I. D. Kalikhman

Hydrazide-Based Hypercoordinate Silicon Compounds

Publisher Summary Hypercoordinate silicon complexes also referred as “hypervalent compounds.” This chapter reviews the introduction and use of a new class of chelating agents, based on the hydrazide functional group. Hydrazide-based silicon complexes are remarkably versatile, forming a variety of novel complexes, penta and hexacoordinate, neutral, cationic, or anionic, as well as zwitterionic compounds. Many of these complexes have intriguing stereochemical properties, show chemical or stereochemical ligand-exchange phenomena, which have been studied extensively using nuclear magnetic resonance (NMR) spectroscopy. Hydrazide-based silicon complexes have been synthesized utilizing a silicon-ligand exchange reaction also known as “transsilylation.” The reaction consists of exchange of ligands between an N- or O-(trimethylsilyl) hydrazide (TMS-hydrazide) and a polyhalosilane, such that the only byproducts are volatile TMS halides, which are readily boiled off. Moreover, the ease and cleanliness of this reaction allow real-time NMR monitoring.

The synthesis and NMR spectra of new pentacoordinate silicon compounds, (OSi)- and (ClSi)-chloro[1-(1,1-dimethyl-2-acylhydrazonium)methyl]dimethylsilanes and (OSi)chloro-[2-(1,1-dimethyl-2-acylhydrazino)methyl]dimethylsilanes

Pentacoordinate silicon compounds containing a “chelate” six-membered silacarbofunctional ring, such as the chloro[1-(1,1-dimethyl-2-acylhydrazonium)methyl]-dimethylsilanes, Me2ClSiCH2NMe2NC(O)R with R = CF3 (Ib), C6H5 (Ic), 4′-MeC6H4 (Id), 4′-MeOC6H4 (Ie), 4′-NO2C6H4 (If) have been synthesized for the first time and were studied by NMR spectroscopy. In contrast to Ib in molecules of Ia (R = Me), IcIf the ClSi bond rather than OSi bond is the weaker component of the hypervalent OSiCl bond, which implies the possibility of Cl → Si coordinative interaction. An irreversible rearrangement of compounds I upon heating leads to (OSi)chloro[2-(1,1-dimethyl-2-acylhydrazino)methyl]dimethylsilanes, Me2ClSiCH2-N[C(O)R]NMe2 (II), and was used for preparative purposes.

Reactions of triethylgermyllithium with N,N-dialkylated carboxamides

Abstract The reactions of triethylgermyllithium with N,N -dialkylated carboxamides have been investigated. N,N -Dimethylacetamide, N,N -dimethylpivalamide, N,N -diethyltrifluoroacetamide and N,N -diethylbenzamide act on triethylgermyllithium, which is made from bis(triethylgermyl)mercury and lithium in hexane, to release the corresponding acyltriethylgermanes as the principal product. The regioselectivity of the present reactions was found to depend markedly on the reaction conditions and the nature of the acyl fragment of amides. Thus, triethylgermyllithium reacts with N,N -dimethylacetamide as metallating agent to give lithium N,N -dimethylacetamide, if the reaction is carried out in tetrahydrofuran solution. Mechanistic interpretations of these results are discussed.

Donor-stabilized silyl cations ☆: Part 7: Neutral hexacoordinate and ionic pentacoordinate silicon chelates with N-isopropylideneimino-acylimidato ligands

Abstract Silicon complexes with a new chelating donor group, the isopropylideneimino-acylimidato(N,O) moiety [OC(R)NNCMe2], have been prepared (9–12, 15), and their properties are compared with previously described silicon chelates (1, 2, and 16, respectively). The new ligand acts as a more powerful donor than the NNMe2 ligand, based on three criteria: (a) the new complexes form directly as pentacoordinate siliconium salts 9–12, i.e., the expected neutral hexacoordinate precursors ionize spontaneously; (b) comparison of crystallographic bond lengths with those of the NNMe2 complexes shows consistently shorter NSi coordination bonds and longer Sihalogen bonds in equally substituted new relative to the previously studied complexes; (c) while in the previous series 1, the dihalo complexes 20–22, 25, resisted ionization at any temperature or solvent, the dibromo-isopropylideneimino complexes 19a and 19b ionize reversibly upon decrease of temperature in chloroform solution. Steric congestion forces the trans-dihalo configuration on the dichloro and bibromo complexes 18, 19. The ionization-resistant complexes 17, 18 and the partly ionized 19 form stable ionic siliconium salts when their counterions are replaced by I−, BPh4−, or by reaction with strong Lewis acids, AlCl3 and AlBr3.

Silicon rehybridization and molecular rearrangements in hypercoordinate silicon dichelates

Hydrazide-based hypercoordinate silicon dichelates are remarkably flexible in terms of geometry and reactivity: this paper demonstrates how rather subtle constitutional changes result in dramatic geometrical and reactivity changes. A change of ligand-donor group from NMe2 to N=CMe2 changes the solid-state geometry from bicapped tetrahedral to octahedral. These two geometries are shown to coexist in solution in dynamic equilibrium. Hexacoordinate complexes are shown to dissociate to pentacoordinate compounds in two distinct ways: ionic or neutral, depending on substitution. Hexacoordinate dichelates with imino-donor groups undergo a skeletal rearrangement (intramolecular aldol-type condensation of imines), catalyzed by their dissociated halide counterions. However, even in the absence of counterions, silacyclobutane dichelates undergo a similar rearrangement.

Silyl- and germylmercurials in organic synthesis. A new route to O-silylated and O-germylated enolates

Abstract The exchange reaction of α-mercurated ketones with bis(triethylsilyl)- (I) and bis(triethylgermyl)mercury (II) leads to the formation of the corresponding triethylsilyl and triethylgermyl enol ethers. O -Silylated and O -germylated enolates derived from ketones and aldehydes can be also prepared by treating mercurials I and II with appropriate α-bromo-carbonyl compounds. This new pathway also represents the best available method for preparing bromo-containing triethylsilyl and triethylgermyl enol ethers. NMR and IR spectral characteristic useful in the identification and characterization of these and related compounds are summarized.

The effect of substituents on the structure and reactivity of organogermanium anions

Abstract The replacement of the ethyl group in Et 3 GeH by a phenyl group was shown by equilibrium metallation to halve the p K a value compared with analogous CH acids. NMR showed that the decreased acceptor effect of the phenyl group in the PhEt 2 Ge − anion is caused by a considerably reduced contribution of the mesomeric effect to anion stabilization compared with what happens in the corresponding carbanions. At the same time, the stabilization of the organogermanium anion increases the role of π-polarization of the aromatic substituent and this contribution is comparable with the mesomeric effect value. A stabilizing effect of organosilicon and organogermanium substituents due to a high degree of polarizability has been shown by concurrent methanolysis of Et 3 GeLi and REt 2 GeLi (R = Me 3 Si, Et 3 Si, Me 3 Ge, Et 3 Ge, t-Bu, Ph). An unexpected reaction of the trimethylsilyl anion with the Me 3 SiGeEt − 2 anion leading to diethylgermane dianion, Et 2 Ge 2− has been revealed. The existence of this process supports the suggested absence of ( d–p )π interaction in germanium anions with organo-silicon and -germanium substituents.

The synthesis and structure of chiral compounds with two hypervalent silicon atoms, Si,Si′-substituted 1,2-bis(dimethylsilylmethyl)diacetylhydrazines

Abstract The multi-stage character of the reaction of 1,2-diacetylhydrazine bis- O , O ′-trimethylsilyl derivative with ClCH 2 SiMe 2 Cl to give bis-( O  Si )-1,2-bis(dimethylchlorosilylmethyl)-1,2-diacetylhydrazine (VIII) was established from NMR spectroscopy. Methanolysis of VIII in the presence of NEt 3 gives the Si , Si ′-dimethoxy derivative (IX). The corresponding Si , Si ′-difluoro derivative (XI) was obtained by treatment of IX with boron trifluoride etherate. The structure, the stereodynamic behaviour, and the exchange reactions of chiral molecules of VIII, IX and XI and their precursors have been studies by multinuclear NMR spectroscopy. Compounds VIII, IX, and XI are the first examples of substances that contain two hypervalent silicon atoms and an XSi(C) 3 O coordination centre.

The aldol-type condensation of methyl diazoacetate with 3-organyl-silyl and -germyl propynals and their carbon analogues

Abstract The optimal conditions for the uncatalyzed, selective addition of methyl diazoacetate (I) to the carbonyl group of the substituted propyn-1-als RCCCHO (II) (R = alkyl, Alk 3 Si, Et 3 Ge) to give the corresponding methyl esters of 2-diazo-3-hydroxy-4-pentyn carboxylic acid RCCCH(OH)C(N 2 )COOMe (III) have been found. Shielding of the triple bond in II by the bulky R group or complexation with dicobalt octacarbonyl, the presence of strong electron acceptor (4-NO 2 C 6 H 4 ) in the 3-position of II, and the low polarity of the solvent result in the selective aldol type condensation. The reaction of γ-hydroxy-propynals R 1 R 2 C(OH)CCHO with methyl diazoacetate also proceeds by the aldol-type addition and involves two molecules of the propynal with the formation of diacetylenic diazotetraoles (VIII). The β-addition of methyl diazoacetate to the triple bond in trialkylsilyl or -germyl propynals leads to the isomeric formyl pyrazoles, IV and V. The presence of the Si- or Ge-containing substituents in the 4-position of IV promotes their dimerization into the tricyclic hemiaminals, VI.

Equilibrium between neutral hexacoordinate silicon complexes and ionic pentacoordinate siliconium salts through fast dissociation–recombination of the Si–Cl bond

Equilibrium between neutral hexacoordinate silicon complexes and ionic siliconium chlorides, which is highly temperature, solvent, counterion, ligand and substituent dependent, was observed by low temperature 29Si NMR and confirmed by crystal analysis.