Valentina Beghetto

Iminophosphine - palladium(0) complexes as catalysts for the Stille reaction

The cross-coupling of iodobenzene with tributylphenylethynylstannane or tributylvinylstannane is efficiently catalysed by iminophosphine–palladium(0)–olefin complexes of the type [Pd(η2-dmf)(P-N)] (dmf, dimethylfumarate; P-N, 1-(PPh2)-C6H4-2-CNR (R=alkyl, aryl)). The catalytic activity depends on the R substituent of the imino group: the highest reaction rates are obtained using aryl-substituted iminophosphines. Equivalent catalytic systems can be obtained using a palladium source such as Pd(OAc)2 or Pd(dba)2 (dibenzylideneacetone, dba) in combination with the iminophosphine ligands. In the coupling of iodobenzene with tributylphenylethynylstannane, the highest reaction rates are obtained using an iminophosphine/palladium molar ratio of 2:1, while in the vinylstannane–iodobenzene coupling the best P-N/Pd ratio is 1:1.

Iminophosphine-palladium(0) complexes as catalysts in the alkoxycarbonylation of terminal alkynes

Abstract In the presence of methanesulfonic acid, the palladium(0)-olefin complexes: [Pd(η 2 -ol)(P N)] [ol=dimethyl fumarate or fumaronitrile, P N=1-(Ph 2 P)C 6 H 4 -2-CH NR (R=CMe 3 or C 6 H 4 OMe-4)] catalyse the alkoxycarbonylation of terminal alkynes. Moderately good rates are obtained when the catalysts are promoted with two equivalents of the free P N ligand and a large excess of acid at 120°C. The catalytic data suggest that derivatives of the type [Pd(alkyne)(P N) n ] ( n =2–3) are the active catalytic species.

Carbonylation of terminal alkynes catalysed by Pd complexes in combination with tri(2-furyl)phosphine and methanesulfonic acid

Pd(OAc)2 in combination with tri(2-furyl)phosphine and methanesulfonic acid is an efficient catalyst for the alkoxycarbonylation of 1-alkynes. Fairly good reaction rates are obtained under mild conditions (50–80°C and P(CO)=20 bar) with high regioselectivity (ca. 95%) towards the formation of 2-substituted acrylic ester.

Esters and N,N-dialkylamides of 2-(trifluoromethyl)acrylic acid (TFMAA) through Pd-catalysed carbonylation of fluorinated unsaturated substrates

Abstract Esters and amides of 2-(trifluoromethyl)acrylic acid (TFMAA) have been synthesised by two different routes involving CO-chemistry. The alkoxycarbonylation of 2-bromo-3,3,3-trifluoropropene was carried out in the presence of PdCl 2 (PPh 3 ) 2 . High substrate conversions were obtained, but the yield in acrylic esters was generally low because the desired unsaturated esters reacted further adding a molecule of alcohol to the C–C double bond. The carbonylation of 2-bromo-3,3,3-trifluoropropene in the presence of secondary amines produced the corresponding unsaturated amides in high yields; the addition of the amine to the C–C double bond also occurred, but this side reaction was minimised by using secondary cyclic amines such as morpholine or piperidine. Alternatively, the acrylic esters can be obtained by hydromethoxycarbonylation of 3,3,3-trifluoropropyne using the catalytic system Pd(OCOCH 3 ) 2 /2-pyridyldiphenylphosphine/CH 3 SO 3 H. In this process the most important side product is the isomeric crotonic ester. The regioselectivity of the reaction can be controlled to a great extent by a suitable choice of the solvent.

Synthesis of new chiral phosphine–phosphites and diphosphites and their application in asymmetric hydroformylation

Abstract A new chiral phosphine–phosphite and two strictly related diphosphites have been synthesised and tested in the asymmetric hydroformylation of styrene. The catalytic efficiency of the phosphine–phosphite appears distinctly higher than that of the two diphosphites; in particular, it furnishes 2-phenylpropanal with very high regioselectivity and enantioselectivity up to 40%.

Synthesis of the chiral diphosphine ligand 2,3-bis(diphenylphosphino)butane (CHIRAPHOS)

Abstract A new synthesis of CHIRAPHOS starting from readily available 2,3-bis(diphenylphosphinyl)-1,3-butadiene has been devised. The key step of the synthesis is the hydrogenation of the diene to rac -2,3-bis(diphenylphosphinyl)butane. This reduction can be conveniently performed by treating 2,3-bis(diphenylphosphinyl)-1,3-butadiene with NaBH 4 in THF/CH 2 Cl 2 mixtures. The racemic diphosphine oxides are subsequently resolved using (2 S ,3 S )-(+)- and (2 R ,3 R )-(−)-2,3- O -dibenzoyltartaric acid [(+)- and (−)-DBTA]. Reduction of the resolved phosphine oxides is carried out using trichlorosilane.

Asymmetric hydroformylation of styrene with PtCl2(atropoisomeric diphosphine)/SnCl2 systems

Abstract The new chiral complex [PtCl2{(S)-(−)-MeOBIPHEP}], where MeOBIPHEP is the atropoisomeric diphosphine 2,2′-bis(diphenylphosphino)-6,6′-dimethoxy-1,1′-biphenyl, has been synthesized. In the presence of SnCl2 this species is an efficient catalyst for the asymmetric hydroformylation of styrene. Asymmetric inductions are higher than those attainable using the system [ PtCl 2 {(R)-(+)- BINAP }] SnCl 2 , where BINAP is 2,2′-bis(diphenylphosphino)-1,1′-binaphthyl. The influence of CO and H2 partial pressures on the catalytic activity of the [ PtCl 2 {(S)-(−)- MeOBIPHEP }] SnCl 2 system has also been studied.

Mechanistic study on the cross-coupling of alkynyl stannanes with aryl iodides catalyzed by η2-(dimethyl fumarate)palladium(0) complexes with iminophosphine ligands

The reactions of [Pd(η2-dmfu)(P–N)] [dmfu = dimethyl fumarate; P–N = 2-(PPh2)C6H4-1-CHNR, R = C6H4OMe-4 (1a), CHMe2 (2a)] and [Pd(η2-dmfu)(P–N)2] with IC6H4CF3-4, ISnBu3 and PhCCSnBu3 have been studied under pseudo-first-order conditions. The oxidative addition of IC6H4CF3-4 yields [PdI(C6H4CF3-4)(P–N)] (1b or 2b). No reaction takes place with PhCCSnBu3 and also with ISnBu3 in the presence of an excess of PhCCSnBu3. In the presence of fumaronitrile (fn), 1b and 2b undergo transmetalation by PhCCSnBu3 followed by fast reductive elimination to yield [Pd(η2-fn)(P–N)]. The same reaction sequence occurs for the system [PdI(C6H4CF3-4)(P–N)]/P–N (1 ∶ 1 molar ratio) to give [Pd(η2-fn)(P–N)2]. The palladium(0) complexes are active catalysts in the cross-coupling of PhCCSnBu3 with aryl iodides ArI (Ar = C6H4CF3-4, Ph). The catalytic efficiency depends on the complex: [Pd(η2-dmfu)(P–N)2] > [Pd(η2-dmfu)(P–N)], and on the substituent R: C6H4OMe-4 > CHMe2. The reactivity and spectroscopic data suggest a catalytic cycle involving initial oxidative addition of ArI to a palladium(0) species, followed by transmetalation of the product and by fast reductive elimination to regenerate the starting palladium(0) compound. For [Pd(η2-dmfu)(P–N)] as catalyst, the oxidative addition is the rate-determining step, while for [Pd(η2-dmfu)(P–N)2] the oxidative addition and the transmetalation steps occur at comparable rate.

(2-Furyl)phenyl(2-pyridyl)phosphine as a new ligand in the alkoxycarbonylation of terminal alkynes

Abstract (2-Furyl)phenyl(2-pyridyl)phosphine has been synthesised by reaction of 2-furyllithium with chlorophenyl(2-pyridyl)phosphine. The new ligand in combination with Pd(OAc)2 and methanesulphonic acid provides a catalytic system highly active in the alkoxycarbonylation of terminal alkynes. The catalytic activity of the system depends on both the metal to ligand and metal to acid ratios. The new ligand seems to be more efficient than the widely used (2-pyridyl)diphenylphosphine.

Asymmetric hydrogenation by an in situ prepared (S)-BINAP–Ru(II) catalytic system

Abstract A catalytic system obtained by in situ mixing [RuCl 2 (benzene)] 2 and ( S )-BINAP has been tested in the asymmetric hydrogenation of various substrates. The best results have been obtained working in methanol: excellent optical purities were achieved in the reduction of substrates such as tiglic acid (83%), geraniol (97%) and methyl 3-oxobutanoate (95%). Some investigations on the nature of the catalyst have been carried out.