Palladium(0)-Catalyzed Cis−Trans Alkene Isomerizations

The cis−trans isomerization of some olefins bearing strong electron-withdrawing substituents (dmma and E-tse) is catalyzed by their mere coordination to a suitable Pd(0) fragment without any external reagents. The choice of spectator ligand is fundamental to the efficiency of the process, and the phosphanyl-quinolines dppq and dppq-Me have proven to be particularly effective. The mechanism of the rearrangement is discussed, and the presence of electron-withdrawing and conjugated substituents on the olefins appears to be an essential prerequisite for the isomerization

Migratory Insertion of Allyl Groups across the Pd−C Bond in Palladium(II) Isocyanide Complexes. The Fundamental Interplay between Temperature and Allyl Hapticity

The 2,6-dimethylphenyl- and tert-butylisocyanides (DIC and TIC, respectively) react in chlorinated solvents with the allyl dimers [Pd(μ-Cl)(η3-C3H3Me2)]2 and [Pd(μ-Cl)(η3-C3H5)]2, giving the insertion products trans-[Pd(DIC)2(CN(2,6-Me2C6H3)CH2CHCMe2)Cl], trans-[Pd(TIC)2(CN(CMe3)CH2CHCMe2)Cl], and trans-[Pd(DIC)2(CN(2,6-Me2C6H3)CH2CHCH2)Cl], respectively. In particular, the reaction between the complex [Pd(μ-Cl)(η3-C3H3Me2)]2 and DIC was studied in detail, and a number of different species involved in the insertion process were identified. A mechanistic network taking into account all the involved derivatives was proposed on the basis of independently measured equilibrium and rate constants.In this respect, two independent equilibria, both involving the formation of η1-allyl intermediates, were detected and the related constants determined. The formation of such intermediates bearing the η1-allyl fragment in cis position to the isocyanide is crucial when the subsequent insertion is considered. The intermediates, however, do not display the same reactivity owing to the different thermodynamic parameters governing their formation. In particular the formation of [Pd(η1-C3H3Me2)(DIC)2Cl] (MD2Cl) represents the privileged path to the product trans-[Pd(DIC)2(CN(2,6-Me2C6H3)CH2CHCMe2)Cl] (P) when the insertion process is carried out at RT. The alternative intermediate [Pd(η1-C3H3Me2)(DIC)3]+Cl− (MD3) hardly contributes to the formation of the product P since it is disfavored by increasing the temperature

Highly Active and Selective Platinum(II)-Catalyzed Isomerization of Allylbenzenes: Efficient Access to (<i>E</i>)-Anethole and Other Fragrances via Unusual Agostic Intermediates

Terminal alkene isomerization reactions can be efficiently catalyzed by PtII complexes bearing a chelating diphosphine and an alkyl or, better, aryl moiety under mild experimental conditions. In particular diphosphines, such as dppb, characterized by a large bite angle in conjunction with a pentafluorophenyl residue coordinated to Pt enable quantitative conversion of the reagent into internal alkenes within few hours at 50 °C in CHCl3 as solvent. E/Z selectivity can be as high as 98:2 for allylbenzene, and the catalytic system can be fruitfully applied to the preparation of E fragrances derived by isomerization of substituted allylbenzene derivatives. The selectivity increases during the progress of the reaction because of a subsequent catalytic step where the Z alkene coordinates to the Pt and is converted into the E isomer. NMR investigation on the catalyst showed formation of agostic Pt···H intermediate species derived by insertion of the substrate into the Pt−aryl bond followed by β-hydride elimination. Formation of such agostic species is promoted by the steric hindrance imparted by the diphosphine characterized by a large bite angle. Kinetic studies and DFT calculations on the possible agostic intermediates shed light on their structure and enable the formulation of a possible catalytic mechanism

Insertion of Substituted Alkynes into the Pd−C Bond of Methyl and Vinyl Palladium(II) Complexes Bearing Pyridylthioethers as Ancillary Ligands. The Influence of Ligand Substituents at Pyridine and Sulfur on the Rate of Insertion

The palladium(II) chloro methyl complexes bearing the bidentate 6-R-C5H3N-2-CH2SR‘ (RN-SR‘; R = H, Me, Cl; R‘ = Me, t-Bu, Ph) and the potentially terdentate 2,6-(CH2SR‘)2-C5H3N (S-N-S(R‘); R‘ = Me, t-Bu, Ph) pyridylthioethers as ancillary ligands were synthesized, characterized, and reacted with substituted alkynes ZC⋮CZ (Z = COOMe, Z‘ = COOt-Bu, Z‘ ‘ = COOEt). The reactions were followed under second-order conditions by 1H NMR technique, and the reaction rates were determined. The corresponding vinyl derivatives were synthesized, and in the case of the complexes [PdCl(ZCCZMe)(MeN-SPh)] and [PdCl(ZCCZMe)(C1N-St-Bu)] (Z = COOMe) reaction rates for alkyne insertion yielding the corresponding butadienyl complexes were also determined. The rate of insertion of the second alkyne on the vinyl complex is more than 3 orders of magnitude lower than the first insertion rate in both the studied complexes, thereby allowing easy separation between vinyl and butadienyl derivatives and an easy preparation of mixed butadienyl esters. Furthermore, the reaction rates are strongly dependent on the steric and electronic features of the ancillary ligands. In particular, the distortion of the complex main coordination plane, induced by the substituent in position 6 of the pyridine ring, was found to significantly influence the substrate reactivity. The structures of the mono-inserted vinyl [PdCl(ZCCZMe)(MeN-St-Bu)] (1) and the bis-inserted butadienyl [PdCl((ZCCZ)2Me)(MeN-St-Bu)] (2) complexes were determined by X-ray diffraction, and the persistence of a structural distortion of the complex skeleton was observed. Moreover, the distortion may be related to facile ancillary ligand displacement, a feature that can be exploited for the synthesis of substrates that would not be easily obtained otherwise

Insertion of Substituted Alkynes into the Pd−C Bond of Methyl and Vinyl Palladium(II) Complexes Bearing Pyridylthioethers as Ancillary Ligands. The Influence of Ligand Substituents at Pyridine and Sulfur on the Rate of Insertion

The palladium(II) chloro methyl complexes bearing the bidentate 6-R-C5H3N-2-CH2SR‘ (RN-SR‘; R = H, Me, Cl; R‘ = Me, t-Bu, Ph) and the potentially terdentate 2,6-(CH2SR‘)2-C5H3N (S-N-S(R‘); R‘ = Me, t-Bu, Ph) pyridylthioethers as ancillary ligands were synthesized, characterized, and reacted with substituted alkynes ZC⋮CZ (Z = COOMe, Z‘ = COOt-Bu, Z‘ ‘ = COOEt). The reactions were followed under second-order conditions by 1H NMR technique, and the reaction rates were determined. The corresponding vinyl derivatives were synthesized, and in the case of the complexes [PdCl(ZCCZMe)(MeN-SPh)] and [PdCl(ZCCZMe)(C1N-St-Bu)] (Z = COOMe) reaction rates for alkyne insertion yielding the corresponding butadienyl complexes were also determined. The rate of insertion of the second alkyne on the vinyl complex is more than 3 orders of magnitude lower than the first insertion rate in both the studied complexes, thereby allowing easy separation between vinyl and butadienyl derivatives and an easy preparation of mixed butadienyl esters. Furthermore, the reaction rates are strongly dependent on the steric and electronic features of the ancillary ligands. In particular, the distortion of the complex main coordination plane, induced by the substituent in position 6 of the pyridine ring, was found to significantly influence the substrate reactivity. The structures of the mono-inserted vinyl [PdCl(ZCCZMe)(MeN-St-Bu)] (1) and the bis-inserted butadienyl [PdCl((ZCCZ)2Me)(MeN-St-Bu)] (2) complexes were determined by X-ray diffraction, and the persistence of a structural distortion of the complex skeleton was observed. Moreover, the distortion may be related to facile ancillary ligand displacement, a feature that can be exploited for the synthesis of substrates that would not be easily obtained otherwise

Highly Active and Selective Platinum(II)-Catalyzed Isomerization of Allylbenzenes: Efficient Access to (<i>E</i>)-Anethole and Other Fragrances via Unusual Agostic Intermediates

Terminal alkene isomerization reactions can be efficiently catalyzed by PtII complexes bearing a chelating diphosphine and an alkyl or, better, aryl moiety under mild experimental conditions. In particular diphosphines, such as dppb, characterized by a large bite angle in conjunction with a pentafluorophenyl residue coordinated to Pt enable quantitative conversion of the reagent into internal alkenes within few hours at 50 °C in CHCl3 as solvent. E/Z selectivity can be as high as 98:2 for allylbenzene, and the catalytic system can be fruitfully applied to the preparation of E fragrances derived by isomerization of substituted allylbenzene derivatives. The selectivity increases during the progress of the reaction because of a subsequent catalytic step where the Z alkene coordinates to the Pt and is converted into the E isomer. NMR investigation on the catalyst showed formation of agostic Pt···H intermediate species derived by insertion of the substrate into the Pt−aryl bond followed by β-hydride elimination. Formation of such agostic species is promoted by the steric hindrance imparted by the diphosphine characterized by a large bite angle. Kinetic studies and DFT calculations on the possible agostic intermediates shed light on their structure and enable the formulation of a possible catalytic mechanism

Chloride-Modulated Insertion Reactions of Dimethylallene across the Pd−C Bond in Palladium Methyl Complexes Bearing Potentially Terdentate Pyridylthioether Ligands

Palladium methyl complexes with potentially terdentate pyridylthioether (S−N−S(R) = 2,6-bis(R-thiomethyl)pyridine, R = Me, t-Bu, Ph; N−S−N = 2[(2-pyridylmethylthio)methyl]pyridine) ligands have been prepared and characterized. Both the bidentate chloride [Pd(Me)(S−N−S(R))]Cl and the terdentate chloride-free [Pd(Me)(S−N−S(R))]+ species are present in solution and display a substantially different reactivity toward allene insertion across the Pd−C bond. The structures of the complexes [Pd(Me)(S−N−S(t-Bu))]OTf and [Pd(Me)(S−N−S(t-Bu))]Cl were determined by X-ray diffraction. The chloride methyl substrates [Pd(Me)(S−N−S(R))]Cl display an enhanced reactivity in solution with respect to the allene insertion, and this reactivity was traced back to the distortion of the main coordination plane induced by the presence of an uncoordinated −CH2−S−R group in position 6 of the coordinating pyridine. The equilibrium position between the terdentate and the bidentate species can be modulated by addition of chloride ion, which therefore controls the overall reactivity of the system

Chloride-Modulated Insertion Reactions of Dimethylallene across the Pd−C Bond in Palladium Methyl Complexes Bearing Potentially Terdentate Pyridylthioether Ligands

Palladium methyl complexes with potentially terdentate pyridylthioether (S−N−S(R) = 2,6-bis(R-thiomethyl)pyridine, R = Me, t-Bu, Ph; N−S−N = 2[(2-pyridylmethylthio)methyl]pyridine) ligands have been prepared and characterized. Both the bidentate chloride [Pd(Me)(S−N−S(R))]Cl and the terdentate chloride-free [Pd(Me)(S−N−S(R))]+ species are present in solution and display a substantially different reactivity toward allene insertion across the Pd−C bond. The structures of the complexes [Pd(Me)(S−N−S(t-Bu))]OTf and [Pd(Me)(S−N−S(t-Bu))]Cl were determined by X-ray diffraction. The chloride methyl substrates [Pd(Me)(S−N−S(R))]Cl display an enhanced reactivity in solution with respect to the allene insertion, and this reactivity was traced back to the distortion of the main coordination plane induced by the presence of an uncoordinated −CH2−S−R group in position 6 of the coordinating pyridine. The equilibrium position between the terdentate and the bidentate species can be modulated by addition of chloride ion, which therefore controls the overall reactivity of the system

Chloride-Modulated Insertion Reactions of Dimethylallene across the Pd−C Bond in Palladium Methyl Complexes Bearing Potentially Terdentate Pyridylthioether Ligands

Palladium methyl complexes with potentially terdentate pyridylthioether (S−N−S(R) = 2,6-bis(R-thiomethyl)pyridine, R = Me, t-Bu, Ph; N−S−N = 2[(2-pyridylmethylthio)methyl]pyridine) ligands have been prepared and characterized. Both the bidentate chloride [Pd(Me)(S−N−S(R))]Cl and the terdentate chloride-free [Pd(Me)(S−N−S(R))]+ species are present in solution and display a substantially different reactivity toward allene insertion across the Pd−C bond. The structures of the complexes [Pd(Me)(S−N−S(t-Bu))]OTf and [Pd(Me)(S−N−S(t-Bu))]Cl were determined by X-ray diffraction. The chloride methyl substrates [Pd(Me)(S−N−S(R))]Cl display an enhanced reactivity in solution with respect to the allene insertion, and this reactivity was traced back to the distortion of the main coordination plane induced by the presence of an uncoordinated −CH2−S−R group in position 6 of the coordinating pyridine. The equilibrium position between the terdentate and the bidentate species can be modulated by addition of chloride ion, which therefore controls the overall reactivity of the system