Electrochemical reduction of dehydroamino acids: Synthesis and photophysical properties of β,β-diarylalanines Paula M.T. Ferreira a ,* Luís S. Monteiro a, * Elisabete M.S. Castanheira b , Goreti Pereira a , Carla Lopes a and Helena Vilaça a a Chemistry Centre (CQ-UM), University of Chemistry, Campus de Gualatr, 4710-057 Braga, Portuga b Centre of Physics (CFUM), University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal ——— * Corresponding author. Tel.: +0-351-253604055; fax: +0-351-253-604382; e-mail: pmf@quimica.uminho.pt ; monteiro@quimica.uminho.pt 1. Introduction The reduction of an isolated double bond is not possible using electrochemical methods, however when the double bond is conjugated with an electron withdrawing group such as a carbonyl it becomes reducible in the accessible potential range. 1 This can be explained by the fact that conjugation lowers the energy of the lowest unoccupied π-orbital which results in a less negative reduction potential. There are a few reports on the electrochemical reduction of activated alkenes such as α,β- unsaturated ketones. 2 In these studies it was demonstrated that the electrochemical reduction of unsaturated ketones involves an initial electron transfer to the ketone producing a radical anion which undergoes protonation followed by other chemical or electrochemical processes. 2 Thus, the radical anion may dimerize or it can be reduced to give the saturated ketone. 1,2 For some years now we have been interested in the synthesis of dehydroamino acid derivatives and in using these compounds as substrates in several types of reactions. 3 As part of this work we have been studying the electrochemical behaviour of dehydroamino acid derivatives. Thus, it was possible to develop an efficient method for the synthesis of 2,5- diaminoadipic acid derivatives by controlled potential electrolysis of N,N-diprotected dehydroalanines. 4 Recently we investigated the electrochemical behaviour of β- halodehydroamino acids and found that these compounds show higher reduction potentials when compared with those of the corresponding dehydroamino acids. 5 Controlled potential electrolysis of the former afforded the E-isomers of the dehalogenated dehydroamino acids. Although there are many reports concerning the chemical reduction of α,β- dehydroamino acids, 6 to the best of our knowledge the electrochemical reduction of β,β-disubstituted dehydroamino acids has not yet been described. Since dehydroamino acids can be considered activated alkenes we decided to study the electrochemical reduction of β,β-disubstituted dehydroalanines obtained by a Suzuki-Miyaura cross-coupling from β- halogenated dehydroamino acids and arylboronic acids. The photophysical properties of some of the saturated amino acids prepared were evaluated in solvents of different polarity and compared with those of the corresponding β,β- diaryldehydroalanines. ARTICLE INFO ABSTRACT Article history: Received Received in revised form Accepted Available online Keywords: Dehydroamino acids Reduction Cyclic voltammetry Electrolysis β,β-diarylalanines Several β,β-disubstituted dehydroalanines were prepared from β,β-dibromo or β-bromo, β- substituted dehydroalanines and aryl boronic acids using a Suzuki-Miyaura cross-coupling reaction. The electrochemical behaviour of these compounds was studied by cyclic voltammetry. All compounds studied showed similar reduction potentials and these were similar to the peak potential of the methyl ester of N-tert-butoxycarbonyl dehydrophenylalanine. Thus, the presence of a second aryl moiety in the dehydroalanine scaffold does not significantly change the reduction potential. Controlled potential electrolysis were preformed at the cathodic peak potential in the presence of triethylammonium chloride as proton donor. The only products isolated in good to high yields were the corresponding β,β-diarylalanines. This reaction was also carried out using a dipeptide containing a β,β-diaryldehydroalanine to give a 1:1 diastereomeric mixture of the reduction product. The photophysical properties of two of the β,β-diaryldehydroalanines and of the corresponding β,β-diarylalanines were studied in three solvents of different polarity. The β,β- diaryldehydroalanines show low fluorescent quantum yields (ΦF < 9%) due to the conjugation of the aromatic moieties with the α,β-double bond and with the carbonyl group, which favours the non-radiative deactivation pathways. The absence of conjugation in the reduction products leads to a significant increase in the fluorescence quantum yields. These results show that the β,β- disubstituted alanines could be used as fluorescent markers. 2009 Elsevier Ltd. All rights reserved.