Olefin metathesis for metal incorporation: Preparation of conjugated ruthenium-containing complexes and polymers

Olefin Metathesis for Metal Incorporation (OMMI) was used for the stoichiometric attachment of ruthenium to both small and large polyenes. The dinuclear complexes (PCy₃)₂C1₂RuCH(CHCH)_nCHRu(PCy₃)₂Cl₂ (n = 1, 2), were prepared by reacting 2 equiv. of the Grubbs first-generation catalyst (PCy₃)₂C1₂Ru(CHPh)) with 1 equiv. of the appropriate polyene (1,3,5-hexatriene for n = 1 and 1,3,5,7-octatetraene for n = 2). Use of excess hexatriene led to the formation of the monoruthenium complex (PCy₃)₂C1₂RuCHCH CHCHCH₂. The mono- and di-ruthenium complexes exhibited marked differences in their spectroscopic and electrochemical properties, in addition to their Z–E isomerization rates. Nucleophilic attack of PCy₃ on the end CH₂ of the mono complex was observed, leading to both isomerization and phosphonium products. Extending the OMMI strategy to the second-generation catalyst was also done, despite the reduced initiation rate. The more reactive catalyst (H₂IMes)RuCl₂(CHPh)(3-bromopyridine)₂ allowed for ruthenium incorporation into polyacetylene, leading to the formation of polymers and oligomers with high ruthenium content.     

Nitrogen- and oxygen-bridged bidentate phosphaalkene ligands

An overview is given on synthesis and structures of new bidentate phosphaalkene ligands [(RMe₂Si)₂CP]₂E (E = O, NR, N^?) and (RMe₂Si)₂CPN(R′)PR′′₂. Exceptional properties of these ligands, extending beyond predictable properties of phosphaalkenes are: (i) the NSi bond cleavage of [(iPrMe₂Si)₂CP]₂NSiMe₃ with Au^I and Rh^I chloro complexes under mild conditions leading to binuclear complexes of the 6π-delocalised imidobisphosphaalkene anion [(iPrMe₂Si)₂CP]₂N^?, and (ii) the chlorotropic formation of molecular 1:2 Pd^II and Pt^II metallochloroylid complexes with novel ylid-type ligands [(RMe₂Si)₂CP(Cl)N(R)PR₂]^?, and the transformation of a P-platina-P-chloroylid complex into a C-platina phosphaalkene by intramolecular chlorosilane elimination. Properties of the heavier congeners [(RMe₂Si)₂CP]₂E (E = S, Se, Te, PR, P^?, As^?) and (RMe₂Si)₂CPEPR′′₂ (E = S, Se, Te) are also described.     

A convenient route to six-membered heterocyclic carbene complexes: Reactions of aminoallenylidene complexes with 1,3-bidentate nucleophiles

Pentacarbonyl dimethylamino(methoxy)allenylidene complexes of chromium and tungsten, [(CO)₅MCCC(NMe₂)OMe] (M = Cr (1a), W (1b)), react with 1,3-bidentate nucleophiles such as amidines and guanidine, H₂N–C(NH)R (R = Ph, C₆H₄NH₂-4, C₆H₄NO₂-3, NH₂), by displacing the methoxy substituent to give exclusively dimethylamino(imino)-allenylidene complexes, [(CO)₅MCCC{NC(NH₂)R}NMe₂] (2a–5a, 2b). Treatment of the chromium complexes 2a–5a with catalytic amounts of hydrochloric acid or HBF₄ gives rise to an intramolecular cyclization. Addition of the terminal NH₂ substituent to the C_α–C_β bond of the allenylidene chain affords pyrimidinylidene complexes 6–9 in high yield. In contrast to the chromium complexes 2a–5a, the corresponding tungsten complex 2b could not be induced to cyclize due to the lower electrophilicity of the α-carbon atom in 2b. The dimethylamino(phenyl)allenylidene complex [(CO)₅CrCCC(NMe₂)Ph] (10) reacts with benzamidine or guanidine similarly to 1a. However, the second reaction step – cyclization to give pyrimidinylidene complexes – proceeds much faster. Therefore, the formation of an imino(phenyl)allenylidene complex as an intermediate is established only by IR spectroscopy. The analogous reaction of 10 with 3-amino-5-methylpyrazole affords, via a formal [3+3]-cycloaddition, a pyrazolo[1,5a]pyrimidinylidene complex 13. Compound 13 is obtained as two isomers differing in the relative position of the N-bound proton (1H or 4H). The related reaction of 10 with thioacetamide yields a thiazinylidene complex and additionally an alkenyl(amino)carbene complex.     

Reaction of isocyanides with iminophosphorane-based carbene ligands: Synthesis of unprecedented ketenimine–ruthenium complexes

The RuC bond of the bis(iminophosphorano)methandiide-based ruthenium(II) carbene complexes [Ru(η⁶-p-cymene)(κ²-C,N-C[P{NP(O)(OR)₂}Ph₂]₂)] (R = Et (1), Ph (2)) undergoes a C–C coupling process with isocyanides to afford ketenimine derivatives [Ru(η⁶-p-cymene)(κ³-C,C,N-C(CNR′)[P{NP(O)(OR)₂}Ph₂]₂)] (R = Et, R′ = Bz (3a), 2,6-C₆H₃Me₂ (3b), Cy (3c); R = Ph, R′ = Bz (4a), 2,6-C₆H₃Me₂ (4b), Cy (4c)). Compounds 3–4a–c represent the first examples of ketenimine–ruthenium complexes reported to date. Protonation of 3–4a with HBF₄ · Et₂O takes place selectively at the ketenimine nitrogen atom yielding the cationic derivatives [Ru(η⁶-p-cymene)(κ³-C,C,N-C(CNHBz)[P{NP(O)(OR)₂}Ph₂]₂)][BF₄] (R = Et (5a), Ph (6a)).     

Hemilability and nonrigidity in metal complexes of bidentate P,PS donor ligands

Hemilability and nonrigidity in a series of mixed P,PS donor ligands has been studied in the complexes [Pd(P,PS)Cl₂], [Pd(η³-C₃H₅)(P,PS)][SbF₆], and [Rh(cod)(P,PS)][SbF₆] (P,PS = Ph₂P-Q-P(S)Ph₂). The effect of bite angle, the rigidity of the ligand backbone, and the role of the ancillary ligands are discussed.     

Understanding the reactivity of [WNAr(CH2tBu)2(CHtBu)] (Ar = 2,6-iPrC6H3) with silica partially dehydroxylated at low temperatures through a combined use of molecular and surface organometallic chemistry

Reaction of [WNAr(CH₂tBu)₂(CHtBu)] (Ar = 2,6-iPrC₆H₃) with silica partially dehydoxylated at 200 °C does not lead only to the expected bisgrafted [(SiO)₂WNAr(CHtBu)] species, but also surface reaction intermediates such as [(SiO)₂WNAr(CH₂tBu)₂]. All these species were characterized by infrared spectroscopy, 1D and 2D solid state NMR, elemental analysis and molecular models obtained by using silsesquioxanes. While a mixture of several surface species, the resulting material displays high activity in the olefin metathesis.     

Molecular structure of caffeine as determined by gas electron diffraction aided by theoretical calculations

The molecular structure of caffeine (3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione) was determined by means of gas electron diffraction. The nozzle temperature was 185 °C. The results of MP2 and B3LYP calculations with the 6-31G⁷ basis set were used as supporting information. These calculations predicted that caffeine has only one conformer and some of the methyl groups perform low frequency internal rotation. The electron diffraction data were analyzed on this basis. The determined structural parameters (r_g and ∠_α) of caffeine are as follows: <r(NC)_ring> = 1.382(3) Å; r(CC) = 1.382(←) Å; r(CC) = 1.446(18) Å; r(CN) = 1.297(11) Å; <r(NC_methyl)> = 1.459(13) Å; <r(CO)> = 1.206(5) Å; <r(CH)> = 1.085(11) Å; ∠N₁C₂N₃ = 116.5(11)°; ∠N₃C₄C₅ = 121. 5(13)°; ∠C₄C₅C₆ = 122.9(10)°; ∠C₄C₅N₇ = 104.7(14)°; ∠N₉–C₄=C₅ = 111.6(10)°; <∠NCH_methyl> = 108.5(28)°. Angle brackets denote average values; parenthesized values are the estimated limits of error (3σ) referring to the last significant digit; left arrow in parentheses means that this parameter is bound to the preceding one.     

Reaction of the RuTp(PR3)Cl fragment with alkynols: Formation of carbene,vinylidene, allenylidene,and carbyne complexes

The reaction of RuTp(COD)Cl (1) with PR₃ (PR₃ = PPh₂ⁱPr, PⁱPr₃, PPh₃) and propargylic alcohols HCCCPh₂OH, HCCCFc₂OH (Fc = ferrocenyl), and HCCC(Ph)MeOH has been studied.In the case of PR₃ = PPh₂ⁱPr, PⁱPr₃ and HCCCPh₂OH, the 3-hydroxyvinylidene complexes RuTp(PPh₂ⁱPr)(CCHC(Ph)₂OH)Cl (2a) and RuTp(PⁱPr₃)(CCHC(Ph₂)OH)Cl (2b) were isolated.With PR₃ = PPh₂ⁱPr and HCCCFc₂OH as well as with PR₃ = PPh₃ and HCCCPh₂OH dehydration takes place affording the allenylidene complexes RuTp(PPh₂ⁱPr)(CCCFc₂)Cl (3b) and RuTp(PPh₃)(CCCPh₂)Cl (3c).Similarly, with PPh₂ⁱPr and HCCC(Ph)MeOH rapid elimination of water results in the formation of the vinylvinylidene complex RuTp(PPh₂ⁱPr)(CCHC(Ph)CH₂)Cl (4).In contrast to the reactions of the RuTp(PR₃)Cl fragment with propargylic alcohols, with HCC(CH₂)_nOH (n = 2, 3, 4, 5) six-, and seven-membered cyclic oxycarbene complexes RuTp(PR₃)(C₄H₆O)Cl (5), RuTp(PR₃)(C₅H₈O)Cl (6), and RuTp(PR₃)(C₆H₁₀O)Cl (7) are obtained. On the other hand, with 1-ethynylcyclohexanol the vinylvinylidene complex RuTp(PPh₂ⁱPr)(CCHC₆H₉)Cl (8) is formed. The reaction of the allenylidene complexes 3a–c with acid has been investigated. Addition of CF₃COOH to a solution of 3a–c resulted in the reversible formation of the novel RuTp vinylcarbyne complexes [RuTp(PPh₂ⁱPr)(C–CHCPh₂)Cl]⁺ (9a), [RuTp(PPh₂ⁱPr)(C–CHCFc₂)Cl]⁺ (9b), and [RuTp(PPh₃)(C–CHCPh₂)Cl]⁺ (9c). The structures of 3a, 3b, and 5b have been determined by X-ray crystallography.     

Infrared spectra and UV-induced photochemistry of methyl aziridine-2-carboxylate isolated in argon and xenon matrices

Methyl aziridine-2-carboxylate (MA2C) has been isolated in low temperature argon and xenon matrices and its structure and photochemistry were studied by FTIR spectroscopy. The reactant as well as the main photoproducts were characterized by comparison of their experimental IR spectra with spectra calculated at the DFT(B3LYP)/6-311++G(d,p) level. The theoretical calculations predicted the existence of two low energy MA2C conformers, differing by the orientation of the OCCN dihedral angle. Both conformers were identified in the studied matrices. Both narrowband tunable and broadband UV irradiations of matrix-isolated MA2C yielded isomerization photoproducts resulting from cleavage of the CC and weakest CN bonds of the aziridine ring. Irradiation with UV laser-light at λ = 235 nm resulted in the formation of the E isomer of methyl 2-(methylimino)-acetate (MMIA) and the Z isomer of methyl 3-iminopropanoate (M3IP). Subsequent irradiation at 290 nm led to observation of new bands resulting from E → Z isomerization of MMIA, while bands due to M3IP remained unchanged. The photoproduced Z isomer of MMIA could be subsequently consumed upon higher-wavelength irradiation (λ = 330 nm). The initially produced MMIA conformer was found to obey the nonequilibrium of excited rotamers (NEER) principle. No photoproducts resulting from the cleavage of the strongest CN bond of the MA2C aziridine ring were observed, nor that of methyl 3-aminoacrylate (M3AA), which could in principle be obtained also by cleavage of the weakest CN bond of the MA2C aziridine ring, but would imply a different H-atom migration simultaneous with the ring opening process. These results indicate that both the differential electronic characteristics of the CN bonds of substituted aziridine rings and the type of required H-atom migration are major factors in determining the specific photochemistries of substituted aziridines. Photofragmentation reactions of MA2C were also observed, through identification of various related products, e.g., acetonitrile, methanol, methane, CO and CO₂.     

‘Pincer’ pyridyl- and bipyridyl-N-heterocyclic carbene analogues of the Grubbs’ metathesis catalyst

The ‘pincer’ pyridine-dicarbene and bipyridyl-carbene ruthenium benzylidene complexes, Ru(C–N–C)Cl₂(CHPh) and Ru(C–N–N)Cl₂(CHPh), (C–N–C) = 2,6-bis(DiPP-imidazol-2-ylidene)pyridine, (C–N–N) = (DiPP-imidazol-2-ylidene)bipyridine, have been prepared and characterised by spectroscopic and diffraction methods. They exhibit moderate metathesis activity. Non-symmetrical linear tridentate ether-functionalised N-heterocyclic carbene pro-ligands are also described.     

A nonenolizable imino-N-heterocyclic carbene ligand and corresponding silver (I) metal complex

The nonenolizable imino-N-heterocyclic carbene ligand precursor [1-t-butylimidazolium-3-{C(Ph)N(Ph)}] chloride has been synthesised. The corresponding silver (I) complex Ag(C∧imine)Cl; where C∧imine = [1-t-butylimidazolium-2-ylidene-3-{C(Ph)N(Ph)}], was prepared by reaction with Ag₂O. All of the compounds have been structurally characterized by single crystal X-ray diffraction.     

Phosphorus-substituted carbene complexes: Chelates,pincers and spirocycles

The syntheses, structural characterizations and reactivity patterns of main group and late transition metal carbene complexes of the bis(phosphoranimino)methandiide, [C(Ph₂PNSiMe₃)₂]²⁻, and the carbodiphosphorane, Ph₃PCPPh₃, are described and compared to previously reviewed early transition metal analogues. Bimetallic spirocyclic aluminum complexes of the former ligand are accessed by spontaneous double deprotonation of the central carbon atom of the parent, CH₂(Ph₂PNSiMe₃)₂, by two equivalents of AlMe₃, whereas the synthesis of platinum complexes requires the intermediacy of the tetralithium dimer, [Li₂C(Ph₂PNSiMe₃)₂]₂, and elimination of LiCl from a metal chloride precursor. In contrast to the early transition metal analogues, which are N,C,N-pincer, Schrock-type alkylidenes, the C,N-chelated platinum complexes are more akin to Fischer carbenes, and their chemistry is dominated by the nucleophilicity of free nitrogen atom and insertions into labile N–Si bonds. Chelated and pincer carbene complexes of rhodium result from single and double orthometallations, respectively, of the phenyl rings in Ph₃PCPPh₃; the latter compounds represent a wholly new class of C,C,C-pincer complexes. Electronic structure calculations show that the metal–carbon interaction in these compounds may be described as a dative, two-electron, C → M σ-bond. The free bis(phosphoranimino)methandiide and carbodiphosphorane ligands, while not having formal six valence electron resonance forms, may be thought of as having “pull–pull” Fischer carbene character, but the metal to which they become coordinated ultimately dictates their chemistry.     

Nickel-catalysed asymmetric SN2′ substitution chemistry of Baylis–Hillman derived allylic electrophiles

Unsymmetrical PhCHCH(CH₂X)(CO₂Me) (X = Cl, OAc) undergoes regioselective α-substitution with AlMe₃ to afford (E)-PhCHCH(Et)(CO₂Me) under Ni(acac)₂ catalysis. On the addition of planar chiral Ferrophite ligands [(R)-CpFe(1,2-C₅H₃Ar{P(OR)₂}) (Ar = Ph, 4-CF₃Ph, 3,5-Me₂Ph, 1-naphthyl; (OR)₂ = 1,1′-binaphthylene, 1,1′-biphenylene)] regioselective methylation γ to the leaving group is possible. It is proposed that the bulky Ferrophite ligand leads to an intermediate nickel allyl species Ni^II(Me)(Ferrophite){η³-PhCHCHCH(CO₂Me)} that adopts a configuration whereby the PhCH terminus of the π-allyl and the Ni–Me are syn leading to good regio (up to 6.4:1) and stereo (up to 94% ee) selectivities.     

Synthesis and mesomorphic investigation of calamitic liquid crystalline system ethyl-[4-(4′-decyloxy)benzoyloxy]-benzoate (4-EDBB): A temperature dependent micro-Raman study and DFT calculations

A new liquid crystalline material containing diester linking group ethyl-[4-(4′-decyloxy)benzoyloxy]-benzoate (4-EDBB) was synthesized. The phase transition temperatures were noted by differential scanning calorimetry (DSC), the texture pattern were observed by polarizing optical microscopy (POM) and temperature dependent Raman study was employed to observe the transitions as well as to understand the molecular rearrangement during phase transition. The transitions were observed with all the three techniques but the Raman signature of crystal → smectic A transition is many fold and more precise and accurate. The correlation between intermolecular interaction and phase behaviour has been discussed using temperature dependence Raman data of CH in-plane bending and CO stretching vibrations. With the help of DFT method the possible dimers of 4-EDBB were optimized and the rotational isomers were also investigated. There exists weak hydrogen bonds at room temperature, which breaks as the temperature is increased causing the CH in-plane bending to shift lower and CO stretching vibrations to shift higher. The discussion of the temperature dependent Raman data reveals that at crystal → smectic A transition as a result of intra-molecular rotation the molecules transform from trans- to cis- conformer.     

1,1-Bis(trifluoromethyl)butadiene-1,3—A new fluorinated building block

Although 1,1-bis(trifluoromethyl)butadiene-1,3 (1) reacts with dimethylamine with selective formation of 1,4-adduct [trans-(CF₃)₂CHCHCHCH₂N(CH₃)₂], halogenation of 1 proceeds with predominant formation (>92%) of 1,2-adducts (CF₃)₂CCHCHXCH₂X (X = Cl or Br). Electrophilic conjugated addition of “ClF” or “BrF” to 1 proceeds exclusively with the formation of 1,2-adducts (CF₃)₂CCHCHFCH₂X (major) and (CF₃)₂CCHCHXCH₂F (X = Cl or Br). Difluorocarbene adds selectively to CHCH₂ moiety of 1 forming thermally stable vinylcyclopropane. In Diels-Alder reaction with linear or cyclic dienes (butadienes, cyclopentadiene, cyclohexadiene-1,3) and quadricyclane compound 1 behaves as dienophile providing for the reaction electron-deficient CHCH₂ bond. The relative rate of cycloaddition of 1 and other fluoroolefins to quadricyclane, measured by high temperature NMR, indicates that (CF₃)₂CCH acts as very strong electron-withdrawing substituent. Synthetic utility of products based on 1 was also demonstrated.     

Cationic organoiron mixed-sandwich hydrazine complexes: Reactivity toward aldehydes,ketones, β-diketones and dioxomolybdenum complexes

This review covers comprehensively the authors work during the present decade based on the chemistry of ionic organometallic hydrazines formulated as [(η⁵-Cp′)Fe(η⁶-Ar-NHNH₂)]⁺PF₆^? (Cp′ = C₅H₅, C₅Me₅; Ar = aryl), that could be considered as a new generation of hydrazines owing to the changes provoked by the coordination of the 12-electron Cp′Fe⁺ fragment both in the electronic properties of the aromatic ring and in the hydrazine group. The reactivity of this new class of hydrazine is obviously centered, as in the classic Fischer's organohydrazines, Ar-NHNH₂, on the –NHNH₂ functional unit which is able to react with aldehydes, RCH(O) (R = alkyl, aryl, ferrocenyl (Fc)) and ketones, RR′CO (R = alkyl, aryl; R′ = alkyl, aryl, Fc), to afford ionic organometallic hydrazones. Likewise, the mixed-sandwich hydrazine precursors react with β-diketones Me–C(O)–CH₂–C(O)–Me to afford ionic organometallic pyrazoles, and with cis-dioxo-molybdenum complexes, e.g. [MoO₂(S₂CNEt₂)₂], to afford ionic organometallic mono-organodiazenido complexes in which the two metal centers are connected by a μ,η⁶:η¹-aryldiazenido bridge. While some ionic hydrazones exhibit NLO properties, the ionic organodiazenido hybrid complexes exhibit charge-transfer features.     

Biotransformation of 3-azidomethyl-4-phenyl-3-buten-2-one and analogs by Saccharomyces cerevisiae: New evidence for an SN2′ mechanism

(Z)-3-XCH₂-4-(C₆H₅)-3-buten-2-one enones (X = SCN, N₃, SO₂Me, OC₆H₅) were synthesized and submitted to biotransformations using whole Saccharomyces cerevisiae cells. The enone (X = SCN) produced (R)-4-(phenyl)-3-methylbutan-2-one (R)-6 with 93% ee and enones (X = N₃, SO₂Me, OC₆H₅) yielded a mixture of (R)-6 and the corresponding CC bond reduction products. Biotransformation with enone (X = N₃) mediated by Saccharomyces cerevisiae resulted in two products via two different routes: (i) the ketone (R)-4-azido-3-benzylbutan-2-one in 28% yield and with >99% ee by CC bond reduction; (ii) ketone (R)-6 in 51% yield and with 95% ee via cascade reactions beginning with azido group displacement by the formal hydride from flavin mononucleotide in an S_N2′ type reaction followed by reduction of the newly formed CC bond.     

Asymmetric cyclopropanation of (5S,SS)-3-p-tolylsulfinyl-5-ethoxyfuran-2(5H)-one with sulfonium ylides: influence of the sulfur ylide substituents on stereoselectivity

The cyclopropanation of title compound 1 with various sulfur ylides has been examined. A very high π-facial selectivity was observed in the reaction with diphenylsulfonium ylides, Ph₂SCRR′ (anti attack with respect to the alkoxy group in 1 is clearly preferred) whereas the endo-selectivity was found when R ≠ R′ is dependent on their relative size. Reactions with dimethylsulfonium ylides Me₂SCRR′ occurred with a moderate π-facial selectivity and exo-selectivity, the former being dependent on the solvent polarity. The stereochemical course of the cyclopropanation reactions is rationalized in terms of steric and electrostatic interactions.     

An improved,efficient route to 2,2-difluoroethenylbenzenes

Treatment of vinylidene fluoride with tert-BuLi at ?115 °C gave a solution of [F₂CCHLi]. Addition of Bu₃SnCl to this lithium reagent at ?110 °C gave an 88% isolated yield of F₂CCHSnBu₃. Reaction of F₂CCHSnBu₃ with substituted aryl iodides under Stille-Liebeskind conditions [Pd(PPh₃)₄/Cu(I)I] at room temperature afforded the 2,2-difluoroethenylbenzines in good yield. In the absence of the Cu(I)I co-catalyst, no reaction occurred. This work provides the most efficient route for the conversion of aryl halides to 2,2-difluorostyrenes.     

The structure and two-dimensional correlation infrared spectroscopy study of a new 2D polyoxomolybdate complex containing β-octamolybdate linked up by potassium ions: (4,4′-Hbpy)2(K2Mo8O26)

A novel compound, (4,4′-Hbpy)₂(K₂Mo₈O₂₆) 1 (bpy = bipydine), was synthesized by hydrothermal method and characterized by X-ray single analysis, thermalgravimetric analysis, one-dimensional (1D) infrared spectroscopy and two-dimensional (2D) correlation infrared spectroscopy under thermal perturbation. In the compound 1, the [Mo₈O₂₆] units link to potassium ions to form layer structure, and the protonated 4,4′-bpy are linked to chains by hydrogen bonds. The 2D IR correlation spectroscopy study indicates that the intensity changes of MoO, NH and CC stretching vibration are sensitive to the temperature variation, and the intensity changes of asymmetry stretching vibration of the terminal MoO occur prior to that of terminal MoO linked by K atom. At the same time, the peaks of asymmetry stretching vibrations of the terminal MoO and the stretching vibrations of NH split into two peaks respectively in 2D IR correlation spectroscopy.