Raffaello Lazzaroni

Selective hydroformylation of open-chain conjugated dienes promoted by mesitylene-solvated rhodium atoms to give β,γ unsaturated monoaldehydes

The hydroformylation of 1,3-butadiene, 2-methyl-1,3-butadiene and 1,3-pentadiene using rhodium vapour-mesitylene cocondesates as a catalytic precursor is reported. The reaction gives β,γ-unsaturated monoaldehydes with high chemoselectivity and regioselectivity. η3-Butenyl complexes, derived from the addition of RhH species to the conjugated double-bound system, are likely to be intermediates, as suggested by deuterioformylation experiments.

1-Hexene rhodium-catalyzed hydroformylation at partial substrate conversion: influence of reaction parameters on the chemoselectivtty and regioselectivity

Abstract The influence of the reaction parameters (temperature, gas pressure) on the chemo- and regioselectivity of 1-hexene hydroformylation in the presence of Rh4(CO)12 as catalytic precursor has been investigated at partial and complete conversion. Both chemo- and regioselectivity are independent of the substrate conversion until 1-hexene is present in the reaction mixture. Chemoselectivity to aldehydes is complete at room temperature, while at higher temperatures it decreases with decreasing pressure and increasing temperature as a consequence of 1-hexene isomerization to (E)- and (Z)-2-hexenes. Similar amounts of heptanal and 2-methylhexanal are obtained at room temperature. At higher temperature and partial substrate conversion, only 1-hexene is converted to aldehydes; the regioselectivity towards the linear isomer increases from 4% at room temperature to 48% at 120 °C and is not affected by the gas pressure. Differing behaviours of the linear and branched alkyl metal intermediates towards the β-hydride elimination under the reaction conditions account for the influence of the reaction parameters on the chemo- and regioselectivity in 1-alkene hydroformylation.

An original approach to 5,6-dihydroindolizines from 1-allylpyrroles by a tandem hydroformylation/cyclization/dehydration sequence

Abstract 6-Methyl-5,6-dihydroindolizine and 3- or 2-ethyl derivatives were obtained via a one-pot hydroformylation/cyclization/dehydration sequence starting from 1-(2-methyl-2-propenyl)pyrroles. 7-Phenyl-5,6-dihydroindolizine and 5-methyl-5,6-dihydroindolizine were similarly synthesized. An easily occurring electrophilic aromatic substitution by the carbon atom of the carbonyl group on the α-position of the pyrrole ring with the formation of the six-membered ring is the key-step of the process.

Rhodium catalyzed deuteroformylation of styrene: (E)- and (Z)-β-deuterostyrene and β,β-dideuterostyrene formation via selective β-hydride elimination from the branched alkylrhodium intermediate

Deuteroformylation of styrene in the presence of Rh4(CO)12 as a catalytic precursor was carried out at 160 atm of CO and D2 1/1 at two temperatures (20 and 90°C) and for times yielding partial or complete conversion. Compounds recovered from the mixture produced by reaction and partial conversion at 90°C include unlabeled styrene, (E)- and (Z)-β-deuterostyrene, C6H5CHCHD, and β,β-dideuterostyrene, C6H5CHCD2, whereas at room temperature the styrene does not take up deuterium. These results indicate that under hydroformylation conditions the branched alkylrhodium intermediate, which affords the branched aldehyde, in part dissociates into rhodium hydride and deuterated olefin. By contrast the linear alkyl intermediate does not dissociate under the same conditions, but instead yields almost completely the corresponding aldehyde.

The Use of 2H NMR in the Elucidation of the Catalytic Pathway of the Hydroformylation Reaction

Abstract Differences in the behaviour of the metal-alkyl intermediates involved in the deuterioformylation of styrene in the presence of Co 2 (CO) 8 or Rh 4 (CO) 12 as catalytic precursors, have been readily revealed by 2 H NMR analysis of the crude mixtures present after partial conversion. The results clearly demonstrate the value of this simple method for mechanistic studies of catalytic reactions.

A simple preparation for (η6-arene)(η4-cyclo-octa-1,5-diene)ruthenium-(0) complexes and their conversion into the corresponding arene–dichlororuthenium(II) complexes

A series of (η6-arene)(η4-cyclo-octa-1, 5-diene)ruthenium(0) complexes have been readily prepared by reaction of (η4-cyclo-octa-1,5-diene)(η6-cyclo-octa-1, 3, 5-triene)ruthenium(0) with arene compounds, under 1 atm H2[arene = C6H6, CH3 C6H5, 1, 4-Me2C6H4, 1,3,5-Me3C6H3, C2H5C6H5, Me2CHC6H5, Et2CHC6H5, C6H5(CH2)3 C6H5,C6H5C6H5, C2H5CH(Me)C6H5, Me2CHCH(Me)C6H5, NH2CH(Me)C6H5, CH3OC6H5, or CH3COC6H5]. These complexes react with aqueous HCI to give in almost quantitative yield the corresponding (η6-arene)-dichlororuthenium(II) complexes.

Rhodium-catalyzed hydroformylation of 1-allylpyrrole as an unexpected way to 5,6-dihydroindolizine synthesis

Abstract When 1-allylpyrrole was subjected to hydroformylation conditions with Rh 4 (CO) 12 as the catalyst precursor, at 120 atm total pressure, at 20 and 100°C, 5,6-dihydroindolizine was found unexpectedly, together with the expected branched aldehyde, the linear isomer being obtained in traces amounts only. An annulation via a nucleophilic attack of the pyrrole C2 carbon atom on the carbonyl group of the linear aldehyde, followed by dehydration of the intermediate alcohol, possibly generates the indolizine structure.

Synthesis of 2-chromanol by hydroformylation of 2-hydroxystyrene derivatives

Abstract 2-Benzyloxy- and 2-tosyloxystyrene were hydroformylated under different reaction conditions with the aim to obtain the corresponding linear aldehydes, valuable intermediates to 2-chromanol, a structural moiety present in several interesting therapeutically active molecules. The best results were obtained by using the catalytic precursor Pt(Xantphos)Cl 2 in toluene or the water-soluble catalytic system Rh(CO) 2 acac/Xantphos(SO 3 Na) 2 in the biphasic medium water/toluene. Rather good regioselectivities were also achieved employing the unmodified complex Rh 4 (CO) 12 at high temperature and low pressure for very short reaction times: unfortunately the chemoselectivity of the process was not satisfactory, due to the extensive formation of the substrate hydrogenation product.

Rhodium-catalyzed hydroformylation of vinylidenic olefins: the different behaviors of the isomeric alkyl–metal intermediates as the origin of the β-regioselectivity

The β-regioselective hydroformylation of the vinylidenic olefins 2-phenylpropene (1a), 2-methylpropene (1b) and 2,3,3-trimethylbutene (1c) was investigated via deuterioformylation experiments [100°C, 100 atm, Rh4(CO)12] carried out at partial substrate conversion. The crude reaction mixtures were directly submitted to 2H NMR analyses. The results obtained allowed to conclude that whereas the primary rhodium–alkyl intermediate undergoes migratory insertion on CO, giving the corresponding linear aldehyde, the tertiary rhodium–alkyl intermediate, when it does form (i.e., in the case of 1a and 1b but not in the case of 1c), undergoes β-hydride elimination exclusively, accounting for the almost complete β-regioselectivity.

INFLUENCE OF THE REACTION TEMPERATURE ON THE REGIOSELECTIVITY IN THE RHODIUM-CATALYZED HYDROFORMYLATION OF VINYLPYRROLES

Abstract The influence of the temperature on the regioselectivity in the hydroformylation of the vinylpyrrole isomers and of the corresponding N -tosylated substrates has been investigated in the range 20–100°C, in the presence of Rh 4 (CO) 12 . At all the temperatures the branched aldehyde was prevailing with respect to the linear isomer for all the substrates ( α -regioselectivity). With increasing temperature, an increase of the linear aldehyde was observed to a different extent in dependence on the substrate nature. 2 H NMR investigation of the crude reaction mixture recovered from deuterioformylation of 3-vinylpyrrole at partial substrate conversion points out that the observed depression of the α -regioselectivity with increasing temperature must be connected to a β -hydride elimination process occurring for the branched alkyl—rhodium intermediates but not for the linear ones.