Fe-catalyzed multicomponent reactions: the regioselective alkoxy allylation of activated olefins and its application in sequential Fe catalysis

We present herein a versatile and broadly applicable Fe-catalyzed regioselective alkoxy allylation of activated double bonds. Substituted allylic carbonates are converted into the corresponding σ-enyl Fe complexes by reaction with Bu(4)N[Fe(CO)(3)(NO)] (TBAFe) at 30?°C. The liberated alkoxide adds to an activated double bond with the generation of a C-nucleophile, which is trapped by the σ-enyl Fe complex in a regioselective manner. Alternatively, the alkoxide acts as a base in deprotonating an external pronucleophile, which undergoes Michael addition. The method is characterized by a broad functional group tolerance, mild reaction conditions, low catalyst loadings, and high regioselectivities in favor of the ipso-substitution product.     

Enamides via Long‐Distance Migration of Double Bonds

A new method for preparation of enamides (N‐(alken‐1‐yl) amides) by means of the ‘long‐distance' migration of the double bond in unsaturated amides in the presence of [Fe(CO)₅] is described. The method is shown to be particularly useful for the isomerization of N‐(but‐3‐enyl)amides, while, in the case of N‐(pent‐4‐enyl) and N‐(hex‐5‐enyl) amides the mixture of products was formed and the yield of the enamide was relatively low.     

Regioselective Insertion of Aluminum(I) in the cyclo‐P5 Ring of Pentaphosphaferrocene

A route to directly access mixed Al–Fe polyphosphide complexes was developed. The reactivity of pentaphosphaferrocene, [Cp*Fe(η⁵‐P₅)] (Cp*=C₅Me₅), with two different low‐valent aluminum compounds was investigated. The steric and electronic environment around the [Al^I] centre are found to be crucial for the formation of the resulting Al–Fe polyphosphides. Reaction with the sterically demanding [Dipp‐BDIAl^I] (Dipp‐BDI={[2,6‐ⁱPr₂C₆H₃NCMe]₂CH}^?) resulted in the first Al‐based neutral triple‐decker type polyphosphide complex. For [(Cp*Al^I)₄], an unprecedented regioselective insertion of three [Cp*Al^III]²⁺ moieties into two adjacent P?P bonds of the cyclo‐P₅ ring of [Cp*Fe(η⁵‐P₅)] was observed. The regioselectivity of the insertion reaction could be rationalized by isolating an analogue of the reaction intermediate stabilized by a strong σ‐donor carbene.     

Intramolecular [2+2] Photocycloaddition of 3‐ and 4‐(But‐3‐enyl)oxyquinolones: Influence of the Alkene Substitution Pattern,Photophysical Studies,and Enantioselective Catalysis by a Chiral Sensitizer

The intramolecular [2+2] photocycloaddition of four 4‐(but‐3‐enyl)oxyquinolones (substitution pattern at the terminal alkene carbon atom: CH₂, Z‐CHEt, E‐CHEt, CMe₂) and two 3‐(but‐3‐enyl)oxyquinolones (substitution pattern: CH₂, CMe₂) was studied. Upon direct irradiation at λ=300 nm, the respective cyclobutane products were formed in high yields (83–95 %) and for symmetrically substituted substrates with complete diastereoselectivity. Substrates with a Z‐ or E‐substituted terminal double bond showed a stereoconvergent reaction course leading to mixtures of regio‐ and diastereomers with almost identical composition. The mechanistic course of the photocycloaddition was elucidated by transient absorption spectroscopy. A triplet intermediate was detected for the title compounds, which–in contrast to simple alkoxyquinolones such as 3‐butyloxyquinolone and 4‐methoxyquinolone–decayed rapidly (τ≈1 ns) through cyclization to a triplet 1,4‐diradical. The diradical can evolve through two reaction channels, one leading to the photoproduct and the other leading back to the starting material. When the photocycloaddition was performed in the presence of a chiral sensitizer (10 mol %) upon irradiation at λ=366 nm in trifluorotoluene as the solvent, moderate to high enantioselectivities were achieved. The two 3‐(but‐3‐enyl)oxyquinolones gave enantiomeric excesses (ees) of 60 and 64 % at ?25 °C, presumably because a significant racemic background reaction occurred. The 4‐substituted quinolones showed higher enantioselectivities (92–96 % ee at ?25 °C) and, for the terminally Z‐ and E‐substituted substrates, an improved regio‐ and diastereoselectivity.     

Is Cyclopropane Really the σ‐Aromatic Paradigm?

Dewar proposed the σ‐aromaticity concept to explain the seemingly anomalous energetic and magnetic behavior of cyclopropane in 1979. While a detailed, but indirect energetic evaluation in 1986 raised doubts—“There is no need to involve ‘σ‐aromaticity’,”—other analyses, also indirect, resulted in wide‐ranging estimates of the σ‐aromatic stabilization energy. Moreover, the aromatic character of “in‐plane”, “double”, and cyclically delocalized σ‐electron systems now seems well established in many types of molecules. Nevertheless, the most recent analysis of the magnetic properties of cyclopropane (S. Pelloni, P. Lazzeretti, R. Zanasi, J. Phys. Chem. A 2007 , 111, 8163–8169) challenged the existence of an induced σ‐ring current, and provided alternative explanations for the abnormal magnetic behavior. Likewise, the present study, which evaluates the σ‐aromatic stabilization of cyclopropane directly for the first time, fails to find evidence for a significant energetic effect. According to ab initio valence bond (VB) computations at the VBSCF/cc‐PVTZ level, the σ‐aromatic stabilization energy of cyclopropane is, at most, 3.5 kcal mol^?1 relative to propane, and is close to zero when n‐butane is used as reference. Trisilacyclopropane also has very little σ‐aromatic stabilization, compared to Si₃H₈ (6.3 kcal mol^?1) and Si₄H₁₀ (4.2 kcal mol^?1). Alternative interpretations of the energetic behavior of cyclopropane (and of cyclobutane, as well as their silicon counterparts) are supported.     

Facile C=O Bond Splitting of Carbon Dioxide Induced by Metal–Ligand Cooperativity in a Phosphinine Iron(0) Complex

New iron complexes [Cp*Fe L ]^? ( 1‐σ and 1‐π , Cp*=C₅Me₅) containing the chelating phosphinine ligand 2‐(2′‐pyridyl)‐4,6‐diphenylphosphinine ( L ) have been prepared, and found to undergo facile reaction with CO₂ under ambient conditions. The outcome of this reaction depends on the coordination mode of the versatile ligand L . Interaction of CO₂ with the isomer 1‐π , in which L binds to Fe through the phosphinine moiety in an η⁵ fashion, leads to the formation of 3‐π , in which CO₂ has undergone electrophilic addition to the phosphinine group. In contrast, interaction with 1‐σ —in which L acts as a σ‐chelating [P,N] ligand—leads to product 3‐σ in which one C=O bond has been completely broken. Such CO₂ cleavage reactions are extremely rare for late 3d metals, and this represents the first such example mediated by a single Fe centre.     

A New Synthesis of Pteridines

A new synthesis of pteridines possessing a (substituted) (Z)‐3‐hydroxyprop‐1‐enyl group at C(6) is based on the acylation of 4‐amino‐5‐nitrosopyrimidines with dienoic acid chlorides, followed by a high‐yielding intramolecular hetero‐Diels–Alder cycloaddition and cleavage of the N? O bond leading to 4 . Thermolysis of the resulting pteridines 4 possessing a benzyloxy group at C(4) led to the products 5 , resulting from isomerisation of the 3‐hydroxyprop‐1‐enyl to an 3‐oxopropyl side chain, while the analogous pteridine 8 possessing an NH₂ group at C(4) remained unaffected.     

Construction of Hexahydrophenanthrenes By Rhodium(I)‐Catalyzed Cycloisomerization of Benzylallene‐Substituted Internal Alkynes through C−H Activation

The treatment of benzylallene‐substituted internal alkynes with [RhCl(CO)₂]₂ effects a novel cycloisomerization by C(sp²)?H bond activation to produce hexahydrophenanthrene derivatives. The reaction likely proceeds through consecutive formation of a rhodabicyclo[4.3.0] intermediate, σ‐bond metathesis between the C(sp²)?H bond on the benzene ring and the C(sp²)?Rh^III bond, and isomerization between three σ‐, π‐, and σ‐allylrhodium(III) species, which was proposed based on experiments with deuterated substrates.     

Boryl‐Functionalized σ‐Alkynyl and Vinylidene Rhodium Complexes: Synthesis and Electronic Properties

The synthesis, reactivity, and properties of boryl‐functionalized σ‐alkynyl and vinylidene rhodium complexes such as trans‐[RhCl(?C?CHBMes₂)(PiPr₃)₂] and trans‐[Rh(C?CBMes₂)(IMe)(PiPr₃)₂] are reported. An equilibrium was found to exist between rhodium vinylidene complexes and the corresponding hydrido σ‐alkynyl complexes in solution. The complex trans‐[Rh(C?CBMes₂)(IMe)(PiPr₃)₂] (IMe=1,3‐dimethylimidazol‐2‐ylidene) was found to exhibit solvatochromism and can be quasireversibly oxidized and reduced electrochemically. Density functional calculations were performed to determine the reaction mechanism and to help rationalize the photophysical properties of trans‐[Rh(C?CBMes₂)(IMe)(PiPr₃)₂].     

Synthesis and Characterization of Cyclopropaosmanaphthalenes Containing a Fused σ‐Aromatic Metallacyclopropene Unit

Metalla‐aromatics are important complexes that show unique properties owing to their highly conjugated systems, which show Hückel or Möbius aromaticity. Recently, several metalla‐aromatics showing spiro‐aromaticity or σ‐aromaticity have been reported. Herein, we report the isolation of the first cyclopropametallanaphthalenes, in which the metallacyclopropene ring shows σ‐aromaticity and weak hyperconjugative aromaticity. The reaction of OsCl₂(PPh₃)₃ with o‐ethynylphenyl alkynes in the presence of PPh₃ followed by protonation with HCl yielded the first cyclopropametallanaphthalenes. The reaction mechanism and the aromaticity were also investigated by density functional theory studies.     

Anionic iron(II) alkoxides as initiators for the controlled ring‐opening polymerization of lactide

Hetero‐bimetallic Fe(II) alkoxide/aryloxides were evaluated as initiators for the ring‐opening polymerization of rac‐lactide. [(THF)NaFe(O^tBu)₃]₂ ( 1 ) and [(THF)₄Na₂Fe(2,6‐diisopropylphenolate)₄] ( 2 ) (THF = tetrahydrofuran) both polymerized lactide efficiently at room temperature, with complex 1 affording better control over the molecular weight parameters of the resultant polymer. At conversions below 70%, a linear increase in molecular weight with conversion was observed, indicative of a well‐controlled polymerization process. Complex 2 is the first example of a dianionic Fe(II) alkoxide and has been structurally characterized to reveal a distorted square planar FeO₄ array in which both Na counterions bridge two aryloxide ligands and are further complexed by two THF ligands. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3798–3803, 2003     

Efficient Fe‐Co‐N‐C Electrocatalyst Towards Oxygen Reduction Derived from a Cationic CoII‐based Metal–Organic Framework Modified by Anion‐Exchange with Potassium Ferricyanide

Fe‐Co‐N‐C electrocatalysts have proven superior to their counterparts (e.g. Fe‐N‐C or Co‐N‐C) for the oxygen reduction reaction (ORR). Herein, we report on a unique strategy to prepare Fe‐Co‐N‐C?x (x refers to the pyrolysis temperature) electrocatalysts which involves anion‐exchange of [Fe(CN)₆]^3? into a cationic Co^II‐based metal‐organic framework precursor prior to heat treatment. Fe‐Co‐N‐C‐900 exhibits an optimal ORR catalytic performance in an alkaline electrolyte with an onset potential (E_onset: 0.97 V) and half‐wave potential (E_1/2: 0.86 V) comparable to that of commercial Pt/C (E_onset=1.02 V; E_1/2=0.88 V), which outperforms the corresponding Co‐N‐C‐900 sample (E_onset=0.92 V; E_1/2=0.84 V) derived from the same MOF precursor without anion‐exchange modification. This is the first example of Fe‐Co‐N‐C electrocatalysts fabricated from a cationic Co^II‐based MOF precursor that dopes the Fe element via anion‐exchange, and our current work provides a new entrance towards MOF‐derived transition‐metal (e.g. Fe or Co) and nitrogen‐codoped carbon electrocatalysts with excellent ORR activity.     

Effect of lithium perchlorate on the spontaneous copolymerization of 4‐vinylpyridine with various electron‐rich vinyl monomers

The spontaneous copolymerization of 4‐vinylpyridine (4‐VP) activated with lithium perchlorate (LiClO₄) with various electron rich monomers (p‐methoxystyrene, MeOSt; p‐methylstyrene, MeSt; styrene, St) was investigated in various solvent systems at 75°C. Increasing the LiClO₄ concentration and the nucleophilicity of the electron rich monomer increased the copolymer yields. Both ¹H‐NMR and elemental analysis confirmed the almost 1:1 copolymer structure for VP/MeOSt system which possessed high molecular weight and narrow polydispersity (PDI). Compared to 4‐VP activated with zinc chloride, LiClO₄ systems showed slightly lower yields and much narrower PDI. We also investigated the spontaneous copolymerization of 4‐VP activated with various protic acids in the reaction with various electron rich comonomers. However, generally protic salt forms showed less solubility in organic solvents and showed low molecular weight polymer products with low yields. The proposed initiation mechanism exhibits the formation of a σ‐bond between the β‐carbons of the two donor‐acceptor monomers, creating the 1,4‐tetramethylene biradical intermediate initiating the copolymerization. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1709–1716, 1999     

DFT studies on the mechanism of Ag2CO3‐catalyzed hydroazidation of unactivated terminal alkynes with TMS‐N3: An insight into the silver(I) activation mode

Silver‐mediated hydroazidation of unactivated alkynes has been developed as a new method for the synthesis of vinyl azides. Density functional theory calculations toward this reaction reveal that terminal alkynes with TMS‐N₃ participated hydroazidation proceed through HN₃ formation, deprotonation and silver acetylides formation, nucleophilic addition, and protonation of terminal carbon by AgHCO₃. It is also found that water molecules and activation modes of Ag (I) have a significant influence on the title reaction mechanism. Initially, catalyst Ag₂CO₃ coordinates preferentially with internal N atom of TMS–N₃ to assist water as hydrogen source and proton‐shuttle in facilitating HN₃ formation. Then, the regioselective anti‐addition of HN₃ to triple bond of active silver‐acetylide or ethynyl carbinols affords product vinyl azide via Ag–C σ‐bond activation or Ag…C π‐coordination activation modes, and the former one is more favorable. The origin of the difference regioselectivity is ascribed to the electronic and orbital effects of the reactive sites. Moreover, Ag₂CO₃ is the critical catalyst, acting as activator, base, and stabilizer to promote the HN₃ and vinyl azide formation. Water molecule plays an important role as proton shuttle to promote HN₃ and key active silver acetylides formation, thus improving the yield of product. © 2017 Wiley Periodicals, Inc.     

Stereoselective Rhodium‐Catalysed [2+2+2] Cycloaddition of Linear Allene–Ene/Yne–Allene Substrates: Reactivity and Theoretical Mechanistic Studies

Allene–ene–allene ( 2 and 5 ) and allene–yne–allene ( 3 and 7 ) N‐tosyl and O‐linked substrates were satisfactorily synthesised. The [2+2+2] cycloaddition reaction catalysed by the Wilkinson catalyst [RhCl(PPh₃)₃] was evaluated. Substrates 2 and 5 , which bear a double bond in the central position, gave a tricyclic structure in a reaction in which four contiguous stereogenic centres were formed as a single diastereomer. The reaction of substrates 3 and 7 , which bear a triple bond in the central position, gave a tricyclic structure with a cyclohexenic ring core, again in a diastereoselective manner. All cycloadducts were formed by a regioselective reaction of the inner allene double bond and, therefore, feature an exocyclic diene motif. A Diels–Alder reaction on N‐tosyl linked cycloadducts 8 and 10 allowed pentacyclic scaffolds to be diastereoselectively constructed. The reactivity of the allenes on [2+2+2] cycloaddition reactions was studied for the first time by density functional theory calculations. This mechanistic study rationalizes the order in which the unsaturations take part in the catalytic cycle, the reactivity of the two double bonds of the allene towards the [2+2+2] cycloaddition reaction, and the diastereoselectivity of the reaction.     

Modification Pathways for Copoly(2‐oxazoline)s Enabling Their Application as Antireflective Coatings in Photolithography

Chromophore‐functionalized copoly(2‐oxazoline)s are successfully evaluated as bottom antireflective coatings (BARCs) in high‐resolution photolithography. With respect to UV light sources used in photolithographic production routines, anthracene is chosen as a chromophore. For application as polymer in BARCs, the copolymer poly(2‐ethyl‐2‐oxazolin)₄₅‐stat‐poly(2‐dec‐9′‐enyl‐2‐oxazolin)₂₀‐stat‐poly(2‐(3′‐(1″‐(anthracen‐9‐ylmethyl)‐1″,2″,3″‐triazol‐4‐yl)propyl)‐2‐oxazolin)₃₅ can be synthesized by the Huisgen cycloaddition click reaction of the copolymer poly(2‐ethyl‐2‐oxazolin)₄₅‐stat‐poly(2‐dec‐9′‐enyl‐2‐oxazolin)₂₀‐stat‐poly(2‐pent‐4′‐inyl‐2‐oxazolin)₃₅ and the corresponding azide‐functionalized anthracenes. These copolymers can be crosslinked by the thermally induced thiol‐ene reaction involving the unsaturated C=C bonds of the poly(2‐dec‐9′‐enyl‐2‐oxazoline) repetition units and a multifunctional thiol as crosslinker. Tests of this BARC in a clean room under production conditions reveal a significant decrease of the swing‐curve of a chemically amplified positive photoresist by more than 50%, hence significantly increasing the resolution of the photoresist.

     

A Square‐Planar Tetracoordinate Oxygen‐Containing Ti4O17 Cluster Stabilized by Two 1,1′‐Ferrocenedicarboxylato Ligands

By introducing steric constraints into molecular compounds, it is possible to achieve atypical coordination geometries for the elements. Herein, we demonstrate that a titanium‐oxo cluster [{Ti₄(μ₄‐O)(μ₂‐O)₂}(OPrⁱ)₆(fdc)₂], which possesses a unique edge‐sharing Ti₄O₁₇ octahedron tetramer core, is stabilized by the constraints produced by two orthogonal 1,1′‐ferrocenedicarboxylato (fdc) ligands. As a result, a square‐planar tetracoordinate oxygen (ptO) can be generated. The bonding pattern of this unusual anti‐van’t Hoff/Le Bel oxygen, which has been probed by theoretical calculations, can be described by two horizontally σ‐bonded 2p_x and 2p_y orbitals along with one perpendicular nonbonded 2p_z orbital. While the two ferrocene units are separated spatially by the ptO with an Fe???Fe separation of 10.4 Å, electronic communication between them still takes place as revealed by the cluster’s two distinct one‐electron electrochemical oxidation processes.     

σ‐Noninnocence: Masked Phenyl‐Cation Transfer at Formal NiIV

Reductive elimination is an elementary organometallic reaction step involving a formal oxidation state change of ?2 at a transition‐metal center. For a series of formal high‐valent Ni^IV complexes, aryl–CF₃ bond‐forming reductive elimination was reported to occur readily (Bour et al. J. Am. Chem. Soc. 2015 , 137, 8034–8037). We report a computational analysis of this reaction and find that, unexpectedly, the formal Ni^IV centers are better described as approaching a +II oxidation state, originating from highly covalent metal–ligand bonds, a phenomenon attributable to σ‐noninnocence. A direct consequence is that the elimination of aryl–CF₃ products occurs in an essentially redox‐neutral fashion, as opposed to a reductive elimination. This is supported by an electron flow analysis which shows that an anionic CF₃ group is transferred to an electrophilic aryl group. The uncovered role of σ‐noninnocence in metal–ligand bonding, and of an essentially redox‐neutral elimination as an elementary organometallic reaction step, may constitute concepts of broad relevance to organometallic chemistry.     

Diels–Alder Additions,Ene Reactions,and Condensations of 4‐(Acylamino)‐5‐nitrosopyrimidines – Synthesis of 8‐Substituted Guanines and of 6‐Substituted Pteridinones

4‐(Acylamino)‐5‐nitrosopyrimidines react either by a reductive condensation to provide 8‐substituted guanines, or by a Diels–Alder cycloaddition, or an ene reaction, to provide 6‐substituted pteridinones, depending on the nature of the acyl group and the reaction conditions. Experimental details are provided for the transformation of (acylamino)‐nitrosopyrimidines to 8‐substituted guanines, and the scope of the reaction is further demonstrated by transforming the trifluoro acetamide 25 to the 8‐(trifluoromethyl)guanine ( 27 ), and the N,N′‐bis(nitrosopyrimidinyl)‐dicarboxamide 29 to the (R,R)‐1,2‐di(guan‐8‐yl)ethane‐1,2‐diol ( 32 ). An intramolecular Diels–Alder reaction of the N‐sorbyl (=N‐hexa‐2,4‐dienoyl) nitrosopyrimidine 10 , followed by a spontaneous elimination to cleave the N,O bond of the initial cycloaddition product provided the pteridinones 14 or 15 , characterized by a (Z)‐ or (E)‐3‐hydroxyprop‐1‐enyl group at C(6). Treatment of 10 with Ph₃P led to the C(8)‐penta‐1,3‐dienyl‐guanine 18 . The ene reaction of the N‐crotonyl (=N‐but‐2‐enoyl) nitrosopyrimidine 19 provided the 6‐vinyl‐pteridinone 20a that dimerized readily to 21a , while treatment of 19 with Ph₃P led in high yield to 8‐(prop‐1‐enyl)guanine ( 23 ). The structure of the dimer 21 was established by X‐ray analysis of its bis(N,N‐dimethylformamidine) derivative 21b . The crystal structure of the nitroso amide 10 is characterized by two molecules in the centrosymmetric unit cell. Intermolecular H‐bonds connect the amino group to the amide carbonyl and to N(1). The crystalline bis(purine) 30 forms a left‐handed helix with four molecules per turn and a pitch of 30.2 Å.     

Computational Mechanistic Elucidation of the Intramolecular Aminoalkene Hydroamination Catalysed by Iminoanilide Alkaline‐Earth Compounds

A comprehensive computational exploration of plausible alternative mechanistic pathways for the intramolecular hydroamination (HA) of aminoalkenes by a recently reported class of kinetically stabilised iminoanilide alkaline‐earth silylamido compounds [{N^N}Ae{N(SiMe₃)₂} ? (thf)_n] ({N^N}=iminoanilide; Ae=Ca, Sr, Ba) is presented. On the one hand, a proton‐assisted concerted N?C/C?H bond‐forming pathway to afford the cycloamine in a single step can be invoked and on the other hand, a stepwise σ‐insertive pathway that involves a fast, reversible migratory olefin 1,2‐insertion step linked to a less rapid, irreversible metal?C azacycle tether σ‐bond aminolysis. Notably, these alternative mechanistic avenues are equally consistent with reported key experimental features. The present study, which employs a thoroughly benchmarked and reliable DFT methodology, supports the prevailing mechanism to be a stepwise σ‐insertive pathway that sees an initial conversion of the {N^N}Ae silylamido into the catalytically competent {N^N}Ae amidoalkene compound and involves thereafter facile and reversible insertive N?C bond‐forming ring closure, linked to irreversible intramolecular Ae?C tether σ‐bond aminolysis at the transient {N^N}Ae alkyl intermediate. Turnover‐limiting protonolysis accounts for the substantial primary kinetic isotope effect observed; its DFT‐derived barrier satisfactorily matches the empirically determined Eyring parameter and predicts the decrease in rate observed across the series Ca>Sr>Ba correctly. Non‐competitive kinetic demands militate against the operation of the concerted proton‐assisted pathway, which describes N?C bond‐forming ring closure triggered by concomitant amino proton delivery at the C?C linkage evolving through a multi‐centre TS structure. Valuable insights into the catalytic structure–activity relationships are unveiled by a detailed comparison of [{N^N}Ae(NHR)] catalysts. Moreover, the intriguingly opposite trends in reactivity observed in intramolecular (Ca>Sr>Ba) and intermolecular (Ca<Sr<Ba) HA catalysis for the studied family of iminoanilide alkaline‐earth amido catalysts are rationalised.