Mechanism‐Driven Elaboration of an Enantioselective Bromocyclopropanation Reaction of Allylic Alcohols

A stereoselective bromocyclopropanation of allylic alcohols using dibromomethylzinc bromide is described. Spectroscopic studies to monitor the formation of transient intermediates not only led to the development of a more‐atom‐economical halocyclopropanation reaction, but also highlighted the unique role of ether additives in the process. The desired bromo‐substituted cyclopropanes were isolated in high yields and excellent diastereo‐ as well as enantioselectivities using readily available reagents.     

Nickel(0)/NaHMDS adduct-mediated intramolecular alkylation of unactivated arenes via a homolytic aromatic substitution mechanism

A variety of polycycles can be synthesized via an intramolecular alkylation cyclization promoted by Ni(PPh(3))(4) and NaHMDS. Mechanistic investigations support the catalytic nature of Ni(0) in the course of TEMPO scavenging experiments and its association with the substrate and NaHMDS to form an adduct by DOSY NMR.     

Simultaneous coherent imaging and strain measurement using coupled photorefractive holography and shearography

By coupling photorefractive holography with speckle shearography, it is possible to simultaneously perform both coherent imaging and strain measurement. Use of the photorefractive effect, which is insensitive to incoherently scattered light, is a significant advantage in coherent imaging as described. Experimental results obtained from a centrally loaded steel plate are presented.     

Hypervalent iodine(III) reagents as safe alternatives to alpha-nitro-alpha-diazocarbonyls

A cyclopropanation reaction involving iodonium ylides generated in situ allows for efficient preparation of substituted 1-nitro-1-carbonyl cyclopropanes. This robust cyclopropanation reaction can be performed in organic solvents, biphasic aqueous media, or under solvent-free conditions with alkene substrates. The iodonium ylides generated in situ display some surprising differences in reactivity when compared to alpha-nitro-alpha-diazocarbonyl compounds. They do not undergo O-H insertion reactions and exhibit reduced reactivity with certain alkenes. [reaction: see text]     

Determination of cobalt and zinc in highpurity niobium, tantalum, molybdenum and tungsten metals by atomicabsorption spectrophotometry after separation by extraction

A method for determining 0.0005-0.05% of cobalt and zinc in high-purity niobium, tantalum, molybdenum and tungsten metals by atomic-absorption spectrophotometry is described. After sample dissolution, cobalt and zinc are separated simultaneously from the matrix materials by chloroform extraction of their thiocyanatediantipyrylmethane ion-association complexes, at pH 3.25, from a citric acid medium approximately 1.2M in sodium thiocyanate. Interference from copper is eliminated with thiourea. Large amounts of iron interfere under the recommended conditions, but moderate amounts may be present in the sample solution without causing appreciable error in the results. Phosphorus (as orthophosphate) interferes in the extraction of cobalt from tungsten solutions. Moderate amounts of other impurities do not interfere in the proposed method.     

Transition metal-catalyzed cyclopropanation of alkenes in water: catalyst efficiency and in situ generation of the diazo reagent

[reaction: see text] A cyclopropanation reaction involving ethyl diazoacetate and olefins proceeds with surprisingly high efficiency in aqueous media using Rh(II) carboxylates. Nishiyama's Ru(II) Py-box and Katsuki's Co(II) Salen complexes that allow for highly enantioselective cyclopropanations in organic solvents can also be applied to aqueous cyclopropanations with similar results. In situ generation of ethyl diazoacetate and cyclopropanation also proceeds efficiently. A chemoselective O-H insertion is also possible in water when hydrophobic catalysts and alcohols are used.     

Photoinduced alkyl group exchange of ethylzinc alkoxides: X-ray crystal structure of an iodomethylzinc methoxide

Irradiation of a solution of ethyl zinc alkoxides and CH2I2 leads to clean formation of iodomethylzinc alkoxides; these intermediates are important species generated in stereoselective cyclopropanation processes; no alkyl group exchange is observed in the absence of irradiation; the solid-state structure of (MeO)8Zn7(CH2I)6 is also reported.     

Iodomethylzinc phosphates: powerful reagents for the cyclopropanation of alkenes

A new family of zinc carbenoids derived from phosphoric acids was developed and used in the cyclopropanation of allylic alcohols and ethers and also of unfunctionalized olefins. The use of the chiral phosphoric acid of a 3,3'-disubstituted BINOL leads to efficient stereocontrol, affording the cyclopropanes of allylic and homoallylic ethers with complete conversions and up to 93% ee. A catalytic version of this reaction using 10 mol % of the chiral phosphate reagent is also disclosed.