Efficient and Systematic Click-Based Synthetic Routes to Amino Acid Functionalized Metallatricarbadecaboranes

Abstract

A general method for the systematic syntheses of amino acid functionalized metallatricarbadecaboranyl complexes has been developed that employs the copper-catalyzed click addition reactions of N-azidoacetyl amino acid methyl esters to a p-ethynylphenyl substituent at a cage carbon of the metallatricarbadecaboranyl cage. The trimethylsilyl-protected p-ethynylphenyl Li+[6-(p-((CH3)3SiC≡C)C6H4)-nido-5,6,9-C3B7H9–] (1) tricarbadecaboranyl anion was synthesized via the reaction of arachno-4,6-C2B7H12– with p-((CH3)3SiC≡C)C6H5CN. Reaction of 1 with (η5-C5H5)Fe(CO)2I and (η5-C5H5)Ru(CH3CN)3PF6 produced 1-(η5-C5H5)-2-(p-((CH3)3SiC≡C)C6H4)-closo-1,2,3,4-MC3B7H9 (M = Fe (2), Ru (3)). Deprotection of 2 and 3 with K2CO3 in MeOH/THF afforded the ethynyl derivatives 1-(η5-C5H5)-2-(p-(HC≡C)C6H4)-closo-1,2,3,4-MC3B7H9 (M = Fe (4), Ru (5)). Click addition of the model compound benzyl azide to 4 and 5 to form the triazole complexes, 1-(η5-C5H5)-2-(p-(1-(C6H5CH2)-1H-1,2,3-N3C2-4-)C6H4)-closo-1,2,3,4-MC3B7H9 (M = Fe (6), Ru (7)) confirmed the reactivity of the p-ethynyl group toward copper-catalyzed click azide addition, with the triazole-linked structure of 7 crystallographically established. Subsequent click addition reactions of the N-azidoacetyl amino acid methyl esters to 4 yielded a wide range of amino acid functionalized ferratricarbadecaboranyl complexes: 1-(η5-C5H5)-2-(p-(1-(Xaa-C(O)CH2)-1H-1,2,3-N3C2-4-)C6H4)-closo-1,2,3,4-FeC3B7H9 (Xaa = PheOMe (8), LeuOMe (9), ValOMe (10), MetOMe (11), AlaOMe (12), TryOMe (13)).