Chemically and electrochemically mediated release of dendrimer end groups

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)-D2O (85:15, v/v, k(obs) = 7 x 10(-6) M s(-1), t(1/2) = 9 min) and THF-d(8)-D2O (76:24, v/v, kobs = 4 x 10(-6) M s(-1), t(1/2) = 15 min). Cyclic voltammetry of 1-5 in DMSO/0.1 M n-Bu4PF6 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 H2O 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, H-1 NMR spectroscopy verified that the redox-promoted disconnection of TLQ from the dendrimers was quantitative.