Giacomo Strappaveccia

Flow approaches towards sustainability

Green chemistry and flow chemistry are ideal partners for accessing novel chemical spaces and define highly efficient synthetic tools. In this review article contributions have been selected according to the advantages offered in terms of features that are not immediately related to classic green metrics such as minimization of reaction time, optimization for time screening, waste minimization, safety improvement, process intensification and easy scale up, energy and cost efficiency. Such features make processes in flow highly interesting in terms of developing a green and sustainable chemistry.

A biomass-derived safe medium to replace toxic dipolar solvents and access cleaner Heck coupling reactions

γ-Valerolactone (GVL) is an excellent reaction medium, derived from biomasses, which can replace classic dipolar aprotic media such as DMF or NMP. In this contribution, we have investigated the use of GVL in the palladium-catalyzed Heck reaction and accessed the clean synthesis of several small molecules. Furthermore, we optimized a GVL-based protocol to synthesize a poly(phenylenevinylene) (PPV) derivative in high yields and purity, and with very low Pd-content. Finally, we demonstrated that GVL is superior to conventional dipolar media in terms of controlling palladium impurities, which may influence the performance of certain optoelectronic devices such as organic solar cells and field-effect transistors.

γ-Valerolactone as an alternative biomass-derived medium for the Sonogashira reaction

γ-Valerolactone (GVL) can be used as an efficient and practical alternative to the banned and commonly used dipolar aprotic solvents. In this contribution GVL has been used as a non-toxic, biodegradable, biomass-derived medium, for the definition of a simple and general protocol for the Sonogashira cross-coupling reaction. The chemical efficiency of GVL as a medium is excellent and the best results have been obtained using DABCO as the base allowing the isolation of products 3a–q in 60–96% yields. These results represent an example which proves that biomass-derived safer solvents can be used efficiently in common transformations reaching higher greenness/sustainability as well as high chemical efficiency.

Efficient synthesis of cyanohydrin trimethylsilyl ethers via 1,2-chemoselective cyanosilylation of carbonyls

Here we report a sustainable protocol for the cyanosilylation of carbonyl compounds 1a–g and 3a–m using trimethylsilyl cyanide and triphenylphosphine supported on polystyrene as a catalyst under solvent-free conditions. It has been shown that a small amount of the catalyst allows the chemoselective 1,2-addition of trimethylsilyl cyanide to α,β-unsaturated carbonyls 1a–g (5 mol%) and to saturated carbonyls 3a–m (2 mol%). The preparation of cyanohydrin trimethylsilyl ethers 2a–g and 4a–m has been accomplished in good yields (72–99%) and very low E-factor values (5–10). Finally, efficiency has been further improved by setting two different flow procedures that have allowed us to perform the representative preparation of cyanohydrin trimethylsilyl ether 4a on a large scale and with the E-factor of 0.16 or 0.47 consisting in a reduction of 90 or 72% of waste compared to our batch conditions.