Heterobimetallic catalysts structures

The first chiral aluminum catalyst for effecting asymmetric Michael addition reactions was reported by Shibasaki and coworkers in 1986 [82], The catalyst was prepared by addition of two equivalents of (i )-BINOL to lithium aluminum hydride which gave the heterobimetallic complex 394. The structure of 394 was supported by X-ray structure analysis of its complex with cyclohexenone in which it was found that the carbonyl oxygen of the enone is coordinated to the lithium. This catalyst was found to result in excellent induction in the Michael addition of malonic esters to cyclic enones, as indicated in Sch. 51. It had previously been reported that a heterobimetallic catalyst prepared from (i )-BINOL and sodium and lanthanum was also effective in similar Michael additions [83-85]. Although the LaNaBINOL catalyst was faster, the LiAlBINOL catalyst 394 (ALB) led to higher asymmetric induction. 

Shibasaki developed chiral lanthanum(iii) sodium(i) tris(binaphtholate) 67, which was prepared from La(Oz-Pr)3, (J )-BINOL, and NaOf-Bu, as the first example of a chiral sodium-containing heterobimetallic catalyst (Scheme 2.39). X-ray analysis of 67 showed that it consists of LaNas-CeoHaeOe 6THF H2O, which contains a central La(iii) atom, three (1 )-BINOL molecules, and three sodium atoms in the core structure. Catalyst 67 efficiently promoted the highly enantioselective Michael reaction... 

Figure 1. Structures of heterobimetallic catalysts and mononuclear model...

Moreover, these rare earth heterobimetallic complexes can be utilized for a variety of efficient catalytic asymmetric reactions as shown in Scheme 7 Next we began with the development of an amphoteric asymmetric catalyst assembled from aluminum and an alkali metal.1171 The new asymmetric catalyst could be prepared efficiently from LiAlH4 and 2 mol equiv of (R)-BINOL, and the structure was unequivocally determined by X-ray crystallographic analysis (Scheme 8). This aluminum-lithium-BINOL complex (ALB) was highly effective in the Michael reaction of cyclohexenone 75 with dibenzyl malonate 77, giving 82 with 99% ee and 88 % yield at room temperature. Although LLB and... 

Fully metalated silsesquioxane derivatives of the type Cy7Si709(0M)3 (M = Li, Na, K) would constitute highly desirable precursors for the construction of realistic catalyst model compounds, including novel heterobimetallic species. However, such alkali metal derivatives of 2-7 were unknown until recently, and structural information on such materials was lacking. There have also been contrasting reports in the literature concerning the metalation of 3 by alkali metal... 

Figure 14.6 A heterobimetallic Ti-Mg silsesquioxane, a homogeneous catalyst for ethylene polymerization after AlEtj activation, a possible model for the heterogeneous catalyst TiCl4/MgCl2/Si02 (one of the proposed surface structures) and a patented route to precatalyst, active after activation with MAO.

This idea was realized very successfully by Shibasaki and Sasai in their heterobimetallic chiral catalysts [17], Two representative well-defined catalysts. LSB 9 (Lanthanum/Sodium/BINOL complex) and ALB 10 (Aluminum/Lithium/BINOL complex), are shown in Figure 8D.2, whose structures were confirmed by X-ray crystallography. In these catalysts, the alkali metal (Na, Li, or K)-naphthoxide works as a Br0nsted base and lanthanum or aluminum works as a Lewis acid. 

Heterobimetallic homogeneous catalysts [e.g. (12)] have been developed26 for the alkylation of a range of aromatic compounds by n-activated alcohols. The superior electrophilicity is attributed to the high-valent T-Sn core in the structures. A review has appeared of reactions involving the hydroxyalkylation and cycloalkylation of arenes by hydrofurans, lactones, and unsaturated acids 27... 

Structural evidence for the formation of alkylated lanthanide metal centers in the ternary catalyst mixtures stems predominantly from ferf.bulanolalc complexes [178-181]. Fully characterized heterobimetallic zzeopentanolate analogues were synthesized in recently [ 142],... 

The catalytic asymmetric nitroaldol reactions promoted by LLB or its derivatives require at least 3.3 mol% of asymmetric catalysts for efficient conversion. However, even in the case of 3.3 mol% of catalyst, reactions are rather slow. Attempts were made to reduce the required catalytic amount and accelerate the reactions, which led to a second-generation heterobimetallic lanthanoid catalyst (LLB-II), prepared from LLB, 1 mol equiv of H20, and 0.9 mol equiv of butyllith-ium. The use of only 1 mol% of LLB-II efficiently promoted catalytic asymmetric nitroaldol reactions and additionally LLB-II (3.3 mol%) accelerated these reactions [32]. A comparison of the efficiency of LLB (or LL(B-a)) and the second-generation catalysts LLB-II (or LL(B-a)-II) is given in Scheme 9. The structure of LLB-II has not yet been unequivocally determined. However, it appears that it is a complex of LLB and LiOH. 

This section is concerned with transition metal clusters that, in addition to metal atoms, contain sulfur atoms in the cluster core (rather than just in the peripheral ligands). Metal-metal bonds often supplement sulfur-atom bridges in stabilizing the structures encountered. Generally, such clusters are likely to resemble the HDS active phases of heterogeneous metal-sulfide catalysts to some extent, e.g. in terms of coordination sphere and metallic oxidation states. Because of the large number of molecular metal-sulfide clusters now known, we shall focus on homometallic clusters of Mo(W) and heterobimetallic clusters of Mo(W)-Co(Ni) (next two sections), i.e. molecular clusters containing the elements that are relevant for industrial... 

Since these initial publications, the considerable efforts of Shibasaki and coworkers have lead to lanthanide element-binaphtholato complexes as being the most developed and potent of asymmetric phospho-aldol catalysts. From their early work on catalysis in the nitro-aldol reaction [28], Shibasaki and co-workers have published widely and in considerable detail on the use of hetero-bimetallic catalysis, developing the field to such a degree that enzyme-like comparisons have been made. Much of the success of the Shibasaki team in hetero-bimetallic catalysis has been reviewed recently [29], the key to which has been the delineation of the solid state structures of the catalytic precursors via single crystal X-ray diffraction studies [30] (Fig. 1) and the development of improved synthetic routes to this class of heterobimetallic system (Scheme 12) which emphasise the important role of added water [31]. 

The single-product heterobimetallic hydroformylation [ iW = Re, Rh, Re Rh ]cBER+UNi mechanism is typically initiated by the combined application of HRe(CO)5 and Rli4(CO)i2 as catalyst precursors to a n-hexane solution containing an alkene, hydrogen and CO at ambient temperature [75-78]. The structure of the system is shown in Fig. 17 where the original form of representation is retained. As mononuclear observable intermediates in the system, both coordi-nately saturated HRe(CO)s and RCORh(CO)4 have been quantified, and as dinuclear observable intermediate in the system, coordinately saturated RhRe (CO)9 has been quantified, by in situ FTIR spectroscopy. 

As expected, suitable selection of a metal combination for each targeted reaction was important to achieve high stereoselectivity. For a sy -selective nitro-Maimich-type reaction, a heterobimetallic complex prepared from Cu(OAc)2, Sm(0-/Pr)3, and dinucleating Schiff base la was the best [12]. Other metal combinations resulted in much lower selectivity. Either Cu or Sm alone also resulted in poor reactivity and selectivity. Thus, both Cu and Sm are essential for high catalytic activity and selectivity. After optimization studies, the addition of achiral phenol source and the use of oxo-samarium alkoxide, Sm50(0-jPr)i3, with a well-ordered structure, gave the superior reactivity and stereoselectivity. Under the optimized reaction cmiditions, 1-10 mol% of the Cu/Sm catalyst promoted asymmetric... 

The first catalytic asymmetric aza-Henry reaction appears to have been reported by Shibasaki in 1999, as part of a general research program focused on heterobimetallic lanthanide complexes and their application in asymmetric catalysis [190]. In the event, a 1 1 3 mixture of KOt-Bu, Yb(Oi-Pr)3, and (i )-BINOL afforded an active catalyst suggested to have the structure 302 (Equation 32) [191]. This catalyst was shown to promote enantioselective additions of nitromethane to N-phosphinoylarylimines, including 300, to provide the corresponding products, such as 303, in 79 % yield and 91 % ee. 

---