Article

Nucleophile assisting leaving groups: a strategy for aliphatic 18F-fluorination.

Molecular Imaging Branch, National Institute of Mental Health, National Institutes of Health, Bethesda, Maryland 20892-1003, USA.
The Journal of Organic Chemistry (impact factor: 4.45). 08/2009; 74(15):5290-6. DOI:10.1021/jo900700j pp.5290-6
Source: PubMed

ABSTRACT A series of arylsulfonate nucleophile assisting leaving groups (NALGs) were prepared in which the metal chelating unit is attached to the aryl ring via an ether linker. These NALGs exhibited significant rate enhancements in halogenation reactions using metal halides. Studies with a NALG containing a macrocyclic ether unit suggest that rate enhancements of these nucleophilic halogenation reactions are facilitated by stabilization of charge in the transition state rather than through strong precomplexation with metal cation. In several cases, a primary substrate containing one of the new leaving groups rivaled or surpassed the reactivity of triflates when exposed to nucleophile but was otherwise highly stable and isolable. These and previously disclosed chelating leaving groups were used in (18)F-fluorination reactions using no-carrier-added [18F]fluoride ion (t(1/2) = 109.7 min, beta+ = 97%) in CH3CN. Under microwave irradiation and without the assistance of a cryptand, such as K2.2.2, primary substrates with select NALGs led to a substantial improvement (2-3-fold) in radiofluorination yields over traditional leaving groups.

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Keywords

18)F-fluorination reactions
 
2-3-fold
 
aryl ring
 
cases
 
ether linker
 
groups rivaled
 
halogenation reactions
 
macrocyclic ether unit
 
metal cation
 
metal chelating unit
 
microwave irradiation
 
NALG
 
NALGs exhibited significant rate enhancements
 
no-carrier-added [18F]fluoride ion
 
nucleophilic halogenation reactions
 
rate enhancements
 
strong precomplexation
 
substantial improvement
 
traditional
 
triflates