Synthesis of Mixed‐Functionalized Tetraacylgermanes

Abstract

Tetraacylgermanes are known as highly efficient photoinitiators. Herein, the synthesis of mixed tetraacylgermanes 4 a–c and 6 a–e with a nonsymmetric substitution pattern is presented. Germenolates are crucial intermediates of these new synthetic protocols. The synthesized compounds show increased solubility compared with symmetrically substituted tetraacylgermanes 1 a–d. Moreover, these mixed derivatives reveal broadened n–π* absorption bands, which enhance their photoactivity. Higher absorption of these new compounds at wavelengths above 450 nm causes efficient photobleaching when using an LED emitting at 470 nm. The quantum yields are in the range of 0.15–0.57, depending on the nature of the aroyl substituents. On the basis of these properties, mixed‐functionalized tetraacylgermanes serve as ideal photoinitiators in various applications, especially in those requiring high penetration depth. The synthesized compounds were characterized by elemental analysis, IR spectroscopy, NMR and CIDNP spectroscopy, UV/Vis spectroscopy, photolysis experiments, and X‐ray crystallography. The CIDNP data suggest that the germyl radicals generated from the new tetraacylgermanes preferentially add to the tail of the monomer butyl acrylate. In the case of 6 a–e only the mesitoyl groups are cleaved off, whereas for 4 a–c both the mesitoyl and the aroyl group are subject to α‐cleavage.

Synthesis, LIFDI Mass Spectrometry and Reactivity of Triacyl‐Germenolates

“…Compound 4 d crystallized in the monoclinic space group P 2 1 / c , and the unit cell contains two molecules. Structural data are in accordance with literature values of other acyl germanium compounds [2,5–7] . The torsion angle between the C=O groups and the aromatic ring planes is a structural feature, which significantly influences the optical properties.…”

“…Based on the accessibility of acylgermanium based radical anions, we could also show that the reduction of tetraacylgermane 1 provides a synthetically attractive reaction pathway towards triacylgermenolate 2 (Scheme 2 SET‐Approach) [5,6] . We could furthermore establish a direct approach towards triacylgermenolate 2 , which circumvents the usage of alkali metals (Scheme 2 Direct Approach).…”

Isolable Geminal Bisgermenolates: A New Synthon in Organometallic Chemistry

“…To explore if the acylgermanes synthesized via the procedure shown in Scheme 3 are suitable photoinitiators, we performed photo CIDNP experiments [23] with 7 g and 7 f . The corresponding 1 H‐NMR/CIDNP spectra in the presence of butyl acrylate clearly reveal the growth of polymer chains with Ge‐substituted end groups based on the expected α‐cleavage of Ge−C(O) bonds upon n–π* excitation via intersystem crossing (see Supporting Information) [7, 10, 22] …”

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

“…However, not to be ignored is the low abundance of germanium in the Earth's crust, which leads to significantly higher costs of germanium-based PIs. [11][12][13] Both compound classes, the acylphosphine oxides as well as the germanium-based PIs follow a Norrish type-I cleavage mechanism upon light irradiation generating radicals and initiating the polymerization process. In the case of the former, a benzoyl-and a phosphinoyl-radical 14 is generated, while in the case of the latter, a benzoyl-and a germyl-radical 15 is generated.…”

“…In 2017 tris- and tetraacylgermanes were synthesized and implemented as long-wavelength PIs. 12,39 During the course of these studies, a synthetic protocol allowing a straightforward access to these highly desirable compounds was developed (see Scheme 12 ). Here, the reaction of a tristrimethylsilyl-substituted germanide with 4 equivalents of acid fluorides leads to the formation of tetraacylgermanes via a multiple silyl abstraction methodology.…”