Iodonium ions, substitution with

Addition is initiated by the positively polarised end (the less electronegative halogen atom) of the unsymmetrical molecule, and a cyclic halonium ion intermediate probably results. Addition of I—Cl is particularly stereoselective (ANTI) because of the ease of formation (and relative stability compared with carbocations) of cyclic iodonium ions. With an unsymmetrical alkene, e.g. 2-methylpropene (32), the more heavily alkyl-substituted carbon will be the more carbocationic (i.e. the less bonded to Br in 33), and will therefore be attacked preferentially by the residual nucleophile, Cle. The overall orientation of addition will thus be Markownikov to yield (34) ... 

Under more basic conditions, a-elimination predominates and insertion of the carbene 40 to the solvent gives racemic 22. Non-basic and poorly nucleophilic conditions allow neighboring group participation to form the rearranged substitution product 23 with complete chirality transfer. The participation can be considered as an intramolecular nucleophilic substitution, and does occur only when it is preferable to the external reactions. Under slightly basic conditions with bases in HFIP, participation is allowed, and the weak base can react with the more electrophilic vinylic cation 21 (but not with the iodonium ion 19). A suitably controlled basicity can result in the formation of cycloalkyne 39, which is symmetrical and leads to racemization. These reactivities are illustrated in Scheme 6. 

Reactions of (ii)-l-decenyl(phenyl)iodonium salt (6a) with halide ions have been examined under various conditions. The products are those of substitution and elimination, usually (Z)-l-halodec-l-ene (6b) and dec-l-yne (6c), as well as iodobenzene (6d), but F gives exclusively elimination. In kinetic studies of secondary kinetic isotope effects, leaving-group substituent effects, and pressure effects on the rate, the results are compatible with the in-plane vinylic mechanism for substitution with inversion. The reactions of four ( )-jS-alkylvinyl(phenyl)iodonium salts with CP in MeCN and other solvents at 25 °C have been examined. Substitution with inversion is usually in competition with elimination to form the alk-l-yne. 

In order to strengthen evidence in favour of the proposition that concerted inplane 5n2 displacement reactions can occur at vinylic carbon the kinetics of reactions of some /3-alkyl-substituted vinyliodonium salts (17) with chloride ion have been studied. Substitution and elimination reactions with formation of (21) and (22), respectively, compete following initial formation of a chloro-A, -iodane reaction intermediate (18). Both (17) and (18) undergo bimolecular substitution by chloride ion while (18) also undergoes a unimolecular (intramolecular) jS-elimination of iodoben-zene and HCl. The [21]/[22] ratios for reactions of (18a-b) increase with halide ion concentration, and there is no evidence for formation of the -isomer of (Z)-alkene (21) iodonium ion (17d) forms only the products of elimination, (22d) and (23). 

Woodward hydroxylation of 256 leads mainly to sterically disfavored products having cis-oriented substituents at C-2 and C-3, and C-2 and C-4. In fact, in this case also, the attack of I4 occurs from the less-hindered side. The iodonium ion (284) is then opened by the acetate anion, to afford iodo acetate 285 which, in the following substitution reaction with silver acetate, gives the more-hindered, c/.y-hydroxylation product 286. 

The reactions of ( )-styryl(phenyl)iodonium salt 37 with halide ions give results similar to those obtained with 50 (Scheme 49, Table 14)." In this case the main product is that of elimination. In aprotic solvents the substitution products (79) are predominantly inverted but in TFE a significant amount of retained 79 was also... 

The examples given in the table are based on chemical oxidation. By contrast, anodic oxidation of iodine in trimethyl orthoformate provides the iodonium ion which is effective for the iodination of appropriate substituted benzenoid compounds (ref.37) and has been considered superior to the iodonium ion developed in acetonitrile solution. Thus the additbn of the anolyte (4 moles) from trimethyl orthoformate (TMOF), elemental iodine and lithium perchlorate trihydrate, by anodic oxidation at ambient temperature in a divided electrolytic cell equipped with a ceramic diaphragm and platinum electrodes, to anisole (1 mole) in TMOF afforded iodoanisoles in 69% yield, in the proportions, 4-iodomethoxybenzene (87%), and the 2-iodo isomer (13%) with none of the 3-isomer. 

The cyclization reaction is a typical two-stage electrophilic addition to an alkene (Chapter 19) with attack by the nucleophile at the more substituted end of the intermediate iodonium ion. The ring opening is a stereospecific 5 2 and the stereochemistry of the alkene will be reproduced in the product. 

Reactions of methyl(vinyl)iodonium ion and its jS-substituted derivatives with chloride ion have been treated by ab initio molecular-orbital calculations (MP2, double-zeta -I- d level). Transition states for 5n2, ligand-coupling substitution (LC), and -elimination ifE) were found. In the gas phase, the barrier to LC is usually the lowest, but the relative barriers for S 2 and change with the substituents. Solvent effects were treated in terms of a dielectric continuum model and found to be large on 5 n2 but small on LC. 

Iodine is only slightly soluble in water and no hydrates form upon dissolution. The solubiHty increases with temperature, as shown in Table 2 (36). Iodine is soluble in aqueous iodide solutions owing to the formation of polyiodide ions. For example, an equiHbrium solution of soHd iodine and KI H2O at 25°C is highly concentrated and contains 67.8% iodine, 25.6% potassium iodide, and 6.6% water. However, if large cations such as cesium, substituted ammonium, and iodonium are present, the increased solubiHty may be limited, owing to precipitation of sparingly soluble polyiodides. Iodine is also more... 

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... 

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. 

Cyclic aliphatic halonium ions (I, Br, Cl) have been observed directly in superacid solution by NMR spectroscopy (B-75MI11900). Cyclic halonium ions with ring size three, five and six are formed from open chain dihalides by reaction with strong Lewis acids such as SbFs. Although numerous iodonium, bromonium and chloronium ions are known, no fluoronium ion has been directly observed. NMR spectra of a solution of 2,3-difluoro-2,3-dimethyl-butane (12) in SbF5-S02 at — 90 °C provide evidence for a rapid interconversion of the two open-chain, substituted /3-fluoroethyl cations (67JA4744). The open-chain cation is about 48.2 kJ mol-1 more stable than the closed fluoronium ion (74JA2665). 

Anions of secondary-sulfonamides, especially N-substituted tosylamidate ions, have emerged as premier partners for C-N bond forming reactions with alkynyliodonium salts. To a much lesser extent secondary-carboxamidate ions have also been used for this purpose. For example, the sequential treatment of -substituted tosylamides with n-butyllithium and phenyl(trimethylsi-lylethynyl)iodonium triflate (26) affords the corresponding N-trimethylsi-lylethynyl-p-toluenesulfonamides, which can be desilylated with tetrabutylam-monium fluoride in wet THF (Scheme 51) [ 151 ]. It is noteworthy that the presence of such groups as n-Bu and CH2 = CH(CH2)2- in the tosylamidate ions did... 

The substitution of alkenyl iodonium salts by halides, using tetrabutylammonium salts, has been studied (Table 9.2). Exclusive inversion of configuration occurred in acetonitrile, so that -precursors gave solely Z-haloalkenes in high yield [35]. In marked contrast, complete retention occurred with cuprous and potassium halides in dichloromethane. Retention of configuration was also noted in reactions of / -substituted alkenyl iodonium salts for example, from /J-phenylsulphonyl decenyl phenyliodonium ion, cis products were formed exclusively in high yield [34],... 

When (Z)-(j5-fluoro-j5-trifluoromethylvinyl)phenyliodonium triflate is mixed with sodium phenoxide or sodium 2-phenylethoxide in dichloromethane, substitution is directed to the jft-carbon atom of the vinyl ligand, while the iodonium function remains intact (equation 200)136. With the 4-ter/-butylbenzenethiolate ion, however, substitution occurs at both vinyl carbon atoms (equation 201)136. 

Azides are formed by the reaction of lithio derivatives with />-toliicncsulfonyl azide, and these in turn can be converted into the corresponding amino compounds by a variety of reductive procedures. Nitro compounds are available by a novel reversal of the general pattern of reaction with electrophiles. This approach requires the initial conversion of the lithio compound into an iodonium salt followed by reaction with nitrite ion. This is illustrated by the preparation of 3-nitrothiophene (Scheme 145). Other nucleophiles, such as thiocyanate ion which yields the 3-thiocyanate, can be employed. The preparative significance of these reactions is again that products not accessible by electrophilic substitution can be obtained. 

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