Reversibility is fundamental for transition metal catalysis, but equally for main group chemistry and especially low-valent silicon compounds, the interplay between oxidative addition and reductive elimination is key for a potential catalytic cycle. Herein, we report a highly reactive acyclic iminosilylsilylene 1, which readily performs an intramolecular insertion into a C═C bond of its aromatic ligand framework to give silacycloheptatriene (silepin) 2. UV-vis studies of this Si(IV) compound indicated a facile transformation back to Si(II) at elevated temperatures, further supported by density functional theory calculations and experimentally demonstrated by isolation of a silylene-borane adduct 3 following addition of B(CF). This tendency to undergo reductive elimination was exploited in the investigation of silepin 2 as a synthetic equivalent of silylene in the activation of small molecules. In fact, the first monomeric, four-coordinate silicon carbonate complex 4 was isolated and fully characterized in the reaction with carbon dioxide under mild conditions. Additionally, the exposure of 2 to ethylene or molecular hydrogen gave silirane 5 and Si(IV) dihydride 6, respectively.

CO2 fixation and reduction to value‐added products is of utmost importance in the battle against rising CO2 levels in the Earth's atmosphere. An organoaluminum complex containing a formal aluminum double bond (dialumene), and thus an alkene equivalent, was used for the fixation and reduction of CO2. The CO2 fixation complex undergoes further reactivity in either the absence or presence of additional CO2, resulting in the first dialuminum carbonyl and carbonate complexes, respectively. Dialumene (1) can also be used in the catalytic reduction of CO2, providing selective formation of a formic acid equivalent via the dialuminum carbonate complex rather than a conventional aluminum–hydride‐based cycle. Not only are the CO2 reduction products of interest for C1 added value products, but the organoaluminum complexes isolated represent a significant step forward in the isolation of reactive intermediates proposed in many industrially relevant catalytic processes.

Rare earth metals show high activities toward C− H bond activation of heteroaromatic substrates and even methane. In this work, we demonstrate the suitability of this synthetic approach to rare earth metallocenes and show the applicability of the resulting complexes as highly efficient initiators for rare earth metal-mediated group transfer polymerization. Bis(cyclopentadienyl)(4,6-dimethylpyridin-2-yl)methyl lanthanide complexes exhibit unprecedented initiation rates for rare earth metal-mediated dialkyl vinylphosphonate polymerization and facilitate an efficient initiation for a broad scope of Michael acceptor-type monomers.

Various polycarbonates with different material properties derived from CO2 and epoxides were prepared utilising a Lewis acid β-diiminato zinc catalyst.

3-Allyl-substituted quinolones undergo at ripletsensitized di-p-methane rearrangement reaction to the corresponding 3-cyclopropylquinolones upon irradiation with visible light (l = 420 nm). Ac hiral hydrogen-bonding sensitizer (10 mol %) was shown to promote the reaction enantioselectively (88-96 %y ield, 32-55 %e e). Surprisingly,i tw as found that the enantiodifferentiation does not occur at the state of initial product formation but that it is the result of aderacemization event. The individual parameters that control the distribution of enantiomers in the photostationary state have been identified.Triplet sensitization represents an efficient and elegant way to promote amolecule from its ground state to atriplet state, in most cases to the lowest-lying triplet state T 1 .T he mechanism of the process has been elucidated, [1] and apart from an energetic driving force,itiscrucial that the sensitizer and the substrate are in close spatial proximity to allow for efficient sensitization. Thei dea to employ triplet energy transfer by ac hiral sensitizer and to perform photochemical reactions enantioselectively was first disclosed in the 1960s. Pioneering studies were directed to the formation of chiral trans-1,2-diphenylcyclopropane (1)from its racemate (rac-1). After initial work by Hammond with as ensitizer that was later shown to operate on the singlet hypersurface, [2] Ouanns and co-workers found in 1973 that chiral ketone 2 induced an otable but small enantioselectivity (3 % ee)i nt he formation of 1 (Scheme 1). [3] Thee fficiency of the reaction suffered from the simultaneous formation of the achiral cyclopropane 3.In the 1980s and 1990s,l ittle attention was paid to the topic of enantioselective triplet-sensitized reactions. [4,5] The few reported studies were discouraging,a nd enantioselectivities did not exceed 20 % ee. [6] With the advent of efficient hydrogen-bonding templates and catalysts, [7] it became evident, however,t hat the issue of insufficient enantioface differentiation could be overcome,a nd the first highly enantioselective triplet-sensitized reaction was disclosed in 2009. [8] Am odified version of the xanthone catalyst used in this and related studies [9] was presented in 2014 and was based on at hioxanthone as the sensitizing unit. [10] Thec ompound, which is available in both enantiomeric forms 4 ( Figure 1) [11] and ent-4,has the advantage to operate with visible light and holds promise to be av ersatile catalyst for several photochemical reactions. [12,13] In the current study,weattempted to use catalyst 4 in an enantioselective [14] di-p-methane rearrangement reaction [15] of 3-allyl-substituted quinolones.A lthough the enantioselectivities of the reaction remained moderate,the mode of action turned out to be remarkable.I tw as found that the enantioenriched cyclopropanes are formed in ad eracemization process,w hich resembles the seminal work on chiral cyclopropanes [2,3] (see Scheme 1), and which likely proceeds via a1 ,3-diradical intermediate.T he results of our preliminary exper...

Hydrogenation of alkenes with C═C bonds is a ubiquitous reaction in organic chemistry. However, this transformation remains unknown for heavier counterparts, disilenes with Si═Si bonds. Here we report the isolation of (Z)-diiminodisilyldisilene 2 featuring a highly trans-bent and twisted structure and the longest silicon-silicon double bond reported to date. In silico studies suggested that the Si═Si bond in 2 is described as very weak double donor-acceptor bond. We utilized the remarkable electronic and structural features of this product to achieve the first demonstration of hydrogen activation by a multiply bonded silicon compound under ambient conditions. Interestingly, NMR and X-ray analysis gave exclusively racemic (RR/SS)-1,2-disilane 3a, indicating a stereospecific trans-hydrogenation of the Si═Si bond. In-depth calculations revealed that in strong contrast to the reactivity of C═C bonds, a concerted anti-addition pathway was favored due to the twisted structure of 2.

The photochemical deracemization of spiro[cyclopropane-1,3'-indolin]-2'-ones (spirocyclopropyl oxindoles) was studied. The corresponding 2,2-dichloro compound is configurationally labile upon direct irradiation at l = 350 nm and upon irradiation at l = 405 nm in the presence of achiral thioxanthen-9-one as the sensitizer.T he triplet 1,3-diradical intermediate generated in the latter reaction was detected by transient absorption spectroscopya nd its lifetime determined (t = 22 ms). Using ac hiral thioxanthone or xanthone,w ith al actam hydrogen bonding site as ap hotosensitizer,a llowed the deracemization of differently substituted chiral spirocyclopropyl oxindoles with yields of 65-98 %and in 50-85 %ee(17 examples). Three mechanistic contributions were identified to co-act favorably for high enantioselectivity:t he difference in binding constants to the chiral thioxanthone,t he smaller molecular distance in the complex of the minor enantiomer, and the lifetime of the intermediate 1,3-diradical.

Solid–liquid equilibria (SLE) of the l-menthol/thymol eutectic system were studied in detail using a combination of differential scanning calorimetry (DSC) and powder X-ray diffraction (XRD). The existence of two cocrystals with stoichiometric ratios 1:3 and 3:2 for l-menthol:thymol was monitored by performing XRD on samples of different compositions. Moreover, the existence of two solid solution regions of l-menthol in thymol and l-menthol in the 1:3 cocrystal was observed. The nonrandom two-liquid (NRTL) and two-suffix Margules models were applied to model the measured SLE data. The two eutectic points of the system were determined at T e1 cal = 271.7 K, x thymol,e1 cal = 0.48 and T e2 cal = 273.1 K, x thymol,e2 cal = 0.33. The complex character of the obtained phase diagram of the system shows that not all deep eutectic systems can be assumed to be of a simple eutectic type with immiscible solid phases. For the accurate determination of the SLE of eutectic systems, a combination of several experimental techniques and thermodynamic modeling is needed.