Derived Catalysts

As in the MBH reactions, / -ICD (44) is also an efficient and remarkably general catalyst in aza-MBH reactions [94, 95]. This catalyst promotes the addition a variety of electron-deficient olefins such as acrylates, enones, and enals with activated aromatic aldimines. Of note, the absolute stereochemistry of the product is generally opposite compared to the analogous MBH reaction with / -ICD catalyst imines gives rise to (S)-enriched adducts, in contrast to aldehydes which afford ( -products [94]. Substitution patterns of the olefin may alter or even invert this trend, however. 

The addition of acrylates to activated aryl imines (103) was studied by the groups of Hatakeyama and Shi [94, 95]. In Hatakeyama s study, various aryl di-phenylphosphinoyl imines and HFIPA, 43, afforded in DMF at low temperature the (S)-product (104) in yields up to 97% and high ee (Table 5.11) [94]. 

Similar to the addition to HFIPA, the /MCD-mediated addition of methyl, phenyl, and naphthyl acrylates to N-sulfonyl imines afforded the (S)-adduct, which is opposite to that observed with aldehydes (Table 5.12) [95]. The best conditions were found using either dichloromethane or acetonitrile as solvent. 

A BINOL-dimethylaminopyridine hybrid was seen to be efficient in mediating the MBH reaction (Table 5.14) [96], with optimal reaction conditions being found as —15 °C with a mixed solvent system consisting of toluene and cyclopentyl methyl ether (CPME) in a 1 9 ratio. The reaction was sensitive to the structure of the catalyst 112, the position of the Lewis base attached to BINOL, the substitution pattern of the amino group, and the length of the spacer. It should be noted that the bulky i-Pr substituent on the amino group showed the best selectivity and kinetic profile (Table 5.14, entry 5) [98]. (For experimental details see Chapter 14.10.4). 

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Interestingly, G jrey et al.", employing a similar tryptophan-derived catalyst (3.4), observed a 99% enantiomeric excess (ee) in the Diels-Alder reaction of 2-bromoacrolein with cyclopentadiene... 

Newer silicone adhesives having solids levels up to 97% are also commercially available [109]. Instead of using silanol condensation reactions, they rely on addition chemistry between vinyl functional silicone oligomers and silicon hydride terminated silicones. This addition reaction is typically facilitated with platinum derived catalysts. This hydrosilation process can be run at reduced oven temperatures, but the finished products typically do not yield the same balance of properties as seen for condensation cure materials. 

The chiral BOX-metal(II) complexes can also catalyze cycloaddition reactions of other ketonic substrates [45]. The reaction of ethyl ketomalonate 37 with 1,3-conju-gated dienes, e.g. 1,3-cyclohexadiene 5c can occur with chiral BOX-copper(II) and zinc(II) complexes, Ph-BOX-Cu(OTf)2 (l )-21a, and Ph-BOX-Zn(OTf)2 (l )-39, as the catalysts (Scheme 4.29). The reaction proceeds with good yield and ee using the latter complex as the catalyst. Compared to the copper(II)-derived catalyst, which affects a much faster reaction, the use of the zinc(II)-derived catalyst is more convenient because the reaction gives 94% yield and 94% ee of the cycloaddition product 38. The cycloaddition product 38 can be transformed into the optically active CO2-... 

In a more recent work the same research group has applied cyclic and acyclic vinyl ethers in the oxazaborolidinone-catalyzed 1,3-dipolar cycloaddition reaction with nitrones [30]. The reaction between nitrone 5 and 2,3-dihydrofuran 6 with 20 mol% of the phenyl glycine-derived catalyst 3c, gave the product 7 in 56% yield as the sole diastereomer, however, with a low ee of 38% (Scheme 6.9). 

Tryptophan derived catalyst 17 also allows the additions of a-unsubstituted enol ethers to aldehydes to be directed in an enantioselective manner38. 

The enantioselection depends greatly on the nature of the R2 group at the boron atom, and the ee values were as high as 97 %. High enantioselectivity was observed in the synthesis of 4-dihydropyranones, based on the Diels-Alder reactions of aldehydes 74 and Danishefsky s diene, catalyzed by a BINOL-Ti(0-i-Pr)4-derived catalyst [75] (Equation 3.23). 

The adduct derived from (a-benzyloxyacetaldehyde (97 % ee) is an important intermediate en route to compactin and mevinolin [76]. In contrast, modest enantioselectivity was attained when the cycloadditions were catalyzed by a chiral BINOL-ytterbium-derived catalyst [77]. Pyridines were used as additives, and the best enantioselection (93% ee) was attained only in the case of p-methoxybenzaldehyde using 2,6-lutidine. 

A MgO-supported W—Pt catalyst has been prepared from IWsPttCOIotNCPh) (i -C5H5)2l (Fig. 70), reduced under a Hs stream at 400 C, and characterized by IR, EXAFS, TEM and chemisorption of Hs, CO, and O2. Activity in toluene hydrogenation at 1 atm and 60 C was more than an order of magnitude less for the bimetallic cluster-derived catalyst, than for a catalyst prepared from the two monometallic precursors. 

These ligands can readily be obtained by a Grignard reaction of aziridine esters, followed by an acidic detritylation (see Scheme 40) [19,55]. These aziridine carbinol-derived catalysts are equally efficient as the Corey ligand 55 derived from proline carbinols (Fig. 4) [55,56]. 

Rhodium Atom-Derived Catalysts in the Hydroformylation of 1,3-Dienes and in the Hydrosilylation of Aromatic Nitriles... 

The use of rhodium catalysts for the synthesis of a-amino acids by asymmetric hydrogenation of V-acyl dehydro amino acids, frequently in combination with the use of a biocatalyst to upgrade the enantioselectivity and cleave the acyl group which acts as a secondary binding site for the catalyst, has been well-documented. While DuPhos and BPE derived catalysts are suitable for a broad array of dehydroamino acid substrates, a particular challenge posed by a hydrogenation approach to 3,3-diphenylalanine is that the olefin substrate is tetra-substituted and therefore would be expected to have a much lower activity compared to substrates which have been previously examined. 

Boratabenzene analogues of commercially significant constrained geometry catalysts have also been investigated.49 For the MAO-activated copolymerization of ethylene/l-octene, the illustrated Ti(IV)-boratabenzene complex is about four times more active than the Zr(IV) complex. The level of 1-octene incorporation is significantly lower than for the corresponding Cp-derived catalysts, due perhaps to the greater steric demand of the amidoboratabenzene framework. 

Withers, H.P., Eliezer, K.F., and Mitchell, J.W. 1990. Slurry-phase Fischer-Tropsch synthesis and kinetic studies over supported cobalt carbonyl derived catalysts. Ind. Eng. Chem. Res. 29 1807-14. 

Dimethylchromene has also proven to be a useful substrate for the assessment of various transition metal complexes as epoxidation catalysts. Chiral Mn(III)-salen complexes are efficient <00CC615 00T417> and can be recycled when used in an ionic liquid <00CC837>. The enantioselective aziridination of a chromene has been achieved using a chiral biaryldiamine-derived catalyst (Scheme 22) <00JA7132>. 

Scheme 12.3 Formation of dihydride intermediates of a cationic Rh complex via displacement of the NMD ligand in the DIPHOS-derived catalyst (S = solvent).

Fig. 26.2 Water-soluble or recyclable BINAP or BIPHEP derivative catalysts.

Recently, a series of pyridine- and quinoline-derived catalysts 31 (Fig. 29.18) have been developed. Ligands 31a-k were obtained by reduction of pyridyl ke-... 

The diamine and TADDOL-derived catalysts 44 were tested on substrates 36-39, giving good enantioselectivities however, high catalyst loadings of 4 mol% were required for full conversion [34]. 

The highest ee-values were obtained with the PHOX catalyst 42 and the pyridine-derived catalyst 60 a (Table 30.14, entries 1 and 2). 

The development of chiral hydrogenation catalysts for unfunctionalized alkenes also allows enantioselective hydrogenation of functionalized olefins where the functionality in the molecule is remote from the double bond. A series of oxazoline-, imidazoline- and pyridine-derived catalysts have been screened for the hydrogenation of unsaturated derivatives of vitamin E (Scheme 30.3). Hy-... 

Pfaltz and co-workers (28) also showed that the semicorrin-derived catalyst is remarkably effective in intramolecular cyclopropanation reactions (28). Cycliza-tion of co-alkenyl diazoketones to form six-membered rings proceeds in high selectivity, while the analogous five-membered rings are somewhat more sensitive to substitution on the pendant alkene, Eqs. 16 and 17. 

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