Amphiphilic Trithiols RC(CH₂SH)₃

Abstract

We report on the synthesis of the amphiphilic molecules 1,1,1-tris(mercaptomethyl)undecane and 1,1,1-tris(mercaptomethyl)dec-9-ene starting from the corresponding tris(hydroxymethyl) compounds. In addition we describe the synthesis of an amphiphilic phosphite ligand possessing only one alkyl chain. These compounds are potential substrates for the monolayer protection of 2D surfaces and nanoparticles.The enormous interest in solution-phase synthesized nanoparticles is a challenge not only for physicists and material scientists but also for chemists working in the field of molecular synthesis. A way to improve and adjust properties of such nanoparticles is the use of well-designed ligands. One major application for nanoparticles is the labeling of functionalized (bio)molecules, which is very often carried out with the help of thiolate groups binding to the nanoparticle surface. 1 However, thiolate ligands have been shown to easily undergo exchange reactions with free thiol functions 2a-f which are usually present in biological media. It is a general observation that compounds with polydentate ligands are thermodynamically and kinetically more stable than those with an equivalent number of comparable monodentate ligands. Thus, ligands of the type RC(CH 2 S -) 3 , where R is a sufficiently long (w-functionalized) alkyl chain, could make use of the chelate effect to bind nearly irreversibly to the 2D surface or to the nanoparticle. The methylene linker allows flexible alignment of the headgroups whereas the whole geometry bears a high packing density on surfaces.One possible approach to the synthesis of surfactants of the type RC(CH 2 SH) 3 is the Tollens reaction with longchain aldehydes which results in the formation of triols of the type RC(CH 2 OH) 3 . 3 The conversion of the alcohol group into other functional groups, however, goes along with some difficulties. The tosylation of the triol 1a could be carried out in good yields, but the tosylate itself is very unreactive. Kolomyjec et al. correlated the reduced reactivity of such tritosylates with the neopentyl unit present in this structure. 4 The halogenation of the long-chain alcohol under different conditions is either not complete or causes decomposition as already found by Huttner in the case of 1b. 5 Treating the triol 1a with PBr 3 6,7 or with a mixture of P(OPh) 3 and PhBr in a modified MichaelisArbuzov reaction 8 did not produce the bromide but the bicyclic phosphite 2. By the simple addition of PBr 3 to 1 in diethylether the phosphite 2 can be synthesized in good yields and purity. 9 Its structure enables a high packing density, 10 and the phosphite headgroup is a better donor than the oxygen atom in phosphine oxides often used in nanoparticle syntheses. In first experiments we could show that well-stabilized cobalt particles can be prepared using the neutral ligand 2. 11 A more elaborate and selective way of C-O activation is the Mitsunobu reaction. 12 This reaction is a versatile tool for the replacement of an alcohol group by any nucleophile. We used...