Frustrated 3-Dimensional Quantum Spin Liquid in CuHpCl

Inelastic neutron scattering measurements are reported for the quantum antiferromagnetic material Cu_2(C_5H_12N_2)_2Cl_4 (CuHpCl). The magnetic excitation spectrum forms a band extending from 0.9 meV to 1.4 meV. The spectrum contains two modes that disperse throughout the a-c plane of the monoclinic unit cell with less dispersion along the unique b-axis. Simple arguments based on the measured dispersion relations and the crystal structure show that a spin ladder model is inappropriate for describing CuHpCl. Instead, it is proposed that hydrogen bond mediated exchange interactions between the bi-nuclear molecular units yield a three-dimensional interacting spin system with a recurrent triangular motif similar to the Shastry-Sutherland Model (SSM). Model independent analysis based on the first moment sum rule shows that at least four distinct spin pairs are strongly correlated and that two of these, including the dimer bond of the corresponding SSM, are magnetically frustrated. These results show that CuHpCl should be classified as a frustration induced three dimensional quantum spin liquid.Comment: 13 pages, 17 figures (Color) ReSubmitted to Phys. Rev. B 9/21/2001 resubmission has new content email comments to [email protected] or [email protected]

34.7.4 Allylic Fluorides (Update 2017)

AbstractThis chapter is an update to the earlier Science of Synthesis contribution (Section 34.7) regarding the synthesis of allylic monofluorides. Herein, literature from 2005–2015 is discussed. Advancements during this time period include the employment of milder fluorinating reagents, methods that favor alkene migration or retention, tactics for catalytic and asymmetric reactions, and the introduction of a creative array of functional-group interconversions.</jats:p

Asymmetric Cycloadditions ofo-Quinone Methides Employing Chiral Ammonium Fluoride Precatalysts

[Image: see text] The catalytic, enantioselective, [4 + 2] cycloaddition reaction of ortho-quinone methides with silyl ketene acetals is described. This mechanistically interesting reaction, initiated by a chiral cinchona alkaloid-derived ammonium fluoride “precatalyst” complex, affords a variety of alkyl- and aryl-substituted 3,4-dihydrocoumarin products in excellent yield and with good enantioselectivity

Evidence for a Symmetrical Fluoronium Ion in Solution

Fluorine Learns to Share Though halides typically coordinate to just one carbon center, their transient coordination to a second carbon (forming a positively charged bridge) explains the spatial dynamics of many reactions. However, unlike chlorine, bromine, and iodine—which can all form such halonium ions—fluorine does not appear to engage in carbon-bridging behavior, presumably because of its very high electronegativity. Struble et al. (p. 57 , see the Perspective by Hennecke ) synthesized a rigid molecule, particularly well-poised to manifest fluoride bridging, and provide evidence for a fluoronium intermediate in a displacement reaction. </jats:p

Asymmetric Cycloadditions of <i>o</i>-Quinone Methides Employing Chiral Ammonium Fluoride Precatalysts

The catalytic, enantioselective, [4 + 2] cycloaddition reaction of ortho-quinone methides with silyl ketene acetals is described. This mechanistically interesting reaction, initiated by a chiral cinchona alkaloid-derived ammonium fluoride “precatalyst” complex, affords a variety of alkyl- and aryl-substituted 3,4-dihydrocoumarin products in excellent yield and with good enantioselectivity

Tricomponent Catalytic α,α-Difluorination of Acid Chlorides

The selective α,α-difluorination of carbonyl compounds remains a challenge in modern organic synthesis; current methods often incorporate stepwise processes and/or harsh conditions, providing unsatisfactory mixtures of mono- and difluorinated products. In this communication, a practical, mild, and one-pot method for the selective α,α-difluorination of readily available acid chlorides is reported in which three separate catalysts act synergistically to form products in outstanding selectivity and fair to excellent yields

Asymmetric Cycloadditions of <i>o</i>-Quinone Methides Employing Chiral Ammonium Fluoride Precatalysts

The catalytic, enantioselective, [4 + 2] cycloaddition reaction of ortho-quinone methides with silyl ketene acetals is described. This mechanistically interesting reaction, initiated by a chiral cinchona alkaloid-derived ammonium fluoride “precatalyst” complex, affords a variety of alkyl- and aryl-substituted 3,4-dihydrocoumarin products in excellent yield and with good enantioselectivity

Catalytic, Asymmetric α-Fluorination of Acid Chlorides: Dual Metal−Ketene Enolate Activation

[Image: see text] In this Communication, we disclose a catalytic, highly enantioselective (up to >99% ee) α-fluorination of acid chlorides to produce a variety of optically active carboxylic acid derivatives from readily accessible and commercially available starting materials. The reaction depends on dually activated ketene enolates generated from two discrete catalysts - a chiral nucleophile and an achiral transition metal complex working in tandem. The active, putative α-fluorobis(sulfonimide) intermediates readily transacylate in situ under mild conditions upon addition of a wide variety of nucleophiles, including complex natural products. As a consequence, the power of this method is witnessed by the broad range of α-fluorinated products that can be accessed efficiently depending on the work up conditions