Final report of WG1

Piero Frediani 

Department of Organic Chemistry, Via della Lastruccia, 13, Florence 50019, Italy

Abstract

1) Hydrogenation of unsaturated organic substrates

The attention on g-butyrolactones as structural elements in organic compounds is growing up in last years. This typical moiety is present in  a large variety of natural and synthetic compounds involved in agrochemicals, food and pharmaceuticals industries. Furthermore some g-butyrolactones due to their smelly proprieties are added to many foodstuffs at concentration often comparable to those present naturally, in order to increase the flavour. The isotope dilution mass spectroscopy provides an accurate and convenient method for the quantification of these compounds if stable isotopomeric compounds are available. So far the possibility to settle a relatively simple method to realise the complete sequence of differently deuterated g‑butyrolactones to be employed as internal standards seems desirable. Anyway despite of a plethora of methods regarding to the synthesis of g-butyrolactones present in the scientific literature only a few procedures have been devoted to the regioselective introduction of multiple deuterium atoms into the lactone ring.

So far we have developed a convenient general synthetic procedure to synthesise isotopomeric g‑butyrolactones containing 2, 4 or 6 deuterium atoms. Three differently deuterated g‑butyrolactones were synthesised in quantitative yield, starting from the saturated acid C4 (succinic) and unsaturated acid C4 (fumaric or maleic and acetylendicarboxylic) in the presence of Ru4H4(CO)8(PBu3)4 using a deuterium pressure of 180 bar at 180 °C (Figure 1). This catalytic system is known to be active in the hydrogenation of carboxylic acids to the corresponding esters. 

It was established, that depending on the pH, the hydrides formed from [{RuCl2(mtppms)2}2] and excess mtppms (Na-salt of monosulfonated triphenylphosphine, 3‑diphenylphosphinobenzensulfonate) catalyzed the selective hydrogenation of disubstituted alkynes either to the corresponding cis- or trans-alkenes. The selectivity decreased upon increasing the H2 pressure to 10 bar or above – the reason of this change is presently unclear. The Joo group has described earlier that the selectivity of the hydrogenation of cinnamaldehyde catalyzed by the water-soluble complex [{RuCl2(mtppms)2}2] (+ excess mtppms) was a sensitive function of the hydrogen pressure. During the D30 Action we made detailed 1H and 31P NMR studies to establish the effects of the pH and the hydrogen pressure on the reaction of [{RuCl2(mtppms)2}2] and excess mtppms with H2, including the determination of the relevant T1 and T2 relaxation times. The results revealed the formation of the following complexes: a) in acidic solutions at 1 bar H2: [RuHCl(mtppms)3] and [{RuHCl(mtppms)2}2]; b) in acidic solutions at elevated H2 pressure: trans-[RuH2(mtppms)4] c) in basic solution at 1 bar H2: cis-[RuH2(H2O)(mtppms)3]; d) in basic solution at elevated pressure: cis-[RuH2(H2)(mtppms)3]. Of these complexes cis-[RuH2(H2)(mtppms)3] is a dihydrogen complex with water-soluble phosphine ligands and trans-[RuH2(mtppms)4] is one of the rare trans-dihydrides. (Furthermore, cis-[RuH2(H2O)(mtppms)3] was previously incorrectly described as cis-[RuH2(mtppms)4].)

2) Hydroformylation of olefins and amides

A series of platinum(II) methyl complexes bearing bis-phosphanyl monosulfides (P^PS) has been used in association with tin(II) chloride for the hydroformylation of 1-octene under 50 bar CO:H2. The best catalyst performance was obtained using the monosulfide of 1,3-bis(diphenylphosphanyl)propane as chelating ligand. Combined conventional and high pressure multinuclear NMR studies established that: i) chelation of the P^PS ligand is retained during catalysis; ii) in this catalytic system, SnCl2 plays three roles: it weakens the Pt–Cl bond in the chloro-Pt(II) precursors; it activates the insertion of CO into the Pt–alkyl bond; and it facilitates the hydrogenolysis of the Pt–acyl intermediate.

A collaboration between the groups of Liverpool and Tarragona has been initiated on the subject “in situ high pressure NMR studies of the hydroformylation of acrylamides”. Preliminary catalytic studies were conducted in Tarragona to facilitate a collaborative high pressure NMR study in Liverpool into the factors determining catalyst performance in a new route to MMA via 3-hydroxy-2-methylpropionamide discovered in Tarragona.  The effect of reaction temperature, pressure, and catalyst concentration, on selectivity and turnover were all investigated and the optimal ranges for the HPNMR study determined. It was found that changing the catalyst concentration had minimal effect on aldehyde regioselectivity, the catalytic system was relatively insensitive to changes in total and partial pressure and that the hydrogenation side reaction could be suppressed at low temperatures.

2) Hydrocarboxylation  of terminal alkenes using Palladium catalysts

Regioselectivity control was studied in palladium catalysed hydroxycarbonylation of styrene in neat water with water-soluble phosphines, mostly trisulfonated triphenylphosphine, TPPTS, but also 4- (N, N, N', N' tetraethyl-diethylene methylenetriamine) phenyl diphenylphosphine, N3P. The factor giving the highest changes in regioselectivity in the TPPTS system, under similar reaction conditions, is the temperature. In the N3P case, only a minor variation in the n/i ratio as a function of temperature is observed. In-situ normal- and high-pressure NMR experiments were performed to obtain further information about the catalytic cycle and the reaction intermediates. Two palladium hydride intermediates, a palladium n3-benzylic complex and both the branched and the linear palladium acyl complexes were identified in the HP NMR experiments. The hydroxycarbonylation in water using styrene as a substrate operates using a hydride mechanism for pathways leading to both linear and branched product. Insertion of styrene in the palladium-hydride bond gives an n3-benzyl compound.  A high CO pressure gives a kinetic preference for the iso-acyl in the next step. In the TPPTS system, at moderate temperatures, the hydrolysis of the iso-acyl is faster than its conversion to the thermodynamically more stable n-acyl. A low n/i therefore requires high pressures and reasonably low temperatures. The N3P ligand always favours the linear product since isomerisation of the iso-acyl to the n-acyl in this system is fast under all conditions investigated.

The kinetics and the mechanism of the catalytic hydrocarboxylation of linear alkenes to obtain carboxylic acids using supercritical carbon dioxide as a solvent was studied. High selectivities in acids have been obtained. The best results were achieved by adding a perfluorinated surfactant to the reaction mixture (93 % conversions and ca 90 % selectivity). Comparative multinuclear high pressure NMR studies in THF-d8 and in supercritical CO2 show the formation of Pd(0) species.

3) Cooperative Effect between Iridium and Platinum in the Carbonylation of Methanol into Acetic Acid

The iodocarbonyl monomer [PtI2(CO)2] promotes the iridium catalyzed carbonylation of methanol to acetic acid at low water contents. Studies based on low pressure or high pressure NMR and the use of labeled reactants were conducted close to the real conditions of catalysis in order to gain a deep insight into this system. Carbonylation of CH3I at low water contents proceeds slowly and the migratory CO insertion step, leading from H[IrI3(CH3)(CO)2] to H[IrI3(COCH3)(CO)2] is rate limiting. The dimer [PtI2(CO)]2 reacts immediately with [PPN][IrI3(CH3)(CO)2] (PPN is Ph3P=N+=PPh3) under nitrogen to afford a mixture of species, among which the key heterobinuclear [Ir-Pt] intermediate [PPN][IrI(CH3)(CO)2(m-I)2PtI2(CO)] has been identified; [PPN][IrI(CH3)(CO)2(m-I)2PtI2(CO)] can in it’s turn lead to the formation of [PPN][PtI3(CO)], [IrI2(CH3)(CO)2(solv)], [Ir2I2(CH3)2(m-I)2(CO)4] and [PPN][Ir2I4(CH3)2(m-I)(CO)4]; all of these species have been characterized. Under CO pressure, [PPN][IrI(CH3)(CO)2(m-I)2PtI2(CO)] is a short-lived species that quickly leads to [IrI2(CH3)(CO)3] and [PPN][PtI3(CO)] showing that the main role of the platinum promoter is to abstract an I- ligand from [PPN][IrI3(CH3)(CO)2]. In the catalytic conditions, I- is abstracted from H[IrI3(CH3)(CO)2] by [PtI2(CO)2] and the rate determining step is accelerated; the relevant species H[IrI3(CH3)(CO)2], H[IrI3(COCH3) (CO)2] and H[PtI3(CO)] have been observed at 30 bar of CO. A catalytic cycle is proposed which depicts cooperative effect between iridium catalyst and platinum promoter.

4) Polymerization of olefins using single site catalysts

A calix[4]arene titanium catalyst has been employed for the synthesis of HDPE in the presence of MAO as co-catalyst. The 25,27-dipropyloxy-calix[4]arene titanium dichloride (Figure 2) has been synthesised and characterized through X-ray diffraction. This complex was employed as catalyst in the polymerization of ethylene under different polymerization conditions. The polymers were characterized through DSC and 13C-NMR and the molecular weight evaluated through GPC and viscometric analyses. The activation step of the catalyst was followed through NMR experiments. A methyltitanium species was evidenced using 1H- and 13C NMR spectroscopy, probably involved in the activation of the titanium complex.

Also the stability of the starting titanium calix[4]arene and the methyltitanium species were confirmed by NMR data, according to those reported in the  literarure. Heating these species up to 373 K the NMR spectra were unaltered. Very high molecular weight were detected also when the reaction temperature was relatively high indicating a high stability of the catalytic system. The joint publication gave rise to the cover of the corresponding Macromolecular issue

 5) Copolymerization of CO with olefins

The WG has continued to examine the preparation of Ni(II) and Pd(II) organometallic complexes containing P,O or P,N chelates capable to achieve the coupling of ethylene/CO/polar monomer, such as methylacrylate. The latter insertion represents a difficult step, the isolation of new complexes in which the coupling has been achieved are being currently characterized, including by X-ray diffraction.

During this period, attempts were made to monitor by IR and NMR techniques the course of reactions such as that described below. To this end, short-term visists to partner’s institutions were performed. Reaction of [Pd(Me)(P,O)(NCMe)]PF6 1 (P,O = Ph2PNHC(O)Me) (Scheme below) was exposed to 1 atm. of CO, the acyl complex [Pd{C(O)Me}(P,O)(NCMe)]PF6 2 was formed quantitatively. Insertion of ethene into the Pd-acyl bond of 2 occurred at ambient temperature, under atmospheric pressure. The reaction was completed after ca. 90 min. (31P{1H} NMR monitoring) and afforded  [Pd{CH2CH2C(O)CH3}(P,O)] 3. When complex 2 was reacted with methylacrylate, 31P{1H} NMR monitoring revealed its complete disappearance after ca. 4 h at ambient temperature and a new resonance appeared at d = 81.9, in the same region as that of 3. The 1H NMR spectrum  of the new complex [Pd{CH[C(O)OMe]CH2C(O)CH3}(P,O)]PF6 4 in acetone-d6 contains three methyl signals at d = 2.45 and 2.61 (Me-C(O)) and 2.91 (OMe), whereas the CH and CH2 protons Ha, Hb and Hc resonate at d = 3.16, 3.22 and 3.44, respectively. Interestingly, the stability of the (C,O) chelate in 3 did not prevent facile CO insertion into the Pd-C bond to give the a-keto chelate  [Pd{C(O)CH2CH2C(O)Me}(P,O)]PF6 5, as evidenced by the occurrence of a new n(CO) vibration at 1708 cm-1 and a 31P{1H} NMR resonance (d = 63.0) (Figure 3). The six-membered metallacycle in 5 inserted ethene under ambient conditions to afford [Pd{CH2CH2C(O)CH2CH2C(O)Me}(P,O)]PF6 6. Complex 5 was also treated with methylacrylate (under a CO atmosphere to avoid decarbonylation to 3) and insertion into the Pd-C(O) bond was achieved after 16 h at ambient temperature (31P{1H} NMR monitoring) and afforded  [Pd{CH[C(O)OMe]CH2C(O)CH2CH2C(O)Me}(P,O)] 7 as the sole product in 75% isolated yield. The use of a dissymmetric P,O ligand in this chemistry offers the advantage over symmetrical P,P or N,N ligands to orient the trans ligands in a more selective manner. The organic ligand in 7 was built-up in a stepwise manner from 1 by successive insertion into the Pd-methyl bond of CO, ethene, CO and methylacrylate. All the intermediates were isolated and characterized and this nicely mimic the desired sequences for terpolymerization reactions with these monomers. Progress were made in such studies but they were not complete enough to give rise to a joint publication.

In the course of this COST D-30 action, another fruitful collaboration developed in the field of cluster chemistry with the group of Bari, triggered by a research stay of Vito Gallo in our Laboratory, using a STSM. He studied in Strasbourg mixed-metal Pt-Co clusters with P,N,P short-bite ligands and discovered an unusual equilibrium between the chelate and bridged forms of such ligands as a functiuon of the steric properties of the N-substituent. A detailed study was undertaken which has given rise to a joint publication, although in a field initially not covered by our action. This work gave rise to the cover of the corresponding Dalton issue(Figure 4).              

6) Carbonates via CO2 –epoxide reactions

In this study the possibilities to react carbon dioxide with epoxides with structurally known transition or main group metal based catalysts are looked for. Coupling of carbon dioxide with epoxides is a desirable alternative route to carbonates especially because most of the industrial processes still use highly toxic phosgene route while manufacturing commodity carbonates. Although, the recent advances in this area have been impressive, sufficiently efficient process to succeed in an industrial setting has not been developed yet. There is also a challenge of competitive products.

The results obtained in the project show that aliphatic epoxides, eg. 1-hexene-epoxide  and cyclohexene epoxide can be homo-and co-polymerized by using heterogenous Zn-catalyst system. We have demonstrated that polycarbonates containing up to 98% polycarbonate linkages in the polymer backbone are formed. Also new catalyst candidates, iron(II) and cobalt(II) complexes bearing tetradentate ligands have been identified for the synthesis of cyclic carbonates.  These complexes were found to be excellent catalysts for the reaction of epoxide and CO2 when used in conjunction with Lewis base co-catalyst (n-Bu4NBr). This catalytic system gives propylene carbonate in high yield under soft reaction conditions (10 bars and 90 ºC) without need of reducing agent (Zn powder). Complex 2a had higher catalytic activity compared to its Fe and Mn analogues (Figure 5).

7) Pauson-Khand reaction catalysed by heterometallic cobalt based clusters

The Pauson-Khand reaction  is the cycloaddition af an alkene, an alkyne and carbon monoxide. We have started a project aimed at studying the catalytic activity of heterometallic cobalt based clusters for the intramolecular cyclocarbonylation of diethyl allylpropargylmalonate. Tri- and tetranuclear clusters containing at least two Co atoms were found active as precatalysts of the Pauson Khand reaction under mild conditions. Among the tetranuclear clusters tested, [RuCo3(CO)12](bmim) and [RuCo3(CO)12](NEt4) gave the best results in terms of productivity, and led to complete conversions of the substrate even at 2% load. Clusters Co4(CO)10(dppm) and Co4(CO)8(dppa)2 containing a diphosphane were almost inactive, while (dppa)PtCo2(CO)7, containing a monodentate phosphane, exhibited a moderate activity. The effect of diphosphane ligands on catalytic activity was found dependent on the metal skeleton. With PtCo2 clusters the productivity was higher when the species favors the chelating coordination mode on the Pt atom, confirming that the presence of P ligands onto Co atoms exerts a detrimental effect.

8) Mechanistic studies of Suzuki coupling by combined NMR and ESI-MS techniques

We have also carried out a detailed mechanistic study on the palladium catalysed cross-coupling reaction between aryldiazonium salts and aryltrifluoroborates. The precatalyst used in this study is bis(m-acetato)bis(4,4’-difluoroazobenzene-C2,N)dipalladium(II) (6. Figure 6). The reaction follows a Pd0/PdII cycle after reduction of 6 to a molecular Pd0 species (I). Based on the combined ESI-MS and 19F NMR techniques the catalytically active Pd0 species I is bearing the arylated azobenzene as a ligand. Oxidative addition by the diazonium salt generates an aryl-PdII intermediate (II) which was also detected in solution. The catalytic cycle is completed with the transmetallation between II and the organoborate, followed by fast reductive elimination of the cross-coupling product, restoring the molecular Pd0 species I. A concurrent activation path was also observed. It consists of the formation of (4,4’-difluoroazobenzene-C2,N)dipalladium(II) tetrafluoroborate (7) by the reaction of 6 with the diazonium salt and subsequent reduction by aryltrifluoroborate to give I (Figure 7).

 9) Dehalogenation of aromatics in ionic liquids

The catalytic hydrocarboxylation of linear alkenes to obtain carboxylic acids using supercritical carbon dioxide as a solvent was studied. High selectivities in acids have been obtained. The best results were achieved by adding a perfluorinated surfactant to the reaction mixture (93 % conversions and ca 90 % selectivity). Comparative multinuclear high pressure NMR studies in THF-d8 and in supercritical CO2 show the formation of Pd(0) species.

10) Supercritical fluid chromatography

In CO2-based chromatography (supercritical fluid chromatography, SFC) both chiral and non-chiral SFC have been developed, especially cryogenic low-temperature chromatography for high-resolution SFC. The technique developed was applied for different types of chiral racemates ( finrozole, guaifenesin and ferulic acid dimer dimethylester). A fast statistical screening method for assessing chiral resolution of racemates has been refined and published .

 

 

 

 

 

 

 

 

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  1. Homogeneously catalyzed high-pressure syntheses of chemicals using small molecules such as carbon monoxide, hydrogen, carbon dioxide, nitrogen, alkenes, or alkynes as reagents

Presentation: Invited at COST D30 Final Evaluation Meeting, by Piero Frediani
See On-line Journal of COST D30 Final Evaluation Meeting

Submitted: 2007-10-02 17:52
Revised:   2009-06-07 00:44
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