Other Stereo-complex Systems

C3-symmetrical tripodal ligands have been used to obtain amido complexes of family (86). If chiral substituents are present in the ligand periphery, some stereo discrimination is observed in the reaction with several chiral ketones and aldehydes. On the other hand, such systems can generate early-late heterobimetallics with metal-metal bonds (equation 41). Reaction of (91) with MeNC as well as the heteroallenes CO2, CS2, RNCO and RNCS led to insertion into the polar metal-metal bond. Tris (pyrazolyl) borate see Tris(pyrazolyI)borates) Zr and Hf complexes are other interesting examples of the type (86). In combination with MAO, they give promising results in ethylene and ethylene/hexene polymerizations. Substitution of these sterically crowded ligands allows adjustements of the environment of the active site to the... 

Our initial studies focused on the transition metal-catalyzed [4+4] cycloaddition reactions of bis-dienes. These reactions are thermally forbidden, but occur photochemically in some specific, constrained systems. While the transition metal-catalyzed intermole-cular [4+4] cycloaddition of simple dienes is industrially important [7], this process generally does not work well with more complex substituted dienes and had not been explored intramolecularly. In the first studies on the intramolecular metal-catalyzed [4+4] cycloaddition, the reaction was found to proceed with high regio-, stereo-, and facial selectivity. The synthesis of (+)-asteriscanoHde (12) (Scheme 13.4a) [8] is illustrative of the utihty and step economy of this reaction. Recognition of the broader utiHty of adding dienes across rc-systems (not just across other dienes) led to further studies on the use of transition metal catalysts to facilitate otherwise difficult Diels-Alder reactions [9]. For example, the attempted thermal cycloaddition of diene-yne 15 leads only... 

Metalloporphyrins and some related metal complexes are effective catalysts in IOB oxidations, as already discussed for alkenes, and acids (Sections 5.1.1.1 and 5.2.4). Also, sulphides have been oxidized to sulphoxides [58]. Some other substrates of various types underwent such catalysed oxidations, because these systems mimic the natural oxidant cytochrome P-450 [2]. From a synthetic point of view, only a few reactions are of importance alkanes were mainly used which underwent regio- and stereo-specific hydroxylation, for instance the methyl group of a pyrrole derivative was converted into hydroxymethyl, leading to one-pot preparation of dipyrro-methanes [59], The preparation of elaborated catalysts is, however, very demanding and precludes a wider use. 

The regio- and stereo-chemical outcome of reductions of cyclic allylic systems is controlled by more complex factors. Reduction of the allyl bromides (6) and (7) occurs exclusively by syn 7-attack (Scheme 9), but other structural features influence the reaction. For example, where the allylic double bond is in a thermodynamically stable position, rearrangement does not normally occur. The reaction has also been studied in complex carbohydrate alkenes. ... 

The combination of triphenylphosphine with esters of trihaloacetic acids provides a reagent system for the stereo- and regio-selective conversion of alcohols into alkyl halides.The bromine-triphenylphosphine adduct has been used at low temperatures (-50 C in dichloromethane) for the removal of the tetrahydropyranyl protecting group from tetrahydropyranyl ethers derived from secondary and tertiary alcohols.The reactions of tertiary phosphines (and other trivalent phosphorus compounds) with iodine in aprotic solvents have received further study, a range of species being identified.The first reported study of the reactions of trivalent phosphorus compounds with monopositive astatine has led to the identification of stable complexes with triphenylphosphine, trioctylphosphine, and triethylphosphite. 

Two possible mechanisms have been proposed for this process. The first one is comparable with the Ni/tartaric acid system. Cinchonidine is adsorbed at the edges of Pt crystals or on ad-atoms on the Pt surfaces. These sites then allow the approach of ethyl pyruvate only with stereo-special configurations, resulting in enantio-selectivity. In the other mechanism a complex-like supramolecule of ethyl pyruvate and cinchonidine forms in the solution and approaches the metal surface afterward. The reaction is carried out as well in stirred vessels under a few megapascals. " ... 

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