Merging Grubbs second-generation catalyst with photocatalysis enables Z-selective metathesis of olefins: scope, limitations, and mechanism

Olefin cross-metathesis is a cornerstone reaction in organic synthesis where stereoselectivity is typically governed by the structure of the catalyst. In this work, we show that merging Grubbs second generation catalyst, a classical E-selective catalyst, with a readily available photocatalyst, enables the exclusive formation of the contra-thermodynamic Z-isomer. The scope and limitations of this unprecedented approach are discussed based on both computational and experimental mechanistic data.

The emission maximum of the light source used is 420 nm ( Figure S1).
Chromatography fractions and stated reactions were monitored by TLC on Merck silica gel 60 F254 aluminum plates. The spots were visualized under UV light at 254 nm. UV-Visible spectra were recorded on a Perkin Elmer Lambda 40 Spectrophotometer. High resolution mass spectrometry (HRMS) analyses were performed on a Q Exactive Mass Spectrometer (Thermofisher) using direct injection. 1 H NMR (300 MHz); 19 F (282 MHz) and 13  Transient absorption spectra were measured with an LP920-KS spectrometer from Edinburgh Instruments.

2-4-General procedure for metathesis reaction (Cross metathesis 8r-8zf):
A flame-dried quartz Schlenk tube was charged under argon with styrene (1.0 eq) and dichloromethane (0.05 M). Next, alkyl acrylate (2.0 to 5.0 eq), Grubbs 2 nd generation catalyst S14 (2 mol %) and 4CzIPN (5 mol %) were added in one portion. The Schlenk tube was closed with a cap and then it stirred at 40 °C under argon for 24h and irradiated at 420 nm. The solvent was removed under reduced pressure and the residue was purified by flash chromatography (silica gel, pentane/Ethyl acetate 98/2) to provide the compound.
Purification (Colorless solid, 64 mg, 0.39 mmol, 56% (Z isomer)). 1    3.83 (s, 3H). 13 Table S1.      Kohn-Sham formalism was used for ground states, while its unrestricted analogue was used for triplet states and oxidized states. SCF convergence was achieved using the DIIS algorithm followed by a semi-quadratic SOSCF converger. The RIJCOSX approximation was applied for all calculations using the def2/J auxiliary basis sets. 12 Frequency calculations were run at the same level of theory. No imaginary frequencies were obtained, except in rare cases in which there was one imaginary frequency originating from methyl rotation (ca -80 cm -1 ).

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In the TD-DFT calculations, 50 singlets and 20 triplets were computed in the framework of the