Transition metal Lewis acid coordination with

In comparison with classic Lewis acids derived from main group halides (e.g., B, Al, Sn), f-elements, and early transition metal halides, late transition metal Lewis acids often are more inert to ubiquitous impurities such as water, offer higher stability, tunable properties by ligand modification, and a well-defined structure and coordination chemistry, thus allowing detailed studies of reaction mechanisms, and a rational basis for catalyst optimization. Among this new class of late transition metal Lewis acids, ruthenium complexes - the subject of this chapter - display remarkable properties... 

Extensive studies on the Lewis acid [( 7 " -C5H5)Re(NO)(PPh3)] have yielded detailed insight into transition metal Lewis acid-Lewis base interactions and the question of a or coordination of carbonyl groups [100]. The Re Lewis acid binds aldehydes via the TT-system whereas ketones coordinate to form M-O cr-complexes. Both react with Et4N CN to give Re-bound cyanohydrins. Diastereoselectivity ranges from 53 to 89 % de for additions to aldehydes and 71 to 99 % de for additions to ketones (Sch. 19) [101]. Diastereoselective reductions to primary and secondary alcohols have also been reported [102]. 

Several successful chiral catalysts have been reported so far for catalytic asymmetric 1,3-dipolar cycloaddition reactions of nitrones with oxazolidinone derivatives (Figure 16.3) [27a,b,c] Recently, Kiindig et al. [27] had reported that the single coordination site Fe and Ru transition-metal Lewis acids can efficiently promote enantioselective 1,3-dipolar cycloadditions of nitrones with a,p-unsaturated aldehydes and represent a rapid access to substrates of high synthetic potential. The Fe catalyst (81) was found to be an excellent catalyst for 1,3-dipolar cycloaddition reactions between methacrolein and nitrone (83) (Scheme 16.24, Table 16.5). Isox-azolidine (84a) was formed as a single region- and diastereoisomer in excellent... 

The Diels-Alder reaction is a key reaction in organic synthesis. Its high versatility in the synthesis of six-membered ring compounds and its potential for the control of up to four stereogenic centers have attracted much attention. Lewis acid catalysis has further enhanced the scope of this reaction. Lewis acids activate the dienophile by coordination to a Lewis basic substituent (usually a carbonyl group) and direct the stereochemistry. Boron Lewis acids are often the catalysts of choice for the Diels-Alder reaction. Early (Ti(IV)) and late (Cu(II)) transition metal complexes in combination with chiral ligands have also been used with much success and the reader is referred to the relevant chapters in this book. 

Covalent Metal Compounds Within molecules, atoms are held together by strong forces called covalent bonds (Lewis 1916). These bonds are formed when a metal as an electron-acceptor (Lewis acid) reacts with an electron-pair donor (Lewis base). Metals that form coordinate bonds most readily are small, and highly charged with empty orbitals, such as the transition metals (Pytkowicz 1983). 

In a number of transition metal complexes, the coordination modes of dialkyl H-phospho-nates correspond to a, b, or e in the above scheme. The coordination mode c is characteristic for silver, alkaline, and earth-alkaline metals. Coordination of type d is encountered in molecular complexes of dialkyl H-phosphonates with Lewis acids as well as in some nontransition and early transition metals as discussed further in the text. 

Not only the highly Lewis acidic early transition metal-based polymerization catalysts suffer from poisoning by coordination of functional groups. Even in late transition metal-based complexes, the possible o-coordination in certain functional groups has a negative impact on polymerization reactions. The prominent example here is the still ongoing search for active acrylonitrile (AN) copolymerization catalysts. This reaction can serve as an ideal example to illustrate the challenges in late transition metal-catalyzed insertion polymerizations with polar functionalized comonomers. The metal-mediated copolymerization of AN has numerous appearances in literature however, in most cases, the reaction mechanism seems to be of ionic or radical nature. 

Chloroaluminate(III) ionic liquid systems are perhaps the best established and have been most extensively studied in the development of low-melting organic ionic liquids with particular emphasis on electrochemical and electrodeposition applications, transition metal coordination chemistry, and in applications as liquid Lewis acid catalysts in organic synthesis. Variable and tunable acidity, from basic through neutral to acidic, allows for some very subtle changes in transition metal coordination chemistry. The melting points of [EMIM]C1/A1C13 mixtures can be as low as -90 °C, and the upper liquid limit almost 300 °C [4, 6]. 

---