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 .

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.

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).

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.

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.

Resonance Raman Spectro-Electrochemistry to Illuminate Photo-Induced Molecular Reaction Pathways.

Electron transfer reactions play a key role for artificial solar energy conversion, however, the underlying reaction mechanisms and the interplay with the molecular structure are still poorly understood due to the complexity of the reaction pathways and ultrafast timescales. In order to investigate such light-induced reaction pathways, a new spectroscopic tool has been applied, which combines UV-vis and resonance Raman spectroscopy at multiple excitation wavelengths with electrochemistry in a thin-layer electrochemical cell to study [RuII(tbtpy)2]2+ (tbtpy = tri-tert-butyl-2,2':6',2''-terpyridine) as a model compound for the photo-activated electron donor in structurally related molecular and supramolecular assemblies. The new spectroscopic method substantiates previous suggestions regarding the reduction mechanism of this complex by localizing photo-electrons and identifying structural changes of metastable intermediates along the reaction cascade. This has been realized by monitoring selective enhancement of Raman-active vibrations associated with structural changes upon electronic absorption when tuning the excitation wavelength into new UV-vis absorption bands of intermediate structures. Additional interpretation of shifts in Raman band positions upon reduction with the help of quantum chemical calculations provides a consistent picture of the sequential reduction of the individual terpyridine ligands, i.e., the first reduction results in the monocation [(tbtpy)Ru(tbtpy•)]⁺, while the second reduction generates [(tbtpy•)Ru(tbtpy•)]0 of triplet multiplicity. Therefore, the combination of this versatile spectro-electrochemical tool allows us to deepen the fundamental understanding of light-induced charge transfer processes in more relevant and complex systems.

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.

Direct Synthesis of Heavy Grignard Reagents: Challenges, Limitations, and Derivatization.

The direct synthesis of organocalcium compounds (heavy Grignard reagents) by the reduction of organyl halides with activated calcium powder succeeded in a straightforward manner for organic bromides and iodides that are bound at sp2 -hybridized carbon atoms. Extension of this strategy to alkyl halides was very limited, and only the reduction of trialkylsilylmethyl bromides and iodides with activated calcium allowed the isolation of the corresponding heavy Grignard reagents. Substitution of only one hydrogen atom of the methylene moiety by a phenyl or methyl group directed this reduction toward the Wurtz-type coupling and the formation of calcium halide and the corresponding C-C coupling product. The stability of the methylcalcium and benzylcalcium derivatives in ethereal solvents suggests an unexpected reaction behavior of the intermediate organyl halide radical anions. Quantum chemical calculations verify a dependency between the ease of preparative access to organocalcium complexes and the C-I bond lengths of the organyl iodides. The bulkiness of the trialkylsilyl group is of minor importance. Chloromethyltrimethylsilane did not react with activated calcium; however, halogen-exchange reactions allowed the isolation of [Ca(CH2 SiMe3 )(thf)3 (µ-Cl)]2 . Furthermore, the metathetical approach of reacting [Ca(CH2 SiMe3 )I(thf)4 ] with KN(SiMe3 )2 and the addition of N,N,N',N'',N''-pentamethyldiethylenetriamine (pmdeta) allowed the isolation of heteroleptic [CaCH2 SiMe3 {N(SiMe3 )2 }(pmdeta)]. In the reaction of this derivative with phenylsilane, the trimethylsilylmethyl group proved to be more reactive than the bis(trimethylsilyl)amido substituent.

Straightforward synthesis of rubidium bis(trimethylsilyl)amide and complexes of the alkali metal bis(trimethylsilyl)amides with weakly coordinating 2,2,5,5-tetramethyltetrahydrofuran.

Rubidium bis(trimethylsilyl)amide (rubidium hexamethyldisilazanide, Rb(hmds)) is accessible on a large scale with excellent yields via a magnetite-catalyzed metalation of hexamethyldisilazane (H(hmds)) in liquid ammonia. Recrystallization of solvent-free alkali metal hexamethyldisilazanides [A(hmds)]n of sodium to cesium from solutions containing 2,2,5,5-tetramethyltetrahydrofuran (Me4THF, thf*) yields the dinuclear complexes [(thf*)A(hmds)]2, which show a rather asymmetric coordination behavior of the bulky ether ligand with strongly bent A-A-O moieties for the heavier K, Rb, and Cs congeners, whereas in the Na complex, the ether ligand is clamped between the trimethylsilyl groups. In hydrocarbon solutions, dissociation of these compounds is observed leading to the liberation of this bulky and weakly binding cyclic ether.

5-Methyl-2-thienylcalcium iodide.

The reduction of 2-bromo- and 3-bromothiophene with calcium powder gives impure thienylcalcium complexes due to interference of various subsequent metalation and calcium-halogen exchange reactions as well as ether degradation. Therefore, calcium-iodine exchange succeeds via the reaction of trimethylsilylmethylcalcium halide with 3-iodothiophene in tetrahydrofuran at -78 °C in the presence of chloro-trimethylsilane, leading to the quantitative formation of 3-(trimethylsilyl)thiophene. The synthesis and isolation of a thienylcalcium halide, a heavy Grignard reagent with a thienyl group, is possible via the reduction of 2-iodo-5-methylthiophene with activated calcium in THF. Substitution of the thf ligands in 5-methyl-2-thienylcalcium iodide ([5-MeThien-2-Ca(thf)4I], 1) by dissolution in tetrahydropyran (THP) leads to the thp complex of 5-methyl-2-thienylcalcium iodide ([5-MeThien-2-Ca(thp)4I], 2). A low field shift of the calcium-bound carbon atom is characteristic of thienylcalcium complexes.

Influence of 18-Crown-6 Ether Coordination on the Catalytic Activity of Potassium and Calcium Diarylphosphinites in Hydrophosphorylation Reactions.

The addition of 18-crown-6 ether (1,4,7,10,13,16-hexaoxacyclooctadecane) to tetranuclear [(thf)K(OPAryl2)]4 and [(thf)4Ca(OPAryl2)2] yields the corresponding mononuclear complexes [(18C6)K(OPAryl2)] [Aryl = Ph (1a), Mes (1b)] and [(18C6)Ca(OPAryl2)2] [Aryl = Ph (2a), Mes (2b)]. The metathesis reaction of [(thf)K(OPAryl2)]4 with CaI2 yields the calciate [K2Ca(thf)x{OPMes2}4]. The addition of dimesitylphosphane oxide and crystallization from a hexane solution gives [K2Ca{OPMes2}4{Mes2P(O)H}] (3). The complexes [(thf)K(OPMes2)]4, [(thf)4Ca(OPMes2)2], 1b, 2b, and the calciate 3 are tested as catalysts in the hydrophosphorylation of isopropylisocyanate with dimesitylphosphane oxide, quantitatively yielding N-isopropyl(dimesitylphosphoryl)formamide. The potassium complexes are more efficient catalysts than the calcium congeners, and coordination of 18-crown-6 decelerates the catalytic conversion.

Coordination behavior of bidentate bis(carbenes) at alkali metal bis(trimethylsilyl)amides.

Lithiation of HN(SiMe3)2 in hexane and crystallization in the presence of 2,2,5,5-tetramethyltetrahydrofuran (Me4THF) allows the isolation of crystalline tetrameric [LiN(SiMe3)2]4 (1) with an average Li-N distance of 208.9 pm. Deprotonation of a methylene-bridged bis(imidazolium) diiodide with NaN(SiMe3)2 in tetrahydrofuran (THF) yields 1,1'-diisopropyl-3,3'-methylene-diimidazole-2,2'-diylidene (H2C(NHCiPr)2) and layering of this orange reaction mixture with pentane yield [{H2C(NHCiPr)2}{(thf)NaI}2]∞ (2). The reaction of this bis(imidazolium) diiodide with excess of KN(SiMe3)2 in THF leads to the formation of [{H2C(NHCiPr)2}KN(SiMe3)2]2 (3). Due to the fact that 2 and 3 are polymeric in the crystalline state, the bulkier 1,1'-bis(2,6-diisopropylphenyl)-3,3'-methylene-diimidazole-2,2'-diylidene (H2C(NHCDipp)2) has been used. Thus, addition of H2C(NHCDipp)2 with the bulky 2,6-diisopropylphenyl (Dipp) groups to AN(SiMe3)2 (A = Li, Na, K) in toluene yields mononuclear [{H2C(NHCDipp)2}LiN(SiMe3)2] (4) and dinuclear [{H2C(NHCDipp)2}AN(SiMe3)2]2 for sodium (5) and potassium (6). The rather short alkali metal-carbon bonds lie in the range of alkylmetal derivatives.

Heavy Grignard Reagents: Synthesis, Physical and Structural Properties, Chemical Behavior, and Reactivity.

The Grignard reaction offers a straight forward atom-economic synthesis of organomagnesium halides, which undergo redistribution reactions (Schlenk equilibrium) yielding diorganylmagnesium and magnesium dihalides. The homologous organocalcium complexes (heavy Grignard reagents) gained interest only quite recently owing to several reasons. The discrepancy between the inertness of this heavy alkaline earth metal and the enormous reactivity of its organometallics hampered a vast and timely development after the first investigation more than 100 years ago. In this overview the synthesis of organocalcium reagents is described as is the durability in ethereal solvents. Aryl-, alkenyl-, and alkylcalcium halides are prepared by direct synthesis. Characteristic structural features and NMR parameters are discussed. Ligand redistribution reactions can be performed by addition of potassium tert-butanolate to ethereal solutions of arylcalcium iodides yielding soluble diarylcalcium, whereas sparingly soluble potassium iodide and calcium bis(tert-butanolate) precipitate. Furthermore, reactivity studies with respect to metalation and addition to unsaturated organic compounds and metal-based Lewis acids, leading to the formation of heterobimetallic complexes, are presented.

Potassium-Mediated Hydrophosphorylation of Heterocumulenes with Diarylphosphane Oxide and Sulfide.

The preparation of the hydrophosphorylation catalysts succeeds via the metalation of dimesitylphosphane oxide and diphenylphosphane sulfide with potassium hydride in ethereal solvents such as tetrahydropyran (THP) and tetrahydrofuran (THF) yielding the tetramers [(thp)K(OPMes2)]4 (1a) and [(thf)3{K(OPMes2)}4] (1b) as well as [(thp)KSPPh2]∞ (2) with a strand-like structure in the crystalline state. In ethereal solution these complexes very slowly degrade into KPAr2 and KE2PAr2 (E = O, S). The catalytic conversion of iPr-N═C═E' (E' = O, S) and of R-N═C═N-R (R = iPr, cHex) to the addition products Ar2P(E)-C(=E')-NHR (Ar = Ph, Mes; E = O, S; E' = O, S, NR) was studied in the presence of catalytic amounts of Ar2PEK (Ar = Ph, Mes; E = O, S). Steric hindrance prevents the addition of dimesitylphosphane oxide to N,N'-diisopropylcarbodiimide, whereas diphenylphosphane oxide and sulfide smoothly add to iPr-N═C═N-iPr yielding Ph2P(E)-C(═N-iPr)-NHiPr (E = O, S).

Surprisingly Different Reaction Behavior of Alkali and Alkaline Earth Metal Bis(trimethylsilyl)amides toward Bulky N-(2-Pyridylethyl)-N'-(2,6-diisopropylphenyl)pivalamidine.

N-(2,6-Diisopropylphenyl)-N'-(2-pyridylethyl)pivalamidine (Dipp-N=C(tBu)-N(H)-C2 H4 -Py) (1), reacts with metalation reagents of lithium, magnesium, calcium, and strontium to give the corresponding pivalamidinates [(tmeda)Li{Dipp-N=C(tBu)-N-C2 H4 -Py}] (6), [Mg{Dipp-N=C(tBu)-N-C2 H4 -Py}2 ] (3), and heteroleptic [{(Me3 Si)2 N}Ae{Dipp-N=C(tBu)-N-C2 H4 -Py}], with Ae being Ca (2 a) and Sr (2 b). In contrast to this straightforward deprotonation of the amidine units, the reaction of 1 with the bis(trimethylsilyl)amides of sodium or potassium unexpectedly leads to a ß-metalation and an immediate deamidation reaction yielding [(thf)2 Na{Dipp-N=C(tBu)-N(H)}] (4 a) or [(thf)2 K{Dipp-N=C(tBu)-N(H)}] (4 b), respectively, as well as 2-vinylpyridine in both cases. The lithium derivative shows a similar reaction behavior to the alkaline earth metal congeners, underlining the diagonal relationship in the periodic table. Protonation of 4 a or the metathesis reaction of 4 b with CaI2 in tetrahydrofuran yields N-(2,6-diisopropylphenyl)pivalamidine (Dipp-N=C(tBu)-NH2 ) (5), or [(thf)4 Ca{Dipp-N=C(tBu)-N(H)}2 ] (7), respectively. The reaction of AN(SiMe3 )2 (A=Na, K) with less bulky formamidine Dipp-N=C(H)-N(H)-C2 H4 -Py (8) leads to deprotonation of the amidine functionality, and [(thf)Na{Dipp-N=C(H)-N-C2 H4 -Py}]2 (9 a) or [(thf)K{Dipp-N=C(H)-N-C2 H4 -Py}]2 (9 b), respectively, are isolated as dinuclear complexes. From these experiments it is obvious, that ß-metalation/deamidation of N-(2-pyridylethyl)amidines requires bases with soft metal ions and also steric pressure. The isomeric forms of all compounds are verified by single-crystal X-ray structure analysis and are maintained in solution.