Regioselective Formation of 3-Selanyl-3-siloxyoxetanes in the Paternò-Büchi Reaction of Silyl O,Se-Ketene Acetals (O,Se-SKA)

Abstract

The photochemical [2+2] cycloaddition (Paternò-Büchi reaction) of silyl O,Se-ketene acetal (SKA) 2 with aromatic aldehyde 3 affords regioselectively 3-selanyl-3-siloxyoxetane 4 (trans:cis ≈ 65:35-85:15) in good to high yields under the aldehyde excitation conditions (hv > 320 nm). A 70:30 mixture of the regioisomers 4aa (trans:cis = 50:50) and 5aa (= 6a), however, is produced in the 9,10-diphenylanthracene (DPA)-sensitized photoreaction (hv > 400 nm) of 2a and 3a. A 1,4-diradical 1,4-DR is proposed for the n-π* excited aldehyde reaction, which regioselectively produces the 3-selanyloxetane 4. The regio-random formation of the oxetanes 4 and 5 (= 6) under the DPA-sensitized conditions is reasonably rationalized by the intervention of radical ion intermediate RI.

Key words

Paternò-Büchi reaction - photochemical [2+2] cycloaddition - oxetane - regioselectivity

References and Notes

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11

Although the irreversible oxidation peaks of 2a,b (2a: E_p ^ox = 1.33 V versus SCE; 2b: E_p ^ox = 1.29 V versus SCE in CH₃CN, tetraethylammonium perchlorate as a supporting electrolyte) were observed, the signals were broad; thus, the values are not accurate for determining the oxidation potentials.

12

The prolonged irradiation led to some decomposition of oxetanes.

17

The fluorescence of DPA was actually quenched by p-cy-anobenzaldehyde 2a (k_q = 1.90 × 10¹⁰ s^-1M^-1), whose rate constant was determined by Stern-Volmer plot.