Gold(i)-catalysed alcohol additions to cyclopropenes

Abstract

Gold(i)-catalysed addition of alcohols to 3,3-disubstituted cyclopropenes occurs in a highly regioselective and facile manner to produce alkyl tert-allylic ethers in good yields. The reaction is tolerant of sterically hindered substituents on the cyclopropene as well as primary and secondary alcohols as nucleophiles. In this full article, we report on the substrate scope and plausible mechanism, as well as the regioselectivity issues arising from subsequent gold(i)-catalysed isomerisation of tertiary to primary allylic ethers.

Gold‐Catalyzed Migrations and Ring Expansions

“…A pioneering example of a gold-catalyzed ring opening of cyclopropenes with alcohols was described by Lee and coworkers 138,139 . The reaction involves the formation of an intermediate gold vinyl carbenoid/cationic intermediate, which then suffers the attack of an alcohol nucleophile to form tert-allylic ethers.…”

Recent advances in the gold-catalyzed additions to C–C multiple bonds

“…The first example of gold-catalyzed ring-opening addition of cyclopropenes has been developed by Lee’s group [31–32]. The reaction of alkyl-disubstituted cyclopropene 36 with a series of alcohols generated the corresponding tert -allylic ethers 37 with high regioselectivity.…”

When cyclopropenes meet gold catalysts

“…Water in the presence of t -BuOH as co-solvent acted as a nucleophile, but the corresponding tertiary alcohol 9i was isolated in only modest yield (34%) (Scheme 5) [18]. Additional results were subsequently reported by Lee et al in a full article in 2010 [19]. Due to their lower nucleophilic character compared to alcohols, phenols could not be used.…”

“…Addition of neopentyl glycol led to a 1:1 mixture of regioisomeric monoethers 9m and 9’m in modest yield (32%) due to the competitive formation of oligomeric by-products. The regioselectivity was found to be highly sensitive to temperature since the tertiary monoether 9m was selectively obtained ( 9m / 9’m > 99:1) (33%) when the reaction was carried out at 10 °C (Scheme 5) [18–19]. …”

“…By lowering the temperature to 10 °C and increasing the quantity of n -BuOH (15 equiv), the tertiary allylic ether 11a (65%) was obtained regioselectively ( 11a / 11’a > 99:1). Curiously, a complete switch of the regioselectivity took place when phenethyl alcohol was employed as a nucleophile, since in this case the primary allylic ether 11’b (65%) was obtained ( 11b / 11’b > 1:99) (Scheme 6) [19]. …”