Semiconductors as sensitisers for the radical addition of tertiary amines to electron deficient alkenes

Siniša Marinković,Norbert Hoffmann

doi:10.1155/s1110662x03000308

Abstract

Using heterogeneous photocatalysis, the radical addition of tertiary amines with electron deficient alkenes can be performed in high yields (up to 98%) and high facial diastereoselectivity. The photochemical induced electron transfer process initiates the radical chain reaction and inorganic semiconductors likeTiO2and ZnS were used. According to the proposed mechanism, the reaction takes place at the surface of the semiconductor and the termination step results from an interfacial electron transfer from the conduction band to the oxoallyl radical intermediate. Frequently, semiconductors are used for the mineralisation of organic compounds in wastewater. However, in this case, they are used in organic synthesis. The process can be performed in a convenient way and is particularly interesting from the ecological and economical point of view. No previous functionalization of the tertiary amines is necessary for C − C bond formation. Further on, the amines are used both as reactant and as solvent. The excess is recycled by distillation and the inexpensive sensitiser can be easily removed by filtration. In this way, products of high interest for organic synthesis are obtained by a diastereoselective radical reaction.

Highlights

Radical reactions have become an important tool in organic chemistry, but the control of reactivity and selectivity still needs considerable efforts [1]
We wondered whether the electron hole h+ of the valence band could be filled by an interfacial electron transfer from a reductive species such as a tertiary amine
The same selectivity and the high yields observed, as in the case of the homogeneous photocatalysis, lead us to conclude that the initiation step occurred at the surface via an interfacial electron transfer followed by a deprotonation

Summary

INTRODUCTION

Radical reactions have become an important tool in organic chemistry, but the control of reactivity and selectivity still needs considerable efforts [1]. We described an efficient procedure involving a photochemical electron transfer to initiate the intermolecular radical addition of tertiary amines to electron deficient alkenes [6, 7]. This method uses homogeneous photocatalysis with electron donor substitued aromatic ketones as sensitisers. We wondered whether the electron hole h+ of the valence band could be filled by an interfacial electron transfer from a reductive species such as a tertiary amine In this way, a radical-cation might be produced and after deprotonation a nucleophilic α-aminoalkyl radical should be obtained

RESULTS AND DISCUSSION

H O O OMen

CONCLUSION