Synthesis and insertion reactions of the cyclometalated palladium-alkyl complexes Pd(CH(2)CMe(2)-o-C(6)H(4))L(2). Observation of a pentacoordinated intermediate in the insertion of SO(2)

A Neophyl Palladacycle as an Air- and Thermally Stable Precursor to Oxidative Addition Complexes

The utilization of isolated Palladium Oxidative Addition Complexes (OACs) has had a significant impact on Pd-catalyzed and Pd-mediated cross-coupling reactions. Despite their importance, widespread utility of OACs has been limited by the instability of their precursor complexes. Herein, we report the use of Cámpora's palladacycle as a new, more stable precursor to Pd OACs. Using this palladacycle, a series of biarylphosphine ligated OACs, including those with pharmaceutical-derived aryl halides and relevance to bioconjugation, were prepared.

Mechanistic Studies of the Palladium-Catalyzed Desulfinative Cross-Coupling of Aryl Bromides and (Hetero)Aryl Sulfinate Salts

Pyridine and related heterocyclic sulfinates have recently emerged as effective nucleophilic coupling partners in palladium-catalyzed cross-coupling reactions with (hetero)aryl halides. These sulfinate reagents are straightforward to prepare, stable to storage and coupling reaction conditions, and deliver efficient reactions, thus offering many advantages, compared to the corresponding boron-derived reagents. Despite the success of these reactions, there are only scant details of the reaction mechanism. In this study, we use structural and kinetic analysis to investigate the mechanism of these important coupling reactions in detail. We compare a pyridine-2-sulfinate with a carbocyclic sulfinate and establish different catalyst resting states, and turnover limiting steps, for the two classes of reagent. For the carbocyclic sulfinate, the aryl bromide oxidative addition complex is the resting state intermediate, and transmetalation is turnover-limiting. In contrast, for the pyridine sulfinate, a chelated Pd(II) sulfinate complex formed post-transmetalation is the resting-state intermediate, and loss of SO2 from this complex is turnover-limiting. We also investigated the role of the basic additive potassium carbonate, the use of which is crucial for efficient reactions, and deduced a dual function in which carbonate is responsible for the removal of free sulfur dioxide from the reaction medium, and the potassium cation plays a role in accelerating transmetalation. In addition, we show that sulfinate homocoupling is responsible for converting Pd(OAc)2 to a catalytically active Pd(0) complex. Together, these studies shed light on the challenges that must be overcome to deliver improved, lower temperature versions of these synthetically important processes.

Dynamics of the efficient cyclometalation of the undercoordinated organoplatinum complex [Pt(COD)(neoPh)]+ (neoPh = 2-methyl-2-phenylpropyl)

The cyclometalation reaction of [Pt(COD)(κ¹-neoPh)]⁺ (neoPh = 2-methyl-2-phenylpropyl) to [Pt(COD)(κ²-neoPh)] was studied experimentally and mechanistically using DFT and MD simulations.

C-H and C-F bond activation reactions of pentafluorostyrene at rhodium complexes

The rhodium(i) complexes [Rh(Bpin)(PEt₃)₃] (1), [Rh(H)(PEt₃)₃] (5) and [Rh(Me)(PEt₃)₃] (14) were employed in reactions with pentafluorostyrene affording coordination of the olefin and C-F or C-H bond activation. Control of the reaction conditions allowed for selective activation reactions at different positions at the fluorinated aromatic ring. The rhodacycle trans-[Rh(F)(CH₂CH₂(2-C₆F₄))(PEt₃)₂] (7) was identified as an intermediate for an activation at the 2-position. Reactivity studies of the latter with CO led to the generation of trans-[Rh(F)(CH₂CH₂C₆F₄)(CO)(PEt₃)₂] (10). Stoichiometric and catalytic hydroboration reactions were achieved using complexes 1 or 5 as catalysts.

Towards Extended Zinc Ethylsulfinate Networks by Stepwise Insertion of Sulfur Dioxide into Zn-C Bonds

The ability to utilize polluting gases in efficient metal-mediated transformations is one of the most pressing challenges of modern chemistry. Despite numerous studies on the insertion of SO₂ into M-C bonds, the chemical reaction of SO₂ with organozinc compounds remains little explored. To fill this gap, we report here the systematic study of the reaction of Et₂ Zn towards SO₂ as well as the influence of Lewis bases on the reaction course. Whereas the equimolar reaction provided a novel example of a structurally characterized organozinc ethylsulfinate compound of general formula [(EtSO₂ )ZnEt]_n , the utilization of an excess of SO₂ led to the formation of the zinc(II) bis(ethylsulfinate) compound [(EtSO₂ )₂ Zn]_n . Moreover, we have discovered that the presence of N-donor Lewis bases represents an efficient tool for the preparation of extended zinc ethylsulfinates, which in turn led to the formation of 1D [(EtSO₂ ZnEt)₂ (hmta)]_n and 2D [((EtSO₂ )₂ Zn)₂ (DABCO)]_n ⋅solv (in which solv=THF or toluene, hmta= hexamethylenetetramine, and DABCO=1,4-diazabicyclo[2.2.2]octane) coordination polymers, respectively. The results of DFT calculations on the reactivity of SO₂ towards selected Zn-C reactive species as well as the role of an N-donor Lewis base on the stabilization of the transition states complement the discussion.

Aerobic C-C and C-O bond formation reactions mediated by high-valent nickel species

Nickel complexes have been widely employed as catalysts in C-C and C-heteroatom bond formation reactions. While Ni(0), Ni(i), and Ni(ii) intermediates are most relevant in these transformations, recently Ni(iii) and Ni(iv) species have also been proposed to play a role in catalysis. Reported herein is the synthesis, detailed characterization, and reactivity of a series of Ni(ii) and Ni(iii) metallacycle complexes stabilized by tetradentate pyridinophane ligands with various N-substituents. Interestingly, while the oxidation of the Ni(ii) complexes with various other oxidants led to exclusive C-C bond formation in very good yields, the use of O₂ or H₂O₂ as oxidants led to formation of appreciable amounts of C-O bond formation products, especially for the Ni(ii) complex supported by an asymmetric pyridinophane ligand containing one tosyl N-substituent. Moreover, cryo-ESI-MS studies support the formation of several high-valent Ni species as key intermediates in this uncommon Ni-mediated oxygenase-type chemistry.

Palladium-Catalyzed Intermolecular [4+1] Spiroannulation by C(sp3 )-H Activation and Naphthol Dearomatization

A novel palladium-catalyzed [4+1] spiroannulation was developed by using a C(sp³ )-H activation/naphthol dearomatization approach. This bimolecular domino reaction of two aryl halides was realized through a sequence of cyclometallation-facilitated C(sp³ )-H activation, biaryl cross-coupling, and naphthol dearomatization, thus rendering the rapid assembly of a new class of spirocyclic molecules in good yields with broad functional-group tolerance. Preliminary mechanistic studies indicate that C-H cleavage is likely involved in the rate-determining step, and a five-membered palladacycle was identified as the key intermediate for the intermolecular coupling.

Csp2–Br bond activation of Br-pyridine by neophylpalladacycle: formation of binuclear seven-membered palladacycle and bipyridine species

A small library of seven-membered palladacycles and asymmetric bipyridines was prepared via C–Br activation and C–C coupling reactions.

C-H Activation in Primary 3-Phenylpropylamines: Synthesis of Seven-Membered Palladacycles through Orthometalation. Stoichiometric Preparation of Benzazepinones and Catalytic Synthesis of Ureas

The dimeric cyclometalated complexes [Pd2{κ(2)C,N-C6H4CH2CH2C(R)(Me)NH2-2}2(μ-Cl)2] (R = Me (1a), H (1b)) are prepared by reacting 1,1-dimethyl-3-phenylpropylammonium or 1-methyl-3-phenylpropylammonium triflate with Pd(OAc)2 in a 1:1 molar ratio and subsequent treatment with excess NaCl. The mononuclear derivatives [Pd{κ(2)C,N-C6H4CH2CH2C(R)(Me)NH2-2}Cl(L)] (L = PPh3, R = Me (3a), H (3b); L = 4-picoline (4-pic), R = Me (4a), H (4b)) were prepared from 1a,b by splitting the chloro bridges with the neutral ligands L. A conformational analysis of the mononuclear palladacycles in solution has been carried out. Insertion of CO takes place into the Pd-C bond of complexes 1a,b, affording Pd(0) and the tetrahydro-benzazepinone 5a or 5b, which possesses potential pharmacological interest. Additionally, the triflate salt A or B undergoes catalytic carbonylation with CO to afford the corresponding N,N'-dialkylurea, using Pd(OAc)2/Cu(OAc)2, in boiling acetonitrile. The crystal structures of 3a·(1)/2H2O, 3b, 4b·CHCl3, and 5a were determined by X-ray diffraction studies.

Pd-Catalyzed C(sp(3))-H Functionalization/Carbenoid Insertion: All-Carbon Quaternary Centers via Multiple C-C Bond Formation

A Pd-catalyzed C(sp(3))-H functionalization/carbenoid insertion is described. The method allows for the rapid synthesis of bicyclic frameworks, generating all-carbon quaternary centers via multiple C-C bond formations in a straightforward manner.

Catalytic Transfer of Magnetism using a Neutral Iridium Phenoxide Complex

A novel neutral iridium carbene complex Ir(κC,O-L1)(COD) (1) [where COD = cyclooctadiene and L1 = 3-(2-methylene-4-nitrophenolate)-1-(2,4,6-trimethylphenyl) imidazolylidene] with a pendant alkoxide ligand has been prepared and characterized. It contains a strong Ir-O bond and X-ray analysis reveals a distorted square planar structure. NMR spectroscopy reveals dynamic solution state behavior commensurate with rapid seven-membered ring flipping. In CD2Cl2 solution, under hydrogen at low temperature, this complex dominates although it exists in equilibrium with a reactive iridium dihydride cyclooctadiene complex. 1 reacts with pyridine and H2 to form neutral Ir(H)2(κC,O-L1)(py)2 which also exists in two conformers that differ according to the orientation of the seven-membered metallocycle and whilst its Ir-O bond remains intact, the complex undergoes both pyridine and H2 exchange. As a consequence, when placed under parahydrogen, efficient polarization transfer catalysis (PTC) is observed via the Signal Amplification By Reversible Exchange (SABRE) approach. Due to the neutral character of this catalyst, good hyperpolarization activity is shown in a wide range of solvents for a number of substrates. These observations reflect a dramatic improvement in solvent tolerance of SABRE over that reported for the best PTC precursor IrCl(IMes)(COD). For THF, the associated 1H NMR signal enhancement for the ortho proton signal of pyridine shows an increase of 600-fold at 298 K. The level of signal enhancement can be increased further through warming or varying the magnetic field experienced by the sample at the point of catalytic magnetization transfer.

Dioxygen activation by an organometallic Pd(ii) precursor: formation of a Pd(iv)–OH complex and its C–O bond formation reactivity

An isolable Pd(iv)–OH complex formed upon aerobic oxidation of an organometallic Pd(ii) precursor exhibits selective C(sp²)–O bond formation reactivity.

Double role of the hydroxy group of phosphoryl in palladium(II)-catalyzed ortho-olefination: a combined experimental and theoretical investigation

Density functional theory calculations have been carried out on Pd-catalyzed phosphoryl-directed ortho-olefination to probe the origin of the significant reactivity difference between methyl hydrogen benzylphosphonates and dimethyl benzylphosphonates. The overall catalytic cycle is found to include four basic steps: C-H bond activation, transmetalation, reductive elimination, and recycling of catalyst, each of which is constituted from different steps. Our calculations reveal that the hydroxy group of phosphoryl plays a crucial role almost in all steps, which can not only stabilize the intermediates and transition states by intramolecular hydrogen bonds but also act as a proton donor so that the η(1)-CH3COO(-) ligand could be protonated to form a neutral acetic acid for easy removal. These findings explain why only the methyl hydrogen benzylphosphonates and methyl hydrogen phenylphosphates were found to be suitable reaction partners. Our mechanistic findings are further supported by theoretical prediction of Pd-catalyzed ortho-olefination using methyl hydrogen phenylphosphonate, which is verified by experimental observations that the desired product was formed in a moderate yield.

Pd(0)-catalyzed sequential C-N bond formation via allylic and aromatic C-H amination of α-methylstyrenes with diaziridinone

A novel Pd(0)-catalyzed sequential C-N bond formation process via allylic and aromatic C-H amination of α-methylstyrenes with di-tert-butyldiaziridinone, giving spirocyclic indolines in good yields, is described. Four C-N bonds and one spiro quaternary carbon are generated in a single operation.

Carbon(sp(3))-fluorine bond-forming reductive elimination from palladium(IV) complexes

Pd(IV)-fluoride complexes, some of which are remarkably insensitive to water, have been synthesized and used in the title reaction, which proceeds with high selectivity to give the product of the C(sp(3))-F coupling. Preliminary mechanistic studies implicate a pathway involving dissociation of pyridine followed by direct C-F coupling at the Pd center.

Cyclic enolates of Ni and Pd: equilibrium between C- and O-bound tautomers and reactivity studies

2-acylaryl complexes of Ni and Pd containing chelating diphosphines react with KtBuO to give metallacyclic enolate complexes. While coordination through the carbon atom is preferred in the case of Pd, the nickel O-enolate compounds are formed as the corresponding O-tautomers. Slow equilibration between O- and C-enolate tautomers is observed for the nickel complex with an unsubstituted enolate function (M-O-C=CH(2)). Theoretical DFT calculations suggest that the barrier for the tautomer exchange has its origin in the rigidity of the metallacycle. Whilst the C-enolate tautomer is unreactive towards aldehydes, the corresponding O-enolate adds to MeCHO and PhCHO, giving rise to products that retain the enolate functionality. The carbonylation of these products cleanly leads to the formation of enol lactones in a highly selective manner.

C–H Bond activation and C–C bond formation in the reaction of 2,5-dimethylthiophene with TpMe2Ir compounds

The bulky 2,5-dimethylthiophene (2,5-Me2T) reacts at 60 degrees C with TpMe2Ir(C2H4)2 to give a mixture of two TpMe2Ir(III) hydride products, 3 and 4, that contain in addition a thienyl (3) or a thienyl-derived ligand (4). For the generation of 3 only sp2 C-H activation is needed, but the formation of 4 requires also the activation of an sp3 C-H bond and the formation of a new C-C bond (between vinyl and thienyl fragments). In the presence of 2,5-Me2T, compound 4 reacts further to produce a complex thiophenic structure (5, characterized by X-ray methods) that derives formally from two molecules of 2,5-Me2T and a vinyl fragment. Compounds 3-5 can be readily protonated by [H(OEt2)2][BAr'4](Ar'= 3,5-C6H3(CF3)2), with initial generation of carbene ligands (in the case of 3 and 5) as a consequence of H+ attack at the beta-carbon of the Ir-thienyl unit. Free, substituted thiophenes, derived from the original 2,5-Me2T, may be isolated in this way.

Insertion of SO2 into a Sulfide-Bridged M−Ge Bond: Synthesis, Characterization, and Reactivity of the O-Germyl-S-sulfoxylate

Addition of SO(2) to [formula: see text](1) results in the insertion of SO(2) into the Pd-Ge bond to give the O-germyl-S-sulfoxylate insertion isomer (Et3P)2Pd(mu-S)S(O)OGe[N(SiMe3)2]2 (2). Heating solutions of 2 results in the formation of (Et3P)Pd(SGe[N(SiMe3)2]2OSOGe[N(SiMe3)2]2S) (3) and a LnPd(0) fragment. X-ray crystal structures of 2 and 3 are reported.