III Salen Complexes

The Jacobsen-Katsuki epoxidation reaction is an efficient and highly selective method for the preparation of a wide variety of structurally and electronically diverse chiral epoxides from olefins. The reaction involves the use of a catalytic amount of a chiral Mn(III)salen complex 1 (salen refers to ligands composed of the N,N -ethylenebis(salicylideneaminato) core), a stoichiometric amount of a terminal oxidant, and the substrate olefin 2 in the appropriate solvent (Scheme 1.4.1). The reaction protocol is straightforward and does not require any special handling techniques. 

To date, a wide variety of structurally different chiral Mn(III)salen complexes have been prepared, of which only a handful have emerged as synthetically useful catalysts. By far the most widely used Mn(III)salen catalyst is the commercially available Jacobsen catalyst wherein R= -C4H8- and R = = i-Bu (Scheme 1.4.1). In... 

In 1990, Jacobsen and subsequently Katsuki independently communicated that chiral Mn(III)salen complexes are effective catalysts for the enantioselective epoxidation of unfunctionalized olefins. For the first time, high enantioselectivities were attainable for the epoxidation of unfunctionalized olefins using a readily available and inexpensive chiral catalyst. In addition, the reaction was one of the first transition metal-catalyzed... 

In most of the successful Diels-Alder reactions reported, dienes containing no heteroatom have been employed, and enantioselective Diels-Alder reactions of multiply heteroatom-substituted dienes, e.g. Danishefsky s diene, are rare, despite their tremendous potential usefulness in complex molecular synthesis. Rawal and coworkers have reported that the Cr(III)-salen complex 15 is a suitable catalyst for the reaction of a-substituted a,/ -unsubstituted aldehydes with l-amino-3-siloxy dienes [21] (Scheme 1.28, Table 1.12). The counter-ion of the catalyst is important and good results are obtained in the reaction using the catalyst paired with the SbFg anion. 

A reagent system that is selective for allylic, benzylic, and cyclopropyl alcohols uses iodosobenzene in conjunction with a Cr(III)(salen) complex.12... 

The insoluble polymer-supported Rh complexes were the first immobilized chiral catalysts.174,175 In most cases, however, the immobilization of chiral complexes caused severe reduction of the catalytic activity. Only a few investigations of possible causes have been made. The pore size of the insoluble support and the solvent may play important roles. Polymer-bound chiral Mn(III)Salen complexes were also used for asymmetric epoxidation of unfunctionalized olefins.176,177... 

Figure 19 Polymer-supported chiral Mn(III)-salen complexes (P = polymer support).

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>. 

Several Ru(III) salen complexes of the type Ruin(salen)(X)(NO) (X=C1-, ONO-, H20 salen = N,AP-bis(salicylidene)-ethylenediamine dianion) have been examined as possible photochemical NO precursors (19). Photo-excitation of the Rum(salen)(NO)(X) complex labilizes NO to form the respective solvento species Ruin(salen)(X)(Sol). The kinetics of the subsequent back reactions to reform the nitrosyl complexes (e.g. Eq. (8)) were studied as a function of the nature of the solvent (Sol) and reaction conditions. The reaction rates are dramatically dependent on the identity of Sol, with values of kNO (298 K, X = C1-) varying from 5 x 10-4 M-1 s-1 in acetonitrile to 4 x 107 M-1 s-1 in toluene, a much weaker electron donor. In this case, Rum Sol bond breaking clearly... 

As pointed out by Hosoya et al.92 the enantiofacial selection of ra-olefins is mainly controlled by the asymmetric centers at the C-8(8 ) carbons, while that of trans-olefins is preferentially controlled by the asymmetric centers at the C-9(9 ) carbons in 119 or 120. Optically active Mn(III)-salen complexes have catalyzed the epoxidation of m-olelins with higher ee (>90%), especially when they are conjugated with an acetylene or phenyl group. However, the epoxidation of trans-olefins with these salen complexes shows rather poor enantio-selectivity (Table 4-18). 

Ironically, oligomeric Co(III) salen complexes 5 (Figme 3) developed by Jacobsen et al. [57, 58] though highly active was non-recyclable. These complexes degraded during HKR of terminal/meso epoxides imder the reaction conditions used. 

Figure 20. Structure of the monomeric Cr(III) salen complex 63 used for impregnation on silica.

Scheme 30. ARO of epoxides catalysed by (Ri )-Cr(III)(salen) complex 64-SLIP.

Table 6 Stirred batch reactor ARO of epoxides catalyzed by a dimeric (R,R)-Cr(III)(salen) complex 65 immobilized in a supported ionic liquid phase compared to the dimeric complex impregnated on silica 64-silica and the reported homogeneous reactions with the monomeric Cr(salen) complex 63...

Gigante, B. Corma, A. Garcia, H. Sabater, M. J. (2000) Assessment of the negative factors responsible for the decrease in the enantioselectivity for the ring opening of epoxides catalyzed by chiral supported Cr(III)-salen complexes Catalysis Lett. 68 113-119. 

Kureshy, R. I. Singh, S. Khan, N. H. Abdi, S. H. R . Agrawal S. Jasra R. V. (2006) Enantioselective aminolytic kinetic resolution (AKR) of epoxides catalyzed by recyclable polymeric Cr(III) salen complexes Tetrahedron Asymmetry 17 1638-1643. 

The effect of structural variation and the use of different caboxylate salts as cocatalysts was investigated by Pietikainen . The epoxidation reactions were performed with the chiral Mn(III)-salen complexes 173 depicted in Scheme 93 using H2O2 or urea hydrogen peroxide as oxidants and unfunctionalized alkenes as substrates. With several soluble carboxylate salts as additives, like ammonium acetate, ammonium formate, sodium acetate and sodium benzoate, good yields (62-73%) and moderate enantioselectivities (ee 61-69%) were obtained in the asymmetric epoxidation of 1,2-dihydronaphthalene. The results were better than with Ai-heterocycles like Ai-methylimidazole, ferf-butylpyridine. 

Table 1 Catalytic data for homogeneous and poly(styrene) supported Co(III) salen complexes in the ffKR of rac-epichlorohydrin.

Poly (norbornene)-supported Co(III)-Salen complexes (44) and poly (styrene)-supported Co(III)-Salen complexes (45,46) were synthesized via newly developed procedures. In particular, a new, high-yielding, one-pot synthesis of non-symmetrical salens was developed (46). Hydrolytic kinetic resolutions were carried out at room temperature and the products were characterized by chiral GC. 

An interesting reversal of chiral induction in chromium(III)-salen complexes using a tartaric derived alicyclic diamine moiety (i.e., 7) has been observed by Mosset, Saalfrank, and co-workers <99T1063>. Thus, epoxidation of the chromene 8 using catalyst 7 and an oxidant consisting of MCPBA/NMO afforded the 3S,4S epoxide 9, whereas the Jacobsen catalyst (1) provided the corresponding 3R,4R enantiomer. A mechanistic rationalization for this curious crossover has not yet been proposed. 

In conclusion, the chiral salen Co(III) complexes immobilized on Si-MCM-41 colud be synthesized by multi-grafting method. The asymmetric synthesis of diols from terminal olefins was applied with success using a hybrid catalyst of Ti-MCM-41/chiral Co(III) salen complexes. The olefins are readily oxidized to racemic epoxides over Ti-MCM-41 in the presence of oxidants such as TBHP, and then these synthesized diols are generated sequentially by epoxide hydrolysis on the salen Co(lll) complexes. This catalytic system may provide a direct approach to the synthesis of enantioselective diols from olefins. 

Katsuki has recently constructed a series of chiral Co(III)-salen complexes 30 and used them as catalysts for cyclopropanation of styrene using tert-butyl diazoacetate [78], Not only is enantiocontrol for addition greater than 90% ee, but diastereoselectivity favors the trans-... 

Discussions to this point rely on the hypothesis that the ligands of oxo(salen)manganese(V) complexes have planar structures by analogy to metalloporphyrin complexes and Mn(III)-salen complexes 11 and 12, the structures of which were determined by the X-ray crystallographic analysis [29a,47]. However, the assumption that the ligand of the oxo-Mn-salen species is planar failed to give a satisfactory explanation for the following stereochemistry observed in the recent study. Trans-cis selectivity in the epoxidation of 1-alkylindenes usually improves as the steric bulk of the catalyst increases. However, the epoxidation of 1-methylindene with the smallest... 

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