Two special pathways to perfluoroaryl iodonium salts: the migration of the nucleophilic aryl group in C 6 F 5 IO and the electrophilic arylation of C 6 F 5 I with [C 6 F 5 Xe] + salts

Two theoretically interesting pathways to the perfluorinated diaryliodonium cation [(C 6 F 5 ) 2 I] + are described: the electrophilic arylation of C 6 F 5 I by [C 6 F 5 Xe] + salts and the nucleophilic migration of a C 6 F 5 group in the iodosyl compound C 6 F 5 IO. The first method allowed also the arylation of C 6 F 5 Br, but failed in the case of C 6 F 5 Cl. Furthermore, the protodeiodination of C 6 F 5 IO and [(C 6 F 5 ) 2 I] + in alkaline medium was investigated.


Introduction
Symmetrical diaryliodonium salts [Ar 2 I] X belong to the oldest examples of polyvalent iodine compounds and cover the largest subgroup of the class of iodonium compounds. 1Only few information has been published on the perfluoroaryl analogues. 2,3,4 Tis is astonishing because the pentafluorophenyl group represents an ideal organo ligand for onium cations which derived from hypervalent molecules.This was most impressively demonstrated in organoxenon chemistry (eq.1). 5 C 6 F 5 XeF + LA ⎯→ [C 6 F 5 Xe] [LA-F] (1) LA = Lewis acid Salts such as [C 6 F 5 Xe] [AsF 6 ] represent thermally stable solids melting (102 °C) without decomposition while the corresponding neutral molecule C 6 F 5 XeF is a low temperature species and decomposes at < 20 °C. 6The pentafluorophenyl group bonded to an onium center of a heavy main group element represents a strong electron-withdrawing ligand without mesomeric interaction and with a high resistance to oxidation.Different to the phenyl group, C 6 F 5 does not take over remarkable positive charge from the cationic center and is therefore not a favored electrophilic leaving group.
The high electronegativity of the C 6 F 5 group of 3.27 which is higher than that of chlorine facilitates its good nucleofugality in anionic species. 7Recently we have published a common synthetic approach to asymmetric iodonium tetrafluoroborates which was proved for a series of pentafluorophenyl(polyfluorophenyl)iodonium tetrafluoroborates.We have investigated their spectroscopic and structural characteristics as well as their thermal stability and have performed ab initio calculations for some representative iodonium cations. 2 The peculiarity of [(C 6 F 5 ) 2 I] + (g) compared to [(C 6 H 5 ) 2 I] + (g) is clearly expressed in the Mulliken charges on I of 1.266 versus 0.906 and on the ipso-C atoms of -0.960 versus -0.441.The significant high differences in charge on I and C(1) in [(C 6 F 5 ) 2 I] [BF 4 ] are responsible for two relatively strong C-I bonds with two very short I⋅⋅⋅F cation-anion contacts of 2.84 Å in average compared to 2.94 Å in the non-fluorinated analogue. 8In this paper we present two specific approaches to [(C 6 F 5 ) 2 I] + salts which cannot be applied to the corresponding [(C 6 H 5 ) 2 I] salts.
The new procedures to [(C 6 F 5 ) 2 I] + salts start from [C 6 F 5 Xe] + salts or from the iodosyl compound C 6 F 5 IO.The latter was first synthesized by Schmeiβer in 1967. 9In 1980 investigations in super acidic media were reported 10 and since 1985 C 6 F 5 IO was introduced in numerous metal-based epoxidation/oxygenation reactions. 11Unfortunately the thermal stability and the structure of 4a discussed in literature is controversial.Thus the decomposition temperatures ranging from 88 to 210 °C. 9,12 o our knowledge nothing is known about the products of thermal decomposition of C 6 F 5 IO.For 2,4,6-C 6 H 2 Cl 3 IO with a less electron-poor aryl group Willgerodt investigated its thermolysis upon steam distillation and reported 2,4,6-C 6 H 2 Cl 3 I, HIO 3 , and CO 2 as decomposition products, no formation of an iodonium compound was mentioned. 13

Results and Discussion
The electrophilic pentafluorophenylation of C 6 F 5 I The aryl group in C 6 F 5 I (1a), different to C 6 H 5 I (1b), resists all attempts of electrophilic substitution because F + does not represent a realistic leaving group by thermodynamic arguments.Despite the higher positive Mulliken charge on iodine in 1a (0.368) compared to 1b (0.164),  2a was consumed within 1 hour and pure 3a was isolated in 20 % yield.The different reactivities in the above mentioned systems of 1a/2 reflect the coordination moiety at Xe.In 2a no naked cation 2 is present.In MeCN solution the hard N-atom is coordinated 14 and in crystalline 2a strong cation-anion contacts were found despite of the weak coordinating character of the anion [AsF 6 ] -. 15 In the melt of 2a slow decomposition proceeded above 125 °C with the formation of equimolar quantities of C 6 F 6 and AsF 5 .The products can be best explained by the intermediate formation of C 6 F 5 XeF which forms C 6 F 6 and Xe in a subsequent step.There is strong evidence that the decomposition of C 6 F 5 XeF proceeds via the homolytic cleavage of the C-Xe bond. 3 Formal addition of the C 6 F 5 and F radical to C 6 F 5 I followed by the abstraction of the fluoride ion by the Lewis acid which is present, resulted in the iodonium salt 3a (Scheme 1).At first glance the negative result in MeCN solution contradicts to an earlier published result that [C 6 F 5 Xe] [(C 6 F 5 ) 3 BF] was able to arylate C 6 F 5 I in MeCN at ambient temperature in the presence of an excess of (C 6 F 5 ) 3 B. 4 [C 6 F 5 Xe] [(C 6 F 5 ) 3 BF] is not stable at room temperature and contains two nucleophilic sites: the ipso-C-atoms and the F-atom bonded to boron.Thus C 6 F 5 XeF may be the decisive intermediate which can add to C 6 F 5 I. Finally the Lewis acid (C 6 F 5 ) 3 B which was present in excess may have allowed the transformation to the corresponding iodonium salt [(C 6 F 5 ) 2 I] [(C 6 F 5 ) 3 BF].
The method of adding formally the [C 6 F 5 ] + cation to a n-base could be successfully extended to C 6 F 5 Br.For C 6 F 5 Cl we were not able to find positive reaction conditions.
melt An excess of C 6 F 5 Br was reacted in the melt of 2a under the same conditions as reported for C 6 F 5 I.In contrast to the completed reaction of 1a in the case of C 6 F 5 Br ca.11 % of initial 2a beside only 6 % of the final product [(C 6 F 5 ) 2 Br] [AsF 6 ] were still found after 1 h.The lower reactivity of the bromo compound with respect to the corresponding iodo analogue and the missing reactivity of C 6 F 5 Cl follows the IP values of C 6 F 5 Hal: I = 9.5 eV, Br = 9.67 eV, and Cl = 9.7 eV. 16Whereas the charge at Hal in C 6 F 5 Hal has no direct influence on the reactivity as proven by the natural charges (NBO, RHF, LAN2DZ basis set) in the series of C 6 F 5 Hal compounds with I = 0.321, Br = 0.169, Cl = 0.023, and F = -0.367.

The formation of [(C 6 F 5 ) 2 I] [IO 3 ] by migration of the nucleophilic aryl group in C 6 F 5 IO
C 6 F 5 IO (4a) like non-fluorinated C 6 H 5 IO (4b) is a polymeric, non-crystalline solid which is obtained by hydrolysis from mixed anhydrides ArIX 2 : X = OAc, Hal, etc.
In analogy to the precursor ArIX 2 we propose a T-shape arrangement at iodine(III) in 4a with a 2c-2e C-I bond and a linear 3c-4e O-I-O triad with a high positive partial charge on iodine and distinct negative partial charges on both bridging oxygen atoms. 174a was prepared from the corresponding pentafluorophenyliodine(III)bis(trifluoroacetate) or -difluoride using saturated NaHCO 3(aq) solutions of pH ≤ 10.To achieve full hydrolysis magnetic stirring was necessary over a very long reaction time of > 1 d.Intensive stirring (Ultra-Turrax) at 0 °C shortened the time of hydrolysis to less than 1 h.A more alkaline medium like a 1 m NaOH (aq) must be avoided to prevent the complete C-I bond cleavage.The nucleophilic attack of OH -on the I-atom initiated a shift of negative charge to the electronegative C 6 F 5 group which leaves the intermediate anion and adds a proton forming C 6 F 5 H.The unstable [IO 2 ] -anion as coproduct disproportionates to I -and [IO 3 ] -(Scheme 2).Scheme 2. The protodeiodination of polymeric C 6 F 5 IO.
The most remarkable property of the non-fluorinated iodosylbenzene 4b is its ability to disproportionate under controlled thermal conditions (steam distillation) yielding iodylbenzene C 6 H 5 IO 2 and iodobenzene (eq.5).Since the early work of Meyer 18 a further redox reaction of 4b under basic conditions is known in aqueous medium (eq.6).
This reaction has been explained by Masson 19 and Johnson 20 based on monomeric 4b by a sequence of dipolar additions or formal redox reactions.In this work we will show that the aimed thermolysis of 4a in the absence of a basic reagent in different media, aprotic included, ends with similar products according to eq. 6 and is best approximated by eq.7: During its thermal decomposition C 6 F 5 IO tends to explode. 12Therefore we decided to investigate this reaction only in gram scale and in the presence of an inert liquid media such as C 6 F 5 I, C 6 F 5 Cl, CCl 4 , C 6 H 6 , or H 2 O, which allow to dissipate the heat of decomposition.We have found optimal conditions for the decomposition at a bath temperature of 100 °C and performed the thermolysis in a rotating flask (rotary evaporator) which contained glass spheres of 4 mm diameter.This precaution prohibited the coagulation of larger amounts of solid 4a and minimized the formation of hot spots.The yield of 6a after isolation and purification was always only in the range of 40 to 60 %, whereas the yield of C 6 F 5 I was ≥ 120 %, both yields were based on eq. 7.In one case we proved the gas phase during the decomposition with an Orsat analyzer for CO 2 and O 2 .The yield of both gaseous products was only 5 and 3 %, respectively.We explain the formation of the iodonium cation [(C 6 F 5 ) 2 I] + by the migration of the nucleophilic C 6 F 5 group at the terminal iodine in the polymeric chain to a positive iodine neighbor center.This process is initiated by the deprotonation of the terminal I-OH group.It should be emphasized that polymeric 4a and 4b differ in their acidity of terminal OH groups.The negatively charged oxygen at the end of the I-O chain activates the nucleofugality of the C 6 F 5 neighbor group and initiates the migration.Scheme 3 explains in the first step the formation of [(C 6 F 5 ) 2 I] [IO 2 ].The redox unstable [IO 2 ] -anion interacts as Lewis acid with the terminal oxygen base of 4a in the direct moiety.In this redox reaction equivalent quantities of 1a and [IO 3 ] -were obtained.
When the thermolysis was performed in the protic medium water no C 6 F 5 H was formed indicating that the migration of the nucleophilic C 6 F 5 group was favored over the addition of a proton.In the aqueous medium no iodine from a disproportionation of [IO 2 ] -was observed which means that no acid was formed.The conversion of [ Additionally to the spectroscopic and analytical methods we proved the constitution of 6a by two metathesis reactions.In the first step 6a was reacted with neat CF 3 CO 2 H and converted to [(C 6 F 5 ) 2 I] [O 2 CCF 3 ] (7a) and HIO 3 (eq.8).
The cation anion ratio of 7a was unambiguously characterized by the integrals in the 19 F NMR spectrum.HIO 3 was identified by its molecular weight, determined by iodometry.The solubility of 7a in aqueous acetone allowed a further metathesis e.g. the convenient substitution of the [CF 3 CO 2 ] -anion by the [ClO 4 ] -anion (eq.9).
When 6a was heated in a 1 m NaOH suspension protodeiodination of the cation proceeded and C 6 F 5 H was formed beside [IO 2 ] -which disproportionate to give I -and [IO 3 ] -, eq. 10, compare Scheme 2.
The different literature statements about the thermal stability of 4a can now be explained.Freshly prepared 4a from C 6 F 5 IX 2 without residual amounts of terminal groups X in the polymer decomposes at ca. 89 °C.4a stored e.g. at 30 °C over a longer period of time (ca. 1 m) undergoes a transformation to 6a and 1a comparable to the fast process described in eq. 7. Thus the reported decomposition of 4a at 210 °C12 can be attributed to 6a.

Thermolysis of C 6 F 5 IO (4a) and formation of [(C 6 F 5 ) 2 I] [IO 3 ] (6a)
a) Thermolysis of 4a in C 6 F 5 I using a rotary evaporator.The rotating flask of the evaporator was charged with glass spheres (65 g), freshly prepared 4a (8.0 g, 26 mmol) and 1a (15 ml) under an atmosphere of dry Ar.Within 30 min.the mixture was heated to 100 °C under rotation and explosion protection.After ca. 3 hours the suspension changed to a yellow solution before precipitation started slowly.After 6 h reaction time the suspension was cooled to 20 °C.The white solid was separated, washed with CCl b) Thermolysis of 4a in water using a rotary evaporator.Using the above procedure 4a (4.2 g, 14 mmol) was heated in water (100 ml) at 100 °C for 6 h.6a (1.7 g, 60 %) was isolated as solid and 1a (1.6 g, 120 %) by extraction of the aqueous phase with CCl 4 .A comparable treatment in water at 50 to 60 °C for 5 h showed no conversion of 4a to 6a. c) Thermolysis of 4a in 1a and monitoring of the gas phase.A flask with 4a (4.0 g, 13 mmol) and 1a (10.8 g) equipped with a condenser which was connected by a three-way-stopcock to a Orsat gas analyzer.After evaporation of the reaction system at -50 °C dry Ar was introduced until normal pressure was achieved.The reaction mixture was then warmed to 100 °C monitoring the pressure.After 6 h the increase of pressure could be neglected.The gas phase was analyzed (ca.0.7 mmol CO 2 and 0.4 mmol O 2 ).6a (1.8 g, 64 % yield) was isolated from the suspension.e) The thermal treatment of C 6 F 5 IO (4a) in alkaline aqueous medium.In a flask with condenser 4a (3.0 g, 9.7 mmol) was heated in 30 ml of 1 m NaOH (aq) at 100 °C for 3 h.After 1h a product started to reflux and all solid disappeared.The volatile resultant was distilled off and identified as C 6 F 5 H by 19 F NMR and IR.The 19 F NMR of the CCl 4 extract of the aqueous phase exhibits no further C 6 F 5 compound.In the aqueous phase the molar ratio of I -to [IO 3 ] -was determined by iodometry to 1 to 2.1.f) The long term storage and decomposition of neat C 6 F 5 IO (4a).A sample of 4a (491 mg) was stored at ca. 30 °C for 1 month.Afterwards all volatile products were pumped off (10 -3 hPa).The weight of the volatile part was 164 (calc.155 mg) and of the residue 327 (calc.336 mg).The calculation was based on eq. 7. The volatile product was identified as 1a by 19 F NMR and the solid residue as 6a by IR.
d) Thermolysis of 4a in C 6 H 6 in the presence of Ag [ClO 4 ].In a further modification a stirred suspension of 4a (1.0 g, 3.2 mmol) and Ag [ClO 4 ] (0.5 g, 2.4 mmol) in dry benzene (5 ml) was refluxed for 2 h.The solid material was separated, washed with C 6 H 6 (0.5 ml), and dried.The white solid was extracted with hot water (60 °C, 3 times 25 ml).The residue consisted of Ag [IO 3 ] (IR, iodometry).The solvent was removed from the extract and the remaining solid was dried in vacuum.It consisted only of [(C 6 F 5 ) 2 I] [ClO 4 ], characterized by IR, Ra and mp (dec.263 °C) in comparison with the metathesis product from the reaction of [(C 6 F 5 ) 2 I] [O 2 CCF 3 ] and Na [ClO 4 ] in acetone/water (see later).