Cu-Catalyzed [3+2] Cycloaddition and Hydrocyanation Reactions of α-Diazoesters

Abstract

$\bf{Cu-Catalyzed [3+2] Cycloaddition and Hydrocyanation Reactions of $\alpha$-Diazoesters}$ $\bf{Part 1. Cu-Catalyzed Reaction of $\alpha$-Aryl Diazoesters with Terminal Alkynes: A Formal [3+2] Cycloaddition Route Leading to Indene Derivatives}$ An efficient method for the preparation of 1-carboxylated 3-aryl indenes has been developed via Cu-catalyzed reaction of $\alpha$-aryl diazoesters with terminal alkynes. It turned out that cationic copper species accelerated the formation of desired indene with the help of silver and sodium salts. Among copper catalysts which had been tested, copper chloride bearing v\bf{N}$-heterocyclic carbene ligands gave the most satisfactory yields. The reaction was not affected by electronic properties of $\alpha$-aryl diazoesters albeit those of alkynes significantly controlled the reaction efficiency. According to the proposed mechanistic pathway, the reaction is believed proceed via tandem processes: alkyne insertion into copper-carbenoid, intramolecular electrophilic attack on the aromatic ring, and then isomerization. The regioselectivity of indene product could be controlled by the addition of stoichiometric amount of base. 3$\bf{H}$-indenes were isomerized to 1$\bf{H}$-indenes during the reaction by potassium acetate. $\bf{Part 2. Acetone Cyanohydrin as a Source of HCN in the Cu-Catalyzed Hydrocyanation of $\alpha$-Aryl Diazoacetates}$ We have developed a mild and efficient protocol of hydrocyanation of $\alpha$-aryl diazoester with acetone cyanohydrin via Cu catalysis. Acetone cyanohydrin could be served as a safe and mild alternative for hydrogen cyanide. The reaction turned out to be sensitive to types of copper catalyst. Only $Cu(CH_3CN)_4PF_6$ and its derivatives could be applied the developed protocol. We also found that catalytic amount of additional TMSCN could accelerate the reaction efficiency. The developed protocol has a wide range of substrate scope. It was revealed that the steric bulkiness...