Sei Otsuka

Acetylene and Allene Complexes: Their Implication in Homogeneous Catalysis

Publisher Summary The chapter discusses facets associated with the nature of the interactions between acetylenes and transition metals and to the insertion reactions of complexes closely related to catalysis. Although only scattered data are available, attempts were made to give a consistent interpretation of the relativities of coordinated acetylene in terms of a qualitative molecular orbital picture. Studies on elementary reactions of acetylenes with metal complexes are now beginning to shed some light on the nature of activation caused by complication. This activation is not a simple process. Many low-valent d 8 -d 10 metal complexes and also some early transition metal compounds with higher oxidation state are capable of activating acetylenes. In the former complexes, interaction would lead to activation of an n 2- acctylcne ligand to an η-acetylene having some radical as well as some anionic character. The chapter also focuses on the complex chemistry and catalytic oligomerizations of allene. It also describes the importance of the role played by auxiliary ligands of transition metals in determining the paths of catalytic oligomerizations. In general the thermal reaction of allene gives a complex mixture of dimers, trimers, and higher oligomers including small amounts of spiro compounds.

Catalytic hydrogenation of nitriles and dehydrogenation of amines with the rhodium(I) hydrido compounds [RhH(PPri3)3] and [Rh2H2(µ-N2){P(cyclohexyl)3}4]

[RhH(PPri3)3] and [Rh2H2(µ-N2){P(cyclohexyl)3}4] are active catalysts for the hydrogenation of nitriles under ambient conditions, producing primary amines selectively; they are also active for the dehydrogenation of amines at higher temperatures to give nitriles or imines.

Preparation of optically active peralkyldiphosphines and their use, as the rhodium(I) complex, in the asymmetric catalytic hydrogenation of ketones

Abstract Two types of the optically active peralkyldiphosphine, 2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis(dialkylphosphino)butane (Rdiop 3) and N-(N′-substituted carbamoyl-4-dicyclohexylphosphino-2-dicyclohexylphosphinomethylpyrrolidine (R-Cycapp 8), have been prepared by various synthetic methods. Rhodium(I) complexes of 3 and 8 showed high catalytic activity for hydrogenation of various kinds of prochiral ketones, which were reduced smoothly to the corresponding optically active hydroxy compounds, under hydrogen at atmospheric pressure and ambient temperature. The neutral rhodium(I) complexes (diphosphine-RhN) hydrogenated α-ketoamides and α-ketopantolactone in fairly high optical yields (66–77%ee). In the hydrogenation of N-(α-ketoacyl)-α-amino esters, the Cydiop-RhN catalyst showed a marked contrast to the diop-RhN system; in the hydrogenation of the methyl ester of N-(phenylglyoxyl)-(S)-α-phenylalanine, 72%de was attained with little double asymmetric induction by the chiral center in the substrate.

Mechanistic aspects of catalytic hydrogenation of ketones by rhodium(I)-peralkyldiphosphine complexes

Mechanistic aspects of the hydrogenation of ketones employing cationic rhodium(I) complexes [Rh((i-Pr)2P(CH2)4P(i-Pr)2)(NBD)]ClO4 (NBD = norbornadiene) and [Rh(CyDIOP)(NBD)]ClO4 (CyDIOP = 2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis(dicyclohexylphosphino)butane) and a neutral complex, “Rh-(CyDIOP)Cl” were studied. The cationic complex-catalyzed hydrogenation of the poor coordinating simple ketone substrates followed a rate equation r0 = kobs[Rh][ketone]0[H2]0 and showed an unusual negative temperature dependence of the reaction rate. The hydrogenation of the chelating substrate PhCOCONHCH2Ph followed a rate equation r0 = kobs[Rh][H2] with the activation parameters Ea 5.51 kcal mol−1, ΔH‡308 4.90 kcal mol−1, ΔS‡308 −32.0 e.u. ([Rh((i-Pr)2P(CH2)4P(i-Pr)2)(NBD)]ClO4 catalyst); Ea 5.36 kcal mol−1, ΔH‡308 4.75 kcal mol−1, ΔS‡308 − 30.9 e.u. ([Rh(CyDIOP)(NBD)]ClO4 catalyst). For the neutral complex-catalyzed hydrogenation of PhCOCONHCH2Ph, the rate equation r0 = [Rh]0.25[ketone]0[H2]0 was obtained with the activation parameters (Ea 3.99 kcal mol−1 ΔH‡308 3.38 kcal mol−1, ΔS‡308 −43.0 e.u.). Several intermediate complexes in the cationic complex-catalyzed hydrogenation were also detected spectroscopically or isolated. On the basis of these observations, a general reaction scheme was proposed.

Asymmetric epoxidation of hydrocarbon olefins by tert-butyl hydroperoxide with molybdenum(VI) catalysts in the presence of optically active diols. Application to the asymmetric synthesis of (3S)-2,3-oxidosqualene

Catalytic enantioselective epoxidation of prochiral olefins without functional groups was achieved by tert-BuOOH using Mo(O)2(acac)2-optically active diols as catalysts.

Reactions of transition—metal dihydrides VI. Stereochemistry of the insertion of substituted acetylenes into the metal-hydrogen bond in dihydrido- bis(η-cyclopentadienyl)-molybdenum and -tungsten

Abstract The thermal reaction of various acetylenes with CP2MH2 (Cp = η-C5H5, M = Mo, W) has been investigated with particular emphasis on the stereochemistry and mechanisms. CF3CCH and Cp2MoH2 give Cp2MoH[C(CF3)CH2] via an exclusive cis insertion as revealed by the use of Cp2MoD2- In contrast, very rapid, trans insertion was observed at low temperature −40 °C) between CF3CCCF3 and (RC5H4)2MH2 (R  H, CH3; M  Mo, W) or Cp2ReH affording (RC5H4)2MH[C(CF3)CHCF3] or Cp2Re[C(CF3)CHCF3]. Investigation of this trans insertion with respect to solvent, steric and kinetic isotope effect suggests an essentially concerted bimolecular reaction between the ground-state Cp2MH2 molecule and the acetylene. The cis insertion requires the excited Cp2MH2 molecule with parallel Cp rings to form the η2-acetylene—metal intermediate.

Chiral metal complexes. Part 5. Cobalt(II) and some other transition-metal complexes of chiral vic-dioximate ligands derived from D-camphor and L-β-pinene

Chiral bis(vic-dioximato)-complexes of FeII, CoII, NiII, and PdII have been prepared from three geometrical isomers (α, β, and δ) of D-camphorquinone dioxime (Hcqd) and two (β and δ) of L-nopinoquinone dioxime (Hnqd). The co-ordination geometry has been investigated by 1H and 13C n.m.r., i.r., and electronic spectroscopy to reveal a novel NO-chelation of the metal with the (E,Z)-dioximate isomers (α- and δ-cqd and δ-nqd). The (E,E)-dioximate isomers (β-cqd and β-nqd ) co-ordinate to a metal with the usual NN-chelation. The c.d. spectra of the free ligands and the corresponding metal complexes are utilized to investigate the chirality of the chromophores at the unco-ordinated and co-ordinated dioximate moiety. Opposite Cotton effects around 20 000 cm–1 have been observed for [Co(α-cqd)2]·H2O and [Co(δ-nqd)2]·H2O indicating their ‘quasi-enantiomeric’ stereochemistries.

Isocyanide insertion in alkyl– and vinyl–metal bonds of square-planar palladium (II) and platinum(II): mechanisms and stereochemistry

Reaction of CNBut with trans-[PdX(R′)(CNBut)2](R′= Me or PhCH2) in the presence of a nucleophile L (CNBut or PPh3) gives the mono-insertion product, [PdX(CR′:NBut)(CNBut)L], and in the absence of L gives a dimer [{PdX(CR′:NBut)(CNBut)}2]. The insertion reaction in the presence of L occurs unimolecularly, being independent of the nature of L. This contrasts to the insertion reaction of CNR (R = But or p-MeC6H4) into trans-[MX(R′)(PPh3)2](M = Pd or Pt) which depends on the nature of CNR, suggesting an associative mechanism. The intramolecular insertion reaction of trans-[MBr(cis-CH:CH·CO2Me)(CNBut)2] gives [{MBr[C(trans-CH:CH·CO2 Me):NBut](CNBut}2], indicating isomerisation of the vinyl group, while insertion of CNC6H4Me-p into trans-[MBr(cis-CH:CH·CO2Me)(PPh3)2] yields trans-[MBr{C(cis-CH:CH·CO2Me):NC6H4Me}(PPh3)2] with retention of the vinyl geometry. Rapid multi-insertion of CNBut occurs with [pdX (R)(Ph2PCH2CH2PPh2)]. In contrast to CNC6H4Me-P, CNBut does not insert into trans-[PtX(cis-CH:CH·CO2H)(PPh3)2]. Various factors which influence the reaction rate and stereochemistry are described and interpretations accommodating all these features are discussed.

Catalytic regular polymerization of propadiene and alkylpropadienes

Abstract Catalytic linear polymerizations of propadiene, 1,2-butadiene, and 3-methyl-1,2-butadiene were achieved with low-valent-nickel complexes as homogeneous catalysts. Interaction of propadiene with some iron, cobalt, and nickel complexes produced propadiene complexes of various stabilities. Isolated complexes involving metal-stabilized-radical-ligands were noted as the possible initiating species. Labile propadiene complexes of Ni(O) and Co(O) atoms are generally active for the catalytic polymerizations. Several factors determining the catalytic activities were discussed. The i.r. and NMR spectra revealed that the polymers obtained with the nickel catalysts are of quite regular structures. The polymer structures of asymmetrically substituted propadienes suggested that a regular propagation takes place preferentially at the unsubstituted double bond which participates in the co-ordination to metal.

2,3,5-Trimethylenehexanediyl (allene trimer) hexacarbonyldi-iron complexes with no iron–iron bond

Reaction of allene with [Fe3(CO)12] at 85–90° gives a yellow, binuclear complex of formula [Fe2(CO)6(C9H12)](I); this can also be prepared from the allene dimer complex, [Fe2(CO)6(C6H8)]. At 120° the reaction affords a second isomer (II), which is orange-yellow and can be alternatively derived by thermal isomerisation of (I). A third isomer (III), which is lemon-yellow is obtained by thermal isomerisation of (I) or (II). The stepwise transformation (I)→(II)→(III) demonstrates the relative thermal stability. The molecular structures for the three diamagnetic complexes were deduced primarily on the basis of 1H n.m.r. spectra and the structure of (II) was confirmed by X-ray analysis.