Tantallaaziridines: from synthesis to catalytic applications

Applications of Vanadium, Niobium, and Tantalum Complexes in Organic and Inorganic Synthesis

Organometallic catalysis is a powerful strategy in chemical synthesis, especially with the cheap and low toxic metals based on green chemistry principle. Thus, the selection of the metal is particularly important to plan relevant and applicable processes. The group VB metals have been the subject of exciting and significant advances in both organic and inorganic synthesis. In this Review, we have summarized some reports from recent decades, which are about the development of group VB metals utilized in various types of reactions, such as oxidation, reduction, alkylation, dealkylation, polymerization, aromatization, protein synthesis, and practical water splitting.

Hydroaminoalkylation for the Catalytic Addition of Amines to Alkenes or Alkynes: Diverse Mechanisms Enable Diverse Substrate Scope

Hydroaminoalkylation is a powerful, atom-economic catalytic reaction for the reaction of amines with alkenes and alkynes. This C-H functionalization reaction allows for the atom-economic alkylation of amines using simple alkenes or alkynes as the alkylating agents. This transformation has significant potential for transformative approaches in the pharmaceutical, agrochemical, and fine chemical industries in the preparation of selectively substituted amines and N-heterocycles and shows promise in materials science for the synthesis of functional and responsive aminated materials. Different early transition-metal, late transition-metal, and photoredox catalysts mediate hydroaminoalkylation by distinct mechanistic pathways. These mechanistic insights have resulted in the development of new catalysts and reaction conditions to realize hydroaminoalkylation with a broad range of substrates: activated and unactivated, terminal and internal, C-C double and triple bonds with aryl or alkyl primary, secondary, or tertiary amines, including N-heterocyclic amines. By deploying select catalysts with specific substrate combinations, control over regioselectivity, diastereoselectivity, and enantioselectivity has been realized. Key barriers to widespread adoption of this reaction include air and moisture sensitivity for early transition-metal catalysts as well as a heavy dependence on amine protecting or directing groups for late transition-metal or photocatalytic routes. Advances in improved catalyst robustness, substrate scope, and regio-/stereoselective reactions with early- and late transition-metal catalysts, as well as photoredox catalysis, are highlighted, and opportunities for further catalyst and reaction development are included. This perspective shows that hydroaminoalkylation has the potential to be a disruptive and transformative strategy for the synthesis of selectively substituted amines and N-heterocycles from simple amines and alkenes.

Direct, Catalytic α-Alkylation of N-Heterocycles by Hydroaminoalkylation: Substrate Effects for Regiodivergent Product Formation

Saturated N-heterocycles are prevalent in pharmaceutical and agrochemical industries, yet remain challenging to catalytically alkylate. Most strategies for C-H activation of these challenging substrates use protected amines or high loadings of precious metal catalysts. We report an early transition-metal system for the broad, robust, and direct alkylation of unprotected amine heterocycles with simple alkenes. Short reaction times are achieved using an in situ generated tantalum catalyst that avoids the use of bases, excess substrate, or additives. In most cases, this catalyst system is selective for the branched reaction product, including examples of products that are generated with excellent diastereoselectivity. Alkene electronic properties can be exploited for substrate-modified regioselectivity to access the alternative linear amine alkylation product with a group 5 catalyst. This method allows for the facile isolation of unprotected N-heterocyclic products, as useful substrates for further reactivity.

Chemistry of group 5 metallaboranes with heterocyclic thiol ligands: a combined experimental and theoretical study

Thermolysis of [(Cp*Nb)₂(B₂H₆)₂], 1b (Cp* = η⁵-C₅Me₅), with 2-mercaptobenzothiazole, C₆H₄NSCSH (MBT), and 2-mercaptobenzoxazole, C₆H₄NOCSH (MBO), yielded hydrogen substituted compounds 2 and 3 with a general formula [(Cp*Nb)₂(B₂H₆)(B₂H₅L)] (2: L = C₆H₄NSCS and 3: L = C₆H₄NOCS). A similar reaction of 1b with Ph₂Se₂ yielded the monosubstituted derivative [(Cp*Nb)₂(B₂H₆){B₂H₅(PhSe)}], 4. All further efforts towards persubstitution of 1b under various drastic conditions were unfruitful. In parallel, in an effort to find a better synthetic route to the known Ta-aziridine complex [Cp*TaBH(C₇H₄NS₂)CH₂S₂NC₆H₄], Cp*TaCl₄ was treated with a 2-mercaptobenzothiazolyl-based borate ligand Na[H₂B(C₆H₄NSCS)₂]. Surprisingly, the reaction led to the formation of the half-sandwich trichloroaryltantalum(v) complex [Cp*TaCl₃{κ²-N,S-C₆H₄NSCS}], 5, containing a heterocyclic thiol ligand. Using an alternative method complex 5 was isolated in good yield when Cp*TaCl₄ was treated with the potassium salt of 2-mercaptobenzothiazole K[C₆H₄NSCS]. All the compounds were characterized by ¹H, ¹¹B{¹H}, and ¹³C{¹H} NMR spectroscopy, and their structures were unequivocally established by crystallographic analysis.

Zirconium-Catalyzed Hydroaminoalkylation of Alkynes for the Synthesis of Allylic Amines

A zirconium-catalyzed hydroaminoalkylation of alkynes to access α,β,γ-substituted allylic amines in an atom-economic fashion is reported. The reaction is compatible with N-(trimethylsilyl)benzylamine and a variety of N-benzylaniline substrates, with the latter giving the allylic amine as the sole organic product. Various internal alkynes with electron-withdrawing and electron-donating substituents were tolerated. Model intermediates of the reaction were synthesized and structurally characterized. Stoichiometric studies on key intermediates revealed that the open coordination sphere at zirconium, imparted by the tethered bis(ureate) ligand, is crucial for the coordination of neutral donors. These complexes may serve as models for the inner-sphere protonolysis reactions required for catalytic turnover.

Cobaltaelectro-Catalyzed C-H Activation with Carbon Monoxide or Isocyanides

Electrochemical oxidative C-H/N-H activations with isocyanides have been realized with a versatile cobalt catalyst. The widely applicable cobalt catalysis manifold further enabled electrooxidative C-H/N-H carbonylations with carbon monoxide under ambient conditions. The C-H functionalizations were efficiently realized with ample scope and outstanding functional group tolerance in a user-friendly undivided cell setup.

XANES/EPR Evidence of the Oxidation of Nickel(II) Quinolinylpropioamide and Its Application in Csp3 -H Functionalization

An in situ generated oxidation species of nickel quinolinylpropioamide intermediate was produced. Characterization by X-ray absorption near edge structure (XANES) and EPR provides complementary insights into this oxidized nickel species. With aliphatic amides and isocyanides as substrates, a nickel-catalyzed facile synthesis of structurally diverse five-membered lactams could be achieved.

Insertion of Isonitriles into the M-C Bonds of Group 4 Dialkyl Complexes

The 2,3-dimethylbutadiene complexes of Group 4 metals with constrained geometry (cg) ligands have been prepared and found to adopt a supine orientation with σ²,π bonding. Treatment of cgTi(2,3-dimethylbutadiene) (1-Ti) with ^tBuNC leads to the formation of a titana-aziridine (3) with a coordinated cyclopentenimine that arises from the formal [4+1] addition of the diene to the isonitrile. In contrast, the reactions of cgZr(2,3-dimethylbutadiene) (1-Zr) or cgHf(2,3-dimethylbutadiene) (1-Hf) with 2 equiv of ^tBuNC or XyNC proceeded in a more sophisticated manner to yield unsymmetrical 2,5-diazametallacyclopentane derivatives (4, 6-Zr, and 6-Hf) or symmetrical 2,5-diazametallacyclopentene complexes (7-Zr and 7-Hf). The unsymmetrical products contain coordinated cyclopropanes; the strength of the interaction is measured by the reduction in the ¹ J_CC of the C-C bond that is coordinated. A detailed mechanistic analysis has been possible with the related cgM(Me)₂ (M = Ti and Hf) complexes. The first insertion is too fast to monitor, but allows complete conversion to an alkyl iminoacyl intermediate. The second isonitrile (RNC) may react with that intermediate by either of two different mechanisms, reductive elimination and coordination/insertion. In the first mechanism (Ti), rate-determining C-C coupling gives a titana-aziridine, followed by fast coordination of the isonitrile. In the second mechanism (Hf), coordination is the slow step; insertion to form a bis(iminoacyl) Hf complex is rapid.

Metal-Mediated Addition of N-Nucleophiles to Isocyanides: Mechanistic Aspects

Despite the long history of the investigation of nucleophilic addition to metal-bound isocyanides, some important aspects of the reaction mechanism remain unclear even for the simplest systems. In this work, the addition of the sp³-N, sp²-N, and mixed sp²/sp³-N nucleophiles (i.e., HNMe₂, HN=CPh₂, and H₂N-N=CPh₂, respectively) to isocyanides C≡NR coordinated to the platinum(II) centers in the complexes cis-[Pt(C≡NCy)(2-pyz)(dppe)]⁺ (2-pyz = 2-pyrazyl, dmpe = Me₂PCH₂CH₂PMe₂) and cis-[PtCl₂(C≡NXyl)(C≡NMe)] was studied in detail by theoretical (DFT) methods. The mechanism of these reactions is stepwise associative rather than concerted and it includes the addition of a nucleophile to the isocyanide C atom, deprotonation of the nucleophilic moiety in the resulting intermediate, and protonation of the isocyanide N atom to give the final product. The calculated activation energy (ΔG≠) of all reactions is in the range of 19.8-22.4 kcal/mol.

Diversity of reactivity modes upon interplay between Au(iii)-bound isocyanides and cyclic nitrones: a theoretical consideration

Metal-bound isocyanides demonstrate a very rich chemistry towards 1,3-dipoles of the allyl anion type such as nitrones. Depending on the metal centre, dipole nature and substituent in isocyanides, cycloadducts, imine + isocyanates or metallacycles may be formed. In this work, the reasons for such diversity, intimate details of the reaction mechanism and main factors determining the chemoselectivity in the reaction between Au(iii)-bound isocyanides ([AuCl₃(C[triple bond, length as m-dash]NR)]) and cyclic nitrones ^-O-N[combining low line]⁺[double bond, length as m-dash]C(H)XXX[combining low line] (XXX = CH₂CH₂CMe₂, CH₂CH₂CH₂, OCH₂CMe₂, CH₂OCMe₂ and CH₂CH₂O) are analysed in detail by theoretical (DFT) methods. The formation of cycloadducts is controlled by the LUMO_{π*(C[triple bond, length as m-dash]N)} of isocyanides and it occurs in a stepwise manner via the initial nucleophilic addition of nitrone at the C atom of C[triple bond, length as m-dash]NR (except for nitronate ^-O-N[combining low line]⁺[double bond, length as m-dash]C(H)CH₂CH₂O[combining low line]). The imine + isocyanate products are formed upon oxygen transfer from nitrones to isocyanides, and N-O bond cleavage is the main factor determining this process. A metallacycle is formed upon decomposition of the cycloadduct, and this process includes deprotonation of the oxadiazoline CH group/N-O bond cleavage and Cl^- elimination/cyclization. The main factor controlling the metallacycle formation is the acidity of the endocyclic CH group in the cycloadduct. Effects of the substituent R in C[triple bond, length as m-dash]NR and the nitrone nature on the reactivity are analysed.

Copper-catalyzed synthesis of arylcarboxamides from aldehydes and isocyanides: the isocyano group as an N1 synthon

For the first time, transition-metal-catalyzed isocyanide group acting as an N1 synthon rather than exhibiting the carbene-like reactivity was discovered, exploiting a new reactivity profile of isocyanides.

Scandium-catalysed intermolecular hydroaminoalkylation of olefins with aliphatic tertiary amines

A homoleptic scandium trialkyl complex in combination with a borate compound served as an excellent catalyst for the C–H addition of aliphatic tertiary amines to various olefins.

A homoleptic scandium trialkyl complex in combination with a borate compound served as an excellent catalyst for the C–H addition of aliphatic tertiary amines to olefins. This highly regiospecific, 100% atom efficient C–H bond alkylation reaction was applicable to a wide variety of tertiary amines and olefins, including functionalised styrenes and unactivated α-olefins. This work represents the first example of rare-earth catalysed olefin hydroaminoalkylation and also the first example of catalytic C–H addition of aliphatic tertiary amines to olefins with any catalyst.

Azaphilic versus Carbophilic Coupling at C=N Bonds: Key Steps in Titanium-Assisted Multicomponent Reactions

Consecutive C- and N-arylation of N-heterocyclic nitriles is mediated by titanium(IV) alkoxides. The carbo- and azaphilic arylation step may be separated by choosing the order in which the two equivalents of aryl transfer reagent are added. In the course of this transformation, the ancillary N-heterocycle acts as both a directing anchor group and electron reservoir. In the selectivity-determining step, the selectivity is governed by a choice between (direct) C- and Ti-arylation; the latter opens up a reaction pathway that allows further migration to the nitrogen atom. The isolation of metal-containing aggregates from the reaction mixture and computational studies gave insights into the reaction mechanism. Subsequently, a multicomponent one-pot protocol was devised to rapidly access complex quaternary carbon centers.

Synthesis and reactivity of cyclometalated triamidophosphine complexes of niobium and tantalum

The triamidophosphine protioligand 1 reacts with the homoleptic pentakis(dimethylamido) precursors of niobium and tantalum [M(NMe2)5, where M = Nb, Ta] to form cyclometalated complexes of the type [N2PCN-κ(5)-N,N,P,C,N]M(NMe2) (2-M). Apart from the three amido donors, one benzylic position of the ligand backbone is deprotonated over the course of this reaction, resulting in the formation of a new M-C bond. As a consequence, a metallaziridine substructure is formed, and the triamidophosphine moiety thus serves as a tetraanionic pentadentate ligand. The dimethylamido complexes 2-M can be converted into the corresponding triflates [N2PCN-κ(5)-N,N,P,C,N]M(OTf) (3-M) and alkyl complexes [N2PCN-κ(5)-N,N,P,C,N]M(CH2SiMe3) (4-M) by treatment with triethylsilyl triflate (Et3SiO3SCF3) followed by (trimethylsilyl)methyllithium (LiCH2SiMe3). The alkyl complexes exhibit interesting reactivities, including a second cyclometalative backbone activation affording the trimethylphosphine-stabilized complexes [NP(CN)2-κ(6)-N,P,C,N,C,N]M(PMe3) (5-M). In the case of tantalum, the formation of a dinuclear hydrido complex (6) is observed upon hydrogenation of 4-Ta. In the case of niobium, the metallaziridine substructure in 4-Nb is prone to ring opening via protonation with triphenylsilylamine (Ph3SiNH2), resulting in formation of the corresponding imido complex [PN3-κ(4)-P,N,N,N]Nb=NSiPh3 (7).