Vinylidene Complexes Research Articles

ChemInform Abstract: Rhodium‐Catalyzed Oxygenative [2 + 2] Cycloaddition of Terminal Alkynes and Imines for the Synthesis of β‐Lactams.

A rhodium-catalyzed oxygenative [2 + 2] cycloaddition of terminal alkynes and imines has been developed, which gives β-lactams as products with high trans diastereoselectivity. In the presence of a Rh(I) catalyst and 4-picoline N-oxide, a terminal alkyne is converted to a rhodium ketene species via oxidation of a vinylidene complex and subsequently undergoes a [2 + 2] cycloaddition with an imine to give rise to the 2-azetidinone ring system. Mechanistic studies suggest that the reaction proceeds through a metalloketene rather than free ketene intermediate. The new method taking advantage of catalytic generation of a ketene species directly from a terminal alkyne provides a novel and efficient entry to the Staudinger synthesis of β-lactams under mild conditions.

ChemInform Abstract: Cyclization of Gold Acetylides: Synthesis of Vinyl Sulfonates via Gold Vinylidene Complexes.

AbstractThis gold‐catalyzed cyclization is suitable for the synthesis of 5‐ and 6‐membered rings.

Chemistry of vinylidene complexes. XXIII. Binuclear rhenium–palladium vinylidene bridged complexes, their reactions with diiron nonacarbonyl

The reaction of [Cp(CO)2ReCCHPh] (1) with [Pd(PPh3)4] followed by the treatment of resulting [Cp(CO)2RePd(μ-CCHPh)(PPh3)2] (2) with diphosphines (dppe, dppp) gave new complexes [Cp(CO)2RePd(μ-CCHPh)(dppe)] (3) and [Cp(CO)2RePd(μ-CCHPh)(dppp)] (4). Treatment of the synthesized complexes with [Fe2(CO)9] led to mixtures of [Cp(CO)2ReFe(μ-CCHPh)(CO)4] (5), [(dppe)PdFe2(CO)8] (6), [CpReFe2(μ3-CCHPh)(CO)8] (7), [CpReFePd(μ3-CCHPh)(CO)5(dppe)] (8) and of 5, [(dppp)PdFe2(CO)8] (9), [CpReFePd(μ3-CCHPh)(CO)5(dppp)] (10), respectively. The main products of both reactions were trinuclear μ3-vinylidene clusters 8 and 10. Reactions of the trinuclear μ3-vinylidene ReFePd clusters 8 and 10 with Fe2(CO)9 were also studied. New complexes were characterized by IR, 1H, 13C, 31P NMR spectroscopy and cyclic voltammetry. The structure of [CpReFe2(μ3-CCHPh)(CO)8] was determined by single-crystal X-ray diffraction.

Photoelectron spectroscopy of CoC2H2(-) and density functional study of Co(n)C2H2 (n = 1-3) anion and neutral clusters.

The anionic and neutral ConC2H2 (n = 1-3) clusters were investigated using anion photoelectron spectroscopy and density functional calculations. The adiabatic detachment energies and vertical detachment energies of ConC2H2(-) (n = 1-3) were determined. Our results show that the most stable geometries of anionic ConC2H2(-) (n = 1-2) and neutral ConC2H2 (n = 1-3) are composed of ConC2H clusters adsorbing a hydrogen atom on the top or bridge sites of Con, whereas Co3C2H2(-) consists of a five-member ring of Co3C2 carbide adsorbing two hydrogen atoms on two bridge sites of Co3. The reaction mechanisms show that the inserted isomer HCoC2H can convert into the vinylidene complex Co═C═CH2 via a side-on isomer M-η(2)-(C2H2).

Coupling Reactions of N-Propargyl Semi-Salen Compounds Induced by Ruthenium Complex

The coupling reaction of N-propargyl semi-salen compound 1d on [Ru]-Cl ([Ru] = Cp(PPh3)2Ru) generates the carbene complex 3d containing a substituted 2H-chromene unit in 7 d. The precursor vinylidene complex 2d is isolated from the reaction of the propargyl group of 1d with [Ru]-Cl in 12 h. Addition of an o-cresol moiety to Cα and Cβ of the vinylidene ligand of 2d takes place in a longer reaction time to yield 3d. Reactions of [Ru]-Cl with other analogous compounds 1a, 1b, and 1c, in excess, also afford carbene complexes 3a, 3b, and 3c, respectively, in 48 h via a similar coupling process. Their precursor vinylidene complexes 2a, 2b, and 2c are also observed in 12 h. Structures of 2 and 3 are determined on the basis of spectroscopic data. The solid state structure of the dppe analogue 3a′ is further confirmed by X-ray diffraction analysis. The added o-cresol part comes from compounds 1, instead of aldehyde which is confirmed by the cross-coupling reactions of 2 and 1 using mass spectrometry. For compariso...

Rhodium-Catalyzed Oxygenative [2 + 2] Cycloaddition of Terminal Alkynes and Imines for the Synthesis of β-Lactams

A rhodium-catalyzed oxygenative [2 + 2] cycloaddition of terminal alkynes and imines has been developed, which gives β-lactams as products with high trans diastereoselectivity. In the presence of a Rh(I) catalyst and 4-picoline N-oxide, a terminal alkyne is converted to a rhodium ketene species via oxidation of a vinylidene complex and subsequently undergoes a [2 + 2] cycloaddition with an imine to give rise to the 2-azetidinone ring system. Mechanistic studies suggest that the reaction proceeds through a metalloketene rather than free ketene intermediate. The new method taking advantage of catalytic generation of a ketene species directly from a terminal alkyne provides a novel and efficient entry to the Staudinger synthesis of β-lactams under mild conditions.

Cyclization of Gold Acetylides: Synthesis of Vinyl Sulfonates via Gold Vinylidene Complexes

Differently substituted terminal alkynes that bear sulfonate leaving groups at an appropriate distance were converted in the presence of a propynyl gold(I) precatalyst. After initial formation of a gold acetylide, a cyclization takes place at the β-carbon atom of this species. Mechanistic studies support a mechanism that is related to that of dual gold-catalyzed reactions, but for the new substrates, only one gold atom is needed for substrate activation. After formation of a gold vinylidene complex, which forms a tight contact ion pair with the sulfonate leaving group, recombination of the two parts delivers vinyl sulfonates, which are valuable targets that can serve as precursors for cross-coupling reactions, for example.

Catalytic Access to α-Oxo Gold Carbenes by N–O Bond Oxidants

Metal carbenes are highly versatile species that mediate various transformations. Recent advances in gold catalysis have allowed catalytic access to α-oxo metal carbenes from the alkyne functionality. Compared with traditional methods that rely upon metal-catalyzed decomposition of diazo precursors, the generation of this synthon occurs in an environmentally more appealing fashion by gold-catalyzed alkyne oxygenation. Hydroxylamine derivatives are typically prepared from hydroxylamine salts that are cheap and can be handled without special precaution. In reactions with an alkyne activated by gold, relatively stable nitrones and related reagents undergo efficient O-atom transfer to form putative α-oxo gold carbenes. The highly reactive nature of these species could be utilized in a variety of cascade transformations. Herein, recent synthetic methods based on this reactivity as well as the currently available mechanistic and structural studies through computational and experimental methods have been discussed. A variety of tandem reactions performed by our laboratory and others have demonstrated the synthetic utility of catalytically generated α-oxo gold carbenes and enabled access to various heterocycles. For example, a reaction between nitrones and alkynes led to azomethine ylides for the [3 + 2] dipolar cycloaddition. Alternatively, α-oxo gold carbenes can be transformed into enolate equivalents through a 1,2-pinacol shift. The addition of hydroxylamine derivatives across triple bonds led to oxoamination, providing α-aminocarbonyl compounds or regioselective Fisher indole-type synthesis. N-O bond cleaving redox chemistry paved the way for intermolecular redox processes, most notably by use of pyridine-N-oxide derivatives with expanding synthetic applications. In closing, other metal-based oxygenations using N-O bond oxidants will be highlighted. One particularly interesting aspect is the process leading to metal vinylidene complexes. Trapping of this intermediate resulted in opposite regioselectivity from gold catalysis in alkyne oxygenation and led to ketene intermediates for use in subsequent cascade transformations.

Boryl‐Functionalized σ‐Alkynyl and Vinylidene Rhodium Complexes: Synthesis and Electronic Properties

The synthesis, reactivity, and properties of boryl-functionalized σ-alkynyl and vinylidene rhodium complexes such as trans-[RhCl(=C=CHBMes2)(PiPr3)2] and trans-[Rh(C≡CBMes2)(IMe)(PiPr3)2] are reported. An equilibrium was found to exist between rhodium vinylidene complexes and the corresponding hydrido σ-alkynyl complexes in solution. The complex trans-[Rh(C≡CBMes2)(IMe)(PiPr3)2] (IMe=1,3-dimethylimidazol-2-ylidene) was found to exhibit solvatochromism and can be quasireversibly oxidized and reduced electrochemically. Density functional calculations were performed to determine the reaction mechanism and to help rationalize the photophysical properties of trans-[Rh(C≡CBMes2)(IMe)(PiPr3)2].

Reactions of Aromatic S- and O-Enynes with Two Methyl Substituents on the Olefinic Unit Assisted by a Ruthenium Complex: Tandem Cyclization and Product Selectivity

Ruthenium-assisted cyclizations of two enynes, HC≡CCH(OH)(C6H4)X-CH2CH=CMe2 (X=S (1a), O (1b)), each of which contains two terminal methyl substituents on the olefinic parts, are explored. The reaction of 1a in CH2Cl2 gives the vinylidene complex 2a from the first cyclization and two side products, 3a and the carbene complex 4a with a benzothiophene ligand. The same reaction in the presence of HBF4 affords 4a exclusively. Air oxidation of 4a in the presence of Et3N readily gives an aldehyde product. In MeOH, tandem cyclizations of 1a generate a mixture of the benzothiochromene compound 10a and the carbene complex 7a also with a benzothiochromene ligand. First, cyclization of 1b likewise proceeds in CH2Cl2 to give 2b. Tandem cyclization of 1b in MeOH yields comparable products 10b and 7b with benzochromene moieties, yet with no other side product. The reaction of [Ru]Cl with HC≡CCH(OH)(C6H4)S-CH2CH=CH2 (1c), which contains no methyl substituent in the olefinic part, in MeOH gives the carbene complex 15c with an unsubstituted thiochromene by means of a C-S bond formation. Structures of 3a and 15c are confirmed by X-ray diffraction analysis. The presence of methyl groups of enynes 1a and 1b promotes sequential cyclization reactions that involve C-C bond formation through carbocationic species.

Synthesis, Structure, and Reactivity of Tungsten Acetylide–Germylene Complexes

The novel acetylide–germylene complexes Cp*(CO)2W(GePh2)(CCR) (7a, R = SiMe3; 7b, R = CMe3) were synthesized by the reactions of Cp*(CO)2W(NCMe)Me with Ph2HGeC≡CR (R = SiMe3, CMe3). X-ray crystal analysis of 7a revealed significantly increased germylene–tungsten and decreased germylene–acetylide interactions in comparison to the corresponding interactions in the previously reported acetylide–silylene complex Cp*(CO)2W(SiPh2)(CCSiMe3) (1). Complexes 7a,b reacted with acetone to give the six-membered cyclic vinylidene complexes Cp*(CO)2W═C═C(R)CMe2OGePh2 (8a, R = SiMe3; 8b, R = CMe3) by acetone insertion reaction, similar to the case of 1 affording Cp*(CO)2W═C═C(SiMe3)CMe2OSiPh2 (2). In the presence of 4-(dimethylamino)pyridine (DMAP), complexes 8a,b gave trans- and cis-DMAP-stabilized germylene acetylide complexes Cp*(CO)2W(GePh2·DMAP)(CCR) (trans- and cis-9a, R = SiMe3; trans- and cis-9b, R = CMe3) and acetone, showing a reactivity different from that of the silicon analogue 2. Complexes cis-9a,b were isolated as crystals from the reaction of 7a,b with DMAP and formed mixtures with trans-9a,b in solutions, respectively. A mixture of cis- and trans-9a reacted with acetone to form an equilibrium mixture with 8a and DMAP. The reactivity of 7a,b toward Me3COH was also investigated to reveal the formation of the vinylidene complexes Cp*(CO)2W{GePh2(OCMe3)}═C═CHR (10a, R = SiMe3; 10b, R = CMe3); 10a is equilibrated with 7a and Me3COH, whereas 10b is further converted to the carbyne complex Cp*(CO)2W≡CCH(CMe3){GePh2(OCMe3)} (11).

Reactivities of Indenylruthenium Complex toward Internal Alkynes: Formation of Disubstituted Vinylidene Complexes and Indenyl–Alkyne Coupling

Reaction of the indenylruthenium complex [(η5-C9H7)RuCl(dppe)] with internal alkynes (MeC≡CR, R = Et, Ph) in the presence of NaBArF4 gives rise to two types of C–C bond activation, i.e., alkyne insertion/β-carbon elimination and internal alkyne/disubstituted vinylidene rearrangement, as reversible processes. At 70 °C, regioisomeric complexes [Ru{C(Me)═C(R)–(η6-C9H7)}(dppe)][BArF4] (2) and [Ru{C(R)═C(Me)–(η6-C9H7)}(dppe)][BArF4] (2′) are formed through insertion of the alkyne into the Ru–indenyl bond followed by haptotropic rearrangement. Complexes 2′ isomerize completely to 2 at this temperature after longer reaction time. At 130 °C, 2 are further converted into the disubstituted vinylidene complexes [(η5-C9H7)Ru{═C═C(R)Me}(dppe)][BArF4] (3) through alkyne–vinylidene rearrangement of the η2-alkyne intermediate. This provides a rare example of direct observation of the β-carbon elimination from an unstrained transition metal alkenyl complex.

Functionalized N-Heterocyclic Carbene Nonspectator Ligands upon Internal Alkyne Activation Reactions

When studying the activation of 3-arylpropiolates by [TpRu(picolyl-(R)I)Cl]/NaBAr(F)4 (picolyl-(Me)I = 3-methyl-1-(2-picolyl)imidazol-2-ylidene (1); picolyl-(Me)BI = 3-methyl-1-(2-picolyl)benzoimidazol-2-ylidene (2)) a migratory insertion of the NHC into a ruthenium-carbon bond and an unprecedented C-N bond activation of the chelating picolyl-NHC ligand take place to give the new ruthenium metallacycles [TpRu(κ(3)-C,N,N'-═C(Ph)-C(CH2Py)(CO2Me)((Me)I)][BAr(F)4] 3a and 4a and [TpRu(κ(3)-C,N,N'-═C(4-CF3Ph)-C(CH2Py)(CO2Me)((Me)I)][BAr(F)4] 3b and 4b. X-ray crystal structures of 3a and 3b are reported, and a mechanistic pathway is proposed. In contrast, activation of internal alkynones by a mixture of [TpRu(picolyl-(Me)I)Cl] complex (1) and NaBAr(F)4 led to isolation and characterization of the corresponding disubstituted vinylidene complexes. Also, structures of [TpRu(picolyl-(Me)I)(═CC(COR)(Ph)][BAr(F)4] (R = Me (6a); Ph (6b)) are reported.

Mechanism and Origins of Z Selectivity of the Catalytic Hydroalkoxylation of Alkynes via Rhodium Vinylidene Complexes To Produce Enol Ethers

We report the first theoretical study of transition-metal-catalyzed hydroalkoxylation of alkynes to produce enol ethers. The study utilizes density functional theory calculations (M06) to elucidate the mechanism and origins of Z selectivity of the anti-Markovnikov hydroalkoxylation of terminal alkynes with a Rh(I) 8-quinolinolato carbonyl chelate (1cat). The chosen system is, without any truncation, the realistic reaction of phenylacetylene and methanol with 1cat. Initiation of 1cat through phenylacetylene substitution for carbonyl generates the active catalyst, a Rh(I) η2-alkyne complex (3), which tautomerizes via an indirect 1,2-hydrogen shift to the Rh(I) vinylidene complex 4. The oxygen nucleophile methanol attacks the electrophilic vinylidene Cα, forming two stereoisomeric Rh(I) vinyl complexes (15 and 16), which ultimately lead to the (Z)- and (E)-enol ether products. These complexes undergo two ligand-mediated proton transfers to yield Rh(I) Fischer carbenes, which rearrange through a 1,2-β-hydrogen shift to afford complexes with π-bound product enol ethers. Final substitution of phenylacetylene gives (Z)- and (E)-PhCH═CHOMe and regenerates 3. The anti-Markovnikov regioselectivity stems from the Rh(I) vinylidene complex 4 with reversed Cα and Cβ polarity. The stereoselectivity arises from the turnover-limiting transition states (TSs) for the Rh(I) carbene rearrangement: the Z-product-forming TS24 is sterically less congested and hence more stable than the E-product-forming TS25. The difference in energy (1.2 kcal/mol) between TS24 and TS25 gives a theoretical Z selectivity that agrees well with the experimental value. Calculations were also performed on the key TSs of reactions involving two other alkyne substrates, and the results corroborate the proposed mechanism. The findings taken together give an insight into the roles of the rhodium–quinolinolato chelate framework in directing phenylacetylene attack by trans effect, mediating hydrogen transfers through hydrogen bonding, and differentiating the energies of key TSs by steric repulsion.

Multistate Photochromism in a Ruthenium Complex with Dithienylethene–Acetylide

The preparation, characterization, and photochromic properties of the ruthenium(II) vinylidene complex 1o and the ruthenium(II) acetylide complex 2o and its oxidized species 2o+ with one dithienylethene (DTE) unit are described. Complexes 1o and 2o can be mutually transformed upon the addition of base or acid due to the interconversion Ru═C═CH–DTE ⇆ Ru–C≡C–DTE. 2o and its oxidized species 2o+ can be interconverted through a reversible redox process. It is found that the ring-closing absorption band of DTE shows a progressive red shift in the order 628 nm (1c) → 641 nm (2c+) → 692 nm (2c). The photocyclization/cycloreversion quantum yields are in the order 1 (Φo→c = 0.0066, Φc→o = 0.001) < 2+ (Φo→c = 0.35, Φc→o = 0.012) < 2 (Φo→c = 0.58, Φc→o = 0.019), implying that the photochemical reactivity exhibits the order 1 < 2+ < 2, coinciding well with the progressively increased electronic density at the reactive carbon atoms. The interconversion among six states is clearly demonstrated by NMR, UV–vis–near-IR, and IR spectral, electrochemical, and computational studies.

Spectroelectrochemistry: A valuable tool for the study of organometallic-alkyne, -vinylidene, -cumulene, -alkynyl and related complexes

This review presents a highly selective summary of spectroelectrochemical methods used in the study of metal alkyne, acetylide, vinylidene and allenylidene complexes. The review is illustrated predominantly by the selected examples from the authors’ group that formed the basis of a lecture at the recent ISE Annual Meeting. Emphasis is placed on the use of spectroelectrochemical methods to study redox-induced ligand isomerisation reactions, and determination of molecular electronic structure, which complement the conventional tools available to the synthetic chemist for characterisation of molecular compounds. The role of computational studies in supporting the interpretation of spectroscopic data is also briefly discussed.

Reduction of carbon monoxide by a tetrakis(aryloxide)diniobium complex having four bridging hydrides

The use of a sterically demanding aryloxide ligand allows us to isolate a tetrakis(aryloxide)diniobium complex bridged by four hydride ligands. This hydride aryloxide undergoes reduction of CO via hydrogenation and deoxygenation, resulting in formation of a methylidene species. Further carbonylation gives a vinylidene complex.

Gold–allenylidenes – an experimental and theoretical study

Herein we describe the isolation and characterisation of the first “gold–allenylidene” complexes [AuCCCR2]+ X− (R = N(CH2)n, OMe; n = 3, 4; X− = OTf−) and present a bonding model for these species based on an experimental and theoretical analysis. Heteroatom stabilisation (oxygen and nitrogen donors) overpowers the ligand effect on gold and has a large impact on the distribution of electron density in the allenylidene fragment. The description of a gold-stabilised propargylic cation rather than a gold–vinylidene complex is favoured, clearly indicated by experimental and theoretical work. Removing this heteroatom stabilisation by means of a theoretical analysis shows that the choice of ligand can fine-tune the electronics ranging from a gold-stabilised carbocation towards a Au–allenylidene. This observation is in excellent agreement with the recent gold “carbene vs. carbocation” discussion and expands the understanding of Au–carbocation interactions to include gold–cumulene systems.

Intermolecular Reactions of Gold–Vinylidene Complexes

Intermolecular Reactions of Gold–Vinylidene Complexes

Infrared Spectra of M-η2-C2H2, HM–C≡CH, and HM–C≡CH– Prepared in Reactions of Laser-Ablated Group 3 Metal Atoms with Acetylene

The major HM-C≡CH and M-η(2)-C(2)H(2) products are observed in the matrix infrared spectra from reactions of laser-ablated group 3 metal atoms with acetylene, while the vinylidene product is not detected. These results reveal that coordination of group 3 metal atoms to the acetylene π-bond and H-migration from C to M readily occur during codeposition and photolysis afterward. The product absorption assigned to the La-vinylidene complex in a previous study is reassigned to one of the absorptions of La-η(2)-C(2)H(2)···C(2)H(2). Two strong Sc-H stretching absorptions are assigned to the free HSc-CCH(-) anion, in accord with a previous study, but a lower frequency counterpart is reassigned to HSc-CCH(-) coordinated to acetylene based on the increasing relative intensity of the low-frequency component at higher acetylene concentration. The group 3 metals evidently form weaker π-complexes than the group 4 metals. The addition of an electron to HM-CCH elongates the M-H and M-C bonds in the anionic species due to the lower ionic contributions to the bonding.