Catalytic stereoselective halogenations

Several alkynols reacted with elemental iodine (or NIS, or NBS) assisted by HTI - in either stoichiometric or catalytic amounts - to afford stereoselectively halogenated enones or enals. Examples of such transformations, some of which involved ring-expansion, are given in Table 7.2 a procedure follows. 

Alkyl chlorides are with a few exceptions not reduced by mild catalytic hydrogenation over platinum [502], rhodium [40] and nickel [63], even in the presence of alkali. Metal hydrides and complex hydrides are used more successfully various lithium aluminum hydrides [506, 507], lithium copper hydrides [501], sodium borohydride [504, 505], and especially different tin hydrides (stannanes) [503,508,509,510] are the reagents of choice for selective replacement of halogen in the presence of other functional groups. In some cases the reduction is stereoselective. Both cis- and rrunj-9-chlorodecaIin, on reductions with triphenylstannane or dibutylstannane, gave predominantly trani-decalin [509]. 

Iridium,204,205 together with osmium, has been not widely used in catalytic hydrogenation. Recently, however, iridium or iridium-based catalysts have been shown to be effective in various hydrogenations, such as in selective hydrogenation of a,P-unsaturated aldehydes to allylic alcohols (Section 5.2), of aromatic nitro compounds to the corresponding hydroxylamines (Section 9.3.6), of halonitrobenzenes to haloanilines without loss of halogen (Section 9.3.2), and in the stereoselective hydrogenation of carbon to carbon double bonds (see, e.g., eqs. 3.25-3.27 and Table... 

Shibatomi, K. Futatsugi, K. Kobayashi, F. Iwasa, S. Yamamoto, H. Stereoselective Construction of Halogenated Quaternary Stereogenic Centers via Catalytic Asymmetric Diels-Alder Reaction. /. Am. Chem. Soc. 2010,132,5625-5627. 

The first three sections of this chapter describe diastereoselective alkylations of chiral enolates including heteroatom-substituted enolates [15, 20]. Section 3.4 deals with the class of enolate alkylations that have typically been included under the rubric of chiral-auxiliary-controlled processes. As suggested by the term, the auxiliary is only transiently utilized and, following alkylation, is subsequently excised. The facile use of chiral auxiliaries in asymmetric enolate alkylations has played and continues to play a pivotal role in the stereoselective formation of new C-C bonds. After a brief survey of the relatively few developments in catalytic enantioselective enolate alkylations (Section 3.5) [21, 22], selected examples of enolate a-hydroxylations (Section 3.6) [23-25] and a-halogenations (Section 3.7) [26, 27] are covered. The corresponding a-aminations of enolates are discussed in Chapter 10, describing stereoselective formation of a-amino acids. 

Kuniyasu H, Takekawa K, Yamashita F, Miyafuji K, Asano S, Takai Y, Ohtaka A, Tanaka A, Sugoh K, Kurosawa H, Kambe N (2008) Insertion of alkynes into an ArS-Pt bond regio- and Stereoselective thermal reactions, facilitation by o-halogen effect and photoirradiation, different alkyne preferences depending on the ancillary ligand, and application to a catalytic reaction. Organometallics 27 4788-4802... 

---