Luigi Toniolo

On the mechanism of hydroesterification of styrene using an in situ-formed cationic palladium complex

Abstract The mechanism of hydroesterification of styrene using in situ-formed Pd(OTs) 2 (PPh 3 ) 2 from Pd(OAc) 2 , PPh 3 and TsOH in methanol has been investigated by isolation and characterisation of catalytically active intermediates. From reaction mixtures, Pd–hydridocarbonyl and Pd–acyl complexes were isolated and characterised, based on which a Pd hydride mechanism has been proposed. Formation of palladium hydride species has also been confirmed by 31 P-NMR experiments.

Metals in organic syntheses: XIV. NMR, IR, and reactivity studies on the olefin hydroformylation catalyzed by Pt-Sn complexes☆

Abstract Among the several hydrides formed when trans-[PtHClL2] (L = PPh3) reacts with Sncl2, only trans-[PtH(SnCl3)L2] rapidly inserts ethylene, at −80°C, to yield cis-[PtEt(SnCl3)L2]. At −10°C, cis-[PtEt(SnCl3)L2] irreversibly rearranges to the trans-isomer, thus indicating that the cis-isomer is the kinetically controlled species, and that the trans-isomer is thermodynamically more stable. At −50°C, a mixture of trans-[PtHClL2] and trans[PtH(SnCl3)L2] reacts with ethylene to give cis-[PtEtClL2] and cis-[PtEt(SnCl3)L2] and this has been attributed to the catalytic activity of SnCl2 which dissociates from cis-[PtEt(SnCl3)L2] at this temperature. Carbon monoxide promotes the cis-trans isomerization of cis[PtEt(SnCl3)L2], which occurs rapidly even at −80°C. This rearrangement is followed by a slower reaction leading to the cationic complex trans-[PtEt(CO)L2]+ SnCl3−. At −80°C, this complex does not react further, but when it is kept at room temperature ethyl migration to coordinated carbon monoxide takes place, to give several Pt-acyl complexes, i.e. trans-[PtCl(COEt)L2], trans-[Pt(SnCl3)(COEt)L2], trans-[PtCl(COEt)l2 · SnCl2], and trans-[Pt(COEt)(CO)L2]+ SnCl3−. This mixture of Pt-acyl complexes reacts with molecular hydrogen to yield n-propanal and the same complex mixture of platinum hydrides as is obtained by treating trans-[PtHClL2] with SnCl2. Trans-[PtH(SnCl3)L2] reacts with carbon monoxide to yield the five-coordinate complex [PtH(SnCl3)(CO)2L2], which has been characterized by NMR and Ir spectroscopy; ethylene does not insert into the PtH bond of this complex at low temperature. At room temperature, trans-[PtH(SnCl3)L2] reacts with a mixture of CO and ethylene to yield the same mixture of Pt-acyl species as is obtained when trans-[PtEt(SnCl3)L2] is allowed to react with CO. The role of a PtSn bond in these reactions is discussed in relation to the catalytic cycle for the hydroformylation of olefins.

Carbon monoxide-ethylene copolymerization catalyzed by a Pd(AcO)2/dppp/TsOH1 system: the promoting effect of water and of the acid

Abstract For the title copolymerization the catalyst productivity (g-polymer/g-Pd · h) is significantly influenced by the presence of water and of the acid as it passes through a maximum upon increasing concentration of H2O and of TsOH. In the presence of 450 ppm of H2O, the maximum productivity is ca. 3.7 times higher than when the copolymerization is carried out in the presence of 1% of trimethylorthoformate, used as H2O scavenger in MeOH as solvent, at 90°C, under 45 atm of total pressure, employing the catalyst precursor in the molar ratio Pd/dppp/TsOH 1/1/2 ([Pd] 5.6 × 10−5 mol · 1−1). Under similar conditions, but under 60 atm of the two monomers, in the presence of 900 ppm of H2O and when employing an excess of the acid ( TsOH Pd 6.4) the productivity reaches a maximum of ca. 11500 g-polymer/g-Pd · h, which is 1.4 times higher than that obtained when the TsOH Pd ratio is 2 1 . The promoting effect of H2O is ascribed to the possibility that a higher concentration of active PdH species, which are proposed to initiate the catalytic process through the insertion of the olefin into a PdH bond, is achieved through the interaction of carbon monoxide with water on the metal center, via a reaction closely related to the water gas shift reaction. It is also proposed that the promoting effect of the acid is due to the reactivation of inactive Pd(0) species, which inevitably form under the reducing reaction conditions, with formation of active PdH species. When the copolymerization is carried out in the presence of benzoquinone (BQ), either under the reaction conditions in which the productivity reaches a maximum or under unfavorable conditions, that is, in the presence of low or relatively high concentrations of water, the productivity has an average value of ca. 7000 g-polymer/g-Pd · h. Since it was found by other research groups that in the presence of BQ the polymer takes origin mainly through the insertion of CO into a PdOCH3 species whose formation is favored in the presence of BQ, the findings presented above give further support to the suggestion that the promoting effect of H2O and of TsOH are due to the possibility that, when present in appropriate amounts, they favour the formation of PdH species which start the catalytic cycle.

Hydroesterification of styrene using an in situ formed Pd(OTs)2(PPh3)2 complex catalyst

Abstract Hydroesterification of styrene to 3-phenyl propionate 1 , and 2-phenyl propionate 2 , has been studied using a Pd(OTs) 2 (PPh 3 ) 2 catalyst formed in situ from Pd(OAc) 2 , PPh 3 and p -toluenesulfonic acid ( p -tsa). Because of the weakly coordinating properties of the TsO − ligand, the catalyst has vacant coordination sites capable of easy activation of reactants. The presence of water is found to be necessary for the reaction and hydrogen enhances the catalytic activity under certain conditions (with Pd: p -tsa=1). The beneficial effect of hydrogen, p -tsa and water is discussed in terms of favoring the formation of a Pd–H species, which initiates the catalytic cycle through the insertion of styrene into this bond with formation of a Pd-alkyl intermediate, which inserts CO to give a Pd-acyl intermediate, which, upon nucleophilic attack of the alkanol on the carbon atom of the acyl ligand, yields the final product and the starting hydride back to the catalytic cycle. p -tsa would favor the formation of a Pd–H species by reactivating any Pd(0) species that may form during the course of catalysis. Water would favor the formation of a Pd–H species through a reaction closely related to the water–gas shift reaction. The effect of various ligands, promoters, solvents and alcohols on catalytic activity as well as selectivity pattern has been studied. Regioselectivity to the branched product, 2 , increases with decrease in basicity of the phosphorous ligands as well as steric bulk around the palladium center and polarity of the medium.

Multistep electron transfer catalytic system for the oxidative carbonylation of phenol to diphenyl carbonate

Abstract The oxidative carbonylation of phenol to diphenyl carbonate is catalyzed by palladium salts in combination with a cocatalyst such as p -benzoquinone (BQ) or a salt of Co, Mn, Cu. The addition of a surfactant such as tetrabutylammonium bromide makes the catalytic system more efficient. The role of each component in the catalytic system is discussed. A catalytic cycle is proposed where, in the first step, diphenylcarbonate is formed from phenol and CO with concomitant reduction of Pd(II) to Pd(0) and formation of two protons. p -Benzoquinone, which is reduced to hydroquinone, in the presence of protons, reoxidizes Pd(0) to Pd(II) while the metal cocatalyst is reduced by hydroquinone which is reoxidized to p -benzoquinone. Oxygen and protons, arising from the last reaction, close the cycle with reoxidation of the reduced metal cocatalyst and formation of water.

Metals in orgranic syntheses : VII. The isolation of trans-[PtCl(COPr-n)(PPh3)2] (I) and trans-[Pt(SnCl 3)(COPr-n)(PPh 3) 2] (II), active intermediates in the hydroformylation of propene catalyzed by a [PtCl 2(PPh 3) 2]-SnCl2, precursor. The crystal and molecular structure of complex i and a comparison with its palladium analog

Trans -[PtCl(COPr-n)(PPh 3 ) 2 ] (I) has been isolated in good yield from the mixtures obtained by treating a mixture of propene, cis -[PtCl 2 (PPh 3 ) 2 ] and SnCl 2 · H 2 O with carbon monoxide in the presence or absence of hydrogen in an alcohol in which no significant hydroformylation or hydroxycarboalkylation actually occurs. The cis -[PECl 2 (PPh 3 2 ]-SnCl 2 · 2 H 2 O system is highly active in the catalytic hydroformylation in methyl isobutyl ketone, and from reaction mixtures in this medium trans -[Pt(SnCl 3 )(COPr-n)(PPh 3 ) 2 ] (II) has been isolated (33% yield). The presence of a PtSn bond in a complex of type II plays a key role in promoting the formation of the aldehyde from the acyl derivative, but it is not necessary for the formation of intermediate I, since this can be isolated in good yield even in the absence of the tin compound. The higher regioselectivity observed using intermediate I or II, compared with that when the precursor is used is discussed in terms of steric effects of the ligands competing for coordination to the platinum atom. The catalytic properties of complex I are compared also with those of its palladium analog, which catalyzes only the hydrocarbo-

Platinum(II) trichlorostannate chemistry. On the importance of the Pt-Sn linkage in hydroformylation chemistry and a novel PtC(OSnCl2)R-carbene

Summary The reaction of trans -PtCl(COR)(PPh 3 ) 2 ( 1 ) (R = a , C 6 H 5 ; b , C 6 H 4 - p -NO 2 ; c C 6 H 4 - p -CH 3 ; d , C 6 H 4 - p -OCH 3 ; e , CH 3 , f , Et; g , Pr n ; h , Hex n ; i , CH 2 CH 2 Ph; j , Bu t ) with SnCl 2 and SnCl 2 plus H 2 are described. The reactions with SnCl 2 alone afford a mixture of trans -Pt(SnCl 3 )(PPh 3 ) 2 ( 2 ), and trans -PtCl(C(OSnCl 2 )-R)(PPh 3 ) 2 ( 3 ) with 3 having tin oxygen bond. For 1f, 1h and 1j , reactions with SnCl 2 plus H 2 give aldehydes and platinum(II) hydride complexes, whereas for 1b and 1d , no aldehydes are obtained. The significance of these results in relation to H 2 activation in the hydroformylation reaction is discussed. 31 P, 119 Sn, 195 Pt and, in a few cases, 13 C NMR data are presented.

On the mechanism of platinum(II)/SnCl2 catalyzed hydroformylation of olefins. Studies of the reactivity of alkyl complexes containing chelating diphosphines

The complexes cis-PtCl(C2H5)(diphosphine) (diphosphine = 1,3-bis(diphenylphosphino)propane and 1,4-bis(diphenylphosphino)butane) have been used as model compounds for the hydroformylation of olefin catalyzed by the system PtCl2/diphosphine/SnCl2. They react with SnCl2 to yield the corresponding trichlorostannate complexes cis-Pt(SnCl3)(C2H5)(diphosphine), which in the absence of free ethylene decompose to form the dichloro species cis-PtCl2(diphosphine) via an unstable hydrido species. Both the chloro- and trichlorostannate-alkyl complexes react with CO to give the acyl species cis-PtX(COC2H5)(diphosphine) (X = Cl or SnCl3). When the diphosphine is 1,4-bis(diphenylphosphino)butane, oligomeric acyl complexes of trans geometry are formed. Preliminary studies of the reactivity of the acyl complexes with molecular hydrogen show that only the complexes bearing the Pt—SnCl3 moiety react at ambient conditions giving propanal as the only observed organic product.

Synthesis, characterization and X-ray structure of [Pd(dppp)(H2O)(TsO)][TsO] (dppp = 1,3-bis(diphenylphosphino)propane; TsO = p-CH3C6H4SO3), a catalytic species in COC2H4 copolymerization

Abstract The title complex was prepared by reacting Pd(AcO) 2 first with dppp and then with TsOH·H 2 O in MeOH at r.t. It is highly active in COC 2 H 4 copolymerization in MeOH. It has been characterized by IR and 1 H and 31 P NMR spectroscopies. The X-ray structure consists of a packing of monomeric palladium cations and tosylate anions in the ratio 1:1. The palladium atom is in rather distorted square coordination.

Metals in organic syntheses: X. Olefin hydroformylation and hydrocarboalkoxylation competitively catalyzed by a [PtCl2(PPh3)2]/SnCl2 system☆

The system [PtCl2(PPh3)2]/SnCl2 significantly catalyzes only the hydroformylation of α-olefins at 100°C in EtOH, at P(CO)  P(H2)  65 atm; hydrocarboalkoxylation does not occur to an apreciable extent, even in the presence of potential activating agents (HCl, LiCl). The catalyst precursor has been recovered from the reaction medium, as the cationic complex [PtH(CO)(PPh3)2](SnCl3), having the SnCl3− anion non-directly bound to the platinum atom, and as trans-[PtCl(COR)(PPh3)2]. The latter complex is a (precursor) intermediate leading to an active catalytic species possessing at least one PtSn bond which plays a key role in the catalysis.