates from neutral precursors.

Conclusions:

• Anionic arylpalladium complexes are kinetically competent intermediates in ligandless Heck-Mizoroki reactions.
• These anionic species are more reactive than their neutral, phosphine-ligated counterparts.
• The findings elucidate the mechanism of Heck reactions catalyzed by bulky phosphine ligands and under ligandless conditions.