New bis(fluoro-ponytailed) bipyridine ligands for Pd-catalyzed Heck reactions under fluorous biphasic catalysis condition

Three highly fluorinated bipyridine derivatives (4,4′-bis(R_fCH₂OCH₂)-2,2′-bpy) {R_f=HCF₂(CF₂)₇ (1a), n-C₈F₁₇ (1b), n-C₁₀F₂₁ (1c)} have been synthesized using 4,4′-bis(BrCH₂)-2,2′-bpy and the corresponding fluorinated alkoxides. The fluorine contents of ligands 1a-c are 58.3, 59.8, and 62.3%, respectively. Despite its high fluorine content, the ligand 1a with a -CF₂H polar terminal group is more soluble in organic solvents. The ligand 1b is a white solid and is still moderately soluble in CH₂Cl₂. The ligand 1c has a high fluorophilicity, the partition ratio being 42:1 for the n-C₈F₁₈/CH₂Cl₂ system. The reaction of ligands 1a-c with [PdCl₂(CH₃CN)₂] results in the novel Pd complexes [PdCl₂(4,4′-bis-(R_fCH₂OCH₂)-2,2′-bpy)] where R_f=HCF₂(CF₂)₇ (2a), n-C₈F₁₇ (2b), n-C₁₀F₂₁ (2c), respectively. The Pd complex 2b is a pale yellow solid, and has been tested unsatisfactorily for FBC. Insoluble in organic solvents, the Pd complex 2c dissolves only in fluorinated solvents, for instance FC77, which is mainly n-C₈F₁₈. The novel Pd complex 2c has been tested as a catalyst in Heck reactions under a fluorous biphasic catalysis condition. It was found that the Pd complex 2c, after an easy separation, keeps its catalytic activity (>90% yield), even after seven runs. The TGA studies indicate that the Pd complexes 2a-c are stable up to 330 °C.     

Synthesis,structure and spectroelectrochemical property of (2,2′-bipyridine)–metal (M = Pt,Pd) dichloride with 4,4′-bis(fluorous-ponytail) on bipyridine

The 4,4′-bis(R_fCH₂OCH₂)-2,2′-bpy ligands [R_f = n-C₃F₇ (1a), HCF₂(CF₂)₃ (1b)] were prepared and then treated with [MCl₂(CH₃CN)₂] (M = Pt or Pd) to result in the corresponding metal complexes, [MCl₂(4,4′-bis(R_fCH₂OCH₂)-2,2′-bpy)] (M = Pt 2a–b; Pd 3a–b). Both ligands and metal complexes were fully characterized by multi-nuclei NMR (¹H, ¹⁹F and ¹³C), FTIR, and mass (GC/MS or HR-FAB) methods. The X-ray structures of 2a–b and 3a–b were studied. With terminal CF₃, the structures of 2a and 3a exhibit disordered polyfluorinated regions in solid state. With terminal HCF₂, the structures of 2b and 3b show a π–π stacking of the bpy planes, five-membered C–H···O hydrogen bond and an unusual intramolecular blue-shifting C–H···F–C hydrogen bond system, whereas without terminal HCF₂, the structures of 2a and 3a show the similar π–π stacking, five-membered C–H···O hydrogen bond and typical orientation of polyfluorinated ponytails, but not the C–H···F–C hydrogen bond system. The CV and UV/Vis studies were also carried out.     

Reactivity studies of (η-arene)ruthenium dimeric complexes towards pyrazoles: isolation of amidines, bis pyrazoles and chloro bridged pyrazole complexes

The complex [(η⁶-p-cymene)Ru(μ-Cl)Cl]₂1 reacts with pyrazole ligands (3a-g) in acetonitrile to afford the amidine derivatives of the type [(η⁶-p-cymene)Ru(L)(3,5-HRR′pz)](BF4)₂ (4a-f), where L = {HNC(Me)3,5-RR′pz}; R, R′ = H (4a); H, CH₃ (4b); C₆H₅ (4c); CH₃, C₆H₅ (4d) OCH₃ (4e); and OC₂H₅ (4f), respectively. The ligand L is generated in situ through the condensation of 3,5-HRR′pz with acetonitrile under the influence of [(η⁶-p-cymene)RuCl₂]₂. The complex [(η⁶-C₆Me₆)Ru(μ-Cl)Cl]₂2 reacts with pyrazole ligands in acetonitrile to yield bis-pyrazole derivatives such as [(η⁶-C₆Me₆)Ru (3,5-HRR′pz)₂Cl](BF₄) (5a-b), where R, R′ = H (5a); H, CH₃ (5b), as well as dimeric complexes of pyrazole substituted chloro bridged derivatives [{(η⁶-C₆Me₆)Ru(μ-Cl) (3,5-HRR′pz)}₂](BF₄)₂ (5c-g), where R, R′ = CH₃ (5c); C₆H₅ (5d); CH₃, C₆H₅ (5e); OCH₃ (5f); and OC₂H₅ (5g), respectively. These complexes were characterized by FT-IR and FT-NMR spectroscopy as well as analytical data. The molecular structures¹ of representative complexes [(η⁶-C₆Me₆)Ru{3(5)-Hmpz}₂Cl]⁺5b, [(η⁶-C₆Me₆)Ru(μ-Cl)(3,5-Hdmpz)]₂²⁺5c and [(η⁶-C₆Me₆)Ru(μ-Cl){3(5)Me,5(3)Ph-Hpz}]₂²⁺5e were established by single crystal X-ray diffraction studies.     

Application and details of photoinduced oxidative cyclization of 5-(4′,9′-methanocycloundeca-2′,4′,6′,8′,10′-pentaenylidene)pyrimidine-2,4,6(1,3,5H)-triones and related compounds

As novel methodology for synthesizing the furan ring, a photoinduced oxidative cyclization of 5-(4′,9′-methanocycloundeca-2′,4′,6′,8′,10′-pentaenylidene)pyrimidine-2,4,6(1,3,5H)-triones (7a-c) and related compounds 9a-c was accomplished to give 5,10-methanocycloundeca[4,5]furo[2,3-d]pyrimidine-2,4(1,3H)-dionylium tetrafluoroborates (8a-c⁺·BF₄⁻) and related compounds 2a-c⁺·BF₄⁻, respectively. In the photoinduced oxidative cyclization, the molecular oxygen in air is used as oxidant and the reaction proceeds under mild conditions to give desired products without byproducts, and thus, it is interesting from the viewpoint of the green chemistry. On the reactions of the mono-substituted derivatives 7d,e and 9e,f, the selectivity of the photoinduced cyclizations were reversed as compared with those of the DDQ-promoted oxidative cyclizations. By the NMR monitoring of the reactions of 7a and deuterated compound 7a-D₂ under degassed conditions, the details of the reaction pathway were clarified and rationalized on the basis of the MO calculation by the 6-31G^∗ basis set of the MP2 levels as well.     

Enantioselective alkynylation to aldimines catalyzed by chiral 2,2′-di(2-aminoaryloxy)-1,1′-binaphthyl-copper(I) complexes

The enantioselective alkynylation of aldimines with terminal acetylenes catalyzed by chiral Cu(I) complexes with (R)-2,2′-di(2-aminoaryloxy)-1,1′-binaphthyl ligands (7) was examined. Chiral C₂-symmetric N,N-ligands 7, which have primary aniline moieties, were readily prepared from inexpensive (R)-1,1′-binaphthol (BINOL) as a chiral source. In particular, the reaction of N-benzylidenebenzeneamine 1a with phenylacetylene 2a proceeded smoothly in the presence of 5 mol % of (CuOTf)₂·C₆H₅CH₃ and 10 mol % of (R)-7d at room temperature for 24 h, and the corresponding propargylamine 3a was obtained with up to 82% ee.     

Phosphite ligands derived from distally and proximally substituted dipropyloxy calix[4]arenes and their palladium complexes: Solution dynamics, solid-state structures and catalysis

Two bisphosphite ligands, 25,27-bis-(2,2′-biphenyldioxyphosphinoxy)-26,28-dipropyloxy-p-tert-butyl calix[4]arene (3) and 25,26-bis-(2,2′-biphenyldioxyphosphinoxy)-27,28-dipropyloxy-p-tert-butyl calix[4]arene (4) and two monophosphite ligands, 25-hydroxy-27-(2,2′-biphenyldioxyphosphinoxy)-26,28-dipropyloxy-p-tert-butyl calix[4]arene (5) and 25-hydroxy-26-(2,2′-biphenyldioxyphosphinoxy)-27,28-dipropyloxy- p-tert-butyl calix[4]arene (6) have been synthesized. Treatment of (allyl) palladium precursors [(η³-1,3-R,R′-C₃H₄)Pd(Cl)]₂ with ligand 3 in the presence of NH₄PF₆ gives a series of cationic allyl palladium complexes (3a-3d). Neutral allyl complexes (3e-3g) are obtained by the treatment of the allyl palladium precursors with ligand 3 in the absence of NH₄PF₆. The cationic allyl complexes [(η³-C₃H₅)Pd(4)]PF₆ (4a) and [(η³-Ph₂C₃H₃)Pd(4)]PF₆ (4b) have been synthesized from the proximally (1,2-) substituted bisphosphite ligand 4. Treatment of ligand 4 with [Pd(COD)Cl₂] gives the palladium dichloride complex, [PdCl₂(4)] (4c). The solid-state structures of [{(η³-1-CH₃-C₃H₄)Pd(Cl)}₂(3)] (3f) and [PdCl₂(4)] (4c) have been determined by X-ray crystallography; the calixarene framework in 3f adopts the pinched cone conformation whereas in 4c, the conformation is in between that of cone and pinched cone. Solution dynamics of 3f has been studied in detail with the help of two-dimensional NMR spectroscopy.The solid-state structures of the monophosphite ligands 5 and 6 have also been determined; the calix[4]arene framework in both molecules adopts the cone conformation. Reaction of the monophosphite ligands (5, 6) with (allyl) palladium precursors, in the absence of NH₄PF₆, yield a series of neutral allyl palladium complexes (5a-5c; 6a-6d). Allyl palladium complexes of proximally substituted ligand 6 showed two diastereomers in solution owing to the inherently chiral calix[4]arene framework. Ligands 3, 6 and the allyl palladium complex 3f have been tested for catalytic activity in allylic alkylation reactions.     

Efficient, recoverable, copper-catalyzed aerobic oxidation of alcohols under FBS and thermomorphic mode

A series of fluorinated bipyridine derivatives, (4,4′-bis(R_fCH₂OCH₂)-2,2′-bpy) {R_f = n-C₈F₁₇ (1a), n-C₉F₁₉ (1b), n-C₁₀F₂₁ (1c), n-C₁₁F₂₃ (1d)} have been successfully synthesized using 4,4′-bis(bromomethylene)-2,2′-bpy and fluorinated alkoxides. Bpy 1a-d have been characterized by multi-nuclei (¹H, ¹⁹F, and ¹³C) NMR, GC/MS and FTIR. The Cu complexes 2a-d could be generated in situ by stirring ligands 1a-d with CuBr·Me₂S at room temperature, respectively. The 3-component systems 3c-d, CuBr·Me₂S/Bpy (1c-d)/2,2,6,6-Tetramethylpiperidine 1-oxyl (TEMPO), were successfully used to the aerobic oxidation of alcohols under the fluorous biphasic system (FBS). The resulting products from FBS could be easily recovered by two phase separation with high yields up to 8 runs (>90%). In order to avoid using the expensive fluorous solvents, systems 3a-d, CuBr·Me₂S/Bpy (1a-d)/TEMPO, were also successfully shown to catalyze the aerobic alcohol oxidation under the thermomorphic condition (in C₆H₅Cl), and the yields of oxidation of 4-nitrobenzyl alcohol were close to 100% even after 8 runs. In particular, 3a was most effective under the thermomorphic mode in the chemoselectivity of aerobic oxidation of aliphatic primary alcohols to aldehydes without any overoxidized acids.     

Synthesis and properties of novel 5-(cyclohepta-2′,4′,6′-trienylidene)pyrimidine-2(1H),4(3H),6(5H)-triones: methodology for synthesizing cyclohepta[b]pyrimido[5,4-d]furan-8(7H),10(9H)-dionylium tetrafluoroborates

Novel condensation reaction of tropone with N-substituted and N,N′-disubstitued barbituric acids in Ac₂O afforded 5-(cyclohepta-2′,4′,6′-trienylidene)pyrimidine-2(1H),4(3H),6(5H)-trione derivatives (8a-f) in moderate to good yields. The ¹³C NMR spectral study of 8a-f revealed that the contribution of zwitterionic resonance structures is less important as compared with that of 8,8-dicyanoheptafulvene. The rotational barriers (ΔG^‡) around the exocyclic double bond of mono-substituted derivatives 8a-c were obtained to be 14.51-15.03 kcal mol⁻¹ by the variable temperature ¹H NMR measurements. The electrochemical properties of 8a-f were also studied by CV measurement. Upon treatment with DDQ, 8a-c underwent oxidative cyclization to give two products, 7 and 9-substituted cyclohepta[b]pyrimido[5,4-d]furan-8(7H),10(9H)-dionylium tetrafluoroborates (11a-c·BF₄⁻ and 12a-c·BF₄⁻) in various ratios, while that of disubstituted derivatives 8d-f afforded 7,9-disubstituted cyclohepta[b]pyrimido[5,4-d]furan-8(7H),10(9H)-dionylium tetrafluoroborate (11d-f·BF₄⁻) in good yields. Similarly, preparation of known 5-(1′-oxocycloheptatrien-2′-yl)-pyrimidine-2(1H),4(3H),6(5H)-trione derivatives (14a-d) and novel derivatives 14e,f was carried out. Treatment of 14a-c with aq. HBF₄/Ac₂O afforded two kinds of novel products 11a-c·BF₄⁻ and 12a,c·BF₄⁻ in various ratios, respectively, while that of 14d-f afforded 11d-f. The product ratios of 11a-c·BF₄⁻ and 12a-c·BF₄⁻ observed in two kinds of cyclization reactions were rationalized on the basis of MO calculations of model compounds 20a and 21a. The spectroscopic and electrochemical properties of 11a-f·BF₄⁻ and 12a-c·BF₄⁻ were studied, and structural characterization of 11c·BF₄⁻ based on the X-ray crystal analysis and MO calculation was also performed.     

Palladium(II)-allyl complexes containing chiral N-donor ferrocenyl ligands

A study of the reactivity of enantiopure ferrocenylimine (S_C)-[FcCHN-CH(Me)(Ph)] {Fc =  (η⁵-C₅H₅)Fe{(η⁵-C₅H₄)-} (1a) with palladium(II)-allyl complexes [Pd(η³-1R¹,3R²-C₃H₃)(μ-Cl)]₂ {R¹ = H and R² = H (2), Ph (3) or R¹ = R² = Ph (4)} is reported. Treatment of 1a with 2 or 3 {in a molar ratio Pd(II):1a = 1} in CH₂Cl₂ at 298 K produced [Pd(η³-3R²-C₃H₄){FcCHN-CH(Me)(Ph)}Cl] {R² = H (5a) or Ph (6a)}. When the reaction was carried out under identical experimental conditions using complex 4 as starting material no evidence for the formation of [Pd(η³-1,3-Ph₂-C₃H₃){FcCHN-CH(Me)(Ph)}Cl] (7a) was found. Additional studies on the reactivity of (S_C)-[FcCHN-CH(R³)(CH₂OH)] {R³ = Me (1b) or CHMe₂ (1c)} with complex 4 showed the importance of the bulk of the substituents on the palladium(II) allyl-complex (2-4) or on the ferrocenylimines (1) in this type of reaction. The crystal structure of 5a showed that: (a) the ferrocenylimine adopts an anti-(E) conformation and behaves as an N-donor ligand, (b) the chloride is in acis-arrangement to the nitrogen and (c) the allyl group binds to the palladium(II) in a η³-fashion. Solution NMR studies of 5a and 6a and [Pd(η³-1,3-Ph₂-C₃H₃){FcCHN-CH(Me)(CH₂OH)}Cl] (7b) revealed the coexistence of several isomers in solution. The stoichiometric reaction between 6a and sodium diethyl 2-methylmalonate reveals that the formation of the achiral linear trans-(E) isomer of Ph-CHCH-CH₂Nu (8) was preferred over the branched derivative (9). A comparative study of the potential utility of ligand 1a, complex 5a and the amine (S_C)-H₂N-CH(Me)(Ph) (11) as catalysts in the allylic alkylation of (E)-3-phenyl-2-propenyl (cinnamyl) acetate with the nucleophile diethyl 2-methylmalonate (Nu⁻) is reported.     

Synthesis and reactivity of asymmetrically substituted ansa-bridged zirconocene complexes: X-ray crystal structures of [Zr{R(H)C(η-C5Me4)(η-C5H4)}Cl2] (R = Bu, Bu) and [Zr{Bu(H)C(η-C5Me4)(η-C5H4)}(CH2Ph)2]

The dialkyl complexes, (R = Prⁱ, R′ = Me (2a), CH₂Ph (3a); R = Buⁿ, R′ = Me (2b), CH₂Ph (3b); R = Bu^t, R′ = Me (2c), CH₂Ph (3c); R = Ph, R′ = Me (2d), CH₂Ph (3d)), have been synthesized by the reaction of the ansa-metallocene dichloride complex, [Zr{R(H)C(η⁵-C₅Me₄)(η⁵-C₅H₄)}Cl₂] (R = Prⁱ (1a), Buⁿ (1b), Bu^t (1c), Ph (1d)), and two molar equivalents of the alkyl Gringard reagent. The insertion reaction of the isocyanide reagent, CNC₆H₃Me₂-2,6, into the zirconium-carbon σ-bond of 2 gave the corresponding η²-iminoacyl derivatives, [Zr{R(H)C(η⁵-C₅Me₄)(η⁵-C₅H₄)}{η²-MeCNC₆H₃Me₂-2,6}Me] (R = Prⁱ (4a), Buⁿ (4b), Bu^t (4c), Ph (4d)). The molecular structures of 1b, 1c and 3b have been determined by single-crystal X-ray diffraction studies.     

Relationships between Fe NMR, Mössbauer parameters, electrochemical properties and the structures of ferrocenylketimines

A comparative study of the electrochemical properties, ⁵⁷Fe NMR and Mössbauer spectroscopic data of compounds [(η⁵-C₅H₅)Fe{(η⁵-C₅H₄)-C(R¹)N-R²}] {R¹ = H, R² = CH₂-CH₂OH (1a), CH(Me)-CH₂OH (1b), CH₂C₆H₅ (1c), C₆H₄-2Me (1d), C₆H₄-2SMe (1e) or C₆H₄-2OH (1f) and R¹ = C₆H₅, R² = C₆H₄-2Me (2d)} is reported. The X-ray crystal structure of [(η⁵-C₅H₅)Fe{(η⁵-C₅H₄)-CHN-C₆H₄-2OH}] (1f) is also described. Density functional theoretical (DFT) studies of these systems have allowed us to examine the effects induced by the substituents of the “-C(R¹)N-R²” moiety or the aryl rings (in 1d-1f) upon the electronic environment of the iron(II) centre.     

Trifluoroethanol: key solvent for palladium-catalyzed polymerization reactions

A series of cationic palladium complexes of general formula [Pd(CH₃)(NCCH₃)(N-N)][X] (N-N = phen 1, 3-sec-butyl-1,10-phenanthroline (3-sBu-phen) 2, bpy 3, (−)-(S,S)-3,3′-(1,2-dimethylethylenedioxy)-2,2′-bipyridine (bbpy) 4, (+)-(R)-3,3′-(1-methylethylenedioxy)-2,2′-bipyridine (pbpy) 5, N,N′-bis(2,6-diisopropylphenyl)-2,3-butanediimine (iso-DAB) 6; , OTf (OTf = triflate) b) containing different nitrogen-donor ligands were prepared from the corresponding neutral chloro derivatives [Pd(CH₃)(Cl)(N-N)] (1c-6c). They were characterized by ¹H NMR spectroscopy and elemental analysis. Single crystals suitable for X-ray determination were obtained for complexes [Pd(CH₃)(NCCH₃)(bbpy)][PF₆] (4a), [Pd(CH₃)(NCCH₃)(iso-DAB)][PF₆] (6a) and [Pd(Cl)₂(bbpy)] (4c′). The latter is the result of an exchange reaction of the methyl group, present in complex 4c, with a chloride, that occurred after dissolution of 4c in CDCl₃, for 1 week at 0 °C. The catalytic behavior of complexes 1a-5a and 1b-5b in the CO/styrene copolymerization was studied in CH₂Cl₂ and 2,2,2-trifluoroethanol (TFE) evidencing the positive effect of the fluorinated alcohol both in terms of productivity and molecular weight values of the polymers obtained. Influence of the nitrogen ligand, the anion and the reaction time in both solvents were investigated and is discussed in detail. Encouraging preliminary results were also obtained in the synthesis of polyethylene, in TFE, catalyzed by [Pd(CH₃)(NCCH₃)(iso-DAB)][PF₆] (6a).     

Synthesis of (3′R,5′S)-3′-hydroxycotinine using 1,3-dipolar cycloaddition of a nitrone

To synthesize (3′R,5′S)-3′-hydroxycotinine [(+)-1], the main metabolite of nicotine (2), cycloaddition of C-(3-pyridyl)nitrones 3a, 3c, and 15 with (2R)- and (2S)-N-(acryloyl)bornane-10,2-sultam [(2R)- and (2S)-8] was examined. Among them, l-gulose-derived nitrone 15 underwent stereoselective cycloaddition with (2S)-8 to afford cycloadduct 16, which was elaborated to (+)-1.     

Recoverable cationic Pd-catalyzed Heck reaction under thermomorphic mode

The reaction of allylpalladium chloride dimer and 4,4′-bis(R_fCH₂OCH₂)-2,2′-bpy, 1a-c, in the presence of AgOT_f resulted in the synthesis of cationic complex, [Pd(η³-allyl)(4,4′-bis-(R_fCH₂OCH₂)-2,2′-bpy)](OT_f), 2a-c where R_f = C₉F₁₉ (a), C₁₀F₂₁ (b), C₁₁F₂₃ (c), respectively. The solubility curves of 2a-c which began from virtually zero below −40 °C and increased dramatically at slightly higher temperature were quantitatively measured. Upon changing from −40-90 °C there was several orders of magnitude increase of solubility for 2a-c. The cationic Pd-catalyzed Heck arylation of methyl acrylate was selected to demonstrate the feasibility of recycling usage with 2b,c as the catalyst using DMF as the solvent at 140 °C for 3 h in each run. At the end of each cycle, the product mixtures were cooled to −30 °C, and then catalysts were recovered by decantation. The products were quantified by GC analysis or by the isolated yield. The Heck reaction of C₆H₅I with methyl acrylate could be catalyzed by 2b with good recycling results for a total of 15 times and also with a 100% selectivity favoring the trans product. To our knowledge, this is the first example of cationic Pd-catalyzed Heck reaction under the thermomorphic mode.     

Synthesis and characterization of palladium(II) complexes with chiral aminophosphine ligands: Catalytic behaviour in asymmetric hydrovinylation. Crystal structure of cis-[PdCl2(PPh((R)-NHCHCH3Ph)2)2]

Optically active ligands of type Ph₂PNHR (R = (R)-CHCH₃Ph, (a); (R)-CHCH₃Cy, (b); (R)-CHCH₃Naph, (c)) and PhP(NHR)₂ (R = (R)-CHCH₃Ph, (d); (R)-CHCH₃Cy, (e)) with a stereogenic carbon atom in the R substituent were synthesized. Reaction with [PdCl₂(COD)₂] produced [PdCl₂P₂] (1) (P = PhP(NHCHCH₃Ph)₂), whose molecular structure determined by X-ray diffraction showed cis disposition for the ligands. All nitrogen atoms of amino groups adopted S configuration. The new ligands reacted with allylic dimeric palladium compound [Pd(η³-2-methylallyl)Cl]₂ to gave neutral aminophosphine complexes [Pd(η³-2-methylallyl)ClP] (2a-2e) or cationic aminophosphine complexes [Pd(η³-2-methylallyl)P₂]BF₄ (3a-3e) in the presence of the stoichiometric amount of AgBF₄. Cationic complexes [Pd(η⁴3-2-methylallyl)(NCCH₃)P]BF₄ (4a-4e) were prepared in solution to be used as precursors in the catalytic hydrovinylation of styrene. ³¹P NMR spectroscopy showed the existence of an equilibrium between the expected cationic mixed complexes 4, the symmetrical cationic complexes [Pd(η³-2-methylallyl)P₂]BF₄ (3) and [Pd(η³-2-methylallyl)(NCCH₃)₂]BF₄ (5) coming from the symmetrization reaction. The extension of the process was studied with the aminophosphines (a-e) as well as with nonchiral monodentate phosphines (PCy₃ (f), PBn₃ (g), PPh₃ (h), PMe₂Ph (i)) showing a good match between the extension of the symmetrization and the size of the phosphine ligand. We studied the influence of such equilibria in the hydrovinylation of styrene because the behaviour of catalytic precursors can be modified substantially when prepared ‘in situ’. While compounds 3 and bisacetonitrile complex 5 were not active as catalysts, the [Pd(η³-2-methylallyl)(η²-styrene)₂]⁺ species formed in the absence of acetonitrile showed some activity in the formation of codimers and dimers. Hydrovinylation reaction between styrene and ethylene was tested using catalytic precursors solutions of [Pd(η³-2-methylallyl)LP]BF₄ ionic species (L = CH₃CN or styrene) showing moderate activity and good selectivity. Better activities but lower selectivities were found when L = styrene. Only in the case of the precursor containing Ph₂PNHCHCH₃Ph (a) ligand was some enantiodiscrimination (10%) found.     

Substituted lithiumtrimethylsiloxysilanides LiSiRR′(OSiMe3) - Investigations of their synthesis, stability and reactivity

The reactions of the trimethylsiloxychlorosilanes (Me₃SiO)RR′SiCl (1a-h: R′ = Ph, 1a: R = H, 1b: R = Me, 1c: R = Et, 1d: R = ⁱPr, 1e: R = ^tBu, 1f: R = Ph, 1g: R = 2,4,6-Me₃C₆H₂ (Mes), 1h: R = 2,4,6-(Me₂CH)₃C₆H₂ (Tip); 1i: R = R′ = Mes) with lithium metal in tetrahydrofuran (THF) at −78 °C and in a mixture of THF/diethyl ether/n-pentane in a volume ratio 4:1:1 at −110 °C lead to mixtures of numerous compounds. Dependent on the substituents silyllithium derivatives (Me₃SiO)RR′SiLi (2b-i), Me₃SiO(RR′Si)₂Li (3a-g), Me₃SiRR′SiLi (4a-h), (LiO)RR′SiLi (12e, 12g-i), trisiloxanes (Me₃SiO)₂SiRR′ (5a-i) and trimethylsiloxydisilanes (6f^∗, 6h^∗, 6i^∗) are formed. All silyllithium compounds were trapped with Me₃SiCl or HMe₂SiCl resulting in the following products: (Me₃SiO)RR′SiSiMe₂R″ (6b-i: R″ = Me, 7c-i: R″ = H), Me₃SiO(RR′Si)₂SiMe₂R″ (8a-g: R″ = Me, 9a-g: R″ = H), Me₃SiRR′SiSiMe₂R″ (10a-h: R″ = Me, 11a-h: R″ = H) and (HMe₂SiO)RR′SiSiMe₂H (13e, 13g-i). The stability of trimethylsiloxysilyllithiums 2 depends on the substituents and on the temperature. (Me₃SiO)Mes₂SiLi (2i) is the most stable compound due to the high steric shielding of the silicon centre. The trimethylsiloxysilyllithiums 2a-g undergo partially self-condensation to afford the corresponding trimethylsiloxydisilanyllithiums Me₃SiO(RR′Si)₂Li (3a-g). (Me₃)Si-O bond cleavage was observed for 2e and 2g-i. The relatively stable trimethylsiloxysilyllithiums 2f, 2g and 2i react with n-butyllithium under nucleophilic butylation to give the n-butyl-substituted silyllithiums ⁿBuRR′SiLi (15g, 15f, 15i), which were trapped with Me₃SiCl. By reaction of 2g and 2i with 2,3-dimethylbuta-1,3-diene the corresponding 1,1-diarylsilacyclopentenes 17g and 17i are obtained.X-ray studies of 17g revealed a folded silacyclopentene ring with the silicon atom located 0.5 Å above the mean plane formed by the four carbon ring atoms.     

Steric and electronic effects in stabilizing allyl-palladium complexes of “P-N-P” ligands, X2PN(Me)PX2 (X = OC6H5 or OC6H3Me2-2,6)

The chemistry of η³-allyl palladium complexes of the diphosphazane ligands, X₂PN(Me)PX₂ [X = OC₆H₅ (1) or OC₆H₃Me₂-2,6 (2)] has been investigated.The reactions of the phenoxy derivative, (PhO)₂PN(Me)P(OPh)₂ with [Pd(η³-1,3-R′,R″-C₃H₃)(μ-Cl)]₂ (R′ = R″ = H or Me; R′ = H, R″ = Me) give exclusively the palladium dimer, [Pd₂{μ-(PhO)₂PN(Me)P(OPh)₂}₂Cl₂] (3); however, the analogous reaction with [Pd(η³-1,3-R′,R″-C₃H₃)(μ-Cl)]₂ (R′ = R″ = Ph) gives the palladium dimer and the allyl palladium complex [Pd(η³-1,3-R′,R″-C₃H₃)(1)](PF₆) (R′ = R″ = Ph) (4). On the other hand, the 2,6-dimethylphenoxy substituted derivative 2 reacts with (allyl) palladium chloro dimers to give stable allyl palladium complexes, [Pd(η³-1,3-R′,R″-C₃H₃)(2)](PF₆) [R′ = R″ = H (5), Me (7) or Ph (8); R′ = H, R″ = Me (6)].Detailed NMR studies reveal that the complexes 6 and 7 exist as a mixture of isomers in solution; the relatively less favourable isomer, anti-[Pd(η³-1-Me-C₃H₄)(2)](PF₆) (6b) and syn/anti-[Pd(η³-1,3-Me₂-C₃H₃)(2)](PF₆) (7b) are present to the extent of 25% and 40%, respectively. This result can be explained on the basis of the steric congestion around the donor phosphorus atoms in 2. The structures of four complexes (4, 5, 7a and 8) have been determined by X-ray crystallography; only one isomer is observed in the solid state in each case.     

Electronically variable imino-phenanthrolinyl-cobalt complexes; synthesis, structures and ethylene oligomerisation studies

The 2-imino-1,10-phenanthroline ligands, 1,10-C₁₂H₇N₂-2-CRN(2,6-i-Pr₂-4-R¹-C₆H₂) [R = R¹ = H (L1); R = H, R¹ = Br (L2); R = H, R¹ = CN (L3); R = H, R¹ = i-Pr (L4); R = Me, R¹ = H (L5); R = Me, R¹ = i-Pr (L6)], have been prepared in high yield from the condensation reaction of 1,10-C₁₂H₇N₂-2-CRO (R = H, Me) with one equivalent of the corresponding 4-substituted 2,6-diisopropylaniline. The molecular structures of L2, L5 and L6 reveal the imino nitrogen atoms to adopt a transoid configuration with respect to the phenanthrolinyl nitrogen atoms. Treatment of Lx with one equivalent of CoCl₂ in n-BuOH at 90 °C gives the high spin complexes, (Lx)CoCl₂ [Lx = L1 (1a), L2 (1b), L3 (1c), L4 (1d), L5 (1e), L6 (1f)], in which the metal centres exhibit distorted square pyramidal geometries. Activation of 1a-1f with excess methylaluminoxane (MAO) gives catalysts that are modestly active for the oligomerisation of ethylene affording mainly linear α-olefins along with some degree of internal olefins. While the donor capability of the 4-position of the N-aryl group does not appear to affect the activity of the catalyst, it does have an influence on the ratio of α-olefins to internal olefins. Single crystal X-ray diffraction studies have been performed on L2, L5, L6, 1a, 1c and 1f.     

Ferrocenoyl peptides with sulfur-containing side chains: synthesis, solid state and solution structures

The synthesis and full characterization of a number of amino acid and dipeptide derivatives with sulfur-containing side chains derived from ferrocene carboxylic acid and ferrocene-1,1′-dicarboxylic acid is presented. In particular, compounds Fc-CO-(Aaa)_n-OMe (4) and Fe[C₅H₄-CO-(Aaa)_n-OMe]₂ (3) with (Aaa)_n = Cys(Bzl) (a), Cys(Bzl)-Cys(Bzl) (b), Cys(p-OMe-Bzl) (c), Cys(p-OMe-Bzl)-Cys(p-OMe-Bzl) (d), Met (e), and Met-Met (f) were prepared. Also, the free acid derivatives Fe[C₅H₄-CO-Met-OH]₂ (6e) and Fc-CO-Met-OH (7e) were prepared and characterized. The solid state structures of 3a, 4b, and 4e were determined by single crystal X-ray diffraction. Compound 3a shows a 1,3′ substitution pattern on the Cp rings in the solid state. Structures in solution were determined by NMR, IR and CD spectroscopy, with particular emphasis on the question of hydrogen bonding and helical chirality of the metallocene. As an example, the full assignment for the Cp signals in the disubstituted derivative 3a was achieved by simulation of the ¹H NMR signals from the cyclopentadienyl ring in combination with 2D-NOESY spectra. In solution, 3a has the known 1,2′ substitution pattern, which is stabilized by intramolecular hydrogen bonds.     

Novel catalytic hydrogenolysis of silyl enol ethers by the use of acidic ruthenium dihydrogen complexes

Treatment of 1-trimethylsilyloxy-1-cyclohexene (1a) in the presence of a catalytic amount of the acidic dihydrogen complex [RuCl(η²-H₂)(dppe)₂]OTf (4a) [dppe=1,2-bis(diphenylphosphino)ethane, OTf=OSO₂CF₃] (10 mol.%) under 1 atm of H₂ in anhydrous ClCD₂CD₂Cl at 50 °C for 8 h afforded cyclohexanone (3a) and Me₃SiH in quantitative NMR yields. Silyl enol ethers such as 1-triethylsilyloxy-1-cyclohexene (1b), 1-t-butyldimethylsilyloxy-1-cyclohexene (1c), and other trimethylsilylethers (1d, 1e, and 1f) reacted similarly with H₂ to afford the corresponding ketones and trialkylsilanes. The direct proton transfer from H₂ to the trimethylsilyl enol ethers (1a and 1d-1f) was confirmed by the experiments employing D₂ gas, where α-monodeuterated ketones (3a′ and 3d′-3f′) were obtained in high yields. The enantioselective protonation of prochiral silyl enol ethers with 1 atm of H₂ by employing [RuCl(η²-H₂)((S)-BINAP)₂]OTf (4e) [BINAP=2,2′-bis(diphenylphosphino)-1,1′-binaphthyl] and [RuCl(η²-H₂)((R, R)-CHIRAPHOS)₂]OTf (4f) [CHIRAPHOS=2,3-bis(diphenylphosphino)butane] showed that no enantioselectivity was observed in either catalytic or stoichiometric protonation reactions under various reaction conditions. The reaction of [RuHCl(dppe)₂] (5a) with one equivalent of Me₃SiOTf under 1 atm of H₂ produced rapidly 4a, concurrent with the formation of Me₃SiH. Based on these studies, the mechanism for this novel hydrogenolysis of silyl enol ethers is proposed which involves heterolytic cleavage of the coordinated H₂ on the ruthenium atom caused by the nucleophilic attack of the oxygen atom of enol ethers to give ketones and Me₃SiOTf, and the subsequent reaction of the resultant complex 5a with Me₃SiOTf under 1 atm of H₂ to regenerate the original dihydrogen complex 4a. On the other hand, the stoichiometric reaction of a lithium enolate 6e with one equivalent of 4e at −78 °C in CH₂Cl₂ under 1 atm of H₂ afforded 2-methyl-1-tetralone (3e) with 75% ee (S) in >95% yield, together with the formation of [RuHCl((S)-BINAP)₂] (5e).