Radical Reactions Using Selenium Precursors

Trapping X‐ray Radiation Damage from Homolytic Se−C Bond Cleavage in BnSeSeBn Crystals (Bn=benzyl, CH 2 C 6 H 5 )

Irradiation of dibenzyl diselenide BnSeSeBn with X‐ray or UV‐light cleaves the Se−C and the Se−Se bonds, inducing stable and metastable radical states. They are inevitably important to all natural and life sciences. Structural changes due to X‐ray‐induced Se−C bond‐cleavage could be pin‐pointed in various high‐resolution X‐ray diffraction experiments for the first time. Extended DFT methods were applied to characterize the solid‐state structure and support the refinement of the observed residuals as contributions from the BnSeSe⋅ radical species. The X‐ray or UV‐irradiated crystalline samples of BnSeSeBn were characterized by solid‐state EPR. This paper provides insight that in the course of X‐ray structure analysis of selenium compounds not only organo‐selenide radicals like RSe⋅ may occur, but also organo diselenide BnSeSe⋅ radicals and organic radicals R⋅ are generated, particularly important to know in structural biology.

Applications of electron paramagnetic resonance spectroscopy to heavy main-group radicals

The exploration of heavy main-group radicals is rapidly expanding, for which electron paramagnetic resonance (EPR) spectroscopic characterisation plays a key role. EPR spectroscopy has the capacity to deliver information of the radical's electronic, geometric and bonding structure. Herein, foundations of electron-nuclear hyperfine analysis are detailed before reviewing more recent applications of EPR spectroscopy to As, Sb, and Bi centred radicals. Additional diverse examples of the application of EPR spectroscopy to other heavy main group radicals are highlighted.

The phenylselenyl radical and its reaction with molecular oxygen

The phenylselenyl radical and its primary oxidation products phenylselenyl peroxy and phenylselenoyl radicals were generated and subsequently isolated in solid noble gas matrices for the first time.

Expanding the Repertoire of Molecular Linkages to Silicon: Si-S, Si-Se, and Si-Te Bonds

Silicon is the foundation of the electronics industry and is now the basis for a myriad of new hybrid electronics applications, including sensing, silicon nanoparticle-based imaging and light emission, photonics, and applications in solar fuels, among others. From interfacing of biological materials to molecular electronics, the nature of the chemical bond plays important roles in electrical transport and can have profound effects on the electronics of the underlying silicon itself, affecting its work function, among other things. This work describes the chemistry to produce ≡Si-E bonds (E = S, Se, and Te) through very fast microwave heating (10-15 s) and direct thermal heating (hot plate, 2 min) through the reaction of hydrogen-terminated silicon surfaces with dialkyl or diaryl dichalcogenides. The chemistry produces surface-bound ≡Si-SR, ≡Si-SeR, and ≡Si-TeR groups. Although the interfacing of molecules through ≡Si-SR and ≡Si-SeR bonds is known, to the best of our knowledge, the heavier chalcogenide variant, ≡Si-TeR, has not been described previously. The identity of the surface groups was determined by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and depth profiling with time-of-flight-secondary ionization mass spectrometry (ToF-SIMS). Possible mechanisms are outlined, and the most likely, based upon parallels with well-established molecular literature, involve surface silyl radicals or dangling bonds that react with either the alkyl or aryl dichalcogenide directly, REER, or its homolysis product, the alkyl or aryl chalcogenyl radical, RE· (where E = S, Se, and Te).

Iron-induced regio- and stereoselective addition of sulfenyl chlorides to alkynes by a radical pathway

The radical addition of the Cl-S σ-bond in sulfenyl chlorides to various C-C triple bonds has been achieved with excellent regio- and stereoselectivity in the presence of a catalytic amount of a common iron salt. The reaction is compatible with a variety of functional groups and can be scaled up to the gram-scale with no loss in yield. As well as terminal alkynes, internal alkynes underwent stereodefined chlorothiolation to provide tetrasubstituted alkynes. Preliminary mechanistic investigations revealed a plausible radical process involving a sulfur-centered radical intermediate via iron-mediated homolysis of the Cl-S bond. The resulting chlorothiolation adducts can be readily transformed to the structurally complex alkenyl sulfides by cross-coupling reactions. The present reaction can also be applied to the complementary synthesis of the potentially useful bis-sulfoxide ligands for transition-metal-catalyzed reactions.

Organic azides: an exploding diversity of a unique class of compounds

Since the discovery of organic azides by Peter Griess more than 140 years ago, numerous syntheses of these energy-rich molecules have been developed. In more recent times in particular, completely new perspectives have been developed for their use in peptide chemistry, combinatorial chemistry, and heterocyclic synthesis. Organic azides have assumed an important position at the interface between chemistry, biology, medicine, and materials science. In this Review, the fundamental characteristics of azide chemistry and current developments are presented. The focus will be placed on cycloadditions (Huisgen reaction), aza ylide chemistry, and the synthesis of heterocycles. Further reactions such as the aza-Wittig reaction, the Sundberg rearrangement, the Staudinger ligation, the Boyer and Boyer-Aubé rearrangements, the Curtius rearrangement, the Schmidt rearrangement, and the Hemetsberger rearrangement bear witness to the versatility of modern azide chemistry.

Solution calorimetric and stopped-flow kinetic studies of the reaction of *Cr(CO)3C5Me5 with PhSe-SePh and PhTe-TePh. Experimental and theoretical estimates of the Se-Se, Te-Te, H-Se, and H-Te bond strengths

The kinetics of the oxidative addition of PhSeSePh and PhTeTePh to the stable 17-electron complex *Cr(CO)3C5Me5 have been studied utilizing stopped-flow techniques. The rates of reaction are first-order in each reactant, and the enthalpy of activation decreases in going from Se (deltaH(double dagger) = 7.0 +/- 0.5 kcal/mol, deltaS(double dagger) = -22 +/- 3 eu) to Te (deltaH(double dagger) = 4.0 +/- 0.5 kcal/mol, deltaS(double dagger) = -26 +/- 3 eu). The kinetics of the oxidative addition of PhSeH and *Cr(CO)3C5Me5 show a change in mechanism in going from low (overall third-order) to high (overall second-order) temperatures. The enthalpies of the oxidative addition of PhE-EPh to *Cr(CO)3C5Me5 in toluene solution have been measured and found to be -29.6, -30.8, and -28.9 kcal/mol for S, Se, and Te, respectively. These data are combined with enthalpies of activation from kinetic studies to yield estimates for the solution-phase PhE-EPh bond strengths of 46, 41, and 33 kcal/mol for E = S, Se, and Te, respectively. The corresponding Cr-EPh bond strengths are 38, 36, and 31 kcal/mol. Two methods have been used to determine the enthalpy of hydrogenation of PhSeSePh in toluene on the basis of reactions of HSPh and HSePh with either *Cr(CO)3C5Me5 or 2-pyridine thione. These data lead to a thermochemical estimate of 72 kcal/mol for the PhSe-H bond strength in toluene solution, which is in good agreement with kinetic studies of H atom transfer from HSePh at higher temperatures. The reaction of H-Cr(CO)3C5Me5 with PhSe-SePh is accelerated by the addition of a Cr radical and occurs via a rapid radical chain reaction. In contrast, the reaction of PhTe-TePh and H-Cr(CO)3C5Me5 does not occur at any appreciable rate at room temperature, even in the presence of added Cr radicals. This is in keeping with a low PhTe-H bond strength blocking the chain and implies that H-TePh < or = 63 kcal/mol. Structural data are reported for PhSe-Cr(CO)3C5Me5 and PhS-Cr(CO)3C5Me5. The two isostructural complexes do not show signs of an increase in steric strain in terms of metal-ligand bonds or angles as the Cr-EPh bond is shortened in going from Se to S. Bond strength estimates of the PhE-H and PhE-EPh derived from density functional theory calculations are in reasonable agreement with experimental data for E = Se but not for E = Te. The nature of the singly occupied molecular orbital of the *EPh radicals is calculated to show increasing localization on the chalcogenide atom in going from S to Se to Te.