Allyl Complexes Research Articles

Formate complexes of titanium(IV) supported by a tris(phenolato)-amine ligand

The synthesis of terminal and bridging formate complexes of titanium(IV) stabilized by a tris(phenolato)-amine ligand is described. The terminal formate complex [(O3N)Ti(OCHO)] (H3(O3N) = tris(4,6-di-tert-butyl-2-hydroxybenzyl)amine) was synthesized by salt metathesis of the chloride precursor [(O3N)TiCl] with sodium formate. Reaction of the terminal formate with the cationic titanium(IV) complex [(O3N)Ti(Et2O)][B(C6F5)4], synthesized by protonolysis of the benzyl complex [(O3N)Ti(CH2Ph)] with [PhMe2NH][B(C6F5)4] gave the bridging formate complex [{(O3N)Ti}2(μ-OCHO–ηO:ηO′)][B(C6F5)4] that was structurally characterized by single crystal X-ray diffraction. Attempts at deprotonation of the formate ligand by KHMDS (HMDS = N(SiMe3)2) to give the CO2 dianion failed but led to the formation of [(O3N)Ti{N(SiMe3)2}] instead.

Nucleophilic 5-endo-trig cyclization of 2-(trifluoromethyl)allylic metal enolates and enamides: Synthesis of tetrahydrofurans and pyrrolidines bearing exo-difluoromethylene units

Ketones and imines bearing a 2-(trifluoromethyl)allylic moiety successfully underwent nucleophilic 5-endo-trig cyclization via their metal enolates and enamides. O- or N-Cyclization proceeded exclusively in each case to afford the corresponding five-membered heterocycles with both exo-difluoromethylene and exo-alkylidene units. On treatment with potassium hexamethyldisilazide (KHMDS) or lithium diisopropylamide (LDA), 2-(trifluoromethyl)allylic ketones or imines provided the corresponding tetrahydrofurans or pyrrolidines bearing a Z-alkylidene group with perfect or substantial stereoselectivity, respectively.

Toluene and α-Olefins as Radical Scavengers: Direct NMR Evidence for Homolytic Chain Transfer Mechanism Leading to Benzyl and “Dormant” Titanium Allyl Complexes

Resting/dormant benzyl- and allyl-type Ti(IV) complexes formed under α-olefin polymerization conditions have been detected and characterized by NMR. Experimental and computational evidence supports a formation mechanism via reversible Ti–C bond homolysis rather than “classical” σ-bond metathesis.

Silylene Complexes of Late 3d Transition Metals Supported by tris-Phosphinoborate Ligands

The complexes [BP3R]MX ([BP3R] = PhB(CH2PR2)3–, R = Ph, iPr; M = Ni, Co, Fe; X = halide) were explored as platforms for generation of first-row metal silylene complexes. Direct silylation of [BP3Ph]NiCl or [BP3iPr]CoCl with (THF)2LiSiHMes2 resulted in formation of the silylene complexes [BP3Ph]Ni(μ-H)(SiMes2) and [BP3iPr]Co(μ-H)(SiMes2), respectively. In contrast, [BP3iPr]FeBr reacted with (THF)2LiSiHMes2 to produce the iron-alkyl [BP3iPr]Fe(CH2-2-(SiH2Mes)-3,5-Me2C6H2), a constitutional isomer of the expected silyl or silylene complex. Preparation of the nickel benzyl complex [BP3Ph]Ni(η2-Bn) allowed for exploration of addition–elimination chemistry for access to silylene complexes from simple primary and secondary silanes. Heating toluene solutions of [BP3Ph]Ni(η2-Bn) in the presence of CySiH3 resulted in the formation of a dimeric μ-silylene complex [Ni(μ-BP2Ph)(μ-SiHCy)]2. In the presence of 4-dimethylaminopyridine (DMAP), these conditions led to exclusive formation of the base-stabilized silylene com...

The Bonding Nature of Cyclometalated Ru Complex: How DFT Study Revealing the Dewar‐Chatt‐Duncanson Model Relates to the Molecular Properties

AbstractThe new intermediate Ru η3‐allyloxapyridyl complex has been studied aiming to illuminate the Dewar–Chatt–Duncanson (donation/back‐donation (D/BD)) model by calculations. A non‐standard, alternative calculation scheme integrating the density functional theory along with the charge decomposition, natural bond orbital (NBO) analysis and the map of electrostatic potential has provided new insights into the electronic structure of the complex presenting it in a Lewis structure. The donating, accepting contribution of Ru to ligand bonding in BD has been estimated by Weinhold's perturbation theory. This allowed quantifying D and BD contribution to the stabilization of the complex Lewis structure by deletion of selected Fock‐matrix elements from the basis set. Lewis energy gets lowered mainly by BD contribution from Ru d NBOs into the C2 atom of allyl ligand. Finally, the outcomes of calculations reveal: that Ru to ligands is bonded in donor‐acceptor fashion; the complex embodies nucleophilic character accumulated on the lone pairs of O atom in oxabenzyl group. The nucleophilic sides suggested by calculations support the reactivity trends of complex in the catalytic cycle. Calculated properties correlate well with experimental observations. The proposed scheme applied to the complex can be helpful for treatment of new metal complex candidates.

Tunable Synthesis of 2-Ene-1,4-diones, 4-Hydroxycyclopent-2-en-1-ones, and 2-(Furan-3-yl)acetamides via Palladium-Catalyzed Cascade Reactions of Allenols.

An efficient and regioselective synthesis of 2-ene-1,4-diones, 4-hydroxycyclopent-2-en-1-ones, or 2-(furan-3-yl)acetamides is successfully realized through palladium-catalyzed one-pot multicomponent reactions of allenols with aryl iodides and carbon monoxide in the presence of tertiary amines. Interestingly, the selectivity depends on the substitution patterns of the allenol substrates. To be specific, from the reaction of allenols with no substituent attached on the internal position of the allenic moiety, 2-ene-1,4-diones or 4-hydroxycyclopent-2-en-1-ones were formed selectively through carbonylation of aryl iodide followed by acylation of allenol with the in situ formed acyl palladium species, β-hydride elimination of the in situ formed allyl palladium complex, and further tautomerization or intramolecular aldol reaction. From the reaction of allenols bearing a substituent at the internal position of the allenic unit, on the other hand, diversely substituted 2-(furan-3-yl)acetamides were formed through a cascade process combining carbonylation of aryl iodide, acylation, and carbonylation of allenol followed by intramolecular condensation and amination by tertiary amine featuring an oxidant-free C-N bond cleavage.

Activity and selectivity of carbon supported palladium catalysts prepared from bis(η3-allyl)palladium complexes in phenylacetylene hydrogenation

Liquid phase hydrogenation of phenylacetylene was investigated on a series of 9.1 wt% Pd/C catalysts prepared from bis(η3-allyl)palladium complexes, as well as commercial Russian Pd/C catalyst (GIPKH-108). Pd/C catalysts were characterized by oxygen absorption and SEM. All prepared Pd/C catalysts demonstrated very high selectivity to styrene. The reduction temperature of palladium precursor has a significant effect on the palladium dispersity, but the substituent near terminal carbon atom of allyl ligand affects the dispersity more essentially than reduction temperature. Pd/C catalyst prepared from bis[(η3-allyl)palladium acetate] showed highest metal dispersity, activity and selectivity in the hydrogenation of phenylacetylene to styrene.

High activity heterogeneous catalysts by plasma-enhanced chemical vapor deposition of volatile palladium complexes on biomorphic carbon

Six new palladium complexes based on allyl and alkenolate ligands were synthesized and structurally characterized. Combination of delocalized allylic sp 2 -hybridized carbon centers and a strongly binding NˆO chelating unit (e.g., 3,3,3-trifluoro(pyridin-2-yl)propen-2-ol) offered a promising combination of high volatility and thermal lability not commonly observed in noble metal precursors. Application of the new Pd compounds in thermal metal–organic and plasma-enhanced chemical vapor deposition demonstrated their clean and efficient decomposition pathways, which in conjunction with their intriguing air stability made them efficient precursors for Pd films and clusters. Plasma-enhanced chemical vapor deposition of the palladium compounds on biomorphic carbon used as a porous substrate with high surface area and interconnected channels delivered recyclable carbon-supported Pd catalysts (Pd@BioC), which showed excellent selectivity, stability, and recyclability in C C coupling reactions.

Synthesis, Characterization, and Ligand Rearrangement of Tungsten Cyclopropyl Complexes

In an effort to study the β-H abstraction/1,3-addition mechanism for CH bond activation, the syntheses of tungsten organometallic complexes featuring a cyclopropyl ligand were undertaken. Attempts to prepare the chloro cyclopropyl complex Cp*W(NO)Cl(c-C3H5) instead afforded the previously reported allyl complex Cp*W(NO)Cl(η3-CH2CHCH2), due to a ring-opening rearrangement of the cyclopropyl ligand. The alkyl cyclopropyl complex Cp*W(NO)(CH2SiMe3)(c-C3H5) (1TMS) was prepared and characterized in solution via NMR spectroscopy but also undergoes a ligand rearrangement process to afford the allyl complex Cp*W(NO)(CH2SiMe3)(η3-CH2CHCH2) (2TMS), which was characterized by NMR spectroscopy and an X-ray diffraction study. Attempts to inhibit the ligand rearrangement or isolate 1TMS in the solid state have been unsuccessful. In order to investigate the mechanism of the conversion of 1TMS to 2TMS, a kinetic study was undertaken, and some preliminary results are discussed.

α-Substituted Benzylic Complexes of Palladium(II) as Precursors of Palladium Hydrides

The adoption of a pseudoallylic (η3) form makes palladium benzylic derivatives a class of stabilized palladium alkyls that can undergo β-H elimination reactions in a more controlled way. α-(Pentafluorophenylmethyl)benzyl palladium complexes have been studied, and they decompose by β-H elimination to give palladium hydrides that, depending on the auxiliary ligands, can: (a) transmetalate to another palladium atom and, by reductive elimination, give hydrogenated products; this process is favored for a combination of bridging ligands (i.e., halogens) and low coordinating ligands. (b) Be used as a hydride source and get trapped by a diene to give palladium allylic derivatives. The presence of carbon monoxide does not induce a β-H elimination reaction, and only CO insertion into the Pd–benzyl bond to give acyl derivatives is observed.

Benzylic Complexes of Palladium(II): Bonding Modes and Pentacoordination for Steric Relief

A large variety of α-(pentafluorophenylmethyl)benzylic palladium complexes with different ligands have been synthesized and characterized. Multinuclear NMR spectroscopic data allow to determine the σ- or η3-benzylic nature of the complexes in solution. The adoption of either coordination mode is a function of the number of ligands coordinated to palladium, and remarkably, the presence of bulky phosphines favors the adoption of a bidentate η3-benzylic mode and pentacoordinated palladium complexes. Experimental data and DFT calculations indicate that this five-coordination could alleviate the steric hindrance of two bulky cis phosphines. The benzylic complexes show a rich fluxional behavior that involves both ligand exchange and σ- to η3-benzylic interconversion.

Indium‐mediated Palladium‐catalyzed Allylic Alkylation of Isatins with Alkynes

AbstractAn unprecedented indium‐mediated palladium‐catalyzed allylic alkylation of isatins with alkynes is disclosed. This reaction provides a new, practical, and straightforward route to access 3‐allyl‐3‐hydroxy‐2‐oxindoles in good yields with broad substrate scope and scalability, exhibiting high atom and step economy. A primary mechanistic study reveals that indium played two roles in the reaction, first as a reductant and second as a Lewis acid. Compared with previous methods, our strategy eliminated the steps for the separation and purification of the reaction intermediates, as well as pre‐installing leaving groups to allylic substrates. Moreover, our reaction did not employ moisture‐sensitive allylic metal species and stoichiometric oxidants.magnified image

Theoretical Investigations of Isoprene Polymerization Catalyzed by Cationic Half-Sandwich Scandium Complexes Bearing a Coordinative Side Arm

Density functional theory studies have been conducted for isoprene polymerization catalyzed by the cationic half-sandwich scandium alkyl species containing a methoxy side arm [(C5Me4C6H4OMe-o)Sc(CH2SiMe3)]+ (1) and that containing a phosphine oxide side arm [{C5Me4SiMe2CH2P(O)Ph2}Sc(CH2SiMe3)]+ (2). It has been found that trans-1,4-polymerization of isoprene by species 1 prefers an insertion–isomerization mechanism: (i) an insertion of cis-isoprene into the metal–alkyl bond to give η3-π-anti-form, (ii) anti-syn isomerization of the resulting 1,2-disubstituted allyl complex to yield a syn-allyl form, (iii) repetitive insertion of cis-isoprene into the metal–syn-allyl bond and subsequent anti–syn isomerization. The resulting η3-π-syn-allyl species is suitable for more kinetically favorable cis-monomer insertion. The stability of the key transition state involved in the most feasible pathway could be ascribed to the smaller deformation of cis-isoprene and stronger interaction between the cis-isoprene moiety ...

Thorium Metallocene Cation Chemistry: Synthesis and Characterization of the Bent [(C5Me5)2Th(C6H5)(THF)][BPh4] and the Parallel Ring [(C5Me5)2Th(NCR)5][BPh4]2 (R = Me, Ph) Complexes

Attempts to synthesize the base-free dication [(C5Me5)2Th]2+ by reaction of the bis(allyl) complex (C5Me5)2Th(C3H5)2 with 2 equiv of [Et3NH][BPh4] in benzene yielded a cationic phenyl complex that, in the presence of THF, crystallized from toluene as [(C5Me5)2Th(C6H5)(THF)][BPh4]. The reaction of the dimethyl complex (C5Me5)2ThMe2 with [Et3NH][BPh4] in toluene in the presence of nitriles RCN generates cations of the formula [(C5Me5)2Th(NCR)5][BPh4]2 (R = Me, Ph) in 40–55% crystalline yield. The molecular structures reveal the first examples of thorium cyclopentadienyl metallocene complexes with parallel rings.

Cu-catalyzed enantioselective synthesis of tertiary benzylic copper complexes and their in situ addition to carbonyl compounds.

Catalytic chemo- and enantioselective generation of tertiary benzylic copper complexes from Cu-B(pin) (pin = pinacolato) additions to 1,1-disubstituted alkenes followed by in situ reactions with ketones and carboxylic acid phenol esters to construct multifunctional alkylboron compounds that contain quaternary stereogenic centers is presented. The method is distinguished by the unprecedented reaction mode of tertiary benzylic Cu complexes, allowing reaction with a wide range of carbonyl electrophiles in good yields and with high chemo-, site-, diastereo- and enantioselectivity. The catalytic protocol was performed with easily accessible chiral ligands and copper salts at ambient temperature. Functionalization of multifunctional alkylboron products provides useful building blocks that are otherwise difficult to access.

Synthesis of homometallic divalent lanthanide organoimides from benzyl complexes.

The reaction of LnI2(thf)2 with benzyl potassium affords the homoleptic benzyl complexes [Ln(CH2Ph)2]n of samarium, europium, and ytterbium. In the cases of Eu and Yb, the treatment of [Ln(CH2Ph)2]n with one equivalent of 2,6-diisopropylaniline gives access to tetrameric organoimide complexes [(thf)Ln(μ3-NDipp)]4, representing the first examples of homometallic Ln(ii) imides. This study revealed that the Yb(ii) organoimide chemistry is significantly different from that of calcium.

Structure of in cell protein crystals containing organometallic complexes.

The molecular structures of in cell protein crystals containing organometallic Pd(allyl) complexes were determined by performing microfocus X-ray diffraction experiments. The coordination sites in a polyhedrin mutant with deletion of selected amino acid residues located at the interface of the polyhedrin trimer are dramatically altered compared to those of the wild-type composite.

Synthesis, structures, and reactivity studies of cyclometalated N-heterocyclic carbene complexes of ruthenium.

An unusual cyclometalation reaction results from a C-C bond activation in Cp*(IPr)RuCl to give Cp*(IPr')Ru(L) featuring a NHC-C(sp2) chelating ligand (5-L; L = propene, N2; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; IPr' = 1-(6-isopropylphenyl)-3-(2,6-diisopropylphenyl)imidazol-2-ylidene). DFT calculations were employed to elucidate the C-C bond activation pathway. Reactions of cyclometalated ruthenium complexes bearing NHC-C(sp2) and NHC-C(sp3) ligands (5-L and Cp*(IXy-H)Ru(N2), 1a, respectively where IXy = 1,3-bis(2,6-dimethylphenyl)-imidazol-2-ylidene; IXy-H is the deprotonated form of (IXy)) are reported. Deprotonation of 1a by an equimolar mixture of benzyl potassium and 18-crown-6 afforded a doubly-cyclometalated complex [Cp*(IXy-2H)Ru][K(18-crown-6)] (7). A lower CO stretching frequency in Cp*(IXy-H)Ru(CO) (8) vs. Cp*(IPr')Ru(CO) (9) suggests that the NHC-C(sp3) ligand is more electron donating. Complexes 5-L reacted with H2 to give the dihydride Cp*(IPr')RuH2 (11). In comparison, after an initial oxidative addition of H2, complex 1a with its more reactive Ru-C(sp3) bond underwent C-H bond reductive elimination, and a second oxidative addition of H2 afforded the trihydride Cp*(IXy)RuH3 (10). Reaction of 1a with B(C6F5)3 resulted in a zwitterionic complex Cp*Ru(IXy'') (12; IXy'' = 1-[2-((C6F5)3BCH2)C6H3-6-methyl]-3-(2,6-dimethylphenyl)imidazol-2-ylidene-1-yl) by the formation of a new C-B bond. In contrast, B(C6F5)3 abstracted a hydride from 5-L and promoted a very unusual C-C bond formation involving insertion of an allyl ligand into a Ru-C bond to form [Cp*Ru(IPr'')][HB(C6F5)3] (IPr'' = 1-[2-(CH2[double bond, length as m-dash]CHCH2)C6H3-6-isopropyl]-3-(2,6-diisopropyl)imidazol-2-ylidene-1-yl) (13).

Reaction of allyl bromide with cyclometallated platinum(II) complexes: Unusual kinetic behavior and a novel case of methyl and allyl C-C bond reductive elimination

Reaction of the known cyclometallated platinum(II) complexes [PtMe(CˆN)(PPh2Me)], in which CˆN is either 2-phenylpyridinate (abbreviated as ppy-H), 1a, or benzo-h-quinolate (abbreviated as bhq-H), 1b, with excess allyl bromide, CH2=CH-CH2Br, in acetone gave the thermodynamic Pt(IV) product complex cis-[PtMe(CH2-CH=CH2)Br(ppy-H)(PPh2Me)], 2a, or cis-[PtMe(CH2-CH=CH2)Br(bhq-H)(PPh2Me)], 2b, respectively, in which the phosphine and the allyl ligands are situated trans to each other. Although these Pt(IV) complexes may possibly take up other isomeric forms such as one in that allyl ligand being situated trans to N atom or one that allyl ligand is located trans to Br atom, our full characterization of the complexes using 1H, 13C and 31P NMR spectroscopy, completely ruled out the latter possibilities. Our attempt to grow crystals of the Pt(IV) complex 2a was ended up to give crystals of the Pt(II) complex [PtBr(ppy-H)(PPh2Me)], 3a, resulting from reductive elimination of Me and allyl from 2a. Also the solution of the Pt(IV) bhq-H analogous complex 2b in acetone at 60 °C gradually gave the Pt(II) complex [PtBr(bhq-H)(PPh2Me)], 3b; these two new complexes 3a and 3b were also prepared directly and fully characterized and their NMR spectra were used to confirm the occurred reductive eliminations. These observations show that during the time a novel case of Me and allyl C-C bond reductive elimination, as determined by 1H, 13C and 31P NMR spectroscopy and X-ray crystallography, is happening. Kinetics of reaction of the Pt(II) complexes 1 with excess ally bromide were investigated by UV–vis spectroscopy and found that while the bhq-H complex 1b reacts by usual second order SN2 mechanism, first order in both the complex and the reagent, the analogous ppy-H complex 1a reacts almost completely by an unusual third order kinetics, first order on 1a and second order on allyl bromide reagent. Suggestions are made for these behaviors and DFT calculations were obtained to support the experimental observations.

Recent Advances in Rare Earth Complexes Bearing Allyl Ligands and Their Reactivity towards Conjugated Dienes and Styrene Polymerization

This mini-review focuses on recent advances on the synthesis, structure, and characterization of allyl-based rare earth organometallic complexes, with emphasis on their ability to catalyze the polymerization of non-polar monomers such as conjugated dienes, styrene, and their related copolymerization.