C-H Borylation of Terminal Alkynes, the Reductive Cyclization of Alkynylboronates, and the Synthesis and Analysis of Pincer-ligated Rhodium Complexes
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Date
2017-08-16
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Abstract
Due to the utility of organoboron reagents in the synthesis of pharmaceuticals and other industrially relevant compounds, the transition metal-catalyzed C-H borylation of organic molecules has become a very popular area of research. Since the Ozerov group’s initial discovery of an iridium complex that can perform catalytic dehydrogenative borylation of terminal alkynes (DHBTA), we have made further investigations into the C-H borylation of terminal alkynes and the synthetic applications of the alkynylboronate products.
Here we describe a pincer-ligated palladium complex as the first group 10 catalyst for DHBTA. Unlike the iridium systems, the palladium catalyst also catalyzed a competing alkyne hydrogenation reaction, which could be suppressed using elemental mercury or phosphines as additives. We then turned our attention to (PNP)Ir complexes as catalysts for the DHBTA of 1,6-enynes and 1,6-diynes. These substrates could be borylated and isolated in moderate to excellent yields, and they could serve as substrates for rhodium-catalyzed reductive cyclization to form five-membered cyclic compounds containing pendant C-B bonds. It was found that borylated 1,6-diynes tethered by amines or an ether linkage would form 3,4-bis(methylpinacolboryl)-substituted pyrroles or furans during reductive cyclization.
Investigations into C-C coupling catalyzed by pincer-ligated rhodium complexes will also be discussed. First, we describe the synthesis of a series of PCP-based pincer-ligated rhodium complexes and how they performed as catalysts for stereoselective alkyne dimerization. While stereoselectivity was not achieved with any of the PCP-ligands on rhodium, these systems were faster and longer-lived than the (PNP)Rh alkyne dimerization catalysts previously reported by our group. Later, the fate of (PNP)Rh as a potential catalyst for Negishi coupling will be discussed. It was observed that although (PNP)Rh could perform all three fundamental steps in Negishi coupling (oxidative addition of an aryl halide, transmetallation, and reductive elimination), it was not a suitable catalyst. This was due to rhodium’s ability to insert into the carbon-zinc bond of the organozinc reagent to form a catalytically inactive species.
Finally, the synthesis, characterization, and electronic structure of complexes of (PNP)Rh bearing small perfluorocarbon ligands will be described.
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Rhodium, palladium, Iridium, catalysis, borylation, Negishi coupling, pyrrole, alkynylboronate, enyne, diyne, pincer ligand, alkyne dimerization,