Cycloisomerization of Conjugated Allenones into Furans under Mild Conditions Catalyzed by Ligandless Au Nanoparticles

Abstract

Au nanoparticles supported on TiO 2 (1 mol %) catalyze the quantitative cycloisomerization of conjugated allenones into furans under very mild conditions. The reaction rate is accelerated by adding acetic acid (1 equiv), but the acid does not participate in the protodeauration step as in the corresponding Au(III)-catalyzed transformation. The process is purely heterogeneous, allowing thus the recycling and reuse of the catalyst effectively in several runs.

Synthesis of 3‐Keto Pyridines from the Conjugated Allenone – Alkynylamine Oxidative Cyclization Catalyzed by Supported Au Nanoparticles

“…Our latest report, regarding the smooth Au/TiO 2 ‐catalyzed cycloisomerization of conjugated allenones into furans, urged us to examine the possible Au‐catalyzed cyclization of N ‐propargylic β‐enaminones ( N ‐1,5‐enynes), compounds that could be formally obtained from an uncatalyzed reaction between allenones and propargylamine . Indeed, propargylamine adds quantitatively to the model allenone 1 in dioxane as solvent within 10 min at 25 °C, yielding exclusively the (Z)‐enaminone 1′ .…”

“…of propargylamine in toluene at 25 °C for 10 min, and then adding the catalyst followed by heating, resulted to the same outcome, as in the case of using directly 1′ . In fact, premixing the reactants for 10 min is more than enough, otherwise unreacted allenone 1 should had been instantaneously isomerized by Au/TiO 2 into 2‐( p ‐tolyl)furan . The catalytic system (Au/TiO 2 ) is recyclable by simple filtration, washing and drying at 90 °C for 2 h. It was reused in five consecutive runs with little loss of activity.…”

“…Allenones 1 – 13 , 15 , 17 and 20 were available from previous studies in our lab . Allenyl ester 18 is commercially available, while compounds 16 and 19 were prepared following a known procedure .…”

“…[2] Thus, simple 1,6-enynes, [3] N-((1H-indol-3-yl)methyl)propiolamides (N-1,6-enynes), [4] aryl propargyl ethers, [5] or ωalkynylfurans [6] undergo cyclization in the presence of catalytic amounts of supported Au nanoparticles. In addition, the activation of an alkyne or allene by Au NPs may trigger inter or intramolecular nucleophilic attack by nitrogen, [7] oxygen, [8] or other [9] nucleophiles.…”

DFT Study on the Gold(I)‐Catalyzed Dehydrogenative Heterocyclization of 2‐(1‐Alkynyl)‐2‐alken‐1‐ones to form 2,3‐Furan‐Fused Carbocycles: Effects of Additives C5H5NO vs. PhNO

“…Furthermore, the additives (such as small molecule bases or acids) also have some significant impacts on the synthesis of furans. Zorba et al declared that HOAc was found as an appropriate additive to effectively promote the cycloisomerization of conjugated allenones into furans under mild conditions catalyzed by ligandless Au‐nanoparticles in yield of 99% . A simple and convenient approach for the construction of 2‐acetyl trisubstituted furans with acetylenic ketones and 1,3‐dicarbonyl compounds via the Cu‐mediated unstrained C (sp 3 )‐C (sp 2 ) bond cleavage has been described by Fan and coworkers, and the furans were isolated with a yield up to 67% when 0.2 equiv.…”