EPR detection of [Cu(mnt)X2]2−—A possible halogenide stabilized intermediate in ligand exchange reactions

During the reaction of [Cu(mnt)₂]^2? with CuX₂ (X = Cl, Br) [Cu(mnt)X₂]^2? species are formed and characterized EPR spectroscopically supporting a dissociative mechanism for ligand exchange reactions between [Cu(mnt)₂]^2? and other Cu(II), Ni(II) and Pd(II) complexes which contain unsaturated dichalcogeno ligands.     

Enantioselectivity and cis/trans-Selectivity in Dirhodium(II)-Catalyzed addition of diazoacetates to olefins

The Rh¹¹-catalyzed carbenoid addition of diazoacetates to olefins was investigated with [Rh₂{(4S)-phox}₄] ( 1 ;phox = tetrakis[(4S)-tetrahydro-4-phenyloxazol-2-one]), [Rh₂{(2S)-mepy}₄] ( 2 ; mepy = tetrakis[methyl (2S)-tetrahydro-5-oxopyrrole-2-carboxylate]), and [Rh₂(OAc)₄] ( 3 ). While catalysis with 2 and 3 afford preferentially trans-cyclopropanecarboxylates, the cis-isomers are the major products with 1 . In general, the enantioselectivities achieved with 1 and 2 are comparable. Additions catalyzed by 1 are strongly sensitive to steric effects. Highly substituted olefins afford cyclopropanes in only poor yield. The preferential cis-selectivity observed in reactions catalyzed by 1 is attributed to dominant interactions between the ligand of the catalyst and the substituents of both olefin and diazoacetate, which overrule the steric interactions between olefin and diazoacetate in the transition state for carbene transfer.     

Effects of electron density shift in five-coordinated Pt(II) complexes with olefins: An XPS study

XP spectra of seven five-coordinated Pt(II) complexes of general formula [Pt(olefin)(NN)Cl₂] reveal significant differences from analogous four-coordinated Pt(II) complexes. Pt4f and N1s signals, give evidence for π-back-donation from Pt onto both the olefin and the nitrogen chelating ligands, provided the latter belong to conjugated moieties. As a result, π-back-donation is more intense and Pt atoms more positive in 5- than in 4-coordinated complexes with similar ligands; this is in keeping with the previously reported decrease of nucleophilic reactivity of coordinated olefins in 5-coordinated Pt(II) complexes. The above electronic shifts are strongly reinforced by conjugation within the coordinated olefin such as with 2-butenedinitrile.     

The isolation of intermediates in hydrogen halide additions to some iridium complexes

The complexes [Ir(cod)L_n]PF₆(I, L = PPh₃, PMePh₂; n = 2. L = PMe₂Ph; n = 3) react with HX to give [IrHX(cod)L₂]PF₆ (II, L = PMePh₂ or PMe₂Ph) or [IrHX₂(cod)(PPh₃)] (III). The intermediates [IrX(cod)L₂] have, in two cases (L = PMePh₂, X = I, Br), been directly isolated from the reaction mixtures at 0°C, and are also formed from I with KX (L = PPh₃, X = Cl; L = PMePh₂, X = Cl, Br, I); these intermediates protonate to give II (L = PMePh₂), or an equimolar mixture of III and I (L = PPh₃, X = Cl). Surprisingly, I₂ reacts with I in MeOH to give III (L = PPh₃). The stereochemistries of II and III were determined by < ¹H NMR and especially by new methods using ¹³C NMR spectroscopy. The complexes I exhibit a Lewis acid reactivity pattern.     

Reactions of dimethyl- and diphenyl-mercury with dihalobis(isocyanide)palladium(II) complexes

The reaction of cis-[PdCl₂(CNR)₂] (R = Ph, p-MeC₆H₄, p-MeOC₆H₄) and trans-[PdI₂(CNPh)₂] with HgR′₂ (R′ = Me, Ph) followed by addition of PPh₃ (Pd/PPh₃, $\text{1}\text{2}$) gives complexes of the type trans- [PdX {C(=NR)C(R′)=NR}(PPh₃)₂] (X = Cl, I) I as main products. These bis(imino) compounds may result from double insertion of the coordinated isocyanides into a PdR′ σ-bond. NaBPh₄ was also found to act like HgPh₂ as a good phenylating agent towards coordinated isocyanide. The reactions of I with methanolic HClO₄ yield cationic compounds: trans- [PdX{C(NHR)C(R′)=NR}(PPh₃)₂]ClO₄; the protonated bis(imino) group may also be formulated as {C(=NR)C(R′)NHR} and a fast equilibrium between the two forms probably exists in solution. The factors influencing the reaction with HgR′₂ and spectroscopic data (IR and ¹H NMR) for the complexes are reported and discussed.     

Diverse reactivities of aromaticity-unsaturation coexisted metalladithiolene rings

This review paper summarizes the reactivities of metal dithiolene complexes based on the ‘coexistence of aromaticity and unsaturation’ in the five-membered metallacycle, the so-called metalladithiolene ring (MS₂C₂). The 16-electron [LM(dithiolene)] (LM = CpM^III, Cp*M^III, (C₆R₆)M^II) complexes are coordinatively unsaturated and usually show M-S centered cycloaddition reactions with nucleophiles (e.g. diazoalkanes, organic azides, quadricyclane) and electrophiles (e.g. tetracyanoethylene oxide, activated acetylene). The resulting metalladithiolene cycloadducts, which have three-membered M-S-C or M-S-N rings, further react with protic acids or PR₃ to undergo the ring-opening reactions involving the M-C bond, M-S bond or M-N bond cleavages. Furthermore, diverse adduct dissociations are observed by thermal, photochemical or electrochemical redox reactions. Such reactions normally produce the original [LM(dithiolene)] complexes (non-adduct) and the eliminated fragments. Among them, the Co-S centered imido adduct [CpCo(dithiolene)(NR)] (R = Ts, Ms) reacted under thermal conditions in the presence of PR₃ to undergo the intramolecular imido migration reaction to the Cp ligand, giving [(C₅H₄-NHR)Co(dithiolene)] complexes. The M-S centered multinuclear cluster complexes are obtained by the reaction of [LM(dithiolene)] with low valent M(CO)_n complexes. The square-planar bis(dithiolene) complexes [M(dithiolene)₂]⁰ (M = Ni, Pd, Pt) or tris(dithiolene) complexes [M(dithiolene)₃]⁰ yield cycloaddition products with olefins. These reactions are due to ligand centered reactions made possible by a molecular orbital overlap between dithiolene LUMO and olefin HOMO. Similar ligand centered adducts are obtained by the reaction of dianionic [M(dithiolene)₂]²⁻ with haloalkanes or dihaloalkanes. Also these adducts of bis(dithiolene) complex are dissociated photochemically and electrochemically. This paper also describes the reactivities of organometallic o-carborane dithiolate complexes, which are generally formulated as [LM(S₂C₂B₁₀H₁₀)] (LM = CpCo, Cp*Rh, Cp*Ir, (p-cymene)Ru and (p-cymene)Os). Diverse addition reactions are reported; in particular, the reaction with acetylene involves B-H bond activation in the carborane moiety.     

Fine‐Tuning the Reactivity and Stability by Systematic Ligand Variations in CpCoI Complexes as Catalysts for [2+2+2] Cycloaddition Reactions

CpCo^I‐olefin‐phosphite and CpCo^I‐bisphosphite complexes were systematically prepared and their reactivity in [2+2+2] cycloaddition reactions compared with highly active [CpCo(H₂C?CHSiMe₃)₂] ( 1 ). Whereas 1 is an excellent precursor for the synthesis of [CpCo(olefin)(phosphite)] complexes ( 2 a – f ), [CpCo(phosphite)₂] complexes ( 3 a – e ) were prepared photochemically from [CpCo(cod)]. The complexes were evaluated in the cyclotrimerization reaction of diynes with nitriles yielding pyridines. For [CpCo(olefin)(phosphite)], as well as some of the [CpCo(phosphite)₂] complexes, reaction temperatures as low as 50 °C were sufficient to perform the cycloaddition reaction. A direct comparison showed that the order of reactivity for the complex ligands was olefin₂>olefin/phosphite>phosphites₂. The complexes with mixed ligands favorably combine reactivity and stability. Calculations on the ligand dissociation from [CpCo(olefin)(phosphite)] proved that the phosphite is dissociating before the olefin. [CpCo(H₂C?CHSiMe₃){P(OPh)₃}] ( 2 a ) was investigated for the co‐cyclization of diynes and nitriles and found to be an efficient catalyst at rather mild temperatures.     

Some trimethyl phosphine and trimethyl phosphite complexes of tungsten(IV)

Direct reduction of WCl₆ with PMe₃ in toluene at 120°C in a sealed tube affords the complexes [WCl₄(PMe₃)_x] (x = 2, 3). [WCl₄(PMe₃)₃] abstracts oxygen from equimolar amounts of water in wet acetone or tetrahydrofuran to give [WOCl₂(PMe₃)₃] in very high yields. This procedure has been successfully applied to the high yield synthesis of other known oxotungsten(IV) complexes, [WOCl₂(PR₃)₃] (PR₃ = PMe₂Ph and PMePh₂). Metathesis reactions of [WOCl₂(PMe₃)₃] with NaX give [WOX₂(PMe₃)₃] (X = NCO, NCS) and [WOX₂(PMe₃)] (X = Me₂NCS₂). The synthesis of the trimethylphosphite analogue, [WOCl₂(P(OMe)₃)₃], is also described and the structures of the new complexes assigned on the basis of IR and ¹H and ³¹P NMR spectroscopy.     

Oxidative addition of MeI to some cyclometalated organoplatinum(II) complexes: Kinetics and mechanism

New cyclometalated platinum(II) complexes [PtMe(C^N)L], 1, in which C^N = deprotonated 2-phenylpyridine (ppy), benzo[h]quinoline (bhq) or 2-(p-tolyl)pyridine (tpy) and L = PPh₃ or PMePh₂, were synthesized by the reaction of [PtMe(C^N)(SMe₂)] with 1 equiv of L. The reaction of complexes 1 with MeI gave the cyclometalated Pt(IV) complexes [PtMe₂I(C^N)L], 3. On the basis of kinetic studies, using Uv–visible spectroscopy, it was suggested that the latter oxidative addition reactions were proceeded by an S_N2 mechanism. The rates of the reactions at different temperatures were measured and consistent with the proposed mechanism, large negative ΔS³ values were found for each reaction. Besides, rate of reactions (in CHCl₃) involving the PPh₃ complexes [PtMe(C^N)(PPh₃)], were almost 3–5 times slower than those involving the PMePh₂ complexes [PtMe(C^N)(PMePh₂)]. This was attributed to the electronic and steric effects of PPh₃ ligand as compared with that of PMePh₂ ligand which was further confirmed using density functional theory (DFT) calculations through finding approximate structures for the described complexes.     

Studies on the reactions of bis(acetylacetonato)platinum(II) with lewis bases : IV. Preparations and characterization of novel acetylacetonato complexes [Pt(C5H6O2)L2]

Novel complexes [Pt(C₅H₆O₂)L₂] (IVa, L = PPh₃; IVb, L = PMePh₂, IVc, L = PMe₂Ph) were prepared by the reactions of [Pt(acac)₂] with tertiary phosphines either at elevated temperature (when L = PPh₃) or at room temperature (L = PMePh₂ and PMe₂Ph), whereas AsPh₃ yielded [Pt(acac)(γ-acac)AsPh₃] (Id) by the reaction with [Pt(acac)₂] even under rigorous conditions. Complexes IV were characterized on the basis of their IR and NMR spectra, elemental analyses and chemical reactions, and a structure which possesses a chelate type “acetylacetonato” ligand involving π-oxoallyl bonding is proposed.     

Synthesis and characterization of binuclear palladium(II) and platinum(II) complexes with a bridging hydroxypyridinate ligand

Summary Binuclear Pd^II and Pt^II complexes of the type [M₂Cl₂(-Opy)₂(PR₃)₂] [M = Pd or Pt; Opy = 2-OC₅H₄N (2-hydroxypyridinate ion); PR₃ = PEt₃, Pn-Bu₃, PMe₂Ph or PMePh₂] were synthesized and characterized by elemental analysis, ¹H- and ³¹P-n.m.r. spectroscopies. The Pd complexes exist in the sym trans form, whereas the corresponding Pt complexes were generated as different isomers.     

Xylyl isocyanide platinum and palladium complexes

Bis(cycloocta-1,5-diene)platinum reacts with isopropyl isocyanide to give the trinuclear complex [Pt₃(CNPrⁱ)₆]. A related palladium compound was prepared by treating either [Pd(dba)₂] or [Pd₂(dba)₂CHCl₃] with 2,6-dimethylphenyl isocyanide. Reactions of the cluster [Pt₃(CNC₆H₃-2,6-Me₂)₆] and its presumed palladium analogue with the olefins (NC)₂C:C(CN)₂, F₂C:CFCl and (CN)₂C:C(CF₃)₂, give the compounds [M(olefin)(CNC₆H₃-2,6-Me₂)₂] (M  Pt, Pd) in which the metals are η²-bonded to the coordinated olefins. The compound [Pd₃(CNC₆H₃-2,6-Me₂)₆] reacts with F₂C:CFBr and with F₂C:CFCl to give the trans complexes [Pd(X)(C₂F₃)(CNC₆H₃-2,6-Me₂)₂] (X  Br, Cl). Similar compounds [M(L)-(CNC₆H₃-2,6-Me₂)₂] (M  Pt, Pd), (L  MeO₂CHC:CHCO₂Me, OOCH: CHCOO) have also been prepared, and characterised. Two platinum complexes [Pt(CH:NC₆H₃-2,6-Me₂)(SiMePh₂)(CNC₆H₃-2,6-Me₂)₂] and [Pt₂(μ-(PhC)₂CO)(CNC₆H₃-2,6-Me₂) ₄] hav been synthesized by treating the complex [Pt₃(CNC₆H₃-2,6-Me₂)₆] with HSiMePh₂ and cyclopropenone, respectively. NMR and IR data for the new species are reported and discussed.     

Ditopic dithiocarbamate ligands for the production of trinuclear species

Reactions of group 10 transition metals with the ditopic ligand dipicolyldithiocarbamate (DPDTC) were performed. Thus, 1:2 reactions of [Ni(CH₃COO)₂], [Pd(COD)Cl₂] or [Pt(COD)Cl₂] with DPDTC produced monomeric complexes of the type [M(κ²-SCS-DPDTC)₂, M = Ni (1), Pd (2) or Pt (3)] with the dithiocarbamate ligand (DTC) coordinated in a typical chelate κ²-SCS fashion. Interestingly, the reaction of [NiCl₂] with DPDTC, under similar conditions, afforded the organic compound 2-(pyridin-2-ylmethyl)imidazo[1,5-a]pyri-dine-3(2 H)-thione (4) as unique product. In order to prove the ditopic nature of the ligand DPDTC, complex [Pd(κ²-SCS-DPDTC)₂] (2) was further reacted with [ZnCl₂] in a 1:2 M ratio to yield the trinuclear complex [Cl₂Zn(κ²-NN-DPDTC-SCS-κ²)Pd(κ²-SCS-DPDTC-NN-κ²)ZnCl₂] (5). The molecular structures of all compounds were determinate by typical analytical techniques including the unequivocal determination of all structures by single crystal X-ray diffraction analysis. As expected, complexes 1–3 are isostructural, and the metal centres exhibiting slightly distorted square-planar geometries. While in 5, the trinuclear nature of the complex in confirmed exhibiting a nice combination of tetrahedral-square planar-tetrahedral geometries for the Zn-Pd-Zn centres respectively.     

Electronic effect of η5-cyclopentadienyl and η3-allyl ligands on the nature of the metal—olefin bond: Reversal of the relative olefin affinity of palladium(II) and platinum(II) ions

The barrier to olefin rotation in [Pt(η³-CH₂CMeCH₂)(olefin)(PPh₃)]PF₆ (3) (olefin = CH₂CH₂, E-MeCHCHMe) has been found to be extremely low compared to those in the other known, 4-coordinate olefin complexes of Pt^II. This can be ascribed to the smaller steric congestion around the olefin in 3. The corresponding barrier in [Pt(η⁵-C₅H₅)(CH₂CH₂)(PPh₃]ClO₄ (2), possessing likewise small steric congestion, was substantially higher than that in 3 (olefin = CH₂CH₂). The ¹³C and ³¹P NMR measurements have revealed much larger J(Pt-C(olefin)) in 2 than that in 3 (olefin = CH₂CH₂), while J(Pt-P) are comparable in these two. Stability constant data suggested that Pd^II ion in the Pd(η⁵-C₅H₅)(PPh₃)⁺ moiety is a better π-donor to olefins than Pt^II ion in the Pt(η³-CH₂CMeCH₂)(PPh₃)⁺ moiety, a reversal of the normal trend in the relative olefin affinity of these metal ions. The above spectral and stability features have been related to the electronic effect of the Cp ligand in enhancing the π back-bond interaction in one particular orientation of the CC bond.     

Some diolefin complexes of iridium(I) and a trans-influence series for the complexes [IrCl(cod)L]

A high-yield synthesis of [IrCl(cod)]₂ (cod = 1,5-cyclooctadiene) is described. The ¹H and ¹³C NMR spectra of a number of complexes [IrCl(cod)L] are interpreted in terms of a trans-effect series Cl^? < sym-collidine < 2-picoline < PCy₃ < P-i-Pr₃ < Pet₃ ～ AsPh₃ < PMe₂Ph < PMePh₂ < PPh₂ <P(MeO)Ph₂ < PClPh₂ < P(OPh)₃ < PCl₂Ph. Some ligand exchange reactions of [IrCl(cod)L] are discussed. A number of complexes of the type [Ir(cod)L_n]PF₆ (L = a variety of amines (n = 2) and phosphines (n = 2 or 3)) are described. Exchange reactions of the sort: [Ir(cod)(PR₃)₂]PF₆ + [Ir(cod)(py)₂]PF₆ ? [Ir(cod)(PR₃)Py]PF₆ are reported in which, surprisingly, the isolable mixed ligand complexes are the only detectable species at equilibrium (py = pyridine).     

A study of the reactions of methionine- and histidine-containing peptides with palladium(II) complexes: The key role of steric crowding on palladium(II) in the selective cleavage of the peptide bond

¹H NMR spectroscopy was applied to study the reactions of palladium(II) complexes, cis-[Pd(dpa)Cl₂] and cis-[Pd(dpa)(H₂O)₂]²⁺ (dpa is 2,2′-dipyridylamine acting as a bidentate ligand) with the dipeptides methionylglycine (Met-Gly) and histidylglycine (His-Gly), and the N-acetylated derivatives of these dipeptides, MeCOMet-Gly and MeCOHis-Gly. All reactions were carried out in the pH range 2.0–2.5 with equimolar amounts of the palladium(II) complex and the peptide at two different temperatures, 25 and 60 °C. In the reactions of cis-[Pd(dpa)Cl₂] and cis-[Pd(dpa)(H₂O)₂]²⁺ with Met-Gly and His-Gly, no hydrolysis of the peptide bond was observed. The final product in these reactions was the [Pd(dpa)₂]²⁺ complex. The square-planar structure of this complex was confirmed by X-ray analysis. The reaction of the cis-[Pd(dpa)(H₂O)₂]²⁺ complex with the MeCOHis-Gly and MeCOMet-Gly peptides under the previously mentioned experimental conditions was remarkably selective in the cleavage of the amide bond involving the carboxylic group of methionine in the side chain. The modes of coordination of cis-[Pd(dpa)Cl₂] and cis-[Pd(dpa)(H₂O)₂]²⁺ in the reactions with the non-acetylated peptides and the total steric inhibition of the hydrolytic reaction between cis-[Pd(dpa)(H₂O)₂]²⁺ and MeCOHis-Gly can be attributed to the steric bulk of the palladium(II) complex. This finding should be taken into consideration in designing new palladium(II) complexes for the regioselective cleavage of peptides and proteins.     

Reactions of 1,4-diaza-3-methylbutadien-2-ylpalladium(II) derivatives with chloro-bridged rhodium(I) complexes

The reactions of the organometallic 1,4-diazabutadienes, RN=C(R′)C(Me)=NR″ [R = R″ = p-C₆H₄OMe, R′ = trans-PdCl(PPh₃)₂ (DAB); R = p-C₆H₄OMe, R″ = Me, R′ = trans-PdCl(PPh₃)₂ (DAB^I; R = R″ = p-C₆H₄OMe, R′ = Pd(dmtc)-(PPh₃), dmtc = dimethyldithiocarbamate (DAB^II); R = R″ = p-C₆H₄OMe, R′ = PdCl(diphos), diphos = 1,2-bis(diphenylphosphino)ethane (DAB^III)] with [RhCl(COD)]₂ (COD = 1,5-cyclooctadiene, Pd/Rh ratio = $\text{1}\text{2}$) depend on the nature of the ancillary ligands at the Pd atom in group R′. In the reactions with DAB and DAB^I transfer of one PPh₃ ligand from Pd to Rh occurs yielding [RhCl(COD)(PPh₃)] and the new binuclear complexes [Rh(COD) {RN=C(R?)-C(Me)=NR″}], in which the diazabutadiene moiety acts as a chelating bidentate ligand. Exchange of ligands between the two different metallic centers also occurs in the reaction with DAB^II. In this case, the migration of the bidentate dmtc anion yields [Rh(COD)Pdmtc] and [Rh(COD) {RN=C(R?)C(Me)=NR″}]. In contrast, the reaction with DAB^III leads to the ionic product [Rh(COD)- (DAB^III)][RhCl₂(COD)], with no transfer of ligands. The cationic complex [Rh(COD)(DAB^III)]⁺ can be isolated as the perchlorate salt from the same reaction (Pd/Rh ratio = 1/1) in the presence of an excess of NaClO₄. In all the binuclear complexes the coordinated 1,5-cyclooctadiene can be readily displaced by carbon monoxide to give the corresponding dicarbonyl derivatives. The reaction of [RhCl(CO)₂]₂ with DAB and/or DAB^I yields trinuclear complexes of the type [RhCl(CO)₂]₂(DAB), in which the diazabutadiene group acts as a bridging bidentate ligand. Some reactions of the organic diazabutadiene RN=C(Me)C(Me)=NR (R = p-C₆H₄OMe) are also reported for comparison.     

Coordination chemistry of the activation of [(triazacyclohexane)CrCl3] with [PhNMe2H][B(C6F5)4] and AlR3

Complexes of N-substituted 1,3,5-triazacyclohexanes with CrCl₃ form 1:1 adducts with [PhNMe₂H][B(C₆F₅)₄] with increased solubility in toluene. Addition of AlⁱBu₃ leads to free PhNMe₂ and a complex with [B(C₆F₅)₄]⁻ weakly coordinated to chromium via a meta-fluorine atom. This complex can polymerise and/or trimerise olefins similar to methyl aluminoxane activated complexes. Decomposition of the active complex involves transfer of the triazacyclohexane to aluminium leading to [(triazacyclohexane)AlⁱBu₂][B(C₆F₅)₄] and [(arene)₂Cr][B(C₆F₅)₄]. These chromium(I) complexes have been characterised by X-ray crystallography and prove that chromium is reduced to the oxidation state +I during the catalysis.     

Quantum chemical calculations of stability constants: study of ligand effects on the relative stability of Pd(II)�Cpeptide complexes

Replacement of [Pd(H₂O)₄]²⁺ by cis-[Pd(en)(H₂O)₂]²⁺, [PdCl₄]^2?, and [Pd(NH₃)₄]²⁺ on the hydrolytic cleavage of the Ace-Ala-Lys-Tyr-Gly?CGly-Met-Ala-Ala-Arg-Ala peptide is theoretically investigated by using different quantum chemical methods both in the gas phase an in water solution. First, we carry out a series of validation calculations on small Pd(II) complexes by computing high-level ab initio [MP2 and CCSD(T)] and Density Functional Theory (B3LYP) electronic energies while solvent effects are taken into account by means of a Poisson-Boltzmann continuum model coupled with the B3LYP method. After having assessed the actual performance of the DFT calculations in predicting the stability constants for selected Pd(II)-complexes, we compute the relative free energies in solution of several Pd(II)?Cpeptide model complexes. By assuming that the reaction of the peptide with cis-[Pd(en)(H₂O)₂]²⁺, [Pd(Cl)₄]^2?, and [Pd(NH₃)₄]²⁺ would lead to the initial formation of the respective peptide-bound complexes, which in turn would evolve to afford a hydrolytically active complex [Pd(peptide)(H₂O)₂]²⁺ through the displacement of the en, Cl^?, and NH₃ ligands by water, our calculations of the relative stability of these complexes allow us to rationalize why [Pd(H₂O)₄]²⁺ and [Pd(NH₃)₄]²⁺ are more reactive than cis-[Pd(en)(H₂O)₂]²⁺ and [PdCl₄]^2? as experimentally found.     

Thermal and photochemical olefin isomerizations with complex [(C5Me4H)2Ti(CH3)2] as initiator

Irradiation of the complex [(C₅Me₄H)₂Ti(Ci(CH₃)₂] in an olefin as a solvent promotes stereospecific photoassisted isomerizations: olefins with terminal double bonds are rapidly isomerized into 2-alkenes with an E-configuration. Kinetic studies of hydrogen migration and of Z-E isomerizations of disubstituted olefins have demonstrated the influence of substitution and of branching of the hydrocarbon chain on the course of the reaction. Formation of paramagnetic complexes of Ti^III that are probably intermediates in these reactions has been confirmed by ESR. The same reactions, but with a lower stereoselectivity, are initiated thermally by the [(C₅Me₄H)₂Ti(CH₃)₂] complex.