Silver tetrafluoroborate activator

This topic has been reviewed [2, pp 94, 100-111, 130-134] All of the standard approaches to the synthesis of a compound like methyl 2-fluorostearate from methyl 2-bromostearate result mall yield of the 2-fluoro ester and the unsaturated esters. Although silver fluoride is not a new reagent, its use moist in wet acetonitrile to convert methyl 2-bromostearate to its fluoro ester is a departure from the traditional set of anhydrous conditions (Procedure 6, p 194) [71] In contrast, silver tetrafluoroborate converts a-chloroketones to their respective fluoroketones under anhydrous conditions. The displacement of less activated halogen groups by silver tetrafluoroborate to form their respective fluorides is novel Although silver tetrafluoroborate could not be used to convert an aliphatic terminal dichloromethyl or trichloromethyl group to its corresponding fluoro derivative, it is an effective fluorine source in other situations [72] (Table 8)... 

Chiral alcohols have also been used in an asymmetric synthesis of sulphoxides based on halogenation of sulphides. Johnson and coworkers have found319 that the reaction of benzyl p-tolyl sulphide with JV-chlorobenzotriazole (NCBT) followed by addition of (—) menthol and silver tetrafluoroborate afforded diastereoisomeric menthoxysulphonium salts 267 which, upon recrystallization and hydrolysis, gave benzyl p-tolyl sulphoxide with 87% optical purity (equation 145). More recently, Oae and coworkers reported320 that optically active diaryl sulphoxides (e.e. up to 20%) were formed either by hydrolysis or thermolysis of the corresponding diaryl menthoxysulphonium salts prepared in situ from diaryl sulphides using ( —) menthol and t-butyl hypochlorite. 

In their important work on the synthesis and properties of optically active chlorosulfurane (+)-177, Martin and Balthazor (195) deduced its absolute configuration by means of chemical correlations. Reaction of (+)-l 77 with silver tetrafluoroborate yields cyclic sulfonium salt 198, which was hydrolyzed without isolation under alkaline conditions to give sulfoxide (-)-(5 -180, from which (+)-177 was obtained. Assuming that the conversion (+)-l 77 -> 198 proceeds with... 

That only the wrong C-16 diastereomer seemed to be produced in this reaction was then demonstrated by the Kutney group, who prepared a series of binary indole-indoline alkaloids using the chloroindolenine approach. The apparent simplicity of this coupling reaction and the rapidity in assembling such binary alkaloids prompted an extensive study of reaction conditions (28), with the desire to find a procedure suitable for generation of the C-16 (S) isomer, required for anticancer activity. Despite the intensive effort involved in this in-depth study, no success could be realized, and it was therefore widely accepted that .. . it is very unlikely that any natural dimer could be obtained in this way (7). At this point it may be noted, however, that we were able to show subsequently that the desired C-16 -C-14 PARF relative stereochemistry can be obtained as a preferential result, albeit only in very low yield [3.6% PARF versus 2.1% PREF (priority reflective)], when the chloroindolenine reaction with vindoline is initiated with silver tetrafluoroborate (13). 

There was particular interest in the analogous reactions of 1-Oacetyl-2,3,5-tri-0-benzoyl-4-bromo-/ -D-ribose (37), because of the potential to prepare from it compounds related to nucleocidin, which is a 4 -fluoroaden-osine derivative having antitrypanosomal activity. With silver fluoride in acetonitrile, this bromide also reacted mainly with inversion of configuration, and gave the l-lyxo fluoride 140 (53% isolated yield), whereas, with silver tetrafluoroborate, much more of the more interesting 1-0-acetyl-2,3,4-tri-0-benzoyl-4-fluoro-/ -D-ribose (141) was obtained.38... 

Another way to activate l-(l-iodoalkyl)-l-silacyclobutanes toward ring expansion is to use silver acetate in acetic acid. In this case, the reaction is believed to proceed via formation of a carbocation a to the silicon. The acetate counterion acts as a nucleophile, attacking the activated SCB with C-Si bond migration (Scheme 35) <1991TL6383>. Silver tetrafluoroborate in dichloromethane induces ring enlargement as well, but shows much lower efficiency (30% yield upon treatment with MeLi) <1994BCJ1694>. 

Again, the authors tested some variations of the metal catalyst the entries with the copper triad metals are listed in Table 12.8. Copper(I) iodide was inactive (entry 1), silver tetrafluoroborate and AUCI3 both delivered the allene without cycloisomerization within 24 h (entries 2 and 4), and potassium tetrachloroaurate was inactive (entry 3). Without activation, phosphanegold(I) chlorides were inactive (entry 5), but after removal of the chloro ligand by silver salts, excellent yields were obtained in short reaction times of 40 min (entries 6-8). 

Despite this they should be considered as possible alternatives in cases where more familiar methods fail. Amongst this group of activators are p-toluenesulfonyl chloride, tiifluoromethanesulfonic anhy-dride,silver tetrafluoroborate, molybdenum oxide, phosphorous pentoxide, trichloromethyl chloroformate, 2-fluoro-l-methylpyridiniumsulfonate, chlorosulfonyl isocyanate, antimony penta-chloride (for which an X-ray structure of the DMSO-SbCb conqilex was obtained) and phenyl dichlo-rophosphate. ... 

A series of easily prepared and exceptionally active ruthenium catalysts for ringopening metathesis polymerization (ROMP) have been reported. As mentioned previously, the reaction of isoprene with RuCh gave a bis(jt-allyl)ruthenium(IV) complex of [RuC1(m-C1)( -CioHi6)]2, which was converted into cationic bis(jt-allyl)-ruthenium(IV) complexes by treatment with silver tetrafluoroborate. All of these complexes are stable in air and in solution for several hours. Although alone they... 

The first total synthesis of the clerodane alkaloid solidago alcohol was achieved in the laboratory of H.-S. Liu, using a highly diastereoselective DIels-Alder cycloaddition as the key step. The installation of the 3-furyl side chain required the conversion of the bicyclic primary alkyl bromide to the corresponding aldehyde. This was accomplished by the modified Kornblum oxidation, which employed silver tetrafluoroborate to activate the substrate. 

The hydrogenation activity which was very low for the Tt-chloro-bridged neutral rhodium(I) complexes 3 could be enhanced tremendously by reaction with silver tetrafluoroborate according to Fig. 3, transforming them into cationic species 4 possessing two additional free coordination sites to bind both the substrates, olefin and hydrogen, in the transition state during the catalytic reaction. 

Nucleophiles can also substitute at the internal carbon position. Reaction of m-benziporphyrin with silver tetrafluoroborate in pyridine under reflux conditions yielded a pyridine-appended macrocyle, with a C-N bond at the core of the ring. The pyridine then forms a second bond at the ortho position with an adjacent meso carbon (70). Recently, we noticed a directly analogous reaction upon the metalation of dicarbahemiporphyrazine with Cu(II) in pyridine (33). In this macrocycle, one of the two core carbon positions is substituted with a pyridine. In both cases, the formation of the C-N bond is believed to result from reductive elimination at the metal. Initial metalation results in the formation of a M-C bond, and the metal is subsequently coordinated by a solvent pyridine. The activation of the C-H bond at the core results in a high valent metal center (Ag(III) and Cu(III) respectively) which can then reductively eliminate to form a monovalent metal center and a C-N bond. 

Owing to the lability of the interflavanyl bond in procyanidins under either acidic or basic conditions, the existing semi-synthetic methods (28, 29) invariably result in an equilibrium between substrates and products. Such a labile bond and the apparent preference of the electrophile for the di- and trimeric products (see Scheme 1), once condensation is initiated, furthermore give poor control over the level of oligomerization. We thus assessed (52) the effectiveness of the thiophilic Lewis acids, dimethyl(methylthio)sulfonium tetrafluoroborate (DMTSF) (55, 54) and silver tetrafluoroborate (AgBF4)(55) to activate the C(4)-S bond in the 4-thioethers of flavan-3-ols towards carbon nucleophiles and hence to generate the interflavanyl bond of procyanidins under neutral conditions. 

In many cases, rather than synthesizing the active Rh complexes for reactions, Rh pre-catalyst systems can be treated with various additives to generate active catalyst species in situ. Chiral phosphorous ligands are used extensively in asymmetric transformations, while silver hexafluoroantimonate (AgSbFg), silver triflate (AgOTf), and silver tetrafluoroborate (AgBF4) are common additives to neutral Rh complexes to generate cationic species in situ. 

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