Transition-metal derivatives stereochemistry

Lower oxidation states are rather sparsely represented for Zr and Hf. Even for Ti they are readily oxidized to +4 but they are undoubtedly well defined and, whatever arguments may be advanced against applying the description to Sc, there is no doubt that Ti is a transition metal . In aqueous solution Ti can be prepared by reduction of Ti, either with Zn and dilute acid or electrolytically, and it exists in dilute acids as the violet, octahedral [Ti(H20)6] + ion (p. 970). Although this is subject to a certain amount of hydrolysis, normal salts such as halides and sulfates can be separated. Zr and are known mainly as the trihalides or their derivatives and have no aqueous chemistry since they reduce water. Table 21.2 (p. 960) gives the oxidation states and stereochemistries found in the complexes of Ti, Zr and Hf along with illustrative examples. (See also pp. 1281-2.)... 

The transition metal-catalyzed allylic substitution using hard or unstabilized nucleophiles has not been extensively studied, particularly with unsymmetrical allylic alcohol derivatives. This may be attributed to the highly reactive and basic nature of these nucleophiles and the inability to circumvent regiochemical infidehty in unsymmetrical systems. Hard nucleophiles may be characterized as those that undergo substitution with net inversion of stereochemistry [29], due to their propensity to add directly to the... 

This is a reaction that seems veiy attractive for synthesis but, in the absence of a transition met catalyst, the yields are very low. We showed in the last chapter how vinyl silanes can be made wit control over stereochemistry and converted into lithium derivatives with retention. Neither of thes vinyl metals couple with vinyl halides alone. But in the presence of a transition metal—Cu (I) for I and Pd(0) for Sn—coupling occurs stereo specifically and in good yield. 

Epoxidation of allylic alcohols with peracids or hydroperoxide such as f-BuOaH in the presence of a transition metal catalyst is a useful procedure for the synthesis of epoxides, particularly stereoselective synthesis [587-590]. As the transition metal catalyst, molybdenum and vanadium complexes are well studied and, accordingly, are the most popular [587-590], (Achiral) titanium compounds are also known to effect this transformation, and result in stereoselectivity different from that of the aforementioned Mo- and V-derived catalysts. The stereochemistry of epoxidation by these methods has been compared for representative examples, including simple [591] and more complex trcMs-disubstituted, rrans-trisubstituted, and cis-trisubstituted allyl alcohols (Eqs (253) [592], (254) [592-594], and (255) [593]). In particular the epoxidation of trisubstituted allyl alcohols shown in Eqs (254) and (255) highlights the complementary use of the titanium-based method and other methods. More results from titanium-catalyzed diastereoselective epoxidation are summarized in Table 25. 

Bicyclo[2.1.0]pentane (62) reacts thermally (120°C, 48 h), with electron-poor olefins via a stepwise, diradical mechanism to afford bicyclo[2.2.1]heptane products derived from the addition of the olefins on the endo side of the bicyclo envelope . On the other hand in the presence of nickel(O) catalysts, the addition occurs under milder reaction conditions and with an alternative stereochemistry, i.e. predominantly on the exo face (equation 42) No class of transition metal catalysed rearrangements has been the subject of more controversy than those of bicyclo butane. A general mechanistic picture, consistent with the experimental facts, has, however, been presented ... 

In most cases the products obtain are derivatives of 1,3-butadiene. Although in this respect the reactions resemble the thermolyses of bicyclobutane, there are two major differences between these two classes of reactions. The first one lies in the reaction conditions. While the thermal reactions necessitate elevated temperatures and are characterized by high activation energies (see Section V.G) most of the reactions with transition metals occur very rapidly at room temperature. The second difference is in the stereochemistry of the reaction. While thermal reactions (be they concerted or not) generally follow the Wood war d-Hoffmann rules, transition metal promoted reactions give products with a different stereochemistry. These can be formally viewed as la + la processes. The following reactions illustrate this point (equations 95-97). Further labeling... 

More recently the introduction of low-valent transition metal and lanthanoid based reducing systems, especially those based on titanium, has provided dramatic advances in efficiency and selectivity. It is now possible to select appropriate conditions for efficient coupling of all types of carbonyl compounds, often with high chemo-, regio- and stereo-selectivity. Moreover, imino- and thio-carbonyl derivatives are also coupled via pinacolic methodology. The coupling of imines to 1,2-diamines is particularly effective, with excellent control of vicinal stereochemistry. 

While copper and iron Lewis acids are the most prominent late transition metal Diels-Alder catalysts, there are reports on the use of other chiral complexes derived from ruthenium [97,98],rhodium [99],andzinc [100] in enantioselective cycloaddition reactions, with variable levels of success. As a comparison study, the reactions of a zinc(II)-bis(oxazoline) catalyst 41 and zinc(II)-pyridylbis(ox-azoline) catalyst 42 were evaluated side-by-side with their copper(II) counterparts (Scheme 34) [101]. The study concluded that zinc(II) Lewis acids catalyzed a few cycloadditions selectively, but, in contrast to the [Cu(f-Bubox)](SbFg)2 complex 31b (Sect. 3.2.1), enantioselectivity was not maintained over a range of temperatures or substitution patterns on the dienophile. An X-ray crystal structure of [Zn(Ph-box)] (01)2 revealed a tetrahedral metal center the absolute stereochemistry of the adduct was consistent with the reaction from that geometry and opposite that obtained with Cu(II) complex 31. 

The stereochemistry of products derived from reactions of coordinated olefins and the stereochemistry of polymers formed in reactions catalyzed by transition metals are ultimately determined by the conformational stability of 7r-complexed intermediates. For example, the cisitrans ratios and the relative amounts of 1,2- versus 1,4-polymer units obtained in diene polymerization are determined by stabilities of syn and anti isomers of vr-allyls and the relative stabilities of various orientations of substituted olefins bound to metals (105). The interconversion rates of these isomers and the thermodynamic preferences of olefin-metal conformations should explain observed product distributions and provide a rational basis for catalyst design. 

---