Unique distal size selectivity with a digold catalyst during alkyne homocoupling.

Area of Science:

• Organometallic Chemistry
• Catalysis
• Synthetic Organic Chemistry

Background:

• Steric hindrance near metal centers typically governs catalytic reactions.
• Distal substituents rarely influence reactivity in metal-catalyzed processes.
• Selective functionalization of long-chain molecules remains a challenge.

Purpose of the Study:

• To investigate the ability of a gold catalyst to differentiate between linear alkynes based on chain length.
• To elucidate the mechanism behind distal size selectivity in gold-catalyzed oxidative homocoupling.
• To determine the kinetics of reductive elimination in the catalytic cycle.

Main Methods:

• Oxidative homocoupling of linear alkynes using a gold catalyst.
• Characterization of reaction products and intermediates.
• Kinetic studies at low temperatures (-78 °C).

Main Results:

• The gold catalyst selectively reacted with decynes (10-carbon alkynes) while leaving dodecynes (12-carbon alkynes) unreactive.
• Experimental evidence suggests distal size selectivity arises from steric constraints in a digold (I, III) acetylide complex, preventing transmetallation of long alkyl chains.
• Reductive elimination of two alkyne molecules from a single Au(III) center was observed to be very rapid (<1 min at -78 °C).

Conclusions:

• Gold catalysts can exhibit remarkable distal size selectivity in alkyne oxidative homocoupling.
• The observed selectivity is attributed to steric limitations in the transmetallation step of the catalytic cycle.
• The reductive elimination step is kinetically facile, contributing to catalytic efficiency.