Asymmetric induction using chiral transition metal catalysts

Both intra- and intermolecular aziridination of alkenes can be accomplished with PhI(OAc)2 and an appropriate nitrogen source (eqs 57 and 58). The former reactions have been described using carbamate, sulfonamide, and sulfamate substrates. Typical catalysts utilized for these processes include Ru, Rh, and Cu complexes. By employing chiral transition metal catalysts, asymmetric induction has been realized in both intra- and inter-molecular reactions. 

Among the transition-metal catalysts that have been used, only those of Pd(II) are productive with diazomethane, which may be the result in cyclopropanation reactions [7,9,21] of a mechanism whereby the Pd-coordinated alkene undergoes electrophilic addition to diazomethane rather than by a metal carbene transformation in any case, asymmetric induction does not occur by using Pd(II) complexes of chiral bis-oxazolines [22],... 

Most commonly used chiral Lewis acids have been derived from main group and early transition series elements. An initial attempt at utilizing optically active catalysts of late transition metal complexes for the enantioselective addition of allyltributylstannane to aldehydes was made by Nuss and Rennels [30]. Employment of Rh(COD)[(-)-DIOP]BF4 (11) as a catalyst, however, resulted in only a small degree of asymmetric induction (17% ee). 

Stereoselective oxycarborative addition is also achieved in cycloaddition and cyclooligomeriza-tion reactions. Thus, hetero-Diels-Alder reactions of dienes and aldehydes are not only catalyzed by main group Lewis acids, but also by transition metal complexes 10°. Tris[3-(heptafluoropropyl-hydroxymethylene)-( + )-camphorato]europium [( + )-Eu(hfc)3] and similar vanadium complexes have been used as the chiral catalyst in [4 + 2] cycloadditions of various achiral and chiral dienes to aldehydes63 67-101. With achiral silyloxydienes only moderate asymmetric inductions are observed, however, with chirally modified dienes, high double diastereoselectivities are achieved. Thus, the reaction of benzaldehyde with 3-terf-butyldimethylsilyloxy-l-(/-8-phenvl-menthoxy)-l.3-butadiene (1) gives (2/ .6/ )-4-wf-bntyldimethylsilyloxy-5,6-dihydro-6-phenyl-2-[(17 ,3/ ,45 )-8-phenylmenthoxy]-2f/-pyran (2) in 95% yield with a diasteieoselectivity of 25 1 ss. After crystallization and hydrolysis with trifluoroacetic acid, optically pure (2/ )-2,3-di-hydro-2-phenyl-4-(4//)-pyranone (3) is obtained in 87% yield. 

This chapter examines reactions that involve molecular rearrangements and cycloadditions. The use of these terms will not be restricted to concerted, pericyclic reactions, however. Often, stepwise processes that involve a net transformation equivalent to a pericyclic reaction are catalyzed by transition metals. The incorporation of chiral ligands into these metal catalysts introduces the possibility of asymmetric induction by inter-ligand chirality transfer. The chapter is divided into two main parts (rearrangements and cycloadditions), and subdivided by the standard classifications for pericyclic reactions e.g., [1,3], [2,3], [4-1-2], etc.). The latter classification is for convenience only, and does not imply adherence to the pericyclic selection rules. Indeed, the first reaction to be described is a net [1,3]-suprafacial hydrogen shift, which is symmetry forbidden if concerted. 

This year has again emphasized the growing importance of organo-transition metal complexes in organic synthesis. In catalysed reactions the major advances have been in asymmetric catalysis with the first reports of chiral induction in catalytic epoxidation and further reports on improved catalysts for asymmetric hydrogenation and allylic alkylation. The formation of carbon-carbon bonds continues to attract attention, and several novel and potentially useful synthetic applications of organometallic complexes have been reported. 

Asymmetric induction in catalytic cyclopropanation reactions with the use of chiral catalysts [8, 21, 29] infers that the chiral ligand associated to the transition metal controls the approach of the olefin to the carbenic centre ... 

From the possibilities to achieve asymmetric induction in homogeneous catalytic reactions, we will discuss only those ones in which the induction is due to a chiral ligand bound to a central transition metal atom or ion. The other possibilities such as use of chiral solvents and auxiliaries will be discussed elsewhere. This article will focus on the peculiarities of the chiral catalysts itself and not on the asymmetric reactions. Any details of the reaction will be discussed only from the point of view of the catalyst (intermediates, precursors, etc) and not from that of the products (including chemo-, regio-, and enantioselectivities of the reaction). Since the number of references concerning the most important applications of some effective ligands is few hundreds, we avoid to refer them in detail. 

---