An Alternative to Precious Metals: Hg(ClO₄)₂·3H₂O as a Cheap and Water-Tolerant Catalyst for the Cycloisomerization of Allenols

Abstract

Hg(ClO4)2·3H2O, a cheap, water-tolerant, and stable salt, catalyzes the cycloisomerization reaction or α-allenols to 2,5-dihydrofurans in an efficient and selective manner. The reaction is general and can be applied to differently functionalized substrates, including alkyl-substituted, aryl-substituted, enantiopure, and tertiary allenols. In addition, density functional theory (DFT) calculations were performed to obtain insight into various aspects of the controlled reactivity of α-allenols under mercury catalysis. They suggest a dual activation of the allenol by the Hg complex that drives the reaction to the chemoselective formation of 2,5-dihydrofurans.

Regioselectivities in Fe(III)‐catalyzed Cycloisomerization Reactions of γ‐Allenyl Alcohols

“…Among various allenols, extensively investigated reactions was on the α‐allenols to be converted into 2,5‐dihydrofurans under silver catalysis (AgBF 4 or AgNO 3 ) . The regioselectivity favored for 5‐ endo products was also observed in other catalysis using AuCl 3 , LAu(I)X, CuCl 2 , PtCl 2 , and Hg(ClO 4 ) 2 metals, and cyclization of chiral α‐allenols resulted the successful axial‐to‐center chirality transfer 2g–i,4c . A more competitive study was performed with allenyl diols, resulting the cyclization of the more hindered hydroxy group through the allene–metal complexation at the less congested face 2h–k .…”

Deciphering the Chameleonic Chemistry of Allenols: Breaking the Taboo of a Onetime Esoteric Functionality

“…Thus, inexpensive and water-tolerant Hg­(ClO 4 ) 2 ·3H 2 O provided the dihydrofuran skeleton 347 in good yields and wide scope, as it happens in sterically hindered tertiary allenols (Scheme , reaction a). In addition, complete selectivity toward the oxycyclization was observed even when electron rich aryl allenols 346a were employed, which normally led to mixtures 348a/347a under gold catalysis (Scheme , reaction b) …”

“…In addition, complete selectivity toward the oxycyclization was observed even when electron rich aryl allenols 346a were employed, which normally led to mixtures 348a/347a under gold catalysis ( Scheme 59 , reaction b). 262 …”

Advances in mercury(II)-salt-mediated cyclization reactions of unsaturated bonds

“…Following a similar strategy, mercury chlorate (Hg(ClO 4 ) 2 ) had been employed successfully as a cheap alternative to precious metals salts in the cyclization of α-allenol derivatives 185 to differently substituted 2,5-dihydrofurans 186 in an efficient and selective manner. It was also shown that from enantiopure allenyl derivatives, the desired pure cyclized product was generated by utilizing the above reaction conditions ( Scheme 57 ) [ 118 ].…”