Halide-Assisted Macrocyclic Ring Formation in Cyclometalated Carbosilane Dendrimers with 1-[C6H3(CH2NMe2)-4-(PdCl)-3] Peripheral Groups: Application as Aldol Condensation Catalysts

Abstract

A series of cyclopalladated carbosilane dendrimers functionalized with monoanionic [C6H4(CH2NMe2)-4]- (= CN) C,N-chelating ligands via a multiple and selective C-H bond activation process have been prepared. The structure of the support, i.e., the branching degree as well as the nature of the alkanediyl linkers present in the carbosilane dendrimer, was varied to investigate its influence on the dimer formation of the peripheral [CN-PdCl] complexes and on the catalytic performance of these Pd(II)-functionalized dendrimers. Me2Si{(CH2)3SiMe2-CN-PdCl}2 (4) and Si{(CH2)2SiMe2-CN-PdCl}4 (7a) were characterized by a single X-ray crystal structure determination. In both cases a unique intramolecular halide-assisted macrocyclic ring structure was present. Variable-temperature NMR spectroscopic investigations indicated that in solutions of 4 and 7a the cis-bridging mode of the peripheral dimers is predominant. For the two asymmetrically branched [G1] palladodendrimers Si{(CH2)2Si(Me){(CH2)2SiMe2-CN-PdCl}2}4 (10a) and Si{(CH2)3Si(Me){(CH2)3SiMe2-CN-PdCl}2}4 (12a) isomeric forms of the [CN-PdCl] dimeric units were observed. In addition, for the [G2] dendrimer Si{(CH2)3Si{(CH2)3Si{(CH2)3SiMe2-CN-PdCl(pyr)}3}3}4, 15b, appended with a maximum of 36 [CN-PdCl(pyr)] complexes, a cis/trans isomerization of the peripheral [CN-PdCl(pyr)] groups is noted. The applicability of the cationic cyclopalladated carbosilane dendrimers, obtained by halide abstraction with AgBF4, as Lewis acid catalysts in a model aldol condensation reaction was demonstrated. Their catalytic performance was compared to that of the parent mononuclear derivative [Pd(C6H3{CH2NMe2}-2-(SiMe3)-5)(pyr)(H2O)]BF4 (16). The obtained catalytic data showed that up to generation [G2], the selectivity of the reaction is not affected. However, the rate of the reaction decreased with increasing steric congestion at the dendrimer periphery.