Themed series in organo-fluorine chemistry

Publisher PDFPeer reviewe

The vicinal difluoro motif : the synthesis and conformation of erythro- and threo-diastereoisomers of 1,2-difluorodiphenylethanes, 2,3-difluorosuccinic acids and their derivatives

Background: It is well established that vicinal fluorines (RCHF-CHFR) prefer to adopt a gauche rather than an anti conformation when placed along aliphatic chains. This has been particularly recognised for 1,2-difluoroethane and extends to 2,3-difluorobutane and longer alkyl chains. It follows in these latter cases that if erythro and threo vicinal difluorinated stereoisomers are compared, they will adopt different overall conformations if the fluorines prefer to be gauche in each case. This concept is explored in this paper with erythro- and threo- diastereoisomers of 2,3-difluorosuccinates. Results: A synthetic route to 2,3-difluorosuccinates has been developed through erythro- and threo- 1,2-difluoro-1,2-diphenylethane which involved the oxidation of the aryl rings to generate the corresponding 2,3- difluorosuccinic acids. Ester and amide derivatives of the erythro- and threo- 2,3-difluorosuccinic acids were then prepared. The solid and solution state conformation of these compounds was assessed by X-ray crystallography and NMR. Ab initio calculations were also carried out to model the conformation of erythro- and threo- 1,2-difluoro-1,2-diphenylethane as these differed from the 2,3-difluorosuccinates. Conclusion: In general the overall chain conformations of the 2,3-difluorosuccinates diastereoisomers were found to be influenced by the fluorine gauche effect. The study highlights the prospects of utilising the vicinal difluorine motif (RCHF-CHFR) as a tool for influencing the conformation of performance organic molecules and particularly tuning conformation by selecting specific diastereoisomers (erythro or threo).Publisher PDFPeer reviewe

The difluoromethylene (CF2) group in aliphatic chains : synthesis and conformational preference of palmitic acids and nonadecane containing CF2 groups

Funding: ERC Advanced Investigator Grant (D.O'H).The syntheses of palmitic acids and a nonadecane are reported with CF2 groups located 1,3 or 1,4 to each other along the aliphatic chain. Specifically 8,8,10,10- and 8,8,11,11-tetrafluorohexadecanoic acids (6b and 6c) are prepared as well as the singly modified analogue 8,8-difluorohexadecanoic acid (6a). Also 8,8,11,11-tetrafluorononadecane (27) is prepared as a pure hydrocarbon containing a 1,4-di-CF2 motif. The modified palmitic acids are characterized by differential scanning calorimetry (DSC) to determine melting points and phase behaviour relative to palmitic acid (62.5 degrees C). It emerges that 6c, with the CF2 groups placed 1,4- to each other, has a significantly higher melting point (89.9 degrees C) when compared to the other analogues and palmitic acid itself. It is a crystalline compound and the structure reveals an extended anti-zig-zag chain. Similarly 8,8,11,11-tetrafluorononadecane (27) adopts an extended anti-zig-zag structure. This is rationalized by dipolar relaxation between the two CF2 groups placed 1,4 to each other in the extended anti-zig-zag chain and suggests a design modification for long chain aliphatics which can introduce conformational stability.Publisher PDFPublisher PDFPeer reviewe

Exploration of a potential difluoromethyl-nucleoside substrate with the fluorinase enzyme

The authors thank EPSRC and the Scottish Imaging Network (SINAPSE) for grants. DO’H thanks the Royal Society for a Wolfson Research Merit Award and ST is grateful to the John and Kathleen Watson Scholarship for financial support.The investigation of a difluoromethyl-bearing nucleoside with the fluorinase enzyme is described. 5’,5’–Difluoro-5’-deoxyadenosine 7 (F2DA) was synthesised from adenosine, and found to bind to the fluorinase enzyme by isothermal titration calorimetry with similar affinity compared to 5’–fluoro-5’-deoxyadenosine 2 (FDA), the natural product of the enzymatic reaction. F2DA 7 was found, however, not to undergo the enzyme catalysed reaction with l–selenomethionine, unlike FDA 2, which undergoes reaction with l-selenomethionine to generate Se-adenosylselenomethionine. A co-crystal structure of the fluorinase and F2DA 7 and tartrate was solved to 1.8 Å, and revealed that the difluoromethyl group bridges interactions known to be essential for activation of fluoride for reaction. An unusual hydrogen bonding interaction between the hydrogen of the difluoromethyl group and one of the hydroxyl oxygens of the tartrate ligand was also observed. The bridging interactions, coupled with the inherently stronger C–F bond in the difluoromethyl group, offers an explanation for why no reaction is observed.PostprintPeer reviewe

Inter- and intramolecular CF···C=O interactions on aliphatic and cyclohexane carbonyl derivatives

The authors thank EaStCHEM and the University of St Andrews for access to a computing facility managed by Dr H. Fruchtl., CNPq, and FAPESP for the studentship (to R.A.C. #2011/01170-1, FAPESP) and for financial support (#2014/25903-6), as well as CNPq for the fellowship (R.R.).The assessment of weak inter- and intra- molecular Cδ+Fδ-...Cδ+=Oδ- interactions were theoretically evaluated in 4 different sets of compounds at different theoretical levels. Intermolecular CH3F...C=O interactions were stabilizing by ca. 1 kcal mol-1 for various carbonyl containing functional groups. Intramolecular CF...C=O interactions were also detected in aliphatic and fluorinated cyclohexane carbonyl derivatives. However, the stabilisation provided by intramolecular CF...C=O interactions was not enough to govern the conformational preferences of compounds 2-4.PostprintPeer reviewe

Selectively fluorinated cyclohexane building blocks : derivatives of carbonylated all-cis-3-phenyl-1,2,4,5-tetrafluorocyclohexane

MA thanks the Egyptian Government for a Scholarship under the Channel Scheme and we are grateful to the support of Professor L. Fathey of the University of Alexandria, Department of Chemistry, Egypt. DO'H thanks the Royal Society for a Wolfson Research Merit Award and we also thank the EPSRC Mass Spectrometry Service at Swansea University, UK for analyses.Palladium catalysed carbonylation reactions using the meta- and para-iodo derivatives of all-cis-3-phenyl-1,2,4,5-tetrafluorocyclohexane ( 4 ) are illustrated as the start point for a variety of functional group interconversions. The resultant benzaldehyde and benzoic acids offer novel building blocks for further derivatisation and facilitate the incorporation of the facially polarised all-cis-1,2,4,5-tetrafluorocyclohexane motif into more advanced molecular scaffolds.Publisher PDFPeer reviewe

Fluorine containing amino acids : synthesis and peptide coupling of amino acids containing the all-cis tetrafluorocyclohexyl motif

MA thanks the Egyptian Government for a Scholarship. DO’H thanks the Royal Society for a Royal Society Wolfson Research Merit Award.A synthesis of two (S)-phenylalanine derivatives is described which have the all-cis, 2,3,5,6-tetrafluorocyclohexyl motif attached to the aromatic ring at the meta and para positions; the para substituted isomer is elaborated into illustrative dipeptides via the free amine and carboxylate respectively demonstrating its utility as a novel amino acid for peptide synthesis and offering a vehicle for incorporation of this unique and facially polarized ring system into bioactive compounds.PostprintPeer reviewe

Particularly strong C-H...pi interactions between benzene and all-cis 1,2,3,4,5,6-hexafluorocyclohexane

The authors thank EaStCHEM and the University of St Andrews for access to a computing facility managed by Dr H. Fruchtl, CNPq and FAPESP for the studentship for R.A.C. (#2011/01170-1, FAPESP) and for financial support (#2014/25903-6), as well as CNPq for the fellowship for R.R. DO’H thanks the Royal Society for a Wolfson Research Merit Award.We present the first high-level ab initio benchmark study of the interaction energy between fluorocyclohexanes and benzene. These compounds form CH...pi interactions with aromatic solvents which causes notable shielding of the axial cyclohexane protons. For the recently synthesised all-cis 1,2,3,4,5,6-hexafluorocyclohexane the interaction energy with benzene amounts to -7.9 kcal/mol and -6.4 kcal/mol at the MP2 and SCS-MP2 levels, respectively (extrapolated to the complete basis set limit), which according to dispersion-corrected density functional calculations, is largely due to dispersion.PostprintPeer reviewe

Synthesis of selectively fluorinated cyclohexanes : the observation of phenonium rearrangements during deoxyfluorination reactions on cyclohexane rings with a vicinal phenyl substituent

A study focused on the synthesis of a derivative of all-cis phenyl-2,3,5,6-tetrafluorocyclohexane, incorporating a methyl group at the benzylic position, found that the fluorination reactions of diepoxide and diol precursors, were susceptible to rearrangement and unexpected products. The origin of these rearranged products can be rationalised by aryl migrations occurring via phenonium ion intermediates, in adventitious pathways occurring during deoxofluorination reactions with hydrogen fluoride reagents.PostprintPeer reviewe

Multicomponent reactions of methyl substituted all-cis tetrafluorocyclohexane aldehydes

The authors thank the EPSRC for supporting this work and DO’H is grateful for a Royal Society Wolfson Research Merit Award.This paper reports the preparation of methyl substituted all-cis tetrafluorocyclohexanes prepared from a Birch reduction of benzoic acid, worked up with a methyl iodide quench. The resultant methylcyclohexadiene carboxylic acid was reduced to the alcohol and protected. The cyclohexadienyl ring was then epoxidised and the C-O bonds sequentially converted through deoxyfluorination reactions to two sets of isomers of all-cis tetrafluorocyclohexane isomers. Deprotection of the benzylic ether and then oxidation gave aldehydes which were then used in Ugi and Passerini multicomponent reactions, allowing this facially polarised cyclohexane to be incorporated into peptidic structural motifs.Publisher PDFPeer reviewe