Michael S. Hill

Intramolecular hydroamination of aminoalkenes by calcium and magnesium complexes: a synthetic and mechanistic study.

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.

Nanostructured Hybrid Polymer−Inorganic Solar Cell Active Layers Formed by Controllable in Situ Growth of Semiconducting Sulfide Networks

Nanostructured composites of inorganic and organic materials are attracting extensive interest for electronic and optoelectronic device applications. In this paper, we introduce a general method for the fabrication of metal sulfide nanoparticle/polymer films employing a low-cost and low temperature route compatible with large-scale device manufacturing. Our approach is based upon the controlled in situ thermal decomposition of a solution processable metal xanthate precursor complex in a semiconducting polymer film. To demonstrate the versatility of our method, we fabricate a CdS/P3HT nanocomposite film and show that the metal sulfide network inside the polymer film assists in the absorption of visible light and enables the achievement of high yields of charge photogeneration at the CdS/P3HT heterojunction. Photovoltaic devices based upon such nanocomposite films show solar light to electrical energy conversion efficiencies of 0.7% under full AM1.5 illumination and 1.2% under 10% incident power, demonstrating the potential of such nanocomposite films for low-cost photovoltaic devices.

Heterofunctionalization catalysis with organometallic complexes of calcium, strontium and barium

Despite the routine employment of Grignard reagents and Hauser bases as stoichiometric carbanion reagents in organic and inorganic synthesis, a defined reaction chemistry encompassing the heavier elements of Group II (M = Ca, Sr and Ba) has, until recently, remained unreported. This article provides details of the recent progress in heavier Group II catalysed small molecule transformations mediated by well-defined heteroleptic and homoleptic complexes of the form LMX or MX2; where L is a mono-anionic ligand and X is a reactive σ-bonded substituent. The intra- and intermolecular heterofunctionalization (hydroamination, hydrophosphination, hydrosilylation and hydrogenation) of alkenes, alkynes, dienes, carbodiimides, isocyanates and ketones is discussed.

Heavier Alkaline Earth Catalysts for the Intermolecular Hydroamination of Vinylarenes, Dienes, and Alkynes

The heavier group 2 complexes [M{N(SiMe(3))(2)}(2)](2)(1, M = Ca; 2, M = Sr) and [M{CH(SiMe(3))(2)}(2)(THF)(2)] (3, M = Ca; 4, M = Sr) are shown to be effective precatalysts for the intermolecular hydroamination of vinyl arenes and dienes under mild conditions. Initial studies revealed that the amide precatalysts, 1 and 2, while compromised in terms of absolute activity by a tendency toward transaminative behavior, offer greater stability toward polymerization/oligomerization side reactions. In every case the strontium species, 2 and 4, were found to outperform their calcium congeners. Reactions of piperidine with para-substituted styrenes are indicative of rate-determining alkene insertion in the catalytic cycle while the ease of addition of secondary cyclic amines was found to be dependent on ring size and reasoned to be a consequence of varying amine nucleophilicity. Hydroamination of conjugated dienes yielded isomeric products via η(3)-allyl intermediates and their relative distributions were explained through stereoelectronic considerations. The ability to carry out the hydroamination of internal alkynes was found to be dramatically dependent upon the identity of the alkyne substituents while reactions employing terminal alkynes resulted in the precipitation of insoluble and unreactive group 2 acetylides. The rate law for styrene hydroamination with piperidine catalyzed by [Sr{N(SiMe(3))(2)}(2)](2) was deduced to be first order in [amine] and [alkene] and second order in [catalyst], while large kinetic isotope effects and group 2 element-dependent ΔS(++) values implicated the formation of an amine-assisted rate-determining alkene insertion transition state in which there is a considerable entropic advantage associated with use of the larger strontium center.

Direct Growth of Metal Sulfide Nanoparticle Networks in Solid‐State Polymer Films for Hybrid Inorganic–Organic Solar Cells

Hybrid metal sulfide/polymer solar cell active layers are fabricated employing an approach based upon the in-situ thermal decomposition of a single source metal xanthate precursor in a semiconducting polymer film. The nanomorphology of the film, the charge photogeneration yield at the donor-acceptor heterojunction and device performance are shown to be dependent upon the annealing temperature. Photovoltaic devices based upon such layers are shown to exhibit power conversion efficiencies of 2.2% under AM1.5 solar illumination thus demonstrating the potential of such nanocomposite films for photovoltaic device applications.

Group 2 Promoted Hydrogen Release from NMe2H⋅BH3: Intermediates and Catalysis

Abstract Both homo‐ and heteroleptic alkyl and amide complexes of the Group 2 elements Mg and Ca are shown to be active for the catalytic dehydrocoupling of Me2NH⋅BH3. Reactions of either magnesium dialkyls or the β‐diketiminate complex [HC{(Me)CN(Dipp)}2MgnBu] with four or two equivalents of Me2NHBH3, respectively, produce compounds containing the [H3BNMe2BH2Me2N]− ion, which coordinates to the magnesium centers through Mg—N and Mg⋅⋅⋅HB interactions in both the solution and solid states. Thermolysis of these compounds at 60 °C produces the cyclic product [(H2BNMe2)2] and, it is proposed, magnesium hydrido species by an unprecedented δ‐hydride elimination process. Calcium‐based species, although less reactive than their magnesium‐based counterparts, are found to engage in similar dehydrocoupling reactivity and to produce a similar distribution of products under thermally promoted catalytic conditions. A mechanism for these observations is presented that involves initial production and insertion of H2B=NMe2 into polarized M—N bonds as the major B—N bond‐forming step. The efficacy of this insertion and subsequent β‐ or δ‐hydride elimination steps is proposed to be dependent upon the charge density and polarizing capability of the participating Group 2 center, providing a rationale for the observed differences in reactivity between magnesium and calcium.

Magnesium-catalysed hydroboration of aldehydes and ketones

The heteroleptic magnesium alkyl complex [CH{C(Me)NAr}(2)Mg(n)Bu] (Ar = 2,6-(i)Pr(2)C(6)H(3)) is reported as a highly efficient pre-catalyst for the hydroboration of aldehydes and ketones with pinacolborane.

Triazenide complexes of the heavier alkaline earths: Synthesis, characterization, and suitability for hydroamination catalysis

A series of triazenide complexes of the heavier alkaline earths, Ca, Sr and Ba, have been synthesized by either protonolysis or salt metathesis routes. Although complexes of the form [{Ar 2N 3}M{N(SiMe 3) 2}(THF) n ] (M = Ca, n = 2; M = Sr, n = 3; Ar = 2,6-diisopropylphenyl) and [{Ar 2N 3}Ca(I)(THF) 2] 2 could be isolated and characterized by X-ray crystallography, solution studies revealed the propensity of these species to undergo Schlenk-like redistribution with the formation of [{Ar 2N 3} 2M(THF) n ] (M = Ca, n = 1; M = Sr, n = 2). The latter compounds have been synthesized independently. In the case of the large barium dication, attempts to synthesize the heaviest analogue of the series, [{Ar 2N 3} 2Ba(THF) n ], failed and led instead to the isolation of the potassium barate complex [K{Ar 2N 3}Ba{N(SiMe 3) 2} 2(THF) 4]. Single crystal X-ray diffraction studies demonstrated that, although in all the aforementioned cases the triazenide ligand binds to the electrophilic group 2 metal centers via symmetrical kappa (2)- N, N-chelates, in the latter compound an unprecedented bridging mode is observed in which the triazenide ligand coordinates through both terminal and internal nitrogen centers. A series of density-functional theory computational experiments have been undertaken to assist in our understanding of this phenomenon. In further experiments, the calcium and strontium amide derivatives [{Ar 2N 3}M{N(SiMe 3) 2}(THF) n ] (M = Ca, n = 2; M = Sr, n = 3) proved to be catalytically active for the intramolecular hydroamination of 1-amino-2,2-diphenylpent-4-ene to form 2-methyl-4,4-diphenylpyrrolidine, with the calcium species demonstrating a higher turnover number than the strontium analogue ( 2a, TOF = 500 h (-1); 2b, TOF = 75 h (-1)). In these instances, because of ambiguities in the structural charcterization of the precatalyst in solution, such quantification holds little value and detailed catalytic studies have not been conducted.

Heavier Group 2 Metals and Intermolecular Hydroamination: A Computational and Synthetic Assessment

A density functional theory assessment of the use of the group 2 elements Mg, Ca, Sr, and Ba for the intermolecular hydroamination of ethene indicated that the efficiency of the catalysis is dependent upon both the polarity and the deformability of the electron density within the metal-substituent bonds of key intermediates and transition states. The validity of this analysis was supplemented by a preliminary study of the use of group 2 amides for the intermolecular hydroamination of vinyl arenes. Although strontium was found to provide the highest catalytic activity, in line with the expectation provided by the theoretical study, a preliminary kinetic analysis demonstrated that this is possibly a consequence of the increased radius and accessibility of this cation rather than a reflection of a reduced barrier for rate-determining alkene insertion.

A hydride-rich magnesium cluster.

High-de-hydride! A straightforward reaction between a magnesium silylamido/N-heterocyclic carbene adduct and phenylsilane provides a {Mg(4)H(6)} cluster molecule that may be regarded as a combination of two magnesium dihydride and two magnesium monohydride moieties.