Microwave‐Assisted Heterogeneous Cross‐Coupling Reactions Catalyzed by Nickel‐in‐Charcoal (Ni/C)

Abstract

A study involving the relatively rare combination of heterogeneous catalysis conducted under microwave conditions is presented. Carbon-carbon bond formation, including Negishi and Suzuki couplings, can be quickly effected with aryl chloride partners by using a base metal (nickel) adsorbed in the pores of activated charcoal. Aminations were also studied, along with cross-couplings of vinyl alanes with benzylic chlorides as a means to stereodefined allylated aromatics. Reaction times for all these processes are typically reduced from several hours to minutes in a microwave reactor.

Microwave‐Assisted Cross‐Coupling and Hydrogenation Chemistry by Using Heterogeneous Transition‐Metal Catalysts: An Evaluation of the Role of Selective Catalyst Heating

“…[13] Importantly, compared to previous studies by the same group employing conventional heating in an oil bath dating back to the late 1990s, [22] the reaction times for most of these transformations could be reduced from several hours to a few minutes by using sealedvessel microwave processing. [13][14][15]21] Choosing the Negishi cross-coupling of aryl chloride 1 with organozinc reagent 2 as our first model reaction (Scheme 1a) we wanted to see if the dramatic rate enhancement on going from 16 h employing oil-bath heating at 60 8C, [23] to 15 min under microwave conditions at 150 8C [13] was due to selective heating of the strongly microwave-absorbing Ni/C catalyst (see below), or could be rationalized by a simple thermal effect as a result of the significantly higher reaction temperature. [24] As a second example, we have selected the Cu/C-promoted Ullmann-type diaryl ether formation involving aryl bromide 4 and phenol 5 (Scheme 1b).…”

“…In recent years, Lipshutz and co-workers have published extensively on the use of these (and related) inexpensive base metal heterogeneous catalysts to perform very efficient microwave-assisted, for example, Ni-catalyzed Suzuki, Negishi, and Kumada carbon-carbon cross-couplings, [13] Cucatalyzed azide-alkyne cycloadditions [21] , and Ullmann-type diarylether formations, [15] in addition to Ni-catalyzed amination, and reduction processes. [13] Importantly, compared to previous studies by the same group employing conventional heating in an oil bath dating back to the late 1990s, [22] the reaction times for most of these transformations could be reduced from several hours to a few minutes by using sealedvessel microwave processing.…”

“…In recent years, Lipshutz and co-workers have published extensively on the use of these (and related) inexpensive base metal heterogeneous catalysts to perform very efficient microwave-assisted, for example, Ni-catalyzed Suzuki, Negishi, and Kumada carbon-carbon cross-couplings, [13] Cucatalyzed azide-alkyne cycloadditions [21] , and Ullmann-type diarylether formations, [15] in addition to Ni-catalyzed amination, and reduction processes. [13] Importantly, compared to previous studies by the same group employing conventional heating in an oil bath dating back to the late 1990s, [22] the reaction times for most of these transformations could be reduced from several hours to a few minutes by using sealedvessel microwave processing. [13][14][15]21] Choosing the Negishi cross-coupling of aryl chloride 1 with organozinc reagent 2 as our first model reaction (Scheme 1a) we wanted to see if the dramatic rate enhancement on going from 16 h employing oil-bath heating at 60 8C, [23] to 15 min under microwave conditions at 150 8C [13] was due to selective heating of the strongly microwave-absorbing Ni/C catalyst (see below), or could be rationalized by a simple thermal effect as a result of the significantly higher reaction temperature.…”

“…Both Ni/C and Cu/C were prepared according to the detailed protocols reported by the Lipshutz group starting from activated charcoal Darco KB-G and the corresponding metal nitrate salts. [13,21] The loading of Ni and Cu within the catalyst preparations was determined by ICP-MS (inductively coupled plasma mass spectroscopy) analysis to be 3.6 % (w/w) for Ni and 8.1 % (w/w) for Cu, respectively. Similar to our recent studies involving Pd/C, [25] the microwave absorbtivity of both catalysts in comparison to the non-impregnated charcoal support was determined by suspending samples of the solid materials in carbon tetrachloride as the microwave-transparent solvent and observing bulk temperature changes on irradiation with constant microwave power.…”

“…The Negishi and Ullmann-type cross-coupling as a model reaction. [13,15] Figure 1. Microwave heating profiles for 100 mg samples of Ni/C, Cu/C, and charcoal (C) suspended in CCl 4 (2.0 mL) in a 10 mL quartz reaction vessel at constant 150 W magnetron output power (single-mode reactor, fiber-optic temperature measurement).…”