Pulsed-field ionization electron spectroscopy and binding energies of alkali metal-dimethyl ether and -dimethoxyethane complexes

Lithium and sodium complexes of dimethyl ether (DME) and dimethoxyethane (DXE) were produced by reactions of laser-vaporized metal atoms with organic vapors in a pulsed nozzle cluster source. The mono-ligand complexes were studied by photoionization and pulsed field ionization zero electron kinetic energy (ZEKE) spectroscopy. Vibrationally resolved ZEKE spectra were obtained for Li(DME), Na(DME) and Li(DXE) and a photoionization efficiency spectrum for Na(DXE). The ZEKE spectra were analyzed by comparing with the spectra of other metal-ether complexes and with electronic structure calculations and spectral simulations. Major vibrations measured for the M(DME) (M=Li,Na) ions were M-O and C-O stretches and M-O-C and C-O-C bends. These vibrations and additional O-Li-O and O-C-C-O bends were observed for the Li(DXE) ion. The M(DME) complexes were in C2v symmetry with the metal atom binding to oxygen, whereas Li(DXE) was in a C2 ring configuration with the Li atom attaching to both oxygen atoms. Moreover, the ionization energies of these complexes were measured from the ZEKE or photoionization spectra and bond dissociation energies were derived from a thermodynamic cycle.     

Assay of89Sr in plant and soil samples by liquid scintillation counting

The Sr in plant and soil samples is converted to SrCl₂ by digestion with HNO₃ and HCl. The recovery of this digestion is 90% with a reproducibility of about 4%. SrCl₂ is solubilized in toluene by forming a complex with di-n-propyl phosphate (DNP). The DNP-toluene reagent was found to be a highly effective carrier of SrCl₂, and to have properties suitable for highly efficient liquid scintillation counting. An efficiency of 95% was obtained for⁸⁹Sr counting. The factors affecting the utility of the method are discussed.     

Studies on the synthesis of the inostamycin natural products: a reductive aldol/reductive Claisen approach to the C10-C24 ketone fragment

[reaction: see text] An approach to the C10-C24 ketone fragment of the inostamycin family of polyether antibiotics is described. The synthetic strategy utilizes an asymmetric Rh-catalyzed reductive aldol reaction and a stereoselective Rh-catalyzed reductive Claisen rearrangement as the key steps in formation of alkene and vinyl iodide synthons, respectively.     

Formation of new organoselenium heterocycles and ring reduction of 10-membered heterocycles into seven-membered heterocycles

Reacting 2,4-bis(phenyl)-1,3-diselenadiphosphetane-2,4-diselenide [{PhP(Se)(μ-Se)}₂], Woollins’ reagent, with an equivalent of aromatic diol in refluxing toluene afforded 10-membered phosphorus-selenium heterocycles 1 and 2 with an O-P(Se)-Se-Se-P(Se)-O linkage. Two equivalents of aromatic diol and Woollins’ reagent in refluxing toluene gave seven-membered phosphorus-selenium heterocycles 3 and 4 with an O-P(Se)-O linkage together with 10-membered phosphorus-selenium heterocycles 1 and 2. It was also found that the diphosphorus species O-P(Se)-Se-Se-P(Se)-O rings 1 and 2 could be readily ring contracted into the monophosphorus rings 3 and 4 in almost quantitative yields via further reaction with another equivalent of corresponding aromatic diol. One representative X-ray structure is reported.     

Synthesis and Structure of Amido‐ and Imido(pentafluorophenyl)borane Zirconocene and Hafnocene Complexes: N?H and B?H Activation

Treatment of Me₂S ? B(C₆F₅)_nH_3?n (n=1 or 2) with ammonia yields the corresponding adducts. H₃N ? B(C₆F₅)H₂ dimerises in the solid state through N? H???H? B dihydrogen interactions. The adducts can be deprotonated to give lithium amidoboranes Li[NH₂B(C₆F₅)_nH_3?n]. Reaction of the n=2 reagent with [Cp₂ZrCl₂] leads to disubstitution, but [Cp₂Zr{NH₂B(C₆F₅)₂H}₂] is in equilibrium with the product of β‐hydride elimination [Cp₂Zr(H){NH₂B(C₆F₅)₂H}], which proves to be the major isolated solid. The analogous reaction with [Cp₂HfCl₂] gives a mixture of [Cp₂Hf{NH₂B(C₆F₅)₂H}₂] and the N? H activation product [Cp₂Hf{NHB(C₆F₅)₂H}]. [Cp₂Zr{NH₂B(C₆F₅)₂H}₂] ? PhMe and [Cp₂Hf{NH₂B(C₆F₅)₂H}₂] ? 4(thf) exhibit β‐B‐agostic chelate bonding of one of the two amidoborane ligands in the solid state. The agostic hydride is invariably coordinated to the outside of the metallocene wedge. Exceptionally, [Cp₂Hf{NH₂B(C₆F₅)₂H}₂] ? PhMe has a structure in which the two amidoborane ligands adopt an intermediate coordination mode, in which neither is definitively agostic. [Cp₂Hf{NHB(C₆F₅)₂H}] has a formally dianionic imidoborane ligand chelating through an agostic interaction, but the bond‐length distribution suggests a contribution from a zwitterionic amidoborane resonance structure. Treatment of the zwitterions [Cp₂MMe(μ‐Me)B(C₆F₅)₃] (M=Zr, Hf) with Li[NH₂B(C₆F₅)_nH_3?n] (n=2) results in [Cp₂MMe{NH₂B(C₆F₅)₂H}] complexes, for which the spectroscopic data, particularly ¹J(B,H), again suggest β‐B‐agostic interactions. The reactions proceed similarly for the structurally encumbered [Cp′′₂ZrMe(μ‐Me)B(C₆F₅)₃] precursor (Cp′′=1,3‐C₅H₃(SiMe₃)₂, n=1 or 2) to give [Cp′′₂ZrMe{NH₂B(C₆F₅)_nH_3?n}], both of which have been structurally characterised and show chelating, agostic amidoborane coordination. In contrast, the analogous hafnium chemistry leads to the recovery of [Cp′′₂HfMe₂] and the formation of Li[HB(C₆F₅)₃] through hydride abstraction.     

Selective Debenzylation of Quaternary Salts. The Benzyl Group as an Excellent Protecting Group for Basic Nitrogen Compounds

In several synthetic problems in our laboratory it became essential to react a basic nitrogen atom with a suitable protecting group which could be subsequently removed under very mild conditions. The presence of sensitive functionality elsewhere in the molecule prevented serious consideration of most of the reagents which have been developed for dealkylation of quaternary ammonium salts^2–12. One of the most recent methods involving the highly nucleophilic system, lithium n-propylmercaptide in hexamethylphosphoramide¹² (HMPA) was thought to be most desirable but even in this instance demethylation of the quaternary system provided difficulties. For example, demethylation of the methiodide derivative of 18β-carbomethoxycleavamine (II, R [dbnd] CH₃, 3,4-double bond) provided only a 5% yield of the desired product. It was clear that a protecting group more susceptible to nucleophilic displacement was essential. During our studies directed to this end it has been established that the benzyl group satisfies the requirements in all respects. We present some of our results since they may be of interest to others involved in research on alkaloids and other basic nitrogen compounds.