Cyclization and Isomerization Reactionsin Silylhydrazine Chemistry

Summary.  This review describes the synthesis and isomerization reactions of cyclic silylhydrazines. Topics of discussion are the ring expansion of the three-membered Si(SiN₂) to the four-membered (SiN)₂ ring by lithiation of the (SiN₂) ring and by thermal silyl group insertion into the N*N bond, the ring expansion of a three-membered (SiN₂) to a five-membered (CSi₂N₂) ring by SiCH₂ insertion into the Si*N bond, the formation of isomeric four- and six-membered (SiN₂)₂ rings, the synthesis of five- and six-membered silylhydrazine rings, and the expansion of a five-membered (N₂Si₂N)N ring to the isomeric six-membered (SiNN)₂ ring. The mechanisms of the isomerizations are explained by quantum chemical calculations, and the results are verified by crystal structure determinations. Received February 23, 2001. Accepted (revised) April 24, 2001     

Calculation of geometric parameters of macrocyclic metal chelates formed by template synthesis in tertiary systems M(II) ion-ethanedithioamide-formaldehyde-ammonia

The geometric parameters of macrotricyclic Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and Zn(II) complexes with 2,8-dithio-3,5,7-triazanonanedithioamide-1,9 with the (N,N,S,S) coordination of the chelant donor centers (formed by template synthesis in the M(II)-ethanedithioamide-formaldehyde-ammonia system) have been calculated by the hybrid B3LYP density functional theory method with the use of the 6-31G(d) basis set and the Gaussian 98 program package. The bond lengths and bond angles in the complexes with the MN₂S₂ coordination core have been reported. Calculations demonstrated that in none of the complexes are the five-membered chelate rings planar and that these rings in the Zn(II) complex are significantly different. For all M(II) ions under consideration, an additional six-membered chelate ring resulting from template cross-linking is turned at a rather large angle to the two five-membered rings and this ring itself is nonplanar.     

Structure of a C26H28 alkene formed via titanium-promoted reductive dimerization of 4-(spirocyclopropyl)-pentacyclo[5.4.0.02,6.03,10.05,9]undecan-8-one

A spiro-fused C₂₆H₂₈ cage≓ dimer hydrocarbon contains two three-membered rings, two four-membered rings, eight five-membered rings, and two six-membered rings. The molecule is calculated by molecular mechanics to have 902 kJ/mol of strain energy distributed primarily between angle strain (457 kJ/mol) and torsional interactions (368 kJ/mol). Molecular mechanics calculations and a geometry-optimized ab initio calculation are used to analyze the bond lengths and bond angles in the molecule. There is one major discrepancy between observed and calculated distances.     

<Emphasis Type="Italic">Ab initio</Emphasis> study of hydrogen bond complexes of ring compounds containing the H<Subscript>2</Subscript>N-C=Y moiety with water

Ab initio calculations, including natural charge population and natural resonance theory analyses, have been carried out to study the two-way effects between hydrogen bonds (H-bonds) and the intramolecular resonance effect by using the H-bonded complexes of ring compounds containing the H₂N-C=Y moiety (C=Y bond is contained in the six-membered or five-membered rings) with water as models. The amino groups in the four monomers of ring compounds (FAYs, Y represents the heavy atoms in the substituent groups, =CH, =N, =SiH, and =P, respectively) can all serve as H-bond donors (HD) and H-bond acceptors (HA) to form stable H-bonded complexes with water. The HD H-bond and resonance effect enhance each other (positive two-way effects) whereas the HA H-bond and resonance effect weaken each other (negative two-way effects). The resonance effect in FAY(1) (C=Y bond is contained in the six-membered rings) is weaker than that in formamide, and those in FAY(2) and FAY(3) (C=Y bonds are contained in the five-membered rings). The two-way effects between H-bond and resonance effect exist in the H-bonded complexes of ring compounds containing the H₂N-C=Y moiety with water.      

Calculation of geometric parameters and energies of macrocyclic metal chelates in the ternary M(II) ion-thiocarbamoylmethanamide-formaldehyde systems

The thermodynamic and geometric parameters of isomeric macrotricyclic Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and Zn(II) complexes that can, in principle, be the products of the reaction of corresponding hexacyanoferrates(II) with thiocarbamoylmethanamide H₂N-C(=S)-C(=O)-NH₂ and formaldehyde in gelatin-immobilized matrix implants have been calculated by the hybrid B3LYP density functional theory method with the use of the 6–31G(d) basis set and the Gaussian 98 program package. For any of the M(II) ions under consideration, the most stable complexes have the MN₄ metal chelate cores. The bond lengths and bond angles in these complexes have been reported, and it has been stated that the five-membered chelate rings in the complexes are not strictly planar. The additional six-membered chelate ring resulting from template cross-linking is also nonplanar and, in some cases, is turned at a rather large angle to the five-membered rings.     

Unexpected Zwitterionic Allenyls from Silylenes and a Fischer Alkynylcarbene: A Remarkable Silylene-Promoted Rearrangement

The silylenes Si(tBu₂bzam)R (tBu₂bzam=N,N′-bis(tertbutyl)benzamidinate; R=mesityl, CH₂SiMe₃) attack the C_carbene atom of the Fischer alkynyl(ethoxy)carbene complex [W(CO)₅{C(OEt)C₂Ph}] to give, after a striking rearrangement, zwitterionic σ-allenyl complexes in which the original carbene C atom forms part of the allene C₃ fragment and also of a Si-C-N-C-N five-membered ring after insertion into a Si−N bond of the original amidinatosilylene. These remarkable allenyl products, which contain two stereogenic groups, are selectively formed as single diastereomers.     

Spin Crossover in a Family of Iron(II) Complexes with Hexadentate ligands: Ligand Strain as a Factor Determining the Transition Temperature

This paper reports the synthesis of a family of mononuclear complexes [Fe(L)]X₂ (X=BF₄, PF₆, ClO₄) with hexadentate ligands L=Hpy-DAPP ({bis[N-(2-pyridylmethyl)-3-aminopropyl](2-pyridylmethyl)amine}), Hpy-EPPA ({[N-(2-pyridylmethyl)-3-aminopropyl][N-(2-pyridylmethyl)-2-aminoethyl](2-pyridylmethyl)amine}) and Hpy-DEPA ({bis[N-(2-pyridylmethyl)-2-aminoethyl](2-pyridylmethyl)amine}). The systematic change of the length of amino-aliphatic chains in these ligands results in chelate rings of different size: two six-membered rings for Hpy-DAPP, one five- and one six-membered rings for Hpy-EPPA, and two five-membered rings for Hpy-DEPA. The X-ray analysis of three low-spin complexes [Fe(L)](BF₄)₂ revealed similarities in their molecular and crystal structures. The magnetic measurements have shown that all synthesized complexes display spin-crossover behavior. The spin-transition temperature increases upon the change from six-membered to five-membered chelate rings, clearly demonstrating the role of the ligand strain. This effect does not depend on the nature of the counter ion. We discuss the structural features accountable for the strain effect on the spin-transition temperature.     

Hetarenes with a bridge nitrogen atom. 6. Ambident properties of the oxazolo[3,2-a]pyridinium nucleus in reactions with nucleophiles: Prediction and experiment

We have studied the problem of ambident opening of five-membered and six-membered rings of the heteroaromatic oxazolo[3,2-a]pyridinium cation. A quantum chemical SINDO1 calculation of the energies of isomeric adducts of this cation with nucleophiles predicts preferential formation of the C₉-adduct in the case of OH and SH ions with opening of the oxazole fragment, while for nitrogen-containing nucleophiles (NH₂, NMe₂) attack at the C₍₅₎ atom is also likely with opening of the pyridine fragment. We have experimentally observed that this cation undergoes opening of the five-membered ring in reaction with NaSH, opening of the six-membered ring in reaction with piperidine, and opening of both rings in reaction with ammonia.For communication 5, see [1].M. V. Lomonosov Moscow State University, Moscow 119899. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 397–402, March, 1997.     

Ab initio study of mechanism of forming a Spiro-Si-heterocyclic ring compound

The mechanism of the cycloaddition reaction of forming a spiro-Si-heterocyclic ring compound between singlet dichloroalkylidenesilylene (Cl₂C=Si:) and ethene has been investigated with CCSD(T)//MP2/6-31G* method. From the potential energy profile, it can be predicted that the reaction has one dominant reaction pathway. The presented rule of this reaction is that the 3p unoccupied orbital of Si in dichloroalkylidene and the π orbital of ethene forming the π → p donor-acceptor bond, resulting in the formation of a three-membered ring intermediate. Ring-enlargement effect make the three-membered ring intermediate isomerizes to a four-membered ring silylene. Due to sp ³ hybridization of Si atom in the four-membered ring silylene, the four-membered ring silylene further combines with ethene to form a spiro-Si-heterocyclic ring compound.     

Structural characterization of the trans-equatorial isomer of (aqua)(ethylenediamine-N,N,N′-tripropionato)chromium(III) trihydrate,[Cr(edtrp)(H2O)] · 3H2O

The structure of trans-equatorial [Cr(edtrp)(H₂O)] · 3H₂O (edtrp^3– is the anion of ethylenediamine-N,N,N-tripropionic acid) was determined by single crystal X-ray diffraction. The chromium(III) ion is surrounded octahedrally by the two nitrogen and three oxygen atoms of the quinquedentate edtrp^3–, forming a five-membered diamine ring and the three six-membered -propionato chelate rings. The remaining coordination position is occupied by the H₂O ligand. The crystal structure conformation is compared to the result of recent molecular mechanics analysis. The ring strain of R and G chelate rings was found to be in agreement with the previously proposed mechanisms for the C—N bond cleavage and recombination.     

Synthese und Struktur der ersten sechsgliedrigen Selena- und Platinatriazaphosphorine

Synthesis and Strukture of the First Six-membered Selena- and Platinatriazaphosphorines The reaction of PhCN with Li[N(SiMe₃)₂], Ph₂PCl and Me₃SiN₃ leads to the starting material (Me₃Si)₂NC(Ph)NP(Ph)₂N(SiMe₃) 1 . 1-Chloro-3,3,5-triphenyl-1λ⁴, 2,4,6,3λ⁵-selenatriazaphosphorine 2 is formed when 1 is treated with SeCl₄. (Ph₃P)₂Pt(C₂H₄) as well as (Ph₃P)₄Pt react with 1 to yield the six-membered platinum containing heterocycles 2 and 4 . The six-membered rings 2 and 4 were characterized by and X-ray single crystal structure analysis.     

Five- and six-membered ring formation from the intramolecular reaction of a protonated oxirane and alkene

Computational studies of competing five- and six-membered cyclisation of alkenyloxiranes 1a-d show that intramolecular reaction of a protonated oxirane and alkene is a concerted, single-step, exothermic process. The reactions proceed via reactant-like transition states, but where the oxirane C-O bond is considerably stretched. Two factors are seen to affect the regiochemistry: (1) stabilisation of the transitory positive charge in the transition state favours cyclisation to the more highly substituted oxirane carbon; and (2) there is an inherent stereoelectronic preference for six-membered cyclisation over five-membered cyclisation. The inherent preference for six-membered cyclisation has a parallel in Baldwin's rules for six-membered ring closure of a carbocation with an alkene, rather than Baldwin's rule for intramolecular nucleophilic reaction of three-membered rings, suggesting that the protonated oxirane mimics a carbocation. The electronic and stereoelectronic effects for cyclisation are modified by steric interactions of axial methyl groups. These systems provide a model for the A-ring cyclisation of oxidosqualene.     

A Quantum Chemical Study of N14 Cluster

Ab initio molecular orbital theory and density functional theory have been employed to study N₁₄ cluster with low spin at the HF/6-31G*, B3LYP/6-31G*, B3PW91/6-31G*, BP86/6-31G*, and BHLYP/6-31G* levels of theory. Twelve isomers were studied, including one previously investigated cage molecule. The most stable isomer of N₁₄ is a C _2h -symmetric molecule that contains two separated five-membered nitrogen rings connected by a —N=N—N=N— bridge. The second, third, and fifth most stable isomers each have one five-membered nitrogen ring. The theoretical results suggest that the five-membered nitrogen ring gives rise to a particularly stable structural unit, and the more side chains that the five-membered nitrogen ring links with, the less stable the structure will become.     

Synthese und Eigenschaften Pentamethylcyclopentadienyl-substituierter PPC- bzw. AsPC-Dreiring-Systeme

Synthesis and Properties of Pentamethylcyclopentadienylsubstituted PPC and AsPC three-membered Rings Via the reaction of bis-(pentamethylcyclopentadienyl)diphosphene [Cp*P?PCp*, 1 ] and 1-(pentamethylcyclopentadienyl)-2-(2,4,6-tri^tbutylphenyl)- diphosphene [Cp*P?PMes*, 2 ] with the diazomethanes N₂CHR [R = H, Si(CH₃)₃] the four new diphosphiranes Cp*PPCp*CHSi(CH₃)₃, 4a , Cp*PPMes*CHSi(CH₃)₃, 4b , Cp*PPCp*CH₂, 5a , Cp*PPMes*CH₂, 5b , are obtained. The formation of 4a results via a 2 + 3-cyclo-addition product, which could be proved by nmr spectroscopy. The reaction of As-(pentamenthylcyclopentadienyl)-P-(2,4,6-tri^tbutylphenyl) arsaphosphene [Cp*As?PMes*, 3 ] with diazomethane leads to 1-(pentamethylcyclopentadienyl)-2-(2,4,6-tri^tbutylphenyl)-1-arsa-2 -phosphacyclopropane [phospharsiran, Cp*AsPMes*CH₂, 6 ]. Analysis of the structures by nmr spectroscopy gives clear evidence for a trans-orientation of the substituents at the El? P bond (El = As, P) in all of the three membered ring systems. For the diphosphirane Cp*PPCp*CH₂ ( 5a ) a Cp*-phosphorus bond cleavage by thermolysis cannot be observed. From the reaction of compound 5a with Cr(CO)₅thf one obtains 1-(pentacarbonylchrom)-1,2-bis(pentamethylcyclopentadienyl)-1,2- diphosphacyclo-propane, 7 .     

Planar Tetracoordinate Silicon in Organic Molecules As Carbenoid-Type Amphoteric Centers: A Computational Study

Designing and synthesizing a stable compound with a planar tetracoordinate silicon (ptSi) center is a challenging goal for chemists. Here, a series of potential aromatic ptSi compounds composed of four conjugated rings shared by a centrally embedded Si atom are theoretically designed and computationally verified. Both Born–Oppenheimer molecular dynamics (BOMD) simulations and potential energy surface scannings verify the high stability and likely existence of these compounds, particularly Si-16-5555 (SiN₄C₈H₈) with 16 π electrons, under standard ambient temperature and pressure. Notably, the Hückel aromaticity rule, which works well for single rings, is inconsistent with the high stability of Si-16-5555 where the 16 p electrons are spread over four five-membered rings fused together. Bonding analyses show that the strong electron donation from the peripheral 12-membered conjugated ring with 16 π electrons to the vacant central atomic orbital Si 3p_z leads to the stabilization for both the ptSi coordination and planar aromaticity. The partial occupation of Si 3p_z results in the peculiar carbenoid-type behaviors for the amphoteric center. By modulating the electron density on the ring with substituent groups, we can regulate the nucleophilic and electrophilic properties of the central Si.     

Ab initio study of mechanism of forming a silicic <Emphasis Type="Italic">bis</Emphasis>-heterocyclic compound between (CH<Subscript>3</Subscript>)<Subscript>2</Subscript>Si=Si: and ethane

The mechanism of the cycloaddition reaction of forming a silicic bis-heterocyclic compound between singlet state (CH₃)₂Si=Si: and ethene has been investigated with the CCSD(T)//MP2/6-31G* method. From the potential energy profile, it can be predicted that the reaction has one dominant reaction pathway. The presented rule of this dominant reaction pathway that the 3p unoccupied orbital of Si: in (CH₃)₂Si=Si: and the π orbital of ethane forming a π → p donor-acceptor bond, resulting in the formation of a three-membered ring intermediate (INT1); Then, INT1 isomerizes to a four-membered ring silylene (P1), which driven by ring-enlargement effect; Due to sp ³ hybridization of Si: atom in the four-membered ring silylene (P1), P1 further combines with ethene to form a silicic bis-heterocyclic compound (P2).     

Size and stability of chelate rings in coordination compounds of cobalt(II), manganese(II), iron(III) and palladium(II) with vicinal oxime-imine ligands

Summary Two types of the Co^II complexes L¹Co (H₂L¹=N,N-ethylenebis(isonitrosoacetylacetoneimine) were prepared. In type (a) the chelate rings are five-membered whereas in type (b) they are six-membered. The type (b) complexes were converted to type (a) in refluxing solutions. Half-ionization of the ligand is observed in the complexes HL¹ Co(O₂CMe) and HL¹MnCl, where the chelate rings are five- and six-membered respectively. The octahedral complex L¹FeCl·H₂O has chelate rings of type (a) as does the complex L²Co (H₂L²=unsymmetric Schiff baseN,N-ethylene(isonitrosoacetylacetoneimineacetylacetoneimine). Twocis complexes (L_a ²L_b ³)Pd and (L_a ³)₂Pd are characterized (HL³=isonitrosoacetylacetoneimine, (a) and (b) denote the type of chelate ring). Structures for the metal complexes and the sizes of the chelate rings are suggested on the basis of analytical and spectral evidence.     

Calculations of silicooxygen ring vibration frequencies

In the work model calculations of the vibrations of ideally isolated silicooxygen rings (using PM3 method) have been carried out. three-, four-, and six-membered rings have been considered. It has been found that that the three-membered silicooxygen rings are flat and practically undeformed showing D_3h symmetry. The rings of higher number of ring members (i.e. n>3) are deformed to some extent. The deformation reveals itself most significantly in the Si–O–Si bond angles distribution. In the case of all the rings the bridging Si–O–Si bonds are ca. 0.02–0.04 Å shorter than the non-bridging Si–O⁻ bonds. Hypothetical IR spectra for all the rings considered have been also calculated. Analysis of these hypothetical spectra leads to the conclusion that the whole spectrum can be divided into four wavenumbers regions, 1200–1100 cm⁻¹ stretching Si–O(Si) vibrations; 1000–800 cm⁻¹ stretching Si–O⁻ vibrations; 800–600 cm⁻¹; the region in which a band characteristic of silicooxygen rings appears, and below 600 cm⁻¹ bending ⁻O–Si–O⁻ and (Si)O–Si–O(Si). It has been also found that as the number of ring members increases the ‘ring band’ shifts to lower wavenumbers: 725 cm⁻¹ for three-membered rings, 650 cm⁻¹ for four-membered rings and 610 cm⁻¹ for six-membered rings. Calculated spectra have been compared with the experimental spectra of cyclosilicates. They showed good agreement in the 1200–600 cm⁻¹ region. In the experimental spectra as well as in the calculated ones, with increasing the number of ring members the ‘ring band’ shifts towards lower wavenumbers.     

Heterocyclic Analogues of Methylenecyclopropanes

Heterocyclic three-membered rings with an exocyclic double bond merit special interest in view of their ring strain and their fascinating chemistry. Their synthesis, thermal decomposition, ring opening, and cycloaddition reactions will be discussed in this review. α-Lactams, alkylideneaziridines, diaziridinones, and diaziridinimines belong to the best known classes, but the chemistry of alkylideneoxiranes and -thiiranes, α-lactones, α-thiolactones, thiiranimines, and aziridinimines has also been studied to some extent. Two elusive three-membered rings are oxiranimines and thiaziridinimines; the former were postulated as reactive intermediates in the thermal decomposition of α-lactams, whereas the latter were trapped efficiently with double and triple bonds during thermolysis of N-sulfonyliminothiatriazolines. In spite of many attempts to prepare thione derivatives of the title compounds, they are still unknown. Included in this review are also some open-chain dipolar species which are isomeric with the heterocyclic three-membered rings.     

Theoretical study of geometrical effect on the deoxygenation of epoxide by singlet carbenes

The mechanism of the abstraction of oxygen from epoxide by carbenes has been investigated using the B3LYP/6-31G* and MP2/6-311++G(d,p)//B3LYP/6-31G* levels of theory. According to our model calculations, the reactivity of carbene decreases in the order: four-membered ring carbene > six-membered ring carbene > three-membered ring carbene > five-membered ring carbene vinylidene. Moreover, the present work shows that the singlet–triplet splitting of a carbene can be used as a guide for the prediction of its reactivity.