The calcium-catalyzed intramolecular hydroamination of alkenes and alkynes is reported. The beta-diketiminato complex [{HC(C(Me)2N-2,6-iPr2C6H3)2}-Ca{N(SiMe3)2}(THF)] affects catalytic cyclization of a range of aminoalkenes and aminoalkynes with activities that are broadly commensurate to those of established rare earth catalysts.

The -diketiminato complex [{HC(C(Me) 2 N-2,6-iPr 2 C 6 H 3 ) 2 }Ca{N(SiMe 3 ) 2 }(THF)] effects intermolecular hydrophosphination of a range of alkenes and alkynes. In behaVior reminiscent of lanthanocene(III) catalysis, a more electrophilic alkene is polymerized to phosphine-terminated macromolecules.Organophosphines, R 3 P, are an important class of compound widely employed in transition metal catalysis and organic synthesis. Hydrophosphination, the addition of the P-H bond of a primary or secondary phosphine to an unsaturated C-C bond, is a potentially powerful and, importantly, atom-efficient route to such compounds. 1 The transformation can be achieved under radical conditions or, alternatively, may be promoted by group 1, 2 late transition metal, 3 or lanthanide based catalysts. 4 On the basis of a proposed analogy between catalytic lanthanide and heavier group 2 metal centers, we have previously reported the -diketiminato-stabilized calcium amide 1 as an effective catalyst for the intramolecular hydroamination of aminoalkenes and aminoalkynes. 5 The reaction was postulated to occur by the generalized catalytic cycle outlined in Scheme 1, via (i) initiation of the precatalyst by a σ-bond metathesis (or protonolysis) of 1 with a primary amine to form a calcium primary amide, (ii) an intramolecular insertion of the alkene into the Ca-N bond, and (iii) the σ-bond metathesis of the resultant calcium alkyl with a further equivalent of amine to liberate the product and regenerate the active catalyst. On the basis of the numerous applications of lanthanide-based catalysts to the heterofunctionalization of unsaturated carboncarbon bonds, we speculated that the observed reactivity was not confined to the intramolecular hydroamination of alkenes and alkynes. Indeed, Harder has very recently shown that homoleptic benzyl alkaline earth complexes may act as precatalysts for the hydrosilylation of alkenes. 6 Lanthanocene catalysts of the form Cp* 2 LnX (X ) H, CH-(SiMe 3 ) 2 , Ln ) La, Sm, Y, Lu) have been applied to the intramolecular hydrophosphination/cyclization of a variety of phosphinoalkenes. 7 In this case, the reaction mechanism has been studied in depth and occurs via a pathway analogous to that depicted in Scheme 1. Both experimental and theoretical studies suggest that the σ-bond metathesis of the Ln-C bond of the intermediate is the rate-determining step (cf. Scheme 1, step iii). Furthermore, the intermolecular hydrophosphination of alkenes with such catalysts has not been achieved; rather, a lanthanocene phosphide mediated polymerization of ethylene has been reported. 8 Although divalent ytterbium catalysts have been applied to the intermolecular variant of this reaction, 4 the reaction mechanism in these cases is potentially complicated by reductive initiation.We now present a preliminary account of the application of 1 to the intermolecular hydrophosphination of unsaturated C-C bonds. In this regard it is noteworthy that limited evidence exists for both σ-bond metathesis and insertion steps requisi...

The beta-diketiminate-stabilized calcium amide complex [{ArNC(Me)CHC(Me)NAr}Ca{N(SiMe(3))(2)}(THF)] (Ar = 2,6-diisopropylphenyl) and magnesium methyl complex [{ArNC(Me)CHC(Me)NAr}Mg(Me)(THF)] are reported as efficient precatalysts for hydroamination/cyclization of aminoalkenes. The reactions proceeded under mild conditions, allowing the synthesis of five-, six-, and seven-membered heterocyclic compounds. Qualitative assessment of these reactions revealed that the ease of catalytic turnover increases (i) for smaller ring sizes (5 > 6 > 7), (ii) substrates that benefit from favorable Thorpe-Ingold effects, and (iii) substrates that do not possess additional substitution on the alkene entity. Prochiral substrates may undergo diastereoselective hydroamination/cyclization depending upon the position of the existing stereocenter. Furthermore, a number of minor byproducts of these reactions, arising from competitive alkene isomerization reactions, were identified. A series of stoichiometric reactions between the precatalysts and primary amines provided an important model for catalyst initiation and suggested that these reactions are facile at room temperature, with the reaction of the calcium precatalyst with benzylamine proceeding with DeltaG(o)(298 K) = -2.7 kcal mol(-1). Both external amine/amide exchange and coordinated amine/amide exchange were observed in model complexes, and the data suggest that these processes occur via low-activation-energy pathways. As a result of the formation of potentially reactive byproducts such as hexamethyldisilazane, calcium-catalyst initiation is reversible, whereas for the magnesium precatalyst, this process is nonreversible. Further stoichiometric reactions of the two precatalysts with 1-amino-2,2-diphenyl-4-pentene demonstrated that the alkene insertion step proceeds via a highly reactive transient alkylmetal intermediate that readily reacts with N-H sigma bonds under catalytically relevant conditions. The results of deuterium-labeling studies are consistent with the formation of a single transient alkyl complex for both the magnesium and calcium precatalysts. Kinetic analysis of the nonreversible magnesium system revealed that the reaction rate depends directly upon catalyst concentration and inversely upon substrate concentration, suggesting that substrate-inhibited alkene insertion is rate-determining.

The potential of the heteroleptic heavier alkaline-earth hexamethyldisilazides [{HC(C(Me)2N-2,6-iPr2C6H3)2}Ae{N(SiMe3)2}(THF)](Ae = Ca, Sr, Ba) as kinetically-stable reagents for further protolytic reaction chemistry has been assessed. Only the previously reported calcium complex was found to be stable to solution dismutation and dynamic ligand exchange. The barium complex was isolated in sufficient purity to enable characterisation by an X-ray analysis. Reactions of the kinetically robust calcium complex with cyclohexylamine and tert-butylamine resulted in displacement of THF and formation of solvated structures, which could be characterised by 1H NMR spectroscopy. Attempts to isolate these solvated complexes were unsuccessful due to redistribution to the homoleptic complex [{HC(C(Me)2N-2,6-iPr2C6H3)2}2Ca]. In contrast, the more acidic amine [H2NCH2CH2OMe] was cleanly deprotonated resulting in the isolation of the first well defined primary amido derivative of a heavier alkaline-earth element, [{HC(C(Me)2N-2,6-iPr2C6H3)2}Ca{NHCH2CH2OMe}]2, which retains its dimeric constitution in solution and is stable to further intermolecular ligand exchange. Reactions of [{HC(C(Me)2N-2,6-iPr2C6H3)2}Ca{N(SiMe3)2}(THF)] with a variety of ortho-disubstituted anilines also resulted in immediate protonation of the hexamethyldisilazide ligand. Only the most sterically demanding 2,6-diisopropylphenyl anilide derivative possessed sufficient kinetic stability to allow isolation of the heteroleptic complex. The crystal structure of [{HC(C(Me)2N-2,6-iPr2C6H3)2}Ca{N(H)-2,6-iPrC6H3}(THF)] was shown to exist as a mononuclear, pseudo-five-coordinate complex in which the coordinative unsaturation of the calcium centre is relieved by a Ca...H-C agostic-type interaction to one of the ortho isopropyl groups of the anilide ligand. This interaction is not maintained in solution however and the complex slowly redistributes to the homoleptic beta-diketiminato species and ill-defined polymeric calcium anilido products.

Multiple spectroscopic and computational methods were used to characterize the ground-state electronic structure of the novel {CoNO}(9) species Tp*Co(NO) (Tp* = hydro-tris(3,5-Me(2)-pyrazolyl)borate). The metric parameters about the metal center and the pre-edge region of the Co K-edge X-ray absorption spectrum were reproduced by density functional theory (DFT), providing a qualitative description of the Co-NO bonding interaction as a Co(II) (S(Co) = 3/2) metal center, antiferromagnetically coupled to a triplet NO(-) anion (S(NO) = 1), an interpretation of the electronic structure that was validated by ab initio multireference methods (CASSCF/MRCI). Electron paramagnetic resonance (EPR) spectroscopy revealed significant g-anisotropy in the S = ½ ground state, but the linear-response DFT performed poorly at calculating the g-values. Instead, CASSCF/MRCI computational studies in conjunction with quasi-degenerate perturbation theory with respect to spin-orbit coupling were required for obtaining accurate modeling of the molecular g-tensor. The computational portion of this work was extended to the diamagnetic Ni analogue of the Co complex, Tp*Ni(NO), which was found to consist of a Ni(II) (S(Ni) = 1) metal center antiferromagnetically coupled to an S(NO) = 1 NO(-). The similarity between the Co and Ni complexes contrasts with the previously studied Cu analogues, for which a Cu(I) bound to NO(0) formulation has been described. This discrepancy will be discussed along with a comparison of the DFT and ab initio computational methods for their ability to predict various spectroscopic and molecular features.