Halonium ion transfer

This intermolecular halonium ion transfer had indeed been postulated earlier as the key factor to account for the absence of self-coupling product 38, when armed and disarmed NPGs, 12 and 36 respectively, were made to compete for one equivalent of NBS (O Scheme 5a). The observed 6-fold difference in the hydrolysis rates of 36 and 12 should have resulted in the presence of 38, in at least 10% [40]. Irreversible reaction of NPGs 55 and 59 with NBS leads to halonium ions 56 and 60, respectively (O Scheme 8). The transfer of halonium (e. g. 60— 55) is reversible and rapid compared with the subsequent steps leading to glycoside formation... 

Halonium ion transfer a key factor in armed-disarmed couplings... 

In searching for the origin of the regioseiectivity observed in the formation of trisaccharides 206 and 212 (O Scheme 28 and Scheme 29) several factors were considered. The reactions in O Scheme 28 and Scheme 29c were carried out with excess NIS promoter, conditions under which the intermolecular halonium ion transfer (responsible for the armed-disarmed effect) is not operative. A study of the three types of -pentenyl donors indicated that their relative reactivities were in the order NPOE > armed > disarmed (e. g. 202a > 64 > 200) [111]. Therefore, the most and the least reactive donors have chosen their preferred —OH in the final trisac-... 

The halonium ion transfer concept in Figure 2 is readily tested. The crucial steady state operation requires that only one equivalent of T be used. A higher amount of NIS would disturb the distribution of intermediates, and this would disrupt the steady state transfer. This is evident in the tabulated data in Schemes 9b and c. Thus an increase to 3 equivalents of NIS, by eliminating the iodonium transfer between glycosyl donors, permits the (almost) simultaneous formation of oxocarbenium ions arising from 52 and 53, with the result that diioglycoside 53 becomes the principal, donor with 55b as major product (23). 

Scheme 9. Halonium ion transfer between n-pentenyl glycosides and...

A simple way to avoid this waste was in fact implicit in the halonium ion transfer concept in Figure 2. The success of diis principle, as noted above, requires use of limited amounts of 1 and addition of all 3 equivalents in one... 

Halogen Transfer Reactions from bis-Amino Halonium Ions to Acceptor Olefins Mechanism and Strategies for Chiral Halogenation... 

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. 

General mechanism for transfer of X+ from is-amino halonium ions to acceptor olefins. 

An elecrophilic Br+ or I+ can be successfully transferred to hydroquinidine (13) and two of its commercially available derivatives (4-chlorobenzoate and 9-phenanthryl ether hydroquinidines) simply by mixing two equivalents of the hydroquinidine with one equivalent of sym(co d ne)2-X+ perchlorate in methylene chloride or acetonitrile. H NMR studies (31) showed that the iodonium ion was associated with the nitrogen at the quinuclidine portion of the hydroquinidine instead of the aromatic nitrogen and also that all of the sym-collidines were removed from the X+ since only free collidine and no collidine-I+ peaks were observed. The (hydroquinidine)2-halonium ion is stable in solution for more than 30 minutes at room temperature these ions (and their parent amines) are more soluble in methylene chloride than in acetonitrile, and having R group other than hydrogen also improves the solubility. 

There are two other mechanistic possibilities, halogen atom abstraction (HAA) and halonium ion abstraction (EL), represented in Schemes 4.4 and 4.5, respectively, so as to display the stereochemistry of the reaction. Both reactions are expected to be faster than outer-sphere electron transfer, owing to stabilizing interactions in the transition state. They are also anticipated to both exhibit antiperiplanar preference, owing to partial delocalization over the C—C—Br framework of the unpaired electron in the HAA case or the electron pair in the EL case. Both mechanisms are compatible with the fact that the activation entropies are about the same as with outer-sphere electron donors (here, aromatic anion radicals). The bromine atom indeed bears three electron pairs located in two orthogonal 4p orbitals, perpendicular to the C—Br bond and in one s orbital. Bonded interactions in the transition... 

Haloperoxidases act as halide-transfer reagents in the presence of halide ions and hydrogen peroxide. In the first step, the halide ion is oxidized to a halonium-ion carrier, from which the positive halogen species is then transferred to the double bond. In an aqueous medium, the intermediary carbocation is trapped and racemic halohydrins are formed (Eq. 7). Selective examples of CPO-cata-lyzed formation of halohydrins are given in Table 9. In CPO-catalyzed reaction. 

R. Madsen and B. Fraser-Reid, Acetal transfer via halonium-ion induced reactions of di-pent-4-enyl acetals Scope and mechanism. J. Org. Chem. 60 772 (1995). 

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. 

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