Treatment of a Methylene‐Bridged Dialuminium Compound with Lithium Alkanides and Alkynides ‐ Single vs. Twofold Terminal Coordination

We have investigated the coordination of alkanide and alkynide anions to the coordinatively unsaturated aluminium atoms of the methylene‐bridged dialuminium compound R₂Al‐CH₂‐AlR₂ [ 1 , R = CH(SiMe₃)₂]. Treatment of 1 with the corresponding lithium derivatives in the presence of a small excess of TMEN (TMEN = tetramethylethylenediamine) yielded mono‐adducts [M]⁺[R₂Al‐CH₂‐AlR₂R']^– [ 2a , M = Li(TMEN)₂, R' = Me; 2b , M = Li(TMEN)₂, R' = n‐Bu; 3a , M = Li(TMEN)₂, R' = C≡C‐SiMe₃; 3b , M = Li(TMEN)₂, R' = C≡C‐t‐Bu; 3d , M = Li(DME)₃, R' = C≡C‐Ph; 3e , M = Li(TMEN)₂, R' = C≡C‐PPh₂)] and bis‐adducts [Li(TMEN)₂]⁺[LiCH₂(AlR₂R')₂]^– [ 4a , R' = C≡C‐CH₂‐NEt₂; 4b , R' = C≡C‐t‐Bu]. In the solid state the mono‐adducts have clearly separated coordinatively saturated (coordination number four) and unsaturated aluminium atoms (coordination number three). In solution the groups R' show a fast exchange between both aluminium atoms as evident from the room temperature NMR spectra that showed in most cases equivalent CH(SiMe₃)₂ groups despite different coordination spheres of the metal atoms. Only 2b gave the expected splitting of resonances at ambient temperature, while cooling was required to prevent the dynamic process for 3a . The dialkynide 4a has a unique molecular structure with one of the lithium cations bonded to the α‐carbon atoms of the alkynido ligands and to the carbon atom of the methylene bridge which is five‐coordinate with a distorted trigonal bipyramidal coordination sphere.     

New Ga/N‐based Active Lewis Pairs,Trifunctional Ga–Ga Compounds,Reactions with Cyanamide and Dual Insertion of Isocyanate

Hydrogallation of Me₃Si–C≡C–NR'₂ with R₂Ga–H (R = tBu, CH₂tBu, iBu) yielded Ga/N‐based active Lewis pairs, R₂Ga–C(SiMe₃)=C(H)–NR'₂ ( 7 ). The Ga and N atoms adopt cis‐positions at the C=C bonds and show weak Ga–N interactions. tBu₂GaH and Me₃Si–C≡C–N(C₂H₄)₂NMe afforded under exposure of daylight the trifunctional digallium(II) compound [MeN(C₂H₄)₂N](H)C=C(SiMe₃)Ga(tBu)–Ga(tBu)C(SiMe₃)=C(H)[N(C₂H₄)₂NMe] ( 8 ), which results from elimination of isobutene and H₂ and Ga–Ga bond formation. 8 was selectively obtained from the ynamine and [tBu(H)Ga–Ga(H)tBu]₂[HGatBu₂]₂. 7a (R = tBu; NR'₂ = 2,6‐Me₂NC₅H₈) and H₈C₄N–C≡N afforded the adduct tBu₂Ga‐C(SiMe₃)=C(H)(2,6‐Me₂NC₅H₈) · N≡C–NC₄H₈ ( 11 ) with the nitrile bound to gallium. The analogous ALP with harder Al atoms yielded an adduct of the nitrile dimer or oligomers of the nitrile at room temperature. The reaction of 7a with Ph–N=C=O led to the insertion of two NCO groups into the Ga–C_vinyl bond to yield a GaOCNCN heterocycle with Ga bound to O and N atoms ( 12 ).     

Synthesis of Tris‐ and Tetrakis(pentafluoroethyl)silanes

The synthesis and complete characterization of functional, highly Lewis acidic tris(pentafluoroethyl)silanes as well as tetrakis(perfluoroalkyl)silanes Si(C₂F₅)₄ and Si(C₂F₅)₃CF₃ by direct fluorination is described. The reaction of SiCl₄ with LiC₂F₅ invariably affords (pentafluoroethyl)fluorosilicates. To avoid silicate formation by fluoride transfer from LiC₂F₅ the Lewis acidity of the silane has to be decreased by electron‐donating substituents, such as dialkylamino groups. The easily accessible Si(C₂F₅)₃NEt₂ is a valuable precursor for a series of tris(pentafluoroethyl)silanes.     

Bis[N,N′‐diisopropylbenzamidinato(−)]silicon(II): Lewis Acid/Base Reactions with Triorganylboranes

Reaction of the donor‐stabilized silylene 1 (which is three‐coordinate in the solid state and four‐coordinate in solution) with BEt₃ and BPh₃ leads to the formation of the Lewis acid/base complexes 2 and 3 , respectively, which are the first five‐coordinate silicon compounds with an Si?B bond. These compounds were structurally characterized by crystal structure analyses and by multinuclear NMR spectroscopic studies in the solid state and in solution. Additionally, the bonding situation in 2 and 3 was analyzed by quantum chemical studies.     

Lewis Acid Catalyzed Selective Reactions of Donor–Acceptor Cyclopropanes with 2‐Naphthols

Lewis acid‐catalyzed reactions of 2‐substituted cyclopropane 1,1‐dicarboxylates with 2‐naphthols is reported. The reaction exhibits tunable selectivity depending on the nature of Lewis acid employed and proceed as a dearomatization/rearomatization sequence. With Bi(OTf)₃ as the Lewis acid, a highly selective dehydrative [3+2] cyclopentannulation takes place leading to the formation of naphthalene‐fused cyclopentanes. Interestingly, engaging Sc(OTf)₃ as the Lewis acid, a Friedel–Crafts‐type addition of 2‐naphthols to cyclopropanes takes place, thus affording functionalized 2‐naphthols. Both reactions furnished the target products in high regioselectivity and moderate to high yields.     

B(C6F5)3‐Catalyzed Transfer of Dihydrogen from One Unsaturated Hydrocarbon to Another

A transition‐metal‐free transfer hydrogenation of 1,1‐disubstituted alkenes with cyclohexa‐1,4‐dienes as the formal source of dihydrogen is reported. The process is initiated by B(C₆F₅)₃‐mediated hydride abstraction from the dihydrogen surrogate, forming a Brønsted acidic Wheland complex and [HB(C₆F₅)₃]^?. A sequence of proton and hydride transfers onto the alkene substrate then yields the alkane. Although several carbenium ion intermediates are involved, competing reaction channels, such as dihydrogen release and cationic dimerization of reactants, are largely suppressed by the use of a cyclohexa‐1,4‐diene with methyl groups at the C1 and C5 as well as at the C3 position, the site of hydride abstraction. The alkene concentration is another crucial factor. The various reaction pathways were computationally analyzed, leading to a mechanistic picture that is in full agreement with the experimental observations.     

Lewis Acid or Brønsted Acid Catalyzed Reactions of Vinylidene Cyclopropanes with Activated Carbon–Nitrogen,Nitrogen–Nitrogen,and Iodine–Nitrogen Double‐Bond‐Containing Compounds

Lewis acid or Brønsted acid catalyzed reactions of vinylidene cyclopropanes (VDCPs), 1 , with activated carbon–nitrogen, nitrogen–nitrogen, and iodine–nitrogen double‐bond‐containing compounds have been thoroughly investigated. We found that pyrrolidine and 1,2,3,4‐tetrahydroquinoline derivatives can be formed in good yields in the reactions of VDCPs 1 with ethyl (arylimino)acetates 2 by a [3+2] cycloaddition or intramolecular Friedel–Crafts reaction pathway. Based on these results, we found that activated carbon–nitrogen and nitrogen–nitrogen double‐bond‐containing compounds, such as N‐toluene‐4‐sulfonyl (N‐Ts) imines 5 and diisopropylazodicarboxylate ( 7 ), can also react with VDCPs 1 to give [3+2] cycloaddition products in moderate to good yields in the presence of a Lewis acid. When N‐tert‐butoxycarbonyl aldimine 9 was used as the substrate, six‐membered cycloaddition products 10 and 11 were formed in moderate yields in the presence of a Brønsted acid, trifluoromethanesulfonic acid (TfOH). The reactions of VDCPs 1 with N‐Ts‐iminophenyliodinane ( 12 ) were also carried out in the presence of (CuOTf)₂ ? C₆H₆ and it was found that nitrogen‐containing indene derivatives 13 were obtained, rather than the aziridination products. Plausible mechanisms for all of these transformations are discussed, based on the obtained results.     

Reactions of Manganese(II) Fluoride in the Superacidic Systems HF/SbF5 and HF/GeF4 – Crystal Structure of Mn(SbF6)2 and Vibrational Spectra of MnGeF6

Mn(SbF₆)₂ was prepared from MnF₂ and SbF₅ in aHF (anhydrous HF) and single crystals were obtained from the respective solution. The compound crystallises in the triclinic space group P 1 (No. 2) with a = 517.3(2) pm, b = 554.9(2) pm, c = 888.2(2) pm, α = 73.98(3)°, β = 89.17(2)°, γ = 62.54(2)° and Z = 1. MnGeF₆ was prepared from MnF₂ and GeF₄ in aHF and by metathetical reaction between solutions of K₂GeF₆ in aHF and Mn(AsF₆)₂ in aHF. Attempt to isolate Mn(GeF₅)₂ prepared by metathetical reaction between solutions of XeF₅GeF₅ in aHF and Mn(AsF₆)₂ in aHF failed, although some slight evidences for its existence were obtained. Vibrational data of MnGeF₆ are in agreement with lowering of the symmetry of GeF₆^2– from O_h to C_3i because of the site symmetry effects.     

Hypervalent Pentafluoroethylgermanium Compounds, [(C2F5)nGeX5−n]− and [(C2F5)3GeF3]2− (X=F,Cl; n=2–5)

This paper gives an account on hypervalent fluoro‐ and chloro(pentafluoroethyl)germanium compounds. The selective synthesis of the tris(pentafluoroethyl)dichlorogermanate salt [PNP][(C₂F₅)₃GeCl₂] as well as its X‐ray structural analysis is described. As a representative example for pentafluoroethylfluorogermanates, the synthesis and structure of 2,4,6‐triphenylpyryliumtris(pentafluoroethyl)difluorogermanate [C₂₃H₁₇O][(C₂F₅)₃GeF₂] is reported. Fluoride‐ion affinities for pentafluoroethylgermanes were calculated using quantum chemical methods, disclosing (C₂F₅)₃GeF as a weaker Lewis acid than (C₂F₅)₃SiF or (C₂F₅)₃PF₂. The theoretical results were confirmed by experiments and give the basis of a synthetic protocol for (C₂F₅)₃GeF. Pentakis(pentafluoroethyl)germanate [PPh₄][Ge(C₂F₅)₅] was detected as an intermediate during the synthesis of [PPh₄][(C₂F₅)₄GeF] starting from tris(pentafluoroethyl)difluorogermanate and LiC₂F₅.     

Formal [4+2]‐Annulation of Vinyl Azides with N‐Unsaturated Aldimines

Highly functionalized quinolines and pyridines could be synthesized by BF₃?OEt₂‐mediated reactions of vinyl azides with N‐aryl and N‐alkenyl aldimines, respectively. The reaction mechanism could be characterized as formal [4+2]‐annulation, including unprecedented enamine‐type nucleophilic attack of vinyl azides to aldimines and subsequent nucleophilic cyclization onto the resulting iminodiazonium ion moieties.     

The Reaction of Transient 1,1‐Bis(trimethylsilyl)silenes with Tris(trimethylsilyl)silyllithium – Synthesis and Structure of Sterically Congested 1‐Trimethylsilylalkylpolysilanes

Tris(trimethylsilyl)silyllithium ( 3 ) reacted with aldehydes and ketones (molar ratio 2 : 1) according to a modified Peterson mechanism under formation of transient silenes, which were immediately trapped by excess 3 to give the organolithium derivatives (Me₃Si)₃SiSi(SiMe₃)₂C(Li)R¹R² ( 7 ). Hydrolysis of 7 afforded the alkylpolysilanes (Me₃Si)₃SiSi(SiMe₃)₂CHR¹R² ( 8 ). Depending on the substituents R¹ and R², 7 proved to be rather unstable in THF solution and underwent a rapid rearrangement, involving a 1,3‐Si,C‐trimethylsilyl migration, resulting in the formation of the lithium silanides (Me₃Si)₂Si(Li)Si(SiMe₃)₂C(SiMe₃)R¹R² ( 9 ), which were hydrolized during the aqueous workup to give the H‐silanes (Me₃Si)₂Si(H)Si(SiMe₃)₂C(SiMe₃)R¹R² ( 10 ). Reaction of 9 with chlorotrimethylsilane produced the 1‐trimethylsilylalkylpolysilanes (Me₃Si)₃SiSi(SiMe₃)₂C(SiMe₃)R¹R² ( 11 ). The structures of the products described were elucidated by comprehensive spectral analyses. The results of X‐ray crystal structure analyses, performed for 8 l (R¹ = H, R² = 2,4,6‐(MeO)₃C₆H₂), 10 d (R¹ = H, R² = Mes) and 11 d (R¹ = H, R² = Mes) are discussed and confirm the expected extreme sterical congestion of the molecules.     

Iron(III) Triflimide as a Catalytic Substitute for Gold(I) in Hydroaddition Reactions to Unsaturated Carbon–Carbon Bonds

In this work it is shown that iron(III) and gold(I) triflimide efficiently catalyze the hydroaddition of a wide array of nucleophiles including water, alcohols, thiols, amines, alkynes, and alkenes to multiple C? C bonds. The study of the catalytic activity and selectivity of iron(III), gold(I), and Brønsted triflimides has unveiled that iron(III) triflimide [Fe(NTf₂)₃] is a robust catalyst under heating conditions, whereas gold(I) triflimide, even stabilized by PPh₃, readily decomposes at 80 °C and releases triflimidic acid (HNTf₂) that can catalyze the corresponding reaction, as shown by in situ ¹⁹F, ¹⁵N, and ³¹P NMR spectroscopy. The results presented here demonstrate that each of the two catalyst types has weaknesses and strengths and complement each other. Iron(III) triflimide can act as a substitute of gold(I) triflimide as a catalyst for hydroaddition reactions to unsaturated carbon–carbon bonds.     

Transient Silenes as Versatile Synthons – The Reaction of Dichloromethyl‐tris(trimethylsilyl)silane with Organolithium Reagents

Treatment of dichloromethyl‐tris(trimethylsilyl)silane (Me₃Si)₃Si–CHCl₂ ( 1 ), prepared by the reaction of tris(trimethylsilyl)silane with chloroform in presence of potassium tertbutoxide, with organolithium reagents (molar ratio 1 : 3) affords the bis(trimethylsilyl)methyl‐disilanes Me₃SiSiR₂–CH(SiMe₃)₂ ( 12 a–d ) ( a : R = Me, b : R = n‐Bu, c : R = Ph, d : R = Mes). The formation of 12 a–d is discussed as proceeding through an exceptional series of isomerization and addition reactions involving intermediate silyl substituted carbenoids and transient silenes. The carbenoid (Me₃Si)₂PhSi–C(SiMe₃)LiCl ( 8 c ) is moderately stable at low temperature and was trapped with water to give (Me₃Si)₂PhSi–CH(SiMe₃)Cl ( 9 c ) and with chlorotrimethylsilane affording (Me₃Si)₂PhSi–CCl(SiMe₃)₂ ( 7 c ). For 12 d an X‐ray crystal structure analysis was performed, which characterizes the compound as a highly congested silane with bond parameters significantly deviating from standard values.     

Preparation and Synthetic Utility of 1-Trimethylsiloxyalkyl-bis(trimethylsilyl)silanes, (Me3Si)2SiH–C(OSiMe3)R1R2

1-Trimethylsiloxyalkyl-bis(trimethylsilyl)silanes ( 5 ), obtained by a base induced isomerization of easily accessable 1-hydroxyalkyl-tris(trimethylsilyl)silanes ( 1 ) were hydrolized to give 1-hydroxyalkyl-bis(trimethylsilyl)silanes ( 6 ), which in presence of sodium hydride underwent a further 1,3-Si,O-trimethylsilyl migration resulting in the formation of 1-trimethylsiloxyalkyl-disilanes Me₃SiSiH₂–C(OSiMe₃)R¹R² ( 7 ). Under acidic conditions, the alkoxysilanes 5 isomerized in a Me₃Si/OSiMe₃ exchange under formation of the 1-trimethylsilylalkyldisiloxanes 10 , which were hydrolyzed affording the silanols 11 . Chlorination of the H-silanes 5 with CCl₄ gave the chlorosilanes 12 , which underwent rapid thermal isomerizations to give via the 1-chloroalkyldisiloxanes 13 the 1-trimethylsilylalkyl-chlorodisiloxanes 15 . Hydrolysis of 12 or 15 , resp., finally afforded the 1-trimethylsilylalkyl-silanediols 18 . Possible mechanisms of the various isomerization processes are discussed. The structures of the products described were elucidated by full spectral analyses. For 18 a the results of an X-ray structural analysis are given.     

Reactions of Silicon Atoms with Methanol: A Combined Matrix‐Spectroscopic and Density Functional Theory Study

The reaction of silicon atoms with methanol ( 4 ) has been studied in an argon matrix at 10 K. In the initial step a triplet n‐adduct T‐5 between a silicon atom and 4 is formed. It cannot be detected directly as long as a low concentration of 4 is used. But T‐5 must be generated since upon simultaneous irradiation during cocondensation methylsilanone ( 15 ) is found. Without irradiation T‐5 undergoes immediately O,H insertion and methoxysilylene ( S‐7‐c ) is isolated, which establishes a photoequilibrium between the s,trans ( S‐7‐t ) and s,cis form ( S‐7‐c ). If a high concentration of 4 is applied the silylenes exist as complexes S‐17 . The next step needs photochemical activation. Products are dimethoxysilane ( 3 ) and (hydroxy)(methoxy)methylsilane ( 19 ). The picture becomes even more complicated when deuteromethanol ( [D]4 ) is treated with silicon atoms. In this case the primarily formed n‐adduct ( [D]T‐5 ) is stable under matrix conditions. As long as a low concentration is applied, the subsequent step has to be induced by long wavelength irradiation and leads – different from the undeuterated case – to O,CH₃ insertion, giving (hydroxy)methylsilylenes ( [D]S‐11‐c ) and ( [D]S‐11‐t ). If a high concentration is used O,D insertion is preferred instead and even without irradiation the first observable products are methanol‐solvated methoxysilylenes ( [D₂]S‐17‐c ) and ( [D₂]S‐17‐t ). Subsequent irradiation of complexes [D₂]S‐17 gives in accordance with the protonated series a mixture of [D₂]3 and [D₂]19 . Our goal, to find a way to dimethylsilanediol ( [D₂]2 ), was finally reached by preparing silylenes [D]S‐11 in a diluted matrix, its specific solvation with [D]4 and final irradiation of complexes [D₂]S‐18 . The structural elucidation of all new species is based on the comparison of the experimental observations with density functional theory calculations. Upon cocondensation of silicon atoms with pure methanol at 77 K dimethoxysilane ( 3 ) and 1,1,2‐trimethoxydisilane ( 25 ) are produced. The also present methoxysilanes 22‐24 have to be regarded as secondary products of 3 .