Elsevier

Tetrahedron

Volume 66, Issue 31, 31 July 2010, Pages 5762-5767
Tetrahedron

A first methodical approach to salts with unsymmetrical fluorophenyl(pentafluorophenyl)difluoroiodonium(V) cations [Rf(RF)IF2]+ (Rf=x-FC6H4, x=2, 3, 4; RF=C6F5)

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Abstract

A promising approach to the unknown type of [Ar′(Ar)IF2]X salts is offered. x-FC6H4IF4 (x=2, 3, 4) reacts with C6F5BF2 in CH2Cl2 and forms [x-FC6H4(C6F5)IF2][BF4] salts in good yields. For [4-FC6H4(C6F5)IF2][BF4] the fluoro-oxidizer property is shown in reactions with weakly reducing agents like E(C6F5)3 (E=P, As, Sb, Bi) and ArI (Ar=4-FC6H4, C6F5). The fluorine/aryl substitution method is also applied to the synthesis of [(4-FC6H4)2IF2][BF4], an example with two identical aryl groups in the difluoroiodonium(V) moiety.

Introduction

The chemistry of organoderivatives of iodine(V) is far less developed than that of iodine(III). While di(aryl)iodonium salts1, 1(a), 1(b) represent the largest number of polyvalent organoiodine(III) compounds, only few examples of the corresponding di(aryl)iodonium(V) salts are known. This is astonishing because Masson2 published a fundamental access to this type of compounds already in 1935. He obtained [(C6H5)2IO]OH by ‘self-condensation’ of C6H5IO2 in the presence of NaOHaq (Eq. 1).2C6H5IO2+OH→[(C6H5)2IO]OH+[IO3]

Beringer3 used this reaction in 1968 to obtain the first examples of [(Ar)2IO]X salts by metathesis (Eq. 2).[(Ar)2IO]OHHOAcF[(Ar)2IO]OAcFMIX[(Ar)2IO]XAr=C6H5, 4-FC6H4, 4-MeC6H4; MI=Na, K; X=F, Cl, Br.

Based on this results, in 1972 Yagupol’skii4 published the syntheses of the first di(aryl)difluoroiodonium(V) salts (Eq. 3).[(Ar)2IO]OAcFSF4[(Ar)2IF2]OAcFSF4[(Ar)2IF2]FAr=C6H5, 4-FC6H4.

The nature of [(C6H5)2IF2]F was elucidated first in 2004 by Hoyer.5 She was able to get the single crystal structure and could prove that no (C6H5)2IF3 was present. Three [(C6H5)2IF2]+ cations are bridged by fluoride ions in distances of 2.5–2.6 Å. Each [(C6H5)2IF2]+ cation has in agreement with the VSEPR notation (AB2C2E) a ψ-trigonal bipyramidal geometry.

Up to now all approaches to salts with cations of the general type [(Ar)2IY2]X were principally based on the ‘self-condensation’ step by Masson and allowed only symmetrical di(aryl) constitutions. In addition it is worth to mention that Masson’s method cannot be applied when a larger number of electron-withdrawing substituents like F or NO2 are present in the aryl group. In such cases the aryl group is eliminated under basic conditions (formation of Ar–H).6 In 2008 we have published a new methodical approach. We obtained the symmetrical example, [(C6F5)2IF2][BF4], by F/C6F5 substitution in C6F5IF4 with C6F5BF2.7 In the present paper we show the potential of this approach for the synthesis of unsymmetrical [Rf(RF)IF2][BF4] salts. Furthermore we want to show that [Ar(Ar′)IF2]+ cations are good fluoro-oxidizers. This property may be useful in future to introduce fluorine into an organic moiety.

Section snippets

Experimental results

x-FC6H4IF4 (x=2, 3, 4) reacted with C6F5BF2 in weakly coordinating solvents like CH2Cl2 under F/C6F5 substitution and the products [x-FC6H4(C6F5)IF2][BF4] precipitated and could be easily isolated in good yields (Eq. 4).x-FC6H4IF4+C6F5BF220°C/3hCH2Cl2[x-FC6H4(C6F5)IF2][BF4]

It is important to mention that few percent of [x-FC6H4(C6F5)I][BF4] were often included in the product. The different channels, which explain the reduction of [x-FC6H4(C6F5)IF2][BF4] are discussed in Ref. 7.

The pair of

Discussion

The interaction of hypervalent element–fluoride moieties with fluoroorganyldifluoroboranes is a widely applicable method for F/R substitution. It was well elaborated for aryl-, alkenyl-, and alkynylxenonium8 and diorganyliodonium(III) salts.9, 10 The hypervalent bond in Ar′IF4 compounds is characterized by significant partial negative charges on fluorine (>−0.5) and strong partial positive charges on iodine(V) (<3.0).11 As a consequence of interaction of the Lewis acidic aryldifluoroborane with

Conclusion

The Lewis acid assisted substitution of a hypervalently bonded fluorine atom by an aryl group can be applied to aryliodine(V) tetrafluorides and delivers [Ar′(Ar)IF2][BF4] iodonium(V) salts with a cation of high electrophilicity. The electrophilic nature of the cation can be deduced from cation–anion interactions in the solid state and from 19F NMR data in MeCN for solutions. This type of salts opens a new application for polyvalent iodine compounds, namely to act as a fluoro-oxidizer.

General

All moisture sensitive compounds were handled under an atmosphere of dry argon. Reactions were carried out in vessels constructed from FEP tubes (o.d.=4.1 mm, i.d.=3.5 mm or o.d.=9.0 mm, i.d=8.0 mm, FEP is a co-polymer of (CF2CF2)n and (CF2C(CF3)F)m). Acetonitrile was refluxed and distilled from KMnO4 and repeatedly refluxed and distilled from P4O10. Dichloromethane was treated in sequence with concd H2SO4, Na2CO3(aq), and H2O and finally refluxed and distilled from P4O10. NMR spectra were recorded

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