Silver tetrafluoroborate, formation

The first asymmetric synthesis of (-)-monomorine I, an enantiomer of the natural alkaloid, by Husson and co-workers starts with the chiral 2-cyano-6-oxazolopiperidine synthon (385) prepared from (-)-phenylglycinol (384), glu-taraldehyde (383), and KCN (443). Alkylation of 385 with an iodo ketal led to the formation of a single product (386). The cyano acetal (386) was treated with silver tetrafluoroborate and then zinc borohydride to afford a 3 2 mixture of C-6 epimeric oxazolidine (387) having the (2S) configuration. Reaction of 387 with... 

Cyclizations with nitrogen nucleophiles involving alkynes and allenes have received little attention until recently. The cyclizations of several a-aminoallenes to 3-pyrrolines with silver tetrafluoroborate was reported by Claesson and coworkers (equation 133).264 A similar cyclization to form A -carba-penems has been reported (equation 134).265 Diastereomeric allenes (R1 R2) were shown to cyclize with complete stereocontrol. Cyclization with palladium chloride in the presence of allyl bromide or electrophilic alkenes allowed for the intermediate vinylpalladium species to be trapped by the electrophile.2651 A related product was obtained by cyclization of an alkynic substrate (equation 13S).265 Other examples of 5-endo cyclization of p-aminoalkynes50 include the formation of indoles by cyclization of 2-alkynylanilines with mercury salts200 or palladium chloride,266a,266b,266c formation of 1-pyrrolines with catalytic palladium chloride (equation 136)198 and formation of pyrroles by cyclization of hydroxy-substituted p-aminoalkynes.198,2666... 

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>. 

To further expand the scope of this new silver(I)-mediated reaction sequence, interrupted Nazarov cyclizations were explored using the halocyclopropane chemistry, an investigation that was prompted by the discovery of an intriguing result. It was found that treatment of the phenethyl-substituted compound 76 with 1.5 equiv of silver tetrafluoroborate in dichloromethane provided benzohydrindenone 77 as the sole product, with no apparent formation of the simple a-chlorocyclopentenone (Scheme 4.24). This prompted an examination of appropriately substituted gem-dichlorocyclopropane substrates in analogous interrupted Nazarov processes to ascertain the scope of this new cascade reaction sequence. 

This reactivity proved to be a general process, providing the unique products in moderate yields following cyclopropanation and immediate treatment with silver tetrafluoroborate. These structures revealed that a cascade sequence was proceeding stereoselectively in every case to furnish a single product as the result of conrotatory 4jt electrocyclization, electrophilic aromatic substitution at the least hindered position on the arene moiety (para to the MeO) in favor of six-membered ring formation, and desilylation with protonation from the exo face of the bicyclic product. Dehydrochlorination to form a second cationic intermediate did not occur in this case, due to structural restrictions imposed by the bridged architecture of 81. 

Fig. 59.—Formation of a cis-(l — 2)-linked glycoside by a silver tetrafluoroborate-catalyzed reaction of glycosyl oxirane with a tributyltin either.343...

The phenylselenenyl chloride/silver tetrafluoroborate induced purino-selenenylation of the unsaturated phosphonate derivatives 50 gives the adducts 52 in modest yields but with a good control of the stereoselectivity (>80% de). This stereoselectivity is in good agreement with preferential formation of the seleniranium salt 51 which undergoes stereoselective opening by a purine base to give the adducts 5296. 

When the chiral allenic amine 1 was cyclized using silver tetrafluoroborate, transfer of chirality occurred from the allenic moiety to the C-2 stereogenic center, as determined by conversion of 2 into the diastcrcomeric mixture of the (+)-(5)-0-methylmandelic amides of the debenzylated product. The high asymmetric induction was ascribed to the formation of a dissymmetric silver-allene complex229. 

Complexes. Silver tetrafluoroborate is insoluble in cyclohexane but very soluble in water, ether, toluene, and nitromethane, and moderately soluble in benzene and cyclohexene. - Solubility in the unsaturated hydrocarbons is attributed to n-bond formation between the unsaturated compound and silver ion. Complexes have been observed with both ir-electron donors and lone-pair donors thus Meerwein prepared stable complexes with benzene, mesitylene, cycloheptatriene, diethyl ether, dimethyl sulfoxide, and a series of acid nitriles. However, the silver tetrafluoroborate-benzene complex is reported variously as a 1 1 complex, a 1 2 complex, and as a 2 3 complex. ... 

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. 

The allene PrCH=C=CHCH2NH2 isomerizes to the 3-pyrroline (122) under the influence of silver tetrafluoroborate. Photolysis of aroyl azides in the presence of diketen (123) yields the hydroxy-pyrrolinones (124).AT-Ray analysis has shown that the adduct of the imine Pr"CH=NPr to N-phenylmaleimide has structure (125). Treatment of dichloromaleimide with ethoxycarbonyl-methylenetriphenylphosphorane affords the Wittig product (126). The formation of the pyrrolidinone (128) in the thermolysis of the iV-cyclohexenylacryl-amide (127) represents an intramolecular ene-reaction (see arrows). The perfluoropyrrolidinone (130) results from the reaction of the cyclobutane (129) with potassium fluoride. Pyrrolidinols (131) are obtained in moderate yields by photochemical cyclization of the amides ArCOCH2CH2NBzCH2Ph. ... 

Scheme 10.60. An example of the Nazarov cycUzation effected on cyclopentenoyl chloride. The example shows a potential rationalization for the observation that treatment of the acid chloride with silver tetrafluoroborate in dichloromethane at -50°C followed by addition of l-(trimethylsilyl)-l-(phenylthio)ethene results in the formation of 4-(thiophenyl)bicyclo[3.3.0] oct-3-ene-2-one. It is argued that the cycUzation step is a concerted process (see Habermas, K. L. Denmark, S. E. Jones, T. K.The Nazarov cyclization, in Paquette, L. A. (ed.). Organic Reactions, Vol. 45, Wiley, Hoboken, NJ, 1994 Magnus, P. Quagliato, D. J. Org. Chem., 1985, 50,1621).

Quaternization of selenides with methanesulphonyl fluoride or methyl iodide-silver tetrafluoroborate, followed by elimination with potassium t-butoxide ki THF or DMSO at room temperature, offers an alternative procedure to selenox-ide elimination. An advantage appears to be the formation of the volatile side product dimethyl selenide, rather than the selenenic, seleninic, or selenoic acids... 

If the reaction was carried out in liquid SO2 at -78it was possible to observe a shift of the allyl and Z-methyltetrahydrofuTeOi protons in an nmr spectrum taken on a HR300 varian nmr Spectrometer but the reaction was very slow. This result is consistent with the formation of the expected oxonium salt. No other products were apparent. However, the spectrum was quite con licated because of the many absorptions due to products and starting materials and we did not attempt to interpret it completely. Lambert and Johnson made similar comments about their nmr spectrum from a reaction of isopropyl bromide, tetrahydrofuran, and silver tetrafluoroborate (9.). ... 

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