A cross-metathesis approach for polymetallic [FeFe]-hydrogenase mimics.

A method has been developed for synthesizing [FeFe]-H2ase mimics with diverse structures and properties, employing cross-metathesis of olefins. Vinylmetallocenes (5 and 6) and vinyl half-sandwich complexes (10 and 11) have been used as cross-metathesis partners with [FeFe]-H2ase mimics (4, 8, and 9) bearing a double bond in the moiety attached to the ADT-bridge nitrogen. Electrochemical studies of these complexes, encompassing metallocene-type (7a-b, 12a-b, and 13a-b) as well as half-sandwich derivatives (12c and 13c-d), have demonstrated that the introduction of a redox unit has a marginal impact on the reduction potential of these [FeFe]-H2ase mimics. The application of this cross-metathesis approach has allowed the synthesis of [FeFe]-H2ase mimics featuring an Ir(III) electrochemical antenna (16-18) as well as systems having an electron-donor-photosensitizer structure (ED-PS) (23). The electrocatalytic properties of these complexes have been elucidated through electrochemical studies.

ß-Lactam and penicillin substituted mesoionic metal carbene complexes.

1,2,3-Triazolylidene MIC M-complexes (M = Au, Pd, Pt) having 2-azetidinones and penicillin G substituents at the triazole ring were prepared by CuAAC on 2-azetidinones having a terminal alkyne tethered at N1, followed by alkylation of the 1,2,3-triazole ring and transmetallation [Au(I), Pd(II) and Pt(II)]. The Au-MIC complexes efficiently catalyze the regioselective cycloisomerization of enynes, while the Pt-MIC complexes were efficient catalysts in hydrosilylation reactions.

Role of imine isomerization in the stereocontrol of the Staudinger reaction between ketenes and imines.

Computational-experimental analysis has allowed determining that the stereochemistry of the Staudinger reaction between ketenes and imines is strongly associated with the nature of the imine, which affects the two steps of the reaction. The first step, namely the nucleophilic attack of the sp2-hybridized nitrogen atom of the imine on the sp-hybridized carbon atom of the ketene, is affected by the energetically accessible in situ isomerization patterns of the imine. The second step consists of a conrotatory electrocyclization of the zwitterionic intermediate formed in the previous step. This latter pericyclic step depends on the inward/outward torquoelectronic effects generated by the substituents of the imine. The impact of these factors on the stereochemistry of this reaction has been analyzed kinetically by numerical methods. The results of these simulations are compatible with the experimental results and support these conclusions.

Revisiting the photochemical synthesis of [FeFe]-hydrogenase mimics: reaction optimization, mechanistic study and electrochemical behaviour.

The photoreaction of [(µ-S)2Fe2(CO)6] and alkenes or alkynes has been optimized to readily obtain functionalized [FeFe]-hydrogenase mimics. Irradiation under low CO pressure in THF produces the corresponding photo-adducts in good/acceptable (alkenes/alkynes) yields, with retention of the starting olefin stereochemistry. DFT-calculations provide plausible reaction pathways in both, singlet and triplet states. The DFT-calculation based in the singlet state is energetically more favorable. The electrochemical behavior of the synthesized compounds is also presented, including studies in acidic media. The electrochemical properties of the products vary in the presence of a double bond (cycloaddition of [(µ-S)2Fe2(CO)6] to alkynes), respect to a single bond (cycloaddition to alkenes).

Triazole-Containing [FeFe] Hydrogenase Mimics: Synthesis and Electrocatalytic Behavior.

Through a Cu-catalyzed Huisgen cycloaddition between terminal alkynes and azides (CuAAC) reaction, azide [(µ-SCH2)2N(4-N3C6H4)Fe2(CO)6] has demonstrated to be a robust and versatile reagent able to incorporate the [(µ-SR)2Fe2(CO)6] fragment on a wide range of substrates, ranging from aromatic compounds to nucleosides, metallocenes, or redox and luminescent markers. The [FeIFeI]/[Fe0FeI] and [Fe0FeI]/[Fe0Fe0] reduction potentials of the triazole derivatives prepared are comparable to those of other aminodithiolate (adt) Fe-Fe hydrogenase mimics. The presence of the triazole linker influences the electrochemical behavior of these complexes depending on the strength of the acid employed.

Bio-Organometallic Derivatives of Antibacterial Drugs.

Overuse and misuse of antibacterial drugs has resulted in bacteria resistance and in an increase in mortality rates due to bacterial infections. Therefore, there is an imperative necessity of new antibacterial drugs. Bio-organometallic derivatives of antibacterial agents offer an opportunity to discover new active antibacterial drugs. These compounds are well-characterized products and, in several examples, their antibacterial activities have been studied. Both inhibition of the antibacterial activity and strong increase in the antibiotic activity of the parent drug have been found. The synthesis of the main classes of bio-organometallic derivatives of these drugs, as well as examples of the use of structure-activity relation (SAR) studies to increase the activity and to understand the mode of action of bio-organometallic antimicrobial peptides (BOAMPs) and platensimicyn bio-organometallic mimics is presented in this article.

Sequential Reactions of Alkynes on an Iridium(III) Single Site.

Sequential insertion of terminal alkynes on IrIII cyclometalated complexes allow the formation of novel metallapolycycles in a controlled and efficient manner. ortho-Methylarylethynyl derivatives led to an unprecedented cascade combination of three fundamental processes (C-C bond formation, C(sp3 )-H activation, and reductive coupling) on a single IrIII center, in a process compatible with functionalized biomolecules and photoactive substrates. The reaction with tert-butylacetylene led to a [6,5,4]-polycycle that incorporates an iridacyclobutenylidene in its structure. The sequence is a multicomponent reaction in which the metal not only promotes the different steps but also determines their stereoselectivity. This is an elegant example of the synergy between a metal-promoting reaction and a symmetry-defined stereochemistry.

Effect of a κ1-Bonded-M-1,2,3-triazole (M = Co, Ru) on the Structure and Reactivity of Group 6 Alkoxy (Fischer) Carbenes.

The [3 + 2] cycloaddition of two different metal-bound azides, [(Me4cyclam)CoII(N3)]ClO4 and (η5-C5H5)(dppe)RuII(N3), (dppe = Ph2PCH2CH2PPh2) with Cr(0) and W(0) (ethoxy)(alkynyl) Fischer carbenes has been efficiently used for the preparation of polymetallic metal-carbene complexes. The presence of the κ1-bonded metal triazole causes a significant influence on the electronic properties, structure, and reactivity of this new class of Fischer alkoxycarbenes. For the Ru(II) derivatives, their chemical behavior is considerably influenced by the interaction of the (η5-C5H5)(dppe)RuII-triazole moiety with the empty p-carbene orbital that provokes a noticeable decrease in the electrophilicity of the M═C carbon (manifested by the shielding of the 13C NMR chemical shifts). In turn, in the Co(II) derivatives, the incorporation of the (Me4cyclam)CoII moiety diminishes the aromaticity of the triazole ring and has a marked effect on the energy and distribution of the LUSO orbital, mostly resident on the Co(II) fragment. The almost negligible participation of the carbene moiety in the LUSO makes this position unable to react with nucleophiles. The reactions reported in this work constitute the first examples of [3 + 2] cycloaddition of azides and alkynyl Fischer carbene complexes in solution.

Mono- and Bimetallic Alkynyl Metallocenes and Half-Sandwich Complexes: A Simple and Versatile Synthetic Approach.

A general process for the synthesis of alkynyl mono and dimetallic metallocenes and half-sandwich complexes has been developed. This approach uses the addition of lithium derivatives of sandwich or half-sandwich complexes to arylsulfonylacetylenes. The reaction occurs in two steps (lithiation and anti-Michael addition to alkynylsulfone followed by elimination of the ArSO2 moiety) to form the corresponding mono- or bimetallic alkynes in clearly higher yields, simpler experimental procedures, and more environmentally benign conditions than those of the so far reported for the synthesis of this type of products. The electrochemical properties of the newly obtained complexes have also been studied.

Silver(I)-Catalyzed Addition of Phenols to Alkyne Cobalt Cluster Stabilized Carbocations.

A smooth catalytic method to use phenols as the nucleophilic partner in the Nicholas reaction has been developed. The method uses either Ag(I) or Au(I) catalysts with AgClO4 or AgBF4 as the most efficient catalysts tested. Neither additional additives nor cocatalysts were required and the formation of the corresponding phenol adducts occurred in excellent yields. The process has the single limitation of the inability of less nucleophilic phenols (4-nitrophenol) to generate the corresponding adducts. Additionally, the reaction is highly diastereoselective. DFT calculations allow a catalytic cycle to be proposed that involves trimetallic intermediates; the rate-determining step of the reaction is hydroxy-group elimination in a cobalt-silver trimetallic intermediate.

2-Azetidinones as Precursors of Pincer Ligands: Preparation, Structure, and Spectroscopic Properties of CC'N-Osmium Complexes.

A metal-promoted degradation of 2-azetidinones to afford CC'N-pincer ligands is reported. The hexahydride complex OsH6(P(i)Pr3)2 (1) reacts with (±)-cis-1-(4-methoxyphenyl)-3-phenoxy-4-(pyridin-2-yl)azetidin-2-one (I), (±)-cis-1-(4-methoxyphenyl)-3-phenoxy-4-(isoquinolin-2-yl)azetidin-2-one (II), and (±)-cis-1-(4-methoxyphenyl)-3-phenoxy-4-(quinolin-2-yl)azetidin-2-one (III) to give the respective OsH2(P(i)Pr3)2(CC'N) (2-4) complexes, which add HBF4·OEt2 to yield [OsH2(P(i)Pr3)2(CC″N)]BF4 (5-7). These salts are the result of the addition of the proton of the acid to the dianionic CC'N-pincer ligand. The hydride ligands of these compounds undergo quantum mechanical exchange coupling, which has been experimentally quantified according to a two-dimensional harmonic oscillator model, where Jex is determined by the separation between the hydrides, their hard sphere radius, and a ν parameter describing the H-M-H vibrational wag mode allowing the movement along the H-H vector. The comparison of the results reveals that the phenomenon is particularly intense for 5-7. Furthermore, in these compounds, the separation between the hydrides is ∼0.1 Å shorter than in the respective neutral species 2-4, whereas the hydride hard sphere radius increases by ∼10%, and the ν value decreases by ∼20%.

Mechanistic Insight into the Facilitation of ß-Lactam Fragmentation through Metal Assistance.

The mechanism of OsH6(PiPr3)2-mediated fragmentation of a 4-(2 pyridyl)-2-azetidinone has been investigated by DFT calculations. The addition of the C4-H bond of the substrate to OsH2(PiPr3)2 allows the active participation of an osmium lone pair in the B-type ß-lactam fragmentation process. This new mechanism makes the N1-C4/C2-C3 fragmentation of the lactamic core thermally accessible through a stepwise process.

A versatile synthesis of ß-lactam-fused oxacycles through the palladium-catalyzed chemo-, regio-, and diastereoselective cyclization of allenic diols.

Chemo-, regio- and stereocontrolled palladium-catalyzed preparations of enantiopure morpholines, oxocines, and dioxonines have been developed starting from 2-azetidinone-tethered γ,δ-, δ,ε-, and ε,ζ-allendiols. The palladium-catalyzed cyclizative coupling reaction of γ,δ-allendiols 2 with allyl bromide or lithium bromide was effective as 8-endo cyclization by attack of the primary hydroxy group to the terminal allene carbon to afford enantiopure functionalized oxocines; whereas the palladium-catalyzed cyclizative coupling reaction of 2-azetidinone-tethered ε,ζ-allendiols 4 furnished dioxonines 16 through a totally chemo- and regioselective 9-endo oxycyclization. By contrast, the palladium-catalyzed cyclizative coupling reaction of 2-azetidinone-tethered δ,ε-allendiols 3 with aryl and alkenyl halides exclusively generated six-membered-ring compounds 14 a and 15 a. These results could be explained through a 6-exo cyclization by chemo- and regiospecific attack of the secondary hydroxy group to the internal allene carbon. Chemo- and regiocontrol issues are mainly influenced by the length of the tether rather than by the nature of the metal catalysts and substituents. This reactivity can be rationalized by means of density functional theory calculations.

Synthesis of new bioorganometallic Ir- and Rh-complexes having ß-lactam containing ligands.

The synthesis (and full spectroscopic and crystallographic characterization) of new classes of bioorganometallic Ir- and Rh-complexes having ß-lactam containing ligands has been achieved in three steps starting from simple precursors. The procedure for preparing these bioorganometallic compounds uses ß-lactams having a phenylpyridyl moiety attached to the C4, N1 or C4 and N1 positions simultaneously, and a directed C-H metal-insertion, in the presence of (MCp*Cl2)2 (M = Ir, Rh). Enantiomerically pure 2-azetidinones can be transformed into diastereomeric (at the metal) mixtures of enantiopure metalla-2-azetidinones. Bimetallic 2-azetidinones are also accessible by this approach. The insertion of electron-poor alkynes into the M-C bond of the bioorganometallic complex occurs regioselectively and in excellent yields. Overall, the sequence imine-ß-lactam-metalla-ß-lactam is a versatile and efficient full methodology to prepare and functionalize unprecedented, novel Ir- and Rh-complexes having ß-lactam containing ligands.

Towards a general synthesis of 3-metal-substituted ß-lactams.

Joining metals and antibiotics: Studies towards a general method for the synthesis of ß-lactams that have a metal complex moiety attached to the C3-position are reported (see scheme). The cis/trans selectivity of the reactions ranges from low in complexes containing the alkyne moiety joined directly to the cyclopentadienyl ring to complete when the metal moiety is separated from the reactive alkyne by an alkynyl-aryl fragment.

The "click" reaction involving metal azides, metal alkynes, or both: an exploration into multimetal structures.

Cu(I) -catalyzed 1,3-cycloaddition of azides and alkynes (CuAAC) is one of the most powerful synthetic methodologies known. However, its use to prepare well-defined multimetallic structures is underdeveloped. Apart from the applications of this reaction to anchor different organometallic reagents to surfaces, polymers, and dendrimers, only isolated examples of CuAAC with metal-η(1) -alkyne and metal-azide complexes to prepare multimetal entities have been reported. This concept sketches the potential of these reactions not only to prepare "a la carte" multimetal 1,2,3-triazole derivatives, but also to discover new and unprecedented reactions.

A click approach to polymetallic chromium(0) and tungsten(0) Fischer-carbene complexes and their use in the synthesis of functionalized polymetallic metal-carbene complexes.

Alkynylamino Cr(0) and W(0) Fischer carbenes undergo a CuAAC reaction with a diverse range of di-, tri-, and tetra-azides to produce polymetallic chromium(0) and tungsten(0) (Fischer)-carbene complexes in good-to-excellent yields. This method is simple, versatile, and is suitable for the preparation of a diverse range of structures with a high level of symmetry. Moreover, the resulting polymetallic carbene complexes are suitable partners for the peripheral functionalization of the metal nuclei, whilst retaining the metal fragment. This fact has been demonstrated in a simultaneous Pauson-Khand reaction, which, in some cases, allows for the generation of four bicyclic [5,5] rings on the periphery of a tetrametallic molecule in a process that involves the formation of 12 new C-C bonds, with four simultaneous CO-insertion processes. The electrochemistry of the polymetallic Fischer carbenes show completely independent behavior for each nucleus, as well as an anomalous observation of the reversible oxidation of the allyl substituents, which has not been reported before in this class of chemistry.

Synthesis of ferrocene tethered open and macrocyclic bis-beta-lactams and bis-beta-amino acid derivatives.

New bioorganometallic ferrocene derivatives are synthesized through a Diversity Oriented Synthesis strategy. Easily available ferrocene bisimines have been transformed into open ferrocenyl bis-beta-lactams. These compounds have demonstrated to be versatile synthons used in further transformations into new ferrocene bis-beta-amino acids. Carefully selected substituents submitted to ring closing metathesis (RCM) and Cu-catalyzed oxidative alkyne coupling conditions have also allowed the conversion of open substrates into ferrocenic macrocyclic bis-beta-lactams.

Arene-chromium tricarbonyl complexes in the Pauson-Khand reaction.

[reactions: see text] We show the use of arene-chromium tricarbonyl complexes in intra- and intermolecular Pauson-Khand reactions. Both styrene and ethynylbenzene complexes react with alkynes and olefins. The synthesis of enynes connected through chromium-complexed aromatic rings is developed. The intramolecular Pauson-Khand reaction occurs in a totally diastereoselective manner.

Tandem enyne metathesis-Diels-Alder reaction for construction of natural product frameworks.

Enynes connected through aromatic rings are used as substrates for metathesis reactions. The reactivity of three ruthenium carbene complexes is compared. The resulting 1,3-dienes are suitable precursors of polycyclic structures via a Diels-Alder process. Some domino RCM-Diels-Alder reactions are performed, suggesting a possible beneficial effect of the ruthenium catalyst in the cycloaddition process. Other examples require Lewis acid cocatalyst. When applied to aromatic ynamines or enamines, a new synthesis of vinylindoles is achieved. Monitorization of several metathesis reactions with NMR shows the different behavior for ruthenium catalysts. New carbenic species are detected in some reactions with an important dependence on the solvent used.