Silver Tetrafluoroborate rearrangements

Methyl a-arylalkanoates. Treatment of methyl ketals of a-halogenoalkyl aryl ketones, prepared in situ, with a number of silver salts, particularly silver tetrafluoroborate or hexafluoroantimonate, in methanol induces a rearrangement to methyl a-arylalkanoates in yields generally around 98%. The rate is influenced by substituents on the aryl group, the oxygens of the ketal group greatly facilitate this reaction (equation I).1... 

Interestingly, these authors53 and later others54 showed the importance of the silver counterion in such rearrangements. While silver tetrafluoroborate or silver nitrate gave the [3,3]-sigmatropic shift product mainly or exclusively, silver trifluoroacetate yielded the dienyl acetate through isomerization from the allenic ester (Scheme 3.33). 

In another related and well-known [3,3]-sigmatropic shift usually performed under thermal conditions, the propargyl-Claisen rearrangement,62 silver salts were also able to catalyze the reaction. Silver tetrafluoroborate and hexafluoroantimonate proved to be the best catalysts for this reaction, leading quantitatively to allenic p-ketoesters when starting from propargyl ethers derived from p-ketoesters (Scheme 3.41).63 64... 

Hiyama et al.69 showed that monoacetylated 1,4-butynediols nicely afforded acetoxy allenols on silver-catalyzed rearrangement. These compounds were ideal substrates for silver-catalyzed cyclization, so the overall sequence could directly be performed in one pot, leading to substituted 2,5-dihydrofurans in high overall yields (Table 3.5). In this process, 5-10 mol% of silver perchlorate or tetrafluoroborate was used in refluxing benzene. 

Extending their work on a-bromoaryl ketones (see Scheme 3.5), Giordano et al. have reported a [l,2]-sigmatropic rearrangement assisted by silver ion.79 Indeed, alkyla-cetals of primary and secondary a-halogenated aryl ketones furnished alkyl esters of a-aryl alkanoic acids in high yields using silver tetrafluoroborate in an alcoholic medium (Table 3.9). 

Treatment of labeled acyloxy alkynones with silver tetrafluoroborate in dichlor-omethane at room temperature exclusively gave a single furan, in which the labeled oxygen atom was connected to the ring carbon (Scheme 3.59). This observation was consistent with a 1,2-acyl migration but not with a [3,3]-sigmatropic rearrangement. 

This rearrangement must occur by heterolytic cleavage of the carbon-chlorine bond in 19 to yield the intermediate cation 6+, which is then attacked by the nucleophilic chloride at one of the partially positively charged cyclopropyl groups. With the less nudeophilic tetrafluoroborate counterion, generated by treating 19 with silver tetrafluoroborate under sol-... 

The case of p-hydroxy-y-alkenyl selenides merits further comments. The rearrangement efficiently takes place using the thallium(I) ethoxide method and the presence of an additional double bond in the reactant does not introduce a serious problem associated with unwanted reaction with the dichlorocarbene intermediate. This is not the case when silver tetrafluoroborate is used. 

Dichlorocarbene (generated in situ), is generally used to induce migration, but silver tetrafluoroborate has also been used, as in the preparation of cuparenone (23 equation 32). Rearrangement occurs only if the selenyl moiety is attached to a fully substituted carbon. For example, the hydroxy selenide (26) does not rearrange but instead yields the epoxide (27 equation 33). ... 

Syntheses of Alkylidene cyclopropanes Via the Selenonium route The selenonium route proved to be more valuable. It has been specifically designed by us to replace the deficient selenoxide route (Scheme 38). It was expected to produce alkylidene cyclopropanes by a mechanism which mimics the selenoxide elimination step but which involves a selenonium ylide in which a carbanion has replaced the oxide. Cyclopropyl selenides are readily transformed to the corresponding selenonium salts on reaction with methyl fluorosulfonate or methyl iodide in the presence of silver tetrafluoroborate in dichloromethane at 20 °C and, as expected, methylseleno derivatives are more reactive than phenyl-seleno analogs. Alkylidene cyclopropanes are, in turn, smoothly prepared on reaction of the selenium salts at 20 °C with potassium tert-butoxide in THF (Scheme 38). Mainly alkyl cyclopropenes form at the beginning of the reaction. They then slowly rearranges, in the basic medium, to the more stable alkylidene cyclopropanes( 6 kcal/mol). In some cases the complete isomerisation requires treatment of the mixture formed in the above reaction with potassium fcrt-butoxide in THF. The reaction seems to occur via a selenonium ylide rather than via a P-elimina-tion reaction promoted by the direct attack of the /crt-butoxide anion on the P-hydrogen of the selenonium salt, since it has been shown in a separate experiment that the reaction does not occur when a diphenylselenonium salt (imable to produce the expected intermediate) is used instead of the phenyl-methyl or dimethyl selenonium analogs. It has also been found that the elimination reaction is the slow step in the process, since styrene oxide is formed if the reaction is performed in the presence of benzaldehyde which traps the ylide intermediately formed... 

SUica gels, 110, 221, 229, 339, 345, 416, 461,505,509 a-Siloxyallylsilanes, 485 Silver acetate, 441-442 Silver carbonate, 441 Silver carbonate-Celite, 441-442 Silver chromate-iodine, 442 Silver cyanide, 442 Silver fluoride, 161 Silver nitrate, 247 Silver(I) oxide, 441, 442-443 Silver tetrafluoroborate, 443-444 Silverll) trifluoroacetate, 444-445 Silver(II) trifluoroacetate, 527 Silylation, 227 Silyl cyclopropanes, 307 Silyl enol ethers, 127, 172, 218, 227, 296, 378, 446, 485, 524-525 Silylvinyl triflates, 410 Simmons-Smith reagent, 172,445 Smiles rearrangement, 243 Sodium, 445... 

Alkylation of 1,4-benzothiazepin-5(4//)-one (82) with methyl iodide/silver tetrafluoroborate gave the sulfonium salt, which rearranged to l-methoxy-4-(methylthio)isoquinoline (83) on heating in acetonitrile (Scheme 27) <88CB2147>. 

PtPh2(bipy)] reacts with HBF4 in acetonitrile to give cationic [Pt(Ph)(bipy)(AN)](Bp4) (020M2088). [Pt(Ph)I(bipy)] reacts with silver tetrafluoroborate in acetone or acetonitrile to yield [Pt(Ph)(bipy)(sol-vent)](Bp4) (solvent = acetone, AN). Under carbon monoxide, the latter transform into the carbonyl complex [Pt(Ph)(CO)(bipy)](Bp4). The acetone complex inserts phenylallene into the platinum-phenyl bond to yield 85. Under carbon monoxide, 85 readily rearranges into cr-allyl 86. [Pt(Ph)(bipy)(AN)](Bp4) oxidatively adds methyl iodide to yield [Pt(Ph)(bipy)(AN)(Me)(l)](BF4). 

Acyl-3.4-benzo-2-azabicyclo[3.2.0]hepta-3,6-dienes 1, on heating at 250-280 C for a short time without solvent, rearrange to the 1-acyl-1-benzazepines 2 (Method A).23-38 In some cases, rearrangement is accompanied by minor amounts of Ar-aeyl-l-naphthylamine and, at higher temperatures, the acylnaphthylatnine can become the major product (see Section 3.2.2.6.). In the presence of silver(I) tetrafluoroborate (Method B) rearrangement takes place at lower temperatures but the yields of benzazepine are inferior as the silver(I) ion also catalyzes the reverse reaction (see Section 3.2.2.1.). 

As a consequence of the cyclobutyl to homoallyl rearrangement, reactions of 1-chloro-, 1-bro-mo-l,2-dimethylcyclobutane, 1-chloro- or l-bromo-l,3-dimethylcyclobutane with silver(I) te-trafluoroborate in diethyl ether gave in each case the same open-chain compound 2-fluoro-4-mcthylpent-4-ene (14) as the major product. Thus, l-bromo-l,3-dimethylcyclobutane (13) was treated with silver(l) tetrafluoroborate in diethyl ether at 0 >C to give 2-fluoro-4-methylpent-4-ene (14).22... 

To improve the thermal tandem [3,3]-sigmatropic rearrangement-enyne allene cyclization, Grissom et al.65 focused their attention on the transformation of 2-propynylvinyl ethers to allenes using silver salts as catalysts. The tetrafluoroborate proved to be the most effective, quantitatively yielding the expected aflenyl aldehyde (Scheme 3.42). However, isomerization problems appeared in some cases (see Scheme 3.33). 

Treatment of bicyclo[4.1.0]heptan-2-ols with perchloric acid in acetic acid caused very clean rearrangement with formation of cyclohept-3-enyl acetates (Table 1). Only in the case of cxo-7-methylbicyclo[4.1.0]heptan-2-ol was the cyclohex-2-enyl acetate the major product probably because the 7-methyl group conferred additional stabilization on the carbocation formed by j0-scission of the outer cyclopropane bond. The same type of reactant could be oxidatively rearranged using pyridinium chlorochromate to afford cyclohepten-4-ones, together with (chloromethyl)cyclohexenes. However, if the chloride in the reagent was replaced with tetrafluoroborate, or if pyridinium chlorochromate was used with silver(I) nitrate, formation of the substituted cyclohexenes was completely suppressed, e.g. formation of 7 from 6, although the reported yields were low. ... 

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