Halogenation catalytic asymmetric

Recent advances in catalytic asymmetric electrophilic a-halogenation of carbonyl compounds are described in two reviews.293,294. 

Oestreich M. Strategies for catalytic asymmetric electrophihc a halogenation of carbonyl compounds. Angew. Chem. Int. Ed. 2005 44 2324-2327. 

The 1/BQ system alone or with Lewis acid additives was also employed for asymmetric catalysis in the synthesis of /3-amino acids235. During an elaboration of the tandem catalytic asymmetric chlorination/esterification process, Lectka and coworkers found that proton sponge 1 competes with ketenes in the reaction with halogenating agents, such... 

Recent Developments in Catalytic, Asymmetric a-Halogenation A New Frontier in Asymmetric Catalysis France, S. Weatherwax, A. Lectka, T. Eur. J. Org. Chem. 2005, 475. 

In 2001, Lectka and co-workers [52] adapted their method for catalytic asymmetric a-halogenation of acid chlorides to include bromination reactions (Scheme 13.24). Through use of the same organocatalyst and shuttle-deprotonation strategy, and a polybromoquinone as the bromine source, the catalytic enantioselective... 

Organocatalytic asymmetric carbon-halogen bond-forming reactions have been combined with other catalytic asymmetric reactions in a single flask to effect cascade reactions. Quintard and Alexakis [78] developed a cascade reaction in which a Cu-catalyzed conjugate addition to an a,p-unsaturated aldehyde was followed by an enamine-catalyzed a-fluorination or chlorination. 

Catalytic asymmetric halogenation reactions are still rarely studied. Togni et al. developed the efficiency of [Ti(TADDOLato)] complexes 18 in combination with the fluorinating agent Selectfluor in the catalytic fluorination of P-ketoesters. In 2004, this group executed the asymmetric chlorination of P-ketoesters using titanium complexes 18 with (dichloroiodo)toluene to generate enantiomerically enriched a-chlorinated products 19 (Scheme 5) [35]. 

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. 

Until recently the catalytic asymmetric halocyclization remained a long-standing problem. Applications of these methods are sparse, and the halogens are often substituted at the later stages. A brief description will be presented here. 

Enantioselective -Functionalization of Aldehydes and Ketones The direct and enantiosective functionalization of enolates or enolate equivalents with carbon-, nitrogen-, oxygen-, sulfur- or halogen-centered electrophiles represents a powerful transformation of chemical synthesis and of fundamental importance to modem practitioners of asymmetric molecule constmction. Independent studies from List, J0rgensen, Cordova, Hayashi, and MacMiUan have demonstrated the power of enamine catalysis, developing catalytic enantioselective reactions such as... 

A similar catalytic procedure for enantioselective formation of C-Br and C-Cl bonds has been reported recently by the Lectka group [83]. The concept of this a-halogenation of carbonyl compounds is tandem asymmetric halogenation and esterification (Scheme 3.28). Inexpensive acyl halides, 74, are used as starting... 

Asymmetric ene Reaction In 1988 Yamamoto and coworkers provided the first indication that asymmetry in ene-reactions could be induced by catalytic amounts of chiral Lewis acids in the presence of 4-A molecular sieves (Scheme 6.64) [88]. They described the first example of asymmetric ene-reaction between prochiral, halogenated aldehydes and alkenes catalyzed by chiral binaphthol-derived aluminum complexes. The hindered 3,3-silyl substituents in the chiral catalyst are essential to achieve good enantioselectivity and high yield. In fact, the use of a catalyst derived from MesAl and 3,3 -biphenylbinaphthol led to the racemic product in a low yield. 

---