Ligand-deficient catalysts

In practice, catalytic networks often involve more than a single cycle. In a very common type of network, a linear pathway is attached to the cycle. This is so, for instance, in reactions with ligand-deficient catalysts. Here, the coordinatively saturated "catalyst" has no activity but, rather, must first lose one of its ligands to provide access for a reactant. Other examples of cycles with attached pathways include systems with inhibition, activation, decay, and poisoning. Also, the network may consist of two or more cycles with a common member or pathway. This situation is typical for reactions yielding different isomeric products. The Christiansen formula is extended to cover such cases. 

Break-up into piecewise simple portions and their reduction. The network 11.1 is "simple" except that aldehyde, an intermediate, builds up to higher than trace concentrations. Thus it can be cut at the aldehyde into two piecewise simple portions which share aldehyde and the ligand-deficient catalyst, cat . After reduction the portions can be written... 

Inhibition. As seen in the previous subsection, the presence of excess ligand reduces the reaction rate produced by a ligand-deficient catalyst because it syphons catalyst material from the active cycle into the inactive external pathway. This can be viewed as a special case of inhibition Although a necessary ingredient of the catalyst system, the ligand depresses the rate. More generally, any substance that reduces the rate through removal of catalyst material from the cycle by reaction with one of the cycle members is called an inhibitor. 

In the previous section we were discussing the case of a ligand-deficient catalyst, as the reaction of the ligand with a catalytic species from the active cycle syphon off the catalyst material from the cycle. This can be considered as a case of inhibition. 

Detailed kinetic studies are essential to help establish that a multicenter complex is not dissociating to an active monomeric catalyst—as is usually the case (/, p. 405). An added problem in some of these ligand-deficient systems (394. 405) is to establish that the catalysis is homogeneous. Even when an active polynuclear system is confirmed, it is likely impossible to demonstrate unequivocally that reactivity does not occur at a single... 

Although dimeric Sharpless ligands as catalysts showed impressive results in related organocatalytic transformations, they provided only limited success in asymmetric MBH reactions (Scheme 5.12) [70]. These compounds are bifunctional catalysts in the presence of acid additives one of the two amine function of the dimers forms a salt and serves as an effective Bronsted acid, while another tertiary amine of the catalyst acts as a nucleophile. Whereas salts derived from (DHQD)2PYR, or (DHQD)2PHAL afforded trace amounts of products in the addition of methyl acrylate 8a and electron-deficient aromatic aldehydes such as 27, (DHQD)2AQN, 56, mediated the same transformation in ee up to 77%, albeit in low yield. It should be noted that, without acid, the reaction afforded the opposite enantiomer in a slow conversion. 

Development of Tetraphosphine Rh(I) catalysts. The tetra(phosphine) rhodium(I) cations, prepared by adding stoichiometric amounts of a monophosphine to the ligand deficient dimers, were subsequently found to be very active hydrosilation catalysts. Although the addition of trisubstituted silanes was slow (1-2 days) and required elevated temperatures ( 55°C), high regioselectivity to the 1,2-hydrosilation products was obtained. Importantly, no products arising from catalyst deactivation in the form of trimeric rhodium(I) complexes were observed. More interestingly, these tetraphosphine rhodium complexes are extremely efficient catalysts for the 1,2-addition of disubstituted silanes to enolizable ketones. Turn-over numbers up to 105/hr at room temperature have been observed for a number of catalyst and ketone/silane combinations. 

Similarly tetravalent Ti and Zr dihydride catalysts are formed from alkyl or aryl precursors. A wide range of Group 8-10 metal hydride catalysts has been isolated or formed in situ from precursor allyl complexes. The systems are generally quite active because the catalysts are necessarily ligand deficient with sites available for substrate coordination. For an 17 -allyl precursor, equation (s) initial dihydride addition to an M(r) -allyl) intermediate appears very plausible ", cf. equations (i)-(l). 

This is a case of competitive inhibition, as the inhibitor and reactant compete directly for the active site. Ligand-deficient catalysis is then a special case of competitive inhibition when the ligand itself acts as an inhibition although it is a necessary ingredient of the catalyst system. 

Analogously to the treatment of ligand deficient catalysis we introduce the concentration of C>u into the total catalyst balance and by defining Cx through Dj, the reaction rate is... 

Ligand-deficient catalysis, inhibition, and corresponding kinetic models were considered in Section 5.4.2. Generation of active sites in catalysis by organometaUic complexes involves a step outside the catalytic cycle, namely the loss of the figand from the catalyst. This step frees the coordinative site, allowing binding of the reactants. 

Future Trends. In addition to the commercialization of newer extraction/ decantation product/catalyst separations technology, there have been advances in the development of high reactivity 0x0 catalysts for the conversion of low reactivity feedstocks such as internal and a-alkyl substituted a-olefins. These catalysts contain (as ligands) ortho-/-butyl or similarly substituted arylphosphites, which combine high reactivity, vastiy improved hydrolytic stabiUty, and resistance to degradation by product aldehyde, which were deficiencies of eadier, unsubstituted phosphites. Diorganophosphites (28), such as stmcture (6), have enhanced stabiUty over similarly substituted triorganophosphites. 

Conventional triorganophosphite ligands, such as triphenylphosphite, form highly active hydroformylation catalysts (95—99) however, they suffer from poor durabiUty because of decomposition. Diorganophosphite-modified rhodium catalysts (94,100,101), have overcome this stabiUty deficiency and provide a low pressure, rhodium catalyzed process for the hydroformylation of low reactivity olefins, thus making lower cost amyl alcohols from butenes readily accessible. The new diorganophosphite-modified rhodium catalysts increase hydroformylation rates by more than 100 times and provide selectivities not available with standard phosphine catalysts. For example, hydroformylation of 2-butene with l,l -biphenyl-2,2 -diyl... 

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