Enantioselectivity encounter complex

STEREOCHEMICAL TERMINOLOGY, lUPAC RECOMMENDATIONS ENANTIOSELECTIVE REACTION ASYMMETRIC INDUCTION ENCOUNTER COMPLEX ENCOUNTER-CONTROLLED RATE DIFFUSION CONTROL FOR BIMOLECULAR COLLISIONS ENDERGONIC PROCESS ENDO-a (or j8)-N-ACETYLGALACTOSAMI-NIDASE... 

Nevertheless, this mechanism cannot fully explain the enantioselectivity encountered. In the same area, Martens et al. have reported the synthesis of numerous oxazaphospholidine borane complexes and their use in the enantiose-... 

Brunner et al. [26] synthesized and applied so-called dendrizymes in enan-tioselective catalysis. These catalysts are based on dendrimers which have a functionalized periphery that carries chiral subunits, (e.g. dendrons functionalized with chiral menthol or borneol ligands). The core phosphine donor atoms can be complexed to (transition) metal salts. The resultant dendron-enlarged 1,2-diphosphino-ethane (e.g. 16, see Scheme 17) Rh1 complexes were used as catalysts in the hydrogenation of acetamidocinnamic acid to yield iV-acetyl-phenylalanine (Scheme 17) [26]. A small retardation of the hydrogenation of the substrate was encountered, pointing to an effect of the meta-positioned dendron substituents. No significantly enantiomerically enriched products were isolated. However, a somewhat improved enantioselectivity (up to 10-11% e.e.) was... 

There are no mechanistic details known from intermediates of copper, like we have seen in the studies on metathesis, where both metal alkylidene complexes and metallacyclobutanes that are active catalysts have been isolated and characterised. The copper catalyst must fulfil two roles, first it must decompose the diazo compound in the carbene and dinitrogen and secondly it must transfer the carbene fragment to an alkene. Copper carbene species, if involved, must be rather unstable, but yet in view of the enantioselective effect of the ligands on copper, clearly the carbene fragment must be coordinated to copper. It is generally believed that the copper carbene complex is rather a copper carbenoid complex, as the highly reactive species has reactivities very similar to free carbenes. It has not the character of a metal-alkylidene complex that we have encountered on the left-hand-side of the periodic table in metathesis (Chapter 16). Carbene-copper species have been observed in situ (in a neutral copper species containing an iminophosphanamide as the anion), but they are still very rare [9],... 

As described in the catalytic enantioselective borane reduction of ketones using oxazaphospholidine-borane complexes, a mechanistic rationale has been proposed. Nevertheless, this mechanism cannot fully explain the enantioselec-tivity encountered. 

Since the diphosphine is appreciably more electron-rich than is BINAP, the major ruthenium complex is a more active hydrogenation catalyst than the parent. Increased electron-rich ligation may be the reason for the success of heterocyclic analogues of BINAP in which the binaphthalene is replaced by a bi(ben-zothiophene) or biindolyl the resulting Ru complexes are effective both in terms of enantioselectivity and reactivity [139]. Readers of the related Chapter 6.1 on the asymmetric hydrogenation of carbonyl compounds will encounter the Ru complexes of ligands in the DUPHOS family, where the ease of modification of the alkyl substituents of the phospholane enhances the power of the system, since it permits the easy optimization of ee for any substrate [140]. 

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-... 

Titanium complexes are often encountered in Lewis acid-catalysed reactions. This is certainly true for catalysed aldol reactions. Mikami and Matsukawa demonstrated that titanium/BINOL complexes e.g. complex (7.20) afforded high yield and enantioselectivity in the aldol reactions of thioester ketene silylacetals with a variety of aldehydes. In contrast to some of the aldol reactions described above, the stereochemistry of the adducts is dependant on the geometry of the enol ether. Thus, reaction of the (B)-enol ether (7.21) with aldehyde (7.22) yields the sy -aldol adduct (7.23) predominantly while the (Z)-e.no ether (7.24) results in isolation of the anti-adduct (7.25) as the major product. The authors invoke a closed silatropic ene transition state (structure (7.26) for syn-transition state), substantiated by suitable crossover experiments, to explain the diastereoselectivities... 

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