G. Cavinato

New alkoxycarbonyl complexes of palladium (II) and their role in carbonylation reactions carried out in the presence of an alkanol

Abstract The new complexes of general formula trans-Pd(COOR)ClL2 (L = PPh3, R = Et, n-Pr, iso-Pr, n-Bu, iso-Bu, sec-Bu, 2-ethoxyethyl, cyclopentyl, cyclohexyl; L = 1,2-diphenylphosphinoethane, R = Et) are prepared by reacting trans-PdCl2L2, suspended in an alkanol, under 10–50 atm of carbon monoxide, at 50–70°C, in presence of a base such as a trialkylamine or a carboxylic acid anion. They have been characterized by IR, 1H and 31P NMR spectroscopy. The complexes with RMe and Et show two absorption bands centered at ca. 1650 cm−1, which are probably due to conformational isomers with cis and trans geometry. The other show only one band at ca. 1650 cm−1. The 1H NMR and 31P NMR spectra of all the monophosphine complexes show that only one isomer is present in solution. Instead, for the diphosphine ethoxycarbonyl complex the 1H NMR spectra suggest that two isomers are present, in the ratio of ca. 1:1, as confirmed by decoupling experiments. The two isomers may take origin from different orientation of the alkoxycarbonyl ligand with respect to the asymmetrical metal centre, because of hindered rotation around the PdC bond due to the partial double bond character. The R group of the alkoxycarbonyl ligand can be exchanged with a different R′ group by reacting the Pd(COOR)Cl(PPh3)2 complex with an excess of R′OH. The reaction is practically quantitative when R is bulkier than R′. The alkoxycarbonyl complexes react with the corresponding alkanol, in the presence of a base such as a trialkylamine, at 90–100°C, under carbon monoxide, yielding Pd(0) carbonyl-phosphine complexes with formation of dimethyl carbonate in an almost quantitative yield. They react also with water giving off CO2 and yielding Pd(0) complexes. The reaction is promoted by an acid of a non-coordinating anion such as HBF4. Instead, the reaction with HCl yields the corresponding dichloride of Pd(II), beside carbon monoxide and the corresponding alkanol. The role of these complexes in catalytic alkoxycarbonylation reactions is discussed.

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-

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.

Metals in organic syntheses. XIII. The isolation and molecular structure of trans-[PdCl(COC6H13-n)(PPh3)2], an intermediate in the hydrocarboalcoxylation of 1-hexene catalyzed by the precursor trans-[PdCl2(PPh3)2]

Abstract The complex trans-[PdCl(COC6H13-n)(PPh3)2] has been isolated in the course of 1-hexene hydrocarboalkoxylation catalyzed by the precursor trans-[PdCl2(PPh)3)2] in EtOH. The catalytic activity of the precursor and that of the acyl complex are practically the same, thus suggesting that the latter complex is an intermediate in the catalysis. The IR spectrum of the acyl complex shows ν(CO) at 1685 cm−1. The crystal and molecular structure of the complex was determined from three dimensional X-ray diffractometer data. The complex crystallizes in the triclinic space group P 1 . Cell parameters were: a = 17.330(8), b = 11.963(7), c = 10.100(7) », α = 112.4(1), β = 97.7(1), γ = 99.8(1)°, Z = 2. Full-matrix least-squares refinement converged at R = 0.067. The coordination about the metal significantly deviates from planarity towards a tetrahedral configuration. No unusual dimensions were shown.

Convenient one-step synthesis of substituted phosphine complexes of platinum(II) directly from hexachloroplatinic acid

Abstract Cis-[PtCl 2 (PPh 3 ) 2 ] and [PtCl 2 (PP)] (PP = Ph 2 P(CH 2 ) 1, 2 or 4 PPh 2 ) are synthesized by treating H 2 [PtCl 6 ]·H 2 O with an excess of phosphine in EtOH; by the same procedure, except for the presence of aqueous formaldehyde, trans -[PtCl 2 (PPh 3 ) 2 ] can be prepared. By pressuring H 2 [PtCl 6 ]· 6H 2 O and an excess of PPh 3 with carbon monoxide and molecular hydrogen in EtOH trans -[PtHCl(PPh 3 ) 2 ] is obtained; in the presence of propene, even without molecular hydrogen, trans -[PtCl(COPrn)(PPh 3 ) 2 ] forms. All the above complexes are prepared in good yield.

Metals in organic syntheses

Abstract The complex trans -[PtCl(COC 6 H 13 -n)(PPh 3 ) 2 ] (I) has been synthesized by treating cis -[PtCl 2 (PPh 3 ) 2 ] and 1-hexene with carbon monoxide under pressure in EtOH at 100°C. When in combination with SnCl 2 -2H 2 O, complex I is an intermediate precursor in the highly regioselective catalytic hydroformylation of 1-hexene, which readily occurs in a solvent such as a ketone. The crystal and molecular structure of complex I has been determined from three dimensional X-ray diffractometer data. The complex crystallizes in the triclinic space group P 1 . Cell parameters are as follows: a 15.869(8), b 12.306(8), c 11.437(7) A, α 109.8(1), β 76.6(1), γ 112.9(1)°, Z  2. Fullmatrix least-squares refinement converged at R  0.058 ( R w  0.064). The structure has approximately square planar geometry, and shows no unusual dimensions.

Isolation and characterization of the acyl complexestrans-[Pt(PPh3)2(COR)Cl] (R =nBu orsBu) and their relevance to the hydroformylation of linear butenes catalyzed by platinum/tin/triphenylphosphine catalytic systems. Molecular structure ofcis- [Pt(PPh3)2Cl(SnCl3)]

Abstract The acyl complex trans -[Pt(PPh 3 ) 2 (CO n Bu)Cl] ( A ) has been synthesized by reaction of [Pt(PPh 3 ) 2 Cl 2 ] with 1-butene under 100 atm of CO at 80–100°C, in ethanol. With 2-butene rather than 1-butene under the same conditions, a mixture of the above acyl complex and of trans - [Pt(PPh 3 ) 2 (CO s Bu)Cl] ( B ) was formed. Complexes A and B do not interconvert. The new acyl complexes A and B have been characterized by IR and 1 H NMR and 13 C NMR spectroscopy. The ratio A / B increases with PPh 3 /Pt ratio and with temperature. The formation of two isomers when 2-butene is used involves an isomerization process which is likely to be limited to the alkyl precursor complexes. The reactivity of complexes A and B has been tested in reactions with SnCl 2 , H 2 , HCl and trans -[Pt(PPh 3 ) 2 HCl]. From the reaction solutions crystals of cis -[Pt(PPh 3 ) 2 Cl(SnCl 3 )] have been obtained. Its molecular structure has been determined by X-ray diffraction. The Pt atom has cis square planar coordination, with angular distortions due to steric factors. The strong trans influence of the SnCl 3 group is confirmed by the lengthening of the trans Pt-P distance. The SnCl 3 group has the pyramidal geometry found in all related compounds.

Hydrocarbo alkoxylation of N-vinylphthalimide catalyzed by palladium complexes

Abstract The hydrocarboalkoxylation of N-vinylphthalimide catalyzed by palladium tertiary phosphine complexes occurs with high selectivity towards the linear isomer when the alcohol is used also as the solvent but towards the branched isomer in the presence of an additional solvent. When triphenylphosphine is employed as the ligand, the yield and the regioselectivity towards the branched isomer increase with increasing pco or decreasing concentration of the phosphine. Reaction in the presence of molecular hydrogen leads to higher yields, with minor changes in regioselectivity. High regioselectivities towards either the linear or the branched isomer are observed also in the presence of chiral di- or mono-phosphines, but the degree of asymmetric induction is very low.

New aspects of the synthesis of dimethyl carbonate via carbonylation of methyl alcohol promoted by methoxycarbonyl complexes of palladium(II)

Abstract [PdCl 2 (PPh 3 ) 2 ] suspended in MeOH reacts with carbon monoxide (40–80 atm, 50°C), in the presence of a base such as NEt 3 to give the methoxycarbonyl complex trans -[PdCl(COOMe)(PPh 3 ) 2 ]. When the carbonylation reaction is carried out at 90–100°C reduction to Pd 0 carbonyl-phosphine complexes occurs, with formation of dimethyl carbonate, selectively and in an almost quantitative yield. The above complexes are less reactive than the acetato-analogues, which give dimethyl oxalate as the main organic carbonylation product even at 50°C.

Palladium-catalyzed carbonylation of benzyl alcohol derivatives to phenylacetic acid derivatives

Abstract Benzyl alcohols are carbonylated to phenylacetic acid derivatives in the presence of a palladium catalyst. Typical reaction conditions are: temperature 9O–120°C;P(CO)=20–80 atm; benzyl alcohol/ROH/Pd=100–200/300–1000/1; [Pd]=0.5×10−2−1 × 1O−2 M; solvent: dioxane, benzene, ethanol; reaction time 1–4 h. Under these experimental conditions high yields are obtained only when the aromatic ring contains a hydroxy substituent at the para position and when the palladium precursor is a chloride used in combination with 2–4 equivalents of PPh3. When the substituent is in a m- or o-position or is a methoxy group, or in the case of benzyl alcohol, only trace amounts of phenylacetic acid derivatives are obtained. The system PdY2(PPh3)2-PPh3 yields the same results as PdY2 with equivalent amounts of phosphine (Y=Cl, Br, I, CH3COO). When the precursor is employed in combination with a base, catalysis does not occur to an appreciable extent. On the contrary, when HCl is added in an amount comparable to that of the palladium precursor (HCl/Pd=2–15), slightly higher yields are obtained. These results suggest that the starting benzyl alcohol reacts with HCl to yield the corresponding chloride, which initiates the catalytic cycle. Moreover, it has been observed that the best results are obtained when the palladium(II) precursor decomposes to metallic palladium. Pd/C is also active, provided that it is employed in combination with HCl and PPh3. Thus, for example, the system Pd/C-HCl-PPh3 is catalytically equivalent to the system that originates from the initial precursor PdCl2(PPh3)-PPh3, eventually in the presence of added HCl. Low catalytic activity is observed in the absence of PPh3 or when this ligand is added in relatively small amounts. The highest yields are obtained when P/Pd=2–3. These facts suggest that the PPh3 ligand eases the oxidative addition step by enhancing the electron density on the metal. Under these conditions, the yield increases with increasing gas pressure. When the ligand is present in relatively large excess, the catalytic activity drops dramatically, probably because PPh3 competes with the coordination to the metal. The catalytic activity is strongly influenced by the nature of the solvent. The yield decreases in the order: dioxane » ethanol ≈ benzene, and depends also on the ROH/solvent ratio; the highest yields are achieved when the EtOH/dioxane ratio is ca. 1/5 (ml). At higher concentrations of EtOH the yield is significantly lower, probably because the equilibrium between benzyl alcohol and hydrochloric acid is less favorable to the formation to benzyl chloride and/or the acid competes with the chloride for the oxidative addition to the metal. A mechanism for the catalytic cycle is proposed: (i) oxidative addition of ArCH2Cl to ‘reduced palladium’, which may be palladium coordinated by other palladium atoms, and/or carbon monoxide, and/or phosphine ligands. (ii) Carbon monoxide ‘insertion’ into Pd—benzyl intermediate with formation of an acyl intermediate. (iii) Nucleophilic attack of EtOH on the carbon atom of the acyl intermediate to give the desired product and return the catalyst back to the catalytic cycle. The promoting effect of the hydroxy substituent in the para position is interpreted in terms of resonance structures, in which deprotonation of this substituent may play an important role in weakening the CCl bond, thus easing the initial step of the catalysis.