CH activation in flow by Dr. Tomothy Noel-June 2017
A Modular Flow Design for the meta-Selective C−H Arylation of Anilines
Described herein is an effective and practical modular flow design for the meta-selective C−H arylation of anilines. The design consists of four continuous-flow modules (i.e., diaryliodonium salt synthesis, meta-selective C−H arylation, inline copper extraction, and aniline deprotection) which can be operated either individually or consecutively to provide direct access to meta-arylated anilines. With a total residence time of 1 hour, the desired product could be obtained in high yield and excellent purity without the need for column chromatography, and the residual copper content meets the standards for parenterally administered pharmaceutical substances.
Merger of Visible-Light Photoredox Catalysis and C–H Activation for the Room-Temperature C-2 Acylation of Indoles in Batch and Flow
A mild and versatile protocol for the C–H acylation of indoles via dual photoredox/transition-metal catalysis was established in batch and flow. The C–H bond functionalization occurred selectively at the C-2 position of N-pyrimidylindoles. This room-temperature protocol tolerated a wide range of functional groups and allowed for the synthesis of a diverse set of acylated indoles. Various aromatic as well as aliphatic aldehydes (both primary and secondary) reacted successfully. Interestingly, significant acceleration (20 to 2 h) and higher yields were obtained under micro flow conditions.
Laboratory of Organic Chemistry, Department of Chemistry, National and Kapodistrian University of Athens, Panepistimiopolis, 15771 Athens, Greece
The omnipresence of C-H bonds in organic compounds renders them highly attractive targets for the installation of functional groups towards the construction of valuable molecular scaffolds. Consequently, C-H activation has extended beyond scientific curiosity and has evolved from being a concept of fundamental interest to constituting an important, modern tool of organic synthesis. The intensity of research efforts and accompanying discussion surrounding this topic has given rise to a plethora of innovative, cutting-edge advancements. These advancements demonstrate the vast potential of the C-H activation approach regarding the design of highly efficient and selective catalytic methodologies for the synthesis of fine chemicals, natural products, and advanced materials. However, the overall sustainable nature of this approach, emanating from some of its main attributes such as atom- and step-economy, is compromised by the frequent need of homogeneous catalysts based on rare, expensive, and even toxic noble transition metals. In order to address this issue and achieve truly sustainable catalytic C-H activation, significant research efforts have focused on the development of homogeneous catalytic systems based on more abundant, first row transition metals. In this respect, various catalytic protocols involving the use of highly abundant, inexpensive, readily available, and also biorelevant metals such as Mg, Ca, Mn, Fe, Cu, and Zn have been elegantly developed in recent years. Catalysts based on the aforementioned sustainable metals exhibit unique behavior in terms of reactivity/selectivity and their use does not only provide an alternative to noble metal catalysis, but also expands the scope of C-H activation. The present review provides a comprehensive examination of selected works that highlight the evolution and growing importance of this merge of two vibrant concepts in modern organic synthesis: sustainable metal catalysis and C-H activation.