Symmetrical and Mixed Tris(acyl)phosphines: Synthesis, Oxidation and Photochemistry.

Herein, we present a convenient synthesis for symmetrical and mixed substituted tris(acyl)phosphines (TAPs) starting from red phosphorus. All TAPs exhibit a phosphaalkene-acylphosphine equilibrium, which was investigated in detail by variable-temperature (VT) NMR spectroscopy supported by density-functional theory (DFT) calculations. Depending on the substituents, two phosphaalkene derivatives and ten acylphosphine derivatives could be isolated. NMR spectroscopy and single-crystal X-ray crystallography enabled a clear structural assignment of these compounds. Oxidation of selected TAPs led to the formation of the corresponding tris(acyl)phosphine oxides (TAPOs). Furthermore, their spectroscopic properties as well as their photochemistry was investigated. Especially, the TAPO compounds were evaluated for their suitability as photoinitiators by CIDNP spectroscopy, photobleaching measurements and by storage stability tests.

Symmetrical and Mixed Tris(acyl)phosphines: Synthesis, Oxidation and Photochemistry.

Invited for the cover of this issue are the groups of M. Haas, G. Gescheidt and H. Grützmacher from the Graz University of Technology and the ETH Zürich. The image depicts a phosphorus mine, where the workers are acid chlorides using their shovels and red phosphorus to provide the chemicals necessary to produce novel reagents. Read the full text of the article at 10.1002/chem.202302535.

Synthesis and Characterization of a Variety of α,ω-Bisacylpolysilanes-A Study on Reactivity and Accessibility.

In this study, a variety of α,ω-bisacylpolysilanes were synthesized via two synthetic protocols. The first method for obtaining these compounds is based on the substitution reaction of bromine either on silica gel or by the use of silver salts. Surprisingly, instead of the expected bromine substitution product PhC(O)(SiMe2)2C(O)Ph 4a, we found the formation of the diastereomer PhC(O)(SiMe2)2CBrPhOCBrPh(SiMe2)2C(O)Ph 4b indicating a more complex reaction cascade. On the other hand, the phenylated compound 3b yielded the expected bromine substitution product PhC(O)(SiPh2)2C(O)Ph 4c. For the second protocol, we utilized the Corey-Seebach approach to isolate other representatives of this compound class. We found that the substituents at the α-silicon atoms influence the selectivity of the dethioketalization. While the ethylated and phenylated disilanes 5b,c yield the expected bisacyldisilanes 6a,b, the methylated disilane 4a undergoes a BF3-induced Si-Si bond breakage followed by an intermolecular sila-aldol reaction. This hitherto unknown sila-aldol reaction results in the formation of the enantiomer PhC(O)SiMe2C(OMe)PhSiMe2F 6c in excellent yields. All isolated compounds were analyzed by a combination of NMR spectroscopy, ultraviolet-visible (UV-vis) spectroscopy, single-crystal X-ray crystallography, and mass spectrometry. Furthermore, the photochemical pathways of two representative examples (4b,c) were examined.

Do germanium-based photoinitiators have the potential to replace the well-established acylphosphine oxides?

In the last few decades, there has been an increasing demand for photoinitiators with growing requirements. Nowadays, photoinitiators need to fulfill several requirements such as a low level of toxicity, biocompatibility, fast polymerization rates, high activities, good photobleaching and much more in order to remain competitive on the market. Accordingly, we compare acylphosphine oxides and acylgermanes, two common classes of photoinitiators, with respect to their various synthetic pathways, toxicity, availability and performance.

Synthesis of Stable Dianionic Cyclic Silenolates and Germenolates.

In this contribution a convenient synthetic method to obtain the previously unknown dianionic cyclic silenolates and germenolates is described. These dianions 2a,b and 4a,b are easily accessible via a one-pot synthetic protocol in high yields. Their structural properties were analyzed by a combination of NMR, single-crystal X-ray crystallography, and DFT quantum mechanical calculations. Moreover, the reactivity of 2a,b and 4a,b with selected examples of electrophiles was investigated. 2a and 4a were reacted with ClSiiPr3 to give new examples of polysilanes and polygermanes with exocyclic double bonds. The reaction of 2b with ClSiMe2SiMe2Cl led to the formation of the acyl bicyclo[2.2.2]octasilane 6. Moreover, the reaction of 2a,b and 4a,b with MeI, as an example of a carbon-centered electrophile, led to selective alkylation reactions at the negatively charged silicon and germanium atoms. The corresponding methylated structures 9a,b and 10a,b were formed in nearly quantitative yields. The competitive reactivity of the silyl and silenolate anion toward 1 equiv of ClSiMe3 showed that the outcome of the reaction was strongly influenced by the substituent at the carbonyl moiety. 2a reacted with 1 equiv of ClSiMe3 to give the corresponding cyclic silenolate S 1 a, which demonstrated that the silyl anion is more nucleophilic than the silenolate with attached aromatic groups. 2b, on the other hand, reacted with 1 equiv of ClSiMe3 to give the bicyclic compound 11 via an intramolecular sila-Peterson alkenation reaction. These findings clearly showed that the alkyl-substituted silenolate is more nucleophilic than the silyl anion. This paper demonstrates that 2a,b and 4a,b have the potential to be used as unique building blocks for complex polysilane and polygermane frameworks.