Chemically and electrochemically mediated release of dendrimer end groups

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Generations 1-5 of poly(propyleneimine) dendrimers (1-5) terminated with redox-labile, trimethyl-locked quinone (TLQ) end groups were shown to release the TLQ units as lactone 6 upon chemical or electrochemical redox action. Sodium dithionite was used as a chemical reducing agent to instantly convert the TLQ units to their deuteroquinone analogues (TLDQ), which were subsequently liberated from the dendrimers following zero-order kinetics in both DMSO-d 6-D 2O (85:15, v/v, k obs = 7 × 10 -6 M s -1, t 1/2 = 9 min) and THF-d 8-D 2O (76:24, v/v, k obs = 4 × 10 -6 M s -1, t 1/2 = 15 min). Cyclic voltammetry of 1-5 in DMSO/0.1 M n-Bu 4PF 6 revealed that the reduction peak potential (E pc) values for the formation of TLQ 2- were generally invariant (-1.85 V vs Fc/Fc +) with dendrimer generation. Thus, bulk electrochemical reduction of 1-5 at an overpotential of -0.3 V vs the E pc of TLQ 2 produced PPI dendrimers containing dianionic TLQ 2- termini. Protonation of the electrochemically generated TLQ 2- end-group units with H 2O produced the transient and structurally labile hydroquinone (TLHQ) intermediates, which were liberated from the dendrimers as lactone 6 in a similar fashion as in the sodium dithionite-induced approach. In both the chemically and electrochemically mediated routes, 1H NMR spectroscopy verified that the redox-promoted disconnection of TLQ from the dendrimers was quantitative. © 2006 American Chemical Society.

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