Ate Complex Formation Research Articles

Rare Earth Metal Complexes Supported by a Tripodal Tris(amido) Ligand System Featuring an Arene Anchor

A new tripodal tris(amido) ligand system featuring an arene anchor was developed and applied to the coordination chemistry of rare earth metals. Two tris(amido) ligands with a 1,3,5-triphenylbenzene backbone were prepared in two steps from commercially available reagents on a gram scale. Salt metathesis and alkane elimination reactions were exploited to prepare mononuclear rare earth metal complexes in moderate to good yields. For salt metathesis reactions, while metal tribromides yielded neutral metal tris(amido) complexes, metal trichlorides led to the formation of ate complexes with an additional chloride bound to the metal center. The new compounds were characterized by X-ray crystallography, elemental analysis, and 1H and 13C nuclear magnetic resonance spectroscopy. The rare earth metal complexes exhibit a trigonal planar coordination geometry for the [MN3] fragment in the solid state rather than a trigonal pyramidal geometry, commonly observed for rare earth metal tris(amido) complexes such as M[N(SiMe3)2]3. Moreover, the arene anchor of the tripodal ligands is engaged in a nonnegligible interaction with the rare earth metal ions. Density functional theory calculations were performed to gain insight into the bonding interactions between the tripodal ligands and the rare earth metal ions. While LUMOs of these rare earth metal complexes are mainly π* orbitals of the arene with a minor component of metal-based orbitals, HOMO-15 and HOMO-16 of a lanthanum complex show that the arene anchor serves as a π donor to the trivalent lanthanum ion.

Coordination capabilities of bis-(2-pyridyl)amides in the field of divalent germanium, tin and lead compounds.

The reactivity of two lithium amides derived from bis-(2-pyridyl)amine (dpa)H or its methyl-substituted congener bis-(6-methyl-2-pyridyl)amine (Me-dpa)H, i.e. (dpa)Li (1) and (Me-dpa)Li (2), toward ECl2 (where E = Ge (dioxane complex) and Sn) is reported. This study produced both heteroleptic complexes (dpa)GeCl (3), [(dpa)SnCl]2 (4), and (Me-dpa)GeCl (5) and homoleptic complexes (dpa)2E (E = Ge (6) or Sn (7)) and (Me-dpa)2E (E = Ge (8) or Sn (9)). The structures of all complexes were established by single-crystal X-ray diffraction analysis showing significant differences depending on the E atom and ligand used. By contrast, in solution, the majority of compounds showed a fluxional behaviour as demonstrated by the NMR study. Finally, it turned out that the Me-dpa ligand, unlike dpa, is able to form ate complexes [(Me-dpa)3E]Li (E = Ge (10), Sn (11) or Pb (12)), whose structures were determined by single-crystal X-ray diffraction analysis. This study revealed the formation of two isomers for Ge and Sn complexes depending on the coordination preference of the lithium atom being coordinated either by nitrogen donors (10-12) or solely by the Ge or Sn electron lone pair (10a and 11a). Furthermore, the NMR experiments proved that the germanium complex 10 exhibits only limited stability in solution and decomposes to germylene 8 and lithium amide 2.

Anionic Palladium(0) and Palladium(II) Ate Complexes.

Palladium ate complexes are frequently invoked as important intermediates in Heck and cross-coupling reactions, but so far have largely eluded characterization at the molecular level. Here, we use electrospray-ionization mass spectrometry, electrical conductivity measurements, and NMR spectroscopy to show that the electron-poor catalyst [L3 Pd] (L=tris[3,5-bis(trifluoromethyl)phenyl]phosphine) readily reacts with Br- ions to afford the anionic, zero-valent ate complex [L3 PdBr]- . In contrast, more-electron-rich Pd catalysts display lower tendencies toward the formation of ate complexes. Combining [L3 Pd] with LiI and an aryl iodide substrate (ArI) results in the observation of the PdII ate complex [L2 Pd(Ar)I2 ]- .

Development of Heteroatom-introducing Methodologies Based on Ate Complex Formation: Perfluoroalkylation, Borylation, Aromatic Hydroxylation, and Amination Reactions.

We have developed regio- and chemoselective heteroelement-introducing reactions based on ate complex formation. This review is focused on novelly designed reactive species for efficient access to perfluoroalkylated aliphatic and aromatic compounds as well as Csp2-borylated compounds.

Yttrium half-sandwich complexes bearing the 2-(N,N-dimethylamino)ethyl-tetramethylcyclopentadienyl ligand

The synthesis of bis(dimethylsilyl)amide and dimethyl half-sandwich complexes of yttrium bearing the 2-(N,N-dimethylamino)ethyl-tetramethylcyclopentadienyl ligand revealed unusual Si‒H activation reactions and ate complex formation, respectively. Protonolysis of complex Y[N(SiHMe2)2]3(thf)2 with cyclopentadiene C5Me4HCH2CH2NMe2 (HCpNMe2) at elevated temperature yielded complex CpNMe2Y[η2-SiMe2(NSiHMe2)2](thf) featuring a four-membered metallacycle. The bis(amido) ligand [η2-SiMe2(NSiHMe2)2]2− is shown to form via separation of H2SiMe2. In contrast, the salt metathesis reaction of YCl3(thf)3 with LiN(SiHMe2)2 and LiCpNMe2 in a 1/2/1 ratio at ambient temperature generated the non-activated complex CpNMe2Y[N(SiHMe2)2]2, exhibiting a distinct ligand activation at elevated temperatures, as evidenced by a comprehensive NMR spectroscopic study. Application of a sequential salt metathesis protocol involving YCl3(thf)3, LiCpNMe2, and LiMe led to the isolation of ate complexes [CpNMe2YCl2]2[LiCl(thf)2] and [CpNMe2YMe2(LiMe)]2. Aforementioned half-sandwich complexes were all characterized by X-ray structure analyses and the respective silylamido and methyl derivatives are demonstrated to produce the half-sandwich yttrium bis(tetramethylaluminate) complex [CpNMe2AlMe3Y(AlMe4)2] in the presence of excess trimethylaluminium, by NMR spectroscopic studies.

Borylated Arylisoquinolines: Photophysical Properties and Switching Behavior of Promising Tunable Fluorophores

A series of nine borylated arylisoquinolines has been prepared with systematic variation in their electronic properties and their photophysical properties were investigated. The color of their fluorescence can be finely tuned by changing the properties of the aryl moiety, which is involved in internal-charge-transfer processes. For example, methoxy-substituted compound 5 showed an intense green emission, whereas dimethylamino-substituted compound 6 showed an orange-red emission. These new fluorophores were tested for their potential as molecular switches with external ionic stimuli, such as protons and fluoride ions. On the one hand, protonation of the isoquinoline moiety led to fluorescence enhancement for compounds that showed weak charge transfer and fluorescence quenching for compounds that showed strong charge transfer. On the other hand, the formation of ate complexes with fluoride led to strong fluorescence quenching in all of the investigated cases.

ChemInform Abstract: Diastereoselective Routes to a 1β-Methylcarbapenem Key Intermediate. Scope of Ate Complex Formation Between Ester Enolates and Organoboranes and -alanes.

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Oligosilylated group 4 titanocenes in the oxidation state +3

Reactions of oligosilylanions with Cp2TiCl2 led to the formation of ate-complexes of silylated titanocenes, which form upon silylation of the intermediate K[Cp2TiCl2].18-crown-6.

Reactions of silyl-stabilised sulfur ylides with organoboranes: enantioselectivity, mechanism, and understanding

The reaction of trimethylsilyl-substituted sulfonium ylides with organoboranes (Ph(3)B, Et(3)B) has been studied and although homologated products were obtained in good yield (after oxidation to the corresponding alcohols), the enantiomeric excesses were low with our camphor-based chiral sulfide (up to 40% ee, cf. corresponding phenyl-substituted sulfonium ylides gave >95% ee). Cross-over experiments were conducted to ascertain the nature of this difference in selectivity. Thus, aryl- and silyl-substituted sulfonium ylides (1 equiv.) were (separately) reacted with Et(3)B (1.5 equiv.) followed by Ph(3)B (1.5 equiv.) The experiments were repeated changing the order of addition of the two boranes. It was found that the aryl-substituted sulfonium ylide only trapped the first borane that was added indicating that ate complex formation was non-reversible and so was the selectivity determining step. In contrast the silyl-substituted sulfonium ylide only trapped Ph(3)B (it is more reactive than Et(3)B) indicating that ate complex formation was reversible and so 1,2-migration was now the selectivity determining step. The reactions have been studied computationally and the experimental observations have been reproduced. They have further revealed that the cause of reversibility in the case of the silyl-substituted sulfonium ylides results from ate complex formation being less exothermic and a higher barrier to 1,2-migration.

Di‐ and Trivalent Lanthanide Complexes Stabilized by Sterically Demanding Aminopyridinato Ligands

AbstractDeprotonation of Ap*H {Ap*H = (2,6‐diisopropylphenyl)‐[6‐(2,4,6‐triisopropylphenyl)pyridin‐2‐yl]amine} and Ap'H {Ap'H = (2,6‐diisopropylphenyl)‐[6‐(2,6‐dimethylphenyl)pyridin‐2‐yl]amine} using KH leads to polymeric [Ap*K]n and [Ap'K]n which undergo clean salt metathesis reactions with NdCl3 in THF forming [Nd(Ap*)Cl2(THF)2]2 and [Nd(Ap')2‐Cl(THF)], respectively. Ethylene polymerization activities of the two chloro complexes (after activation with MAO) were studied. Derivatization of the chloro compounds proceeds without ate complex formation, for instance the reaction of [Nd(Ap')2Cl(THF)] with one equiv. of [K{N(SiMe3)2}] leads to the “THF‐free” silylamide [Nd(Ap')2{N(SiMe3)2}]. Furthermore rare examples of heteroleptic amido‐iodo complexes of selected divalent lanthanides can be stabilized by deprotonated Ap*H. Reaction of [Ap*K]n with [LnI2(THF)3] (Ln = Yb, Sm) in THF leads, after workup in hexane, to [Yb(Ap*)I‐(THF)2]2 and [Sm(Ap*)I(THF)2]2. All lanthanide complexes, four of them are paramagnetic, were characterized by X‐ray crystal structure analysis. These compounds exhibit an excellent solubility in nonpolar solvents like hexane. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Computer-assisted design of chiral boron enolates: The role of ate complexes in determining aldol stereoselectivity.

Transition-state modelling for the aldol reaction of chiral Z and E enol borinates ( 1 and 2, Scheme 1) bearing mixed ligands (L 1 = Ipc, L 2 = 9) predicted higher enantioselectivities than those calculated and experimentally tested with C 2 symmetric systems (L 1 = L 2 = Ipc, L 1 = L 2 = 9). Reagent 8 was prepared and used to generate E enol borinates 24, which reacted with aldehydes to give the anti aldol products 25–28 with substantially lower enantiomeric excesses than predicted. This unexpected suggested that ate complex formation may be an important factor in controlling the selectivity of the boron-mediated aldol reaction. In particular, the presence of two different ligands on boron makes it a prostereogenic centre, and two diastereomeric ate complexes ( 29 and 30) can be formed on aldehyde complexation. These ate complexes are calculated to display different re: si face selectivities. The experimental results are similar to the ones predicted if the aldol reaction proceeds via the less selective ate complex 29.

Stereochemistry in carbenoid formation by bromine/lithium and bromine/zinc exchange reactions of gem-dibromo compounds

Stereochemistry in generation of lithium and zincate carbenoids by bromine/metal exchange reactions of gem-dibromo compounds with BuLi and lithium triorganozincates, respectively, has been investigated. Both lithium and zincate carbenoids derived from 1,1-dibromoalkenes are demonstrated to be configurationally stable at low temperatures whereas, in the presence of the unreacted starting dibromoalkenes, the lithium carbenoids, but not the zincate carbenoids, undergo facile isomerization at the carbenoid carbons. Zincate carbenoids derived from 1,1-dibromocyclopropanes undergo isomerization slowly at the carbenoid carbons in the presence of the starting dibromocyclopropanes. The isomerization reactions of the lithium and zincate carbenoids are proved to proceed through a mechanism involving a bromine/metal exchange between the carbenoids and the starting dirbromo compounds. Highly stereoselective formation of the lithium carbenoids is observed in the reaction of 1,1-dibromoalkenes under the thermodynamic conditions. Under kinetically controlled conditions, both the bromine/lithium and bromine/zinc exchange reactions take place preferentially at the sterically more hindered bromine atom of the starting gem-dibromo compounds. The observation is rationalized by an exchange mechanism involving a linear transition state of either an ate complex formation or an S N2 reaction at the more hindered bromine atom where strain relief due to the elongation of the carbon-bromine bond is expected.

Hindered organoboron groups in organic chemistry. 18. The production of boron stabilised carbanions

Details are given of processes leading to boron stabilised carbanions. By use of large aromatic groups on boron, ate-complex formation by bases can be inhibited and α-proton abstraction can be successfully achieved. By judicious choice of the aromatic groups and the bases, chemistry can be carried out despite the steric hindrance around the boron atom.

Diastereoselective routes to a 1.beta.-methylcarbapenem key intermediate. Scope of ate complex formation between ester enolates and organoboranes and -alanes

Two diastereoselective syntheses of the 1β-methylcarbapenem key intermediate 5 are described. Triethylborane-mediated epimerization of the α-methyl diastereomer 4 proceeds with high stereoselectivity to give pure 5 in good yield. Triethylaluminum-mediated methylation of the desmethyl analogue 3 gives 5 with modest stereoselectivity. Low-temperature FT-IR studies of reaction intermediates demonstrated that the epimerization proceeds via an ate comples, while the methylation does not involve ate complex formation

ChemInform Abstract: Nucleophilic Addition Reactions of Hindered Unsaturated Boranes. A New Synthesis of Organoboranes.

AbstractVinylboranes (I) and (VII), bearing the bulky mesityl substituents to preclude ate complex formation, undergo nucleophilic addition on reaction with organolithium compounds (II).

ChemInform Abstract: DIALKYL(3‐PYRIDYL)BORANES

AbstractAus dem Brompyridin (I) werden auf den im Formelschema angegebenen Wegen verschiedene PyridyIbo‐rane wie (IV) und die Pyridylboronsäure (VIII) hergestellt.

The dimesitylboron group in organic synthesis 1. Introduction.

The bulky dimesitylboron group encourages the formation of an α-carbanion from dimesitylalkylboranes by proton abstraction by a relatively hindered base, as compared with ate complex formation. It also stabilises the carbanion so formed. However the dimesitylboron group allows ate complex formation with unhindered bases, so allowing oxidative release of organic moieties from alkyldimesitylboranes. This desirable combination of properties has great potential for organic synthetic methodology.

Dialkyl(3-pyridyl)boranes.

Dialkyl (3-pyridyl) boranes (2) were synthesized by the iodination of lithium trialkyl (3-pyridyl) borates, prepared from trialkylboranes and 3-lithiopyridine. Although 2 was resistant to quaternization or ate-complex formation, betaines (4b-e) were obtained by the reaction of 2a with cyanide ion and alkylating agents in high yields. Further, protonolysis of 2a with propionic acid gave 3-pyridylboronic acid (5). Proton and carbon-13 nuclear magnetic resonance data for 2a, b and 4b are presented.

NMR studies of organometallic complexes and their etherates

NMR spectroscopy was used for investigating the formation of ate complexes between ethyllithium and the ethyl compounds of three metals of Group IIB. The changes of the methylic triplet and methylenic quadruplet were followed as a function of the initial ratio of the reagents in the solution. It was observed that: (a) ethyllithium and diethylmercury in benzene exchange their ethyl groups very slowly; (b) a fast exchange reaction takes place between ethyllithium and diethylcadmium in benzene, without however, formation of any detectable ate complex; on the addition of a small amount of diethylether (Et 2O) or tetrahydrofuran (THF), a 1:1 complex, lithium triethylcadmium, is formed which is more stable in the presence of the cyclic ether; (c) a 1.1 complex, (C 2H 5) 3ZnLi, is formed in pure benzene as well as in a mixed reaction medium: benzene+diethylether or benzene+THF. On the addition of various amounts of ether to a lihiumtriethylzinc solution in benzene, the changes of the ethyl signals demonstrate the existence of the following species: LiEt 3Zn·Et 2O, LiEt 3Zn·(THF) 2. LiEt 3Zn-DME. Similarly, the formation of LiEt 3Cd(THF) 2 by the addition of THF to a benzene solution of EtLi and Et 2Cd has been investigated.