Vinylidene Compounds from the Reactions of Me3SiC⋮CSiMe3 with TpMe2Ir Precursors. Protonation to Alkylidene and Iridabenzene Structures

Isomerization of a cis-(2-Borylalkenyl)Gold Complex via a Retro-1,2-Metalate Shift: Cleavage of a C-C/C-Si Bond trans to a C-Au Bond

This manuscript describes the first example of an alkyne insertion to the Au-B bond of a di(o-tolyl)borylgold complex to afford a cis-2-borylalkenylgold complex, and its isomerization to result in interchanging substituents on the alkenyl carbon atom and the boron atom. The former reaction is the first example of an alkyne insertion to a Au-B bond. In the latter reaction, the regiochemistry of the isomerized alkenylgold products varied depending on the substituents. DFT calculations revealed the formation of gold alkynylborates as a common intermediate via a "retro-1,2-metalate shift", which can be considered as an anti-β-carbon/silicon elimination, and identified a subsequent 1,2-metalate shift as the regiochemistry-determining step. Relative energies of the transition states to each isomer and natural-bond-orbital (NBO) analyses were used to clearly rationalize the regiochemistry of the products.

Metallaaromatic Chemistry: History and Development

Since the prediction of the existence of metallabenzenes in 1979, metallaaromatic chemistry has developed rapidly, due to its importance in both experimental and theoretical fields. Now six major types of metallaromatic compounds, metallabenzenes, metallabenzynes, heterometallaaromatics, dianion metalloles, metallapentalenes and metallapentalynes (also termed carbolongs), and spiro metalloles, have been reported and extensively studied. Their parent organic analogues may be aromatic, non-aromatic, or even anti-aromatic. These unique systems not only enrich the large family of aromatics, but they also broaden our understanding and extend the concept of aromaticity. This review provides a comprehensive overview of metallaaromatic chemistry. We have focused on not only the six major classes of metallaaromatics, including the main-group-metal-based metallaaromatics, but also other types, such as metallacyclobutadienes and metallacyclopropenes. The structures, synthetic methods, and reactivities are described, their applications are covered, and the challenges and future prospects of the area are discussed. The criteria commonly used to judge the aromaticity of metallaaromatics are presented.

Isolation of a C3-metalated indolizine complex and a phosphonium ring-fused bicyclic metallafuran from the osmium-induced transformation of pyridine-tethered alkynes

The first C3-metalated indolizine complex was prepared from the reaction between cis-[Os(dppm)2Cl2] (dppm = 1,1-bis(diphenylphosphino)methane) and propargylic pyridine, HC[triple bond, length as m-dash]CC(OH)(2-py)2. A phosphonium ring-fused bicyclic osmafuran complex was also prepared from the reaction between cis-[Os(dppm)2Cl2] and pyridyl ynone, HC[triple bond, length as m-dash]C(C[double bond, length as m-dash]O)(2-py). The formation of these two products revealed the intermediacy of metal-vinylidene species regarding [Os(dppm)2Cl]+-mediated alkyne transformations. Comparison of the d6 transition-metal precursors employed to activate HC[triple bond, length as m-dash]CC(OH)(2-py)2 suggests that precursors with higher π-basicity favor the vinylidene-involving pathway.

Intermolecular Dehydrative Coupling and Intramolecular Cyclization of Ruthenium Vinylidene Complexes Formed from Aromatic Propynes Containing Carbonyl Functionalities

A remarkable intermolecular dehydrative coupling reaction with the formation of a C-C bond was found for the vinylidene complex 2 a, yielding the dinuclear bisvinylidene complex 4 a. Complex 2 a containing 1-hydroxyindan moiety was first formed from the reaction of o-propynyl benzaldehyde 1 a with [Ru]-Cl ([Ru]=Cp(PPh₃ )₂ Ru) by a cyclization process. For analogous aldehyde 1 b containing an additional 1,3-dioxolane group on the aryl ring, similar intermolecular coupling yields the dinuclear bisvinylidene complex 4 b. However, the fluoro group on the aryl ring in aldehyde 1 c inhibits the coupling reaction, giving only the vinylidene complex 2 c. For the reactions of [Ru]-Cl in MeOH with compounds 1 f, 1 g and 1 h, each with a ketone functionality, cyclization gives vinylidene complexes 2 f, 2 g and 2 h, respectively. However, no similar intermolecular coupling was observed, instead, the intramolecular dehydration yields 8 f, 8 g and 8 h, respectively. In CDCl₃ , catalytic cyclization is observed for the o-propynylphenyl ketone 1 h with [Ru]-Cl at 50 °C giving the isochromene products 14 h. Furthermore, treatment of the o-propynylaryl α,β-unsaturated ketones 1 k-m and 1 n with [Ru]-Cl in MeOH affords the corresponding vinylidene complexes 10 k-m and 11 n each with 1-benzosuberone moiety in the presence of NH₄ PF₆ . These intramolecular cyclization products were formed by the addition of Cβ onto the terminal carbon of the alkene moiety. All these reaction products were characterized by spectroscopic methods. In addition, structures of complexes 4 a, and 10 l were confirmed by single crystal X-ray diffraction analysis.

Facile synthesis of polycyclic metallaarynes

Metalla-analogs of polycyclic arynes represent an interesting class of metallaaromatics with a formal M[triple bond, length as m-dash]C bond within the ring. The first examples of a bicyclic β-metallaaryne and tricyclic metallaarynes, including a metallaanthracyne and a metallaphenanthryne, were obtained in good yields by reactions of OsCl2(PPh3)3 with alkyne-functionalized phosphorus ylides.

Recent Advances in Metallaaromatic Chemistry

Metallaaromatics can be broadly defined as aromatic compounds in which one of the ring atoms is a transition metal. The metallabenzenes are one important class of these compounds that has undergone extensive study recently. Closely related species such as fused-ring metallabenzenes, heterometallabenzenes, π-coordinated metallabenzenes and metallabenzynes have also attracted considerable attention. Although many metallaaromatics can be considered as metalla-analogues of classic organic aromatic compounds, this is not always the case. Recent seminal studies have shown that metallapentalenes and metallapentalynes, which are metalla-analogues of the anti-aromatic compounds pentalene and pentalyne, are in fact aromatic and highly stable. Very unusual spiro-metallaaromatic compounds have also recently been isolated. In this concepts article, key features of all these intriguing metallaaromatic compounds are discussed with reference to the structural, spectroscopic, reactivity and theoretical studies that have been undertaken. These compounds continue to generate much interest, not only because of the contributions they make to fundamental chemical understanding, but also because of the promise of possible practical applications.

Ruthenium-Catalyzed Cycloisomerization of 2-Alkynylanilides: Synthesis of 3-Substituted Indoles by 1,2-Carbon Migration

We developed ruthenium-catalyzed cycloisomerization of alkynylanilides that gave 3-substituted indoles in high yields. The reaction proceeded via the disubstituted vinylidene ruthenium complex that was formed by the 1,2-carbon migration.

Amide-Directed Formation of Five-Coordinate Osmium Alkylidenes from Alkynes

The amide-directed synthesis of five-coordinate osmium alkylidene derivatives from alkynes is reported. These types of complexes, which have been elusive until now because of the tendency of osmium to give hydride alkylidyne species, are prepared by reaction of the dihydride OsH₂Cl₂(PⁱPr₃)₂ (1) with terminal alkynes containing a distal amide group. Complex 1 reacts with N-phenylhex-5-ynamide and N-phenylhepta-6-ynamide to give OsCl₂{=C(CH₃)(CH₂)_nNH(CO)Ph}(PⁱPr₃)₂ (n = 3 (2), 4 (3)). The relative position of carbonyl and NH groups in the organic substrates has no influence on the reaction. Thus, treatment of 1 with N-(pent-4-yn-1-yl)benzamide leads to OsCl₂{=C(CH₃)(CH₂)₃NHC(O)Ph}(PⁱPr₃)₂ (4). The new compounds are intermediate species in the cleavage of the C–C triple bond of the alkynes. Under mild conditions, they undergo the rupture of the C_α–CH₃ bond of the alkylidene, which comes from the alkyne triple bond, to afford six-coordinate hydride–alkylidyne derivatives. In dichloromethane, complex 2 gives a 10:7 mixture of OsHCl₂{≡C(CH₂)₃C(O)NHPh}(PⁱPr₃)₂ (5) and OsHCl₂{≡CCH(CH₃)(CH₂)₂C(O)NHPh}(PⁱPr₃)₂ (6). The first complex contains a linear separation between the alkylidyne C_α atom and the amide group, whereas the spacer is branched in the second complex. In contrast to the case for 2, complex 4 selectively affords OsHCl₂{≡C(CH₂)₃NHC(O)Ph}(PⁱPr₃)₂ (7). In spite of their instability, these compounds give the alkylidene–allene metathesis, being a useful entry to five-coordinate vinylidene complexes, including the dicarbon-disubstituted OsCl₂(=C=CMe₂)(PⁱPr₃)₂ (8) and the monosubstituted OsCl₂(=C=CHCy)(PⁱPr₃)₂ (9).

Synthesis of Ir(III) complexes with Tp(Me2) and acac ligands and their reactivity with electrophiles

The reaction of the bis(ethylene) complex [Tp(Me2)Ir(C2H4)2] () (Tp(Me2) = hydrotris(3,5-dimethylpyrazolyl)borate) with an excess of 2,4-pentanedione (acetylacetone, Hacac) at 70 °C produced a mixture of the Ir(iii) complex [Tp(Me2)Ir(acac)(C2H5)] () as a major product (67% yield) and two other side complexes [Tp(Me2)Ir(acac)(H)] () and [Tp(Me2)Ir(C9H14O2)] () in 20 and 13% yields, respectively. According to the proposed reaction mechanism and DFT calculations, complexes and are generated from an 18e(-) intermediate [Tp(Me2)Ir(C2H4)(acac)(C2H3)] () which undergoes either hydrogen insertion or β-hydride elimination followed by the subsequent loss of a molecule of ethylene. The lowest yielding complex which features a 2-iridafuran is presumably generated from an unusual thermal coupling between one vinylic and one acac moiety. The availability of the acidic α-proton of the acac ligand was tested by the treatment of complex with Br2 and Cu(NO3)2 rendering the substitution complexes [Tp(3-Br,Me2)Ir(3-Br-acac)Br] () and [Tp(Me2)Ir(3-NO2-acac)(C2H5)] () in good yields. The series of heteroleptic iridium(iii) compounds display air and moisture stability and have been characterized by NMR, IR, and elemental analyses, and, in the case of , and , by single-crystal X-ray diffraction analyses.

Redox-induced umpolung of transition metal carbenes

Metal carbene complexes have been at the forefront of organic and organometallic synthesis and are instrumental in guiding future sustainable chemistry efforts. While classical Fischer and Schrock type carbenes have been intensely studied, compounds that do not fall within one of these categories have attracted attention only recently. In addition, applications of carbene complexes rarely take advantage of redox processes, which could open up a new dimension for their use in practical processes. Herein, we report an umpolung of a nucleophilic palladium carbene complex, [{PC(sp²)P} ^tBuPd(PMe₃)] ({PC(sp²)P} ^tBu = bis[2-(di-iso-propylphosphino)-4-tert-butylphenyl]methylene), realized by successive one-electron oxidations that generated a cationic carbene complex, [{PC(sp²)P} ^tBuPdI]⁺, via a carbene radical, [{PC˙(sp²)P} ^tBuPdI]. An EPR spectroscopic study of [{PC˙(sp²)P} ^tBuPdI] indicated the presence of a ligand-centered radical, also supported by the results of reactions with 9,10-dihydroanthracene and PhSSPh. The cationic carbene complex shows electrophilic behavior toward nucleophiles such as NaH, ^p TolNHLi, PhONa, and PMe₃, resulting from an inversion of the electronic character of the Pd-C_carbene bond in [{PC(sp²)P} ^tBuPd(PMe₃)]. The redox induced umpolung is reversible and unprecedented.

Organometallic chemistry of ethynyl boronic acid MIDA ester, HCCB(O2CCH2)2NMe

The reactions of HCCBMIDA (BMIDA = B(O₂CCH₂)₂NMe) with a range of ruthenium complexes examples of σ-alkynyl, σ-alkenyl and vinylidene complexes bearing 4-coordinate boron substituents.

Building a parent iridabenzene structure from acetylene and dichloromethane on an iridium center

Parenthood: The reaction of [TpIr(C2H4)2] (1) (Tp=hydrotris(pyrazolyl)borate) with acetylene in CH2 Cl2 affords a 1:1 mixture of the "parent" metallabenzene 2 (that is, all the ring carbon centers are CH units) and the β-Cl substituted vinyl species 3. Generation of 2 is by the coupling of an iridacyclopentadiene (formed from two acetylene molecules at the Ir center) with the dichloromethane-derived chlorocarbene ":C(H)Cl" and a subsequent α-Cl elimination event.

Electrophilic substitution reactions of metallabenzynes

The electrophilic substitution reactions of metallabenzynes Os(≡CC(R)═C(CH(3))C(R)═CH)Cl(2)(PPh(3))(2) (R = SiMe(3), H) were studied. These metallabenzynes react with electrophilic reagents, including Br(2), NO(2)BF(4), NOBF(4), HCl/H(2)O(2), and AlCl(3)/H(2)O(2) to afford the corresponding bromination, nitration, nitrosation, and chlorination products. The reactions usually occur at the C2 and C4 positions of the metallacycle. These observations support the notion that metallabenzynes exhibit aromatic properties.

New highly stable metallabenzenes via nucleophilic aromatic substitution reaction

Treatment of the ruthenabenzene [Ru{CHC(PPh(3))CHC(PPh(3))CH}Cl(2)(PPh(3))(2)]Cl (1) with excess 8-hydroxyquinoline in the presence of CH(3)COONa under air atmosphere produced the S(N)Ar product [(C(9) H(6)NO)Ru{CHC(PPh(3))CHC(PPh(3))C}(C(9)H(6)NO)(PPh(3))]Cl(2) (3). Ruthenabenzene 3 could be stable in the solution of weak alkali or weak acid. However, reaction of 3 with NaOH afforded a 7:1 mixture of ruthenabenzenes [(C(9)H(6)NO)Ru{CHC(PPh(3))CHCHC}(C(9)H(6)NO)(PPh(3))]Cl (4) and [(C(9)H(6)NO)Ru{CHCHCHC(PPh(3))C}(C(9)H(6)NO)(PPh(3))]Cl (5), presumably involving a P-C bond cleavage of the metallacycle. Complex 3 was also reactive to HCl, which results in a transformation of 3 to ruthenabenzene [Ru{CHC(PPh(3))CHC(PPh(3))C}Cl(2)(C(9)H(6)NO)(PPh(3))]Cl (6) in high yield. Thermal stability tests showed that ruthenabenzenes 4, 5, and 6 have remarkable thermal stability both in solid state and in solution under air atmosphere. Ruthenabenzenes 4 and 5 were found to be fluorescent in common solvents and have spectral behaviors comparable to those organic multicyclic compounds containing large π-extended systems.