Homogeneous stereocontrol

Burgess, K. Ohlmeyer, M. J. Stereocontrol in Catalysed and Uncatalysed Hydroborations. In Homogeneous Transition Metal Catalyzed Reactions Moser, W. R. Slocum, D. W., Eds. Advances in Chemistry Series 230 American Chemical Society Washington DC, 1992 pp 163-177. 

Section 3 will deal with catalytic systems whose stereospecificity is mainly controlled by the chirality of the environment of the transition metal, independently of the possible chirality of the growing chain (chiral site stereocontrol). In particular, in Section 3.1 the chirality and stereospecificity of homogeneous catalytic systems based on metallocenes of different symmetries and in different experimental conditions will be reviewed. In Section 3.2 the chirality of model catalytic sites, which have been supposed for isospecific first-generation TiCl3-based and high-yield MgC -supported catalysts, is described. In Section 3.3 we will present a comparison between model catalytic sites proposed for heterogeneous and homogeneous stereospecific site-controlled catalysts. 

Diastereoselective radical cyclizations A= stereocontrol element B = substrate (homogeneous)... 

We also tried to trap the presumed intermediate radical A using acrylonitrile, butyl vinyl ether or allyltributylstannane as the trapping reagents. In the first two cases mixtures of the 2-deoxy-lactone 65 and the C-2-alkylated product were obtained. Treatment of 20 with allyltributylstannane gave 68 as a homogeneous product (Scheme 14) in a fully stereocontrolled reaction [45,98b]. 

In view of the data concerning propylene polymerisation in the presence of homogeneous vanadium-based Ziegler-Natta catalysts, the syndiospecificity of the polymerisation is believed [387,395] to arise from steric repulsions between the last inserted monomer unit of the growing chain and the methyl group of coordinated propylene molecule, i.e. chain end stereocontrol is postulated to play the essential role in the stereoregulation. 

Over the years two mechanisms have been proposed as being responsible for the stereocontrol of the growing polymer chain firstly the site-control mechanism and secondly the chain-end control mechanism. In the site-control mechanism the structure of the catalytic site determines the way the molecule of 1-alkene will insert (enantiomorphic site-control). Obviously, the Cossee mechanism belongs to this class. In the chain-end control mechanism the last monomer inserted determines how the next molecule of 1-alkene will insert. Several Italian schools have supported the latter mechanism. For heterogeneous catalysts it would seem that site control was strongly gaining preference [32], at least until the more detailed work on homogeneous catalysts became known. 

The potential utility of the stereocontrol available through exo [6 + 4] cycloaddition in the tropone series can be seen in Garst s stereoselective cyclodecene syndiesis. This methodology is predicate on the efficient translation of the double bond geometry from stereochemically homogeneous ( ) and (Z)-1-acetoxybutadiene via cycloadducts (30) and (35) into ( )-cyclodecene (36) and (Z)-cyclodecene (37), respectively. Reduction of cycloadducts (30) and (35) to the corresponding fully saturated ketoacetates, followed by conversion to the mesylates and hydri reduction, led to the desired 10-membered ring products (36) and (37) in 78% and 40% yields, respectively. 

Having established the positive effect of the presence of water, let us examine which protocols are to be adopted and which are deleterious in terms of catalytic activity and overall stereocontrol.The first protocols reported in DMSO did not focus on the presence of water but it is likely that the required trace amount was present as contaminant of the solvent used. The procedure to avoid is a homogeneous reaction in water as solvent using water-soluble carbonyl compounds in this case reactions likely proceed under general base catalysis mechanisms, resulting in very poor conversions and lack of enantio-control. Thus, two useful approaches for reactions in water are available in the... 

The hydroxyl coordination with catalysts affords an efficient stereocontrol of the homogeneous hydrogenation of hydrindanone 17. ... 

Homogeneous catalysis can be effected vsrith alkyllithiums, but the results depend on the purity of the reaction system. On the one hand, Braun et al. (1960) and Kem (1960) reported that isotactic polymer was formed, supposedly by homogeneous catalysis. On the other hand, Worsfold and Bywater (1963) showed that pure butyllithium gave syndiotactic polystyrene. The formation of isotactic polymer was presumably caused by lithium hydroxide. This was shown by adding traces of moisture to react with part of the butyllithium. In previously reported examples, it appears that either the solvent or monomer was contaminated with moisture. It is not known whether the lithium hydroxide was dissolved or dispersed colloidally. If dispersed colloidally, it could exert stereocontrol through surface effects. If dissolved, the lithium hydroxide could effect stereocontrol as an ion-pair associated with growing chain end. 

Although significant advances in stereoselective epoxide polymerization have been achieved over the last half-century, few known catalysts are capable of excellent levels of stereocontrol. Historically, most catalysts for epoxide polymerization have been of the heterogeneous variety and have exhibited poor selectivity. It is our opinion that the most fertile area for future catalyst exploration involves homogeneous, discrete catalysts that are capable of involving multiple metal centers in the polymerization mechanism. If the spatial environment of the active catalyst is precisely controlled, new generations of stereoselective epoxide polymerization catalysts will become available. Our current research focuses on the search for such catalysts. 

With the advent of these homogeneous catalysts, extensive investigations followed on the influence of the catalyst geometry on the stereospecificity of olefin polymerization and the origins of stereocontrol. Zambelli employed the chiral metallocene (Hla) to study the... 

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