The Mechanism of Rh(I)-Catalyzed Coupling of Benzotriazoles and Allenes Revisited: Substrate Inhibition, Proton Shuttling, and the Role of Cationic vs Neutral Species

Direct coupling of benzotriazole to unsaturated substrates such as allenes represents an atom-efficient method for the construction of biologically and pharmaceutically interesting functional structures. In this work, the mechanism of the N2-selective Rh complex-catalyzed coupling of benzotriazoles to allenes was investigated in depth using a combination of experimental and theoretical techniques. Substrate coordination, inhibition, and catalyst deactivation was probed in reactions of the neutral and cationic catalyst precursors [Rh(μ-Cl)(DPEPhos)]2 and [Rh(DPEPhos)(MeOH)2]+ with benzotriazole and allene, giving coordination, or coupling of the substrates. Formation of a rhodacycle, formed by unprecedented 1,2-coupling of allenes, is responsible for catalyst deactivation. Experimental and computational data suggest that cationic species, formed either by abstraction of the chloride ligand or used directly, are relevant for catalysis. Isomerization of benzotriazole and cleavage of its N-H bond are suggested to occur by counteranion-assisted proton shuttling. This contrasts with a previously proposed scenario in which oxidative N-H addition at Rh is one of the key steps. Based on the mechanistic analysis, the catalytic coupling reaction could be optimized, leading to lower reaction temperature and shorter reaction times compared to the literature.

Mechanistic Versatility at Ir(PSiP) Pincer Catalysts: Triflate Proton Shuttling from 2-Butyne to Diene and [3]Dendralene Motifs

The five-coordinate hydrido complex [IrH(OTf)(PSiP)] (1) catalytically transforms 2-butyne into a mixture of its isomer 1,3-butadiene, and [3]dendralene and linear hexatriene dimerization products: (E)-4-methyl-3-methylene-1,4-hexadiene and (3Z)-3,4-dimethyl-1,3,5-hexatriene, respectively. Under the conditions of the catalytic reaction, benzene, and 363 K, the hexatriene further undergoes thermal electrocyclization into 2,3-dimethyl-1,3-cyclohexadiene. The reactions between 1 and the alkyne substrate allow isolation or nuclear magnetic resonance (NMR) observation of catalyst resting states and possible reaction intermediates, including complexes with the former PSiP pincer ligands disassembled into PSi and PC chelates, and species coordinating allyl or carbene fragments en route to products. The density functional theory (DFT) calculations guided by these experimental observations disclose competing mechanisms for C–H bond elaboration that move H atoms either classically, as hydrides, or as protons transported by the triflate. This latter role of triflate, previously recognized only for more basic anions such as carboxylates, is discussed to result from combining the unfavorable charge separation in the nonpolar solvent and the low electronic demand from the metal to the anion at coordination positions trans to silicon. Triflate deprotonation of methyl groups is key to release highly coordinating diene products from stable allyl intermediates, thus enabling catalytic cycling.

Interplay between the Directing Group and Multifunctional Acetate Ligand in Pd-Catalyzed anti-Acetoxylation of Unsymmetrical Dialkyl-Substituted Alkynes

The cooperative action of the acetate ligand, the 2-pyridyl sulfonyl (SO₂Py) directing group on the alkyne substrate, and the palladium catalyst has been shown to be crucial for controlling reactivity, regioselectivity, and stereoselectivity in the acetoxylation of unsymmetrical internal alkynes under mild reaction conditions. The corresponding alkenyl acetates were obtained in good yields with complete levels of β-regioselectivity and anti-acetoxypalladation stereocontrol. Experimental and computational analyses provide insight into the reasons behind this delicate interplay between the ligand, directing group, and the metal in the reaction mechanism. In fact, these studies unveil the multiple important roles of the acetate ligand in the coordination sphere at the Pd center: (i) it brings the acetic acid reagent into close proximity to the metal to allow the simultaneous activation of the alkyne and the acetic acid, (ii) it serves as an inner-sphere base while enhancing the nucleophilicity of the acid, and (iii) it acts as an intramolecular acid to facilitate protodemetalation and regeneration of the catalyst. Further insight into the origin of the observed regiocontrol is provided by the mapping of potential energy profiles and distortion–interaction analysis.

Preparation of Neutral trans - cis [Ru(O2CR)2P2(NN)], Cationic [Ru(O2CR)P2(NN)](O2CR) and Pincer [Ru(O2CR)(CNN)P2] (P = PPh3, P2 = diphosphine) Carboxylate Complexes and their Application in the Catalytic Carbonyl Compounds Reduction

The diacetate complexes trans-[Ru(κ¹-OAc)₂(PPh₃)₂(NN)] (NN = ethylenediamine (en) (1), 2-(aminomethyl)pyridine (ampy) (2), 2-(aminomethyl)pyrimidine (ampyrim) (3)) have been isolated in 76-88% yield by reaction of [Ru(κ²-OAc)₂(PPh₃)₂] with the corresponding nitrogen ligands. The ampy-type derivatives 2 and 3 undergo isomerization to the thermodynamically most stable cationic complexes [Ru(κ¹-OAc)(PPh₃)₂(NN)]OAc (2a and 3a) and cis-[Ru(κ¹-OAc)₂(PPh₃)₂(NN)] (2b and 3b) in methanol at RT. The trans-[Ru(κ¹-OAc)₂(P₂)₂] (P₂ = dppm (4), dppe (5)) compounds have been synthesized from [Ru(κ²-OAc)₂(PPh₃)₂] by reaction with the suitable diphosphine in toluene at 95 °C. The complex cis-[Ru(κ¹-OAc)₂(dppm)(ampy)](6) has been obtained from [Ru(κ²-OAc)₂(PPh₃)₂] and dppm in toluene at reflux and reaction with ampy. The derivatives trans-[Ru(κ¹-OAc)₂P₂(NN)] (7-16; NN = en, ampy, ampyrim, 8-aminoquinoline; P₂ = dppp, dppb, dppf, (R)-BINAP) can be easily synthesized from [Ru(κ²-OAc)₂(PPh₃)₂] with a diphosphine and treatment with the NN ligands at RT. Alternatively these compounds have been prepared from trans-[Ru(OAc)₂(PPh₃)₂(NN)] by reaction with the diphosphine in MEK at 50 °C. The use of (R)-BINAP affords trans-[Ru(κ¹-OAc)₂((R)-BINAP)(NN)] (NN = ampy (11), ampyrim (15)) isolated as single stereoisomers. Treatment of the ampy-type complexes 8-15 with methanol at RT leads to isomerization to the cationic derivatives [Ru(κ²-OAc)P₂(NN)]OAc (8a-15a; NN = ampy, ampyrim; P₂ = dppp, dppb, dppf, (R)-BINAP). Similarly to 2, the dipivalate trans-[Ru(κ¹-OPiv)₂(PPh₃)₂(ampy)] (18) is prepared from [Ru(κ²-OPiv)₂(PPh₃)₂] (17) and ampy in CHCl₃. The pincer acetate [Ru(κ¹-OAc)(CNN^OMe)(PPh₃)₂] (19) has been synthesized from [Ru(κ²-OAc)₂(PPh₃)₂] and HCNN^OMe ligand in 2-propanol with NEt₃ at reflux. In addition, the dppb pincer complexes [Ru(κ¹-OAc)(CNN)(dppb)] (CNN = AMTP (20), AMBQ^Ph (21)) have been obtained from [Ru(κ²-OAc)₂(PPh₃)₂], dppb, and HAMTP or HAMBQ^Ph with NEt₃, respectively. The acetate NN and pincer complexes are active in transfer hydrogenation with 2-propanol and hydrogenation with H₂ of carbonyl compounds at S/C values of up to 10000 and with TOF values of up to 160000 h^-1.

DFT calculations bring insight to internal alkyne-to-vinylidene transformations at rhodium PNP- and PONOP-pincer complexes

Through DFT calculations, the equilibrium between Rh–alkyne and Rh–vinylidene species of PXNXP pincer ligated Rh cationic complexes is shown to be tuned by the P–Rh–P bite angle, which in turn is dictated by the nature of the X moiety of the pincer ligand.

Five-Membered Ruthenacycles: Ligand-Assisted Alkyne Insertion into 1,3-N,S-Chelated Ruthenium Borate Species

Building upon previous work, the chemistry of [(η⁶ -p-cymene)Ru{P(OMe)₂ OR}Cl₂ ], (R=H or Me) has been extended with [H₂ B(mbz)₂ ]^- (mbz=2-mercaptobenzothiazolyl) using different Ru precursors and borate ligands. As a result, a series of 1,3-N,S-chelated ruthenium borate complexes, for example, [(κ² -N,S-L)PR₃ Ru{κ³ -H,S,S'-H₂ B(L)₂ }], (2 a-d and 2 a'-d'; R=Ph, Cy, OMe or OPh and L=C₅ H₄ NS or C₇ H₄ NS₂ ) and [Ru{κ³ -H,S,S'-H₂ B(L)₂ }₂ ], (3: L=C₅ H₄ NS, 3': L=C₇ H₄ NS₂ ) were isolated upon treatment of [(η⁶ -p-cymene)RuCl₂ PR₃ ], 1 a-d (R=Ph, Cy, OMe or OPh) with [H₂ B(mp)₂ ]^- or [H₂ B(mbz)₂ ]^- ligands (mp=2-mercaptopyridyl). All the Ru borate complexes, 2 a-d and 2 a'-d' are stabilized by phosphine/phosphite and hemilabile N,S-chelating ligands. Treatment of these Ru borate species, 2 a'-c' with various terminal alkynes yielded two different types of five-membered ruthenacycle species, namely [PR₃ {C₇ H₄ S₂ -(E)-N-C=CH(R')}Ru{κ³ -H,S,S'-H₂ B(L)₂ }], (4-4'; R=Ph and R'=CO₂ Me or C₆ H₄ NO₂ ; L=C₇ H₄ NS₂ ) and [PR₃ {C₇ H₄ NS-(E)-S-C=CH(R')}Ru{κ³ -H,S,S'-H₂ B(L)₂ }], (5-5', 6 and 7; R=Ph, Cy or OMe and R'=CO₂ Me or C₆ H₄ NO₂ ; L=C₇ H₄ NS₂ ). All these five-membered ruthenacycle species contain an exocyclic C=C moiety, presumably formed by the insertion of a terminal alkyne into the Ru-N and Ru-S bonds. The new species have been characterized spectroscopically and the structures were further confirmed by single-crystal X-ray diffraction analysis. Theoretical studies and chemical-bonding analyses established that charge transfer occurs from phosphorus to ruthenium center following the trend PCy₃ <PPh₃ <P(OPh)₃ <P(OMe)₃ .

One catalyst, multiple processes: ligand effects on chemoselective control in Ru-catalyzed anti-Markovnikov reductive hydration of terminal alkynes

The ligand effect through kinetic and thermodynamic control on the chemoselectivity of one-catalyst multi-step catalysis.

Insertion of terminal alkyne into Pt-N bond of the square planar [PtI2(Me2phen)] complex

The reactivity of [PtX₂(Me₂phen)] complexes (X = Cl, Br, I; Me₂phen = 2,9-dimethyl-1,10-phenanthroline) with terminal alkynes has been investigated. Although the dichlorido species [PtCl₂(Me₂phen)] exhibits negligible reactivity, the bromido and iodido derivatives lead in short time to the formation of five-coordinate Pt(ii) complexes of the type [PtX₂(Me₂phen)(η²-CH[triple bond, length as m-dash]CR)] (X = Br, I; R = Ph, n-Bu), in equilibrium with the starting reagents. Similar to analogous complexes with simple acetylene, the five coordinate species can also undergo dissociation of an halido ligand and formation of the transient square-planar cationic species [PtX(Me₂phen)(η²-CH[triple bond, length as m-dash]CR)]⁺. This latter can further evolve to give an unusual, sparingly soluble square planar product where the former terminal alkyne is converted into a :C[double bond, length as m-dash]C(H)(R) moiety with the α-carbon bridging the Pt(ii) core with one of the two N-donors of coordinated Me₂phen. The final product [PtX₂{κ²-N,C-(Z)-N[combining low line]1-N10-C[combining low line][double bond, length as m-dash]C(H)(R)}] (N1-N10 = 2,9-dimethyl-1,10-phenanthroline; X = Br, I) contains a Pt-N-C-C-N-C six-membered chelate ring in a square planar Pt(ii) coordination environment.

Computational Studies of Carboxylate-Assisted C-H Activation and Functionalization at Group 8-10 Transition Metal Centers

Computational studies on carboxylate-assisted C-H activation and functionalization at group 8-10 transition metal centers are reviewed. This Review is organized by metal and will cover work published from late 2009 until mid-2016. A brief overview of computational work prior to 2010 is also provided, and this outlines the understanding of carboxylate-assisted C-H activation in terms of the "ambiphilic metal-ligand assistance" (AMLA) and "concerted metalation deprotonation" (CMD) concepts. Computational studies are then surveyed in terms of the nature of the C-H bond being activated (C(sp²)-H or C(sp³)-H), the nature of the process involved (intramolecular with a directing group or intermolecular), and the context (stoichiometric C-H activation or within a variety of catalytic processes). This Review aims to emphasize the connection between computation and experiment and to highlight the contribution of computational chemistry to our understanding of catalytic C-H functionalization based on carboxylate-assisted C-H activation. Some opportunities where the interplay between computation and experiment may contribute further to the areas of catalytic C-H functionalization and applied computational chemistry are identified.

Rapid Markovnikov addition of HCl to a pendant alkyne: evidence for a quinoidal cumulene

The reaction of the vinylidene cations trans-[Ru(CCH–C₆H₂-2,5-R₂-4-CCH)Cl(dppm)₂]⁺ (R = H, Me) with chloride results in Markovnikov addition of HCl to the terminal alkyne, suggesting the intermediacy of a quinoidal cumulene complex.

Mechanistic insight into the ruthenium-catalysed anti-Markovnikov hydration of alkynes using a self-assembled complex: a crucial role for ligand-assisted proton shuttle processes

A combined computational and experimental study into the mechanism of the anti-Markovnikov hydration of phenylacetylene by a self-assembled ligand complex.

Reactivity studies of iridium pyridylidenes [Tp(Me2)Ir(C(6)H(5))(2)(C(CH)(3)C(R)NH] (R=H, Me, Ph)

The reactivity of a series of iridiumpyridylidene complexes with the formula [Tp(Me2) Ir(C6 H5 )2 (C(CH)3 C(R)NH] (1 a-1 c) towards a variety of substrates, from small molecules, such as H2 , O2 , carbon oxides, and formaldehyde, to alkenes and alkynes, is described. Most of the observed reactivity is best explained by invoking 16 e(-) unsaturated [Tp(Me2) Ir(phenyl)(pyridyl)] intermediates, which behave as internal frustrated Lewis pairs (FLPs). H2 is heterolytically split to give hydridepyridylidene complexes, whilst CO, CO2 , and H2 CO provide carbonyl, carbonate, and alkoxide species, respectively. Ethylene and propene form five-membered metallacycles with an IrCH2 CH(R)N (R=H, Me) motif, whereas, in contrast, acetylene affords four-membered iridacycles with the IrC(CH2 )N moiety. C6 H5 (CO)H and C6 H5 CCH react with formation of IrC6 H5 and IrCCPh bonds and the concomitant elimination of a molecule of pyridine and benzene, respectively. Finally the reactivity of compounds 1 a-1 c against O2 is described. Density functional theory calculations that provide theoretical support for these experimental observations are also reported.