Origins of the Selectivity for Borylation of Primary over Secondary C−H Bonds Catalyzed by Cp*-Rhodium Complexes

Catalyst Design for Rh-Catalyzed Arene and Alkane C-H Borylation: The NHC Affects the Induction Period, and Indenyl is Superior to Cp

In order to establish design criteria for Rh C-H borylation catalysts, analogues of the successful catalyst [Rh(Ind)(SIDipp)(COE)] (Ind = η5-indenyl, SIDipp = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene, and COE = cis-cyclooctene) were synthesized by changing the indenyl and carbene ligands. [RhCp(SIDipp)(COE)] (1) formed alongside the C-C activated, cyclometalated byproduct [RhCp(κ2CAr,Ccarbene-SIDipp')(iPr)] (rac-2; SIDipp' = 1-(6-isopropylphenyl)-3-(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene). Computational modeling of COE dissociation showed that both C-C and C-H activation of the SIDipp aryl group is thermally attainable and reversible under experimental conditions, with the C-C activation products being the more thermodynamically stable species. Oxidative addition of 1 with SiH(OEt)3 gave the Rh silyl hydride [RhCp(H){Si(OEt)3}(SIDipp)] (rac-3). [Rh(Ind)(IDipp)(COE)] (4; IDipp = 1,3-bis(2,6-diisopropylphenyl)-imidazole-2-ylidene), the carbonyl analogue [Rh(Ind)(IDipp)(CO)] (5; νCO = 1940 cm-1, cf. 1944 cm-1 for [Rh(Ind)(SIDipp)(COE)]), and [Rh(Ind)(IMe4)(COE)] (6; IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene) were also characterized, but attempts to synthesize Rh carbene complexes with fluorenyl or 1,2,3,4-tetrahydrofluorenyl ligands were not successful. For the catalytic C-H borylation of benzene using B2pin2, 1 was inactive at 80 °C, and [Rh(Ind)(SIDipp)(COE)] was superior to all other complexes tested due to the shortest induction period. However, the addition of HBpin to precatalyst 4 eliminated the induction period. Catalytic n-alkane C-H borylation using [Rh(Ind)(NHC)(COE)] gave yields of up to 21% alkylBpin, but [RhCp*(C2H4)2] was the better catalyst.

Molecular Dihydrogen Activation by (C5Me5)M/N (M=Rh, Ir) Transition Metal Frustrated Lewis Pairs: Reversible Proton Migration to, and Proton Abstraction from, the C5Me5 Ligand

The masked transition-metal frustrated Lewis pairs [Cp*M(κ3N,N',N''-L)][SbF6] (Cp*=η5-C5Me5; M=Ir, 1, Rh, 2; HL=pyridinyl-amidine ligand) reversibly activate H2 under mild conditions rendering the hydrido derivatives [Cp*MH(κ2N,N'-HL)][SbF6] observed as a mixture of the E and Z isomers at the amidine C=N bond (M=Ir, 3Z, 3E; M=Rh, 4Z, 4E). DFT calculations indicate that the formation of the E isomers follows a Grotthuss type mechanism in the presence of water. A mixture of Rh(I) isomers of formula [(Cp*H)Rh(κ2N,N'-HL)][SbF6] (5 a-d) is obtained by reductive elimination of Cp*H from 4. The formation of 5 a-d was elucidated by means of DFT calculations. Finally, when 2 reacts with D2, the Cp* and Cp*H ligands of the resulting rhodium complexes 4 and 5, respectively, are deuterated as a result of a reversible hydrogen abstraction from the Cp* ligand and D2 activation at rhodium.

Highly Selective Nickel-Catalyzed Isomerization-Hydroboration of Alkenes Affords Terminal Functionalization at Remote C-H Position

We report herein the synthesis and characterization of nickel complexes supported by tridentate and bidentate phosphino(silyl) ancillary ligands, along with the successful application of these complexes as precatalysts for the hydroboration of terminal and internal alkenes using pinacolborane (HBPin). These reactions proceeded with low nickel loadings of 2.5-5 mol % in the absence of co-solvent, and in some cases at room temperature. Isomerization to afford exclusively the terminal hydroboration product was obtained across a range of internal alkenes, including tri- and tetra-substituted examples. This reactivity is unprecedented for nickel and offers a powerful means of achieving functionalization at a C-H position remote from the C=C double bond. Nickel-catalyzed deuteroboration experiments using DBPin support a mechanism involving 1,2-insertion of the alkene and subsequent chain-walking, which results in isotopic scrambling.

Iridium-Catalyzed, Site-Selective Silylation of Secondary C(sp3)-H Bonds in Secondary Alcohols and Ketones

We report the iridium-catalyzed, stereoselective conversion of secondary alcohols or ketones to anti-1,3-diols by the silylation of secondary C-H bonds γ to oxygen and oxidation of the resulting oxasilolane. The silylation of secondary C-H bonds in secondary silyl ethers derived from alcohols or ketones is enabled by a catalyst formed from a simple bisamidine ligand. The silylation occurs with high selectivity at a secondary C-H bond γ to oxygen over distal primary or proximal secondary C-H bonds. Initial mechanistic investigations suggest that the source of the newly achieved reactivity is a long catalyst lifetime resulting from the high binding constant of the strongly electron-donating bisamidine ligand.

Exploration of Ligand-Centered Hydride Transfer in La/Y-Catalyzed Deoxygenative Reduction of Tertiary Amides with Pinacolborane

A number of rare-earth metals and actinides have proven to be active in a wide variety of atom-efficient transformations. As compared to the related organometallic catalysts, the detailed mechanisms for the rare-earth metal-catalyzed reactions remain largely unexplored. Herein, the detailed catalyst activation process and reaction mechanisms of deoxygenative reduction of amides with pinacolborane (HBpin) catalyzed by Y[N(TMS)₂]₃ and La[N(TMS)₂]₃ complexes as well as a La₄(O)acac₁₀ cluster are investigated by density functional theory calculations. The M(III)-hemiaminal complex is disclosed to be the active catalyst for both the complexes and the cluster. During catalyst activation for both the Y and La complexes, the H-B bond polarity results in the formation of a transient M(III)-hydride intermediate, which is converted into an on-cycle M(III)-hemiaminal complex via facile migratory insertion. However, this kind of La(III)-hydride species cannot be formed for the La cluster. Starting from the M(III)-hemiaminal complex, the reaction proceeds via the ligand-centered hydride transfer mechanism that involves B-O bond formation, hydride transfer to B, C-O cleavage within the hemiaminal borane, hydride transfer to C, and σ-bond metathesis. The additional HBpin molecule is vital for the first hydride transfer that leads to the formation of [H₂Bpin]^- species. Our calculations reveal several important cooperative effects of the HBpin component during the hydride transfer processes. The improved mechanistic insights will be helpful for further development of selective C═O reduction.

Kinetics and mechanism of sequential ring methyl C-H activation in cyclopentadienyl rhodium(III) complexes

We have studied activation of the methyl C-H bonds in the cyclopentadienyl ligands of half-sandwich Rh(III) complexes [η⁵-Cp^XRh(N,N')Cl]⁺ by observing the dependence of sequential H/D exchange on variations in Cp^X = Cp* (complexes 1 and 2), Me₄PhCp (Cp^XPh, 3) or Me₄PhPhCp (Cp^XPhPh, 4), and chelated ligand N,N' (bpy, 1; phen, 2-4). H/D exchange was fastest in d₄-MeOD (t_1/2 = 10 min, 37 °C, complex 1), no H/D exchange was observed in DMSO/D₂O, and d₄-MeOD enhanced the rate in CD₃CN. The proposed Rh(I)-fulvene intermediate was trapped by [4 + 2] Diels-Alder reactions with conjugated dienes and characterized. The Rh(I) oxidation state was confirmed by X-ray photoelectron spectroscopy (XPS). Influence of solvent on the mechanisms of activation and Diels-Alder adduct formation was modelled using DFT calculations with the CAM-B3LYP functional and CEP-31 g basis set, and influence on the reaction profile of the dimiine ligand and phenyl substituent using the larger qzvp basis set. The Rh(III)-OH intemediate is stabilised by H-bonding with methanol and a Cp* CH₃ hydrogen. The Rh(I)(Me₄fulvene) species, stabilised by interaction of methanol with a coordinated water, again by two H-bonds H₂O-HOMe (1.49 Å) and fulvene CH₂ (1.94 Å), arises from synchronous transfer of the methanol OH proton to a Rh(III)-OH ligand and Cp* methyl hydrogen to the methanol oxygen. Additionally, the observed trend in catalytic activity for complexes 1-4 was reproduced by DFT calculations. These complexes form a novel class of catalytic molecular motors with a tunable rate of operation that can be stalled in a given state. They provide a basis for elucidation of the effects of ligand design on the contributions of electronic, rotational and vibrational energies to each step in the reaction pathway at the atomic level, consideration of which will enhance the design principles for the next generation of molecular machines.

Activation of H-H, HO-H, C(sp2)-H, C(sp3)-H, and RO-H Bonds by Transition-Metal Frustrated Lewis Pairs Based on M/N (M = Rh, Ir) Couples

Reaction of the dimers [(Cp*MCl)₂(μ-Cl)₂] (Cp* = η⁵-C₅Me₅) with Ph₂PCH₂CH₂NC(NH(p-Tolyl))₂ (H₂L) in the presence of NaSbF₆ affords the chlorido complexes [Cp*MCl(κ²N,P-H₂L)][SbF₆] (M = Rh, 1; Ir, 2). Upon treatment with aqueous NaOH, solutions of 1 and 2 yield the corresponding complexes [Cp*M(κ³N,N′,P-HL)][SbF₆] (M = Rh, 3; Ir, 4) in which the ligand HL presents a fac κ³N,N′,P coordination mode. Treatment of THF solutions of complexes 3 and 4 with hydrogen gas, at room temperature, results in the formation of the metal hydrido-complexes [Cp*MH(κ²N,P-H₂L)][SbF₆] (M = Rh, 5; Ir, 6) in which the N(p-Tolyl) group has been protonated. Complexes 3 and 4 react with deuterated water in a reversible fashion resulting in the gradual deuteration of the Cp* group. Heating at 383 K THF/H₂O solutions of the complexes 3 and 4 affords the orthometalated complexes [Cp*M(κ³C,N,P-H₂L_-H)][SbF₆] [M = Rh, 7; Ir, 8, H₂L_-H = Ph₂PCH₂CH₂NC(NH(p-Tolyl))(NH(4-C₆H₃Me))], respectively. At 333 K, complexes 3 and 4 react in THF with methanol, primary alcohols, or 2-propanol giving the metal-hydrido complexes 5 and 6, respectively. The reaction involves the acceptorless dehydrogenation of the alcohols at a relatively low temperature, without the assistance of an external base. The new complexes have been characterized by the usual analytical and spectroscopic methods including the X-ray diffraction determination of the crystal structures of complexes 1–5, 7, and 8. Notably, the chlorido complexes 1 and 2 crystallize both as enantiopure conglomerates and as racemates. Reaction mechanisms are proposed based on stoichiometric reactions, nuclear magnetic resonance studies, and X-ray crystallography as well as density functional theory calculations.

In solution, masked transition-metal frustrated Lewis pairs (TMFLPs) give rise to the corresponding TMFLP species which activate dihydrogen, water, and alcohols following FLP reaction pathways. When D₂O or alcohols with deuterated OH groups were employed, H/D exchange at the Cp* ligand (involving C(sp³)−H activation) was observed. C(sp²)−H bond activation involving orthometalation of the p-Tolyl ring was also observed.

Recent Trends in Group 9 Catalyzed C–H Borylation Reactions: Different Strategies To Control Site-, Regio-, and Stereoselectivity

Organoboron compounds continue contributing substantially to advances in organic chemistry with their increasing role as both synthetic intermediates and target compounds for medicinal chemistry. Particularly attractive methods for their synthesis are based on the direct borylation of C–H bonds of available starting materials since no additional pre-functionalization steps are required. However, due to the high abundance of C–H bonds with similar reactivity in organic molecules, synthetically useful C–H borylation protocols demand sophisticated strategies to achieve high regio- and stereoselectivity. For this purpose, selective transition-metal-based catalysts have been developed, with group 9 centered catalysts being among the most commonly utilized. Recently, a multitude of diverse strategies has been developed to push the boundaries of C–H borylation reactions with respect to their regio- and enantioselectivity. Herein, we provide an overview of approaches for the C–H borylation of arenes, alkenes, and alkanes based on group 9 centered catalysts with a focus on the recent literature. Lastly, an outlook is given to assess the future potential of the field.

1 Introduction

1.1 Mechanistic Considerations

1.2 Selectivity Issues in C–H Borylation

1.3 Different Modes of Action Employing Directing Group Strategies in C–H Borylation

1.4 Scope and Aim of this Short Review

2 Trends in C–H Borylation Reactions

2.1 Photoinduced Catalysis

2.2 Transfer C–H Borylation

2.3 Lewis Acid Mediated C–H Borylation

2.4 Directed Metalation

2.5 Miscellaneous C–H Borylation Reactions

2.6 Electrostatic Interactions

2.7 Hydrogen Bonding

3 Conclusion and Outlook

Computational Study of Bridge Splitting, Aryl Halide Oxidative Addition to PtII , and Reductive Elimination from PtIV : Route to Pincer-PtII Reagents with Chemical and Biological Applications

Density functional theory computation indicates that bridge splitting of [Pt^II R₂ (μ-SEt₂ )]₂ proceeds by partial dissociation to form R₂ Pt^a (μ-SEt₂ )Pt^b R₂ (SEt₂ ), followed by coordination of N-donor bromoarenes (L-Br) at Pt^a leading to release of Pt^b R₂ (SEt₂ ), which reacts with a second molecule of L-Br, providing two molecules of PtR₂ (SEt₂ )(L-Br-N). For R=4-tolyl (Tol), L-Br=2,6-(pzCH₂ )₂ C₆ H₃ Br (pz=pyrazol-1-yl) and 2,6-(Me₂ NCH₂ )₂ C₆ H₃ Br, subsequent oxidative addition assisted by intramolecular N-donor coordination via Pt^II Tol₂ (L-N,Br) and reductive elimination from Pt^IV intermediates gives mer-Pt^II (L-N,C,N)Br and Tol₂ . The strong σ-donor influence of Tol groups results in subtle differences in oxidative addition mechanisms when compared with related aryl halide oxidative addition to palladium(II) centres. For R=Me and L-Br=2,6-(pzCH₂ )₂ C₆ H₃ Br, a stable Pt^IV product, fac-Pt^IV Me₂ {2,6-(pzCH₂ )₂ C₆ H₃ -N,C,N)Br is predicted, as reported experimentally, acting as a model for undetected and unstable Pt^IV Tol₂ {L-N,C,N}Br undergoing facile Tol₂ reductive elimination. The mechanisms reported herein enable the synthesis of Pt^II pincer reagents with applications in materials and bio-organometallic chemistry.

Rhodium Indenyl NHC and Fluorenyl-Tethered NHC Half-Sandwich Complexes: Synthesis, Structures and Applications in the Catalytic C-H Borylation of Arenes and Alkanes

Indenyl (Ind) rhodium N-heterocyclic carbene (NHC) complexes [Rh(η⁵ -Ind)(NHC)(L)] were synthesised for 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene (SIPr) with L=C₂ H₄ (1), CO (2 a) and cyclooctene (COE; 3), for 1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene (SIMes) with L=CO (2 b) and COE (4), and 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) with L=CO (2 c) and COE (5). Reaction of SIPr with [Rh(Cp*)(C₂ H₄ )₂ ] did not give the desired SIPr complex, thus demonstrating the "indenyl effect" in the synthesis of 1. Oxidative addition of HSi(OEt)₃ to 3 proceeded under mild conditions to give the Rh silyl hydride complex [Rh(Ind){Si(OEt)₃ }(H)(SIPr)] (6) with loss of COE. Tethered-fluorenyl NHC rhodium complexes [Rh{(η⁵ -C₁₃ H₈ )C₂ H₄ N(C)C₂ H_x NR}(L)] (x=4, R=Dipp, L=C₂ H₄ : 11; L=COE: 12; L=CO: 13; R=Mes, L=COE: 14; L=CO: 15; x=2, R=Me, L=COE: 16; L=CO: 17) were synthesised in low yields (5-31 %) in comparison to good yields for the monodentate complexes (49-79 %). Compounds 3 and 1, which contain labile alkene ligands, were successful catalysts for the catalytic borylation of benzene with B₂ pin₂ (Bpin=pinacolboronate, 97 and 93 % PhBpin respectively with 5 mol % catalyst, 24 h, 80 °C), with SIPr giving a more active catalyst than SIMes or IMes. Fluorenyl-tethered NHC complexes were much less active as borylation catalysts, and the carbonyl complexes were inactive. The borylation of toluene, biphenyl, anisole and diphenyl ether proceeded to give meta substitutions as the major product, with smaller amounts of para substitution and almost no ortho product. The borylation of octane and decane with B₂ pin₂ at 120 and 140 °C, respectively, was monitored by ¹¹ B NMR spectroscopy, which showed high conversions into octyl and decylBpin over 4-7 days, thus demonstrating catalysed sp³ C-H borylation with new piano stool rhodium indenyl complexes. Irradiation of the monodentate complexes with 400 or 420 nm light confirmed the ready dissociation of C₂ H₄ and COE ligands, whereas CO complexes were inert. Evidence for C-H bond activation in the alkyl groups of the NHC ligands was obtained.

Homogeneous catalytic C(sp3)-H functionalization of gaseous alkanes

The conversion of light alkanes into bulk chemicals is becoming an important challenge as it effectively avoids the use of prefunctionalized alkylating reagents. The implementation of such processes is, however, hampered by their gaseous nature and low solubility, as well as the low reactivity of the C-H bonds. Efforts have been made to enable both polar and radical processes to activate these inert compounds. In addition, these methodologies also benefit significantly from the development of a suitable reactor technology that intensifies gas-liquid mass transfer. In this review, we critically highlight these developments, both from a conceptual and a practical point of view. The recent expansion of these mechanistically-different methods have enabled the use of various gaseous alkanes for the development of different bond-forming reactions, including C-C, C-B, C-N, C-Si and C-S bonds.

Reversible Hydride Migration from C5Me5 to RhI Revealed by a Cooperative Bimetallic Approach

The use of cyclopentadienyl ligands in organometallic chemistry and catalysis is ubiquitous, mostly due to their robust spectator role. Nonetheless, increasing examples of non-innocent behaviour are being documented. Here, we provide evidence for reversible intramolecular C-H activation at one methyl terminus of C5Me5 in [(η-C5Me5)Rh(PMe3)2] to form a new Rh-H bond, a process so far restricted to early transition metals. Experimental evidence was acquired from bimetallic rhodium/gold structures in which the gold center binds either to the rhodium atom or to the activated Cp* ring. Reversibility of the C-H activation event regenerates the RhI and AuI monometallic precursors, whose cooperative reactivity towards polar E-H bonds (E=O, N), including the N-H bonds in ammonia, can be understood in terms of bimetallic frustration.

Methane Borylation Catalyzed by Ru, Rh, and Ir Complexes in Comparison with Cyclohexane Borylation: Theoretical Understanding and Prediction

Methane borylation catalyzed by Cp*M(Bpin)_n (M = Ru or Rh; HBpin = pinacolborane; n = 2 or 3) and (TMPhen)Ir(Bpin)₃ (TMPhen = 3,4,7,8-tetramethyl-1,10-phenanthroline) was investigated by DFT in comparison with cyclohexane borylation. Because Ru-catalyzed borylation has not been theoretically investigated yet, its reaction mechanism was first elucidated; Cp*Ru(Bpin)₃ 1-Ru is an active species, and Cp*Ru(Bpin)₃(H)(CH₃) 4-Ru is a key intermediate. In 4-Ru, the Ru is understood to have an ambiguous oxidation state between +IV and +VI because it has a H··Bpin bonding interaction with a bond order of about 0.5. Methane borylation occurs through oxidative addition of methane C-H bond followed by reductive elimination of borylmethane in all of these catalysts. The catalytic activity for methane borylation increases following the order Cp*Ru(Bpin)₃ < (TMPhen)Ir(Bpin)₃ < Cp*Rh(Bpin)₂. Cyclohexane borylation occurs in the same mechanism except for the presence of isomerization of a key intermediate. Chemoselectivity of methane over cyclohexane increases following the order Ir < Ru < Rh. In all of these catalysts, the rate-determining step (RDS) of cyclohexane borylation needs a larger ΔG°^‡ than the RDS of methane borylation because the more bulky cyclohexyl group induces larger steric repulsion with the ligand than methyl. One reason for the worse chemoselectivity of the Ir catalyst is its less congested transition state of the reductive elimination of borylcyclohexane. Herein, use of a strongly electron-donating ligand consisting of pyridine and N-heterocyclic carbene with bulky substituents is computationally proposed as a good ligand for the Ir catalyst; actually, the Ir complex of this ligand is calculated to be more active and more chemoselective than Cp*Rh(Bpin)₂ for methane borylation.

Diverse functionalization of strong alkyl C-H bonds by undirected borylation

The selective functionalization of strong, typically inert carbon-hydrogen (C-H) bonds in organic molecules is changing synthetic chemistry. However, the undirected functionalization of primary C-H bonds without competing functionalization of secondary C-H bonds is rare. The borylation of alkyl C-H bonds has occurred previously with this selectivity, but slow rates required the substrate to be the solvent or in large excess. We report an iridium catalyst ligated by 2-methylphenanthroline with activity that enables, with the substrate as limiting reagent, undirected borylation of primary C-H bonds and, when primary C-H bonds are absent or blocked, borylation of strong secondary C-H bonds. Reactions at the resulting carbon-boron bond show how these borylations can lead to the installation of a wide range of carbon-carbon and carbon-heteroatom bonds at previously inaccessible positions of organic molecules.

Iridium-Catalyzed Silylation of Five-Membered Heteroarenes: High Sterically Derived Selectivity from a Pyridyl-Imidazoline Ligand

The steric effects of substituents on five-membered rings are less pronounced than those on six-membered rings because of the difference in bond angles. Thus, the regioselectivities of reactions of five-membered heteroarenes that occur with selectivities dictated by steric effects, such as the borylation of C-H bonds, have been poor in many cases. We report that the silylation of five-membered-ring heteroarenes occurs with high sterically derived regioselectivity when catalyzed by the combination of [Ir(cod)(OMe)]2 (cod=1,5-cyclooctadiene) and a phenanthroline ligand or a new pyridyl-imidazoline ligand that further increases the regioselectivity. The silylation reactions with these catalysts produce high yields of heteroarylsilanes from functionalization at the most sterically accessible C-H bonds of these rings under conditions that the borylation of C-H bonds with previously reported catalysts formed mixtures of products or products that are unstable. The heteroarylsilane products undergo cross-coupling reactions and substitution reactions with ipso selectivity to generate heteroarenes that bear halogen, aryl, and perfluoroalkyl substituents.

Iridium-Catalyzed sp3 C-H Borylation in Hydrocarbon Solvent Enabled by 2,2'-Dipyridylarylmethane Ligands

Iridium-catalyzed alkane C-H borylation has long suffered from poor atom economy, resulting from both the inclusion of only 1 equiv of boron from the diboron reagent and a requirement for neat substrate. An appropriately substituted dipyridylarylmethane ligand was found to give highly active alkane borylation catalysts that facilitate C-H borylation with improved efficiency. This system provides for complete consumption of the diboron reagent, producing 2 molar equivalents of product at low catalyst loadings. The superior efficacy of this system also enables borylation of unactivated alkanes in hydrocarbon solvent with a reduced excess of substrate and improved functional group compatibility. The effectiveness of this ligand is displayed across a selection of functional groups, both under traditional borylation conditions in neat substrate and under atypical conditions in cyclohexane solvent. The utility of this catalytic system is exemplified by the borylation of substrates containing polar functionality, which are unreactive toward C-H borylation under neat conditions.

Iridium-Catalyzed Silylation of Five-Membered Heteroarenes: High Sterically Derived Selectivity from a Pyridyl-Imidazoline Ligand

The steric effects of substituents on five-membered rings are less pronounced than those on six-membered rings because of the difference in bond angles. Thus, the regioselectivities of reactions that occur with selectivities dictated by steric effects, such as the borylation of C-H bonds, have been poor in many cases. We report that the silylation of five-membered ring heteroarenes occurs with high sterically derived regioselectivity when catalyzed by the combination of [Ir(cod)(OMe)]₂ and a phenanthroline ligand or a new pyridyl-imidazoline ligand that further increases the regioselectivity. The silylation reactions with these catalysts produce high yields of heteroarylsilanes from functionalization at the most sterically accessible C-H bonds of these rings under conditions that the borylation of C-H bonds with previously reported catalysts formed mixtures of products or products that are unstable. The heteroarylsilane products undergo cross-coupling reactions and substitution reactions with ipso selectivity to generate heteroarenes that bear halogen, aryl and perfluoroalkyl substituents.

DFT studies of two-electron oxidation, photochemistry, and radical transfer between metal centres in the formation of platinum(iv) and palladium(iv) selenolates from diphenyldiselenide and metal(ii) reactants

Excitation by light to the M–M antibonding orbital of [M^IIIMe₂(2,2′-bipyridine)(SePh)]₂ releases 2[M^IIIMe₂(bipy)(SePh)]˙; the doublets react to give M^IVMe₂(bipy)(SePh)₂ and M^IIMe₂(bipy).

Nickel(II/IV) Manifold Enables Room-Temperature C(sp3)-H Functionalization

This Article demonstrates a mild oxidatively induced C(sp³)-H activation at a high-valent Ni center. In contrast with most C(sp³)-H activation reactions at Ni^II, the transformation proceeds at room temperature and generates an isolable Ni^IV σ-alkyl complex. Density functional theory studies show two plausible mechanisms for this C-H activation process involving triflate-assisted C-H cleavage at either a Ni^IV or a Ni^III intermediate. The former pathway is modestly favored over the latter (by ∼3 kcal/mol). The Ni^IV σ-alkyl product of C-H cleavage reacts with a variety of nucleophiles to form C(sp³)-X bonds (X = halide, oxygen, nitrogen, sulfur, or carbon). These stoichiometric transformations can be coupled using N-fluoro-2,4,6-trimethylpyridinium triflate as a terminal oxidant in conjunction with chloride as a nucleophile to achieve a proof-of-principle Ni^II/IV-catalyzed C(sp³)-H functionalization reaction.

Nickel(IV)-Catalyzed C-H Trifluoromethylation of (Hetero)arenes

This Article describes the development of a stable Ni^IV complex that mediates C(sp²)-H trifluoromethylation reactions. This reactivity is first demonstrated stoichiometrically and then successfully translated to a Ni^IV-catalyzed C-H trifluoromethylation of electron-rich arene and heteroarene substrates. Both experimental and computational mechanistic studies support a radical chain pathway involving Ni^IV, Ni^III, and Ni^II intermediates.

DFT mechanistic investigation into phenol dearomatization mediated by an iodine(iii) reagent

Density functional theory (DFT) was utilized to investigate the mechanistic aspects of the oxidative dearomatization of phenols mediated by an iodine(iii) reagent. In this article, we will show that the conventional mechanism in which an iodine(iii) phenolate is proposed as the key intermediate is not operative, and the process is promoted if the phenolate ligand is dearomatized on the iodine(iii) center. The dearomatized phenolate is calculated to be a more potent reductant than phenolate itself. In such a case, the reaction is capable of proceeding via two competitive mechanisms (dissociative and associative). Consistent with the experimental findings, we found that while the less polar solvents considerably disfavor the dissociative mechanism, they have an insignificant effect on the associative one. The energetic order of these two mechanisms is calculated to be influenced by the nature of the counter anion coordinated to the iodine(iii) center.

Protic additives or impurities promote imine reduction with pinacolborane

We report here that addition of stoichiometric amounts of alcohols or water to mixtures of imines and pinacolborane promote reduction reactions. The reactions of several imines were examined, revealing that alkyl imines were reduced, while aniline derived imines were not effectively reduced. The use of binol as an additive resulted in modest enantioinduction, however other chiral additives that were screened gave negligible enantioinduction. While the reactions described herein are not competitive in conversion with established imine reduction technologies, this work reveals that the presence of protic impurities must be considered as a promoter of side reactions in catalyzed imine hydroborations. Amines also promote imine reduction in certain cases, raising the possibility of a slow autocatalytic reaction. The ability of water or other protic impurities to promote the reduction of imines with pinacolborane represents an important identification of a potential source of background reaction in catalyzed reductions of imines.

Recent Synthesis Developments of Organoboron Compounds via Metal-Free Catalytic Borylation of Alkynes and Alkenes

Diboron reagents have been traditionally regarded as "Lewis acids", which can react with simple Lewis base to create a significant nucleophilic character in one of boryl moieties. In particular, bis(pinacolato)diboron (B₂pin₂) reacts with simple Lewis bases, such as N-heterocyclic carbenes (NHCs), phosphines and alkoxides. This review focuses on the application of trivalent nucleophilic boryl synthon in the selective preparation of organoboron compounds, mainly through metal-free catalytic diboration and the β-boration reactions of alkynes and alkenes.

Cobalt Pincer Complexes in Catalytic C-H Borylation: The Pincer Ligand Flips Rather Than Dearomatizes

The mechanism for the borylation of an aromatic substrate by a cobalt pincer complex was investigated by density functional theory calculations. Experimental observations identified trans-(^iPrPNP)CoH₂(BPin) as the resting state in the borylation of five-membered heteroarenes, and 4-BPin-(^iPrPNP)Co(N₂)BPin as the resting state in the catalytic borylation of arene substrates. The active species, 4-R-(^iPrPNP)CoBPin (R=H, BPin), were generated by reductive elimination of H₂ in the former, through Berry pseudorotation to the cis isomer, and N₂ loss in the latter. The catalytic mechanism of the resulting Co(I) complex was computed to involve three main steps: C-H oxidative addition of the aromatic substrate (C₆H₆), reductive elimination of PhBPin, and regeneration of the active complex. The oxidative addition product formed through the most favorable pathway, where the breaking C-H bond of C₆H₆ is parallel to a line between the two phosphine atoms, leaves the complex with a distorted PNP ligand, which rearranges to a more stable complex via dissociation and re-association of HBPin. Alternative pathways, σ-bond metathesis and the oxidative addition in which the breaking C-H bond is parallel to the Co-B bond, are predicted to be unlikely for this Co(I) complex. The thermodynamically favorable formation of the product PhBPin via reductive elimination drives the reaction forward. The active species regenerates through the oxidative addition of B₂Pin₂ and reductive elimination of HBPin. In the overall reaction, the flipping (refolding) of the five-membered phosphine rings, which connects the species with two phosphine rings folded in the same direction and that with them folded in different directions, is found to play an important role in the catalytic process, as it relieves steric crowding within the PNP ligand and opens Co coordination space. Metal-ligand cooperation based on the ligand's aromatization/dearomatization, a common mechanism for heavy-metal pincer complexes, and the dissociation of one phosphine ligand, do not apply in this system. This study provides guidance for understanding important features of pincer ligands with first-transition-row metals that differ from those in heavier metal complexes.

Siloxane-Based Nanoporous Polymers with Narrow Pore-size Distribution for Cell Imaging and Explosive Detection

Porous polymers are among the most promising porous materials for various application because they show the combined advantages of fluorescent porous materials and polymers. This study developed a cell imaging technique based on luminescent porous organosilicon polymers (LPOPs) that were synthesized via Friedel-Crafts reaction of octaphenylcyclotetrasiloxane with octavinylsilsesquioxanes. The porous organosilicon polymers possessed narrow pore-size distribution, high surface area, and monomodal nanopores centered at approximately 0.59 nm. The excellent properties to the porous polymers can be attributed to the fine structures of LPOPs. LPOP-2 owned the highest fluorescence intensity and micropore volume ratio in LPOPs and showed high selectivity for Fe³⁺ detection and excellent sensitivity to nitroaromatic compound detection. Interestingly, these porous polymers still exhibited excellent responsiveness to Fe³⁺ ion even when inside of living cells. We also fabricated a paper-based sensor using LPOP-2 to develop a simple method for visual detection of explosives. This rapid and visual paper sensor demonstrates promising application for explosive detection and can be expanded for the detection of other analytes.

New activation mechanism for half-sandwich organometallic anticancer complexes

The Cp ^x C-H protons in certain organometallic Rh^III half-sandwich anticancer complexes [(η⁵-Cp ^x )Rh(N,N')Cl]⁺, where Cp ^x = Cp*, phenyl or biphenyl-Me₄Cp, and N,N' = bipyridine, dimethylbipyridine, or phenanthroline, can undergo rapid sequential deuteration of all 15 Cp* methyl protons in aqueous media at ambient temperature. DFT calculations suggest a mechanism involving abstraction of a Cp* proton by the Rh-hydroxido complex, followed by sequential H/D exchange, with the Cp* rings behaving like dynamic molecular 'twisters'. The calculations reveal the crucial role of p_π orbitals of N,N'-chelated ligands in stabilizing deprotonated Cp ^x ligands, and also the accessibility of Rh^I-fulvene intermediates. They also provide insight into why biologically-inactive complexes such as [(Cp*)Rh^III(en)Cl]⁺ and [(Cp*)Ir^III(bpy)Cl]⁺ do not have activated Cp* rings. The thiol tripeptide glutathione (γ-l-Glu-l-Cys-Gly, GSH) and the activated dienophile N-methylmaleimide, (NMM) did not undergo addition reactions with the proposed Rh^I-fulvene, although they were able to control the extent of Cp* deuteration. We readily trapped and characterized Rh^I-fulvene intermediates by Diels-Alder [4+2] cyclo-addition reactions with the natural biological dienes isoprene and conjugated (9Z,11E)-linoleic acid in aqueous media, including cell culture medium, the first report of a Diels-Alder reaction of a metal-bound fulvene in aqueous solution. These findings will introduce new concepts into the design of organometallic Cp* anticancer complexes with novel mechanisms of action.

Electronic and Steric Tuning of a Prototypical Piano Stool Complex: Rh(III) Catalysis for C-H Functionalization

The history of transition metal catalysis is heavily steeped in ligand design, clearly demonstrating the importance of this approach. The intimate relationship between metal and ligand can profoundly affect the outcome of a reaction, often impacting selectivity, physical properties, and the lifetime of a catalyst. Importantly, this metal-ligand relationship can provide near limitless opportunities for reaction discovery. Over the past several years, transition-metal-catalyzed C-H bond functionalization reactions have been established as a critical foundation in organic chemistry that provides new bond forming strategies. Among the d-block elements, palladium is arguably one of the most popular metals to accomplish such transformations. One possible explanation for this achievement could be the broad set of phosphine and amine based ligands available in the chemist's toolbox compatible with palladium. In parallel, other metals have been investigated for C-H bond functionalization. Among them, pentamethylcyclopentadienyl (Cp*) Rh(III) complexes have emerged as a powerful mode of catalysis for such transformations providing a broad spectrum of reactivity. This approach possesses the advantage of often very low catalyst loading, and reactions are typically performed under mild conditions allowing broad functional group tolerance. Cp*Rh(III) is considered as a privileged catalyst and a plethora of reactions involving a C-H bond cleavage event have been developed. The search for alternative cyclopentadienyl based ligands has been eclipsed by the tremendous effort devoted to exploring the considerable scope of reactions catalyzed by Cp*Rh(III) complexes, despite the potential of this strategy for enabling reactivity. Thus, ligand modification efforts in Rh(III) catalysis have been an exception and research directed toward new rhodium catalysts has been sparse. Recently, chiral cyclopentadienyl ligands have appeared allowing enantioselective Rh(III)-catalyzed C-H functionalization reactions to be performed. Alongside chiral ligands, an equally important collection of achiral cyclopentadienyl-derived ligands have also emerged. The design of this new set of ligands for rhodium has already translated to significant success in solving inherent problems of reactivity and selectivity encountered throughout the development of new Rh(III)-catalyzed transformations. This Account describes the evolution of cyclopentadienyl ligand skeletons in Rh(III)-catalysis since the introduction of pentamethylcyclopentadienyl ligands to the present. Specific emphasis is placed on reactivity and synthetic applications achieved with the new ligands with the introduction of achiral mono-, di-, or pentasubstituted cyclopentadienyl ligands exhibiting a stunning effect on reactivity and selectivity. Furthermore, an underlying question when dealing with ligand modification strategies is to explain the reason one ligand outperforms another. Conjecture and speculation abound, but extensive characterization of their steric and electronic properties has been carried out and information about electronic and steric properties of the ligands all contribute to our understanding and give crucial pieces to solve the puzzle.

Schmid T, Toupet L, Dorcet V, Mauduit M, Baslé O. Directed ortho C–H borylation catalyzed using Cp*Rh(iii)–NHC complexes

Cp*Rh(NHC) complexes with bulky chiral bidentate NHC-carboxylate ligands demonstrated high selectivity in the pyridine-directed C–H borylation of arenes under mild conditions.

Computational study of selectivity in the [PtIICl4]2--catalysed arylation of arenes by diaryliodonium reagents: arene activation at PtIV centres

The mechanism for the [Pt^IICl₄]^2--catalysed reaction of Ph₂I^III(TFA) (TFA = trifluoroacetate) with naphthalene (NapH) to give β-phenylation over α-phenylation of naphthalene has been examined by computational methods. In support of the mechanism proposed by Sanford based on experimental evidence, reaction commences by oxidative phenyl transfer from [Ph₂I]⁺ to [Pt^IICl₄]^2-, giving trans-[PhPt^IVCl₄(TFA)]^2-. Transformation to cis-[PhPt^IVCl₃(TFA)]^- leads to reaction with NapH at the β-position in an inner-sphere Concerted Metalation Deprotonation (CMD) manner to give cis-[Ph(Nap)Pt^IVCl₃]^- and trifluoroacetic acid. Reductive elimination yields β-PhNap, and coordination of chloride regenerates [Pt^IICl₄]^2- for subsequent catalytic cycles. The selectivity for β-phenylation over α-phenylation is attributable to steric factors in the CMD Pt^IV transition state containing a higher coordination number than that occurring for related reactions in Pd^II catalysis that gives α-phenylation.

Large-Scale Selective Functionalization of Alkanes

Great progress has been made in the past several decades concerning C-H bond functionalization. But despite many significant advances, a commercially viable large-scale process for selective alkane functionalization remains an unreached goal. Such conversions will require highly active, selective, and long-lived catalysts. In addition, essentially complete atom-economy will be required. Thus, any reagents used in transforming the alkanes must be almost free (e.g., O₂, H₂O, N₂), or they should be incorporated into the desired large-scale product. Any side-products should be completely benign or have value as fuels (e.g., H₂ or other alkanes). Progress and promising leads toward the development of such systems involving primarily molecular transition metal catalysts are described.

Diboron(4) Compounds: From Structural Curiosity to Synthetic Workhorse

Although known for over 90 years, only in the past two decades has the chemistry of diboron(4) compounds been extensively explored. Many interesting structural features and reaction patterns have emerged, and more importantly, these compounds now feature prominently in both metal-catalyzed and metal-free methodologies for the formation of B-C bonds and other processes.