Catalysts tethering

Table 16.1 Homogeneous catalysts, tethered single-site catalysts, and biphase catalysts for the hydrogenation of aromatic hydrocarbons.

Kani, I. Omaty, M. A. Rawashdeh-Omaiy, M. A. Lopez-Castillo, Z. K. Flores, R Akgerman, A. Fackler, Jr., J. P. Homogeneous catalysis in supercritical carbon dioxide with rhodium catalysts tethering fluoroacrylate polymer ligands. Tetrahedron, 2002,58,3923-3928. 

Each catalyst was tested in three successive catalytic runs of 2 h at a reaction temperature of 55 °C in order to take advantage of the thermomorphic behavior of the solvent mixture. This allowed the actual catalytic conversion to take place in a homogeneous reaction medium under these conditions, with the phases separating again upon cooling to ambient temperature at the end of the reaction. The catalysts tethered to the hyperbranched PEIs were also tested under similar reaction conditions, and the results obtained are represented in Figure 19.3. 

Gao H., Angelici R.J. Combination catalysts consisting ofa homogeneous catalyst tethered to a silica-supported palladium heterogeneous catalyst Arene hydrogenation. J. Am. Chem. Soc. 1997 119 6937-6938... 

Hydroformylation of 1-hexene in supercritical carbon dioxide was investigated using a rhodium-phosphane catalyst tethered to a silica support. The performance of the tethered catalyst was compared with a homogeneous rhodium-phosphane... 

A relatively new area of research is the immobilisation of Pd-NHC complexes on graphene oxide (GO). The complexes have been covalently tethered via a number of different organic bridges, or through -interaction between GO and a pyrene group attached to the NHC. The recyclable nature of these catalysts was demonstrated in Suzuki coupling, although the number of recycles was relatively low (5-6). " The catalyst tethered by... 

Intramolecular cycloadditions of substrates with a cleavable tether have also been realized. Thus esters (37a-37d) provided the structurally interesting tricyclic lactones (38-43). It is interesting to note that the cyclododecenyl system (w = 7) proceeded at room temperature whereas all others required refluxing dioxane. In each case, the stereoselectivity with respect to the tether was excellent. As expected, the cyclohexenyl (n=l) and cycloheptenyl (n = 2) gave the syn adducts (38) and (39) almost exclusively. On the other hand, the cyclooctenyl (n = 3) and cyclododecenyl (n = 7) systems favored the anti adducts (41) and (42) instead. The formation of the endocyclic isomer (39, n=l) in the cyclohexenyl case can be explained by the isomerization of the initial adduct (44), which can not cyclize due to ring-strain, to the other 7t-allyl-Pd intermediate (45) which then ring-closes to (39) (Scheme 2.13) [20]. While the yields may not be spectacular, it is still remarkable that these reactions proceeded as well as they did since the substrates do contain another allylic ester moiety which is known to undergo ionization in the presence of the same palladium catalyst. 

Although, as stated above, we wiU mostly focus on hydrolytic systems it is worth discussing oxidation catalysts briefly [8]. Probably the best known of these systems is exemphfied by the antitumor antibiotics belonging to the family of bleomycins (Fig. 6.1) [9]. These molecules may be included in the hst of peptide-based catalysts because of the presence of a small peptide which is involved both in the coordination to the metal ion (essential co-factor for the catalyst) and as a tether for a bisthiazole moiety that ensures interaction with DNA. It has recently been reported that bleomycins will also cleave RNA [10]. With these antibiotics DNA cleavage is known to be selective, preferentially occurring at 5 -GpC-3 and 5 -GpT-3 sequences, and results from metal-dependent oxidation [11]. Thus it is not a cleavage that occurs at the level of a P-O bond as expected for a non-hydrolytic mechanism. 

REMP, the acronym for ring expansion metathesis polymerisation is a special case of ROMP, where the growing polymer chain stays attached to the catalyst at both ends nntil a macrocycle is released. This requires that the active carbene be tethered... 

Pyridine compounds 45 can also be produced by the NHC-Ni catalysed cycloaddition between nitriles 43 and diynes 44 (Scheme 5.13) [16]. The SIPr carbene was found to be the best ligand for the nickel complex in this reaction. The reaction required mild reaction conditions and low catalyst loadings, as in the case of cycloaddition of carbon dioxide. In addition to tethered aUcynes (i.e. diynes), pyridines were prepared from a 3-component coupling reaction with 43 and 3-hexyne 23 (Scheme 5.13). The reaction of diynes 44 and nitriles 43 was also catalysed by a combination of [Ni(COD)J, NHC salts and "BuLi, which generates the NHC-Ni catalyst in situ. The pyridines 45 were obtained with comparable... 

Vinyl cyclopropanes tethered to an aUcyne chain 127 were also subjected to the cycloisomerisation reaction in presence of the NHC-Ni catalyst system (Scheme 5.34) [39], The product formation depends on the substrate used and the NHC hgand. When SIPr carbene is used, three different products were obtained depending on the size of the R group attached to the alkyne moiety. If R is small (like a methyl) product 128 is obtained exclusively. If R is Et or Pr a mixture of 128 and 129 is obtained in 3 2 to 1 2 ratio, respectively. However, when R is large groups such as Bu or TMS only product 130 is obtained. When IfBu carbene 131 is used as the ligand, cycloisomerisation of 127 afforded product 128 exclusively, regardless of substituent size (Scheme 5.34) [39]. 

Regarding bis-NHC chelating ligands, several structures that differ in the motifs used for the enlargement of the tether have been proposed as catalysts for the Mizoroki-Heck reaction. They range from non-functionalised aliphatic chains [23-25] to phenyl [26], biphenyl [27], binaphthyls [28] and to chains containing additional coordination positions like ethers [29], amines [30], and pyridines in an evolution towards pincer complexes [31-35], In most cases, the activity of aryl bromides in Mizoroki-Heck transformations was demonstrated to be from moderate to high, while the activation of chlorides was non-existent or poor (Scheme 6.7). 

Figure 2.27. Tethered organic bases as solid catalysts. 

Ansa-metallocenes with a bridged tethered donor functionality, R(Me)Si(C5Me4)2Ln CH(SiMe3)2 (Ln = Y, Sm, R = (CH2)nOMe), have been synthesized [139]. Use of these new catalyst precursors results in a significantly enhanced activity (2- to 5-fold) while the diastereoselectivity (e.g., for the IH of 2-amino-5-hex-... 

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