Photochemical generation of highly destabilized vinyl cations: the effects of alpha- and beta-trifluoromethyl versus alpha- and beta-methyl substituents

UVA Light-promoted Catalyst-free Cyclization of Vinyl Selenides: Green and Efficient Synthesis of C3-Unsubstituted 2-Selanyl Benzochalcogenophenes

A metal- and catalyst-free photo-promoted cyclization of properly substituted vinyl selenides was developed using UVA irradiation. A total of eighteen new C3-unsubstituted 2-selanyl benzochalcogenophenes (benzofurans, benzothiophenes and benzoselenophenes) were prepared in 30-86% yield after irradiation with UVA at room temperature. The usefulness of the title compounds was demonstrated in the easy functionalization of the remaining free C-H bond of the benzochalcogenophenes to form new C-Se and C-Br bonds by simple procedures. Furthermore, the reaction can be performed under natural sunlight irradiation and the solvent is easily reused further in several subsequent runs.

CF3-substituted carbocations: underexploited intermediates with great potential in modern synthetic chemistry

"The extraordinary instability of such an "ion" accounts for many of the peculiarities of organic reactions" - Franck C. Whitmore (1932). This statement from Whitmore came in a period where carbocations began to be considered as intermediates in reactions. Ninety years later, pointing at the strong knowledge acquired from the contributions of famous organic chemists, carbocations are very well known reaction intermediates. Among them, destabilized carbocations - carbocations substituted with electron-withdrawing groups - are, however, still predestined to be transient species and sometimes considered as exotic ones. Among them, the CF3-substituted carbocations, frequently suggested to be involved in synthetic transformations but rarely considered as affordable intermediates for synthetic purposes, have long been investigated. This review highlights recent and past reports focusing on their study and potential in modern synthetic transformations.

Palladium-catalyzed geometrically selective hydrogenation of (Z)-trifluoromethyl alkenyl triflate: an efficient approach to (Z) or (E)-3,3,3-trifluoropropenyl derivatives

Synthesis of (Z)- or (E)-3,3,3-trifluoropropenyl derivatives with excellent divergent geometric control via hydrogenation of (Z)-trifluoromethyl alkenyl triflates.

Synthesis and properties of novel polyimide fibers containing phosphorus groups in the main chain

Different diamines ratios synthesized co-polyimide (co-PI) fibers and these fibers had good thermal properties and mechanical performances with increased DAMPO contents.

Methylenation of perfluoroalkyl ketones using a Peterson olefination approach

An operationally simple, inexpensive, and rapid route for the olefination of a wide array of trifluoromethyl ketones to yield 3,3,3-trifluoromethylpropenes is reported. Using a Peterson olefination approach, the reaction gives good to excellent yields of the alkene products and can be performed without purification of the β-hydroxysilyl intermediate. The reaction can be extended to other perfluoroalkyl substituents and is easily scaled up. The alkenes prepared can be readily transformed into a variety of other perfluoroalkyl-containing compounds.

Copper-mediated radical trifluoromethylation of unsaturated potassium organotrifluoroborates

Copper-mediated trifluoromethylation of unsaturated organotrifluoroborates with the Langlois reagent (NaSO2CF3) and TBHP allows the introduction of trifluoromethyl groups into a variety of organic substructures. The reactions are easy to set up, the conditions are mild and general, and the process provides access to trifluoromethylated alkynes, alkenes, arenes, and heteroarenes in fair to good yields.

Phosphonium iodide as a donor liquid electrolyte for dyesensitized solar cells

An efficient triphenylmethylphosphonium iodide-based liquid electrolyte was synthesized and used for the first time as an electrolyte in dye-sensitized solar cells (DSSCs). With the as-synthesized electrolyte, the DSSC yielded an overall light to electricity conversion efficiency of 5.34 % to 7.10 %, when the radiant power was tuned from 100 mW cm-2 to 10 mW cm-2. This may be attributed to the limitation of mass transport in the DSSC. The electronic and ionic processes in the DSSC were investigated by electrochemical impedance spectroscopy and linear voltammography, respectively.

Vinyl cations substituted with beta pi-donors have triplet ground states

Computations at the CASPT2, CBS-QB3, and B3LYP levels of theory demonstrate that beta-substitution of vinyl cations with pi-donors switches the ground state of these ions from the familiar closed-shell singlet state to a carbene-like triplet state similar to the electronic state of triplet phenyl cations. Although the parent vinyl cation is a ground-state singlet species with a very large energy gap to the lowest energy triplet state, substituting the beta hydrogens with just one strong pi-donor (e.g., NH(2), NMe(2), OMe) or two moderate pi-donors (e.g., F, OH, Ar, vinyl) makes the triplet state the computed ground electronic state. In many cases, the singlet states for these beta pi-donor-substituted vinyl cations are prone to rearrangements, although such rearrangements can be inhibited through incorporation of the pi-donors into rings. For example, a vinyl cation based on 1,3-dimethyl-2-methylene imidazolidine (32) is predicted to show a substantial barrier to singlet state rearrangement as well as possess a triplet ground state with a large energy gap. In contrast to the singlet states, the stabilized triplet states appear to be well behaved and more immune to rearrangements. These triplet ions may exhibit substantially different properties and reaction chemistry than those seen for typical closed-shell vinyl cations.

Fluorous tolerance of the estrogen receptor alpha as probed by 11-polyfluoroalkylestradiol derivatives

The concern of this work was to try to delineate factors, inherent to fluorination, susceptible to influence estradiol binding to the estrogen receptor alpha (ERalpha). For this purpose, fluorinated chains were linked at 11beta position of the steroid (i.e., C(6)F(13), CH(2)CH(2)C(4)F(9), CH(2)CH(2)C(8)F(17)). Relative binding affinity (RBA) for ERalpha of these compounds and of other related fluorinated derivatives was compared to those of non-fluorinated analogs. Despite being relatively well accepted by the receptor, investigated compounds exhibited lower RBA values at 0 degrees C than their non-fluorinated counterparts. Nevertheless, heavily fluorinated chains were tolerated in so far as they are not too long (C-4) and insulated from the steroidal core by a two methylene spacer unit. Increase of the temperature of our binding assay (25 degrees C) failed to change the RBA values of two selected polyfluorohexyl derivatives while it drastically enhanced the value of the corresponding non-fluorinated analogs. Rigidity of the chain induced by fluorination as well as the oleophilic (fluorophobic) nature of the estradiol binding cavity of ERalpha is proposed to explain these properties.

Photochemical generation of six- and five-membered cyclic vinyl cations

The photochemical solvolyses of 4-tert-butylcyclohex-1-enyl(phenyl)iodonium tetrafluoroborate (1) and cyclopent-1-enyl(phenyl)iodonium tetrafluoroborate (2) in methanol yield vinylic ethers and vinylic cycloalkenyliodobenzenes and cycloalkenylbenzene, which are the trapping products of the geometrically destabilized C6-ring and C5-ring vinyl cation with the solvent and with the leaving group iodobenzene. Iodonium salt 2 also yields an allylic ether and allylic cyclopentenyliodobenzenes and cyclopentenylbenzene, which are the trapping products of the C5-ring allylic cation produced from the C5-ring vinyl cation by a hydride shift in a typical carbocationic rearrangement.

Synthesis of 11beta-perfluorohexylestradiol

[reaction: see text] We report the synthesis of 11beta-perfluorohexylestradiol 1e using a perfluoroorganometallic reagent for the introduction of the fluorous part. This compound is useful for biological studies and for imaging the ERalpha estradiol receptor distribution in the whole cell by secondary ion mass spectrometry (SIMS). The key step of this synthesis involves the radical reduction of an 11beta-oxalate derivative. The stereochemical outcome of this reaction was studied for a range of C11 substituents, and we attempted to rationalize the apparent abnormal behavior of the phenyl group.

Probing the intermediates of halogen addition to alkynes: bridged halonium versus open halovinyl cation; a theoretical study

[Reaction: see text]. Intermediates formed in halogen addition (X = Br, Cl, F) to alkynes (ethyne, propyne, 2-butyne, trifluoromethylethyne, trimethylsilylethyne, and 1-trimethylsilylpropyne) were studied computationally by MP2 at the MP2/6-311++G(3df,3pd) level and/or by DFT at the B3LYP/6-31+G(d) level. Structure optimization and frequency calculations were performed to identify the minima and to obtain their relative energies. PCM calculations (with H2O, CH2Cl2, and CCl4 as model solvents) were employed to examine solvation effects on the relative stabilities in the resulting bridged halonium, -halovinyl, or -halovinyl cations. GIAO-MP2 and GIAO-DFT calculations were employed to compute NMR chemical shifts (13C, 19F, and 29Si as appropriate). In selected cases, PCM-GIAO calculations were also performed to investigate the extent of solvent effects on the computed NMR shifts. The NPA-derived charges and the GIAO shifts were examined in comparative cases to shed light on structural features. In several cases, structure optimization starting with the -halovinyl cations resulted in -halovinyl cations (via formal hydride shift or trimethylsilyl shift). With the CF3 derivative (when X = F), a formal F shift results in polyfluoroallyl cation generation from fluorovinyl cation as starting geometry.