The unique role of fluorine in the design of active ingredients for modern crop protection

Ecotoxicology of organofluorous compounds

Organofluorous compounds have been developed for myriad purposes in a variety of fields, including manufacturing, industry, agriculture, and medicine. The widespread use and application of these compounds has led to increasing concern about their potential ecological toxicity, particularly because of the stability of the C-F bond, which can result in chemical persistence in the environment. This chapter reviews the chemical properties and ecotoxicology of four groups of organofluorous compounds: fluorinated refrigerants and propellants, per- and polyfluorinated compounds (PFCs), fluorinated pesticides, and fluoroquinolone antibiotics. These groups vary in their environmental fate and partitioning, but each raises concern in terms of ecological risk on both the regional and global scale, particularly those compounds with long environmental half-lives. Further research on the occurrence and toxicities of many of these compounds is needed for a more comprehensive understanding of their ecological effects.

Hydrofluoroarylation of alkynes with fluoroarenes

A combination of Ni(cod)(2) and PCyp(3) is found to be an effective catalyst for chemoselective activation of the C-H bond of fluoroarenes over C-F bonds followed by insertion of alkynes to allow direct alkenylation of the electron-deficient arenes. The characteristics of the reactions are: a C-H bond ortho to a fluorine substituent is selectively activated; the reactivity of fluorobenzenes is roughly proportional to the number of fluorine atoms. The reaction conditions tolerate a broad range of both alkynes and fluoroarenes containing both electron-withdrawing and -donating groups, thus allowing efficient synthesis of a variety of substituted ethenes containing a fluoroaryl motif in high regio- and stereoselective manners. Mechanistic studies including both labeling experiments and stoichiometric reactions reveal that oxidative addition of C-H bonds in fluoroarenes to nickel(0) is kinetically highly facile whereas that of C-F bonds is thermodynamically favoured.

N-H...F hydrogen bonds in fluorinated benzanilides: NMR and DFT study

Using (19)F and (1)H-NMR (with (14)N decoupling) spectroscopic techniques together with density functional theoretical (DFT) calculations, we have investigated weak molecular interactions in isomeric fluorinated benzanilides. Simultaneous presence of through space nuclear spin-spin couplings ((1h)J(N-HF)) of diverse strengths and feeble structural fluctuations are detected as a function of site specific substitution of fluorine atoms within the basic identical molecular framework. The transfer of hydrogen bonding interaction energies through space is established by perturbing their strengths and monitoring the effect on NMR parameters. Multiple quantum (MQ) excitation, up to the highest possible MQ orders of coupled protons, is utilized as a tool for accurate (1)H assignments. Results of NMR studies and DFT calculations are compared with the relevant structural parameters taken from single crystal X-ray diffraction studies.

Silver-catalyzed late-stage fluorination

Carbon-fluorine bond formation by transition metal catalysis is difficult, and only a few methods for the synthesis of aryl fluorides have been developed. All reported transition-metal-catalyzed fluorination reactions for the synthesis of functionalized arenes are based on palladium. Here we present silver catalysis for carbon-fluorine bond formation. Our report is the first example of the use of the transition metal silver to form carbon-heteroatom bonds by cross-coupling catalysis. The functional group tolerance and substrate scope presented here have not been demonstrated for any other fluorination reaction to date.

Mechanism of C-F reductive elimination from palladium(IV) fluorides

The first systematic mechanism study of C-F reductive elimination from a transition metal complex is described. C-F bond formation from three different Pd(IV) fluoride complexes was mechanistically evaluated. The experimental data suggest that reductive elimination occurs from cationic Pd(IV) fluoride complexes via a dissociative mechanism. The ancillary pyridyl-sulfonamide ligand plays a crucial role for C-F reductive elimination, likely due to a kappa(3) coordination mode, in which an oxygen atom of the sulfonyl group coordinates to Pd. The pyridyl-sulfonamide can support Pd(IV) and has the appropriate geometry and electronic structure to induce reductive elimination.

Synthesis and insecticidal activity of heptafluoroisopropyl-containing benzoylphenylurea structures

Fourteen novel heptafluoroisopropyl-containing benzoylphenylureas were designed and synthesized. Their insecticidal activities against armyworm ( Pseudaletia separata Walker) were examined and compared with the commercial product diflubenzuron. Three compounds (IIi, IIj, and IIk) showed excellent insecticidal effect, and their activity resembled that of diflubenzuron. Compound IIi also showed nearly the same insecticidal activity as novaluron on African cotton leafworm ( Spodoptera littoralis ). Furthermore, results from a field trial indicated that 5% EC IIi exhibited similar efficacy in comparison with chlorfluazuron and hexaflumuron against imported cabbage worm ( Pieris rapae L.) and diamondback moth ( Plutella xylostella ), respectively.

Diastereoselective synthesis of 2,3,4,5,6-pentafluoroheptanes

A stereocontrolled synthesis of alkanes containing five contiguous fluorine atoms is presented. The compounds were prepared by sequential fluorination of diastereoisomeric alcohol-diepoxides. The chemistry involved epoxide ring-opening with HF.NEt(3) and deshydroxyfluorination reactions of free alcohols with Deoxo-Fluor. The fluorination reactions were all highly stereospecific, with all five fluorines being incorporated in three sequential steps. Three different diastereoisomers of the 2,3,4,5,6-pentafluoroheptyl motif were prepared as heptane-1,7-diol derivatives, a structural format amenable for incorporation of the vicinal pentafluoro scaffold into larger molecular architectures.

The unique role of halogen substituents in the design of modern agrochemicals

The past 30 years have witnessed a period of significant expansion in the use of halogenated compounds in the field of agrochemical research and development. The introduction of halogens into active ingredients has become an important concept in the quest for a modern agrochemical with optimal efficacy, environmental safety, user friendliness and economic viability. Outstanding progress has been made, especially in synthetic methods for particular halogen-substituted key intermediates that were previously prohibitively expensive. Interestingly, there has been a rise in the number of commercial products containing 'mixed' halogens, e.g. one or more fluorine, chlorine, bromine or iodine atoms in addition to one or more further halogen atoms. Extrapolation of the current trend indicates that a definite growth is to be expected in fluorine-substituted agrochemicals throughout the twenty-first century. A number of these recently developed agrochemical candidates containing halogen substituents represent novel classes of chemical compounds with new modes of action. However, the complex structure-activity relationships associated with biologically active molecules mean that the introduction of halogens can lead to either an increase or a decrease in the efficacy of a compound, depending on its changed mode of action, physicochemical properties, target interaction or metabolic susceptibility and transformation. In spite of modern design concepts, it is still difficult to predict the sites in a molecule at which halogen substitution will result in optimal desired effects. This review describes comprehensively the successful utilisation of halogens and their unique role in the design of modern agrochemicals, exemplified by various commercial products from Bayer CropScience coming from different agrochemical areas.

Biotransformation of fluorobiphenyl by Cunninghamella elegans

The fungus Cunninghamella elegans is a useful model of human catabolism of xenobiotics. In this paper, the biotransformation of fluorinated biphenyls by C. elegans was investigated by analysis of the culture supernatants with a variety of analytical techniques. 4-Fluorobiphenyl was principally transformed to 4-fluoro-4'-hydroxybiphenyl, but other mono- and dihydroxylated compounds were detected in organic extracts by gas chromatography-mass spectrometry. Additionally, fluorinated water-soluble products were detected by (19)F NMR and were identified as sulphate and beta-glucuronide conjugates. Other fluorobiphenyls (2-fluoro-, 4,4'-difluoro- and 2,3,4,5,6-pentafluoro-biphenyl) were catabolised by C. elegans, yielding mono- and dihydroxylated products, but phase II metabolites were detected from 4,4'-difluorobiphenyl only.

Fluorinated 1-phenyl-1H-tetrazol-5-yl sulfone derivatives as general reagents for fluoroalkylidene synthesis

Julia-Kocienski olefination reagents 1-fluoropropyl, (cyclopropyl)fluoromethyl, 1-fluoro-2-methyl-2-propenyl, and 1-fluoro-5-hexenyl 1-phenyl-1H-tetrazol-5-yl (PT) sulfones were prepared by metalation followed by electrophilic fluorination. Although metalation-fluorination of n-propyl, 5-hexenyl, and (cyclopropyl)methyl PT-sulfones proceeded under homogeneous conditions, fluorination of 2-methyl-2-propenyl PT-sulfone required heterogeneous fluorination conditions. Condensation reactions of fluoro PT-sulfones with aldehydes resulted in fluoroalkylidenes in high yields. Screening of olefination conditions showed that stereoselectivity depended on reagent and carbonyl structure and can in many cases be tuned either toward E- or Z-selectivity. For example, LHMDS-mediated condensations of 1-fluoropropyl PT-sulfone in the presence of MgBr(2) x OEt(2) were Z-selective with electron-rich aromatic aldehydes, a hindered aromatic aldehyde, and cinnamaldehyde. Low-temperature KHMDS-mediated condensations were E-selective with electron-rich and electron-deficient aromatic aldehydes and Z-selective with n-octanal. Dialkyl, aryl alkyl, and diaryl ketones reacted as well to give fluoro olefin products in 71-99% yields.

Formation of ArF from LPdAr(F): catalytic conversion of aryl triflates to aryl fluorides

Despite increasing pharmaceutical importance, fluorinated aromatic organic molecules remain difficult to synthesize. Present methods require either harsh reaction conditions or highly specialized reagents, making the preparation of complex fluoroarenes challenging. Thus, the development of general methods for their preparation that overcome the limitations of those techniques currently in use is of great interest. We have prepared [LPd(II)Ar(F)] complexes, where L is a biaryl monophosphine ligand and Ar is an aryl group, and identified conditions under which reductive elimination occurs to form an Ar-F bond. On the basis of these results, we have developed a catalytic process that converts aryl bromides and aryl triflates into the corresponding fluorinated arenes by using simple fluoride salts. We expect this method to allow the introduction of fluorine atoms into advanced, highly functionalized intermediates.

Fluorination of boronic acids mediated by silver(I) triflate

A regiospecific Ag-mediated fluorination reaction of aryl- and alkenylboronic acids and esters is reported. The fluorination reaction uses commercially available reagents, does not require the addition of exogenous ligands, and can be performed on a multigram scale. This report discloses the first practical reaction sequence from arylboronic acid to aryl fluorides.

Methyl carbonium ion migration during the reaction of 4-chloro-5-methoxyl-3(2H)-pyridazinone with trifluoroethylation agents

To synthesize 4-chloro-5-methoxy-2-(β-trifluoroethyl)-3(2H)-pyridazinone (4), the reactions of 4-chloro-5-methoxy-3(2H)-pyridazinone (5) with RCH₂CF₃ (R = I, TsO, MsO, TfO) in different solvents were studied. It was found that methyl group migration took place during this reaction. An oxonium salt 9 was suggested as the active intermediate for the formation of the byproduct 4-chloro-5-methoxy-2-methyl-3(2H)-pyridazinone (7) and 4-chloro-2-methyl-5-(β-Trifluoroethoxy)-3(2)-pyridazinone (8).

Enantiopure quaternary alpha-trifluoromethyl-alpha-alkoxyaldehydes from L-tartaric acid derived ketoamides

The diastereoselective nucleophilic trifluoromethylation of a range of ketoamides derived from L-tartaric acid has been studied. TMSCF3 in the presence of a catalytic amount of K2CO3 in DMF has been identified as the conditions leading to the highest diastereoselectivities. A sequential one-pot reaction trifluoromethylation-etherification of the trifluoromethylcarbinol has been developed. Only one further one-pot reaction, ketal hydrolysis-oxidative cleavage, led to the final alpha-trifluoromethylated alpha-alkoxyaldehydes. This procedure was applied to the preparation of a series of enantiopure aryl, heteroaryl, and alkyl alpha-trifluoromethyl-alpha-alkoxyaldehydes.

Fragmentation of carbohydrate anomeric alkoxyl radicals: new synthesis of chiral 1-fluoro-1-halo-1-iodoalditols

A new general methodology for the synthesis of 1,1,1-trihaloalditols by starting from 1,5-anhydro-2-deoxy-hex-1-enitol derivatives (glycals) is described. The halogens are introduced sequentially in each of the three different steps of the process. The fluorine is introduced in the first step by electrophilic fluorination of the starting glycal; next, hydroxyhalogenation of the resulting vinyl fluoride allows the addition of any halogen (F, Cl, Br or I) at will, and finally, an iodine atom is inserted through an alkoxyl radical fragmentation reaction. This methodology allows the preparation of diverse types of 1,1,1-trihalogenated compounds (R--CF(2)I, R--CFI(2), R--CFClI and R--CFBrI) under mild conditions compatible with sensitive substituents. In some cases, the diastereomeric mixtures generated from R--CFClI and R--CFBrI can be chromatographically separated, and their configuration determined by X-ray crystallographic analysis. The synthetic usefulness of these compounds has been preliminarily assessed by examining the reactivity of the fluorinated radical generated by rupture of the C--I bond.