Lewis acid mediated diastereoselective keto-ene cyclization on chiral perhydro-1,3-benzoxazines: synthesis of enantiopure cis-3,4-disubstituted 3-hydroxypyrrolidines

Chiral 2-acyl-3-allyl-perhydro-1,3-benzoxazines derived from (−)-8-aminomenthol were easily cyclized in the presence of Lewis acids at 0 °C. The diastereoselectivity of the cyclization was dependent on the nature of the Lewis acid. The cyclization compounds can be transformed into enantiopure cis-3,4-disubstituted 3-hydroxypyrrolidines by ring opening of the N,O-acetal moiety and subsequent elimination of the menthol appendage.     

New cationic palladium (II) and rhodium (I) complexes of [Ph2PCH2C(Ph)N(2,6-Me2C6H3)]

Treatment of the bulky iminophosphine ligand [Ph₂PCH₂C(Ph)N(2,6-Me₂C₆H₃)] (L) with [M(CH₃CN)₂(ligand)]⁺ⁿ, where for M = Pd(II): ligand = η³-allyl, n = 1, and for M = Rh(I), ligand: 2(C₂H₄), 2(CO) or cod, n = 0, yields the mono-cationic iminophosphine complexes [Pd(η³-C₃H₅)(L)][BF₄] (1), [Rh(cod)(L)][BF₄] (2), [Rh(CO)(CH₃CN)(L)][BF₄] (3), and cis-[Rh(L)₂][BF₄] (4). All the new complexes have been characterised by NMR spectroscopy and X-ray diffraction. Complex 1 shows moderate activity in the copolymerisation of CO and ethene but is inactive towards Heck coupling of 4-bromoacetophenone and n-butyl acrylate.     

Density functional studies of influences of Ni triad metals and solvents on oxidative addition of MeI to [M(CH3)2(NH3)2] complexes and C-C reductive elimination from [M(CH3)3(NH3)2I] complexes

Three metal square planar complexes of the type [M(CH₃)₂(NH₃)₂] (M = Ni, Pd, Pt), with a systematic variation in the metals, are chosen to investigating their S_N2-type oxidative addition reactions with methyl iodide by using the B3LYP levels of theory. The oxidative addition was found to take place via a transition state with a nearly linear arrangement of the I-CH₃-M moiety. Solvation effects in these oxidative addition reactions were also investigated. Considering the nature of the metal centre and solvation effects, the following conclusions emerge: (i) addition of MeI is exothermic for all three metals, and Pt is predicted to react with a much lower barrier than either Pd or Ni. The results describe that the MeI addition would be expected to be more favourable with the complex bearing the third-row metal (platinum) as compared to the other triad metals, nickel or palladium, in which case a more strongly bound MeI adduct is formed with a lower activation barriers and the reaction being more exothermic; (ii) the reaction is very difficult to occur in low polar solvents, such as benzene, due to the high barrier which is induced by dissociation of iodide anion from methyl group, but the reaction easily occurs in polar solvents, such as acetonitrile; this is attributed to the ability of polar solvents to solvate and therefore stabilize the related polar intermediate ion pair. Ethane reductive elimination from the M(VI) complexes fac-[M(CH₃)₃(NH₃)₂I] were also studied, indicating that the Ni(IV) and Pd(IV) complexes are very prone to undergo the reductive elimination while the Pt(IV) analogous is less reactive towards the reductive elimination. The results indicate that in contrast to the Me-Me reductive elimination, the S_N2 oxidative addition reaction of MeI to M(II) is much less sensitive to the nature of the metal centre, suggesting that the nucleophilicity of M(II) in [M(CH₃)₂(NH₃)₂] does not change significantly as one moves from M = Ni to Pt.     

Nickel(0)-catalyzed disilylative and silastannylative cyclizations of 1,3-diene and tethered aldehyde

Nickel(0)-catalyzed bismetallative cyclization of 1,3-diene and a tethered aldehyde in the presence of PhF₂SiSiMe₃ or Me₃SiSnBu₃ gave the corresponding cyclized product having an allylsilyl or an allylstannyl unit in the side chain in good yields. The cyclized product obtained from the reaction in the presence of Me₃SiSnBu₃ had reactivity as an allylstannane derivative, and the coupling reaction with benzaldehyde proceeded in a diastereoselective manner. When the silastannylative cyclization was carried out in the presence of a chiral monodentate phosphine ligand, the cyclized product was produced as an optically active form with modest enantiomeric excess.     

The molecular structure of a three-coordinate palladium(II)-styrene complex [Pd(η5-C5H5)(PEt3)(styrene)]BF4

The molecular structure of a three-coordinate palladium(II)-styrene complex, [Pd(η⁵-C₅H₅)(PEt₃)(styrene)]BF₄ has been determined by means of X-ray diffraction. The crystal belongs to the monoclinic system, space group P2₁/c, with four formula units in a cell of dimensions: a 10.229(3), b 11.262(3), c 18.760(5) Å and β 103.77(2)°. The structure was solved by the heavy atom method, and refined by the least-squares procedure to R = 0.050 for 3635 observed reflections. The palladium atom is surrounded by the cyclopentadienyl group, the triethylphosphine ligand and the olefinic bond of styrene in the cationic complex. In the palladiumstyrene bonding, the olefinic bond is inclined by 77.3° to the coordination plane defined by the Pd and P atoms and the center of the cyclopentadienyl ring (PdC(1) 2.176(6), PdC(2) 2.234(5) and C(1)C(2) 1.369(8) Å).     

Theoretical studies of the oxidative addition of PhBr to Pd(PX3)2 and Pd(X2PCH2CH2PX2) (X = Me, H, Cl)

The density functional theory calculations were used to study the influence of the substituent at P on the oxidative addition of PhBr to Pd(PX₃)₂ and Pd(X₂PCH₂CH₂PX₂) where X = Me, H, Cl. It was shown that the C_ipso-Br activation energy by Pd(PX₃)₂ correlates well with the rigidity of the X₃P-Pd-PX₃ angle and increases via the trend X = Cl < H < Me. The more rigid the X₃P-Pd-PX₃ angle is, the higher the oxidative addition barrier is. The exothermicity of this reaction also increases via the same sequence X = Cl < H < Me. The trend in the exothermicity is a result of the Pd(II)-PX₃ bond strength increasing faster than the Pd(0)-PX₃ bond strength upon going from X = Cl to Me. Contrary to the trend in the barrier to the oxidative addition of PhBr to Pd(PX₃)₂, the C_ipso-Br activation energy by Pd(X₂PCH₂CH₂PX₂) decreases in the following order X = Cl > H > Me. This trend correlates well with the filled d_π orbital energy of the metal center. For a given X, the oxidative addition reaction energy was found to be more exothermic for the case of X₂PCH₂CH₂PX₂ than for the case of PX₃. This effect is especially more important for the strong electron donating phosphine ligands (X = Me) than for the weak electron donating phosphine ligands (X = Cl).     

Synthesis of 3-substituted benzo[g]isoquinoline-5,10-diones: a convenient one-pot Sonogashira coupling/iminoannulation procedure

A series of 3-substituted 5,10-dimethoxybenzo[g]isoquinolines were prepared by coupling of terminal alkynes with the tert-butylimine of 3-bromo-1,4-dimethoxy-2-naphthaldehyde in the presence of a Pd-catalyst and subsequent Cu-catalyzed cyclization of the intermediate 3-alkynyl-2-naphthylcarbaldehyde. A CAN-mediated oxidative demethylation yielded the corresponding 2-azaanthraquinones in excellent yields. Since this methodology proved to be limited to alkynes bearing aromatic groups, an alternative and more general Pd-catalyzed coupling procedure was developed, starting from 3-bromo-1,4-dimethoxy-2-naphthaldehyde. For more acidic terminal alkynes, like phenylacetylene, a combination of Pd(OAc)₂/P(t-Bu)₃/CuI (2/6/1) with potassium carbonate in DMF gave a complete conversion within 24 h. For less acidic acetylenes, 2 equiv of alkyne and caesium carbonate as a base were required in order to obtain complete conversion of the starting material within 24 h. These altered Sonogashira conditions also allowed the isolation of a benzo[f]indenone as an interesting side product in case Bu₄NCl was added to the reaction mixture. The 3-alkynyl-1,4-dimethoxy-2-naphthaldehyde acquired after completion of the Pd-catalyzed coupling could be cyclized by adding a solution of ethanolic ammonia and an extra equivalent of potassium carbonate to the reaction mixture. As such, this consecutive one-pot coupling/iminoannulation procedure was a convenient alternative to the Larock isoquinoline procedure, enabling the isolation of a series of 3-substituted 5,10-dimethoxybenzo[g]isoquinolines.     

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.     

Synthesis and structure of the heterotrinuclear chromium-palladium clusters (RC5H4CrCl)2-(μ3S)(μ-SCMe3)2Pd(PPh3) with ferromagnetic exchange interactions over the CrSCr system

Reactions of (RC₅H₄)₂Cr₂(SCMe₃)₂S(I, R = H; II, R = Me) with (PPh₃)₂PdCl₂ in benzene at 20°C gives trinuclear complexes (RC₅H₄)₂Cr₂Cl₂(μ₃-S)(μ-SCMe₃)₂Pd(PPh₃)(III, R = H; IV, R = Me). The structure of IV as a monobenzene solvate is established by an X-ray analysis (black-green triclinic crystals space group P$\text{1}$ with a = 11.403(4), b = 14.933(5), c = 14.131(5) Å, α = 99.13(3), β = 112.72(3), γ = 95.65(3)°, V = 2201.6 Å, Z = 2; IV·C₆H₆). The structure was solved by direct methods and refined in an anisotropic approximation to R = 0.046, R_w = 0.058 for 7643 reflections with I ? 2σ(I). In the molecule of IV metal atoms are separated by non-bonding distances (Cr … Cr 4.079(I), Cr … Pd 3.230(I) and 3.380(I) Å) but linked by the bridging tridentate sulphur atom (CrS 2.339(2) and 2.329(2), PdS 2.327(2) Å), and two SCMe₃ groups between Pd and Cr (CrS 2.396(2) and 2.403(2), PdS 2.350(2) and 2.381(2) Å, Cr?Pd 85.14(6) and 89.92(6)°). The Cl atoms are transferred from Pd to Cr atoms (CrCl 2.308(2) Å) and being terminally coordinated are in trans-positions to each other (as well as η-CH₃C₅H₄ rings) with respect to the Cr₂Pd plane. Cr atoms in III and IV exhibit ferromagnetic exchange interactions over the Cr?Cr system (+2J = 28 and 11 cm^?1, respectively).     

The Staudinger reaction of platinum(II)- and palladium(II)-coordinated 2-(azidomethyl)phenyl isocyanide. X-ray structure of trans-[PtCl{(H)}(PPh3)2][BF4] · CDCl3 · H2O

2-(Azidomethyl)phenyl isocyanide, 2-(CH₂N₃)C₆H₄NC (AziNC), coordinates to some cationic Pt(II) and Pd(II) species to afford isocyanide complexes of the type trans-[MCl(AziNC)(PPh₃)₂][BF₄] (M=Pt, l; Pd, 2). AziNC is coordinated also in some neutral Pt(II) and Pd(II) species such as [MCl₂(AziNC)₂] (M=Pt, 3; Pd, 4) derived from the reactions of 2 equiv. of AziNC with [PtCl₂(COD)] and [PdCl₂(MeCN)₂], respectively. Complexes 1 and 2 react with 1 equiv. of PPh₃ affording the heterocyclic carbene complexes trans-[MCl{(H)}(PPh₃)₂][BF₄] (M=Pt, 5; Pd, 6). Complexes 3 and 4 react with 1 equiv. of PPh₃ displacing the isocyanide with the formation of the complexes cis-[MCl₂(AziNC)(PPh₃)] (M=Pt, 7; Pd, 8). These latter ones react with 2 equiv. of PPh₃ affording as the final products the cationic carbene species trans-[MCl{(H)}(PPh₃)₂][Cl] (M=Pt, 9; Pd, 10). Complex 5 was also characterized by single crystal X-ray diffraction. The carbene complex is square-planar and the angle formed between the platinum square plane and the heterocyclic carbene ligand is 87.9(2)°. The C(1)-N(1) and C(1)-N(2) bond distances in the latter of 1.32(2) and 1.30(2) Å, respectively, are short for a single bond and indicate extensive π-bonding between the nitrogen atoms and the carbene carbon.     

[Pd(H)(SnCl3)L2]: The key active species in the catalyzed alkoxycarbonylation reaction of terminal alkenes

The four complexes [Pd(H)(Cl)L₂] and [Pd(H)(SnCl₃)L₂], L = PPh₃, PCy₃, have been synthesized and fully characterized by multinuclear NMR. They represent the active species of the hydride palladium-catalyzed alkoxycarbonylation of terminal alkenes. Isolation of the model acylplatinum complex, resulting from the carbonylation of dihydromyrcene, clearly shows that SnCl₂ as co-catalyst produces a SnCl₃ ligand which modulates the metal center electron density.     

Ge2H2 a π-ligand in organometallic chemistry

η² π-Complexes of Ge₂H₂ with the organometallic fragments V(PH₃)₂(I)(CO)₂, Cr(CO)₄, Co(PH₃)₂(Cl) and M(PH₃)₂ (M = Ni, Pd, Pt) have been studied at the B3LYP level using the SBKJC relativistic effective core potentials and their associated basis sets on metals and iodine, and the 6-31G(d) basis set on all other elements. The transition metal fragments of V, Cr, Co, Ni, Pd and Pt were chosen based on known alkyne compounds. All the complexes are local minima for both the HGeGeH and GeGeH₂ isomers of the Ge₂H₂ ligand. The complexes containing GeGeH₂ isomer as a ligand are lower in energy than those with the HGeGeH ligand (except in the V complex in which the difference is only 1.0 kcal/mol). There is a net charge transfer from ligand to metal in complexes V-Co and from metal to ligand in late transition metal complexes (Ni-Pt).     

Luminescence properties of efficient X-ray phosphors of YBa3B9O18, LuBa3(BO3)3, α-YBa3(BO3)3 and LuBO3

The samples of YBa₃B₉O₁₈, LuBa₃(BO₃)₃, α-YBa₃(BO₃)₃ and LuBO₃ powders have been synthesized by the solid-state reaction methods at high temperature and their X-ray excited luminescent properties were investigated. All the studied materials show a broad emission band in the wavelength range of 300-550 nm with the peak centers at about 385 nm for YBa₃B₉O₁₈ and LuBa₃(BO₃)₃, 415 nm for α-YBa₃(BO₃)₃ and 360 nm for LuBO₃ powders, respectively. Even though those compounds have the different atomic structures, they have the common structural feature of each yttrium or lutetium ion bonded to six separate BO₃ groups, i.e., octahedral RE(BO₃)₆ (RE=Lu or Y) moiety. This octahedral RE(BO₃)₆(RE=Lu or Y) moiety seems to be an important structural element for efficient X-ray excited luminescence of those compounds, as are the edge-sharing octahedral TaO₆ chains for tantalate emission.     

Synthesis of Co2Pt, Co2Pd and MoPd2 mixed-metal clusters with the P–N–P assembling ligands (Ph2P)2NH (dppa) and (Ph2P)2NMe (dppaMe). Crystal structure of [Co2Pt(μ3-CO)(CO)6(μ-dppa)]

Heterometallic triangular platinum–cobalt, palladium–cobalt and palladium–molybdenum clusters stabilized by one or two bridging diphosphine ligands such as Ph₂PNHPPh₂ (dppa) or (Ph₂P)₂NMe (dppaMe) or by mixed ligand sets Ph₂PCH₂PPh₂ (dppm)/dppa have been prepared with the objectives of comparing the stability and properties of the clusters as a function of the short-bite diphosphine ligand used and of the metal carbonyl fragment they contain. Ligand redistribution reactions were observed during the purification of [Co₂Pd(μ₃-CO)(CO)₄(μ-dppa)(μ-dppm)] (4) by column chromatography with the formation of [Co₂Pd(μ₃-CO)(CO)₄(μ-dppm)₂] and the dinuclear complex [(OC)₂ Cl] (5). The latter was independently prepared by reaction of [Pd(dppa-P,P′)₂](BF₄)₂ with Na[Co(CO)₄]. Attempts to directly incorporate the ligand (Ph₂P)₂N(CH₂)₃Si(OMe)₃ (dppaSi) into a cluster or to generate it by N-functionalization of coordinated dppa were unsuccessful, in contrast to results obtained recently with related clusters. The crystal structure of [Co₂Pt(μ₃-CO)(CO)₆(μ-dppa)] (1) has been determined by X-ray diffraction.     

Phenoxide-assisted P-C bond cleavage in PdCl2(PPh3)2 under very mild conditions

PdCl₂(PPh₃)₂ reacted with NaOAr (Ar = Ph, p-tolyl) at 0 °C to afford PdCl(Ph)(PPh₃)₂, instead of PdCl(OAr)(PPh₃)₂, in 12-16% isolated yields based on Pd. The structure was confirmed by NMR and X-ray crystallography. GC-MS analysis of the reaction solution revealed that OPPh₂(OAr), OPPh(OAr)₂, and OP(OAr)₃ are formed, while NMR studies indicated that PdCl(Ph)(PPh₃)₂ is produced when PdCl(OAr)(PPh₃)₂ decomposes. The reaction of PdCl₂(PPh₃)₂ with Bu₃Sn(OC₆H₄-p-OMe) also gave PdCl(Ph)(PPh₃)₂ in 8% isolated yield. These results suggest that PdCl(OAr)(PPh₃)₂ is highly labile and the aryloxy ligand exchanges with the phenyl groups in triphenylphosphine even under very mild conditions.     

Novel formation of 1,3-oxazepine heterocycles via palladium-catalyzed intramolecular coupling reaction

The oxazepine ring systems containing pyridazinone moiety were constructed via palladium-catalyzed intramolecular coupling reaction. The best conditions for this reaction were Pd(OAc)₂ as a palladium source, 1,1′-bis(diphenylphosphino)-ferrocene (DPPF) as the ligand, and K₂CO₃ as base at 80 °C in toluene. The products have potential applications as biological and medicinal relevant compounds.     

Synthesis of novel 3-bromo-1,2-dihydroquinolines via palladium mediated intramolecular cyclization of N-tosyl-N-propargyl anilines

Facile synthesis of novel 3-bromo-1,2-dihydroquinolines by the intramolecular cyclization of N-tosyl-N-propargyl anilines catalyzed by Pd(OAc)₂ in conjunction with CuBr₂ and LiBr.     

Aspects of transmetallation reactions of 2-Me2NCH2C6H4- and 2,6-(Me2NCH2)C6H3-Metal (Pd,Pt, Hg,Tl) complexes with metal carboxylates and low-valent metal (Pd,Pt) complexes

A study has been made of reactions involving organometallic compounds containing ortho-Me₂NCH₂ substituted aryl ligands. The single step syntheses of the new compounds [(2-Me₂NCH₂C₆H₄)₂TlCl], [ [{(S)-2-Me₂NCH(Me)C₆H₄}₂TlCl], [{(S)-2-Me₂NCH(Me)C₆H₄}TlCl₂], [{2,6-(Me₂NCH₂)₂C₆H₃}TlClBr] and [{2,6-(Me₂NCH₂)₂C₆H₃}HgCl] are described. Stable internal NTl coordination at low temperatures has been established for the C-chiral thallium compounds. Reactions of the other Tl and Hg compounds and of [(2-Me₂NCH₂C₆H₄)₂Hg] with Pd(O₂CMe)₂, and also of the reverse reaction of cis-[(2-Me₂NCH₂C₆H₄)₂Pd] with Hg(O₂CR)₂ or Tl(O₂CR)₃, gave transmetallation of one organo ligand and led to a single mono-organopalladium compound and corresponding by-products. Reaction of cis-[(₂-Me₂NCH₂C₆H₄)₂Pd] with Pd(O₂CR)₂ gave the dimeric compound [{(2-Me₂NCH₂C₆H₄)Pd(O₂CR)}₂]. cis-[(2-Me₂NCH₂C₆H₄)₂Pt] did not react with Pd(O₂CMe)₂, while reaction of trans-[(2-Me₂NCH₂C₆H₄)₂Pt] or cis-[(2-Me₂NC₆H₄CH₂)₂Pt] with Pd(O₂CMe)₂ resulted in decomposition. Upon heating, trans-[(2-Me₂NCH₂C₆H₄)₂Pt] was isomerized to cis-isomer. A redox reaction between [(2-Me₂NCH₂C₆H₄)₂Hg] and [Pt(COD)₂] (COD  1,5-cyclo-octadiene) and [Pd₂(DBA)₃] (DBA  dibenzylideneacetone) gave the cis-isomers of [(2-Me₂NCH₂C₆H₄)₂M] (M  Pd, Pt).The results are discussed in terms of influence of internal coordination of the CH₂NMe₂ group. It is concluded that although internal coordination of the CH₂NMe₂ ligand can stabilize metal—carbon bonds it cannot prevent cleavage of such bonds by electrophiles. In this respect, there is no difference in the behaviour of Hg(O₂CR)₂ and Tl(O₂CR)₃. The reactions are influenced by the metal—nitrogen bond strength, which follows the order PtN > PdN > HgN, TlN. The reactivity of Pt compounds is greatly influenced by their structure and type of ligand. It is proposed that cleavege of PdC bonds occurs mainly by a mechanism involving direct electrophilic attack at the carbon centre.     

Pd(PBu 3 t ) Adducts of Os3(CO)12. Synthesis and Structures of Pd2Os3(CO)12(PBu 3 t )2 and Pd3Os3(CO)12 (PBu 3 t )3

Two new compounds Pd₂Os₃(CO)₁₂ , 13 and Pd₃Os₃(CO)₁₂ , 14 have been obtained from the reaction of with Os₃(CO)₁₂ at room temperature. The products were formed by the addition of two and three groups to the Os–Os bonds of Os₃(CO)₁₂. Compounds 13 and 14 interconvert between themselves by intermolecular exchange of the groups in solution. Compounds 13 and 14 have been characterized by single crystal X-ray diffraction analyses.Dedicated to Professor Brian F. G. Johnson on the occasion of his retirement – 2005.     

Low-valent zirconocene-mediated cyclization of γ,δ-unsaturated oximes

γ,δ-Unsaturated O-methyl oximes were cyclized to dihydropyrrole by the treatment of (1-butene)ZrCp₂3 prepared by Negishi’s procedure (reaction with Cp₂ZrCl₂ and two equivalents of n-BuLi). In this cyclization, the geometry of oximes was affected and syn-oximes were cyclized efficiently. However, it was found that the anti-oxime is not suitable for the cyclization.