Hydrogenation core-functionalized dendrimers

A set of core-functionalized dendrimers was synthesized by Van Leeuwen et al. and one compound was applied in continuous catalysis. [45] The dendritic dppf, Xantphos and triphenylphosphine derivatives (Figures 4.22, 4.30 and 4.31) were active in rhodium-catalyzed hydroformylation and hydrogenation reactions (performed batch-wise). Dendritic effects were observed which are discussed in paragraph 4.5. The dendritic rhodium-dppf complex was applied in a continuous hydrogenation reaction of dimethyl itaconate. 

BINAP core-functionalized dendrimers were synthesized by Fan et al. (36), via condensation of Frechet s polybenzyl ether dendritic wedges to 5,5 -diamino-BINAP (26—28). The various generations of BINAP core-functionalized dendrimers were tested in the ruthenium-catalyzed asymmetric hydrogenation of 2-[p-(2-methyl-propyl)phenyl]acrylic acid in the presence of 80 bar H2 pressure and in a 1 1 (v/v) methanol/toluene mixture. As later generations of the in situ prepared cymeneruthe-nium chloride dendritic catalysts were used, higher activities were observed (TOF values were 6.5, 8.3, and 214 h respectively). Relative to those of the BINAP... 

Since the pioneering studies of asymmetric catalysis with core-functionalized dendrimers reported by Brunner (88) and Bolm (89), several noteworthy investigations have been described in this field. Some examples of the dendritic effects observed in enantioselective catalysis with dendrimers having active sites in the core were discussed in Section II, such as the catalytic experiments with TADDOL-cored dendrimers described by Seebach et al. (59) the asymmetric addition of Et2Zn to aldehydes catalyzed by core-functionalized phenylacetylene-containing dendrimers reported by Hu et al (42)-, the asymmetric hydrogenation investigations with (R)-BINAP core-functionalized dendrimers synthesized by Fan et al. (36) or the results... 

Dendrimers having phosphines as branching points (Figure 39 Rh = [Rh(cod)Cl]) were synthesized by acid-base hydrolysis of aminosilanes with alcohols. This organometallic compound could be considered as a periphery- and core-functionalized dendrimer. Interestingly, the complexes can be formed before or after the acid-base reaction that forms the dendrimer. The Rh(l) metallodendrimer of Figure 39 was catalytically active in the hydrogenation of... 

Two interesting reports by Chan and co-workers of dendritic core-functionalized Ru-BINAP (BINAP = 2,2/-bis(diphenylphosphino)-l,l/-binaphthyl) catalysts, which were employed in asymmetric hydrogenations and which were fully recoverable, have appeared recently [40,41]. In particular, such dendrimers containing long alkyl chains in the periphery were synthesized and... 

Brunner et al. [26] synthesized and applied so-called dendrizymes in enan-tioselective catalysis. These catalysts are based on dendrimers which have a functionalized periphery that carries chiral subunits, (e.g. dendrons functionalized with chiral menthol or borneol ligands). The core phosphine donor atoms can be complexed to (transition) metal salts. The resultant dendron-enlarged 1,2-diphosphino-ethane (e.g. 16, see Scheme 17) Rh1 complexes were used as catalysts in the hydrogenation of acetamidocinnamic acid to yield iV-acetyl-phenylalanine (Scheme 17) [26]. A small retardation of the hydrogenation of the substrate was encountered, pointing to an effect of the meta-positioned dendron substituents. No significantly enantiomerically enriched products were isolated. However, a somewhat improved enantioselectivity (up to 10-11% e.e.) was... 

FIGURE 4.16 The formation of a dendrimer using the divergent approach involving Michael addition of acrylonitrile to an amine core. Each addition step is followed by a hydrogenation to reform amine functions for subsequent addition of acrylonitrile. 

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