Coordination chemistry of alkali metal dimesityl-thio- and dimesityl-selenophosphinites [(L)2A-EPMes2]2 (A = Li, Na, K; E = S, Se; L = THF, THP) and [(18C6)K-SPMes2].

The reactions of dimesitylphosphane oxide Mes2P(O)H with Lawessons reagent and dimesitylphoshane with selenium yield Mes2P(E)H with E = S (1a) and E = Se (1b), respectively, with moderate yields. Metalation of dimesitylphosphane sulfide 1a with n-butyllithium, sodium hydride or potassium hydride in THF allows the isolation of dinuclear dimesityl-thiophosphinites of the type [(thf)2A-S-PMes2]2 [A = Li (4), Na (5), K (2a)] with central four-membered A2S2 rings. The weaker base THP leads to the very similar aggregate [(thp)2K-S-PMes2]2 (3a) as has also been observed for the homologous potassium dimesityl-selenophosphinites of the type [(L)2K-Se-PMes2]2 [L = thf (2b), thp (3b)]. Addition of 18-crown-6 ether leads to deaggregation and expectedly to formation of mononuclear [(18C6)K-S-PMes2] (6). Moderate yields have been obtained due to dismutation reactions that yield the corresponding phosphinates AE2PMes2 and phosphanides APMes2, a degradation process which has been observed earlier also for Li-O-PMes2. This side reaction hampers the application of these thio- and selenophosphinites as catalysts in the addition of phosphane sulfides and selenides across alkynes.

Group IV Complexes with Sterically Congested N-Aryl-adamantylcarbamidate Ligands.

Metalation of N-(2,6-dibenzhydryl-4-tolyl)adamantane-1-carboxamide (1, Ar*N(H)-C(O)-Ad) with M(NMe2)4 (M = Ti, Zr, Hf) yields amidate complexes Ar*N=C(Ad)-O-Ti(NMe2)3 (2) as well as bis(amidate) compounds (Ar*N=C(Ad)-O)2M(NMe2)2 (M = Zr (3), Hf (4)). In 2, the amidate ligand acts as a monodentate base via the oxygen atom with the Ti center in a slightly distorted tetrahedral environment. The steric requirement of the amidate ligand stabilizes the small coordination number of four of the Ti atom. In congeners 3 and 4, two bidentate amidate ligands exist in the coordination spheres, leading to hexacoordinate group IV metal atoms. The small bite angles of the Zr- and Hf-bound amidate ligands lead to severe distortion of the octahedral environments of the Zr and Hf centers. Titanium compound 2 is an unsuitable choice to catalyze hydrofunctionalization of alkynes with amines and phosphane oxides and despite the significantly smaller pKa value of the carboxylic amide, formation of carboxamide 1 is the dominant reaction upon addition of amines or phosphane oxides to release intramolecular steric strain introduced by the very bulky adamantylamidato ligand.

In Situ Generation of Magnesium- and Calcium-Based Grignard Reagents for Amide Synthesis.

The alkaline-earth metals Mg and Ca are too inert for the direct metalation of primary and secondary amines. Consequently, activation prior to use is required. Alternatively, the Grignard reagents RMgX (R=alkyl, aryl, X=halide) can be applied in metalation of amines. However, such a straightforward procedure for the synthesis of alkylcalcium reagents is disadvantageous due to diverse side reactions, including Wurtz-type C-C coupling and ether degradation reactions. Therefore, suspensions of magnesium or calcium with amine can be treated in a smooth reaction with ethyl bromide in an ethereal solvent at room temperature. Intermediately formed RAeX (Ae=alkaline-earth metal, i. e., Mg, Ca) either metalates amines yielding the corresponding amides in an in situ Grignard metalation method (iGMM) or adds across C=N bonds of imines in an in situ Grignard addition method (iGAM). The amides R'2 N-AeX (Ae=Mg: Hauser bases) undergo Schlenk-type ligand exchange reactions yielding homoleptic Ae(NR'2 )2 and potentially sparingly soluble AeX2 .

Synthesis of Sterically Encumbered Alkaline-Earth Metal Amides Applying the In Situ Grignard Reagent Formation.

Magnesium and calcium are too inert to deprotonate amines directly. For the synthesis of bulky amides alternative strategies are required and in the past, N-bound trialkylsilyl groups have been used to ease metalation reactions. The in situ Grignard reagent formation in stirred suspensions of magnesium or calcium with hydryl halide and imine in THF allows the synthesis of a plethora of amides with bulky silyl-free substituents. Ball milling protocols partially favor competitive side reactions such as aza-pinacol coupling reactions. Calcium is the advantageous choice for the in situ Grignard reagent formation and subsequent addition onto the imines yielding bulky calcium bis(amides) whereas the stronger reducing heavier alkaline-earth metals strontium and barium are less selective and hence, the aza-pinacol coupling reaction becomes competitive. Exemplary, the solid-state molecular structures of [(Et2 O)Mg(N(Ph)(CHPh2 )2 ] and [(Et2 O)2 Ca(N(Ph)(CHPh2 )2 ] have been determined.

In situ Grignard Metalation Method, Part II: Scope of the One-Pot Synthesis of Organocalcium Compounds.

The in situ Grignard Metalation Method (iGMM) is a straightforward one-pot strategy to synthesize alkaline-earth metal amides in multi-gram scale with high yields via addition of bromoethane to an ethereal suspension of a primary or secondary amine and magnesium (Part I) or calcium (Part II). This method is highly advantageous because no activation of calcium is required prior to the reaction. Contrary to the magnesium-based iGMM, there are some limitations, the most conspicuous one is the large influence of steric factors. The preparation of Ca(hmds)2 succeeds smoothly within a few hours with excellent yields opening the opportunity to prepare large amounts of this reagent. Side reactions and transfer of the iGMM to substituted anilines and N-heterocycles as well as other H-acidic substrates such as cyclopentadienes are studied. Bulky amidines cannot be converted directly to calcium amidinates via the iGMM but stoichiometric calciation with Ca(hmds)2 enables their preparation.

In Situ Grignard Metalation Method for the Synthesis of Hauser Bases.

The in situ Grignard Metalation Method (iGMM) is a straightforward one-pot procedure to quickly produce multigram amounts of Hauser bases R2 N-MgBr which are valuable and vastly used metalation reagents and novel electrolytes for magnesium batteries. During addition of bromoethane to a suspension of Mg metal and secondary amine at room temperature in an ethereal solvent, a smooth reaction yields R2 N-MgBr under evolution of ethane within a few hours. A Schlenk equilibrium is operative, interconverting the Hauser bases into their solvated homoleptic congeners Mg(NR2 )2 and MgBr2 depending on the solvent. Scope and preconditions are studied, and side reactions limiting the yield have been investigated. DOSY NMR experiments and X-ray crystal structures of characteristic examples clarify aggregation in solution and the solid state.

Sterically shielded primary anilides of the alkaline-earth metals of the type (thf)nAe(NH-Ar*)2 (Ae = Mg, Ca, Sr, and Ba; Ar* = bulky aryl).

Metalation of 2,4,6-triphenylphenylamine (H2N-C6H2-2,4,6-Ph3, 1a) and 4-methyl-2,6-bis(diphenylmethyl)aniline (2,6-bis(diphenylmethyl)-p-toluidine, H2N-C6H2-4-Me-2,6-(CHPh2)2, 2a) with dibutylmagnesium and Ae[N(SiMe3)2]2 with a stoichiometric 1 : 2 ratio in THF at room temperature yields the corresponding primary anilides [(thf)nAe{N(H)-C6H2-2,4,6-Ph3}2] (Ae/n = Mg/2 (1b), Ca/2 (1c), Sr/3 (1d), and Ba/3 (1e)) and [(thf)nAe{N(H)-C6H2-4-Me-2,6-(CHPh2)2}2] (Ae/n = Mg/2 (2b), Ca/3 (2c) and Sr/2 (2d)), respectively. The 1 : 1 reaction of Mg(n/sBu)2 and MgPh2 with 2a leads to the formation of heteroleptic [(thf)2Mg(R){N(H)-C6H2-4-Me-2,6-(CHPh2)2}] (R = n/sBu (2bBu), Ph (2bPh)). At 50 °C, the strontium complex 2d liberates one equivalent of 2avia intramolecular deprotonation of the triarylmethyl functionality yielding dinuclear [(thf)2Sr{N(H)-C6H2-4-Me-2-(CPh2)-6-(CHPh2)2}]2 (2d'). The barium compound is significantly more reactive and regardless of applied stoichiometry the isotypic barium congener [(thf)2Ba{N(H)-C6H2-4-Me-2-(CPh2)-6-(CHPh2)2}]2 (2e') forms. The molecular structures of 1c, 2d, 2d', and 2e' are stabilized by metal-phenyl π-interactions.

Synthesis and Oligonuclear Structures of Strontium and Barium Complexes with Protonated and Deprotonated N-Mesityl-P,P-diphenylphosphinic Amide Ligands.

Metalation of N-mesityl-P,P-diphenylphosphinic amide Ph2P(O)-NHMes (HL, I) with MgBu2 and Ae{N(SiMe3)2}2 (Ae = Ca, Sr, and Ba) yields alkaline-earth metal complexes with the compositions of [(thf)nAe(L·HL)2] [Ae/n = Mg/0 (II), Ca/2 (III)] as well as of [Sr2L3(L·HL)(HL)] (1), [Ba2L3(L·HL)(HL)] (2), [Ba3L6] (3), and [(thf)2Ba3L6] (4). In III, 1, 2, and 3, the alkaline-earth metal atoms are in severely distorted octahedral environments, and the structural distortions are partially caused by the small O-Ae-N bite angles of the chelating Ph2P(O)-NMes anions. The substructures (L·HL) contain N-H···N hydrogen bridges, stabilizing the arrangement of the ligands in complexes II, III, 1, and 2. In the trinuclear barium complex [Ba(µ-L)3Ba(µ-L)3Ba] (3), a rigid adjustment of the anionic L bases leads to a C3-symmetric molecule in the crystalline state with bridging oxygen atoms. Due to the small O-Ba-N bite angles of the chelating anions, vacant coordination sites are available at the outer barium centers. Coordination of thf bases in these gaps yields the complex [(thf)Ba(µ-L)3Ba(µ-L)3Ba(thf)] (4). However, THF is unable to deaggregate the trinuclear complexes into smaller barium-containing moieties. Increasing the radius of the alkaline-earth metals and increasing the nuclearity of these compounds lead to decreasing solubility in common organic solvents. NMR studies verify that the molecular structures of these alkaline-earth metal complexes are maintained in ethereal solvents and toluene.

One-Step Synthesis and Schlenk-Type Equilibrium of Cyclopentadienylmagnesium Bromides.

In the in situ Grignard metalation method (iGMM), the addition of bromoethane to a suspension of magnesium turnings and cyclopentadienes [C5 H6 (HCp), C5 H5 -Si(iPr)3 (HCpTIPS )] in diethyl ether smoothly yields heteroleptic [(Et2 O)Mg(CpR )(µ-Br)]2 (CpR =Cp (1), CpTIPS (2)). The Schlenk equilibrium of 2 in toluene leads to ligand exchange and formation of homoleptic [Mg(CpR )2 ] (3) and [(Et2 O)MgBr(µ-Br)]2 (4). Interfering solvation and aggregation as well as ligand redistribution equilibria hamper a quantitative elucidation of thermodynamic data for the Schlenk equilibrium of 2 in toluene. In ethereal solvents, mononuclear species [(Et2 O)2 Mg(CpTIPS )Br] (2'), [(Et2 O)n Mg(CpTIPS )2 ] (3'), and [(Et2 O)2 MgBr2 ] (4') coexist. Larger coordination numbers can be realized with cyclic ethers like tetrahydropyran allowing crystallization of [(thp)4 MgBr2 ] (5). The interpretation of the temperature-dependency of the Schlenk equilibrium constant in diethyl ether gives a reaction enthalpy ΔH and reaction entropy ΔS of -11.5 kJ mol-1 and 60 J mol-1 , respectively.

Photoisomerization Neutralizes Vasoconstrictive Activity of a Heme Degradation Product.

Delayed cerebral ischemia (DCI) caused by cerebral vasospasm is the leading determinant of poor outcome and mortality in subarachnoid hemorrhage (SAH) patients, but current treatment options lack effective prevention and therapy. Two substance families of heme degradation products (HDPs), bilirubin oxidation end products (BOXes) and propentdyopents (PDPs), are elicitors of pathologic cerebral hypoperfusion after SAH. Z-configured HDPs can be photoconverted into the corresponding E-isomers. We hypothesize that photoconversion is a detoxification mechanism to prevent and treat DCI. We irradiated purified Z-BOXes and Z-PDPs with UV/Vis light and documented the Z-E photoconversion. E-BOX A slowly reisomerizes to the thermodynamically favored Z-configuration in protein-containing media. In contrast to vasoconstrictive Z-BOX A, E-BOX A does not cause vasoconstriction in cerebral arterioles in vitro and in vivo in wild-type mice. Our results enable a critical assessment of light-induced intrathecal photoconversion and suggest the use of phototherapy to prevent and cure HDP-mediated cerebral vasospasms.

Fe2+-Mediated Activation of BKCa Channels by Rapid Photolysis of CORM-S1 Releasing CO and Fe2.

Heme catabolism by heme oxygenase (HO) with a decrease in intracellular heme concentration and a concomitant local release of CO and Fe2+ has the potential to regulate BKCa channels. Here, we show that the iron-based photolabile CO-releasing molecule CORM-S1 [dicarbonyl-bis(cysteamine)iron(II)] coreleases CO and Fe2+, making it a suitable light-triggered source of these downstream products of HO activity. To investigate the impact of CO, iron, and cysteamine on BKCa channel activation, human Slo1 (hSlo1) was expressed in HEK293T cells and studied with electrophysiological methods. Whereas hSlo1 channels are activated by CO and even more strongly by Fe2+, Fe3+ and cysteamine possess only marginal activating potency. Investigation of hSlo1 mutants revealed that Fe2+ modulates the channels mainly through the Mg2+-dependent activation mechanism. Flash photolysis of CORM-S1 suits for rapid and precise delivery of Fe2+ and CO in biological settings.

Stripping and Plating a Magnesium Metal Anode in Bromide-Based Non-Nucleophilic Electrolytes.

A non-nucleophilic Hauser base hexamethyldisilazide (HMDS) magnesium electrolyte possesses inherent properties required for a magnesium-sulfur battery. However, the development of full cell batteries using HMDSCl-based electrolytes is still hampered by a low coulombic efficiency. A new electrolyte formulation of non-nucleophilic HMDS magnesium containing bromide as a halide instead of chloride was obtained through a simple and straightforward synthesis route. The electrochemistry of magnesium was investigated through plating and stripping in three different HMDSBr-based electrolytes: Mg(HMDS)Br, Mg(HMDS)Br-BEt3 , and Mg(HMDS)Br-AlEt3 dissolved in tetrahydrofuran. The different magnesium species present in the electrolytes were determined using NMR. Weak electron-withdrawing Lewis acids, BEt3 and AlEt3 were used intentionally and their impact was investigated. Contrary to expectation, the substitution of chloride by bromide does not drastically narrow the electrochemical stability window. HMDSBr-based electrolytes demonstrated long-term (1000 cycles) stable reversibility and highly efficient (≈99 %) magnesium plating/tripping without a high ratio of bromide compared with the MgHMDSCl-based electrolytes. The aprotic electrolyte shows comparatively high anodic stability (≈2.4 V vs. Mg/Mg2+ ) and high ionic conductivity of 1.16 mS cm-1 at room temperature. Plating of magnesium with low overpotential (<188 mV) revealed a morphology dependence on the electrolyte type with a shiny metallic homogenous layer, suggesting a rational balance between the nucleation and growth process in HMDSBr-based electrolytes.

Scope and Limitations of the s-Block Metal-Mediated Pudovik Reaction.

The hydrophosphorylation of phenylacetylene with di(aryl)phosphane oxides Ar2 P(O)H (Pudovik reaction) yields E/Z-isomer mixtures of phenylethenyl-di(aryl)phosphane oxides (1). Alkali and alkaline-earth metal di(aryl)phosphinites have been studied as catalysts for this reaction with increasing activity for the heavier s-block metals. The Pudovik reaction can only be mediated for di(aryl)phosphane oxides whereas P-bound alkyl and alcoholate substituents impede the P-H addition across alkynes. The demanding mesityl group favors the single-hydrophosphorylated products 1-Ar whereas smaller aryl substituents lead to the double-hydrophosphorylated products 2-Ar. Polar solvents are beneficial for an effective addition. Increasing concentration of the reactants and the catalyst accelerates the Pudovik reaction. Whereas Mes2 P(O)H does not form the bis-phosphorylated product 2-Mes, activation of an ortho-methyl group and cyclization occurs yielding 2-benzyl-1-mesityl-5,7-dimethyl-2,3-dihydrophosphindole 1-oxide (3).

Pyrrolic and Dipyrrolic Chlorophyll Degradation Products in Plants and Herbivores.

The degradation of chlorophyll, the omnipresent green pigment, has been investigated intensively over the last 30 years resulting in many elucidated tetrapyrrolic degradation products. With a comparison to the degradation of the structurally similar heme, we hereby propose a novel additional chlorophyll degradation mechanism to mono- and dipyrrolic products. This is the first proof of the occurrence of a family of mono- and dipyrrols in leaves that are previously only known as heme degradation products. This product family is also found in spit and feces of herbivores with specific metabolomic patterns reflecting the origin of the samples. Based on chromatographic and mass spectrometric evidence as well as on mechanistic considerations we also suggest several tentative new degradation products. One of them, dihydro BOX A, was fully confirmed as a novel natural product by synthesis and comparison of its spectroscopic data.

Structure-Solubility Relationship of 1,4-Dioxane Complexes of Di(hydrocarbyl)magnesium.

Systematic variation of the 1,4-dioxane (dx) concentration during the precipitation of sparingly soluble [MgBr2 (dx)2 ] from ethereal Grignard solutions of RMgBr has allowed the structural investigation of crystallized [R2 Mg(dx)n ] (n=1, 1.5, 2, and 3), which form during this dioxane method, depending on the bulkiness of R. The numbering of the complexes explored in this study is based on the number n of dioxane molecules per magnesium atom, followed by the substituent R; an apostrophe denotes coordination polymers. The following derivatives were studied by X-ray crystal-structure determination and NMR spectroscopy: n=1: [Me2 Mg(µ-dx)]∞ (1'-Me) and [nPr2 Mg(µ-dx)]∞ (1'-nPr); n=1.5: [{iPr2 Mg(dx)}2 (µ-dx)] (1.5-iPr), [{oTol2 Mg(dx)}2 (µ-dx)] (1.5-oTol), and [(Me3 Si-C≡C)2 Mg(dx)1.5 ]∞ (1.5'-C2 SiMe3 ); n=2: [tBu2 Mg(dx)2 ] (2-tBu) and [oTol2 Mg(dx)2 ] (2-oTol); n=3: [Ph2 Mg(dx)3 ] (3-Ph). In the structure types 1', 1.5, and 2, the magnesium atom exhibits the coordination number 4, whereas pentacoordinate metal atoms are observed in types 3 and 1.5'. The structure type 2' is realized for [(Ph-C≡C)2 Mg(dx)2 ]∞ (2'-C2 Ph), [MgCl2 (dx)2 ]∞ (2'-Cl), and [MgBr2 (dx)2 ]∞ (2'-Br) with hexacoordinate metal atoms. The solubility of the dioxane adducts in common organic solvents strongly depends on the degree of aggregation with the solubility decreasing from molecular to strand to layer structures.

Total syntheses of the bilirubin oxidation end product Z-BOX C and its isomeric form Z-BOX D.

Oxidative degradation products of bilirubin (BOXes) are biologically highly active and certain BOXes cause long-lasting narrowing of cerebral blood vessels presumably with a significant role in subarachnoid hemorrhage. Due to the fact that mode of action as well as fate of these BOXes is widely unknown, larger amounts of these bilirubin degradation end products are required. The total synthesis of colorless (Z)-3-(5-(2-amino-2-oxoethylidene)-4-methyl-2-oxo-2,5-dihydro-1H-pyrrol-3-yl)propanoic acid (BOX C) succeeds via a seven-step procedure with a total yield of 20%. Its isomeric form (Z)-3-(2-(2-amino-2-oxoethylidene)-4-methyl-5-oxo-2,5-dihydro-1H-pyrrol-3-yl)propanoic acid (BOX D) can be prepared via a five-step protocol with a yield of 30%. NMR and crystallographic studies reveal that charge delocalization within the conjugated π-systems of BOXes C and D is negligible. Exposure of solutions of Z-BOX C and Z-BOX D to bright sunlight leads to Z/E-isomerization and mixtures of the respective E/Z-BOXes C and D.

Bis(trimethylsilyl)amide complexes of s-block metals with bidentate ether and amine ligands.

The synthesis of the bistrimethylsilylamide complexes of alkali and alkaline-earth metals with bidentate ether and amine bases 1,2-bis(dimethylamino)ethane (tetramethylethylenediamine, tmeda), dimethyl-methoxyethylamine (dmmea), and 1,2-dimethoxyethane (dme) succeeds via addition of these bases to coligand-free complexes or via ligand exchange of thf adducts. The lithium and sodium complexes are mononuclear (exceptions: [(dme)LiN(SiMe3)2]2 and [(tmeda)NaN(SiMe3)2]2) whereas the heavier congeners form dinuclear compounds with central four-membered M2N2 rings. The alkaline-earth metal complexes crystallize as mononuclear complexes i.e. as [(L)M{N(SiMe3)2}2] with four-coordinate metal centers or as [(L)2M{N(SiMe3)2}2] with six-coordinate metal atoms. Intramolecular steric repulsion leads to distortions such as an asymmetric coordination of L and/or significantly different proximal and distal M-N-Si bond angles.

Propentdyopents as Heme Degradation Intermediates Constrict Mouse Cerebral Arterioles and Are Present in the Cerebrospinal Fluid of Patients With Subarachnoid Hemorrhage.

RATIONALE: Delayed ischemic neurological deficit is the most common cause of neurological impairment and unfavorable prognosis in patients with subarachnoid hemorrhage (SAH). Despite the existence of neuroimaging modalities that depict the onset of the accompanying cerebral vasospasm, preventive and therapeutic options are limited and fail to improve outcome owing to an insufficient pathomechanistic understanding of the delayed perfusion deficit. Previous studies have suggested that BOXes (bilirubin oxidation end products), originating from released heme surrounding ruptured blood vessels, are involved in arterial vasoconstriction. Recently, isolated intermediates of oxidative bilirubin degradation, known as PDPs (propentdyopents), have been considered as potential additional effectors in the development of arterial vasoconstriction. OBJECTIVE: To investigate whether PDPs and BOXes are present in hemorrhagic cerebrospinal fluid and involved in the vasoconstriction of cerebral arterioles. METHODS AND RESULTS: Via liquid chromatography/mass spectrometry, we measured increased PDP and BOX concentrations in cerebrospinal fluid of SAH patients compared with control subjects. Using differential interference contrast microscopy, we analyzed the vasoactivity of PDP isomers in vitro by monitoring the arteriolar diameter in mouse acute brain slices. We found an arteriolar constriction on application of PDPs in the concentration range that occurs in the cerebrospinal fluid of patients with SAH. By imaging arteriolar diameter changes using 2-photon microscopy in vivo, we demonstrated a short-onset vasoconstriction after intrathecal injection of either PDPs or BOXes. Using magnetic resonance imaging, we observed a long-term PDP-induced delay in cerebral perfusion. For all conditions, the arteriolar narrowing was dependent on functional big conductance potassium channels and was absent in big conductance potassium channels knockout mice. CONCLUSIONS: For the first time, we have quantified significantly higher concentrations of PDP and BOX isomers in the cerebrospinal fluid of patients with SAH compared to controls. The vasoconstrictive effect caused by PDPs in vitro and in vivo suggests a hitherto unrecognized pathway contributing to the pathogenesis of delayed ischemic deficit in patients with SAH.

Synthesis and catalytic activity of tridentate N-(2-pyridylethyl)-substituted bulky amidinates of calcium and strontium.

Metalation of the formamidine Dipp-N[double bond, length as m-dash]C(H)-N(H)-C2H4-Py (1a) and benzamidine Dipp-N[double bond, length as m-dash]C(Ph)-N(H)-C2H4-Py (1b) with [(thf)2M{N(SiMe3)2}2] (M = Ca, Sr) yields the corresponding homoleptic complexes [M{Dipp-N[double bond, length as m-dash]C(R)-N-C2H4-Py}2] (M/R = Ca/H (2a), Ca/Ph (2b), and Sr/Ph (3b)) regardless of the applied stoichiometry. Only during calciation of Dipp-N[double bond, length as m-dash]C(H)-N(H)-C2H4-Py (1a), the heteroleptic intermediate [{(Me3Si)2N}Ca{Dipp-N[double bond, length as m-dash]C(R)-N-C2H4-Py}]2 (2a') has been observed. The formamidinate complex of strontium crystallizes as a tmeda adduct of the type [(tmeda)Sr{Dipp-N[double bond, length as m-dash]C(H)-N-C2H4-Py}2] (3a). Metalation of the pivalamidine Dipp-N[double bond, length as m-dash]C(tBu)-N(H)-C2H4-Py (1c) leads to the formation of heteroleptic mononuclear [{(Me3Si)2N}M{Dipp-N[double bond, length as m-dash]C(tBu)-N(H)-C2H4-Py}] (M = Ca (2c) and Sr (3c)) with a side-on bonding of the Dipp group to the alkaline earth metals. Calciation of chiral Dipp-N[double bond, length as m-dash]C(tBu)-N(H)-CH2CH(Me)-Py (R)-1d and (S)-1d with [(thf)2Ca{N(SiMe3)2}2] yields the homoleptic complexes [Ca{Dipp-N[double bond, length as m-dash]C(tBu)-N-CH2CH(Me)-Py}2] with the enantiomeric forms (R,R)-2d and (S,S)-2d regardless of the applied stoichiometry. The complexes 2c and 2d catalyze the intramolecular hydroamination of the aminoalkene 2,2-diphenylpent-4-enylamine yielding 2-methyl-4,4-diphenylpyrrolidine but the stereochemistry cannot be influenced by the chiral compounds (R,R)-2d and (S,S)-2d.

Alkaline-Earth Metal Bis[bis(trimethylsilyl)amide] Complexes with Weakly Coordinating 2,2,5,5-Tetramethyltetrahydrofuran Ligands.

The reaction of [(Me3Si)2N-Ae{µ-N(SiMe3)2}]2 with 2,2,5,5-tetramethyltetrahydrofuran in pentane yields the mononuclear complexes [(Me4thf)Ae{N(SiMe3)2}2] (Ae = Mg (1), Ca (2), Sr (3), and Ba (4)) with three-coordinate alkaline-earth metal centers. With increasing radius of the alkaline-earth metal atoms, the N-Ae-N bond angles decrease. These ether adducts significantly enhance the solubility of the bis(trimethylsilyl)amides of the alkaline-earth metals in hydrocarbon solvents. Contrary to the magnesium derivative 1, the heavier congeners dissociate into mononuclear [Ae{N(SiMe3)2}2] and free Me4THF without formation of sparingly soluble dinuclear [(Me3Si)2N-Ae{µ-N(SiMe3)2}]2.