Hon Man Lee

Catalytic cross-coupling reactions mediated by palladium/nucleophilic carbene systems

Abstract In this mini-review, we present a summary of our recent work in the field of palladium-catalyzed cross-coupling reactions, with emphasis on the use of nucleophilic N-heterocyclic carbenes (NHC) as ancillary ligand. The palladium-mediated coupling reactions investigated include the Suzuki–Miyaura, Kumada–Tamao–Corriu, Heck, Sonogashira, Stille, Hiyama and aryl amination reactions.

Pd(0)-Catalyzed Decarboxylative Coupling and Tandem C–H Arylation/Decarboxylation for the Synthesis of Heteroaromatic Biaryls

An effective Pd(0) carbene complex was successfully employed in the decarboxylative coupling of the heteroaromatic carboxylic acids (imidazo[1,2-a]pyridine and isoxazole) with aryl halides. For carboxyindoles, either decarboxylative coupling or tandem C-H arylation and decarboxylation occurred, leading to the formation of C2-monoarylated indoles.

One-Step Synthesis of Isocoumarins and 3-Benzylidenephthalides via Ligandless Pd-Catalyzed Oxidative Coupling of Benzoic Acids and Vinylarenes

A straightforward synthetic method for the preparation of isocoumarins and 3-benzylidenephthalides via C-H olefination and oxidative coupling of readily available benzoic acids and vinylarenes was developed. The directing effect of the substituents on the benzoic acid allows for the synthesis of both types of lactone in pure form.

Efficient Pd-catalyzed direct arylations of heterocycles with unreactive and hindered aryl chlorides.

A highly electron-rich Pd complex can efficiently catalyze the direct arylation of heteroaromatics with unreactive and sterically congested aryl chlorides.

Bis-alkoxycarbonylation of styrene by pyridinimine palladium catalysts

Pyridinimine-modified Pd(II) complexes of general formulae (N-N′)Pd(Y)2 catalyze the methoxycarbonylation of styrene to give dimethyl phenylsuccinate as the largely major product [N-N′ = py-2-C(R)N(2,6-R′C6H3), R = H, Me; R′ = Me, i-Pr; 6-Mepy-2-C(H)N[2,6-(i-Pr)2C6H3]; py-2-C(H)N(C6H5); Y = acetate, trifluoroacetate]. The influence of various catalytic parameters on the overall conversion of styrene to carbonylated products and on the product selectivity has been studied by systematically varying the type of palladium initiator, the concentrations of organic oxidant (1,4-benzoquinone) and protic acid (p-toluenesulfonic acid), and the CO pressure. By an appropriate choice of the structure of the pyridinimine ligand and of the reaction parameters, turn-over numbers as high as 96 and selectivities in dimethyl phenylsuccinate as high as 98% were obtained. In particular, the overall conversion of styrene is controlled by the steric properties of the alkyl substituents on the imine aryl group, while the nature of the substituent (H or Me) on the imine carbon influences the selectivity. The addition of 2 equivalents of TsOH to the catalytic mixtures generally increased the styrene conversion but lowered the selectivity in dimethyl phenylsuccinate due to greater production of methyl 3,6-diphenyl-4-oxohexanoate. Further additions of TsOH (up to 6 equivalents) resulted in better selectivities and lower conversions for all precursors.

Copolymerization of carbon monoxide with ethene catalyzed by bis-chelated palladium(II) complexes containing diphosphine and dinitrogen ligands

Several bis-chelated palladium(II) complexes, [Pd(P-P)(N-N)x](PF6)2, containing binary combinations of diphosphine and dinitrogen ligands have been prepared and characterized. The diphosphine ligands comprise 1,3-bis(diphenylphosphino)propane (dppp), meso-2,4-bis(diphenylphosphino)pentane (meso-bdpp), rac-2,4-bis(diphenylphosphino)pentane (rac-bdpp) and 2,2′-bis(diphenylphosphinoethyl)pentane (Etdppp), while the dinitrogen ligands are either 2,2′-bipyridine (bipy; x=1) or 1,8-naphthyridine (napy; x=2). The structure of [Pd(meso-bdpp)(N,N′-bipy)](PF6)2·CH2Cl2 has been determined by an X-ray structural analysis. All the Pd(II) complexes have been tested as catalyst precursors for the copolymerization of carbon monoxide and ethene in methanol solution in either autoclaves or high-pressure sapphire NMR tubes. The combination of meso-bdpp and bipy at palladium, in conjuction with both 1,4-benzoquinone and p-toluenesulfonic acid, has shown the best catalytic performance. The different catalytic activities exhibited by the stereoisomers [Pd(meso-bdpp)(N,N′-bipy)]- (PF6)2 and [Pd(rac-bdpp)(N,N′-bipy)](PF6)2 has been interpreted in terms of the different spatial distribution of the phenyl rings around the metal center determined by the conformation of the six-membered metallaring.

New structurally rigid palladium catalysts for the alternating copolymerization of carbon monoxide and ethene

Decreasing the flexibility of the Ph2PCH2CH2PPh2 (dppe) backbone by making the central carbon atoms part of a cyclobutane ring leads to the formation of Pd(II) catalysts for CO/ethene copolymerization that are more efficient than those of dppe by a factor of ten.