GeCl<Subscript>2</Subscript> cycloaddition reactions to unsaturated organic compounds taking ethylene,buta-1,3-diene,and hexa-1,3,5-triene as examples: a quantum chemical study

The cycloaddition reactions of dichlorogermylene GeCl₂ to ethylene, buta-1,3-diene, and hexa-1,3,5-triene were studied within the framework of the density functional theory (PBE and B3LYP density functionals) and by the ab initio CBS-QB3 method. The energy characteristics of the reaction of GeCl₂ with ethylene were refined and non-empirical quantum chemical calculations of reaction pathways in the GeCl₂ + buta-1,3-diene and GeCl₂ + hexa-1,3,5-triene systems were carried out for the first time. It was shown that the [2+1] cycloaddition reactions are kinetically hindered and thermodynamically unfavorable, while the [4+1] and [6+1] cycloaddition reactions are characterized by low barriers and result in thermodynamically favorable products. For the [4+1] cycloaddition to buta-1,3-diene and [6+1] cycloaddition to hexa-1,3,5-triene, the most energetically favorable reaction pathways involve a suprafacial and antarafacial approach of reactants, respectively.     

Ab initio studies of

Ab initio calculations of the [1,5]-H shift in (3Z)-penta-1,3-diene and other substituted pentadienes and heteroanalogues using the hybrid density functional Becke3LYP with the 6-31G basis set are presented. Electron-donating substituents, such as methoxy in (3Z)-3-methoxypenta-1,3-diene 1, or heteroatoms such as a nitrogen atom in (Z)-ethylidenevinylamine 2, (1Z)-buta-1,3-dienylamine 3, (2Z)-but-2-enylideneamine 4, (Z)-allylidenemethylamine 5, and methylene-(Z)-propenylamine 6 are introduced. The electron-withdrawing fluoride is substituted for the hydrogen atoms in (3Z)-3-fluoropenta-1,3-diene 7, (3Z)-2,4-difluoropenta-1,3-diene 8, (3Z)-1,1',2,3,4,5,5'-heptafluoropenta- 1,3-diene 10, (1E,3E)-1,3,5-trifluoropenta-1,3-diene 11, and (1Z,3E)-1,3,5- trifluoropenta-1,3-diene 13. A detailed analysis of the geometries, energies, and electronic characteristics of the sigmatropic transposition compared to those of the unsubstituted case provides insights into substituent effects of this prototype of pericyclic reaction. The inductive and mesomeric effects of heteroatoms or heterosubstituents are of a great importance and in a continuous balance in the energetics of the transformation. Sterics can also play an important role due to the geometrical constraints of the reaction. As a general trend, decreasing the electron density of the phi system destabilizes the aromatic transition structure and increases the activation energy, and vice versa.     

Substituent Effects on Fe+-Mediated [4+2] Cycloadditions in the Gas Phase

The Fe⁺-mediated [4+2] cycloaddition of dienes with alkynes has been examined by four-sector ion-beam and ion cyclotron resonance mass spectrometry. Prospects and limitations of this reaction were evaluated by investigating several Me-substituted ligands. Me Substitution at C(2) and C(3) of the diene, i.e., 2-methylbuta-1,3-diene, 2,3-dimethylbuta-1,3-diene, hardly disturbs the cycloaddition. Similarly, variation of the alkyne by use of propyne and but-2-yne does not affect the [4+2] cycloaddition step, but allows for H/D exchange processes prior to cyclization. In contrast, Me substituents in the terminal positions of the diene moiety (e.g., penta-1,3-diene, liexa-2,4-diene) induce side reactions, namely double-bond migration followed by [3+2] and [5+2] cycloadditions, up to almost complete suppression of the [4+2] cycloaddition for 2,4-dimethylhexa-2,4-diene. Similarly, alkynes with larger alkyl substituents (pent-1-yne, 3,3-dimethylbut-1-yne) suppress the [4 + 2] cycloaddition route. Stereochemical effects have been observed for the (E)- and (Z)-penta-1,3-diene ligands as well as for (E,E)- and (E,Z)-hexa-2,4-diene. A mechanistic explanation for the different behavior of the stereoisomers in the cyclization reaction is developed. Further, the regiochemical aspects operative in the systems ethoxyacetylene/pentadiene/Fe⁺ and ethoxyacetylcne/isoprene/Fe⁺ indicate that substituents avoid proximity.     

1,3-Anionic Cycloadditions of Organolithium Compounds: An Initial Survey. New synthetic methods (4)

In the field of [3 + 2]-cycloaddition reactions, 1,3-anionic cycloadditions have now joined 1,3-dipolar cycloadditions as a second reaction type of major general importance. The cycloaddition in question has been found to occur with 2-azaallyllithium compounds, allyllithium compounds (with an electron acceptor located on C²), and 1,3-diphenylpropargyllithium. Compounds with CC, CN, NN, and CS double bonds and those with CC and CN triple bonds were found to be 1,3-anionophilic. 1,3-Anionic cycloaddition opens new routes to 1-aza-, 1,2-diaza-, 1,3-diaza-, 1,2,4-triaza-, and 1-thia-3-azacyclopentane derivatives, to pyrroles and imidazoles, as well as (and this appears to be of special importance) to cyclopentane derivatives. Currently available data attribute stereospecificity to the 1,3-anionic cycloadditions, which occupy an interesting intermediate position between 1,3-dipolar cycloadditions and transition-metal-catalyzed cyclizations. A two-step mechanism has been demonstrated in one case.     

Role of singlet diradicals in reactions of 2-carbenabicyclo[3.2.1]octa-3,6-diene

The generation of 2-carbenabicyclo[3.2.1]octa-3,6-diene (1) results in the formation of C(8)H(8) hydrocarbons endo-6-ethynylbicyclo[3.1.0]hex-2-ene (4), semibullvalene (5), and 5-ethynyl-1,3-cyclohexadiene (6), and C(8)H(10) hydrocarbons bicyclo[3.2.1]octa-2,6-diene (7), tricyclo[3.2.1.0(4,6)]oct-2-ene (8), and tetracyclo[3.3.0.0(2,8)0(4,6)]octane (9). Focus is placed on three mechanistic pathways for the formation of the C(8)H(10) hydrocarbon fraction: (a) abstraction of hydrogen by triplet carbene 1T to produce an equilibrating set of monoradicals, (b) interconversion of triplet carbene 1T into tricyclic triplet diradical 19T and tetracyclic triplet diradical 20T, and (c) interconversion of singlet 1S with analogous singlet diradical 19S and 20S. Ab initio calculations at the (U)B3LYP/6-311+G(3df,2p)//(U)B3LYP/6-31G(d,p) and broken spin symmetry UBS B3LYP/6-311+G(3df,2p)//B3LYP/6-31G(d,p) levels rule out choices (a) and (b) and are consistent with the singlet diradical process.     

Pt(II)-mediated nitrile-tetramethylguanidine coupling as a key step for a novel synthesis of 1,6-dihydro-1,3,5-dihydrotriazines

The coupling between tetramethylguanidine, HN=C(NMe2)2, and coordinated organonitriles in the platinum(II) complexes cis/trans-[PtCl2(RCN)2] (R = Me, Et, Ph) proceeds rapidly under mild conditions to afford the diimino compounds containing two N-bound monodentate 1,3-diaza-1,3-diene ligands [PtCl2{NH=C(R)N=C(NMe2)2}2] (R = Et, trans-1; R = Ph, trans-2; R = Me, cis-3; R = Et, cis-4), and this reaction is the first observation of metal-mediated nucleophilic addition of a guanidine to ligated nitrile. Complexes 1-4 were characterized by elemental analyses (C, H, N), X-ray diffraction, FAB mass spectrometry, IR, and 1H and 13C{1H} NMR spectroscopies; assignment of signals from E/Z-forms of 1,3-diaza-1,3-diene ligands and verification of routes for their Z right harpoon over left harpoon E isomerization in solution were performed using 2D 1H,1H-COSY, 1H,13C-HETCOR, and 1D NOE NMR experiments. The newly formed and previously unknown 1,3-diaza-1,3-dienes NH=C(R)N=C(NMe2)2 were liberated from the platinum(II) complexes [PtCl2{NH=C(R)N=C(NMe2)2}2] (1-3) by substitution with 2 equiv of 1,2-bis-(diphenylphosphino)ethane (dppe) to give the uncomplexed HN=C(R)N=C(NMe2)2 species (5-7) in solution and the solid [Pt(dppe)2](Cl)2. The former were utilized in situ, after filtration of the latter, in the reaction with 1,3-di-p-tolylcarbodiimide, (p-tol)N=C=N(tol-p), in CDCl3 to generate (6E)-N,N-dimethyl-1-(4-methylphenyl)-6-[(4-methylphenyl)imino]-1,6-dihydro-1,3,5-triazin-2-amines) (8-10) due to the [4 + 2]-cycloaddition accompanying elimination of HNMe2. The formulation of 8-10 is based on ESI-MS, 1H, 13C{1H} NMR, and X-ray crystal structures determined for 9 and 10. The reaction of 1,3-diaza-1,3-dienes with 1,3-di-p-tolylcarbodiimide, described in this article, constitutes a novel synthetic approach to a useful class of heterocyclic species like 1,6-dihydro-1,3,5-triazines.     

The Suzuki reaction in aqueous media promoted by P, N ligands

The synthesis and structure of palladium complexes of trisubstituted PTA derivatives, PTA(R3), are described. Water-soluble phosphine ligands 1,3,5-triaza-7-phosphaadmantane (PTA), tris(aminomethyl)phosphine trihydrobromide, tri(aminomethyl) phosphine, 3,7-dimethyl-1,5,7-triaza-3-phosphabicyclo[3,3,1]nonane (RO-PTA), 3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane (DAPTA), lithium 1,3,5-triaza-7-phosphaadamantane-6-carboxylate (PTA-CO?Li), 2,4,6-triphenyl-1,3,5-triaza-7-phosphatricyclo [3.3.1.1]decane, and 2,4,6-triphenyl-1,3,5-triaza-7-phosphatricyclo[3.3.1.1]decane were used as ligands for palladium catalyzed Suzuki reactions in aqueous media. RO-PTA in combination with palladium acetate or palladium chloride was the most active catalyst for Suzuki cross coupling of aryl bromides and phenylboronic acid at 80 °C in 1:1 water:acetonitrile. The activity of Pd(II) complexes of RO-PTA is comparable to PPh?(m-C?H?SO?Na) (TPPMS) and P(m-C?H?SO?Na)? (TPPTS) and less active than tri(4,6-dimethyl-3-sulfonatophenyl)phosphine trisodium salt (TXPTS). Activated, deactivated, and sterically hindered aryl bromides were examined, with yields ranging from 50% to 90% in 6 h with 5% palladium precatalyst loading. X-ray crystal structures of (RO-PTA)PdCl?, (PTA(R3))?PdCl? (R = Ph, p-tert-butylC?H?), and PTA(R3) (R = p-tert-butylC?H?) are reported.     

Preparation and reactions of 1,3-diphosphacyclobutane-2,4-diyls that feature an amino substituent and/or a carbonyl group

The preparation and properties of a 1-amino-1,3-diphosphacyclobutane-2,4-diyl and a 1-benzoyl-1,3-diphosphacyclobutane-2,4-diyl, which can be regarded as functionalized cyclic biradical derivatives, were investigated. Hydrolysis of 1-diisopropylamino-3-methyl-2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyl (7), which is formed by reaction of Mes*C[triple chemical bond]P (4; Mes*=2,4,6-tBu(3)C(6)H(2)) with lithium diisopropylamide and iodomethane, resulted in ring-opening of the 1,3-diphosphacyclobutane-2,4-diyl skeleton, as well as de-aromatization of one of the Mes* rings. 3-Oxo-1,3-diphosphapropene 8 and 7-phosphabicyclo[4.2.0]octa-1(8),2,4-triene 9 were the resultant products, and these were subsequently characterized. Isomerization and oxidation of 7 occurred in the presence of TEMPO (2,2,6,6-tetramethyl-1-piperidinoxy) to give the first example of a cyclic dimethylenephosphorane derivative, namely 3-oxo-1,3-diphospha-1,4-diene 10. 1-Benzoyl-3-tert-butyl-2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclobutane-2,4-diyl (12) was isolated and characterized from the reaction of 4 with tert-butyllithium and benzoyl chloride. Compound 12 was subsequently heated and underwent rearrangement of the benzoyl group and ring-expansion to afford 1-oxo-1H-[1,3]diphosphole 13. Reaction of 4 with lithium diisopropylamide and benzoyl chloride afforded the 2H-[1,2,4]oxadiphosphinine 15, which was probably formed through the 1,3-diphosphacyclobutane-2,4-diyl intermediate 14. Thermolysis of 15 afforded 1-oxo-1H-[1,3]diphosphole 16 in an Arbuzov-type rearrangement.     

Preparation of 2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclobutenes from 2-(2,4,6-tri-tert-butylphenyl)-1-phosphaethyne and alkyllithiums

2-(2,4,6-Tri-tert-butylphenyl)-1-phosphaethyne was allowed to react with 0.5 eq. of alkyllithium to afford 2,4-bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphacyclobutenes through an intramolecular cyclisation of a 1,3-diphosphabuta-1,3-diene intermediate.     

Darstellung und Molekülstruktur von 1,3-Diaza-2,4-disilacyclobutanen

Synthesis and Molecular Structures of 1,3-Diaza-2,4-disilacyclobutanes Lithio-aminofluorosilanes are obtained from the reaction of aminofluorosilanes with butyllithium. The thermal elimination of LiF from lithio-aminofluorosilanes affords a simple synthetic route to four-membered silicon-nitrogen rings. The structures of 1,3-bis(3,5-dimethylphenyl)-2,2,4,4-tetramethyl- and 2,4-di-tert-butyl-2,4-difluoro-1,3-bis(2,4,6-trimethylphenyl)-1,3-diaza-2,4-disilacyclobutanes have been determined by X-ray diffraction.     

Synthesis of cyclo-2,4,6-triarsa-1,3,5-triazanes from cyclo-2,4-diarsa-1,3-diazanes demonstrating the general influence of substituent steric strain on the relative stability of pnictazane oligomers

2,4-Dichloro-1,3-diaryl-cyclo-2,4-diarsa-1,3-diazanes (aryl = 2,6-dimethylphenyl, Dmp, or 2,6-diisopropylphenyl, Dipp) have been transformed into the corresponding 2,4,6-trichloro-cyclo-2,4,6-triarsa-1,3,5-triazanes on reaction with GaCl(3) followed by 4-(dimethylamino)pyridine (DMAP). The nitrogen bound Dmp and Dipp substituents impose "medium" substituent steric strain on the heterocycles influencing the relative thermodynamic stability of potential oligomers in favor of the trimers. This ring expansion disproportionation reaction is initiated by chloride ion abstraction, and the intermediate 2,4-dichloro-1,3,5-tris(2,6-diisopropylphenyl)-cyclo-2,4-diarsa-1,3,5-triazane-6-arsenium tetrachlorogallate has been isolated and structurally characterized. Subsequent reaction with 4-(dimethylamino)pyridine (DMAP) effects release of chloride ion from the gallate anion and consequential formation of a covalent As-Cl bond in the trimer. The observations are analogous to those for the phosphorus derivatives demonstrating a general applicability of this new synthetic procedure for the development and diversification of cyclopnictazane chemistry.     

Reliable predictions of the thermochemistry of boron-nitrogen hydrogen storage compounds: BxNxHy, x = 2, 3

Thermochemical data calculated using ab initio molecular orbital theory are reported for 16 BxNxHy compounds with x = 2, 3 and y > or = 2x. Accurate gas-phase heats of formation were obtained using coupled cluster with single and double excitations and perturbative triples (CCSD(T)) valence electron calculations extrapolated to the complete basis set (CBS) limit with additional corrections including core/valence, scalar relativistic, and spin-orbit corrections to predict the atomization energies and scaled harmonic frequencies to correct for zero point and thermal energies and estimate entropies. Computationally cheaper calculations were also performed using the G3MP2 and G3B3 variants of the Gaussian 03 method, as well as density functional theory (DFT) using the B3LYP functional. The G3MP2 heats of formation are too positive by up to approximately 6 kcal/mol as compared with CCSD(T)/CBS values. The more expensive G3B3 method predicts heats of formation that are too negative as compared with the CCSD(T)/CBS values by up to 3-4 kcal/mol. DFT using the B3LYP functional and 6-311+G** basis set predict isodesmic reaction energies to within a few kcal/mol compared with the CCSD(T)/CBS method so isodesmic reactions involving BN compounds and the analogous hydrocarbons can be used to estimate heats of formation. Heats of formation of c-B3N3H12 and c-B3N3H6 are -95.5 and -115.5 kcal/mol at 298 K, respectively, using our best calculated CCSD(T)/CBS approach. The experimental value for c-B3N3H6 appears to be approximately 7 kcal/mol too negative. Enthalpies, entropies, and free energies are calculated for many dehydrocoupling and dehydrogenation reactions that convert BNH6 to alicyclic and cyclic oligomers and H2(g). Generally, the reactions are highly exothermic and exergonic as well because of the release of 1 or more equivalents of H2(g). For c-B3N3H12 and c-B3N3H6, available experimental data for sublimation and vaporization lead to estimates of their condensed phase 298 K heats of formation: DeltaHf degrees [c-B3N3H12(s)] = -124 kcal/mol and DeltaHf degrees [c-B3N3H6(l)] = -123 kcal/mol. The reaction thermochemistries for the dehydrocoupling of BNH6(s) to c-B3N3H12(s) and the dehydrogenation of c-B3N3H12(s) to c-B3N3H6(l) are much less exothermic compared with the gas-phase reactions due to intermolecular forces which decrease in the order BNH6 > cyclo-B3N3H12 > cyclo-B3N3H6. The condensed phase reaction free energies are less negative compared with the gas-phase reactions but are still too favorable for BNH6 to be regenerated from either c-B3N3H12 or c-B3N3H6 by just an overpressure of H2.     

A computational study of the mechanism for the transmetalation of 2-trimethylstannylbuta-1,3-diene with SnCl4

Electronic structure calculations were performed at the B3LYP/6-31G level to identify the stationary structures on the potential energy surfaces for the transmetalation of 2-trimethylstannylbuta-1,3-diene with SnCl(4). The reaction pathways were characterized by locating the transition states on the intrinsic reaction coordinate. The calculations showed that the reaction between the reactant and SnCl(4), which generates 1-trichlorostannylbuta-2,3-diene via transmetalation, has a low energy barrier of 78.1 kJ.mol(-)(1). The following isomerization process is the rate-controlling step. It turned out that the isomerization process from 1-trichlorostannylbuta-2,3-diene to 2-trichloro-stannylbuta-1,3-diene via transmetalation with SnCl(4) is more energetically favorable than other possible isomerization processes.     

Divergent and solvent dependent reactions of 4-ethoxycarbonyl-3-methyl-1-tert-butoxycarbonyl-1,2-diaza-1,3-diene with enamines

Starting from 1-tert-butyloxycarbonyl-3-methyl-4-ethoxycarbonyl-1,2-diaza-1,3-diene and β,β,β and α,β-substituted enamines a careful choice of solvents and temperatures allows the divergent synthesis of 5,6-dihydro-4H-pyridazines, 2-(1-N-boc-hydrazono-ethyl)-4-pyrrolidin-1-yl-but-3-enoic acid ethyl ester, and 1-amino-pyrroles. Moreover, some interesting conclusions about the mechanism(s) of the reaction have been drawn by careful analysis of products' structure and distribution. Thus, the reaction may proceed through a stereospecific [4+2] cycloaddition mechanism giving rise to 5,6-dihydro-4H-pyridazines or by simple addition or domino addition/cyclization pathways affording, respectively, 2-(1-N-boc-hydrazono-ethyl)-4-pyrrolidin-1-yl-but-3-enoic acid ethyl ester and 1-amino-pyrroles (formally the [3+2] cycloaddition product).     

On the fluxional behaviour of a polycyclic (4.4.2) propella-2,4-diene

Existence of bicyclo (4.2.0) octa-2,4-diene ? 1,3,5-cyclooctatriene type of valence isomerisation in a polycyclic propellane framework has been demonstrated through cycloaddition and thermolysis reactions.     

Quantum-chemical study of mechanisms of reaction between 1,3,5-trimethyl-1H-pyrazole-4-carbaldehyde with 2-mercaptoethanol

By quantum-chemical method MP2/6-311+G**//B3LYP/6-31+G** competing reaction channels were studied of the formation of pyrazolyl-1,3-oxathiolane and/or bis(2-hydroxyethyl)dithioacetal from 1,3,5-trimethyl-1H-pyrazole-4-carbaldehyde and 2-mercaptoethanol. The kinetically controlled formation of bis(2-hydroxyethyl) dithioacetal is governed by the concentration in the reaction mixture of 2-mercaptoethanol dimers. The reaction by stages of 1,3,5-trimethyl-1H-pyrazole-4-carbaldehyde with the molecules of mercaptoethanol results in the prevailing formation of 2-(pyrazol-4-yl)-1,3-oxathiolane structures.     

A density functional theory study on the reaction mechanism of hydrazones with α-oxo-ketenes: Comparison between stepwise 1,3-dipolar cycloaddition and Diels–Alder pathways

Density functional theory calculations have been performed at the B3LYP/6-311+G(d,p) and M06-2X/6-31G(d,p) levels to obtain an insight into the nature of the stepwise cycloaddition reactions of hydrazones with α-oxo-ketenes. Three reaction pathways are possible, two Diels–Alder reactions and a 1,3-dipolar cycloaddition (1,3-DC) reaction. Despite the high energy required for 1,2-hydrogen shift in hydrazone to form an azomethine imine, 1,3-DC reaction among the possible pathways is the most favorable. The mechanism has been explained on the basis of transition state stabilities, global and local reactivity indices of the reactants, intrinsic reaction coordinate calculation, and the electron localization function topological analysis of the bonding changes along the 1,3-DC reaction. The computed free energies and enthalpies agree with the experimental outcome.     

Computational study of reaction pathways in the course of interaction of deactivated silylenes with buta-1,3-diene

The PESs of systems including deactivated silylenes (SiHHal, SiHal₂, Hal = F, Cl, and 2-silaimidazol-2-ylidene, SiN₂H₂C₂H₂) and buta-1,3-diene have been studied using G3(MP2)//B3LYP method. Two major reaction channels, (2 + 1) and (4 + 1) cycloaddition reactions, leading to 2-vinylsiliranes and silacyclopent-3-enes, respectively, as well as [1,3]-sigmatropic rearrangements between 2-vinylsiliranes and the corresponding silacyclopent-3-enes, have been considered in detail. Reactivity of silylenes toward buta-1,3-diene decreases in the following series: SiHHal > SiHal₂ > SiN₂H₂C₂H₂, which is reflected in increase of the reaction barriers for both cycloaddition reactions and in decrease of exothermicity of the formation of the corresponding products. The (4 + 1) cycloaddition is preferable for SiHal₂ and SiN₂H₂C₂H₂ and can compete with (2 + 1) cycloaddition for SiHHal. [1,3]-Sigmatropic rearrangement is important for isomerization of 2-vinylsiliranes to the corresponding silacyclopent-3-enes for all systems studied, except the SiCl₂ system.     

A computational study of the cycloaddition of thiobenzophenone S-methylide to thiobenzophenone

The cycloaddition of thiobenzophenone S-methylide to thiobenzophenone, an experimentally well-known reaction, was studied, using (U)HF/3-21G* for finding stationary points and (U)B3LYP/6-31G*//(U)HF/3-21G* single-point calculations for energies. Some optimizations were performed by (U)B3LYP/ 6-31G* to check the reliability of the calculations. The comparison of the concerted pathways and stepwise reactions via C,C-biradicals and C,S-zwitterions showed that the formation of a tetraphenyl-substituted C,C-biradical and its ring closure to 4,4,5,5-tetraphenyl-1,3-dithiolane constitutes the energetically most probable pathway of product formation, despite the fact that the regioisomeric 2,2,4,4-tetraphenyl-substituted product is more favorable by 17 kcal mol(-1). Model calculations on bond dissociation energies showed that (U)B3LYP with various basis sets overestimates radical stabilization, whereas CBS-QB3 closely reproduced experimental values. Results with the BLYP functional are similar to those with B3LYP. The consequences of the overestimation of radical stability for the cycloaddition mechanism involving biradicals are discussed. Thiobenzophenone S-methylide, if not captured by a dipolarophile, dimerizes to 2,2,3,3-tetraphenyl-1,4-dithiane. Calculation disclosed likewise a tetraphenyl-substituted C,C-biradical as intermediate.     

The reductive dimerization of some 1,3-dienes and of 1,3,5-cycloheptatriene in the presence of trimethylchlorosilane: a DFT investigation

Treatment of 1,3-dienes and 1,3,5-cycloheptatriene by chlorotrimethylsilane in the presence of wire of lithium led mainly to reductive dimerization with formation of bis(allylsilane) derivatives. Bis-silyl compounds obtained: from 1,3-butadiene, 1,8-bis(trimethylsilyl)-2,6-octadiene (70%); from isoprene, (Z,Z)-2,7-dimethyl-1,8-bis(trimethylsilyl)-2,6-octadiene (44%) and 2,6-dimethyl-1,8-bis(trimethylsilyl)-2,6-octadiene (19%); from butadiene-isoprene mixture (1:1), 3-methyl-1,8-bis(trimethylsilyl)-2,6-octadiene (55%); from 2,3-dimethylbutadiene, (E,E)-2,3,6,7-tetramethyl-1,8-bis(trimethylsilyl)-2,6-octadiene (36%), from 1,3-cyclohexadiene, 4,4′-bis(trimethylsilyl)-bicyclohexyl-2,2′-diene (48%); from 1,3,5-cycloheptatriene, 1,1′-bi[(S^∗,S^∗)-6-(trimethylsilyl)cyclohepta-2,4-dien-1-yl] (53%). The structure of the various intermediates (radical anion, dianion, silylated radical, silylated anion) has been established by calculations at the B3LYP/6-311++G(d,p) level of theory with zero-point energy correction. These results are in accordance with a pathway including the formation of a radical anion, its silylation furnishing to a γ-silylated allylic radical followed by a dimerization reaction in the head to head manner.