Catalyst salen-based

Langanke J, Leitner W (2008) Regulated Systems for Catalyst Immobilisation Based on Supercritical Carbon Dioxide. 23 91-108 Larock R (2005) Palladium-Catalyzed Annulation of Alkynes. 14 147-182 Larrow JF, Jacobsen EN (2004) Asymmetric Processes Catalyzed by Chiral (Salen)Metal Complexes 6 123-152... 

Dialkylzincs are much less reactive than phenyl or alkynylzincs. In 2002, Kozlowski et al. developed a chiral salen-based catalyst 62 that can promote the diethylzinc addition to a-ketoesters in high yield, [Eq. (13.38)]. In their catalysis, titanium acts as a Lewis acid, and amine nitrogen acts as a Lewis base (63). The enantioselectivity was up to 78% ee ... 

Katsuki and coworkers reported that Zr(salen)-based derivatives served as efficient catalysts for asymmetric BV reactions using UHP as terminal oxidant . Complex 117 in chlorobenzene at 0 °C showed the best catalytic activity with enantiomeric excesses higher than 80%, in most cases. Similarly to what was proposed for Ti(salen) derivatives (equation 51), the treatment of Zr(salen) complexes with H2O2 would give a Zr-peroxo (salen) complex 118, prone to ring-opening with formation of a Criegee adduct 119 and evolution to products (equation 81). 

Very few methods developed recently are applicable to the cyanation of not only electron-rich but also electron-deficient aromatic, as well as aliphatic aldehydes. Titanium(IV)-derived complex 1 (Fig. 1), reported by Uang, catalyzes the hydro cyanation of aromatic, a, 3-unsaturated, and aliphatic aldehydes with high enantioselectivity (>88% ee for all substrates reported) [15]. Similarly, Ti(IV)-catalyst 2 developed by Choi has proved to be highly enantioselec-tive for the cyanation of various classes of aldehydes (>90% ee for most substrates) [16]. Belokon and co-workers reported two chiral salen-based systems (salen)VO catalyst 3a and [(salen)TiO]2 3b, both of which provided moderate levels of enantioselection when applied to the asymmetric cyanation of diverse... 

Chiral (salen)Mn(III)Cl complexes are useful catalysts for the asymmetric epoxidation of isolated bonds. Jacobsen et al. used these catalysts for the asymmetric oxidation of aryl alkyl sulfides with unbuffered 30% hydrogen peroxide in acetonitrile [74]. The catalytic activity of these complexes was high (2-3 mol %), but the maximum enantioselectivity achieved was rather modest (68% ee for methyl o-bromophenyl sulfoxide). The chiral salen ligands used for the catalysts were based on 23 (Scheme 6C.9) bearing substituents at the ortho and meta positions of the phenol moiety. Because the structures of these ligands can easily be modified, substantia] improvements may well be made by changing the steric and electronic properties of the substituents. Katsuki et al. reported that cationic chiral (salen)Mn(III) complexes 24 and 25 were excellent catalysts (1 mol %) for the oxidation of sulfides with iodosylbenzene, which achieved excellent enantioselectivity [75,76]. The best result in this catalyst system was given by complex 24 in the formation of orthonitrophenyl methyl sulfoxide that was isolated in 94% yield and 94% ee [76]. 

Another example of catalyst modification was achieved by synthesizing a Co(salen)-based catalyst with pendent ammonium salts (Figure 8.27) [54], This complex was able to catalyze the copolymerization of C02 and PO at TOF-values of up to 26000h 1, with high molecular weights and low polydispersities. Upon completion of the reaction the polymer/catalyst mixture was filtered through a pad of silica which yielded the purified polymer product. When the Co(salen) complex had been recovered from the silica it was recycled several times, with little to no loss of activity. 

Application of Salen-based Catalysts to Asymmetric Catalysis... 

Scheme 1 Selected reactions of chiral salen-based catalysts...

The salen-based catalysts mentioned above are not soluble in water, which constitutes a limitation this is overcome by the preparation of new amphiphilic salen-type transition metal complexes. Therefore, several bulky salen-type SB ligands containing both tert-butyl and methyl(triphenylphosphonium) substituents have been prepared.122 The introduction of both lipophilic and ionic substituents in the ligands increases the solubility of the complexes of these ligands, which are found to be soluble both in water and in most common organic solvents and this may enhance the catalytic properties of the complexes. 

Besides mono-Cp systems,958,959,973-989 other half-sandwich complexes with different aromatic ligands were investigated.969-971,982,983,985,990-993 Other studies were concerned with polymerization conditions, including catalyst heterogenization.990,994-1006 Other catalysts such as mono-276 and bis-benzamidinate1007 complexes, bis(phenolato) Ti complexes, 8 and salen-based Ti complexes,1009 are also able to polymerize styrene to a... 

The more thoroughly developed salen-based catalysts have also been studied in the context of aziridination, albeit with limited success. While Burrows observed no measurable enantioselection in the aziridination of styrene derivatives using simple chiral (salen)Mn catalysts derived from 1,2-phenylethylenedi-amine [14], Katsuki encountered some success (up to 28% ee in the azidination of styrene) with more complex derivatives of the same diamine [15]. Substantially improved enantioselectivities were observed with a less hindered diamine backbone associated with highly optimized chiral salicylide elements. Thus, up to 94% ee has been obtained in the aziridination of styrene with a 2,3-diaminob-utane-derived catalyst (Scheme 6) [16]. Incorporation of catalytic levels of a py-... 

Asymmetric epoxidationJacobsen s salen-based catalyst 1, derived from (R,R)-or (S,S)-diphcnyl-l,2-diaminoethane (16,157) can effect asymmetric epoxidation of alkenes with NaOCI, but the enantioselectivity is generally only moderate ( 70% ee) in the case of ci.v-alkencs. Subsequently, this group has examined salen-based catalysts... 

Work has been done with salen catalysts to enable enantioselective epoxidations with m-CPBA. Highly enantioselective K.ochi-Jacobsen-K.atsuki epoxidation of unfunctionalized olefins with Mn(III)-(salen)-based chiral catalyst provides an efficient route to optically active epoxides. It has been noted that m-CPBA in the presence of JV-methylmorpholine N-oxide at low temperature (e.g., -78 °C) suppresses bond rotation leading to high enantioselectivity even when the substrate is acyclic.10,15... 

Another interesting asymmetric catalyst for epoxidation of simple olefins such as cis-PhCH=CHMe was reported by Jacobsen and coworkers. In this example the catalyst was a chiral manganese salen-based complex (3.54) (Figure 3.22), and the oxidant used was NaOCl high yields and ee of > 90% were achieved. 

Asymmetric reactions that can exhibit this type of behavior include atom and group transfer reactions, such as the asymmetric oxidation of sulfides, some asymmetric epoxidations of olefins, " asymmetric aziridination of olefins, - and as)rmmetric cyclo-propanation of olefins. In the asymmetric oxidation of sulfides, a non-racemic, cliiral, low-valent metal complex is oxidized, in this case by iodosobenzene, to generate a highly reactive 0x0 intermediate. The 0x0 is then transferred directly to the sulfur to form the sulfoxide in the enantioselectivity-determining step. A representative example is illustrated in Equation 14.12 that involves a chiral salen-based catalyst. ... 

Recently, immobilization of salen-based catalysts has been demonstrated both on solid supports [83] and on dendritic molecular frameworks, which allow for en-antioselective catalysis with good to excellent ee over several cycles [84]. 

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