Abstract
Three orthogonal cascade C—H functionalization processes are described, based on ruthenium-catalyzed C—H alkenylation. 1-Indanones, indeno indenes, and indeno furanones were accessed through cascade pathways by using arylacetophenones as substrates under conditions of catalytic [{Ru(p-cymene)Cl2}2] and stoichiometric Cu(OAc)2. Each transformation uses C—H functionalization methods to form C—C bonds sequentially, with the indeno furanone synthesis featuring a C—O bond formation as the terminating step. This work demonstrates the power of ruthenium-catalyzed alkenylation as a platform reaction to develop more complex transformations, with multiple C—H functionalization steps taking place in a single operation to access novel carbocyclic structures.
Highlights
Three orthogonal cascade CÀH functionalization processes are described, based on ruthenium-catalyzed CÀH alkenylation. 1-Indanones, indeno indenes, and indeno furanones were accessed through cascade pathways by using arylacetophenones as substrates under conditions of catalytic [{Ru(p-cymene)Cl2}2] and stoichiometric Cu(OAc
After an initial coarse screen of solvents and catalysts, we were delighted to observe the formation of two cascade CÀH functionalization products, 6 a and 7 a, when using [{Ru(p-cymene)Cl2}2] (5 mol %), AgBF4 (20 mol %), Cu(OAc)2·H2O (2 equiv) in DCE at 100 8C (Table 1, entry 1)
The initial cyclized indanone product 6 undergoes oxidative oxyacylation at the enolic CÀH position, a transformation that has been reported on simpler substrates by using hypervalent iodine oxidants,[14] peroxide oxidation of enamine-type intermediates,[15] and heavy-metal oxidants such as Tl(OAc)3 and Pb(OAc)4.[16]. Oxidative oxyacylation by using simple CuII salts and a Brønsted acid has not been reported, so we were pleased to find that yields could be considerably enhanced by using a silver catalyst as additive in the initial cyclization as before, and adding a second charge of Cu(OAc)2·H2O/HBF4 to the reaction vessel and heating for a further 16 h
Summary
Three orthogonal cascade CÀH functionalization processes are described, based on ruthenium-catalyzed CÀH alkenylation. 1-Indanones, indeno indenes, and indeno furanones were accessed through cascade pathways by using arylacetophenones as substrates under conditions of catalytic [{Ru(p-cymene)Cl2}2] and stoichiometric Cu(OAc)2. We chose the readily available a-phenyl acetophenone (4 a) as our starting substrate, and studied its reaction with methyl acrylate (5 a) under ruthenium catalysis with the aim of uncovering cascade CÀH functionalization processes (Table 1).
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