Synthesis and Chemistry of Titanacyclopentane and Titanacyclopropane Rings Supported by Aryloxide Ligation

Reductive Retrocyclization of a Mangana(II)cyclopentasilane to Form Manganese(0) Bis(η2-disilene) Complexes

Ligand-exchange reactions on a mangana(II)cyclopentasilane complex that contains two THF ligands with aryl isocyanides led to the formation of manganese(0) bis(η2-disilene) complexes via a retrocyclization. In stark contrast, ligand-exchange reactions with CNtBu, an N-heterocyclic carbene, or pyridine-based ligands furnished manganese(II) complexes wherein the manganacyclopentasilane framework remained intact. The thermolysis of the obtained bis(η2-disilene) complex in the presence of mesityl isocyanide led to the formation of a cyclotetrasilane via the formal dimerization of the two η2-disilene moieties. The insertion of a mesityl isocyanide into the Mn-Siβ bond results in the formation of a manganese(II) complex supported by a [SiCSi]-type tridentate ligand scaffold.

Umpolung of an Aliphatic Ketone to a Magnesium Ketone‐1,2‐diide Complex with Vicinal Dianionic Charge

The new β‐diketimine ^iPrDipnacnacH, HC(iPrCNDip)₂H, Dip=2,6‐iPr₂‐C₆H₃, was converted to the magnesium(I) complex [{(^iPrDipnacnac)Mg}₂] and reaction with 2‐adamantanone (OAd) afforded the ketone‐1,2‐diide complex [{(^iPrDipnacnac)Mg}₂(μ‐OAd)]. The complex contains the first stable dianion of an aliphatic ketone with an electropositive metal and shows an OAd²⁻ unit with long C−O bond and pyramidal carbon centre. DFT studies reveal an anionic charge on both neighbouring C and O atoms. Reductions of aliphatic ketones with magnesium(I) complexes show that these likely proceed via highly reactive dianions and afforded a 1 : 1 mixture of an alkoxide and an enolate when an enolisable ketone was used, and rapid CH activations reactions, e.g., of stabilising ligand moieties, when non‐enolisable ketones were employed.

Ti-Catalyzed Diastereoselective Cyclopropanation of Carboxylic Derivatives with Terminal Olefins

Cyclopropanols and cyclopropylamines not only serve as important structural motifs in medicinal chemistry but also show diverse reactivities in organic synthesis. Owing to the high ring strain energy, the development of a general protocol from stable and readily available starting materials to afford these cyclopropyl derivatives remains a compelling challenge. Herein, we describe that a Ti-based catalyst can effectively promote the diastereoselective syntheses of cyclopropanols and cyclopropylamines from widely accessible carboxylic derivatives (acids, esters, amides) with terminal olefins. To the best of our knowledge, this method represents the first example of direct converting alkyl carboxylic acids into cyclopropanols. Distinct from conventional studies in Ti-mediated cyclopropanations with reactive alkyl Grignard reagents as nucleophiles or reductants, this protocol utilizes Mg and Me₂SiCl₂ to turn over the Ti catalyst. Our method exhibits broad substrate scope with good functional group compatibility and is amenable to late-stage synthetic manipulations of natural products and biologically active molecules.

Ti-Catalyzed and -Mediated Oxidative Amination Reactions

Titanium is an attractive metal for catalytic reaction development: it is earth-abundant, inexpensive, and generally nontoxic. However-like most early transition metals-catalytic redox reactions with Ti are difficult because of the stability of the high-valent Ti^IV state. Understanding the fundamental mechanisms behind Ti redox processes is key for making progress toward potential catalytic applications. This Account details recent progress in Ti-catalyzed (and -mediated) oxidative amination reactions that proceed through formally Ti^II/Ti^IV catalytic cycles.This class of reactions is built on our initial discovery of Ti-catalyzed [2 + 2 + 1] pyrrole synthesis from alkynes and azobenzene, where detailed mechanistic studies have revealed important factors that allow for catalytic turnover despite the inherent difficulty of Ti redox. Two important conclusions from mechanistic studies are that (1) low-valent Ti intermediates in catalysis can be stabilized through coordination of π-acceptor substrates or products, where they can act as "redox-noninnocent" ligands through metal-to-ligand π back-donation, and (2) reductive elimination processes with Ti proceed through π-type electrocyclic (or pericyclic) reaction mechanisms rather than direct σ-bond coupling.The key reactive species in Ti-catalyzed oxidative amination reactions are Ti imidos (Ti≡NR), which can be generated from either aryl diazenes (RN═NR) or organic azides (RN₃). These Ti imidos can then undergo [2 + 2] cycloadditions with alkynes, resulting in intermediates that can be coupled to an array of other unsaturated functional groups, including alkynes, alkenes, nitriles, and nitrosos. This basic reactivity pattern has been extended into a broad range of catalytic and stoichiometric oxidative multicomponent coupling reactions of alkynes and other reactive small molecules, leading to multicomponent syntheses of various heterocycles and aminated building blocks.For example, catalytic oxidative coupling of Ti imidos with two different alkynes leads to pyrroles, while stoichiometric oxidative coupling with alkynes and nitriles leads to pyrazoles. These heterocycle syntheses often yield substitution patterns that are complementary to those of classical condensation routes and provide access to new electron-rich, highly substituted heteroaromatic scaffolds. Furthermore, catalytic oxidative alkyne carboamination reactions can be accomplished via reaction of Ti imidos with alkynes and alkenes, yielding α,β-unsaturated imine or cyclopropylimine building blocks. New catalytic and stoichiometric oxidative amination methods such as alkyne α-diimination, isocyanide imination, and ring-opening oxidative amination of strained alkenes are continuously emerging as a result of better mechanistic understanding of Ti redox catalysis.Ultimately, these Ti-catalyzed and -mediated oxidative amination methods demonstrate the importance of examining often-overlooked elements like the early transition metals through the lens of modern catalysis: rather than a lack of utility, these elements frequently have undiscovered potential for new transformations with orthogonal or complementary selectivity to their late transition metal counterparts.

Synthesis of a titanium ethylene complex via C–H-activation and alternative access to Cp2Ti(η2-Me3SiC2SiMe3)

The synthesis of a room temperature stable titanacyclopropane in a one-step-two-transformation protocol is presented. Additionally, a novel one-pot procedure toward Cp₂Ti(η²-Me₃SiC₂SiMe₃) was subsequently developed.

Three-Coordinate Iron(0) Complexes with N-Heterocyclic Carbene and Vinyltrimethylsilane Ligation: Synthesis, Characterization, and Ligand Substitution Reactions

Low-coordinate iron(0) species are implicated as intermediates in a range of iron-catalyzed organic transformations. Isolable iron(0) complexes with coordination numbers of less than four, however, are rarely known. In continuing with our interests in three-coordinate iron(0) complexes with N-heterocyclic carbene (NHC) and alkene ligation, we report herein the synthesis and ligand substitution reactivity of three-coordinate iron(0) complexes featuring monodentate alkene ligands, [(NHC)Fe(η²-vtms)₂] (vtms = vinyltrimethylsilane, NHC = 1,3-bis(2',6'-diisopropylphenyl)-imidazol-2-ylidene (IPr), 1; 1,3-bis(2',6'-diisopropylphenyl)-4,5-tetramethylene-imidazol-2-ylidene (cyIPr), 2; 1,3-bis(2',6'-diisopropylphenyl)-4,5,6,7-tetrahydro-1,3-diazepin-2-ylidene (7-IPr), 3). Complexes 1-3 were synthesized from the one-pot reactions of ferrous dihalides with the N-(2,6-diisopropylphenyl)-substituted NHC ligands, vtms, and KC₈. Reactivity study of 1 revealed its facile ligand substitution reactions with terminal aryl alkynes, ketones, isocyanides, and CO, by which iron(0) complexes [(IPr)Fe(η²-HCCAr)] (Ar = Ph, 5; p-CH₃C₆H₄, 6; 3,5-(CF₃)₂C₆H₃, 7), [(IPr)Fe(η²-OCPh₂)₂] (8), [(IPr)Fe(CNR)₄] (R = 2,6-Me₂C₆H₃, 9; Bu^t, 10), and (IPr)Fe(CO)₄ (11) were prepared in good yields. These iron(0) complexes have been characterized by ¹H NMR, solution magnetic susceptibility measurement, single-crystal X-ray diffraction study, ⁵⁷Fe Mössbauer spectroscopy, and elemental analysis. Characterization data and computational studies suggest S = 1 ground-spin states for three-coordinate iron(0) complexes 1-3 and 5-8 and S = 0 ground states for 9-11. Theoretical studies on the three-coordinate complexes 1, 6, and 8 indicated pronounced metal-to-ligand backdonation from occupied Fe 3d orbitals to the π* orbitals of the C═C, C≡C, and C═O moieties of the π ligands. In addition, 1 proved an effective precatalyst for the cyclotrimerization reaction of alkynes.

Isolation and structural characterization of a titanacyclopropane as key intermediate in the double aryl Grignard addition to 2-(arylethynyl)pyridine derivatives

A titanacyclopropane species, which is a key reaction intermediate in the Ti(OiPr)₄-mediated double aryl Grignard addition to 1,2-di(pyridin-2-yl)ethyne and related alkynes, was isolated and fully characterized.

C(sp3 )-H Oxidative Addition and Transfer Hydrogenation Chemistry of a Titanium(II) Synthon: Mimicry of Late-Metal Type Reactivity

Two-electron reduction of the Ti^IV compound (^ket guan)(Im^Dipp N)Ti(OTf)₂ (3) gives the arene-masked complex (^ket guan)(η⁶ -Im^Dipp N)Ti (1) in excellent yield. Upon standing in solution, 1 converts to a Ti^IV metallacycle (4) through dehydrogenation of a pendant isopropyl group. Spectroscopic evidence shows this transformation initially proceeds via the oxidative addition of a C(sp³ )-H bond and can be reversed upon exposure of 4 to H₂ . Interestingly, treatment of 1 with cyclohexene gives cyclohexane and 4 via a titanium-mediated transfer hydrogenation reaction, a process that can be extended to catalytically hydrogenate other unsaturated hydrocarbons under mild conditions. These results, rare for the early-metals, suggest 1 possesses chemical characteristics reminiscent of noble, late-metals.

Stereoselective ring-opening polymerization of rac-lactides catalyzed by titanium complexes containing N,N-bidentate phenanthrene derivatives

Titanium complexes bearing N,N-bidentate diamido derived from Schiff bases were investigated as catalysts for lactide polymerization.

Metallacycle-Mediated Cross-Coupling with Substituted and Electronically Unactivated Alkenes

This perspective surveys the history of- and recent advances in- metallacycle-mediated coupling chemistry of substituted alkenes. While the reaction of preformed metal-π complexes with ethylene was reported nearly 30 years ago, the generalization of this mode of bimolecular C-C bond formation to the regio- and stereoselective union of complex substrates has only recently begun to emerge. This perspective discusses early observations in this area, the challenges associated with controlling such processes, the evolution of a general strategy to overcome these challenges, and a summary of highly regio- and stereoselective convergent coupling reactions that are currently available by metallacycle-mediated cross-coupling with substituted alkenes.

Preparation of stereodefined homoallylic amines from the reductive cross-coupling of allylic alcohols with imines

Regio-, diastereo-, and enantioselective coupling reactions between imines and allylic alcohols have been developed. These coupling reactions deliver complex homoallylic amine products through a convergent C-C bond forming process that does not proceed through intermediate allylic organometallic reagents. In general, convergent coupling, by exposure of an allylic alkoxide to a preformed Ti-imine complex, occurs with allylic transposition in a predictable and stereocontrolled manner. While simple diastereoselection in these reactions is high, delivering anti-products with ≥20:1 selectivity, the organometallic transformation described is compatible with a diverse range of functionality and substrates (including aliphatic and aromatic imines, allylic silanes, trisubstituted alkenes, vinyl- and aryl halides, trifluoromethyl groups, thioethers, and aromatic heterocycles). Alkene geometry of the products is a complex function of the allylic alcohol structure and is consistent with a mechanistic proposal based on syn-carbometalation followed by syn-elimination by way of a boat-like transition state geometry. Single asymmetric coupling reactions provide a means to translate the stereochemical information of the allylic alcohol to the homoallylic amine or to control diastereoselection in the coupling reactions of achiral allylic alcohols with chiral imines. Double asymmetric coupling reactions are also described that afford a unique means to control stereoselection in these complex convergent coupling processes. Finally, empirical models are proposed that are consistent with the observed stereochemical course of these coupling reactions en route to chiral homoallylic amines possessing di- or trisubstituted alkenes and anti- or syn- relative stereochemistry at the allylic and homoallylic positions.

Syntheses and structures of cationic and neutral, homo- and heteroleptic tert-butoxides of the group 4 metals

The utility of tri-tert-butoxystannate as a chelating tridentate ligand for group 4 metals was investigated. The highly Lewis acidic metals degraded the stannate ion in a series of tert-butoxide abstraction steps to produce a variety of group 4 tert-butoxides. A total of 1 equiv of NaSn(O(t)Bu)(3) reacted with cis-MCl(4)(THF)(2) [M = Zr (1), Hf (2)] in THF solutions to furnish the salts fac-{[M(O(t)Bu)(3)(THF)(3)](SnCl(3))}, which are separated ion pairs featuring weakly coordinating trichlorostannate ions. Neutral complexes, namely, [M(O(t)Bu)(2)Cl(2)(THF)(2)] [M = Zr (3), Hf (4)], were isolated when 2/3 equiv of sodium stannate was used in these reactions. Titanium tetrachloride formed analogues neither of 1 and 2 nor of 3 and 4, but Ti(O(t)Bu)(3)Cl reacted with silver triflate to give [Ti(O(t)Bu)(2)(OTf)(2)(THF)(2)] (5). Anion exchange of triflate for trichlorostannate transformed 1 to the contact ion pair fac-[Zr(O(t)Bu)(3)OTf(THF)(2)] (6). A total of 2 equiv of NaSn(O(t)Bu)(3) reacted with cis-MCl(4)(THF)(2) to give the complexes fac-[Sn(mu-O(t)Bu)(3)M(O(t)Bu)(3)] [M = Zr (7), Hf (8)]. Tri-tert-butoxystannate may be used as a selective alkoxylating agent for group 4 metals, and it can be transferred to these metals intact if their Lewis acidity is appropriately attenuated as in fac-{[M(O(t)Bu)(3)(THF)(3)](SnCl(3))}. Single-crystal X-ray studies revealed distorted octahedral coordination geometries for all compounds (1-8), with 1, 2, 7, and 8 being crystallographically C(3) symmetric.

Regioselective Reductive Cross-Coupling Reactions of Unsymmetrical Alkynes

The present microreview summarizes our progress over the last few years in defining regioselective reductive cross-coupling reactions of unsymmetrical alkynes with terminal- and internal alkynes, aldehydes, and imines. We begin with a brief historical perspective of metal-mediated reductive dimerization reactions of aromatic alkynes and discuss the challenges associated with "crossed" versions of this mode of reactivity. Next, a collection of available methods that allow for regioselective reductive cross-coupling of internal alkynes with terminal and internal alkynes, aldehydes, and imines is summarized. After an examination of the requirements for regioselectivity in these cases, the logic behind our design of alkoxide-directed titanium-mediated reductive cross-coupling reactions is presented. A nomenclature is introduced to delineate the presumed mechanistic origin of regioselection associated with each reaction design, and a presentation of alkoxide-directed regioselective reductive cross-coupling reactions of alkynes follows. Throughout, principal issues related to reactivity and selectivity are discussed to assess scope and limitations of available methods and to describe the broad challenges that exist for defining complex fragment union reactions based on reductive cross-coupling chemistry.

Conversion of allylic alcohols to stereodefined trisubstituted alkenes: a complementary process to the Claisen rearrangement

A stereoselective method for the conversion of allylic alcohols to (Z)-trisubstituted alkenes is presented. Overall, the reaction sequence described is stereochemically complementary to related Claisen rearrangement reactions--processes that typically deliver the stereoisomeric trisubstituted alkene containing products.

Nickel-catalyzed [2+2+2] cycloaddition of two alkynes and an imine

The reaction of N-benzenesulfonylbenzaldimine with Ni(cod)2 and PCy3 gave the corresponding η2-iminenickel complex quantitatively. Diphenylacetylene reacted with the η2-iminenickel complex to generate five-membered aza-nickelacycle. Insertion of second alkynes into the five-membered aza-nickelacycle led to the formation of the corresponding seven-membered aza-nickelacycles. Heating the solution of the seven-membered aza-nickelacycles induced the reductive elimination to give 1,2-dihydropyridine. In the presence of 10 mol % of Ni(cod)2 and PMetBu2 at 100 °C, the intermolecular [2+2+2] cycloaddition of N-benzenesulfonylbenzaldimine and 2-butyne occurred to give the expected 1,2-dihydropyridine in 87 % yield. In the presence of PCy3, the reaction also proceeded catalytically, however, PMetBu2 gave better results. Less bulky or less basic phosphine, PnBu3 or P(o-tol)3, was not efficient for the reaction. Although Ni(0)-NHC complex was a good catalyst for [2+2+2] cycloaddition of two alkynes and a ketone or an aldehyde, this reaction did not proceed in the presence of an NHC ligand, 1,3-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene.