The mechanism of formation of cyclic triphosphenium ions; detection of transient intermediates in solution

The mechanism of formation of cyclic triphosphenium ions [-(CH(2))(n)P(R(2))PP(R(2))-](+) 3 from diphosphanes R(2)P(CH(2))(n)PR(2) and phosphorus(III) halides PX(3)(X = Cl or Br) has been unequivocally established for the six-membered heterocycles with R = Et, (i)Pr or c-Hex, n= 3, and for five-membered rings with R = Et, n= 2. The initial stage is the formation of an acyclic species, [R(2)P(CH(2))(n)P(R(2))PX(2)](+)X(-) 1. The cation of this species cyclises to a symmetrical dication [-R(2)P(CH(2))(n)P(R(2))P(X)-](2+) 2 by loss of halide, where the middle P atom has an X group attached and is still formally P(iii). The rate-determining step is then a redox reaction to form the final cyclic monocationic product 3, with a 'bare' middle P atom. Several transient intermediate species, including the precursor cyclic dication 2 in each case, have been identified by means of (31)P NMR solution-state spectroscopy.     

Silanones and silanethiones from the reactions of transient silylenes with oxiranes and thiiranes in solution. The direct detection of diphenylsilanethione

The transient silylenes SiMe(2) and SiPh(2) react with cyclohexene oxide (CHO), propylene oxide (PrO), and propylene sulfide (PrS) in hydrocarbon solvents to form products consistent with the formation of the corresponding transient silanones and silanethiones, respectively. Laser flash photolysis studies show that these reactions proceed via multistep sequences involving the intermediacy of the corresponding silylene-oxirane or -thiirane complexes, which are formed with rate constants close to the diffusion limit in all cases and exhibit UV absorption spectra similar to those of the corresponding complexes with the nonreactive O- and S-donors, tetrahydrofuran and tetrahydrothiophene. The SiMe(2)-PrO and SiPh(2)-PrO complexes both exhibit lifetimes of ca. 300 ns, and are longer-lived than the corresponding complexes with CHO, which are both in the range of 230-240 ns. On the other hand, the silylene-PrS complexes are considerably shorter-lived and vary with silyl substituent; the SiMe(2)-PrS complex decays with the excitation laser pulse (i.e., τ ≤ 25 ns), while the SiPh(2)-PrS complex exhibits τ = 48 ± 3 ns. The decay of the SiPh(2)-PrS complex affords a long-lived transient product exhibiting λ(max) ≈ 275 nm, which has been assigned to diphenylsilanethione (Ph(2)Si═S) on the basis of its second order decay kinetics and absolute rate constants for reaction with methanol, tert-butanol, acetic acid, and n-butyl amine, for which values in the range of 1.4 × 10(8) to 3.2 × 10(9) M(-1) s(-1) are reported. The experimental rate constants for decay of the SiMe(2)-epoxide and -PrS complexes indicate free energy barriers (ΔG(?)) of ca. 8.5 and ≤7.1 kcal mol(-1) for the rate-determining steps leading to dimethylsilanone and -silanethione, respectively, which are compared to the results of DFT (B3LYP/6-311+G(d,p)) calculations of the reactions of SiH(2) and SiMe(2) with oxirane and thiirane. The calculations predict a stepwise C-O cleavage mechanism involving singlet biradical intermediates for the silylene-oxirane complexes, and a concerted mechanism for silanethione formation from the silylene-thiirane complexes, in agreement with earlier ab initio studies of the SiH(2)-oxirane and -thiirane systems.     

Photochemical myers-saito and C2-C6 cyclizations of enyne-allenes: direct detection of intermediates in solution

Several examples of the photochemical Myers-Saito and C2-C6 cyclization of enyne-allenes are described. The presence of a triplet sensitizing unit at the allene terminus and laser flash photolysis results suggest that the cyclization proceeds along the triplet manifold. An intermediate with tau = 33 +/- 5 mus was tentatively assigned to a singlet biradical.     

Synthesis and characterization of rhodium complexes with phosphine-stabilized germylenes

The reaction of phosphine-stabilized germylenes (1a,b) with dimer complex [Rh(2)(μ-Cl)(2)(COD)(2)] leads to the corresponding phosphine-germylene-Rh(I) complexes (2a,b). Interestingly, the stability of these complexes depends strongly on the nature of the substituent of the germylene fragment. Indeed, the complex (2a) with the chloro-germylene ligand isomerizes into a metallacycle rhodium complex (3a) via germylene insertion into the Rh-Cl bond, while the complex with the phenyl-substituted germylene (2b) was isolated and represents the first stable Rh(I)-germylene complex with a Rh-Cl bond.     

Di- and trivalent organogermanium reactive intermediates. Kinetics and mechanisms of some reactions of diphenylgermylene and tetraphenyldigermene in solution

The reactivity of diphenylgermylene (Ph2Ge) with several classes of germylene scavengers has been studied in hexane solution at 23 degrees C by laser flash photolysis of 3,4-dimethyl-1,1-diphenyl-1-germacyclopent-3-ene (1a), a clean and highly efficient precursor to the germylene and its dimer, tetraphenyldigermene (2a). The reactions studied include M-H insertion reactions with Group 14 hydrides (M = Si, Ge, Sn), halogen atom abstractions from bromo- and chlorocarbons, Lewis acid-base complexation with 1 degrees, 2 degrees, and 3 degrees aliphatic amines, and reaction with an aliphatic alkene, alkyne and diene, and oxygen. Absolute rate constants for (irreversible) scavenging of the germylene could be obtained by direct measurement of the germylene decay kinetics for all but the least efficient scavengers (triethylsilane, oxygen, chloroform, and 1-bromopentane), for which estimates of the rate constants were obtained by Stern-Volmer analysis of the reduction in digermene yield as a function of scavenger concentration. Distinctly different kinetic behavior is observed for scavenging of Ph2Ge by isoprene, 4,4-dimethyl-1-pentene, and triethylamine; in these cases, the results suggest that reaction is rapid (k(Q) = 3-6 x 10(9) M(-1)s(-1)) but reversible (K(eq) = 2500 - ca. 20,000 M(-1)) over the range of scavenger concentrations studied. The reactions with the C-C unsaturated compounds proceed via the intermediacy of long-lived transient species absorbing at <290 nm, which are tentatively assigned to the corresponding three-membered germanocycles on the basis of their UV spectra and lifetimes. Upper limits for the absolute rate constants for reaction of tetraphenyldigermene (2a) toward many of these reagents are also reported.     

Synthesis and characterization of three new thermally stable N-heterocyclic germylenes

Three new stable germylenes, rac-1,3-di-tert-butyl-4,5-dimethyl-1,3-diaza-2-germacyclopentane-2-ylide (1), 1,3-di-tert-butyl-4,4-dimethyl-1,3-diaza-2-germacyclopentane-2-ylide (2), and rac-1,3,4-tri-tert-butyl-1,3-diaza-2-germacyclopentane-2-ylide (3) have been synthesized by the reaction of their corresponding germyl dichlorides with elemental lithium. Full synthetic procedures and characterizations are described.     

Role of reactive intermediates in the radiolytic degradation of Acid Red 1 in aqueous solution

The role of reactive intermediates of water radiolysis (e_aq⁻, H, HO, O₂⁻/HO₂) in decoloration and mineralization of aqueous solutions of Acid Red 1 dye was investigated. The decoloration is highly effective in the reactions of e_aq⁻ and H, and less effective in HO reactions. The O₂⁻/HO₂ pair does not take part in decoloration. For mineralization, which is an oxidative degradation, HO radicals are needed: the efficiency increases with the dissolved oxygen concentration. The reactions of the O₂⁻/HO₂ radical pair slightly increase the rate of mineralization. Iron and copper ions (possible constituents of waste waters) in low concentration do not influence the reactions.     

有机锗倍半氧化物及倍半硫化物的合成

有机锗化合物具有广谱药理活性,特别是抗肿瘤活性,其中有机锗倍半氧化物和倍半硫化物具有显著的抗癌活性。在我国这方面的研究却很少,为进一步研究有机锗的药理活性,我们合成了未见报道的一系列有机锗倍半氧化物和倍半硫化物,并分离了它们的中间体。     

Stabilisation of reactive intermediates in molecular sieves

This paper presents studies on paramagnetic intermediates, free atoms and radicals produced in γ-irradiated molecular sieves and their reactions with adsorbate molecules or exchangeable cations. Four different systems have been investigated using EPR spectroscopy, Na-A/CH₄, AgNa-A/CH₃OH, Ag-SAPO-11/C₂H₄ and AgCs-rho/NH₃. It was found that methyl radicals are formed in two different sites in Na-A/CH₄ and in one of them they are stable at room temperature. The formation of Ag·CH₂OH⁺ radical cation with one-electron bond between silver and carbon has been established in AgNa-A/CH₃OH by EPR experiments with [¹³C]CH₃OH and DFT calculations. In Ag-SAPO-11/C₂H₄ the stabilisation of biligand silver/ethylene complex, Ag⁰(C₂H₄)₂ was postulated based on EPR and DFT results. Tetrameric silver clusters (Ag ₄ ³⁺ ) produced radiolytically in AgCs-rho/NH₃ strongly interact with two ammonia molecules as was deduced from the changes in superhyperfine structure of high-field EPR line of Ag ₄ ³⁺ pentet for zeolite exposed to [¹⁴N]NH₃ and [¹⁵N]NH₃. The presented examples clearly show that the combination of radiation methods with EPR technique is very useful to study the structure and reactivity of paramagnetic intermediates.     

A theoretical study of electronic effects in alkyne‐linked reactive intermediates: (nitrenoethynyl)methylenes, (nitrenoethynyl)silylenes,and (nitrenoethynyl)germylenes

Acetylene‐linked reactive intermediates of (nitrenoethynyl)‐X‐methylenes, (nitrenoethynyl)‐X‐silylenes, and (nitrenoethynyl)‐X‐germylenes are almost experimentally unreachable (X–M–C≡C–N; X=H ( 1 ), CN ( 2 ), OH ( 3 ), NH₂ ( 4 ), NO₂ ( 5 ), and CHO ( 6 ); M=C, Si, and Ge). The effects of the electron‐donating and electron withdrawing groups were compared and contrasted at seven levels of theory. All singlet species as ground states with one local open‐shell singlet carbene subunit (π¹π¹) and another local open‐shell singlet nitrene subunit (π¹π¹) were found to be more stable than their corresponding triplets including one local open‐shell singlet carbene (δ¹π¹) (or one local closed‐shell singlet carbene [δ²π⁰]) and another local triplet nitrene subunit (π¹π¹) with 45.94–77.996 kcal/mol singlet–triplet energy gap (ΔE_s‐t). Their relative silylenes and germylenes made reduction of ΔE_s‐t, so the triplet ground states were found for species 3 _Si , 4 _Si , 5 _Si , 2 _Ge , 3 _Ge , 4 _{Ge ,} and 5 _Ge . All the singlet silylenes/germylenes formed by one local closed‐shell singlet silylenes/germylenes (δ²π⁰) and one local closed‐shell singlet nitrene subunit (π²π⁰). Also, one local closed‐shell singlet silylene/germylene subunit (δ²π⁰) and one local triplet nitrene subunit (π¹π¹) were observed for triplet silylenes/germylenes. The singlet and triplet species 3 _Si , 4 _Si , 3 _Ge , and 4 _Ge , due to their electrophilic (Si₄/Ge₄) and nucleophilic (X₅) centers, could be identified as intermediates in chemical reactions.     

Development of an ion attachment mass spectrometer for direct detection of intermediates in combustion flames

A system with Li+ ion attachment (IA) ionization has been developed for the direct detection of intermediates formed in burning flames by mass spectrometry. Dimethyl ether (DME) among alternative fuels was selected as a test substance to examine the capability of the system. As a result, intermediates generated in a premixed DME-air flame were directly detectable as Li+ adduct ions. By moving the burner on an X-Y stage, spatial distribution profiles of different species, including unburned DME and formaldehyde, were obtained for three types of flames: diffusion, partially premixed, and premixed.     

Studies of dithiiranes and thiosulfines as reactive intermediates

Indirect, but compelling evidence for the intermediacy of dithiiranes 1 and thiosulfines 2 has been found by us and is also provided by literature data. The intriguing dithiirane → dithioester rearrangement as well as most of the interrelations and conversions postulated in Scheme 1 have been demonstrated experimentally. In a number of cases acetyl α-chloroalkyl disulfides 16 (X = Cl, Y = COCH₃) are useful starting materials for the synthesis of thioamides such as 40.     

Electrospray mass spectrometric detection of products and short-lived intermediates in aqueous aerosol microdroplets exposed to a reactive gas

The intermediates ISO3- (m/z=207) and IS2O3- (m/z=239) generated in aqueous (NaI/Na2S2O3) microdroplets traversing dilute O3 gas plumes are detected via online electrospray mass spectrometry within approximately 1 ms, and their stabilities gauged by collisionally induced dissociation. The simultaneous detection of anionic reactants and the S2O62-, HSO4-, IO3-, and I3- products as a function of experimental conditions provides evidence of genuinely interfacial reaction kinetics. Although O3(aq) reacts about 3 times faster with I- than with S2O32- in bulk solution, only S2O32- is significantly depleted in the interfacial layers of [I-]/[S2O32-]=10 microdroplets below [O3(g)] approximately 50 ppm.     

Photolysis of alpha-azidoacetophenones: direct detection of triplet alkyl nitrenes in solution

We report the first detection of triplet alkyl nitrenes in fluid solution by laser flash photolysis of alpha-azido acetophenone derivatives, 1. Alphazides 1 contain an intramolecular triplet sensitizer, which ensures formation of the triplet alkyl nitrene by bypassing the singlet nitrene intermediate. At room temperature, azides 1 cleave to form benzoyl and methyl azide radicals in competition with triplet energy transfer to form triplet alkyl nitrene. The major photoproduct 3 arises from interception of the triplet alkyl nitrene with benzoyl radicals. The triplet alkyl nitrene intermediates are also trapped with molecular oxygen to yield the corresponding 2-nitrophenylethanone. Laser flash photolysis of 1 reveals that the triplet alkyl nitrenes have absorption around 300 nm. The triplet alkyl nitrenes were further characterized by obtaining their UV and IR spectra in argon matrices. (13)C and (15)N isotope labeling studies allowed us to characterize the C-N stretch of the nitrene intermediate at 1201 cm(-)(1).     

Trapping of reactive intermediates in dienone photochemistry by amines

Irradiation of 4-methyl-4-trichloromethyl-2,5-cyclohexadienones in amine solvents leads to the formation of the corresponding dichloromethyl cyclohexadienones by a process involving electron transfer from the amines to zwitterion intermediates.