High substrate/catalyst ratios

We report here a number of examples of the use of this anchored catalyst for the hydrogenation of different substrates at moderate to high substrate/catalyst ratios along with a direct comparison of these results with those obtained using the homogeneous Wilkinson under the same conditions. Also presented will be some examples of the use of the anchored catalyst in long term continuous reactions. Reaction rates, selectivities and the extent of metal loss will be presented where appropriate. 

Tandem ATRA/ATRC of 1,6-dienes with CCI4 were devised using copper catalysis (see Part 2, Sect. 3.3.3, Fig. 42, 169 >170) [284—286, 296]. Very high substrate/catalyst ratios can be achieved when these reactions were performed in the presence of catalytic amounts of azo initiators AIBN or V-70 or ascorbic acid 222 to reduce accumulating Cu(II) back to Cu(I) [285, 286, 292]. Tandem ATRC/ ATRC of indoles 228 were tested recently the yields of tetracyclic spiro benzoindolizidines 229, however, remained moderate (Fig. 59) [318]. 

Most recently, these BlNAP-cored dendrimers were further employed in the Ir-catalyzed asymmetric hydrogenation of quinolines by Fan et al. (Scheme 4.3) [33]. Unlike the asymmetric hydrogenation of prochiral olefins, ketones and imines, the hydrogenation of heteroaroniatic compounds proved to be rather difficult [34—37]. All four generations of dendrimer catalysts generated in situ from BlNAP-cored dendrimers and [lr(COD)Cl]2 were found to be effective, even at an extremely high substrate catalyst ratio in the asymmetric hydrogenation of quinaldine with... 

Several new, water-soluble atropisomeric diphosphines in the biphenyl series, such as (S)-(+)- and (R)-(—)-MeO-BIPHEP tetrasulfonate (32), were prepared and used as components of Ru(II)- and Rh(I)-based hydrogenation catalysts [98]. Several C=C unsaturated substrates (enamides, unsaturated acids) could be hydrogenated biphasically with high rates and enantioselectivities (Eqs. 33 and 34). In some cases high substrate/catalyst ratios could be used (up to 10000 1), a strong requirement for practical applications [99]. 

Allylic alcohols, such as gcraniol (42) and nerol (43), can be converted to citronellol (44) with high efficiency and excellent enantioselectivity by hydrogenation using ruthenium BINAP complexes108 or related catalysts76. Enantiomeric excesses between 96-98%, essentially quantitative yields, and very high substrate/catalyst ratios (up to 50000 1) are attractive attributes of... 

The Tedicyp (36)/[PdCl(CjH5)]2 system catalyzes the Suzuki crossheteroaryl bromides with arylboronic acids with a very high substrate/catalyst ratio in good yields. Substrates such as pyridines, quinolines, thiophenes, an indole, a pyrimidine or a fiiran have been used successfully (Equation 64) [86]. 

A palladium/tetraphosphine system catalyzed allylic amination in water in good yields, with a very high substrate/catalyst ratio and a turnover number of 980,000. ° ... 

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