The polystyrene-supported N-alkylimidazole-based dendritic catalysts for the Baylis-Hillman reaction exhibit one of the strongest beneficial effects of multivalent architecture ever reported for an organocatalyst. The yields in the model reaction of methyl vinyl ketone with p-nitrobenzaldehyde are more than tripled when a non-dendritic catalyst is replaced by a second- or third-generation analogue. Moreover, the reaction of the less active substrates will not occur with the non-dendritic catalyst and will proceed to a significant extent only with the analogous catalysts of higher generations. A substantial additional enhancement of the reaction yield could be achieved by increasing the content of water in the reaction solvent. The plausible cause of the dendritic effect is the assistance of the second, nearby imidazole moiety in the presumably rate-determining proton transfer in the intermediate adduct, after the first imidazole unit induced the formation of the new carbon-carbon bond.

While comparing analogousp olystyrenesupported and homogeneous catalysts for the Baylis-Hillmanr eaction, we hypothesized that the hydrophobice nvelopment of the imidazole catalytic sites of the former is responsible for the significantly better chemoselectivity exhibited by the heterogeneous catalysts compared to theirh omogeneousc ounterparts.I no rder to test this hypothesis,w ep repared as eries of branched/dendritic homogeneousc atalysts,w ith an imidazole active site near the focal point andf lexiblet ails of various lengthsa nd polarities,c apable of providing partials hieldingo ft his site.T he design of the catalysts was based on a5 -hydroxyisophthalate scaffold,a nd they were prepared throughanumber of multistep synthetic pathways. Thec omparison of the catalysts under av arietyo f conditions in am odel Baylis-Hillman reactiond emonstrated that long hydrophobic tales enhance the chemoselectivity parameter of the catalysis,w hile reducing the rate of the consumptiono ft he substrates, and that the chemoselectivity is further improvedb y the presence of af ree phenolic moiety in the vicinity of the catalytic imidazole unit. Moreover, in secondgeneration catalysts,t he peripherall ong tails could be either hydrophobic or polar, since the dendritic inner backbone itself presumably partially provides the necessary isolation of the catalytic site.T hus,e xperimental results support our hypothesis.Synthesis of 1,3-bis(methoxymethyl)-5-(methoxymethoxy)benzene: Thec ompound was prepared according to the typical procedure for the benzylic alcohol alkylation, using the followingq uantities:i odomethane (1.68 mL, 28.0 mmol, 4.0 equiv.), [5-(methoxymethoxy)-1,3-phenylene]dimethanol

While steric hindrance can be particularly large in dendritic molecules, it is usually implicated with difficulties in the synthesis of higher generation structures and restricted access of reagents, including bond-cleaving agents, to the dendritic interior. A different situation, where the steric hindrance is translated into a steric strain within the dendritic molecule and, consequently, causes enhanced decomposition of the dendron-containing structure, has only occasionally been reported and exclusively for dendronized polymers. In this work we describe post-synthetic cleavage of sterically congested third-generation oligoether dendrons from solid supports, followed by their disassembly into monomeric building blocks under acidic conditions. This disassembly was monitored by H NMR, revealing the intermediate fragment structures and the exact order of bond cleavage within the dendritic molecule. These conclusions were supported by MS analysis of the cleavage mixtures. Though distinguishing between steric and electronic reasons of molecular disassembly can be a challenging task, we were able to analyze these factors separately by monitoring the dendron disassembly and comparing the rates of "decay" of parent dendrons and their fragments. This comparison reveals that while the electron donation of the steric congestion-inducing alkyl substituents is a prerequisite for the disassembly of the structures, this disassembly is very strongly accelerated in the sterically crowded dendrons and intermediates, where both steric and electronic factors contribute in a synergistic way to the disassembly phenomenon.

Capitalizing on our experience in constructing branched and dendritic monomers with functionalizable aromatic moiety at the focal point and various terminal groups, we prepared lipophilic and water-soluble dendrimers with BODIPY fluorophore in the core, emitting in the green and near IR spectral regions. The dendrimers, incorporating aliphatic tails, exhibited excellent fluorescence properties in apolar organic solvents, while those with the triethylenglycol-based tails were highly fluorescent in polar organic solvents as well. The water solubility induced by the terminal hexaethyleneglycol tails enables fluorescence of the dyes also in aqueous media. The phenolic hydroxyls in the molecules enable attachment of linkers for prospective conjugation of additional units.