Halonium ions acidic

Addition of chlorine or bromine in the presence of water can yield compounds containing haUde and hydroxyl on adjacent carbon atoms (haloalcohols or halohydrins). The same products can be obtained in the presence of methanol (13) or acetic acid (14). As expected from the halonium ion intermediate, the addition is anti. As expected from Markovnikov s rule, the positive halogen goes to the same carbon that the hydrogen of a protic reagent would. 

Studies of the transfer of Br+ and I+ from amine-coordinated halonium ions to acceptor l-co-alkenols have been undertaken to determine the mechanism in an effort to assist in the development of chiral transfer reagents. Transfer of Br+ and I+ from two commercially available dimeric hydroquinine and hydroquinidine ligands ((DHQ)2PHAL and (DHQD)2PHAL) to various 1, (o-alkenols and l,co-alkenoic acids is shown to provide enantiomeric excesses of 4-47% depending on the acceptor alkene. 

Table 3. Enantiomeric excesses obtained from halocyclizations of various l,o-alkenols or l,co-alkenoic acids with halonium ions of dimeric hydroquinidine or dimeric hydroquinine species 14,15,16. ... 

Epoxides, like cyclic halonium ions, undergo ring opening through rearside attack of nucleophiles (see Section 6.3.2). Two mechanisms are shown, for both basic and acidic conditions. Under acidic conditions, protonation of the epoxide oxygen occurs first. The epoxidation-nucleophilic attack sequence also adds substituents to the double bond in an anti sense. 

Chlorine, bromine, and iodine act as cationogens in the presence of the more active Lewis acids such as trialkylalumi nu tn or dialkylaluminum halide [DiMaina et al., 1977 Magagnini et al., 1977]. The initiating species is the halonium ion X+ present in low concentration via the equilibrium reaction between Lewis acid and halogen. 

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). 

Treatment of either cis- or rrans-stilbene-2-carboxylic acids with chlorine or bromine leads to 4-halogeno-3,4-dihydro-3-phenylisocoumarins (58T<4)393). The reactions are stereospecific and are thought to involve intramolecular attack by the carboxyl group on a halonium ion. Ring closure to the corresponding 4-hydroxy compound also occurs stereo-specifically using peroxyphthalic acid (59JOC934). 

Therefore, it must be concluded that earlier attempts to prove the existence of stable, well-defined alkyl cations were unsuccessful in experiments using sulfuric acid solutions and inconclusive in the interaction of alkyl halides with Lewis acid halides. Proton elimination reactions or dialkyl halonium ion formation may have affected the early conductivity studies. 

The halogen atom in organic halonium ions is generally bound to two carbon atoms, although in the case of acidic halonium ions—that is, protonated alkyl halides—one ligand is hydrogen. 

Halonium ions are an important class of onium ions.43 The dialkylchloro, bromo, and iodohalonium ions can be prepared and even isolated as stable salts (i.e., 46), as shown by Olah et al. by reacting an excess of haloalkane with strong Lewis acid halides in solvents of low nucleophilic-ity (eq 14). In superacid solution, dialkylhalonium ions show enhanced alkylating reactivity.44 It is considered that this enhanced reactivity is due to further protolytic (or electrophilic) activation involving the non-bonded... 

Oxyselenides of terpenes have been electrochemically synthesized in high yields and with high regioselectivity in an MeOH (or AcOH, HjO/MeChO-f NX (X = Cl, Br, I)—(PhSe)2—(Pt) system in the presence of a small amount of sulfuric acid (Scheme 3-17). The oxyselenation proceeds through regenerating a halonium ion... 

The study of gas-phase acid-induced nucleophilic displacement on 2,3-dihalobutanes has provided stereochemical evidence for the occurrence of cyclic chloronium and bromo-nium ions (X = Cl, Br), but not fluoronium ions17. Protonation or methylation of the neutral 2,3-dihalobutane by a suitable acid GA+ produces a halonium intermediate 2, which in the presence of water ultimately leads to the corresponding halohydrin neutral product (Scheme 4). Analysis of these neutral products indicated that the reaction proceeds with retention of configuration when X = Cl, Br and with inversion of configuration when X = F. The results were rationalized by the mechanisms sketched in Scheme 4, namely direct bimolecular nucleophilic displacement by H20 on 2 when X= F and intramolecular nucleophilic displacement to convert 2 into the cyclic halonium ion 3 (with inversion of configuation) followed by bimolecular nucleophilic displacement on 3 (with inversion of configuration) when X = Cl and Br. 

The gas-phase base-induced elimination reaction of halonium ions was thoroughly investigated in radiolytic experiments22. Radiolytically generated acids C/JH5+ (n = 1,2) were allowed to react at 760 Torr with selected 2,3-dihalobutanes to form the halonium intermediates which, in the presence of trimethylamine, undergo base-induced bimolecu-lar elimination as shown in Scheme 6. This elimination reaction occurs in competition with unimolecular nucleophilic displacement to the cyclic halonium ion and subsequent rearrangement. Isolation and identification of the neutral haloalkenes formed and kinetic treatment of the experimental results indicated that 3-halo-1 -butene is formed preferentially with respect to the isomeric 2-halo-2-butenes and that the bimolecular elimination process occurs predominantly via a transition state with an anti configuration22. 

Transfer of a halonium ion to l,o)-alkenols or l,co-alkenoic acids which are capable of undergoing halocyclization is a key method to generate 4-, 5- and 6-membered heterocyclic rings via the so-called halocyclization processes 1,2,3,4,5,6,7). This synthetic method would be far more valuable if the halogen transfer to an achiral alkene could be conducted in a chiral fashion (exemplified in Figure 1) as it would produce optically active heterocycles that could be further functionalized through manipulation of the halomethyl group. 

The reaction with A -bromosuccinimide or Af-bromoacetamide with or without added acid, on the other hand, leads predominantly to the same epoxide trans-6 as formed in the reaction with peracids. It is proposed that in the formation of cis-6 with acetyl hypobromite, hypobromous acid or A -chlorosuccinimide, nucleophilic attack of the halonium ion is fast. The epoxide ratio therefore is governed by the kinetically favored tran.v-halohydrin. However, in the reaction with A -bromosuccinimide or A -bromoacetamide the nucleophilic attack on the bromonium ion species becomes rate determining, attack of the nucleophile therefore follows the path of lowest activation energy. This is the attack on the c/.v-bromonium ion distant to the bulky tert-butyl group leading to the trans-diaxial bromohydrin, according to the Fiirst-Plattner rule, hence giving trans-6. 

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