Fluoride-promoted reactions

In case of o-carboxylic ester groups, such as in 623, ring closure to give 3,1-benzoxazin-4-ones 624 is observed. The yields in this tetrabutylammonium fluoride promoted reaction are 50-55 %. 

Treatment of 2 with 3 equiv of 3-phenyl-2-propenal in refluxing toluene-cfe while the reaction progress was monitored by H NMR spectroscopy resulted in the disappearance of the aldehyde hydrogen peak (5 1.56). The IR spectrum of 4 shows a new absorption due to a vC-o stretch at 1448 cm 1. The mass spectrum of the product shows a molecular ion at m/z 538. To our surprise, an X-ray study of 4 showed it to be the insertion product of the two carbonyl ligands into the C-Si bond in 2. The reaction has the potential for developing a new method for double C-C bond formation between the carboranyl unit and carbonyl compounds. Such an insertion of the carbonyl functionality into the o-carborane has been observed in Yamamoto s work on the chemoselective addition of o-carborane to the aldehyde groups by a palladium-catalyzed9 or a fluoride-promoted reaction.10... 

The fluoride-promoted reaction of allyltrifluorosilanes with aldehydes to generate homoallylic alcohols was first reported by Sakurai in 1987 [48]. The reactions are... 

Fluoride-promoted reaction of the substrates in Eq. [4] (Scheme 56) gives only desilylated products (111). Lewis acidic conditions, however, lead to cis-fused products in good yields. The stereochemistry at the vinyl stereocenter was not established, but remarkable diastereoselection is claimed. 

Halogenation and dehalogenation are catalyzed by substances that exist in more than one valence state and are able to donate and accept halogens freely. Silver and copper hahdes are used for gas-phase reactions, and ferric chloride commonly for hquid phase. Hydrochlorination (the absoration of HCl) is promoted by BiCb or SbCl3 and hydrofluorination by sodium fluoride or chromia catalysts that form fluorides under reaction conditions. Mercuric chloride promotes addition of HCl to acetylene to make vinyl chloride. Oxychlori-nation in the Stauffer process for vinyl chloride from ethylene is catalyzed by CuCL with some KCl to retard its vaporization. 

Allylic transposition is observed in fluoride-induced reactions of allyl(trifluoro)silanes50 and allyl(trichloro)- and allyl(trialkoxy)silanes in the presence of hydroxylic promoters, e.g., 1,2-benzenediol51,52. Pentacoordinated silicates are believed to be involved53. 

The synthesis of polyhalide salts, R4NX , used in electrophilic substitution reactions, are described in Chapter 2 and H-bonded complexed salts with the free acid, R4NHX2, which are used for example in acid-catalysed cleavage reactions and in electrophilic addition reactions with alkenes, are often produced in situ [33], although the fluorides are obtained by modification of method I.I.I.B. [19, 34], The in situ formation of such salts can inhibit normal nucleophilic reactions [35, 36]. Quaternary ammonium chlorometallates have been synthesized from quaternary ammonium chlorides and transition metal chlorides, such as IrClj and PtCl4, and are highly efficient catalysts for phase-transfer reactions and for metal complex promoted reactions [37]. 

Trimethylsilylation of enolizable carbonyl compounds and alcohols has also been accomplished by the fluoride ion promoted reaction with hexamethyldisilane and ethyl trimethylsilylacetate [48, 49], with high stereospecificity giving Z-enol ethers from ketones [50]. l-Trimethylsilyl-(l-trimethylsilyloxy)alkanes, produced from the reaction of aldehydes with hexamethyldisilane, undergo acid-catalysed hydrolysis during work up to yield the trimethylsilylcarbinols [51]. In the case of aryl aldehydes, the initially formed trimethylsiloxy carbanion produces the pinacol (Scheme 3.1). 

Optimum yields of (3-vinyl-y-butyrolactols from the Pd(II) promoted reaction of vinyl triflates with Z-but-2-en-l,4-diol (Scheme 6.33) are attained when tetra-n-butylammonium chloride is added (47]. The lactol is conveniently oxidized to the lactone with celite-supported silver carbonate. The corresponding arylbutyrolactols are obtained in high yield (70-80%) from an analogous reaction of iodoarenes with the enediol. The yields of 2-alkenyl-2,5-dihydrofurans, resulting from the Pd(0) catalysed reaction of cyclic alkynylcarbonates with acrylic esters via tandem C-C and C-0 bond forming reactions, are enhanced by the presence of tetra-n-butyl-ammonium fluoride (e.g. Scheme 6.33) (48]. 

Unsaturated fluorocarbons are much more reactive toward nucleophiles than then hydrocarbon counterparts owing to fluorine s ability to both stabilize carban ions and inductively increase the electrophihcity of multiple bonds and aromauc rings Nucleophilic attack dominates the chemistry of unsaturated fluorocarbons, and the role of fluoride ion in fluorocarbon chemistry is analogous to that of the proton in hydrocarbon chemistry [729] Lake the related electrophilic reactions for hydrocarbons, there are fluoride-promoted isomenzations and dimerizations (equation 9), oligomerizations (equation 10), additions (equation 11), and amomc Fnedel-Crafts alkylations (equation 12) that all proceed via carbamomc intermediates [729 141]... 

The Pandey group has developed a silver fluoride-promoted desily lation of tertiary bis(silyl)amines as an interesting alternative method to access azomethine ylides (Scheme 2.10).18 Notably, this method allows the generation of nonstabilized azomethine ylides under essentially neutral conditions. The starting materials are prepared by a three-step process, sometimes coupled into a single operation. For example, Boc-protected pyrrolidine 36 can be sequentially deprotonated and silylated twice in a one-pot reaction (Scheme 2.10). Removal of the Boc group and alkylation of the free amine leads to bis(silyl)amine 37. When this compound is treated with 2 equiv of silver fluoride in the presence of phenyl vinyl sulfone, rapid formation of products 39 as single stereoisomers results. 

The desired extent of fluorination of the starting material may dictate the fluorinating agent to be used. For example, fluorine converts silicon tetrachloride to silicon tetra-fluoride, whereas antimony(III) fluoride promotes the reactions which produce the chlorofluorides, SiCl3F, SiCl2F2, and SiCIFs. 

Calcium fluoride has also been used in metathetical reactions. Usually, heat is required to promote reactions of this type. Booth and Dutton4 passed vapors of a volatile halide [phosphorus(V) oxychloride] over a bed of heated calcium fluoride to produce the chlorofluoride and fluoride derivatives. Sodium fluoride, potassium fluoride, titanium (IV) fluoride, and zinc fluoride have been used in similar metathetical fluorination reactions. 

Silyl enolates react with acyl cation equivalents to give the C- and/or O-acylated products (Equation (90)).333 Fluoride-catalyzed reaction using acyl fluorides is valuable for O-acylation of silyl enolates derived from aldehydes and ketones.334 CuCl also promotes the 0-acylation with acyl chlorides.335 The CuCl-promoted reaction of ester silyl enolates results in exclusive (7-acylation. Combined use of BiCfl and Znl2 (or Nal) effects catalytic (7-acylation of ketone silyl enolates with acyl chlorides. 

Among the fluoride ion promoted reactions which occur in dipolar non-HBD solvents are alkylations of alcohols and ketones, esterifications, Michael additions, aldol and Knoevenagel condensations as well as eliminations for a review, see reference [600]. In particular, ionic fluorides are useful in the dehydrohalogenation of haloalkanes and haloalkenes to give alkenes and alkynes (order of reactivity R4N F > K ([18]crown-6) F > Cs F K F ). For example, tetra-n-butylammonium fluoride in AjA-dimethylformamide is an effective base for the dehydrohalogenation of 2-bromo-and 2-iodobutane under mild conditions [641] cf Eq. (5-123). 

Tetrabutylammonium fluoride promotes the intramolecular nucleophilic attack of the ester group. Oxazoline-5-ones 622 are obtained from carbodiimide 621 in this manner. The reaction is conducted at room temperature in THE and yields of 55-75 % are obtained. ... 

A highly exo-selective asymmetric hetero Diels-Alder reaction was the key step in D.A. Evans total synthesis of (-)-epibatidine. The bicyclic cycloadduct was then subjected to a fluoride-promoted fragmentation that afforded a (f-keto ester, which was isolated exclusively as its enol tautomer. The removal of the ethoxycarbonyl functionality was achieved using the Krapcho decarboxylation. Interestingly, the presence of a metal salt was not necessary in this transformation. Simply heating the substrate in wet DMSO gave rise to the decarboxylated product in quantitative yield. 

Denmark, S. E., Lee, W. Investigations on Transition-State Geometry in the Lewis Acid- (Mukaiyama) and Fluoride-Promoted Aldol Reactions. J. Org. Chem. 1994, 59, 707-709. 

Cohen has used a soluble version of the Simmons-Smith reagent to promote the 3,2-rearrangement of allylic sulfides (Scheme 52). He has also applied this methodology to the synthesis of sarkomycin (225). The allylic sulfide (221), easily available via a Petrow reaction, was first alkylated and the sulfo-nium ylide was then generated by a fluoride-promoted desilylation. Rearrangement of the ylide (223) gave the homoallylic sulfide (224) which was eventually transformed into sarkomycin (Scheme 53). 

The most intriguing results are those from the reaction promoted by fluoride. The reaction of fluoride with an allylmetal reagent is thought to proceed through either an allyl anion or an allyl fluorosiliconate intermediate [24]. Allyl anions [25] and pentacoordinate silicon species [26] have been proposed as intermediates in the fluoride-induced allylation of a carbonyl compound. The results obtained with the deuterium model rule out the intermediacy of a free allyl anion since the ratio 14/15 is different for the syn compared to the anti Sp/ pathways. 

Fluoride-promoted fragmentation of 2,3-bis(2-trimethylsilylethyl)cyclopropenethione resulted in elimination of fluorotrimcthylsilane and ethenc to give the tetrabutylammonium salt of 3-thioxocycloprop-1 -cne-1,2-dithiolate, which, by addition of sodium perchlorate solution, was converted into the sodium salt in 62% yield.The reaction of 2,3-bis(/ert-butylsulfanyl)cyclo-propenethione with two equivalents of phenyllithium in tetrahydrofuran at — 80 C for three hours gave 2-(tect-butylsulfanyl)-3-phcnylcyclopropenethionc in 85% yield. 

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