Transfer Semihydrogenation of Alkynes Catalyzed by a Zero‐Valent Palladium N‐Heterocyclic Carbene Complex

Abstract

Beyond Lindlar and without hydrogen: Transfer hydrogenation of internal alkynes catalyzed by a palladium(0) catalyst containing an N‐heterocyclic carbene ligand gives Z alkenes without over‐reduction to alkanes (see scheme). Contrary to most transfer hydrogenations, ketones are not reduced. As such, this is the first catalyst that shows excellent stereo‐ and chemoselectivity for the semihydrogenation of alkynes without the need for hydrogen gas.

Synthesis of palladium(0) and -(ii) complexes with chelating bis(N-heterocyclic carbene) ligands and their application in semihydrogenation

“…15,41,47 We applied complex 3a in this reaction, using 1-phenyl-1-propyne as the substrate and formic acid as a hydrogen donor at 70°C in acetonitrile. Unfortunately, this complex showed poor activity of only 18% conversion for this system (Table 2, entry 1).…”

Mechanism of Pd(NHC)-Catalyzed Transfer Hydrogenation of Alkynes

“…3 Therefore, the transfer semihydrogenation of alkynes was developed with in situ generated Pd 0 (IMes)(MA)-complexes, 14 where the open coordination site is occupied by solvent. As the reaction is slower but more selective in MeCN than in THF, we postulated that in acetonitrile a [Pd 0 (IMes)(MA)(MeCN)] species is the source of active catalyst.…”

“…The over-reduction to alkanes is fully inhibited when formic acid is used as hydrogen donor in strongly coordinating solvents, 14,15 whereas the use of hydrogen gas still gives overreduction. This increased chemoselectivity strongly hints at different modes of operation for the formic acid-mediated transfer hydrogenation versus hydrogenation with molecular hydrogen.…”

Recent Developments in the Applications of Palladium Complexes Bearing N-Heterocyclic Carbene Ligands

“…The reaction was highly stereoselective to the Z alkene and almost no over-reduction to the alkane was observed. 138 Chelating complexes as 56 showed similar semihydrogenation activity and performed very well in the coupling of phenylboronic acid with aryl bromides. 137 Compound 57, which under molecular hydrogen undergoes an oxidative addition to form a trans-hydride, was found very active in the hydrogenation of alkenes and alkynes under mild reaction conditions (RT, low Pd loadings and 1 atm of H 2 ).…”

“…137,138 The hydrogen donor source used was HCOOH/NEt 3 , so the process is best described as a transfer hydrogenation being CO 2 the only secondary product generated. Under these conditions, catalyst 55 showed excellent activity in the reduction of aromatic, aliphatic, internal and terminal alkynes.…”