Showing papers by "George M. Whitesides published in 1975"

Reaction of n-butyllithium and 2,2,6,6-tetramethylpiperidine nitroxyl

Abstract: Stable nitroxyl radicals2-a are widely used as radical scavengersa and as probes for certain types of molecular motion.s In the course of other problems that utilized ni

Synthesis of fatty acids using organocopper (I) ate complexes derived from Grignard reagents.

Abstract: Fatty acid esters have been synthesized in good yield by reaction between copper(I) ate complexes formed from methylcopper(I) and primary or secondary Grignard reagents and esters of primary iodoalkylcarboxylic acids. The synthetic method is i l lustrated with proced.ures for CHr:gH(CHz)rgCO2C2H5, CeHsCHzO(CH2)16COzCzHs, CH3(CH2)25CO2C2H5, CH3O2C(CH)I2CO2CHI, CzHsOzC(CHz)szCO2C2H5, and CHg(CH2)4CH(CHs)(CHz)roCOzCHs. The reaction sequence uses starting Grignard reagent with high efficiency, provides product mixtures that are conveniently worked up, and tolerates a variety of functional groups. It provides the most direct route presently available to a variety of representative classes of simple fatty acids.

Dynamic nuclear magnetic resonance study of fluorine exchange in liquid sulfur tetrafluoride

Abstract: The temperature dependence of . the NMR spectrum of SF+ has been reexamined, and the observed l ine shapes in the region of intermediate xchange compared with theoretical l ine shapes calculated f ior a number of dif ferent methods of permu-t ing axial and equatorial f l r ior ine uto*r. Careful ly puri f ied SFa yields experimental spectra in good agreement with ihose calJulated assuming the intramolecular f luorine exchange characterist ic of Berry pseudorotat ion; these experimental spectra do not closely resemble those calculated on the basis of uny intermolecular permutation we have examined' The ex.^hung\" rate in unpu.i f i .d SF+ is substantial ly higher than in carefui ly puri f ied material. The l ine shapes observed for unpurified material can be matched to those calculated assuming permutations characteristic of several plausible bimolecular mechanisms; avai lable data do not uniquely define the mechanism of this impurity-catalyzed exchange. At low temperatures, l iquid SFa displays an AzBz NMR spectrum characteristic of a Czv molecule;3 as the temperature is raised, fluorine site exchange leads to line broadening and eventual coalescence to a single sharp resonance.3 This spectral temperature dependence has been extensively examined, but a unique mechanistic interpretation of this dependence has proved elusive. In 1958, Cotton, George, and Waugha surmised that fluoride impurities induced fluorine exchange in their substantially contaminated samples. Muetterties and Phillipss subsequently published similar spectra, and, noting a dependence of fluorine exchange rate on sample concentration, advocated intermolecular mechanisms involving dimeric or ionic intermediates. In a later publication,6 however, they emphasized the possibility that traces of HF might be catalyzing fluorine exchange and also expressed the belief that intramolecular exchange does occur. but at a rate lower than the observed higher-order processes. The hypothesis of impurity catalysis was supported by Bacon, Gil lespie, and Quail,T and more recently by Gibson, Abbott, and Janzen,8 who retarded the rate of fluorine exchange by careful sample purification. The latter authors, however, were unable to establish that all impurit ies had been removed. Other physical studies of sulfur tetrafluoride have failed to clarify the relative importance of intraand intermolecular paths for the fluorine exchange. Redington and cowork\"rr itrongly favored bimolecular mechanisms on the basis of matrix-isoiation infrared spectra,e'10 and variations in NMR coupling constants and chemical shifts with medium have been interpreted as indicating intermolecular association of l iquid SFa.rr Gas phase studies, employing both farinfrared spectroscopyl 2 and electron diffraction,l3 implicated intramolecular exchange processes. Theoretical studiesl4'ls also propose intramolecular exchange in the gas phase, thus lending credence to Muetterties and Phil l ips' hypothesis that intramolecular exchange might be observed in the liquid phase if higher-order processes could be suppressed. The present dynamic NMR study was undertaken to delineate the site exchange scheme responsible for f luorine exchange in sulfur tetrafluoride, and thus unambiguously determine which, if any,. of the proposed exchange mechanisms are consistent with experiments. The results of this investigation are reported in three parts: (i) generation of all possible site exchange schemes implied by intramolecular, impurity catalyzed, and bimolecular mechanisms, Whitesides et al. / NMR Study of F Exchange in Liquid SFa

Large-scale synthesis of diammonium acetyl phosphate

Abstract: A detailed procedure for the large-scale synthesis of diammonium acetyl phosphate (1) is presented. Ketene is used to acylate 1007o phosphoric acid in ethyl acetate at -10o, and the resulting mixture of monoand polyacetyl phosphoric acids converted to I by treatment with anhydrous ammonia in ethyl acetate-methanol at -10o. The product is obtained as an easily fi ltered, crystall ine solid in ca. 90% yield and ca. 90% purity.

Mechanisms of Thermal Decomposition of Transition Metal Organometallic Compounds

Abstract: Transition metal alkyls are intermediates in a variety of catalytic processes. These intermediate metal alkyls are often thermally unstable: in fact, their thermal instability -- that is, their propensity to transform into different compounds rapidly and in high yield at low temperature -- is frequently the characteristic that makes them catalytically important. Knowledge of the detailed mechanisms of these thermal decompositions is important both in the rational application of known catalytic reactions, and in the development of new ones.