α-Photooxygenation of chiral aldehydes with singlet oxygen

Organocatalytic α-oxygenation of chiral aldehydes with photochemically generated singlet oxygen allows synthesizing chiral 3-substituted 1,2-diols. Stereochemical results indicate that the reaction in the presence of diarylprolinol silyl ethers is highly diastereoselective and that the configuration of a newly created stereocenter at the α-position depends predominantly on the catalyst structure. The absolute configuration of chiral 1,2-diols has been unambiguously established based on electronic circular dichroism (ECD) and TD-DFT methods.


Photochemical reactions -equipment
Photochemical reactions were performed in homemade photoreactors with LED bulbs, LED tapes or high power single LED depending on the scale and the colour of light required.
Photooxygenation reactions performed on a scale larger than 0.5 mmol were conducted in a PCV box with two warm white LED bulbs. To assure constant flow of oxygen, effective cooling, and mixing, a three-neck round-bottom flask was equipped with a 'cold finger', capillary and outer joint ( Figure S1a).
Aldehyde 1 was obtained according to the Melchiorre method [S1] in a violet LED stripes photoreactor or in reactor with eight violet 1 W LEDs (Figure S1b or c) depending on the scale. Aldehydes 2 and 3 were obtained according to the MacMillan methods [S2,S3] using violet or blue LED tape photoreactors, respectively ( Figure S1b).
Small-scale photooxygenation reactions were conducted in vials in a photoreactor with green high power LED. A coolant circulated inside the aluminium block allowed to control the reaction temperature, while constant flow of the oxygen was provided by teflon capillaries ( Figure S1d).

Additional experiments conducted in order to find the cause of decrease in efficiency of the 'one-pot' process
In a 'one-pot' β-benzylation and α-oxygenation process, both yields and selectivities were lower than that observed in the two-step experiment. In this section we describe experiments performed in order to find out which reagents or solvents present in the reaction mixture after the first step have a negative effect on subsequent photooxygenation (Table S1, entries 1-3). These results were compared with the analogous α-photooxygenation with the most efficient catalyst -ether 17 preformed in absence and presence of buffer (Table S1, entry 4 and 5, also Table 1, entry 3 in the article). Table S1. Parameters influencing the yield and stereoselectivity of the α-photooxygenation of 3,4-diphenylbutanal (1).