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Title

New Selective Haloform-type Reaction Yielding 3-Hydroxy-2,2-difluoroacids: Theoretical Study of the Mechanism

AuthorsOlivella, Santiago ; Solé, Albert; Jiménez, Óscar; Bosch, María Pilar ; Guerrero, Ángel
KeywordsHaloform-type Reaction
Organic molecules
Yielding 3-Hydroxy-2,2-difluoroacids
Zero-point vibrational energies
Issue Date4-Feb-2005
PublisherAmerican Chemical Society
CitationJournal of the American Chemical Society 127(8): 2620–2627 (2005)
AbstractExperimental results of an unprecedented haloform-type reaction in which 4-alkyl-4-hydroxy-3,3-difluoromethyl trifluoromethyl ketones undergo base-promoted selective cleavage of the CO−CF3 bond, yielding 3-hydroxy-2,2-difluoroacids and fluoroform, are rationalized using DFT (B3LYP) calculations. The gas-phase addition of hydroxide ion to 1,1,1,3,3-pentafluoro-4-hydroxypentan-2-one (R) is found to be a barrierless process, yielding a tetrahedral intermediate (INT), involving a ΔGr(298 K) of −61.4 kcal/mol. The CO−CF3 bond cleavage in INT leads to a hydrogen-bonded [CH3CHOHCF2CO2H···CF3]- complex by passage through a transition structure (TS1) with a ΔG(298 K) of 20.8 kcal/mol and a ΔGr(298 K) of 9.8 kcal/mol. This complex undergoes a proton transfer between its components, yielding a hydrogen-bonded [CH3CHOHCF2CO2···CHF3]- complex. This process has associated with it a ΔG(298 K) of only 3.1 kcal/mol and a ΔGr(298 K) of −43.3 kcal/mol. The CO−CF2 bond cleavage in INT leads to a hydrogen-bonded [CH3CHOHCF2···CF3CO2H]- complex by passage through a transition structure (TS3) with a ΔG(298 K) of 29.2 kcal/mol and a ΔGr(298 K) of 25.1 kcal/mol. The lower energy barrier found for CO−CF3 bond cleavage in INT is ascribed to the larger number of fluorine atoms stabilizing the negative charge accumulated on the CF3 moiety of TS1, as compared to the number of fluorine atoms stabilizing the negative charge on the CH3CHOHCF2 moiety of TS3. The solvent-induced effects on the two pathways, introduced within the SCRF formalism through PCM calculations, do not reverse the predicted preference of the CO−CF3 over the CO−CF2 bond cleavage of R in the gas phase.
Description8 pages, 5 figures, 4 tables.-- PMID: 15725018 [PubMed].-- Printed version published Mar 2, 2005.-- Supporting information available at: http://pubs.acs.org/doi/suppl/10.1021/ja043522d
Publisher version (URL)http://dx.doi.org/10.1021/ja043522d
URIhttp://hdl.handle.net/10261/15598
DOI10.1021/ja043522d
ISSN0002-7863
E-ISSN1520-5126
Appears in Collections:(IQAC) Artículos
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