Allylation fluoride-induced

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. 

Scheme 4.54 Formation of chiral allene via fluoride-induced elimination of chiral allylic silyl triflate213.

Allylic selenocyanates and thiocyanates are prepared from the reaction of allylsilanes with KSeCN 2CuBr2 and KSCN 2CuBr2, respectively. Both cyanoselenation and cyan-othianation take place at the a-position of allylsilanes (equation 106). Presumably, the electrophile XCN generated in situ is first added to the /-position, followed by a [l,3]-migration196. Fluoride induced reactions of allylsilanes with thioketones yield the corresponding allyl sulfides197. Displacement via an Se route is noted. 

Fluoride-induced rearrangements of terminal to internal isomers of perfluoro-alkenes are well established [70-72], for example the rapid isomerisation of perfluoro-l-pentene to the thermodynamically more stable isomer occurs in the presence of cesium fluoride (Scheme 33) [73], Allylic displacement reactions occur readily and with a stereochemistry suggesting steric control (Scheme 34) [74],... 

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. 

Scheidt recently reported that a-silyl alkoxides generated from a-silyl silylethers 79 by fluoride-induced desilylation instead of deprotonation of a-silyl alcohol can be trapped by primary alkyl and by allylic and benzylic electrophiles via a Brook rearrangement (Scheme... 

The syntheas of eiy/Aro-diols by Sharpless a mmetric difaydroxylation of /-but dimetl silyl-protected allylic alcohols, followed by cyclic sul te formation and fluoride induced Payne-like rearrangement, is illustrated in Scheme 1. ... 

General Considerations. A sequence involving metalation, alkylation, and fluoride-induced elimination of benzenesulfinate allows the conversion of (2) to a terminal alkene. An analogous sequence involving a double alkylation of (2) provides a 1,1-disubstituted alkene (eq 2). The Uthio derivative of (2) has also been used to prepare cyclopropyUdene derivatives, homo-allylic alcohols, and aUyl silanes via the Julia alkenation. ... 

The proposed catalytic cycle [42] is analogous to that shown in Scheme 5.6, except for the additional release of an enolate anion due to the fluoride-induced desilylation. Oxidative addition of allyl carbonates leads to the formation of the allyl complex 78, COj, and an alkoxide RO . The fluoride source and the alkoxide RO are capable of liberating an enolate anion by desilylation. This explains why substoichiometric amounts of Bu4NPhgSiF2 are sufficient to maintain the catalytic cycle that is displayed in Scheme 5.25 for the allylation of 2-methylcyclohexanone through the silyl enol ether. The carbon-carbon bond-forming step is assumed to occur by a collapse of the ion pair 79 consisting of the cationic allylpalladium complex and the enolate anion. Aside from these ionic species, covalently bound palladium enolates were also discussed. 

Mixtures of anhydrous hydrogen fluoride and tetrahydrofuran are successfully used as fluorinating agents to convert 1,1,2-trifluoro-l-allcen-3-ols, easily prepared from bromotrifluoroethene via lithiation followed by the reaction with aldehydes or ketones, to 1,1,1,2-tetrafluoro-2-alkenes The yields are optimal with a 5 1 ratio of hydrogen fluoride to tetrahydrofuran The fluorination reaction involves a fluonde lon-induced rearrangement (Sf,j2 mechanism) of allylic alcohols [65] (equation 40)... 

Allylsilanes react with carbonyl compounds to transfer the allyl group with 1,3-transposition, in the presence of Lewis acids, typically titanium(IV) chloride47. Recently this reaction has been carried out under super-acid catalysis48. Transfer of the allyl group is also induced by tetrabutylammonium fluoride, but in this case reaction takes place regioselectively at the less substituted end of the allyl fragment49. 

Reaction of allylic silanes with aldehydes and ketones can also be induced by fluoride ion, which is usually supplied by the THF-soluble salt tetrabutylammonium fluoride (TBAF). Fluoride adds at silicon to form a hypervalent anion with much enhanced nucleophilicity.73 An alternative reagent to TBAF is tetrabutylammonium triphenyldi-fluorosilicate.74... 

Ruthenium complexes mediate the hydroamination of ethylene with pyridine.589 The reaction, however, is not catalytic, because of strong complexation of the amine to metal sites. Iridium complexes with chiral diphosphine ligands and a small amount of fluoride cocatalyst are effective in inducing asymmetric alkene hydroamination reaction of norbomene with aniline [the best enantiomeric excess (ee) values exceed 90%].590 Strained methylenecyclopropanes react with ring opening to yield isomeric allylic enamines 591... 

The authors explained the formation of these compounds as follows. The O-nucleophile initially attacks the C=C bond of the perfluoroolefin, forming a carbanion. The latter may be stabilized by elimination of the fluoride ion from the y-position of the CF2 fragment, giving a multiple bond. The olefin has a mobile fluorine atom in the allyl position, leading to intramolecular nucleophilic cyclization induced by the O- or N-nucleophiles. This process may be regarded as an example of the important role of the intermediate from the highly active perfluoroolefin, determining the direction of the subsequent attack of the O- and N-nucleophiles. 

Fluoride- or Lewis acid-induced cyclization of alkylidene 1,3-dioxo compounds with an allylic silane moiety has also been studied. When la was used very high trans preference was observed. 

The nucleophilic reactivity of allylic silanes can also be induced by fluoride ion. Fluoride adds at silicon to form a hypervalent structure with enhanced nucleophilicity. 

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