Aqueous magnetic fluids were synthesized by a sequential process involving the chemical coprecipitation of Fe(II) and Fe(III) salts with ammonium hydroxide (NH4OH) followed by resuspension of the ultrafine particles in water using fatty acids. This procedure produced Fe3O4 nanoparticles stabilized against agglomeration by bilayers of n-alkanoic acids with 9−13 carbons encapsulating the metal particles. The magnetic properties and particle size and size distributions of these magnetic fluids, characterized by transmission electron microscopy and superconducting quantum interference device, indicated the formation of single-domain nanoparticles of mean diameter ∼9.3 and ∼7.5 nm, respectively; the difference in values determined by the two methods implies the presence of a nonmagnetic layer on the particle surface. Thermogravimetric analysis measurements showed the existence of two distinct populations of surfactants on the particle surface, each having surfactant coverage of ∼21−24 Å2/molecule, that was consistent with highly organized surfactant bilayer structures. Differential scanning calorimetry indicated the presence of a phase transition for the bilayer-coated particles that suggests partial interpenetration of the hydrocarbon tails of the primary and secondary surfactants.
Small-angle neutron scattering and dynamic and static light scattering measurements were used to probe the structures of aqueous and organic-solvent-based magnetic fluids comprising dispersed magnetite nanoparticles (∼10 nm in diameter) stabilized against flocculation by adsorbed alkanoic acid layers. A core−shell model fitted to a set of neutron scattering spectra obtained from contrast variation experiments allowed the determination of the iron oxide core size and size distribution, the thicknesses of the surfactant shells, and the spatial arrangement of the individual particles. The magnetic colloidal particles appear to form compact fractal clusters with a fractal dimension of 2.52 and a correlation length of ∼350 Å in aqueous magnetic fluids, consistent with the structures of clusters observed directly using cryo-TEM (transmission electron microscopy), whereas chainlike clusters with a fractal dimension of 1.22 and a correlation length of ∼400 Å were found for organic-solvent-based magnetic fluids. The differences in cluster structure were attributed to the relative strengths of the particle−particle interaction energies. Weak interactions in the organic-solvent-based systems dictate the formation of small structures for which the apparent fractal dimensions are naturally small, whereas significantly stronger interparticle interactions in aqueous magnetic fluids result in larger, more compact clusters with higher fractal dimensions. The growth of the aqueous clusters beyond a certain size was inhibited by an increasingly high energy barrier (balance between repulsive electrostatic and attractive van der Waals interactions) with increasing cluster size. The aqueous clusters were stable against further growth when diluted with a surfactant solution but grew in time when diluted with pure water. In the latter case, the loss of part of the stabilizing secondary surfactant layer to the aqueous phase to satisfy thermodynamic partitioning constraints led to a destabilization in its dispersion. Light scattering studies indicated a change in the fractal dimension from 2.52 to about 1.20 as the clusters grew.
This paper describes the development of a continuous, high yielding, and scalable enolization, oxidation, and quench process for the hydroxylation of the azapirone psychtropic agent buspirone to afford 6-hydroxybuspirone (6-hydroxy-8-[4-(4-pyrimidin-2-yl-piperazin-1-yl)-butyl]-8-aza-spiro[4.5]decane-7,9-dione). Two feed streams were reacted continuously using an in-line static mixer followed by oxidation in a continuous flow trickle-bed reactor. The laboratory reactor operation was demonstrated at steady state for over 40 h. The process was scaled up using both volumetric (enolization) and numbering-up (oxidation) scale-up strategies. A pilot-plant reactor was developed and successfully implemented in a three-batch campaign (47 kg input per batch).
This manuscript details the process research and development of a convergent and safe approach to 1 on a multikilo scale. Specific highlights of the process development efforts will be described, including the development of a dehydrogenation method for dihydropyrimidines and a thermochemically safe synthesis of a 1,2,4-aminotriazole fragment. A key feature of the synthesis is the use and optimization of a modified Julia-Kocienski olefination reaction. Specifically, we report an unprecedented dependence of the product olefin geometry on reaction temperature, where an E:Z ratio as high as 200:1 can be obtained. Initial insights into the mechanistic rationale for this observation are also provided. Finally, a purity upgrade sequence via an intermediate crystalline form is highlighted as a method of controlling the final API quality.
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