Sunscreen and Ruthenium Olefin Metathesis Catalysts: A One-Pot, Two-Step Photochemical Synthesis of Coumarins

We have recently demonstrated a novel type of chromatic selectivity by exploiting differences in molar absorption coefficients. Thus, selective removal of photolabile protecting groups (PPGs) using internal or external "sunscreens" was achieved using a single light source.¹ Herein, the sunscreen methodology is applied to the catalytic photo-induced cross metathesis (CM) reaction of 2-vinyl phenol derivatives protected with 2-nitrobenzyl PPG. 2-vinyl phenols are impractical for olefin metathesis reactions due to stable chelate formation with the ruthenium catalyst.^2-4 By protection of the phenol moiety with a 2-nitrobenzyl PPG we can exploit the large difference in the molar absorption coefficients at 380 nm UV light between the ruthenium catalysts developed by our group^5-7 and the 2-nitrobenzyl chromophore, to selectively activate the ruthenium catalyst without removal of the 2-nitrobenzyl PPG in the presence of an external sunscreen solution. Thus, we can now carry out a light triggered cross metathesis (CM) reaction of the acrylate esters and 2-nitrobenzyl protected 2-vinyl phenols in 380 nm light, in the presence of an external solution of pyrene carboxaldehyde as a sunscreen. While subsequent irradiation of the reaction vessel with 254 nm light after removal of the external sunscreen prompts a chain of three reactions to yield the coumarin derivatives.

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