Halide-assisted macrocyclic ring formation in cyclometalated carbosilane dendrimers with 1-[C6H3 (CH2NMe2)-4-(PdCl)-3] peripheral groups: Applicationas aldol condensation catalysts

A series of cyclopalladated carbosilane dendrimers functionalized with mono anionic [C6H4(CH2NMe2)-4](-) (= CN) C,N-chelating ligands via a multiple an d selective C-H bond activation process have been prepared. The structure o f the support, i.e., the branching degree as well as the nature of the alka nediyl linkers present in the carbosilane dendrimer, was varied to investig ate its influence on the dimer formation of the peripheral [CN-PdCl] comple xes and on the catalytic performance of these Pd(II)-functionalized dendrim ers. Me2Si{(CH2)(3)SiMe2-CN-PdCl}(2) (4) and Si{(CH2)(2)SiMe2-CN-PdCl}(4) ( 7a) were characterized by a single X-ray crystal structure determination. I n both cases a unique intramolecular halide-assisted macrocyclic ring struc ture was present. Variable-temperature NMR spectroscopic investigations ind icated that in solutions of 4 and 7a the cis-bridging mode of the periphera l dimers is predominant. For the two asymmetrically branched [G1] palladode ndrimers Si{(CH2)(2)Si(Me){(CH2)(2)SiMe2-CN-PdCl}(2)}(4) (10a) and Si{(CH2) (3)Si(Me){(CH2)(3)-SiMe2-CN-PdCl}(2)}(4) (12a) isomeric forms of the [CN-Pd Cl] dimeric units were observed. In addition, for the [G2] dendrimer Si{(CH 2)(3)Si{(CH2)(3)Si{(CH2)(3)SiMe2-CN-PdCl(pyr)}(3)}(3)}(4), 15b, appended wi th a maximum of 36 [CN-PdCl(pyr)] complexes, a cis/trans isomerization of t he peripheral [CN-PdCl(pyr)] groups is noted. The applicability of the cati onic 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{CH2-NMe2}-2-(SiMe3)-5)(pyr)(H2O)]BF4 (16). The obtained catalytic data showed that up to generation [G2], the s electivity of the reaction is not affected. However, the rate of the reacti on decreased with increasing steric congestion at the dendrimer periphery.