Arkadiusz Listkowski

CROWN ETHER ANALOGS FROM SUCROSE

A convenient synthesis of 1′,2,3,3′4,4′-hexa-O-benzylsucrose (4) from the free disaccharide is presented. Diol 4 and previously obtained 1′-O-benzyloxymethyl-2,3,3′,4,4′-penta-O-benzylsucrose (3) served as precursors for chiral crown ether analogs containing a sucrose backbone. Deprotection of macrocyclic compounds (removal of the benzyl blocks) was possible under hydrogenolysis conditions.

Tautomerism in porphycenes: analysis of rate-affecting factors.

Double hydrogen transfer occurring in both ground and the lowest electronically excited singlet states was studied for a series of 19 differently substituted porphycenes. The rates of tautomerization have been determined using femtosecond pump-probe spectroscopy with polarized light. The values vary by over 3 orders of magnitude, suggesting the importance of tunneling. Good correlation exists between the values of the rates and the parameters characterizing the strength of two intramolecular hydrogen bonds: proton NMR shift, distance between the hydrogen-bonded nitrogen atoms, and the NH stretching frequency. While hydrogen-bond strength is the main factor determining the rate of double hydrogen transfer, other factors, such as static and dynamic symmetry breaking and the population of low-frequency vibrations also have to be taken into account.

Evidence for Dominant Role of Tunneling in Condensed Phases and at High Temperatures: Double Hydrogen Transfer in Porphycenes.

Investigation of the double hydrogen transfer in porphycene, its 2,7,12,17-tetra-tert-butyl derivative, and their N-deuterated isotopologues revealed the dominant role of tunneling, even at room temperature in condensed phase. Ultrafast optical spectroscopy with polarized light employed in a wide range of temperatures allowed the identification and evaluation of contributions of two tunneling modes: vibrational ground-state tunneling, occurring from the zero vibrational level, and vibrationally activated, via a large amplitude, low-frequency mode. Good correspondence was found between the rates of incoherent tunneling occurring in condensed phase and the values estimated on the basis of tunneling splittings observed in molecules isolated in supersonic jets or helium nanodroplets. The results provide solid experimental insight into widely proposed quantum facets of ubiquitous hydrogen-transfer phenomena.

The long and winding road to new porphycenes

We describe attempts — not always successful — made over the years to improve the efficiency of porphycene synthesis and to produce novel compounds, custom-designed for specific purposes. New porphycenes are reported, some of them obtained rather unexpectedly as by-products of the planned reactions. Structure and energy computations of possible tautomeric forms in porphycenes substituted by one, two, three, and four tert-butyl groups lead to predictions regarding the kinetics and mechanisms of intramolecular double hydrogen transfer. The occurrence of tautomerization in single molecules of tert-butyl-substituted porphycenes is demonstrated by using fluorescence polarization techniques.

Sucrose Analogs Containing Sulfur Functionalities

Regioselective synthesis of sucrose monosulfides, with sulfur functionality placed either at the C-6 (glucose part) or C-6′ (fructose) ends, is presented. The performed model dimerization of the mercaptane into the disulfide opens a convenient route to the open-chain sucrose dimers.