Halogenation of enamines

The halogenation of enamines is formally analogous to protonation with salt formation. Thus the steroidal enamine (164) undergoes bromination (7/5) to give the j8-bromo iminium bromide (165), which is readily hydrolyzed to the /5-bromo aldehyde (166). 

Halogenation of enamines to give stable halo enamines [162]. 

The most extensive use of enamine halogenations has, hctwever, been in the attachment of fluorine to the steroid skeleton (499-503). The formation of a ]6-fluoro-17-ketosteroid by the reaction of perchlorofluoride with a 17-enamide has also been reported (504). 

Such functional groups as OR, OH, NH2, SMe, halogen, and COOR may be present in the molecule," but not groups that are reducible by borane. Hydroboration of enamines with 9-BBN provides an indirect method for reducing an aldehyde or ketone to an alkene, for example. 

Instead of direct halogenation of ketones, reactions with more reactive derivatives such as silyl enol ethers and enamines have advantages in certain cases. 

Another example of halogen-atom transfer by CPO is the vinylic halogenation of cyclic enaminones and enamines (Eq. 10, Table 11). 

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... 

Mechanism 22-4 Base-Catalyzed Keto-EnolTautomerism 1047 Mechanism 22-5 Acid-Catalyzed Keto-EnolTautomerism 1047 22-3 Alkylation of Enolate Ions 1050 22-4 Formation and Alkylation of Enamines 1051 22-5 Alpha Halogenation of Ketones 1054... 

A similar competition between the substitution of the allylic halogen and rearrangement with loss of halogen leading to bicyclic compounds, i.e. 6-alkyl-6-pyrrolidinobicyclo[3.1.0]hexanes was observed upon treatment of a-halogenated cyclic enamines with organometallic reagents (Mg, Li, Cu, hydrides) (Table 2). ° ... 

ASYMMETRIC HALOGENATION OF KETONES VIA ENAMINES OF L-PROLENE ESTERS... 

Halogenation of a Cxe -macroUde. Reaction of the pyrrolidine enamine of the antibiotic leucomycin A3 (1) with NBS yields, after hydrolysis, the 17-bromo derivative (62%). A similar reaction is observed with NCS (66%, yield). In contrast, reaction with perchloryl fluoride in limited amounts results in fluorination at Ci, and Cm. The Cm-rnonofluoride was made indirectly by reaction first with NBS and then with perchloryl fluoride the bromofluoro... 

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