Photoinduced Metallonitrene Formation by N2 Elimination from Azide Diradical Ligands

Transition-metal nitrides/nitrenes are highly promising reagents for catalytic nitrogen-atom-transfer reactivity. They are typically prepared in situ upon optically induced N₂ elimination from azido precursors. A full exploitation of their catalytic potential, however, requires in-depth knowledge of the primary photo-induced processes and the structural/electronic factors mediating the N₂ loss with birth of the terminal metal-nitrogen core. Using femtosecond infrared spectroscopy, we elucidate here the primary molecular-level mechanisms responsible for the formation of a unique platinum(II) nitrene with a triplet ground state from a closed-shell platinum(II) azide precursor. The spectroscopic data in combination with quantum-chemical calculations provide compelling evidence that product formation requires the initial occupation of a singlet excited state with an anionic azide diradical ligand that is bound to a low-spin d⁸-configured Pt^II ion. Subsequent intersystem crossing generates the Pt-bound triplet azide diradical, which smoothly evolves into the triplet nitrene via N₂ loss in a near barrierless adiabatic dissociation. Our data highlight the importance of the productive, N₂-releasing state possessing azide ππ* character as a design principle for accessing efficient N-atom-transfer catalysts.     

Stable Oxindolyl‐Based Analogues of Chichibabin's and Müller's Hydrocarbons

Chichibabin's and Müller's hydrocarbons are classical open‐shell singlet diradicaloids but they are highly reactive. Herein we report the successful synthesis of their respective stable analogues, OxR‐2 and OxR‐3 , based on the newly developed oxindolyl radical. X‐ray crystallographic analysis on OxR‐2 reveals a planar quinoidal backbone similar to Chichibabin's hydrocarbon, in accordance with its small diradical character (y₀=11.1 %) and large singlet–triplet gap (ΔE_S‐T=−10.8 kcal mol⁻¹). Variable‐temperature NMR studies on OxR‐2 disclose a slow cis/trans isomerization process in solution through a diradical transition state, with a moderate energy barrier (ΔG^≠_298K=15–16 kcal mol⁻¹). OxR‐3 exhibits a much larger diradical character (y₀=80.6 %) and a smaller singlet–triplet gap (ΔE_S‐T=−3.5 kcal mol⁻¹), and thus can be easily populated to paramagnetic triplet diradical. Our studies provide a new type of stable carbon‐centered monoradical and diradicaloid.     

A theoretical study of electronic effects in alkyne‐linked reactive intermediates: (nitrenoethynyl)methylenes, (nitrenoethynyl)silylenes,and (nitrenoethynyl)germylenes

Acetylene‐linked reactive intermediates of (nitrenoethynyl)‐X‐methylenes, (nitrenoethynyl)‐X‐silylenes, and (nitrenoethynyl)‐X‐germylenes are almost experimentally unreachable (X–M–C≡C–N; X=H ( 1 ), CN ( 2 ), OH ( 3 ), NH₂ ( 4 ), NO₂ ( 5 ), and CHO ( 6 ); M=C, Si, and Ge). The effects of the electron‐donating and electron withdrawing groups were compared and contrasted at seven levels of theory. All singlet species as ground states with one local open‐shell singlet carbene subunit (π¹π¹) and another local open‐shell singlet nitrene subunit (π¹π¹) were found to be more stable than their corresponding triplets including one local open‐shell singlet carbene (δ¹π¹) (or one local closed‐shell singlet carbene [δ²π⁰]) and another local triplet nitrene subunit (π¹π¹) with 45.94–77.996 kcal/mol singlet–triplet energy gap (ΔE_s‐t). Their relative silylenes and germylenes made reduction of ΔE_s‐t, so the triplet ground states were found for species 3 _Si , 4 _Si , 5 _Si , 2 _Ge , 3 _Ge , 4 _{Ge ,} and 5 _Ge . All the singlet silylenes/germylenes formed by one local closed‐shell singlet silylenes/germylenes (δ²π⁰) and one local closed‐shell singlet nitrene subunit (π²π⁰). Also, one local closed‐shell singlet silylene/germylene subunit (δ²π⁰) and one local triplet nitrene subunit (π¹π¹) were observed for triplet silylenes/germylenes. The singlet and triplet species 3 _Si , 4 _Si , 3 _Ge , and 4 _Ge , due to their electrophilic (Si₄/Ge₄) and nucleophilic (X₅) centers, could be identified as intermediates in chemical reactions.     

Elevated Catalytic Activity of Ruthenium(II)–Porphyrin‐Catalyzed Carbene/Nitrene Transfer and Insertion Reactions with N‐Heterocyclic Carbene Ligands

Bis(NHC)ruthenium(II)–porphyrin complexes were designed, synthesized, and characterized. Owing to the strong donor strength of axial NHC ligands in stabilizing the trans M?CRR′/M?NR moiety, these complexes showed unprecedently high catalytic activity towards alkene cyclopropanation, carbene C? H, N? H, S? H, and O? H insertion, alkene aziridination, and nitrene C? H insertion with turnover frequencies up to 1950 min^?1. The use of chiral [Ru(D₄‐Por)(BIMe)₂] ( 1 g ) as a catalyst led to highly enantioselective carbene/nitrene transfer and insertion reactions with up to 98 % ee. Carbene modification of the N terminus of peptides at 37 °C was possible. DFT calculations revealed that the trans axial NHC ligand facilitates the decomposition of diazo compounds by stabilizing the metal–carbene reaction intermediate.     

Olefin cis‐Dihydroxylation and Aliphatic C?H Bond Oxygenation by a Dioxygen‐Derived Electrophilic Iron–Oxygen Oxidant

Many iron‐containing enzymes involve metal–oxygen oxidants to carry out O₂‐dependent transformation reactions. However, the selective oxidation of C H and CC bonds by biomimetic complexes using O₂ remains a major challenge in bioinspired catalysis. The reactivity of iron–oxygen oxidants generated from an Fe^II–benzilate complex of a facial N₃ ligand were thus investigated. The complex reacted with O₂ to form a nucleophilic oxidant, whereas an electrophilic oxidant, intercepted by external substrates, was generated in the presence of a Lewis acid. Based on the mechanistic studies, a nucleophilic Fe^II–hydroperoxo species is proposed to form from the benzilate complex, which undergoes heterolytic O O bond cleavage in the presence of a Lewis acid to generate an Fe^IV–oxo–hydroxo oxidant. The electrophilic iron–oxygen oxidant selectively oxidizes sulfides to sulfoxides, alkenes to cis‐diols, and it hydroxylates the C H bonds of alkanes, including that of cyclohexane.     

Innentitelbild: Catalytic Asymmetric Functionalization of Aromatic C?H Bonds by Electrophilic Trapping of Metal‐Carbene‐Induced Zwitterionic Intermediates (Angew. Chem. 48/2014)

Asymmetric functionalization of aromatic C H bonds of N,N‐disubstituted anilines with diazo compounds and imines is reported for the efficient construction of α,α‐diaryl benzylic quaternary stereocenters in good yields with high diastereoselectivities and excellent enantioselectivities. This Rh^II/chiral phosphoric acid cocatalyzed transformation is proposed to proceed through a metal‐carbene‐induced zwitterionic intermediate which undergoes electrophilic trapping. To the best of our knowledge, this is the first asymmetric example of metal carbene‐induced intermolecular functionalization of aryl C H bonds.     

Rhodium(II)-Catalyzed CH Insertions with {[(4-Nitrophenyl)sulfonyl]imino}phenyl-λ3-iodane

The [Rh₂(OAc)₄]-catalyzed decomposition of {[(4-nitrophenyl)sulfonyl]imino}phenyl-λ³-iodane (NsN?IPh) resulted in formal insertions into CH bonds, activated by phenyl or vinyl groups, or by O-substituents. Scope and limitations of the reaction were investigated. Yields of up to 84% were achieved in the most favorable cases. Yields were enhanced by electron-releasing substituents and decreased by steric hindrance. Aziridination competed with allylic insertion with olefinic substrates. The insertion reaction proceeded with retention of configuration. With chiral Rh^II catalysts, a modest asymmetric induction was observed. A mechanism involving direct insertion by a Rh-complexed nitrene into the CH bond is proposed.     

Amide‐Functionalized Naphthyridines on a RhII–RhII Platform: Effect of Steric Crowding,Hemilability, and Hydrogen‐Bonding Interactions on the Structural Diversity and Catalytic Activity of Dirhodium(II) Complexes

Ferrocene‐amide‐functionalized 1,8‐naphthyridine (NP) based ligands {[(5,7‐dimethyl‐1,8‐naphthyridin‐2‐yl)amino]carbonyl}ferrocene (L¹H) and {[(3‐phenyl‐1,8‐naphthyridin‐2‐yl)amino]carbonyl}ferrocene (L²H) have been synthesized. Room‐temperature treatment of both the ligands with Rh₂(CH₃COO)₄ produced [Rh₂(CH₃COO)₃(L¹)] ( 1 ) and [Rh₂(CH₃COO)₃(L²)] ( 2 ) as neutral complexes in which the ligands were deprotonated and bound in a tridentate fashion. The steric effect of the ortho‐methyl group in L¹H and the inertness of the bridging carboxylate groups prevented the incorporation of the second ligand on the {Rh^II–Rh^II} unit. The use of the more labile Rh₂(CF₃COO)₄ salt with L¹H produced a cis bis‐adduct [Rh₂(CF₃COO)₄(L¹H)²] ( 3 ), whereas L²H resulted in a trans bis‐adduct [Rh₂(CF₃COO)₃(L²)(L²H)] ( 4 ). Ligand L¹H exhibits chelate binding in 3 and L²H forms a bridge‐chelate mode in 4 . Hydrogen‐bonding interactions between the amide hydrogen and carboxylate oxygen atoms play an important role in the formation of these complexes. In the absence of this hydrogen‐bonding interaction, both ligands bind axially as evident from the X‐ray structure of [Rh₂(CH₃COO)₂(CH₃CN)₄(L²H)₂](BF₄)₂ ( 6 ). However, the axial ligands reorganize at reflux into a bridge‐chelate coordination mode and produce [Rh₂(CH₃COO)₂(CH₃CN)₂(L¹H)](BF₄)₂ ( 5 ) and [Rh₂(CH₃COO)₂(L²H)₂](BF₄)₂ ( 7 ). Judicious selection of the dirhodium(II) precursors, choice of ligand, and adaptation of the correct reaction conditions affords 7 , which features hemilabile amide side arms that occupy sites trans to the Rh–Rh bond. Consequently, this compound exhibits higher catalytic activity for carbene insertion to the C?H bond of substituted indoles by using appropriate diazo compounds, whereas other compounds are far less reactive. Thus, this work demonstrates the utility of steric crowding, hemilability, and hydrogen‐bonding functionalities to govern the structure and catalytic efficacyof dirhodium(II,II) compounds.     

Aluminum five‐ and six‐coordination in bis(acetylacetonato)aminoalkoxides

Reactions of bis(acetylacetonato)aluminum(III)‐di‐μ‐isopropoxo‐di‐isopropoxo aluminum(III), [(CH₃COCHCOCH₃)₂Al(μ‐OPrⁱ)₂Al(OPrⁱ)₂] with aminoalcohols, (HO R NR¹R²) in 1:1 and 1:2 molar ratios in refluxing anhydrous benzene yielded binuclear complexes of the types [(CH₃COCHCOCH₃)₂Al(μ‐OPrⁱ)₂Al(O R NR¹R²)(OPrⁱ)] and [(CH₃COCHCOCH₃)₂Al(μ‐OPrⁱ)₂Al(O R NR¹R²)₂] (R   (CH₂)₃ , R¹ = R² = H; R =  CH₂C(CH₃)₂ , R¹ = R² = H; R =  (CH₂)₂ , R¹ = H, R² =  CH₃; and R   (CH₂)₂ , R¹ = R² = CH₃), respectively. All these compounds are soluble in common organic solvents and exhibit sharp melting points. Molecular weight determinations reveal their binuclear nature in refluxing benzene. Plausible structures have been proposed on the basis of elemental analysis, molecular weight measurements, IR, NMR (¹H, ¹³C, and ²⁷Al), and FAB mass spectral studies. ²⁷Al NMR spectra show the presence of both five‐ and six‐coordinated aluminum sites. © 2003 Wiley Periodicals, Inc. Heteroatom Chem 14:518–522, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10184     

Construction of α-phosphonolactams via rhodium (II)-catalyzed intramolecular C?H insertion reactions

The Rh(II)-catalyzed intramolecular C H insertion reactions of N,N-dialkyl-α-diazo-α-(diethylphosphono)acetamides 2a , f–j in CHCl₃ or ClCH₂CH₂Cl were found to give monocyclic and bicyclic α-phosphono-β-lactams, 3a and 3f–j , in 43–67% yields via regiospecific α-C H insertion of the N-alkyl groups. Similar treatment of N-benzyl-N-isopropyl-α-diazo-α-(diethylphosphono)acetamide ( 2b ) and the corresponding N-isobutyl-N-methylacetamide 2d in ClCH₂CH₂Cl afforded mixtures of β-lactams 3b (35%) and and 3b ′ (16%), β-lactam 3d (47%), and γ-lactam 4d (10%), respectively, each of which is formed by the competitive C H insertion reaction between benzylic and isopropyl α-C H bonds and between methyl α-C H and methine β-C H bonds, respectively. For the formation of β-lactams, the selectivity in the rhodium-mediated C H insertion in ClCH₂CH₂Cl follows the order methyl > methine > benzylic α-C H bond on N-substituents. The N,N-dibutyl-α-diazo homologue 2c and Nα[α-diazo-α-(diethylphosphono)acetyl]-2-methylindoline ( 2k ) exclusively produced γ-lactams 4c (67%) and 4k (81%) via insertion into the methylene β-C H and methyl β-C H bonds. tert-Butyl N-[α-diazo-α-(dibenzylphosphono)acetyl]-piperidine-2-carboxylate ( 2m ) on similar treatment, followed by deprotection of the benzyl ester afforded the 7-phosphono carbacepham 6 in 32% overall yield. Similar Rh(II)-catalyzed cyclization of N-methyl-N[4-benzyloxy-α-(diethylphosphono)-phenyl(diethyl-phosphono)methyl]-α-diazo-acetamide ( 2n ) led to 1-[4′-benzylphenyl(diethylphosphono)methyl] -3-(diethyl-phosphono)azetidin-2-one ( 3n ) in 78% yicld. The phosphono group at C-7 of 3f was converted into the acetylamino group via a four-step reaction. Application of chiral rhodium(II) carboxylates 12a–c to the insertion reactions of 2b , c produced α-phosphono-β-and γ-lactams, 3b and 4c , in 6–24% ee and 25–29% ee, respectively.     

Crystalline Diradical Dianions of Pyrene‐Fused Azaacenes

Although diradicals and azaacenes have been greatly attractive in fundamental chemistry and functional materials, the isolable diradical dianions of azaacenes are still unknown. Herein, we describe the first isolation of pyrene‐fused azaacene diradical dianion salts [(18‐c‐6)K(THF)₂]⁺[(18‐c‐6)K]⁺? 1 ^2?.. and [(18‐c‐6)K(THF)]²⁺? 2 ^2?.. by reduction of the neutral pyrene‐fused azaacene derivatives 1 and 2 with excess potassium graphite in THF in the presence of 18‐crown‐6. Their electronic structures were investigated by various experiments, in conjunction with theoretical calculations. It was found that both dianions are open‐shell singlets in the ground state and their triplet states are thermally readily accessible owing to the small singlet–triplet energy gap. This work provides the first examples of crystalline diradical dianions of azaacenes with considerable diradical character.     

Dehydrative Direct C?H Allylation with Allylic Alcohols under [Cp*CoIII] Catalysis

The unique reactivity of [Cp*Co^III] over [Cp*Rh^III] was demonstrated. A cationic [Cp*Co^III] catalyst promoted direct dehydrative C H allylation with non‐activated allyl alcohols, thus giving C2‐allylated indoles, pyrrole, and phenyl‐pyrazole in good yields, while analogous [Cp*Rh^III] catalysts were not effective. The high γ‐selectivity and C2‐selectivity indicated that the reaction proceeded by directing‐group‐assisted C H metalation. DFT calculations suggested that the γ‐selective substitution reaction proceeded by C H metalation and insertion of a C C double bond, with subsequent β‐hydroxide elimination. The [Cp*Co^III] catalyst favored β‐hydroxide elimination over β‐hydride elimination.     

Rhodium‐Catalyzed Alkene Difunctionalization with Nitrenes

The Rh^II‐catalyzed oxyamination and diamination of alkenes generate 1,2‐amino alcohols and 1,2‐diamines, respectively, in good to excellent yields and with complete regiocontrol. In the case of diamination, the intramolecular reaction provides an efficient method for the preparation of pyrrolidines, and the intermolecular reaction produces vicinal amines with orthogonal protecting groups. These alkene difunctionalizations proceed by aziridination followed by nucleophilic ring opening induced by an Rh‐bound nitrene generated in situ, details of which were uncovered by both experimental and theoretical studies. In particular, DFT calculations show that the nitrogen atom of the putative [Rh]₂=NR metallanitrene intermediate is electrophilic and support an aziridine activation pathway by N ??? N=[Rh]₂ bond formation, in addition to the N ??? [Rh]₂=NR coordination mode.     

Observation of a Thermally Accessible Triplet State Resulting from Rotation around a Main‐Group π Bond

We report the first direct spectroscopic observation by electron paramagnetic resonance (EPR) spectroscopy of a triplet diradical that is formed in a thermally induced rotation around a main‐group π bond, that is, the Si?Si double bond of tetrakis(di‐tert‐butylmethylsilyl)disilene ( 1 ). The highly twisted ground‐state geometry of singlet 1 allows access to the perpendicular triplet diradical 2 at moderate temperatures of 350–410 K. DFT‐calculated zero‐field splitting (ZFS) parameters of 2 accurately reproduce the experimentally observed half‐field transition. Experiment and theory suggest a thermal equilibrium between 1 and 2 with a very low singlet–triplet energy gap of only 7.3 kcal mol^?1.     

Azavinylidenephosphoranes: A Class of Cyclic Push–Pull Carbenes

The synthesis of a novel family of cyclic push–pull carbenes, namely, azavinylidene phosphoranes, is described. The methodology is based on a formal [3+2] cycloaddition between terminal alkynes and phosphine–imines followed by an oxidation/deprotonation step. Carbenes 6 , obtained by simple deprotonation, exhibit typical transient carbene reactivity like the intramolecular C?H insertion reaction and a pronounced ambiphilic character exemplified by [2+1] cycloaddition with electron‐poor methyl acrylate. Owing to the cyclic structure, carbenes 6 also exhibit an excellent coordination ability toward transition metals. Rh^I complex 10 was obtained in excellent yield and was fully characterized by multinuclear NMR spectroscopy and X‐ray crystallography. The corresponding Rh^I–carbonyl complex was also prepared; this indicates that carbenes 6 belong to the strongest σ‐donating ligands to date. DFT calculations confirmed the high σ‐donation ability of 6 and their classification as push–pull carbenes with a relatively small singlet–triplet energy gap of 23.2–24.3 kcal mol^?1.     

Communication: The insertion of silylene in C?H bonds; rate constant limits and the energy barrier

The technique of laser flash photolysis has been used to set limits on the rate constants for the bimolecular reactions of SiH₂ with methane (CH₄) and tetramethylsilane (SiMe₄) at both ambient and elevated temperatures (ca 600 K). These limits show that the energy barriers to insertion reactions of SiH₂ in the C H bonds of CH₄ are at least 45(±6) kJ mol⁻¹ and in the C H and/or Si C bonds of SiMe₄ are at least 23(±6) kJ mol⁻¹. The best thermochemical estimate of the activation energy for SiH₂+CH₄ is 59(±12) kJ mol⁻¹. Reasons for the greatly diminished reactivity of SiH₂ with C H as compared with Si H bonds are discussed. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 393–395, 1999     

Rhodium(III)‐Catalyzed C?C and C?O Coupling of Quinoline N‐Oxides with Alkynes: Combination of C?H Activation with O‐Atom Transfer

[Cp*Rh^III]‐catalyzed C H activation of arenes assisted by an oxidizing N O or N N directing group has allowed the construction of a number of hetercycles. In contrast, a polar N O bond is well‐known to undergo O‐atom transfer (OAT) to alkynes. Despite the liability of N O bonds in both C H activation and OAT, these two important areas evolved separately. In this report, [Cp*Rh^III] catalysts integrate both areas in an efficient redox‐neutral coupling of quinoline N‐oxides with alkynes to afford α‐(8‐quinolyl)acetophenones. In this process the N O bond acts as both a directing group for C H activation and as an O‐atom donor.     

C?H Bond Activation by Metal–Superoxo Species: What Drives High Reactivity?

Metal–superoxo species are ubiquitous in metalloenzymes and bioinorganic chemistry and are known for their high reactivity and their ability to activate inert C H bonds. The comparative oxidative abilities of M–O₂^.− species (M=Cr^III, Mn^III, Fe^III, and Cu^II) towards C H bond activation reaction are presented. These superoxo species generated by oxygen activation are found to be aggressive oxidants compared to their high‐valent metal–oxo counterparts generated by O⋅⋅⋅O bond cleavage. Our calculations illustrate the superior oxidative abilities of Fe^III– and Mn^III–superoxo species compared to the others and suggest that the reactivity may be correlated to the magnetic exchange parameter.     

Nitrenes and the Decomposition of Carbonylazides

The decomposition of organic carbonylazides can lead to the formation of nitrenes. Ethoxycarbonylnitrene is formed in the photolytic and thermal decomposition of ethyl azidoformate and by α-elimination from N-(p-nitrobenzenesulfonyloxy)urethan. Both of the possible electronic states of this nitrene take part in intermolecular reactions. Pure singlet nitrene is formed by α-elimination from the urethan and on thermal decomposition of ethyl azidoformate, but changes so rapidly into the triplet form that the reactions of both forms are observed. Singlet ethoxycarbonylnitrene undergoes selective and stereospecific insertion into C? H bonds and adds stereospecifically to olefins. Triplet ethoxycarbonylnitrene, however, does not undergo insertion into C? H bonds, and adds to olefins with complete loss of the geometric configuration. By following quantitatively the stereospecificity of the addition reaction and by selective interception of the triplet and singlet forms of the nitrene, it can be shown that the photolysis of ethyl azidoformate leads directly to nitrene of which one third is in the triplet state. In the decomposition of aryl- and alkylcarbonylazides (acid azides), the removal of nitrogen is accompanied by a synchronous rearrangement to isocyanates (Curtius rearrangement). In this system, nitrenes are obtained only by photolysis. They add to double bonds and undergo very selective insertion into C? H bonds, but do not rearrange at a measurable rate to isocyanates. The photolytic Curtius rearrangement is also a concerted reaction.     

1,1‐Dilithioethylene: Toward Spectroscopic Identification of the Definitive Singlet Ground Electronic State of a Peculiar Structure

1,1‐Dilithioethylene is a prototypical carbon–lithium compound that is not known experimentally. All low‐lying singlet and triplet structures of interest were investigated by using high‐level theoretical methods with correlation‐consistent basis sets up to pentuple ζ. The coupled cluster methods adopted included up to full triple excitations and perturbative quadruples. In contrast to earlier studies that predicted the twisted C_2v triplet to be the ground state, we found a peculiar planar C_s singlet ground state in the present research. The lowest excited electronic state of 1,1‐dilithioethylene, the twisted C_s triplet, was found to lie 9.0 kcal mol^?1 above the ground state by using energy extrapolation to the complete basis set limit. For the planar C_s singlet and twisted C_s triplet states of 1,1‐dilithioethylene, anharmonic vibrational frequencies were reported on the basis of second‐order vibrational perturbation theory. The remarkably low (2050 cm^?1) C?H stretching fundamental (the C?H bond near the bridging lithium) of the singlet state was found to have very strong infrared intensity. These highly reliable theoretical findings may assist in the long‐sought experimental identification of 1,1‐dilithioethylene. Using natural bond orbital analysis, we found that lithium bridging structures were strongly influenced by electrostatic effects. All carbon–carbon linkages corresponded to conventional double bonds.