The nickel-catalyzed direct carboxylation of alkenes with the cheap and abundantly available C1 building block carbon dioxide (CO2 ) in the presence of a base has been achieved. The one-pot reaction allows for the direct and selective synthesis of a wide range of α,β-unsaturated carboxylates (TON>100, TOF up to 6 h(-1) , TON=turnover number, TOF=turnover frequency). Thus, it is possible, in one step, to synthesize sodium acrylate from ethylene, CO2 , and a sodium salt. Acrylates are industrially important products, the synthesis of which has hitherto required multiple steps.
We report the first catalyst based on palladium for the reaction of CO2, alkene and a base to form sodium acrylate and derivatives. A mechanism similar to a previously reported Ni(0)-catalyst is proposed based on stoichiometric in situ NMR experiments, isolated intermediates and a parent palladalactone. Our palladium catalyst was applied to the coupling of CO2 with conjugated alkenes.
The mechanism of the Pd-catalyzed vinylation of aryl halides with vinylalkoxysilanes in water has been studied using different catalytic precursors. The NaOH promoter converts the initial vinylalkoxysilane into a highly reactive water-soluble vinylsilanolate species. Similarly, deuteriumlabeling experiments have shown that, irrespective of the catalytic precursor used, vinylation occurs exclusively at the CH vinylic functionality via a Heck reaction and not at the C− Si bond via a Hiyama cross-coupling. The involvement of a Heck mechanism is interpreted in terms of the reduced nucleophilicity of the base in water, which disfavors the transmetalation step. The Heck product (β-silylvinylarene) undergoes partial desilylation, with formation of a vinylarene, by three different routes: (a) hydrolytic desilylation by the aqueous solvent (only at high temperature); (b) transmetalation of the silyl olefin on the PdH Heck intermediate followed by reductive elimination of vinylarene; (c) reinsertion of the silyl olefin into the PdH bond of the Heck intermediate followed by β-Si syn-elimination. Both the Hiyama and Heck catalytic cycles and desilylation mechanisms b and c have been computationally evaluated for the [Pd(en)Cl 2 ] precursor in water as solvent. The calculated Gibbs energy barriers support the reinsertion route proposed on the basis of the experimental results.
Water-soluble Pd(II) complexes of
general formula Na
x
[(NHC)PdCl2L] (L = Cl– (1), PPh3 (2), PPh2(C6H4-m-SO3Na)
(3), and 4-Mepy (4)), where NHC is a dianionic
sulfonated and sterically hindered N-heterocyclic carbene, have been
prepared. The new complexes are active catalysts for the Suzuki–Miyaura
cross-coupling of aryl chlorides and boronic acids in mixtures of
isopropyl alcohol/water or, in the case of water-soluble aryl chlorides,
in pure water. The trichloride complex 1, the crystal
structure of which is reported, catalyzes the coupling of inactivated
and sterically hindered substrates under mild conditions (60 °C,
0.1 mol % Pd). The corresponding biphenyls are obtained in almost
quantitative yields, except in the case of some of the most hindered
aryl chlorides, for which small amounts of arene are also formed by
a competitive hydrodehalogenation process.
We present here the first example of the telomerisation of isoprene with glycerol and polyethylene glycol (PEG-200), opening a facile route to new terpene structures, based on a combination of renewable and petroleum-based feedstocks. The reaction is catalysed by a palladium-carbene complex. Significantly, this system gives > 99% of linear monotelomer products. The factors that govern substrate conversion, dimerisation/telomerisation selectivity, and catalyst activity are studied and discussed.
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