Dendritic effects in catalysis by Pd complexes of bidentate phosphines on a dendronized support: Heck vs. carbonylation reactions

Abstract

Bidentate phosphine ligands have been prepared on polystyrene beads modified with polyether dendron spacers. When complexed to Pd(0), these systems exhibited a negative dendritic effect on Heck catalysis (contrary to the analogous monodentate phosphine systems), but mostly a positive influence on carbonylation. This opposite influence of the dendronization falls into line with other differences in the optimal ligand structure for the two reactions. The negative effect on the Heck catalysis with bidentate phosphines may indicate that dendrimer-induced reduction in the cross-linking upon Pd complexation is responsible for the positive effect in the corresponding monodentate phosphine systems.

Cyclic 2:1 and 1:2 Aldehyde‐to‐Acetone Byproduct Adducts in Aldol Reactions Promoted by Supported Proline‐Incorporated Catalysts

“…The use of catalyst 13 in the model reaction led to a slight improvement in the conversion and a notable increase in the chemoselectivity towards 7. In contrast, 6Ј, a catalyst analogous to 6 but prepared on firstgeneration dendronized chloromethyl-terminated resin (vs. Wang Bromo PS, Scheme 1), [9] displayed significantly higher activity, but much lower selectivity toward 7. p-Cya- nobenzaldehyde (replacing p-nitrobenzaldehyde) reacted similarly with acetone in the presence of 6 to form cyclic byproduct 14 along with the main aldol adduct.…”

Dendritic Molecules on Solid Support: Solid‐Phase Synthesis and Applications

“…Indeed, Fr echet-type polyether dendrimers prepared in solution demonstrate a broad spectrum of chemical and coordinative compatibilities. 64 Accordingly, polyether dendron synthesis on solid support was developed, initially by Bradley and coworkers and later by Portnoy and Dahan 65,66,68 Instead of the 3,5-dihydroxy benzyl alcohol derivatives used in the soluble Fr echet-type dendrimers, a more SPOS suitable 3,5-bis(hydroxymethyl)phenol unit was conceived as a building block for the dendron construction.…”

“…67 To avoid this obstacle, Portnoy and coworkers changed the synthetic scheme (Scheme 15.17b), replacing the Mitsunobu coupling by chlorodehydroxylation-nucleophilic substitution sequence, thus adding a third synthetic step during each generation assembly but eliminating the need for the Mitsunobu special reagent. 68 The same group introduced a tetrafurcated building block that can be incorporated into the dendritic structures. 69 Tetra-, octa-, and hexadecaol dendritic grafts were prepared, starting from Wang resin or the aforementioned first-and second-generation polyether dendronized resins (Scheme 15.18).…”

“…108a Portnoy and coworkers synthesized bidentate phosphine ligands on a polyether dendronized polystyrene using an alternative approach (Scheme 15.37). 68 Serinol was immobilized on the chloromethyl-terminated dendrons through its amino group. Subsequently, the chlorodehydroxylation followed by the substitution of chlorides by diphenylphosphide anion led to the formation of chelating bisphosphine ligands on the polyether dendronized polystyrene.…”

Elucidation of Architectural Requirements from a Spacer in Supported Proline‐Based Catalysts of Enantioselective Aldol Reaction

“…As in the case of the perfect dendritic structures, the branching units of the spacers were derived from dimethyl-5-hydroxyisophthalate building block, which is immobilized via nucleophilic substitution and activated by a reduction-chlorodehydroxylation sequence. [12] The linear units were derived from 3-hydroxybenzyl alcohol, which was immobilized via nucleophilic substitution and activated by a chlorodehydroxylation reaction. The preparation of the azide-terminated spacers and the propargyl-carrying protected hydroxyproline as well as the cycloaddition and deprotection reactions, leading to the active supported catalytic systems, were carried out by procedures used for the perfect dendrons without substantial changes.…”