Chiral metal complexes ligand transformation

The continuous availability of trillions of independent microreactors greatly multiplied the initial mixture of extraterrestrial organics and hydrothermal vent-produced chemicals into a rich variety of adsorbed and transformed materials, including lipids, amphiphiles, chiral metal complexes, amino add polymers, and nudeo-tide bases. Production and chiral amplification of polypeptides and other polymeric molecules would be induced by exposure of absorbed amino adds and organics to dehydration/rehydration cydes promoted by heat-flows beneath a sea-level hydro-thermal field or by sporadic subaerial exposure of near-shore vents and surfaces. In this environment the e.e. of chiral amino adds could have provided the ligands required for any metal centers capable of catalyzing enantiomeric dominance. The auto-amplification of a small e.e. of i-amino adds, whether extraterrestrially delivered or fluctuationally induced, thus becomes conceptually reasonable. 

In this overview, the opportunities and problems associated with the industrial application of chiral metal complexes will be analyzed in detail. In Section 3.11.2, the critical factors are discussed which affect the feasibility of an enantioselective catalyst. In the following Sections, important families of chiral ligands are listed and finally about 40 types of catalytic transformations are described and characterized regarding enantioselectivity, catalyst activity, and productivity, and their potential for technical applications is assessed. 

The BV transformations with chiral metal complexes are based on either transition metals such as Cu [37b, 40, 41], Pt [42, 43], Co [44], Pd [45], Zr [46], Hf [47] or nontransition metals such as Mg [48] and Al [49]. For example, Kocovsky and coworkers have developed a series of new terpene-derived pyridinephosphine ligands, whose complexes with Pd(II) have been proven to catalyze the BV oxidation. Prochiral cyclobutanones (1) were oxidized at low temperature (—40°C), with 5 mol % catalyst loading and the urea-H202 complex as the stoichiometric oxidant to give lactones 2 in good yields and up to 81% ee (Eq. (10.5)) [50],... 

The ferrocene-derived Josiphos ligands (137) first reported by Togni and Spindler [99] are another class of chiral diphosphine ligands that are of immense importance both in asymmetric hydrogenations and in other asymmetric transformations. A variety of substrates can be reduced with high enantioselectivity in the presence of chiral metal complexes derived from these ligands. The reduction of dimethyl itaconate (136) furnishes 138 in quantitative yield and 98-99 % ee (Equation 39) [99]. 

Ghosh et al. [70] reviewed a few years ago the utihty of C2-symmetric chiral bis(oxazoline)-metal complexes for catalytic asymmetric synthesis, and they reserved an important place for Diels-Alder and related transformations. Bis(oxazoline) copper(II)triflate derivatives have been indeed described by Evans et al. as effective catalysts for the asymmetric Diels-Alder reaction [71]. The bis(oxazoline) Ugand 54 allowed the Diels-Alder transformation of two-point binding N-acylimide dienophiles with good yields, good diastereos-electivities (in favor of the endo diastereoisomer) and excellent ee values (up to 99%) [72]. These substrates represent the standard test for new catalysts development. To widen the use of Lewis acidic chiral Cu(ll) complexes, Evans et al. prepared and tested bis(oxazoHnyl)pyridine (PyBOx, structure 55, Scheme 26) as ligand [73]. 

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