Ruthenium Tris(pyrazolyl)borate Complexes. 1. Synthesis and Reactivity of Ru(HB(pz)3)(COD)X (X = Cl, Br) and Ru(HB(pz)3)(L2)Cl (L = Nitrogen and Phosphorus Donor Ligands)

Titanium-catalyzed highly stereoselective anti-Markovnikov intermolecular hydroalkoxylation of alkynes to prepare Z-enol ethers

Enol ethers are essential synthetic frameworks and widely applied in organic synthesis; however, high regio- and stereo-selective access to enol ethers remains challenging. Herein, we report a titanium-catalyzed stereospecific anti-Markovnikov hydroalkoxylation reaction of alkynes for the synthesis of Z-enol ethers with excellent functional group tolerance and yields. Mechanistic studies showed that the titanium coordinates with the alkyne and then an oxygen anion attacks the π-bond of the alkyne from the backside to provide a trans-oxygen metallation intermediate, which accounts for the high Z-stereoselectivity. Furthermore, Z-enol ethers could be applied as a kind of synthon for late-stage transformations and gram-scale synthesis, which demonstrates their potential value in organic synthesis.

Synthesis of Fulvene Vinyl Ethers by Gold Catalysis

Gold-catalyzed cyclization of 1,5-diynes with ketones as reagents and solvent provides diversely substituted vinyl ethers under mild conditions. The regioselectivity of such gold-catalyzed cyclizations is usually controlled by the scaffold of the diyne. Herein, we report the first solvent-controlled switching of regioselectivity from a 6-endo-dig- to 5-endo-dig-cyclization in these transformations, providing fulvene derivatives. With respect to the functional-group tolerance, aryl fluorides, chlorides, bromides, and ethers are tolerated. Furthermore, the mechanism and selectivity are put to scrutiny by experimental studies and a thermodynamic analysis of the product. Additionally, 6-(vinyloxy)fulvenes are a hitherto unknown class of compounds. Their reactivity is briefly evaluated, to give insights into their potential applications.

Transition Metal Vinylidene- and Allenylidene-Mediated Catalysis in Organic Synthesis

With their mechanistic novelty and various modalities of reactivity, transition metal unsaturated carbene (alkenylidene) complexes have emerged as versatile intermediates for new reaction discovery. In particular, the past decade has witnessed remarkable advances in the chemistry of metal vinylidenes and allenylidenes, leading to the evolution of a diverse array of new catalytic transformations that are mechanistically distinct from those developed in the previous two decades. This review aims to provide a survey of the recent achievements in the development of organic reactions that make use of transition metal alkenylidenes as catalytic intermediates and their applications to organic synthesis.

Hydrophenoxylation of internal alkynes catalysed with a heterobimetallic Cu-NHC/Au-NHC system

A dual catalytic process based on a heterobimetallic system, consisting of Cu-NHC and Au-NHC complexes, was shown to be highly active and selective for the hydrophenoxylation of internal alkynes.

Reactions of a Ruthenium Complex with Substituted N-Propargyl Pyrroles

In an investigation into the chemical reactions of N-propargyl pyrroles 1 a-c, containing aldehyde, keto, and ester groups on the pyrrole ring, with [Ru]-Cl ([Ru]=Cp(PPh3 )2 Ru; Cp=C5 H5 ), an aldehyde group in the pyrrole ring is found to play a crucial role in stimulating the cyclization reaction. The reaction of 1 a, containing an aldehyde group, with [Ru]-Cl in the presence of NH4 PF6 yields the vinylidene complex 2 a, which further reacts with allyl amine to give the carbene complex 6 a with a pyrrolizine group. However, if 1 a is first reacted with allyl amine to yield the iminenyne 8 a, then the reaction of 8 a with [Ru]-Cl in the presence of NH4 PF6 yields the ruthenium complex 9 a, containing a cationic pyrrolopyrazinium group, which has been fully characterized by XRD analysis. These results can be adequately explained by coordination of the triple bond of the propargyl group to the ruthenium metal center first, followed by two processes, that is, formation of a vinylidene intermediate or direct nucleophilic attack. Additionally, the deprotonation of 2 a by R4 NOH yields the neutral acetylide complex 3 a. In the presence of NH4 PF6 , the attempted alkylation of 3 a resulted in the formation the Fischer-type amino-carbene complex 5 a as a result of the presence of NH3, which served as a nucleophile. With KPF6, the alkylation of 3 a with ethyl and benzyl bromoacetates afforded the disubstituted vinylidene complexes 10 a and 11 a, containing ester groups, which underwent deprotonation reactions to give the furyl complexes 12 a and 13 a, respectively. For 13 a, containing an O-benzyl group, subsequent 1,3-migration of the benzyl group was observed to yield product 14 a with a lactone unit. Similar reactivity was not observed for the corresponding N-propargyl pyrroles 1 b and 1 c, which contained keto and ester groups, respectively, on the pyrrole ring.

Computational study of the mechanism and selectivity of ruthenium-catalyzed hydroamidations of terminal alkynes

Density functional theory calculations were performed to elucidate the mechanism of the ruthenium-catalyzed hydroamidation of terminal alkynes, a powerful and sustainable method for the stereoselective synthesis of enamides. The results provide an explanation for the puzzling experimental finding that with tri-n-butylphosphine (P(Bu)3) as the ligand, the E-configured enamides are obtained, whereas the stereoselectivity is inverted in favor of the Z-configured enamides with (dicyclohexylphosphino)methane (dcypm) ligands. Using the addition of pyrrolidinone to 1-hexyne as a model reaction, various pathways were investigated, among which a catalytic cycle turned out to be most advantageous for both ligand systems that consists of: (a) oxidative addition, (b) alkyne coordination, (c) alkyne insertion (d) vinyl-vinylidene rearrangement, (e) nucleophilic transfer and finally (f) reductive elimination. The stereoselectivity of the reaction is decided in the nucleophilic transfer step. For the P( n Bu)3 ligand, the butyl moiety is oriented anti to the incoming 2-pyrolidinyl unit during the nucleophilic transfer step, whereas for the dcypm ligand, steric repulsion between the butyl and cyclohexyl groups turns it into a syn orientation. Overall, the formation of E-configured product is favorable by 4.8 kcal mol-1 (Δ‡GSDL) for the catalytic cycle computed with P(Bu)3 as ancillary ligand, whereas for the catalytic cycle computed with dcypm ligands, the Z-product is favored by 7.0 kcal mol-1 (Δ‡GSDL). These calculations are in excellent agreement with experimental findings.

Stereoselective Gold(I)-Catalyzed Intermolecular Hydroalkoxlation of Alkynes

We report the use of cationic gold complexes [Au(NHC)(CH₃CN)][BF₄] and [{Au(NHC)}₂(μ-OH)][BF₄] (NHC = N-heterocyclic carbene) as highly active catalysts in the solvent-free hydroalkoxylation of internal alkynes using primary and secondary alcohols. Using this simple protocol, a broad range of (Z)-vinyl ethers were obtained in excellent yields and high stereoselectivities. The methodology allows for the use of catalyst loadings as low as 200 ppm for the addition of primary alcohols to internal alkynes (TON = 35 000, TOF = 2188 h^-1).

cis-1,3,5-Triaminocyclohexane as a facially capping ligand for ruthenium(II)

Reaction of cis-[RuCl2(DMSO-S)3(DMSO-O)] with cis-1,3,5-triaminocyclohexane (tach) results in the formation of [RuCl(tach)(DMSO-S)2]Cl, a valuable precursor for a wide range of other tach-containing Ru complexes. Reaction of [RuCl(tach)(DMSO-S)2]Cl with the chelating nitrogen-based ligands (N-N = bipyridine, phenanthroline, and ethylenediamine) affords [Ru(N-N)(DMSO-S)2(tach)][Cl]2. A similar reaction between [RuCl(tach)(DMSO-S)]Cl with the chelating phosphorus-based ligands (P-P = dppm, dppe, dppp, dppb, dppv, and dppben) leads to the formation of [RuCl(P-P)(tach)]Cl. The structures of 10 examples of the tach-containing complexes have been determined by single crystal X-ray diffraction. An examination of the structural metrics obtained from these studies indicates that the tach ligand is a strong sigma donor. In addition, the presence of the NH2 groups in the tach ligand allow for participation in hydrogen bonding further modulating the coordinative properties of the ligand.

Picolyl-NHC hydrotris(pyrazolyl)borate ruthenium(II) complexes: synthesis, characterization, and reactivity with small molecules

Ruthenium(II) hydrotris(pyrazolyl)borate chloro complexes bearing picolyl-functionalized N-heterocyclic carbenes [TpRu(κ(2)-C,N-picolyl-(R)I)Cl] (picolyl-(Me)I = 3-methyl-1-(2-picolyl)imidazol-2-ylidene) (1a), picolyl-(iPr)I = 3-isopropyl-1-(2-picolyl)imidazol-2-ylidene (1b), picolyl-(Me)45DClI = 3-methyl-1-(2-picolyl)-4,5-dichloroimidazol-2-ylidene (1c), picolyl-(Ph)I = 3-phenyl-1-(2-picolyl)imidazol-2-ylidene (1d), picolyl-(Me)BI = 3-methyl-1-(2-picolyl)benzoimidazol-2-ylidene (1e)) have been synthesized and characterized. Furthermore, cationic carbonyl derivatives 2a-e have been prepared, characterized, and used to study the donor properties of the picolylcarbene ligands (picolyl-(R)I) via infrared spectroscopy. Also, the reactivity of the 16-electron species [TpRu(κ(2)-C,N-picolyl-(R)I)](+), in situ generated using NaBAr(F)4 (Ar(F) = 3,5-bis(trifluoromethyl)phenyl) as a halide scavenger, toward N2, CH3CN, H2, CH2CH2, S8, and O2 was studied indicating a strong influence of the NHC wingtip and backbone substituents in the product distribution. The crystal structures of [TpRu(κ(2)-C,N-picolyl-(iPr)I)Cl] (1b), [TpRu(κ(2)-C,N-picolyl-(Me)I)CO][BAr(F)4] (2a), [TpRu(κ(2)-C,N-picolyl-(Ph)I)CO][BAr(F)4] (2d), [{TpRu(κ(2)-C,N-picolyl-(Me)I}2(μ-N2)][BAr(F)4]2 (3'a), [{TpRu(κ(2)-C,N-picolyl-(Ph)I)}2(μ-N2)][BAr(F)4]2 (3'd), [TpRu(κ(2)-C,N-picolyl-(iPr)I)(η(2)-CH2CH2)][BAr(F)4] (5b), and [{TpRu(κ(2)-C,N-picolyl-(Me)I)}2(μ-S2)][BAr(F)4]2 (6) are reported.

Chlorido[hydridotris(pyrazol-1-yl-κN)borato](1H-pyrazole-κN)(triphenyl-phosphine-κP)ruthenium(II)

In the title compound, [Ru(C(9)H(10)BN(6))Cl(C(3)H(4)N(2))(C(18)H(15)P)], the Ru(II) atom is coordinated by an N,N',N''-tridentate hydrido-trispyrazolylborate (Tp) ligand, a pyrazole (HPz) mol-ecule, a chloride ion and a triphenyl-phosphine ligand, resulting in a distorted RuClPN(4) octa-hedral coordination for the metal ion: the tridentate N atoms occupy one octa-hedral face and the Cl and P atoms are cis. One of the phenyl rings is disordered over two orientations in a 0.547 (10):0.453 (10) ratio, and a weak intra-molecular N-H⋯Cl hydrogen bond generates an S(5) ring.

(Benzonitrile-κN)chlorido[hydridotris(pyrazol-1-yl-κN2)borato](triphenylphosphine-κP)ruthenium(II) ethanol solvate

The reaction of [Ru(C₉H₁₀BN₆)Cl(C₁₈H₁₅P)₂] with benzo­nitrile leads to crystals of the title compound, [Ru(C₉H₁₀BN₆)Cl(C₁₈H₁₅P)(C₇H₅N)]·C₂H₅OH. In the crystal structure, the environment about the ruthenium metal center corresponds to a slightly distorted octa­hedron with an average N—Ru—N bite angle of the Tp ligand of 86.6 (2)°.

(Dithio-benzoato-κS,S')[hydridotris(pyrazol-1-yl-κN)borato](triphenyl-phosphine-κP)ruthenium(II)

Reaction of [Ru(Tp)Cl(PPh(3))(2)] (Tp = hydridotrispyrazolyl-borate) with ammonium dithio-benzoate in methanol leads to the formation of the title compound, [Ru(C(9)H(10)BN(6))(C(7)H(5)S(2))(C(18)H(15)P)]. In the crystal structure, the Ru atom is coordinated by three N atoms of the Tp ligand, one P atom of the triphenyl-phosphine ligand and the two S atoms of the dithio-benzoate ligand within a slightly distorted octa-hedron. The Ru-S bonds are slightly different [2.321 (1) and 2.396 (1) Å] and the average N-Ru-N angle is 86.31°.

Azido-(1,1-diphenyl-methanimine-κN)[hydridotris(pyrazolyl-κN)borato](triphenyl-phosphine-κP)ruthenium(II) diethyl ether solvate

The reaction of [RuCl(C(9)H(10)BN(6))(C(18)H(15)P)(2)] with benzo-phenone imine in methanol, in the presence of sodium azide, leads to the formation of the title compound, [Ru(C(9)H(10)BN(6))(N(3))(HN=CPh(2))(C(18)H(15)P)]·C(4)H(10)O, which crystallizes as the diethyl ether solvate. In the crystal structure, the Ru atom is coordinated by three N atoms of one hydridotris(pyrazoly)borate anion, one P atom of one triphenyl-phosphine ligand, one N atom of the azide anion and one N atom of the benzophenone-imine ligand in a slightly distorted octa-hedral geometry. The azide anion is almost linear [177.0 (5)°], with an Ru-N-N angle of 125.9 (3)°. There is a small difference between the N-N distances [1.200 (5) and 1.164 (5) Å], the longer bond being adjacent to the Ru atom.

(Benzophenone imine-κN)chlorido(hydridotripyrazolylborato)(triphenylphosphine)ruthenium(II) diethyl ether solvate

The reaction of RuCl(Tp)(Ph₃P)₂, where Tp is [(CH)₃N₂]₃BH, with benzophenone imine leads to the formation of the title compound, [Ru(C₉H₁₀BN₆)Cl(C₁₃H₁₁N)(C₁₈H₁₅P)]·C₄H₁₀O. The environment about the Ru atom corresponds to a slightly distorted octa­hedron and the bite angle of the Tp ligand produces an average N—Ru—N angle of 86.3 (9)°. The three Ru—N(Tp) bond lengths [2.117 (2), 2.079 (2) and 2.084 (2) Å] are slightly longer than the average distance (2.038 Å) in other ruthenium–Tp complexes.

(O,O'-Diethyl dithio-phosphato-κS,S')(hydridotripyrazol-1-ylborato-κN,N,N)(triphenyl-phosphine-κP)ruthenium(II)

Reaction of [Ru(Tp)Cl(PPh(3))(2)] {where Tp is hydridotri-pyrazol-yl-borate, BH[C(3)H(3)N(2))(3))]} with NH(4)[S(2)P(OEt)(2)] in methanol afforded the title compound, [Ru(C(9)H(10)BN(6))(C(4)H(10)O(2)PS(2))(C(18)H(15)P)], in which the Ru(II) ion is in a slightly disorted octa-hedral coordination environment. The [S(2)P(OEt)(2)](-) ligand coordinates in a chelating mode with two similar Ru-S bond lengths and a slightly acute S-Ru-S angle. The atoms of both -OCH(2)CH(3) groups of the diethyl dithio-phosphate ligand are disordered over two sites with approximate occupancies of 0.76 and 0.24.

Metal vinylidenes as catalytic species in organic reactions

Organic vinylidene species have found limited use in organic synthesis owing to their inaccessibility. In contrast, metal vinylidenes are much more stable and may be readily accessed through transition-metal activation of terminal alkynes. These electrophilic species may be trapped by a number of nucleophiles. Additionally, metal vinylidenes can participate in pericyclic reactions and processes that involve migration of a metal ligand to the vinylidene species. This Focus Review addresses the reactions and applications of metal vinylidenes in organic synthesis.