Bromonium Bromosuccinimide

In analogy with the peracid attack on steroidal double bonds, the formation of the bromonium ion, e.g., (81a), occurs from the less hindered side (usually the a-side of the steroid nucleus) to give in the case of the olefin (81) the 9a-bromo-l l -ol (82). Base treatment of (82) provides the 9 5,1 l S-oxide (83). Similarly, reaction of 17/3-hydroxyestr-5(10)-en-3-one (9) with A -bromosuccinimide-perchloric acid followed by treatment with sodium hydroxide and sodium borohydride furnishes the 3, 17 5-dihydroxy-5a,l0a-oxirane. As mentioned previously, epoxidation of (9) with MPA gives the 5, 10 -oxirane. °... 

The elements of bromine azide have been added to steroid olefins. The addition can be rationalized as proceeding through a positive bromonium ion under the ionic conditions of Hassner and Boerwinkle (bromine plus sodium azide and hydrochloric acid in nitromethane-dichloromethane) or Ponsold (A-bromosuccinimide or A-bromoacetamide in chloroform contain-... 

When electron-withdrawing groups are attached to the double bond, the reaction is strongly inhibited and may fail completely. In such cases, the bromide anion, produced by the reaction of dimethyl sulfoxide with N-bromosuccinimide, competes with the dimethyl sulfoxide for the bromonium (or bromo carbonium) ion, an intermediate of the reaction. Thus, dibromide may accompany recovered alkene or any bromohydrin formed. Similarly, exogenous anions often compete with dimethyl sulfoxide for the cation. ... 

The regiochemistry of the bromination (Scheme 20) of a,/l-unsaturated ketones101 (in methanol) by bromine is affected by the presence of A-bromosuccinimide, which also depresses the overall rate of bromination. A possible explanation of the observed behaviour is the presence of a complex (50) involving the C=C double bond of olefin, TV-bromosuccinimide and bromine, as shown in Scheme 21. Probably, NBS acts as a hydrogen bromide scavenger, thereby causing a change in the bromination mechanism from an acid-catalysed pathway to a bromonium ion mechanism. 

The efficient azidobromination of cycloalkenes [Eq. (5.342)] and open-chain alkenes to give /3-bromoalkyl azides with iV-bromosuccinimide and azidotrimethylsi-lane is catalyzed by Nafion-H.919 Terminal alkenes and alkenes with bulky substituents do not react, whereas 2,3-trimethylbutene-2 reacts without catalysis. The stereochemistry of the process suggests the involvement of a bromonium ion intermediate. 

The cr,/3-unsaturated amides 396 (where R = aryl or heteroaryl) cyclized in the presence of sodium acetate and iV-bromosuccinimide, presumably through the bromonium ion intermediate 397, to furnish N-unsubstituted... 

Halohydrins are /I-halogenated alcohols. They can be obtained in H20-containing solvents from olefins and reagents, which transfer Hal+ ions. TV-Bromosuccinimide (transfers Br+ Figures 3.33 and 3.34 as well as 3.36), chloramine-T (transfers Cl+ Figure 3.35), and elemental iodine (transfers I+ Figure 3.36) have this ability. Bromonium and chloronium ions are then attacked by H20 according to an SN2 mechanism. This furnishes the protonated bromo- or chlorohydrins, which are subsequently deprotonated. 

The reagent used to form the bromonium ion here is not bromine, and may be new to you. It is called N-bromosuccinimide, or NBS for short. Unlike the noxious brown liquid bromine, NBS is an easily handled crystalline solid, and is perfect for electrophilic addition of bromine to alkenes when the bromonium ion is not intended to be opened by Br-. It works by providing a very small concentration of Br2 in solution a small amount of HBr is enough to get the reaction going, and thereafter every addition reaction produces another molecules of HBr which liberates more Br2 from NBS. In a sense, NBS is a source of Br+>. 

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. 

In a medium of 8-10 M lithium acetate at pH 4, however, the extent of cleavage of the model peptides with both V-bromosuccinimide and V-bro-moacetamide increased markedly (Table VI). This increase in yield is probably due to the greatly decreased activity of water in concentrated salt solutions (cf. Robinson and Stokes, 1955). Under such conditions opening of the bromonium intermediate by water competes poorly with intramolecular participation of the imidole group. 

Unlike base-mediated cyclizations, halocyclizations do not depend on the presence of Michael acceptors. As shown in Scheme 7.94, Armstrong and Teegarden [234] treated a hydroxyolefin (derived from a Wittig reaction applied to a protected arabinose) with A-bromosuccinimide and catalytic bromine to activate the illustrated cyclization. The reaction took place through an intermediate bromonium ion, providing a 52% yield of the (3 anomer. This observation is consistent with the preference for a anomers observed in base-mediated cyclizations. Moreover, the isolation of a bromo-substituted C-glycoside allows the preparation of new and novel structures. This chemistry was further adapted to the use of iodine instead of bromine [235]. 

When treating dimethyl-1,5-nonadicne with A -bromosuccinimide the bromonium ion attacks the nonhalogenated. electron-rich double bond. After Markovnikov ring closure the resulting cation is stabilized by a [1,2] suprafacial hydrogen shift, and dibromo-m-hydrindanone 1 is formed stereoselectively32. 

The introduction of a methoxy or ethoxy group to the 6-position of 2-(dimethylamino)pteridin-4(3//)-one (5) can be carried out using a two-step process requiring iV-bromosuccinimide and the appropriate alcohol. The pathway involves the addition of bromonium ion across the 5,6-bond, then addition of the alcohol and the elimination of hydrogen bromide. The 6-bromo derivative is not an intermediate in the reaction since it is stable to the alcohol under reflux. Some of the 6-bromo derivative is also formed in the reaction but it can be separated chromato-graphically.253... 

Bromonium ion. See Halonium ion (R)- and (5)-2-Bromooctane, stereochemistry of hydrolysis of, 307—308, 319 A-Bromosuccinimide, reagent for allylic bromination, 371, 391 benzylic bromination, 415—416, 435 Brpnsted, Johannes, 134 Brpnsted acid. See Acidity Brpnsted base. See Basicity Brown, Herbert C., 228 Buckminsterfullerene, 410—411 1,3-Butadiene... 

Alternatively, aciyUc moieties can be introduced directly to the imsaturated fatty oil groups through a bromonium cyclic intermediate (72). N-Bromosuccinimide is used for the bromination. The bromine moiety is removed from the nitrogen by an heterolytic fission as a bromonium ion and a succinimide anion is formed. The bromonium ion is attacked by the 7i-bonds of the alkene. At the... 

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