Synthesis of Chiral Poly(silyl ether)s via CuH-Catalyzed Asymmetric Hydrosilylation Polymerization of Diketones with Silanes

Poly(silyl ether)s as Degradable and Sustainable Materials: Synthesis and Applications

Polymer research is currently focused on sustainable and degradable polymers which are cheap, easy to synthesize, and environmentally friendly. Silicon-based polymers are thermally stable and can be utilized in various applications, such as columns and coatings. Poly(silyl ether)s (PSEs) are an interesting class of silicon-based polymers that are easily hydrolyzed in either acidic or basic conditions due to the presence of the silyl ether Si-O-C bond. Synthetically, these polymers can be formed in several different ways, and the most effective and environmentally friendly synthesis is dehydrogenative cross coupling, where the byproduct is H2 gas. These polymers have a lot of promise in the polymeric materials field due to their sustainability, thermal stability, hydrolytic degradability, and ease of synthesis, with nontoxic byproducts. In this review, we will summarize the synthetic approaches for the PSEs in the recent literature, followed by the properties and applications of these materials. A conclusion and perspective will be provided at the end.

Iron-Catalyzed Alkoxylation, Dehydrogenative-Polymerization and Tandem Hydrosilylative-Alkoxylation

Alkoxylation, hydrosilylative-alkoxylation, and dehydrogenative-polymerization are some of the most widely used transformations in synthetic chemistry. However, these transformations are traditionally catalyzed by precious, and rare late-transition metals. Presented here is a molecularly defined iron complex that catalyzes alkoxylation, tandem hydrosilylative-alkoxylation, and dehydrogenative polymerization of silanes under mild conditions. The iron complex [Fe(CO)4 (H)(SiPh3 )] 1 catalyzes a direct Si-O coupling reaction between an array of silanes and alcohols to produce desired alkoxysilanes in excellent yield, with H2 as the only byproduct. The iron catalyst tolerates various functional groups and provides access to 20 alkoxysilanes, including essential molecules such as β-citronellol and cholesterol. Further, complex 1 catalyzes the polymerization of renewable diol and silane monomer to produce a renewable and degradable poly(isosorbide-silyl ether). Remarkably, complex 1 catalyzes a tandem hydrosilylative-alkoxylation of alkynes under mild conditions to yield unsaturated silyl ethers. The synthetic utility has been demonstrated by gram-scale alkoxylation and hydrosilylative-alkoxylation reactions.

Dehydrocoupling Polymerization: Poly(silylether) Synthesis by Using an Iron β-Diketiminate Catalyst

We describe the iron-catalyzed polymerizations of diol and silane monomers to obtain fourteen different poly(silylether) products with number average molecular weights (M_n ) up to 36.3 kDa. The polymerization reactions developed in this study are operationally simple and applicable to 1° and 2° silane monomer substrates and a range of benzylic and aliphatic diol substrates as well as one polyol example. The polymers were characterized by IR spectroscopy, DSC and TGA and, where solubility allowed, ¹ H, ¹³ C{¹ H}, ²⁹ Si{¹ H} NMR spectroscopies, GPC and MALDI-TOF were also employed. The materials obtained displayed low T_g values (-70.6 to 19.1 °C) and were stable upon heating up to T_-5%,Ar 421.6 °C. A trend in T_-5%,Ar was observed whereby use of a 2° silane leads to higher T_-5%,Ar compared to those obtained using a 1° silane. Reaction monitoring was undertaken by in situ gas evolution studies coupled with GPC analysis to follow the progression of chain-length growth which confirmed a condensation polymerization-type mechanism.

Enantioselective Intermolecular C-H Silylation of Heteroarenes for the Synthesis of Acyclic Si-Stereogenic Silanes

Intermolecular C-H silylation for the synthesis of acyclic silanes bearing a silicon-stereogenic center in one enantiomeric form remains unknown to date. Herein, we report the first enantioselective intermolecular C-H silylation of heteroarenes for the synthesis of acyclic silicon-stereogenic heteroarylsilanes. This process undergoes a rhodium-catalyzed direct intermolecular dehydrogenative Si-H/C-H cross-coupling, giving access to a variety of acyclic heteroarylated silicon-stereogenic monohydrosilanes, including bis-Si-stereogenic silanes, in decent yields with excellent chemo-, regio-, and stereo-control, which significantly enlarge the chemical space of the optically active silicon-stereogenic monohydrosilanes.

Highly Efficient Synthesis of Poly(silylether)s: Access to Degradable Polymers from Renewable Resources

The design of new materials with tunable properties and intrinsic recyclability, derived from biomass under mild conditions, stands as a gold standard in polymer chemistry. Reported herein are platinum complexes which catalyze the formation of poly(silylether)s (PSEs) at low catalyst loadings. These polymers are directly obtained from dual-functional biobased building blocks such as 5-hydroxymethylfurfural (HMF) or vanillin, coupled with various dihydrosilanes. Access to different types of copolymer architectures (statistical or alternating) is highlighted by several synthetic strategies. The materials obtained were then characterized as low T_g materials (ranging from -60 to 29 °C), stable upon heating (T_-5% up to 301 °C) and resistant towards uncatalyzed methanolysis. Additionally, quantitative chemical recycling of several PSEs could be triggered by acid-catalyzed hydrolysis or methanolysis. These results emphasize the interest of biobased poly(silylether)s as sustainable materials with high recycling potential.

Polymer Skeletal Editing via Anionic Brook Rearrangements

This report communicates the first example of polymer backbone metamorphosis affected by anionic 1,2-Brook rearrangement of acyl silane moieties. Introduction of the acyl silane functionality into a polymer backbone was achieved via acyclic diene metathesis copolymerization (ADMET) of diene 1 and two dienes. We demonstrate that, using organolithium species and cyanide as nucleophiles, the backbones of resulting copolymers can be triggered to undergo highly efficient 1,2-Brook rearrangement, which transforms the poly(acyl silane)s into poly(silyl ether)s. Furthermore, the carbanion intermediate of the 1,2-Brook rearrangement can be intercepted by ketone electrophiles to give rise to polymers with quaternary stereogenic centers in the backbone and pendant functionality. Such structural editing of polymer backbones enables a new retrosynthetic paradigm for silicon-containing polymers that could not be accessed by traditional means.

Catalytic Enantioselective Dehydrogenative Si-O Coupling to Access Chiroptical Silicon-Stereogenic Siloxanes and Alkoxysilanes

A rhodium-catalyzed enantioselective construction of triorgano-substituted silicon-stereogenic siloxanes and alkoxysilanes is developed. This process undergoes a direct intermolecular dehydrogenative Si-O coupling between dihydrosilanes with silanols or alocohols, giving access to a variety of highly functionalized chiral siloxanes and alkoxysilanes in decent yields with excellent stereocontrol, that significantly expand the chemical space of the silicon-centered chiral molecules. Further utility of this process was illustrated by the construction of CPL-active (circularly polarized luminescence) silicon-stereogenic alkoxysilane small organic molecules. Optically pure bis-alkoxysilane containing two silicon-stereogenic centers and three pyrene groups displayed a remarkable g_lum value with a high fluorescence quantum efficiency (g_lum = 0.011, Φ_F = 0.55), which could have great potential application prospects in chiral organic optoelectronic materials.