Functionalizing Thiosemicarbazones for Covalent Conjugation

Abstract

Thiosemicarbazones (TSCs) with their modular character (thiosemicarbazides + carbonyl compound) allow broad variation of up to four substituents on the main R¹R²C=N(1)-NH-C(S)-N(4)R³R⁴ core and are thus interesting tools for the formation of conjugates or the functionalization of nanoparticles (NPs). In this work, di-2-pyridyl ketone was introduced for the coordination of metals and 9-anthraldehyde for luminescence as R¹ and R² to TSCs. R³ and R⁴ substituents were varied for the formation of conjugates. Amino acids were introduced at the N4 position to produce [R¹R²TSC-spacer-amino acid] conjugates. Further, functions such as phosphonic acid (R-P(O)(OH)₂), D-glucose, o-hydroquinone, OH, and thiol (SH) were introduced at the N4 position producing [R¹R²TSC-spacer-anchor group] conjugates for direct NP anchoring. Phenyl, cyclohexyl, benzyl, ethyl and methyl were used as spacer units. Both phenyl phosphonic acid TSC derivatives were bound on TiO₂ NPs as a first example of direct NP anchoring. [R¹R²TSC-spacer-end group] conjugates including OH, S-Bn (Bn = benzyl), NH-Boc (Boc = tert-butyloxycarbonyl), COOtBu, C≡CH, or N₃ end groups were synthesized for potential covalent binding to functional molecules or functionalized NPs through amide, ester, or triazole functions. The synthesis of the thiosemicarbazides H₂NNH-C(S)-NR₃R₄ starting from amines, including amino acids, SCCl₂ or CS₂, and hydrazine and their condensation with dipyridyl ketone and anthraldehyde led to 34 new TSC derivatives. They were synthesized in up to six steps with overall yields ranging from 10 to 85% and were characterized by a combination of nuclear magnetic resonance spectroscopy and mass spectrometry. UV-vis absorption and photoluminescence spectroscopy allowed us to easily trace the dipyridyl imine and anthracene chromophores.

Keywords: amino acids; functionalization; glucose; o-hydroquinone; phosphonate; thiosemicarbazones.

Scheme 1

(A) Schematic representation of thiosemicarbazones (TSC) in their thione and thiol forms (R, R′, R″ = H, alkyl, or aryl) with numbering of the TSC backbone (in green). (B) N^S thione coordination of a TSC to metal fragments [M]. (C) 2-acetylpyridine-thiosemicarbazones (R² = CH₃) showing tridentate N^N^S thiolate binding. (D) Formation of a TSC from a thiosemicarbazone and a carbonyl.

Scheme 2

Schematic approaches to [TSC–X anchor] (approach A) and [TSC–Z–Y–X anchor] (approach B) conjugates for covalent anchoring of nanoparticles (NPs), with an example from this study. Shown is the [dipy–TSC–Ph–phos–TiO₂] conjugate (dipy = di-2-pyridyl).

Scheme 3

Targeted functionalization of TSC in this work. (A) [R¹R²TSC–spacer–amino acid] conjugates (B) [R¹R²TSC–spacer–anchor] (B1) and [R¹R²TSC–spacer–end group] conjugates (B2).

Scheme 4

Components of the [TSC–X anchor] and [TSC–Z–Y–X anchor] conjugates in this work.

Scheme 5

General procedure for the synthesis of TSCs in this work.

Scheme 6

Summary of functionalized TSCs reported in this work (the labels A, B1, and B2 refer to Scheme 3).

Figure 1

Synthesis of TSCs derived from amino acids.

Figure 2

Synthesis of TSCs starting with diamines introducing hydrocarbon spacers.

Figure 3

Synthesis of TSCs from acetobromo-α-D-glucose. TBAB = tetra-n-butylammonium bromide.

Figure 4

Synthesis of TSCs starting from trans-4-hydroxy-cyclohexanol.

Figure 5

Synthesis of TSCs starting from dopamine.

Figure 6

Synthesis of TSCs derived from amino thiols.

Figure 7

Attempted synthesis of thiosemicarbazones with an alkyne function starting from propargyl amine.

Scheme 7

Proposed cyclization reaction preventing the formation of propargyl amine thiosemicarbazide. ??? denotes the formation of mixtures of unidentifiable reaction products.

Figure 8

Synthesis of synthesis of TSCs with alkyne functions on rigid phenylene spacers.

Figure 9

Attempted synthesis of thiosemicarbazones with an azide function.

Figure 10

Synthesis of TSCs with azide functions using the rigid dimethyl-phenylene spacer.

Figure 11

Synthesis of TSCs with a phosphonate group.

Figure 12

Anchoring of TSC-phosphonates on TiO₂ forming the conjugates [Dipy–TSC–Ph–phos–TiO₂] (top) and [Anthr–TSC–Ph–phos–TiO₂] (bottom). TMBS = trimethylbromosilane.