Direct observation of o-benzyne formation in photochemical hexadehydro-Diels–Alder (hν-HDDA) reactions

Reactive ortho-benzyne derivatives are believed to be the initial products of liquid-phase [4 + 2]-cycloadditions between a 1,3-diyne and an alkyne via what is known as a hexadehydro-Diels–Alder (HDDA) reaction. The UV/VIS spectroscopic observation of o-benzyne derivatives and their photochemical dynamics in solution, however, have not been reported previously. Herein, we report direct UV/VIS spectroscopic evidence for the existence of an o-benzyne in solution, and establish the dynamics of its formation in a photoinduced reaction. For this purpose, we investigated a bis-diyne compound using femtosecond transient absorption spectroscopy in the ultraviolet/visible region. In the first step, we observe excited-state isomerization on a sub-10 ps time scale. For identification of the o-benzyne species formed within 50–70 ps, and the corresponding photochemical hexadehydro-Diels–Alder (hν-HDDA) reactions, we employed two intermolecular trapping strategies. In the first case, the o-benzyne was trapped by a second bis-diyne, i.e., self-trapping. The self-trapping products were then identified in the transient absorption experiments by comparing their spectral features to those of the isolated products. In the second case, we used perylene for trapping and reconstructed the spectrum of the trapping product by removing the contribution of irrelevant species from the experimentally observed spectra. Taken together, the UV/VIS spectroscopic data provide a consistent picture for o-benzyne derivatives in solution as the products of photo-initiated HDDA reactions, and we deduce the time scales for their formation.

We report the transient ultraviolet/visible absorption spectrum of an o-benzyne species in solution for the first time.     

Rhodium catalysed dehydrogenative borylation of vinylarenes and 1,1-disubstituted alkenes without sacrificial hydrogenation--a route to 1,1-disubstituted vinylboronates

The complex trans-[Rh(Cl)(CO)(PPh3)2] (1) is an efficient catalyst precursor for the dehydrogenative borylation of alkenes without consumption of half the alkene substrate by hydrogenation, giving useful vinylboronate esters including 1,1-disubstituted derviatives that cannot be made by alkyne hydroboration.     

Electronic couplings in organic mixed-valence compounds: the contribution of photoelectron spectroscopy

We show that the electronic coupling in strongly coupled organic mixed-valence systems can be effectively probed by means of gas-phase ultraviolet photoelectron spectroscopy (UPS). Taking six diamines as examples, the UPS estimates for the electronic couplings H(ab) are compared with the corresponding values determined from the intervalence charge-transfer absorption bands and from electronic structure calculations. Similar trends are observed for the H(ab) values estimated from UPS and optical spectra; this provides support for the applicability of Hush theory to strongly coupled organic mixed-valence systems. The UPS electronic couplings are found to be somewhat smaller than those from optical spectroscopy, which is attributed to the role of vibronic coupling to symmetrical modes; when corrected for this vibronic coupling, the UPS H(ab) estimates confirm that triarylamine-based mixed-valence systems are close to the class-II/class-III borderline.     

Fully Borylated Methane and Ethane by Ruthenium‐Mediated Cleavage and Coupling of CO

Many transition‐metal complexes and some metal‐free compounds are able to bind carbon monoxide, a molecule which has the strongest chemical bond in nature. However, very few of them have been shown to induce the cleavage of its C?O bond and even fewer are those that are able to transform CO into organic reagents with potential in organic synthesis. This work shows that bis(pinacolato)diboron, B₂pin₂, reacts with ruthenium carbonyl to give metallic complexes containing borylmethylidyne (CBpin) and diborylethyne (pinBC≡CBpin) ligands and also metal‐free perborylated C₁ and C₂ products, such as C(Bpin)₄ and C₂(Bpin)₆, respectively, which have great potential as building blocks for Suzuki–Miyaura cross‐coupling and other reactions. The use of ¹³CO‐enriched ruthenium carbonyl has demonstrated that the boron‐bound carbon atoms of all of these reaction products arise from CO ligands.     

Stabilisation of a heptamethine cyanine dye by rotaxane encapsulation

The crystal structure of a cyanine dye rotaxane shows that the cyclodextrin is tightly threaded round the polymethine bridge of the dye; encapsulation dramatically increases the kinetic chemical stability of the radicals formed on oxidation and reduction of the dye, making it possible to observe the rotaxane radical dication by ESR and UV-vis-NIR spectroscopy.     

Electron affinities of 1,1-diaryl-2,3,4,5-tetraphenylsiloles: direct measurements and comparison with experimental and theoretical estimates

We present a comprehensive experimental and theoretical characterization of the electronic structure of four 1,1-diaryl-2,3,4,5-tetraphenylsiloles (aryl = phenyl, 2-(9,9-dimethylfluorenyl), 2-thienyl, pentafluorophenyl). Solid-state electron affinities and ionization potentials of these siloles were measured using inverse-photoelectron spectroscopy (IPES) and photoelectron spectroscopy (PES), respectively; the density of electronic states obtained from calculations performed at the density functional theory (DFT) level corresponds very well to the PES and IPES data. The direct IPES measurements of electron affinity were then used to assess alternative estimates based on electrochemical and/or optical data. We also used DFT to calculate the reorganization energies for the electron-transfer reactions between these siloles and their radical anions. Additionally, optical data and ionization potential and electron affinity data were utilized to estimate the binding energies of excitons in these siloles.