Reduction of imines by hydroxycyclopentadienyl ruthenium hydride: intramolecular trapping evidence for hydride and proton transfer outside the coordination sphere of the metal

Reduction of imines by [2,5-Ph2-3,4-Tol2(eta(5)-C4COH)]Ru(CO)2H (2) produces kinetically stable ruthenium amine complexes. Reduction of an imine by 2 in the presence of an external amine trap gives only the complex of the newly generated amine. Reaction of 2 with H2N-p-C6H4N=CHPh (11), which contains an intramolecular amine trap, gave a 1:1 mixture of [2,5-Ph2-3,4-Tol2(eta(4)-C4CO)](CO)2RuNH(CH2Ph)(C6H4-p-NH2) (8), formed by coordination of the newly generated amine to the ruthenium center, and [2,5-Ph2-3,4-Tol2(eta(4)-C4CO)](CO)2RuNH2C6H4-p-NHCH2Ph (9), formed by coordination of the amine already present in the substrate. These results require transfer of hydrogen to the imine outside the coordination sphere of the metal to give a coordinatively unsaturated intermediate that can be trapped inside the initial solvent cage. Amine diffusion from the solvent cage must be much slower than coordination to the metal center. Mechanisms requiring prior coordination of the substrate to ruthenium would have led only to 8 and can be eliminated.     

Electron trapping in polar-solvated zeolites

Of current interest in our laboratory is the nature of photoinduced processes in the cavities of zeolites completely submerged in polar solvents, or polar-solvated zeolites (PSZ). The present study addresses the nature of electron trapping in PSZ with emphasis on the zeolites NaX and NaY. Free electrons were generated by two-photon, pulsed-laser excitation of either pyrene or naphthalene included in zeolite cavities. Trapped electrons were monitored by diffuse transmittance, transient absorption spectroscopy at visible wavelengths. In anhydrous alcohols, electron trapping by Na(4)(4+) ion clusters was observed in both NaX and NaY. The resulting trapped electrons decayed over the course of tens of milliseconds. No evidence for alcohol-solvated electrons was found. More varied results were observed in solvents containing water. In NaX submerged in CH(3)OH containing 5% or higher water, species having microsecond lifetimes characteristic of solvated electrons were observed. By contrast, a 2 h exposure of NaY to 95/5 CH(3)OH/H(2)O had no effect on electron trapping relative to anhydrous CH(3)OH. The difference between NaX and NaY was explained by how fast water migrates into the sodalite cage. Prolonged exposure to water at room temperature or exposure to water at elevated temperatures was necessary to place water in the sodalite cages of NaY and deactivate Na(4)(4+) as an electron trap. Additional studies in NaY revealed that solvent clusters eventually become lower energy traps than Na(4)(4+) as the water content in methanol increases. In acetonitrile-water mixtures, electron trapping by Na(4)(4+) was eliminated and no equivalent species characteristic of solvated electrons in methanol-water mixtures was observed. This result was explained by the formation of low energy solvated electrons which cannot be observed in the visible region of the spectrum. Measurements of the rate of O(2) quenching in anhydrous solvents revealed rate constants for the quenching of ion cluster trapped electrons that were 2-4 times higher than that for pyrene triplets. In NaX, the rate constant in methanol was 10(4) times smaller than that in cyclohexane, showing greater inhibition of O(2) reactivity in the medium of PSZ. The results of this study point out the conditions under which Na(4)(4+) is active as an electron trap in PSZ and that water must be present in the sodalite cage to produce solvated electrons in the supercage.     

The solvent cage effect: is there a spin barrier to recombination of transition metal radicals?

This investigation explored whether there is a spin barrier to recombination of first- and second-row transition metal-centered radicals in a radical cage pair. To answer this question, the recombination efficiencies of photochemically generated radical cage pairs (denoted as FcP) were measured in the presence and absence of an external heavy atom probe. Two methods were employed for measuring the cage effect. The first method was femtosecond pump-probe transient absorption spectroscopy, which directly measured FcP from reaction kinetics, and the second method (referred to herein as the "steady-state" method) obtained FcP from quantum yields for the radical trapping reaction with CCl4 as a function of solvent viscosity. Both methods generated radical cage pairs by photolysis (lambda = 515 nm for the pump probe method and lambda = 546 nm for the steady-state method) of Cp'2Mo2(CO)6 (Cp' = eta(5)-C5H4CH3). In addition, radical cage pairs generated from Cp'2Fe2(CO)4 and Cp*2TiCl2 (Cp* = eta(5)-C5(CH3)5) were studied by the steady-state method. The pump-probe method used p-dichlorobenzene as the heavy atom perturber, whereas the steady-state method used iodobenzene. For both methods and for all the radical caged pairs investigated, there were no observable heavy atom effects, from which it is concluded there is no spin barrier to recombination.     

Mechanistic study of hydrogen transfer to imines from a hydroxycyclopentadienyl ruthenium hydride. Experimental support for a mechanism involving coordination of imine to ruthenium prior to hydrogen transfer

Reaction of [2,3,4,5-Ph(4)(eta(5)-C(4)COH)Ru(CO)(2)H] (2) with different imines afforded ruthenium amine complexes at low temperatures. At higher temperatures in the presence of 2, the complexes decomposed to give [Ru(2)(CO)(4)(mu-H)(C(4)Ph(4)COHOCC(4)Ph(4))] (1) and free amine. Electron-rich imines gave ruthenium amine complexes with 2 at a lower temperature than did electron-deficient imines. The negligible deuterium isotope effect (k(RuHOH)/k(RuDOD) = 1.05) observed in the reaction of 2 with N-phenyl[1-(4-methoxyphenyl)ethylidene]amine (12) shows that neither hydride (RuH) nor proton (OH) is transferred to the imine in the rate-determining step. In the dehydrogenation of N-phenyl-1-phenylethylamine (4) to the corresponding imine 8 by [2,3,4,5-Ph(4)(eta(4)-C(4)CO)Ru(CO)(2)] (A), the kinetic isotope effects observed support a stepwise hydrogen transfer where the isotope effect for C-H cleavage (k(CHNH)/k(CDNH) = 3.24) is equal to the combined (C-H, N-H) isotope effect (k(CHNH)/k(CDND) = 3.26). Hydrogenation of N-methyl(1-phenylethylidene)amine (14) by 2 in the presence of the external amine trap N-methyl-1-(4-methoxyphenyl)ethylamine (16) afforded 90-100% of complex [2,3,4,5-Ph(4)(eta(4)-C(4)CO)]Ru(CO)(2)NH(CH(3))(CHPhCH(3)) (15), which is the complex between ruthenium and the amine newly generated from the imine. At -80 degrees C the reaction of hydride 2 with 4-BnNH-C(6)H(9)=NPh (18), with an internal amine trap, only afforded [2,3,4,5-Ph(4)(eta(4)-C(4)CO)](CO)(2)RuNH(Ph)(C(6)H(10)-4-NHBn) (19), where the ruthenium binds to the amine originating from the imine, showing that neither complex A nor the diamine is formed. Above -8 degrees C complex 19 rearranged to the thermodynamically more stable [Ph(4)(eta(4)-C(4)CO)](CO)(2)RuNH(Bn)(C(6)H(10)-4-NHPh) (20). These results are consistent with an inner sphere mechanism in which the substrate coordinates to ruthenium prior to hydrogen transfer and are difficult to explain with the outer sphere pathway previously proposed.     

Reaction of InCl3 with various reducing agents: InCl3-NaBH4-mediated reduction of aromatic and aliphatic nitriles to primary amines

While alternative methods of preparing dichloroindium hydride (HInCl(2)) via the in situ reduction of InCl(3) using lithium amino borohydride (LAB) were explored, generation of HInCl(2) from the reduction of InCl(3) by sodium borohydride (NaBH(4)) was also re-evaluated for comparison. The reductive capability of the InCl(3)/NaBH(4) system was found to be highly dependent on the solvent used. Investigation by (11)B NMR spectroscopic analyses indicated that the reaction of InCl(3) with NaBH(4) in THF generates HInCl(2) along with borane-tetrahydrofuran (BH(3)·THF) in situ. Nitriles underwent reduction to primary amines under optimized conditions at 25 °C using 1 equiv of anhydrous InCl(3) with 3 equiv of NaBH(4) in THF. A variety of aromatic, heteroaromatic, and aliphatic nitriles were reduced to their corresponding primary amine in 70-99% isolated yields. Alkyl halide and nitrile functional groups were reduced in tandem by utilizing the reductive capabilities of both HInCl(2) and BH(3)·THF in a one-pot reaction. Finally, the selective reduction of the carbon bromine bond in the presence of nitriles was achieved by generating HInCl(2) via the reduction InCl(3) with NaBH(4) in CH(3)CN or with lithium dimethylaminoborohydride (MeLAB) in THF.     

Mechanistic aspects of oxidation of N-substituted 5H-dibenz[c,e]azepines by aldehyde oxidase

5H-Dibenz[c,e]azepine ( 2 ) and its N-ethyl and N-(2-ethoxyethyl) analogues 3 and 4 were prepared and evaluated as substrates for aldehyde oxidase. Quaternization of 2 with ethyl iodide furnished 3 , while 4 was prepared by lithium aluminum hydride reduction of N-(2-ethoxy)ethyldiphenimide followed by mercuric acetate oxidation of the resultant amine 6 . The rates of oxidation of 2 and 3 were similar, suggesting a lack of selectivity by the enzyme for the respective imine and iminium functional groups in these compounds. The rate of oxidation of 3 decreased with increasing pH while the extent of “hydration” of this substrate increased over a similar pH range, signifying a preference by the enzyme for 3 over its carbinolamine equilibrium partner. Experiments with deuterium labelled analogues of 2 and 3 indicated that azomethine hydrogen loss from these substrates during enzymatic oxidation was not rate determining. Thus 5H-dibenz[c,e]azepine-5,5,7-d₃ ( 7 ), prepared by lithium aluminum deuteride reduction of diphenimide ( 5 ), and its N-ethyl analogue 8 , had respective enzymatic oxidation rates which did not differ from those of their non-deuterated counterparts.     

Loss of benzaldehyde in the fragmentation of protonated benzoylamines: Benzoyl cation as a hydride acceptor in the gas phase

In electrospray ionization tandem mass spectrometry of protonated 1‐benzoylamines (1‐benzoylpiperadine, 1‐benzoylmorpholine, and 1‐benzoyl‐4‐methylpiperazine), the dominant fragmentation pathway was amide bond cleavage to form benzoyl cation and neutral amine. Meanwhile, in their fragmentations, an interesting loss of benzaldehyde (106 Da) was observed and identified to derive from hydride transfer reaction between the benzoyl cation and amine. A stepwise mechanism for loss of 106 Da (benzene and CO) could be excluded with the aid of deuterium labeling experiment. Theoretical calculations indicated that hydride transfers from amines (piperadine, morpholine, and 1‐methylpiperazine) to benzoyl cation were thermodynamically permitted, and 1‐methylpiperazine was the best hydride donor among the 3 amines. The mass spectrometric experimental results were consistent with the computational results. The relative abundance of the iminium cation (relative to the benzoyl cation) in the fragmentation of protonated 1‐benzoyl‐4‐methylpiperazine was higher than that in the fragmentation of the other 2 protonated 1‐benzoylamines. By comparing the fragmentations of protonated 1‐benzyl‐4‐methylpiperazine and protonated 1‐benzoyl‐4‐methylpiperazine and the energetics of their hydride transfer reactions, this study revealed that benzoyl cation was a hydride acceptor in the gas phase, but which was weaker than benzyl cation.     

Direct crystallographic observation of a coordinatively unsaturated transition-metal complex in situ generated within a self-assembled cage

In situ crystallographic observation of an extremely labile coordinatively unsaturated transition-metal complex, Cp'Mn(CO)2 (Cp' = methylcyclopentadienyl), was achieved by generating and trapping it within the confined cavity of a self-assembled cage in the crystalline state at a low temperature. The labile target Cp'Mn(CO)2 was formed by the photodissociation of the CO ligand from stable Cp'Mn(CO)3, which was accommodated within the cavity of the coordination cage. Being trapped in the cage at a low temperature, Cp'Mn(CO)2 showed sufficient stability to be analyzed by X-ray crystallography. The diffraction study revealed that the 16-electron unsaturated Mn complex adopted a pyramidal geometry rather than a planar geometry.     

Synthesis of 4-hydroxy-2-methylisocarbostyril-3-carboxanilides

A procedure for preparing the title compounds is reported. These compounds represent the first 2-substituted-4-hydroxyisocarbostyrils, a structural type which could not be obtained by N-alkylation of the previously known 2H-4-hydroxyisocarbostyril. Although 3-carboxanilides in this series could not be made by aminolysis of the corresponding 3-ester, they were successfully prepared from 4-hydroxy-2-methylisocarbostyril using aryl isocyanates and sodium hydride in hexamethylphosphoramide as solvent. Evidence is presented for an enolic (“4-hydroxy”) form of the title compounds and preference for O-alkylation was observed. Contrary to previous reports, data was obtained to support the enolic structure of the previously known 4-hydroxyisocarbostyril.     

"Inverse sodium hydride": a crystalline salt that contains H(+) and Na(-)

A crystalline salt has been synthesized that contains H(+) and Na(-) rather than the usual hydride oxidation states of H(-) and Na(+). The key is irreversible encapsulation of H(+) within the cage of 3(6)adamanzane (Adz). The internal proton is kinetically inert to reduction by Na(-) in solution in NH(3)-MeNH(2) mixtures. Synthesis of the sodide is accomplished by a metathesis reaction between Na and AdzH(+)X(-) in which X(-) is a sacrificial anion such as glycolate, isethionate, or nitrate. Reduction or deprotonation of the sacrificial anion forms insoluble byproducts and AdzH(+)Na(-) in solution. After solvent removal, the sodide is dissolved in dimethyl ether and transferred through a frit into a separate chamber for crystallization. The compound was characterized as the sodide by analysis, NMR spectra, and optical absorption spectroscopy.     

First enantioselective synthesis of a 1-(trimethylsilyl)alkylamine

Reduction of n-propanoyltrimethylsilane NH-imine with the complex hydride LiBH₄/diethyltartrate in THF yielding enantiomerically enriched 1-(trimethylsilyl)butylamine with a 60% enantiomeric excess is the first example of the enantioselective synthesis of such an amine. Other combinations of solvent (ether or THF), hydride (NaBH₄ or LiAlH₄) and chiral inductor (mono- or diol) failed.     

Study on stable coatings for determination of lead by flow-injection hydride generation and in situ concentration in graphite furnace atomic absorption spectrometry

Lead hydride was generated by flow-injection from 0.05 M oxalic acid sample solution by using 0.1 M HCl carrier solution and the reaction with 1.5% sodium borohydride in the presence of 2% potassium hexacyanoferrate(III) as a mild oxidizing agent. Pb was determined by in situ concentration in graphite furnace AAS. The hydride generation by flow-injection and trapping in the graphite tube coated with a highly stable trapping reagent (e.g. tungsten) allows automatic Pb determination. In a systematic investigation, the in situ concentration of Pb was studied in the temperature range 50–600° on graphite tubes coated with noble metals (Ir, Ir/Mg, Pd/Ir), and with W or Zr. The highest response was found on the Ir coatings at trapping temperatures of 200–300°, followed by the W and Zr coatings. The radiotracer ²¹⁰Pb was used to measure hydride generation (95%) and trapping efficiency (71%) on a W-coated tube. Signal stability and reproducibility was tested over 400 trapping and atomization cycles, and the better performance was found with the W and Zr coatings at a precision of 3%. Trapping temperatures above 450°C can lead to errors in absorbance values owing to an adsorptive “carry-over” effect. A characteristic mass of about 21 pg Pb for W-coated tube (283.3 nm) and a detection limit (3σ) of about 0.25 ng was obtained with a 0.5 ml sample loop. The problem with Pb hydride generation is the relatively high reagent blank (1.3 ng in 30 s trapping time) even using chemicals of the highest purity. The method has been tested by applying it to the determination of Pb in a sediment certified reference material.     

Intramolecular trapping of an intermediate in the reduction of imines by a hydroxycyclopentadienyl ruthenium hydride: support for a concerted outer sphere mechanism

Reduction of imines by [2,5-Ph2-3,4-Tol(2)(eta(5)-C(4)COH)]Ru(CO)2H (1) produces kinetically stable ruthenium amine complexes. Reduction of an imine possessing an intramolecular amine was studied to distinguish between inner sphere and outer sphere mechanisms. 1,4-Bn(15)NH(c-C(6)H(10))=NBn (12) was reduced by 1 in toluene-d8 to give 85% of [2,5-Ph2-3,4-Tol(2)(eta(4)-C(4)CO)](CO)(2)RuNHBn(c-C(6)H(10))(15)NHBn (16-RuN,15N), resulting from coordination of the newly formed amine to the ruthenium center, and 15% of trapping product [2,5-Ph2-3,4-Tol(2)(eta(4)-C(4)CO)](CO)(2)Ru(15)NHBn(c-C(6)H(10))NHBn (16-Ru(15)N,N), resulting from coordination of the intramolecular trapping amine. These results provide support for an outer sphere transfer of hydrogen to the imine to generate a coordinatively unsaturated intermediate, which can be trapped by the intramolecular amine. An opposing mechanism, requiring coordination of the imine nitrogen to ruthenium prior to hydrogen transfer, cannot readily explain the observation of the trapping product 16-Ru(15)N,N.     

Rhodium complexes of a chelating ligand with imidazol-2-ylidene and pyridin-2-ylidene donors: the effect of C-metalation of nicotinamide groups on uptake of hydride ion

Rhodium complexes of the imidazolylidene (C-im) N-heterocyclic carbene (NHC) ligand, C-im-pyH(+), bearing a nicotinamide cation substituent (pyH(+)) have been targeted for ligand-centered uptake and delivery of hydride ion. This work reveals that rhodium(I) complexes such as [Rh(C-im-pyH(+))(COD)X][PF(6)] (1, a: X = Cl, b: X = I) undergo facile C-metalation of the nicotinamide ring to afford rhodium complexes of a novel chelate ligand, C,C'-im-py, with coordinated imidazolylidene (C(im)) and pyridylidene (C(py)) NHC-donors. Seven examples were characterized and include rhodium(III) monomers of the general formula [Rh(C,C'-im-py)L(x)I(2)](z+) (2: z = 1, L = H(2)O or solvent, x = 2; 3, 5, 7: z = 0, L = carboxylate, x = 1) and novel rhodium(II) dimers, the anti/syn-isomers of [Rh(2)(C,C'-im-py)(2)(μOAc)(2)I(2)] (4-anti/syn). The NMR data, backed by DFT calculations, is consistent with attribution of the C,C'-im-py ligand as a bis(carbene) donor. Single crystal X-ray diffraction studies are reported for 2, 3, 4-anti, 4-syn and 7. Consistently, within the each complex, the Rh-C(im) bond length is shorter than the Rh-C(py) bond length, which is the opposite trend to that expected based on simple electronic considerations. It is proposed that intramolecular steric interactions imposed by different rings in the rigid C,C'-im-py chelate ligand dictate the observed Rh-C(NHC) bond lengths. Attempts to add hydride to the C-metalated nicotinamide ring in 3 were unsuccessful. The redox behavior of 3 and 4 and, for comparison, an analogous bis(imidazolylidene)rhodium(III) monomer (8), were characterized by cyclic voltammetry, electron paramagnetic resonance (EPR), and UV-vis spectroelectrochemistry. In 3 and 4, the C-metalated nicotinamide ring is found to exhibit a one-electron reduction process at far lower potential (-2.34 V vs. Fc(+)/Fc in acetonitrile) than the two-electron nicotinamide cation-dihydronicotinamide couple found for the corresponding nonmetalated ring (-1.24 V). The C,C'-ligand is electrochemically silent over a large potential range (from -2.3 V to the anodic solvent limit), thus for both 3 and 4 the first reduction processes are metal-centered. For 4-anti, the cyclic voltammetry and UV-vis spectrochemical results are consistent with a diamagnetic [Rh(I)Rh(II)](2) tetrameric reduction product. Density functional theory (DFT) calculations were used to further probe the uptake of hydride ion by the nicotinamide ring, both before and after C-metalation. It is found that C-metalation significantly decreases the ability of the nicotinamide ring to take up hydride ion, which is attributed to the "carbene-like" character of a C-metalated pyridylidene ring.     

一锅法合成1,4-二叔丁氧羰基亚精胺

以丁二胺和丙烯腈为原料,采用一锅法制得N1,N4-二叔丁氧羰基-N4-(2-氰乙基)-1,4-丁二胺(3);以甲基叔丁基醚为溶剂,3经LiAlH4还原合成了1,4-二叔丁氧羰基亚精胺,其结构经1H NMR和MS确证。     

Mechanism for the reduction of the mixed-valent MnIIIMnIV[2-OHsalpn]2+ complex by tertiary amines

The mixed-valent dimanganese(III/IV) complex MnIIIMnIV(2-OHsalpn)2+, 1, is cleanly reduced in acetonitrile by aliphatic tertiary amines to give the dimanganese(III) product MnIII2(2-OHsalpn)2, 2. Thorough characterization of the organic reaction products shows that tributylamine is converted to dibutylformamide and propionaldehyde. Kinetic studies and radical trapping experiments suggest that this occurs via initial single-electron transfer from the amine to 1 coupled with C-H alpha proton transfer from the oxidized amine. EPR spectroscopy and base inhibition studies indicate that coordination of the amine to 1 is a critical step prior to the electron transfer step. Rate data and its dependence on the amine indicate that the ability of the amine to reduce 1 is correlated to its basicity rather than to its reduction potential. Weakly basic amines were unable to reduce 1 irrespective of their reduction potential. This was inferred to indicate that proton transfer from the amine radical cation is also important in the reduction of 1 by tertiary amines. Comparison of the activation energy with reaction thermodynamics indicates that proton transfer and electron transfer must be concerted to explain the rapidity of the reaction. The fate of the amine radical is dependent on the presence of oxygen, and labeling studies show that oxygen in the organic products arises from dioxygen, although incorporation from trace water was also observed. These data indicate that inhibition of the hydrolytic quenching of the amine radical in an aprotic solvent results in a different fate for the amine radical when compared to amine oxidation reactions in aqueous solution. The proposed mechanism gives new insight into the ability of amines with high reduction potential to reduce metal ions of lower potential. In particular, these data are consistent with the ability of small amines and certain amine-containing buffers to inhibit manganese-dependent oxygen evolution in photosynthesis, which arises in some cases as a result of manganese reduction and its concomitant loss from the PS II reaction center.     

Photochemical generation of six- and five-membered cyclic vinyl cations

The photochemical solvolyses of 4-tert-butylcyclohex-1-enyl(phenyl)iodonium tetrafluoroborate (1) and cyclopent-1-enyl(phenyl)iodonium tetrafluoroborate (2) in methanol yield vinylic ethers and vinylic cycloalkenyliodobenzenes and cycloalkenylbenzene, which are the trapping products of the geometrically destabilized C6-ring and C5-ring vinyl cation with the solvent and with the leaving group iodobenzene. Iodonium salt 2 also yields an allylic ether and allylic cyclopentenyliodobenzenes and cyclopentenylbenzene, which are the trapping products of the C5-ring allylic cation produced from the C5-ring vinyl cation by a hydride shift in a typical carbocationic rearrangement.     

Phenonium ions from the addition of phenyl cations to alkenes. Photochemical synthesis of (rearranged) aminoalkylanilines from haloanilines in the presence of alkenes and amines

beta-Aminoalkylanilines are smoothly obtained by irradiation of 4-chloro- and 4-fluoroanilines (as well as the N,N-dimethyl derivatives) in the presence of alkenes (1-hexene, cyclohexene) and amines (butylamine, piperidine) in polar, protic solvents such as trifluoroethanol (yield 40-75%). The reaction involves photoheterolysis of the haloaniline, addition of the resulting phenyl cation to the alkene and trapping of the phenonium cation by amine. A fraction (up to ca. 20%) of aminoalkylanilines resulting from Wagner-Meerwein rearrangement of the phenonium cation is obtained in some cases. Reduction and direct trapping of the phenyl cation by the amine compete with the above three-component synthesis in a less stabilizing solvent such as acetonitrile, but not in CF(3)CH(2)OH.     

Automated determination of tin by hydride generation using in situ trapping on stable coatings in graphite furnace atomic absorption spectrometry

Conditions have been studied for the determination of Sn by coupling of hydride generation and graphite furnace atomic absorption spectrometry. Sequestering and in situ concentration of Sn hydride in the graphite furnace requires just a single application of a long-term stable trapping reagent for automated analyses. In a systematic study it is shown that effective trapping of stannane is possible on graphite tubes or platforms coated with a carbide-forming element such as Zr, Nb, Ta, or W at trapping temperatures of 500 to 600°C. Trapping temperatures should not be higher than 600°C (the “critical temperature”) because otherwise at temperatures higher than 700°C errors in absorbance values could occur by an adsorptive “carry-over effect”. Signal stability and reproducibility are tested over more than 400 complete trapping and atomization cycles, and a precision of 2% is observed. Narrow peaks are obtained for all coatings except for Nb- and Ta-coated platforms where double peaks occur. Ir- or Pd/Ir-coated surfaces allow trapping of stannane at lower temperatures but the signal stability (especially in the case of Pd/Ir coating) is lower than with the carbide-forming element coatings. The highest sensitivity is found for Zr- and W-coated tubes with a characteristic mass of about 17 and 20 pg, respectively, and the calibration curves are linear up to 2 ng Sn on Zr-treated tubes (peak height) and 4 ng on Zr-coated platforms (integrated absorbance) using the 286.3 nm line. The detection limit is 25 pg for a 1 ml sample volume, and the reagent blank is still significant with the purest available chemicals. The method is tested by determination of Sn in low alloy steel samples.     

Dramatic Solvent Effect in the Reduction of 2,3—Allenoic Acid Esters.A Simple Synthesis of 2,3—Allenols from 2,3—Allenoates

2，3－Allenols were prepared conveniently from the reduction reaction of 2,3-allenoates with DIBAL-H(Diisobutylaluminum hydride) in toluene.A dramatic solvent effect was observed for this reaction.