Charge Effects in PCP Pincer Complexes of NiII bearing Phosphinite and Imidazol(i)ophosphine Coordinating Jaws: From Synthesis to Catalysis through Bonding Analysis

This contribution reports on a new family of Ni^II pincer complexes featuring phosphinite and functional imidazolyl arms. The proligands ^RPIMC^HOP^R′ react at room temperature with Ni^II precursors to give the corresponding complexes [(^RPIMCOP^R′)NiBr], where ^RPIMCOP^R=κ^P,κ^C,κ^P‐{2‐(R′₂PO),6‐(R₂PC₃H₂N₂)C₆H₃}, R=iPr, R′=iPr ( 3 b , 84 %) or Ph ( 3 c , 45 %). Selective N‐methylation of the imidazole imine moiety in 3 b by MeOTf (OTf=OSO₂CF₃) gave the corresponding imidazoliophosphine [(^iPrPIMIOCOP^iPr)NiBr][OTf], 4 b , in 89 % yield (^iPrPIMIOCOP^iPr=κ^P,κ^C,κ^P‐{2‐(iPr₂PO),6‐(iPr₂PC₄H₅N₂)C₆H₃}). Treating 4 b with NaOEt led to the NHC derivative [(NHCCOP^iPr)NiBr], 5 b , in 47 % yield (NHCCOP^iPr=κ^P,κ^C,κ^C‐{2‐(iPr₂PO),6‐(C₄H₅N₂)C₆H₃)}). The bromo derivatives 3–5 were then treated with AgOTf in acetonitrile to give the corresponding cationic species [(^RPIMCOP^R)Ni(MeCN)][OTf] [R=Ph, 6 a (89 %) or iPr, 6 b (90 %)], [(^RPIMIOCOP^R)Ni(MeCN)][OTf]₂ [R=Ph, 7 a (79 %) or iPr, 7 b (88 %)], and [(NHCCOP^R)Ni(MeCN)][OTf] [R=Ph, 8 a (85 %) or iPr, 8 b (84 %)]. All new complexes have been characterized by NMR and IR spectroscopy, whereas 3 b , 3 c , 5 b , 6 b , and 8 a were also subjected to X‐ray diffraction studies. The acetonitrile adducts 6 – 8 were further studied by using various theoretical analysis tools. In the presence of excess nitrile and amine, the cationic acetonitrile adducts 6 – 8 catalyze hydroamination of nitriles to give unsymmetrical amidines with catalytic turnover numbers of up to 95.     

Brønsted Acid Catalyzed Asymmetric Hydroamination of Alkenes: Synthesis of Pyrrolidines Bearing a Tetrasubstituted Carbon Stereocenter

The first highly enantioselective Brønsted acid catalyzed intramolecular hydroamination of alkenes enables the efficient construction of a series of chiral (spirocyclic) pyrrolidines with an α‐tetrasubstituted carbon stereocenter with excellent functional group tolerance. A unique feature of this strategy is the use of a thiourea group acting as both the activating and the directing group through cooperative multiple hydrogen bonding with a Brønsted acid and the double bond. The utility of this method is highlighted by the facile construction of chiral synthetic intermediates and important structural motifs that are widely found in organic synthesis.     

Amidate ligand design effects in zirconium-catalyzed enantioselective hydroamination of aminoalkenes

In situ generated axially chiral zirconium biphenyl amidate complexes are efficient precatalysts for the enantioselective intramolecular hydroamination of aminoalkenes, generating α-substituted pyrrolidines and piperidine with up to 74% ee. Five new chelating amide proligands and three new zirconium amidate complexes have been prepared and fully characterized in this investigation of ligand structure/catalyst function. Solid-state molecular structures of the complexes suggest that the observed moderate and highly variable enantioselectivities are a consequence of the multiple isomers accessible to this family of complexes, including a κ²-(O,O)-bonding motif. Thermal stability studies of the complexes further revealed the tendency of these complexes to undergo diastereoselective dimerization to afford homochiral dimers. These dimeric precatalysts are less efficient when used for the cyclization of aminoalkenes in comparison to their monomeric precursors. These results illustrate the variable coordination modes accessible to amidate ligands and suggest steric factors that must be considered in advanced ligand design.     

Does a Concerted Non‐Insertive Mechanism Prevail over a σ‐Insertive Mechanism in Catalytic Cyclohydroamination by Magnesium Tris(oxazolinyl)phenylborate Compounds? A Computational Study

The present study comprehensively explores alternative mechanistic pathways for intramolecular hydroamination of 2,2-dimethyl-4-penten-1-amine (1) by [{To(M)}MgMe] (To(M)=tris(4,4-dimethyl-2-oxazolinyl)phenylborate) (2) with the aid of density functional theory (DFT) calculations. A single-step amidoalkene → cycloamine conversion through a concerted proton transfer associated with N-C ring closure has been explored as one possible mechanism; its key features have been described. This non-insertive pathway evolves via a six-centre TS structure featuring activation of the olefin unit towards nucleophilic amido attack outside the immediate vicinity of the metal centre by amino proton delivery and describes a viable mechanistic variant for alkaline-earth metal-mediated aminoalkene hydroamination. However, herein is presented sound evidence for the operation of the Mg-N amido σ-bond insertive mechanism, its turnover-limiting activation barrier is found to be 5.0 kcal mol(-1) lower than for the non-insertive mechanism, for the cyclohydroamination of 2,2-disubstituted 4-aminoalkenes by a [{To(M)}Mg-NHR] catalyst. The operative mechanism involves rapid equilibria of the {To(M)}Mg-amidoalkene resting state 3 with its amine adduct, easily accessible and thermodynamically disfavoured, hence reversible, 1,2-olefin insertion into the Mg-N amido σ-bond with ring closure at 3, linked to turnover-limiting Mg-C azacycle tether aminolysis by an adduct substrate molecule, followed by facile cycloamine liberation to regenerate the active catalyst species 3. The following aspects are in support of this scenario: 1) the derived rate law is consistent with the experimentally obtained empirical rate law; 2) the reasonable agreement between the computationally estimated and the observed value of the primary KIE; 3) the assessed effective activation barrier for turnover-limiting aminolysis matches empirically determined Eyring parameters remarkably well; and 4) the observed resistance of isolated 3 to undergo amidoalkene cycloamine/cycloamido transformation until further quantities of substrate is added is consistently explained. The herein unveiled insights into the structure-reactivity relationships will undoubtedly govern the rational design of alkaline-earth metal-based catalysts and likely facilitate further advances in the area.     

DFT study of the mechanism of hydroamination of ethylene with ammonia catalyzed by diplatinum(II) complexes: Inner‐ or outer‐sphere?

A detailed analysis of the reaction profiles of the hydroamination reaction between ethylene and ammonia catalyzed by the diplatinum(II) [{Pt(NH₂)(μ‐H)(PPh₃)}₂] complex is presented herein using density functional theory computational techniques. The coordinatively unsaturated 14e T‐shaped [Pt(NH₂)(PPh₃)H] species resulted from the dissociation of the diplatinum [{Pt(NH₂)(μ‐H)(PPh₃)}₂] precatalyst are identified as the active catalytic species. All possible reaction pathways that constitute the entire catalytic cycle have exhaustively been investigated. Overall, the rate‐determining step of all catalytic cycles constructed was found to be the oxidative addition of ammonia that leads to the regeneration of the catalyst. According to the energy span model, the outer‐sphere mechanism for the hydroamination of ethylene with ammonia catalyzed by the diplatinum complexes is favored over the inner‐sphere one, whereas TOF values are in favor of the inner‐sphere mechanism. © 2012 Wiley Periodicals, Inc.     

Synthesis, structure, and catalytic activity of binuclear lanthanide complexes with chiral NOBIN-based NNO ligands

Two new amido binuclear complexes {(1)YN(SiMe₃)₂}₂ · C₇H₈ (3 · C₇H₈) and {(2)SmN(SiMe₃)₂}₂ · C₆H₁₄ (4 · C₆H₁₄) have been readily prepared in good yields by amine elimination reaction between Ln[N(SiMe₃)₂]₃ (Ln = Sm, Y) and chiral NNO ligands, (S)-2-(pyridin-2-ylmethylamino)-2′-hydroxy-1,1′-binaphthyl (1H₂) and (S)-5,5′,6,6′,7,7′,8,8′-octahydro-2-(pyrrol-2-ylmethyleneamino)-2′-hydroxy-1,1′-binaphthyl (2H₂), respectively. They both have been characterized by various spectroscopic techniques, elemental analyses, and X-ray diffraction analyses. They are active catalysts for asymmetric hydroamination/cyclization of aminoalkenes and ring-opening polymerization of rac-lactide, affording cyclic amines in excellent conversions with moderate ee values and isotactic-rich polylactides, respectively.     

Indole Synthesis Using Silver Catalysis

Indoles are amongst the most important classes of heteroaromatics in organic chemistry, commonly found in biologically active natural products and therapeutically useful compounds. The synthesis of indoles is therefore important and several methods for their synthesis that make use of silver(I) catalysts and reagents have been developed in recent years. This Minireview contains, to the best of our knowledge, a comprehensive coverage of silver‐mediated indole forming reactions since the first reaction of this type was reported in 2004.     

Cationic Zinc Organyls as Precatalysts for Hydroamination Reactions

The cationic zinc triple‐decker complex [Zn₂Cp*₃]⁺[BAr^F₄]^? (BAr^F₄=B(3,5‐(CF₃)₂C₆H₃)₄) exhibits catalytic activity in intra‐ and intermolecular hydroamination reactions in the absence of a cocatalyst. These hydroaminations presumably proceed through the activation of the C?C multiple bond of the alkene or alkyne by a highly electrophilic zinc species, which is formed upon elimination of the Cp* ligands. The reaction of [Zn₂Cp*₃]⁺[BAr^F₄]^? with phenylacetylene gives the hydrocarbonation product (Cp*)(Ph)CCH₂, which might be formed via a similar reaction pathway. Additionally, several other structurally well‐defined cationic zinc organyls have been examined as precatalysts for intermolecular hydroamination reactions without the addition of a cocatalyst. These studies reveal that the highest activity is achieved in the absence of any donor ligands. The neutral complex [ZnCp^2S₂] (Cp^2S=C₅Me₄(CH₂)₂SMe) shows a remarkably high catalytic activity in the presence of a Brønsted acid.