Iodonium tetrafluoroborates

The add-catalyzed cyclodehydration of (Z)- and ( )-6-hydroxy-a,p-unsaturared ketones offers a mild synthesis of substituted furans. In the case of ( )-olefins, photochemical isomerisation was found to accelerate the reaction <96TL6065>. Reaction of alkynyl(phenyl)iodonium tetrafluoroborates with tropolone in the presence of a base yields 2-substituted furotropones (Scheme 16, <96TL5539>). 

Electrophilic iodine reagents are extensively employed in iodocyclization (see Section 4.2.1). Several salts of pyridine complexes with 1+ such as bis-(pyridinium)iodonium tetrafluoroborate and b/.s-(collidine)iodonium hexafluorophos-... 

An interesting hydroiodination reaction occurs when a mixture of cyclohexene and triethylsilane in dichloromethane is treated with a mixture of bis(pyridine) iodonium tetrafluoroborate and tetrafluoroboric acid in diethyl ether (Eq. 125). A 50% yield of iodocyclohexane is produced after one hour at 20°.268... 

Cyclopentene annelation.1 The reaction of the anion of a 1,3-dicarbonyl compound with l-decynyl(phenyl)iodonium tetrafluoroborate results in an annelated 3-pentylcyclopentene in reasonable yield. The product is considered to result from... 

When 4-/-butylcyclohex-1 -enyl(phenyl)iodonium tetrafluoroborate (3) is heated at 60 °C in chloroform, 1-fluorocyclohexene 4, 1-chlorocyclohexene 5 and l-(o-iodophenyl)cyclohexene 6 are formed with accompanying iodobenzene leaving group (eq 2).3 These three substitution products are best accounted for by formation of an ion pair involving cyclohexenyl cation 7. The cyclohexenyl cation 7 formed picks up fluoride from tetrafluoroborate and chloride from chloroform solvent, and recombines with the iodobenzene generated (eq 3). This kind of reactions with a counteranion and solvent are characteristic of unstable carbocations and are known in the case of phenyl cation generated from the diazonium salt in the Schiemann-type reaction.4... 

Reactions of 3 in alcoholic and aqueous solvents result in a normal solvolysis product 8 (and cyclohexanone 9) as well as the recombination product 6 (eq 4).5 This is again rationalized by an ion-pair mechanism. 2-Methylcyclohex-l-enyl(phenyl)iodonium tetrafluoroborate (10) undergoes solvolysis about 250 times as fast as 3, and gives some rearranged product 12 in accord with the SnI solvolysis mechanism (Scheme 1). 

Interesting results concerning phenyl group participation were observed with ( )-styryl(phenyl)iodonium tetrafluoroborate (26) using a deuterated substrate (eq 12)16 When 26-ad was heated in trifluoroethanol (TFE) at 60 °C, slow reaction gave die E isomer of substitution product 28 quantitatively, but the deuterium was completely scrambled between the a and p positions. This strongly indicates that a symmetrical intermediate is involved during the reaction and the most reasonable one is vinylenebenzenium ion (27) formed by phenyl participation. This intermediate also explain the exclusive formation of the retained ( )-28. 

Photosolvolysis of some vinyl iodonium substrates can be compared with thermal solvolysis of the same substrates. Photochemical reaction of (E)- and (Z)-2-phenylprop-l-enyl(phenyl)iodonium tetrafluoroborate (31) in methanol and TFE gave products of heterolytic cleavage of vinylic C-I bond as summarized in Table III (for the product structures see eq 14.17 In contrast to... 

Comparisons of product distributions in thermal and photochemical solvolyses show that the primary vinyl cation is not involved in thermolysis but is formed photochemically. The chirality probe approach using optically active 4-methylcyclohexylidenmethyl(aryl)iodonium tetrafluoroborate 19 was applied to the photosolvolysis.24 The rearranged product 4-methylcycloheptanone retained some optical activity, but the enantiomeric product in slight excess has a different structure depending on the iodoarene leaving group Arl of the substrate. The results indicate that the primary vinyl cation involved is not in a free, dissociated achiral form. 

Cyclohexenyl and cyclopentenyl iodonium tetrafluoroborates were also photolyzed in methanol. Ring-strained five-membered cyclic vinyl cation could be generated photochemically as well as the six-membered cyclic vinyl cation.25... 

Bis(pyridinium)iodonium tetrafluoroborate [I(Py)2BF4] reacts readily with alkenes to afford 1,2-disubstituted products arising from addition of iodine and pyridine. Synthetically more important is, however, the reaction of unsaturated systems with I(Py)2BF4 in the presence of nucleophiles, which provides a general method for vicinal iodofunction-alization of alkenes. In this regard, the addition of a stoichiometric amount of tetrafluo-roboric acid to the reaction medium is often required to avoid the competitive formation of products resulting from pyridine acting as a nucleophile. 

Typical procedure. To a stirred solntion of Ph2TeCl2 (716 mg, 1.40 mmol), PdClj (25 mg, 0.14 mmol) and NaOMe (220 mg, 4.20 mmol) in CHjCN/MeOH (20 mL) under nitrogen atmosphere was added p-methoxyphenyl(phenyl)iodonium tetrafluoroborate (556 mg, 1.40 mmol) at room temperatnre. The reaction mixture was stirred at room temperature for 7 h. The reaction mixture was extracted with diethyl ether (3x20 mL). The organic layer was dried over anhydrous sodium sulphate and evaporated in vacuo. The crude product was separated by Si02 column chromatography (hexane, Rf=0.l7) to afford the conpled product (219 mg, 85%). 

Electrophilic iodine reagents have also been employed in iodocyclization. Several salts of pyridine complexes with I+ such as bis(pyiidinium)iodonium tetrafluoroborate and bis(collidine)iodonium hexafluorophosphate have proven especially effective.61 y-Hydroxy- and d-hydroxyalkcncs can be cyclized to tetrahydrofuran and tetrahydropyran derivatives, respectively, by positive halogen reagents.62 (see entries 6 and 8 in Scheme... 

When cyclohexene (5) is treated with bis(pyridine)iodonium tetrafluoroborate (Ipy2BF4, 6) in the presence of tetrafluoroboric acid in dichloromethane, irans- -fluoro-2-iodocyclohexane (7) is obtained in 89% yield, with the tetrafluoroborate counter anion acting as the source of fluoride ion.59 60... 

Iodonium tetrafluoroborate, biphenylene-X-ray structures, 1, 566 Iodopropenylation alkyl halides, 1, 469 Ionenes uses, 1, 289 Ion exchange resins pyridine polymers and, 1, 308-309 Ionization potentials pyridines, 2, 135 cts- 3-Ionol synthesis, 3, 666 cis-/3-Ionone... 

It has been shown that selective a-vinylation of enolate anions derived from 1,3-dicarbonyl compounds can be achieved by reaction with 4-/-butyl-1 -cyclohcxcnyl-(aryl)iodonium and 1 -cyclopentenyl(aryl)iodonium tetrafluoroborates without competing a-arylation, provided that the alkenyliodonium salt used bears a / -mcthoxyphcnyl, rather than phenyl, group.24... 

In a recent series of studies focused on the synthetic utility of alkenyliodo-nium salts, ( )-/J-phenylethenyl(phenyl)- and ( )-l-hexenyl(phenyl)iodonium tetrafluoroborates, 22 and 23, were utilized for alkenylations of a range of soft, anionic nucleophiles (Scheme 45) [128-135]. In all cases but one, alkenylations with 22 occurred with retention of configuration, while alkenylations with 23 occurred with inversion of configuration. Only the dialkyl phosphoroselenolate salts gave mixtures of (Z)- and (fj)-products with 22 [132]. Furthermore, although cuprous iodide was used to catalyze the reactions of 22 and 23 with the phosphorothioate and -dithioate salts, the stereochemical results were the same [131,133]. It was generally assumed that retention was an outcome of the ligandcoupling or addition-elimination pathways, while stereochemical inversion was attributed to the vinylic... 

Sn2 mechanism (Scheme 47) [138,139]. Concerted,in-plane nucleophilic substitutions of this type are facilitated by the exceptional nucleofugality of iodoben-zene, and appear to be operative with weakly basic nucleophiles. Similar reactions of ( )-/ phenylvinyl(phenyl)iodonium tetrafluoroborate are slower, less stereoselective, and more prone to competitive alkyne formation. 

Alkynylbenzaldehydes cyclize in the presence of bis(pyridine)iodonium tetrafluoroborate (IPy2BF4) to form the intermediate 4-iodoisochromenylium species 141, which can be trapped by various nucleophiles to afford 4-iodo-l//-isochromene derivatives (Scheme 45) <2003JA9028>. 

The treatment of (trimethylsilyl)acetylene with iodosylbenzene and boron trifluoride etherate does not give ethynyl(phenyl)iodonium tetrafluoroborate, but leads instead to (E)-1 -trimethylsilyl-2-ethoxyethynyl(phenyl)iodonium tetrafluoroborate (equation 26)79. 

However, when [(trimethylsilyl)ethynyl]phenyliodonium tetrafluoroborate is treated with 46% aqueous hydrofluoric acid, protodesilylation occurs, and ethynyl(phenyl)iodonium tetrafluoroborate is obtained in high yield (equation 27)27. 

The alkyl chain in such MCI reactions need not be restricted to the alkynyliodonium component. For example, the treatment of sodium 2-n-hexyl- 1,3-indandionate with 1-propynyl(phenyl)iodonium tetrafluoroborate in THF affords the spiro-nonene system shown in equation 4228. 

When y-CH bonds are present in the R group of the alkynyliodonium ion, cyclopentenyl sulfones predominate. For example, the treatment of 5-phenyl-1-pentynyl(phenyl)-iodonium tetrafluoroborate with te/ra- -butylammonium benzenesulfinate in THF (i.e. homogeneous conditions) affords a moderate yield of l-phenylsulfonyl-3-phenylcy-clopentene and a low yield of the corresponding alkynyl sulfone (equation 51)32. With appropriately constructed alkynyliodonium ions, annulated cyclopentenyl sulfones are obtained (equations 52 and 53)32. 

Various alkynyl(phenyl)iodonium tetrafluoroborates react with aliphatic sodium carboxylates in aqueous THF to give a-(acyloxy)ketones (equation 91)30. The presence of... 

When / -dicarbonyl enolates are allowed to react with alkynyliodonium salts, typically in ter/-butyl alcohol or THF, alkynyl- and/or cyclopentenyl- -dicarbonyl compounds are obtained. The product compositions are largely regulated by the migratory aptitude of R in the alkynyl moiety and the availability of alkyl side chains for the MC-insertion (MCI) pathway (equation 45). These divergent modes of reactivity are nicely illustrated by the reactions of the 2-phenyl-1,3-indandionate ion with ethynylfphenyl)- and 4-methyl-1-hexynyl(phenyl)iodonium tetrafluoroborates (equation 1 15)27 2. 

Because the hydrogen atom and phenyl group migrate so readily, the reactions of / -dicarbonyl enolates with ethynyl- and (phenylethynyl)iodonium salts can be expected to result in alkynylation. It has already been noted that the 2- -hexyl-l,3-indandionate ion undergoes alkynylation with (phenylethynyl)phenyliodonium tetrafluoroborate (equation 43), despite the availability of the -hexyl group for [2 + 3] annulation. Ethynylations of six / -dicarbonyl enolates and the anion of 2-nitrocyclohexane with ethynyl(phenyl)-iodonium tetrafluoroborate in THF have also been reported27. For example, admixture of the ethynyliodonium salt and the anion of ethyl 2-cyclopentanone-l-carboxylate in THF affords the 1-ethynyl derivative in 71% isolated yield (equation 124)27. 

The treatment of alkynyl(phenyl)iodonium tetrafluoroborates in dichloromethane with aqueous sodium periodate affords the corresponding alkynyliodonium periodates24. However, except for the ter t-buty analog, which has been characterized by X-ray analysis (Table 3), these compounds are unstable to autooxidation and readily decompose to carboxylic acids (equation 150). 

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