Real-Time Monitoring of a Sol-Gel Reaction for Polysilane Production Using Inline NIR Spectroscopy.

The sol-gel process is an effective method for the preparation of homogeneous structured nanomaterials whose physico-chemical properties strongly depend on the experimental conditions applied. The control of a three-component reaction with silanes showing multiple reaction sites revealed the need for an analytical tool that allows a rapid response to ongoing transformations in the reaction mixture. Herein, we describe the implementation of near-infrared (NIR) spectroscopy based on compact, mechanically robust, and cost-efficient micro-optomechanical system technology in the sol-gel process of three silanes with a total of nine reaction sites. The NIR-spectroscopically controlled reaction yields a long-time stable product with reproducible quality, fulfilling the demanding requirements for further use in coating processes. 1H nuclear magnetic resonance measurements are used as reference values for the calibration of a partial least squares (PLS) regression model. The precise prediction of the desired parameters from collected NIR spectroscopy data acquired during the sol-gel reaction proves the applicability of the calibrated PLS regression model. The determined shelf-life and further processing tests verify the high quality of the sol-gel and the produced highly cross-linked polysilane.

Trisubstituted Highly Activated Benzo[d]thiazol-2-yl-sulfone-Containing Olefins as Building Blocks in Organic Synthesis.

In this paper, we report the formation of highly electrophilic 1,1-deactivated olefins, their use as novel synthetic building blocks, and their transformation to structurally diverse molecular scaffolds. Synthesis of 1,1-deactivated olefins substituted with a BT-sulfonyl group and a carbonyl or nitrile, respectively, consists of unusual Ti(OPri)4-mediated Knoevenagel-type condensation and proceed in good to excellent yields. Generated olefins can be further transformed in a highly stereoselective manner and in good yields to various polyfunctionalized heterocycles and acyclic molecular scaffolds. Overall, the obtained structures are accessed in two to four steps starting from the (mostly) commercially available aldehydes. In addition, the presence of the BT-sulfonyl group in prepared structures allows for further chemoselective functionalization/post-synthetic transformations to provide structurally diverse final compounds.

Formal (4+1)-Cyclization of Ammonium Ylides with Vinylogous para-Quinone Methides.

The formal (4+1)-cyclization between in situ generated achiral carbonyl-stabilized ammonium ylides and vinylogous p-quinone methides can be carried out under operationally simple conditions, leading to two different double bond regioisomers depending on the used conditions. Besides these racemic approaches, also a first proof-of-concept for enantioselective versions of these reactions has been obtained.

Ammonium Ylide Mediated Cyclization Reactions.

The use of readily accessible ammonium ylides for (asymmetric) transformations, especially cyclization reactions, has received considerable attention over the past two decades. A variety of highly enantioselective protocols to facilitate annulation reactions have recently been introduced as an alternative to other common methods including S-ylide-mediated strategies. It is the intention of this short review to provide an introduction to this field by highlighting the potential of ammonium ylides for (asymmetric) cyclization reactions as well as to present the limitations and challenges of these methods.

Synthesis of Trifluoroacetyl-Substituted Cyclopropanes Using Onium Ylides.

The use of carbonyl-stabilized ammonium- and sulfonium ylides allows for the synthesis of highly-functionalized trifluoroacetyl-substituted cyclopropanes. It turned out that the diastereoselectivities strongly depend on the nature of the chosen ylide and the employed base. The products could be obtained in good yields under operationally simple conditions.

Enantioselective Spirocyclopropanation of para-Quinone Methides Using Ammonium Ylides.

The use of Cinchona alkaloid-based chiral ammonium ylides allows for the first highly enantioselective and broadly applicable spirocyclopropanation reactions of para-quinone methides. This strategy provides a straightforward protocol toward the chiral spiro[2.5]octa-4,7-dien-6-one skeleton, which is a frequently found structural motif in important biologically active molecules.

Asymmetric Synthesis of 2,3-Dihydrobenzofurans by a [4+1] Annulation Between Ammonium Ylides and In Situ Generated o-Quinone Methides.

A highly enantio- and diastereoselective [4+1] annulation between in situ generated ammonium ylides and o-quinone methides for the synthesis of a variety of 2,3-dihydrobenzofurans has been developed. The key factors controlling the reactivity and stereoselectivity were systematically investigated by experimental and computational means and the energy profiles obtained provide a deeper insight into the mechanistic details of this reaction.

Benzylic Ammonium Ylide Mediated Epoxidations.

A high yielding synthesis of stilbene oxides using ammonium ylides has been developed. It turned out that the amine leaving group plays a crucial role as trimethylamine gives higher yields than DABCO or quinuclidine. The amine group also influences the diastereoselectivity, and detailed DFT calculations to understand the key parameters of these reactions have been carried out.

Towards a General Understanding of Carbonyl-Stabilised Ammonium Ylide-Mediated Epoxidation Reactions.

The key factors for carbonyl-stabilised ammonium ylide-mediated epoxidation reactions were systematically investigated by experimental and computational means and the hereby obtained energy profiles provide explanations for the observed experimental results. In addition, we were able to identify the first tertiary amine-based chiral auxiliary that allows for high enantioselectivities and high yields for such epoxidation reactions.