Dearomatized BIAN Alkaline-Earth Alkyl Catalysts for the Intramolecular Hydroamination of Hindered Aminoalkenes

Reaction of a sterically encumbered bis­(imino)­acenapthene (dipp-BIAN) with either potassium alkyl or the heavier alkaline-earth dialkyl [Ae­{CH­(SiMe₃)₂}₂(THF)₂] (Ae = Mg, Ca, Sr) reagents results in dearomatization of the aromatic ligand. The heteroleptic alkaline-earth alkyl species show enhanced stability toward Schlenk-type redistribution but undergo solution exchange when the bis­(trimethylsilyl)­methyl substituent is replaced by an anionic ligand of lower overall steric demands. In contrast, analogous reactions performed with [Ba­{CH­(SiMe₃)₂}₂(THF)₂] evidenced facile solution redistribution and resulted in an unusual C–C coupling reaction which is suggested to result from a sterically induced reductive process. An assessment of the Mg, Ca, and Sr alkyl compounds as precatalysts for the intramolecular hydroamination of aminoalkenes evidenced enhanced reactivity, which is ascribed to the greater solution stability of the catalytically active species. Most notably the calcium species may even be applied to the high-yielding cyclization of substrates bearing alkyl substitution at either of the alkenyl positions

Dearomatized BIAN Alkaline-Earth Alkyl Catalysts for the Intramolecular Hydroamination of Hindered Aminoalkenes

Reaction of a sterically encumbered bis­(imino)­acenapthene (dipp-BIAN) with either potassium alkyl or the heavier alkaline-earth dialkyl [Ae­{CH­(SiMe₃)₂}₂(THF)₂] (Ae = Mg, Ca, Sr) reagents results in dearomatization of the aromatic ligand. The heteroleptic alkaline-earth alkyl species show enhanced stability toward Schlenk-type redistribution but undergo solution exchange when the bis­(trimethylsilyl)­methyl substituent is replaced by an anionic ligand of lower overall steric demands. In contrast, analogous reactions performed with [Ba­{CH­(SiMe₃)₂}₂(THF)₂] evidenced facile solution redistribution and resulted in an unusual C–C coupling reaction which is suggested to result from a sterically induced reductive process. An assessment of the Mg, Ca, and Sr alkyl compounds as precatalysts for the intramolecular hydroamination of aminoalkenes evidenced enhanced reactivity, which is ascribed to the greater solution stability of the catalytically active species. Most notably the calcium species may even be applied to the high-yielding cyclization of substrates bearing alkyl substitution at either of the alkenyl positions

Cesium Reduction of a Lithium Diamidochloroberyllate

Room temperature reaction of elemental cesium with the dimeric lithium chloroberyllate [{SiNDipp}BeClLi]2 [{SiNDipp} = {CH2SiMe2N(Dipp)}2, where Dipp = 2,6-di-isopropylphenyl, in C6D6 results in activation of the arene solvent. Although, in contrast to earlier observations of lithium and sodium metal reduction, the generation of a mooted cesium phenylberyllate could not be confirmed, this process corroborates a previous hypothesis that such beryllium-centered solvent activation also necessitates the formation of hydridoberyllium species. These observations are further borne out by the study of an analogous reaction performed in toluene, in which case the proposed generation of formally low oxidation state beryllium radical anion intermediates induces activation of a toluene sp3 C–H bond and the isolation of the polymeric cesium benzylberyllate, [Cs({SiNDipp}BeCH2C6H5)]∞

Stoichiometric and Catalytic Reactivity of <i>tert</i>-Butylamine–Borane with Calcium Silylamides

The primary amine–borane <i>t</i>-BuNH₂·BH₃ reacts with a β-diketiminate-supported silylamido calcium complex with elimination of HN­(SiMe₃)₂ and formation of the corresponding primary amidoborane complex, in which the deprotonated amine–borane is attached to the alkaline-earth center via its nitrogen atom and anagostic interactions with the boron-bound hydrides. Catalytic dehydrocoupling reactions employed with this β-diketiminate precatalyst are found to be slow and complicated by protonation of the supporting ligand and the formation of a number of boron-containing products, all of which have been positively identified. In common with previous studies of group 2 catalyzed secondary amine–borane dehydrogenation, the first formed major product of the catalysis is identified by solution NMR and solid-state single-crystal X-ray studies to be a cyclic diborazane, [H^tBuN–BH₂]₂, the formation of which is accompanied by variable proportions of diamidoborane and aminoborane products. The active calcium species is also observed to be depleted during the catalysis due to the formation of hydrocarbon-insoluble [Ca­(BH₄)₂·THF]_∞, which has also been structurally characterized. Continued heating of these reaction mixtures results in the formation of cyclic trimeric 1,3,5-tri-<i>tert</i>-butylborazine, which is proposed to form through the intermediacy of [H^tBuN–BH₂]₂ by an, as yet, undefined sequence of borazane dehydrogenation and ring expansion reactions

Heavier Alkaline Earth Catalyzed Ene-yne Cyclizations: Atom-Efficient Access to Tetrahydroisoquinoline Frameworks

Tetrahydroisoquinoline frameworks may be accessed with 100% atom efficiency through the alkaline earth catalyzed addition of primary amines to ene-yne substrates through a sequence of intermolecular alkene and intramolecular alkyne hydroamination steps

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

The heavier group 2 complexes [M­{N­(SiMe₃)₂}₂]₂ (<b>1</b>, M = Ca; <b>2</b>, M = Sr) and [M­{CH­(SiMe₃)₂}₂(THF)₂] (<b>3</b>, M = Ca; <b>4</b>, 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, <b>1</b> and <b>2</b>, 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, <b>2</b> and <b>4</b>, were found to outperform their calcium congeners. Reactions of piperidine with <i>para</i>-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 η³-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(SiMe₃)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 Δ<i>S</i>^⧧ 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

Kinetically Directed Reactivity of Magnesium Dihydropyridides with Organoisocyanates

Although reactions between β-diketiminato magnesium species containing monocyclic 1,4-dihydropyridide ligands and the representative ketone benzophenone are observed to provide reduction by formal hydride transfer to yield magnesium diphenylphenoxide, similar reactions with organic isocyanates display only a very limited propensity toward hydride transfer and reduction to amidate species. In these latter systems, reaction primarily takes place with Mg–N insertion to afford O-bound ureide complexes. Further reactions of a magnesium dihydro-isoquinolide complex, which is constrained to hydride dearomatization at the 2-position only, display more variable behavior. Although similar Mg–N insertion and ureide formation is observed for reactions with isocyanates bearing less sterically demanding N-substitution, more bulky isocyanates provide unusual enamide C–C coupling reactivity

Alkaline Earth Catalysis of Alkynyl Alcohol Hydroalkoxylation/Cyclization

Heavier alkaline earth bis­(trimethylsilyl)­amides [Ae­{N­(SiMe₃)₂}₂]₂ (Ae = Ca, Sr, Ba) are shown to act as effective precatalysts for the regioselective intramolecular hydroalkoxylation/cyclization of a wide range of alkynyl and allenyl alcohols. In the majority of cases, cyclization of alkynyl alcohols produces mixtures of the possible endo- and exocyclic enol ether products, rationalized as a consequence of alkynylalkoxide isomerization to the corresponding allene derivatives. Cyclization rates for terminal alkynyl alcohols were found to be significantly higher than for substrates bearing internal alkynyl substituents, while the efficacy of cyclization was in general found to be determined by a combination of stereoelectronic influences and the thermochemical viability of the specific alkaline earth metal catalysis, which we suggest is determined by the individual M–O bond strengths. Kinetic studies have provided a rate law pertaining to a pronounced catalyst inhibition with increasing [substrate], indicating that turnover-limiting insertion of C–C unsaturation into the M–O bond requires the dissociation of substrate molecules away from the Lewis acidic alkaline earth center

Mononuclear Three-Coordinate Magnesium Complexes of a Highly Sterically Encumbered β‑Diketiminate Ligand

The highly sterically encumbered chelating β-diketiminate ligand, [HC­{C­(Me)­N­(2,6-CHPh₂-4-MeC₆H₂)}₂]⁻, ^ArL^–, has been used to prepare a series of heteroleptic three-coordinate magnesium complexes. Both the bis­(imine) and imine-enamine tautomers of the ligand precursor, ^ArLH, as well as the diethyl ether adduct of the bromide complex [^ArLMgBr­(OEt₂)], the monomeric methyl complex [^ArLMgMe], the THF-solvated and unsolvated <i>n</i>-butylmagnesium complexes [^ArLMg^<i>n</i>Bu­(THF)] and [^ArLMg^<i>n</i>Bu], and the 1-hexynyl analogue [^ArLMgCC^<i>n</i>Bu] have been crystallographically characterized. Both <i>n</i>-butylmagnesium complexes showed remarkable stability in air, both in the solid state and in solution. Single crystals of the highly sensitive magnesium hydride, [^ArLMgH], underwent partial hydrolysis by solid-state water diffusion to the isostructural hydroxide compound [^ArLMgOH]

Three-Coordinate Beryllium β‑Diketiminates: Synthesis and Reduction Chemistry

A series of mononuclear, heteroleptic beryllium complexes supported by the monoanionic β-diketiminate ligand [HC­{CMeNDipp}₂]⁻ (<b>L</b>; Dipp = 2,6-diisopropylphenyl) have been synthesized. Halide complexes of the form [LBeX] (X = Cl, I) and a bis­(trimethylsilyl)­amide complex were produced via salt metathesis routes. Alkylberyllium β-diketiminate complexes of the form [LBeR] (R = Me, ^<i>n</i>Bu) were obtained by salt metathesis from the chloride precursor [LBeCl]. Controlled hydrolysis of [LBeMe] afforded an air-stable, monomeric β-diketiminatoberyllium hydroxide complex. [LBeMe] also underwent facile protonolysis with alcohols to form the corresponding β-diketiminatoberyllium alkoxides [LBeOR] (R = Me, ^<i>t</i>Bu, Ph). High temperatures and prolonged reaction times were required for protonolysis of [LBeMe] with primary amines to yield the β-diketiminatoberyllium amide complexes [LBeNHR] (R = ^<i>n</i>Bu, CH₂Ph, Ph). No reactions were observed between [LBeMe] and silanes, terminal acetylenes, or secondary amines. All compounds were characterized by ¹H, ¹³C, and ⁹Be NMR spectroscopy and, in most cases, by X-ray crystallography. Reduction of the beryllium chloride complex with potassium metal resulted in apparent hydrogen-atom transfer between two β-diketiminate backbones, yielding two dimeric, potassium chloride bridged diamidoberyllium species. X-ray analysis of a cocrystallized mixture of the 18-crown-6 adducts of these species allowed unambiguous identification of the two reduced diketiminate ligands, one of which had been deprotonated at a backbone methyl substituent and the other reduced by hydride addition to the β-imine position. It is proposed that this process occurs by the formation of an unobserved radical anion species and intermolecular hydrogen-atom transfer by a radical-based hydrogen abstraction mechanism