Complexes bimetallic—salen

A very successful example for the use of dendritic polymeric supports in asymmetric synthesis was recently described by Breinbauer and Jacobsen [76]. PA-MAM-dendrimers with [Co(salen)]complexes were used for the hydrolytic kinetic resolution (HKR) of terminal epoxides. For such asymmetric ring opening reactions catalyzed by [Co(salen)]complexes, the proposed mechanism involves cooperative, bimetallic catalysis. For the study of this hypothesis, PAMAM dendrimers of different generation [G1-G3] were derivatized with a covalent salen Hgand through an amide bond (Fig. 7.22). The separation was achieved by precipitation and SEC. The catalytically active [Co "(salen)]dendrimer was subsequently obtained by quantitative oxidation with elemental iodine (Fig. 7.22). 

Mirkin and coworkers reported on catalytic molecular tweezers used in the asymmetric ring opening of cyclohexene oxide. In this case the early transition metal is the catalyst and rhodium functions as the structural inductor metal. The catalyst consists of two chromium salen complexes, the reaction is known to be bimetallic, and a switchable rhodium complex, using carbon monoxide as the switch. Indeed, when the salens are forced in dose proximity in the absence of CO the rate is twice as high and the effect is reversible [77]. 

Fig. 24 Initiation mechanisms for a model salen complex bimetallic pathway (a) monometallic pathway (b) binary pathway (c)...

North, M. Synthesis of Cyclic Carbonates from Epoxides and Carbon Dioxide Using Bimetallic Aluminium (Salen) Complexes, ARKTVOC, no. i (2012) 610-628. 

The asymmetric addition of trimethylsilyl cyanide to aldehydes was accomplished in good enantioselectivities in the presence of a bimetallic alu-minium(salen) complex catalyst using triphenylphosphine oxide as the cocatalyst. ... 

Finally, innovative bimetallic complexes as dissymmetric ligands can easily be built, giving birth to an unprecedented class of metal Schiff base salen complexes that can be successfully electropolymerized concomitantly with an overoxidation process. The electrooxidation of 8-(3-acetylimino-6-methyl-2,... 

Cobalt(III)-SALEN complexes (see Fig. 20) were found to be efficient catalysts for asymmetric cyclopropanation (184). Co(acac)2 in the presence of chiral amino alcohols (derived from camphor) has been employed as a catalyst for the enan-tioselective addition of diethylzinc to chalcone (185). Axially chiral SALEN-type ligands possessing biphenyl-core as an element of chirality are efficient ligands for the enantioselective addition of diethylzinc to aldehydes. The formation of bimetallic species forming a chiral pocket was shown (186). 

In 2009, the North group found that bimetallic aluminium(salen) complex 4 was a highly effective catalyst for the asymmetric addition of trimethylsilyl cyanide to aldehydes (Scheme 19.2). In the presence of a phosphine oxide cocatalyst and 2 mol% of the catalyst (l ,l )-[(salen)Al]20 4, the corresponding adducts were obtained in 53-96% enantiomeric excess, which were comparable to those obtained using mononuclear (salen)AlCl complexes. An analysis of the reaction kinetics showed that the reactions exhibited first-order kinetics, with the reaction rate being independent of the aldehyde... 

Scheme 19.3 Possible mechanism for asymmetric cyanosilylation of aldehydes catalysed by a bimetallic Al(salen) complex.

Subsequently, the authors used sodium azide, an inexpensive and easily handled hydrazoic acid precursor, in combination with concentrated aqueous hydrochloric acid, in the conjugate addition to a,p-unsaturated ketones. Chiral bimetallic aluminium(salen) complex (5, 5 )-[(salen)Al]20 ent-4 proved efficient (Scheme 19.32). 

In 2001, North et al. reported the development of bimetallic Ti(IV) and V(IV) salen complexes (100) and (102) as the optimal catalysts for the asymmetric addition of trimethylsilyl cyanide to ketones and aldehydes (Scheme 16.28) [32]. Both catalysts predominantly gave the (S)-enantiomer of the cyanohydrin derivative. Complex (100) proved to be optimal for quantitative conversion of both electron rich and electron deficient ketones into the corresponding cyanohydrin trimethylsilyl ethers... 

Clegg W, Harrington RW, North M, Pasquale R (2010) Cyclic carbonate synthesis catalysed by bimetallic aluminium-salen complexes. Chem Eur J 16 6828-6843... 

Very recently North s group revealed that the bimetallic aluminum(salen) complex 1 (Scheme 9) when used in conjunction with tetrabutylammonium bromide constitutes the only catalyst system capable of catalyzing the insertion of carbon dioxide into oxirane at 1 atm (760 mmHg) and at ambient temperature (25-30... 

North and coworkers recently described an efficient and wide-scope catalytic synthesis of both heterocycles of type A and C (Figure 17) by variation of the reaction temperature [138]. In an initial screening phase, a bimetallic Al(salen) complex was used as a Lewis acid activator in conjxmction with Bu4NBr as nucleophilic cocatalyst at 70 °C in a sealed tube using 2-7 equivalent of CS2 and propylene oxide as substrate. A reaction temperature of 50 °C was found to be optimal considering both the chemical yield of the process as well as the chemoselectivity for the l,3-oxathiolane-2-thione product (96%, ratio A/C 8.0) using only 1.8 equivalent of CS2-... 

The bimetallic A1 complex was able to mediate the synthesis of a series of l,3-oxathiolane-2-thione products with moderate-to-high chemoselectivity and good yields under mild and neat reaction conditions. Even more recently, the same group used a Ti(IV)-salen-based catalyst at somewhat higher temperatures (90 °C) but employing much lower (co)catalyst loadings (lmol%) [139]. 

W. Clegg, R.W. Harrington, M. North, R Villuendas, A bimetallic aluminum(salen) complex for the synthesis of l,3-oxathiolane-2-thiones and l,3-dithiolane-2-thiones, J. Org. Chem. 75 (2010) 6201-6207. 

As a recent progress on chiral aluminum Lewis acids, chiral Al(salen) complexes (25) developed by Jacobsen s group were found to be one of the most useful asymmetric catalysts for enantioselective 1,4-addition to a,P-unsaturated carbonyls (Figure 6.4) [110]. Jha and Joshi also reported the Al-Na bimetallic catalyst prepared from SALEN and Red-Al for Michael addition of malonic diesters to cyclic enones. [Ill]... 

Enantioselective Michael addition of malonates to a,p-unsaturated carbonyls with Al(salen) complex was firstly reported by Jha and Joshi in 2001. They reported the formation of Al-Na bimetallic Al(salen) complex (73b) from (R,R)-salen and NaAlH2(OCH2CH20Me), and catalytic activity for catalytic enantioselective Michael addition of malonic diesters to cyclic a,P-enones (Scheme 6.93) [111]. In the reaction of cyclopentenone with various malonates, this catalyst resulted in good chemical yield and moderate enantioselectivity. 

Subsequently, Jacobsen observed that the Cr-salen complex 138 (X = C1) effects asymmetric additions of TMSN3 to meso-oxiranes in high optical purity (Scheme 9.16) [123]. Mechanistic studies have implicated a key step that involves the intervention of bimetallic catalysis [124]. In such a system, one Cr-salen complex - 139 (X Nj) - serves as the delivery vehicle for the... 

In contrast, Cozzi and Umani-Ronchi found the (salen)Cr-Cl complex 2 to be very effective for the desymmetrization of meso-slilbene oxide with use of substituted indoles as nucleophiles (Scheme 7.25) [49]. The reaction is high-yielding, highly enantioselective, and takes place exclusively at sp2-hybridized C3, independently of the indole substitution pattern at positions 1 and 2. The successful use of N-alkyl substrates (Scheme 7.25, entries 2 and 4) suggests that nucleophile activation does not occur in this reaction, in stark contrast with the highly enantioselective cooperative bimetallic mechanism of the (salen)Cr-Cl-catalyzed asymmetric azidolysis reaction (Scheme 7.5). However, no kinetic studies on this reaction were reported. 

Bi, W.-Y.,Lii, X.-Q., Chai, W.-L., etal. (2008) Construction and NIR luminescent property of hetero-bimetallic Zn—Nd complexes from two chiral salen-type Schiff-base Ugands. Journal of Molecular Structure, 891, 450. 

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