Intermediates three-membered ring halogenation

Halonium ion (Section 7.2) A species containing a positively charged, divalent halogen. Three-membered-ring bromonium ions are implicated as intermediates in the electrophilic addition of Br2 to alkenes. 

The two types of stabilization are not eguivalent the cation and the bromonium ion are different molecules with different shapes, while the two representations of the oxonium ion are just that—they aren t different molecules. This stabilization of an adjacent cationic centre by a heteroatom with at least one lone pair to form a three-membered ring intermediate is not restricted to bromine or the other halogens, but is also an important aspect of the chemistry of compounds containing oxygen, sulfur, or selenium, as you will see in Chapter 27. 

HC1, HBr, and HI add to alkenes by a two-step electrophilic addition mechanism. Initial reaction of the nucleophilic double bond with H+ gives a carbo-cation intermediate, which then reacts with halide ion. Bromine and chlorine add to alkenes via three-membered-ring bromonium ion or chloronium ion intermediates to give addition products having anti stereochemistry. If water is present during the halogen addition reaction, a halohydrin is formed. 

A reactive, cationic intermediate with a three-membered ring containing a halogen atom usually, a chloronium ion, a bromonium ion, or an iodonium ion. (p. 349)... 

Bridged halonium ions resemble carbocations in that they are short-lived intermediates that react readily with nucleophiles. Carbocations are inherently unstable because only six electrons surround carbon, whereas halonium ions are unstable because they contain a strained three-membered ring with a positively charged halogen atom. 

The mercurinium ion is attacked by the nucleophilic solvent—water, in the present case— to yield the addition product. This attack is back-side (unless prevented by some structural feature) and the net result is anti addition, as in the addition of halogens (Sec. 7.12). Attack is thus of the Sn2 type yet the orientation of addition shows that the nucleophile becomes attached to the more highly substituted carbon— as though there were a free carbonium ion intermediate. As we shall see (Sec. 17.15), the transition state in reactions of such unstable three-membered rings has much SnI character. 

Based on the previous analysis, 3-methyl-l-hexene reacts with mercuric acetate to give secondary carbocation intermediate 72. Is this carbocation unusual relative to previously discussed carbocations, such as 30 in Section 10.3 Note that mercury is a transition metal with d-orbitals that can donate electron density to the carbocation, stabilizing that positive center via what is known as back-donation (similar to what is observed with the halogens in the formation of halonium ions). Despite the manner in which is it drawn, this carbocation is not a three-membered ring, but rather a secondary carbocation that is stabilized by back-donation from the mercury atom. The dashed line (--) indicates significant coordination between the carbon and mercury. 

So, a modification of our earlier addition mechanism suffices to explain the reaction. In contrast to the addition to aikenes we saw in Chapter 9 (HX, carbocations, and boranes), in alkene halogenation (bromination and chlorination) a three-membered ring is produced as an intermediate. In some, very spedalized cases the bromonium ion can even be isolated, and it seems its intermediacy in most brominations is assured. 

The absence of rearrangement and the anti stereochemistry of the addition product must be accommodated in a proposed mechanism for the addition of a halogen to an alkene involving a carbocation intermediate. The first step of the reaction mechanism is the electrophilic addition of bromine to the Jt bond to give a three-membered ring called a tydic bromonium ion. 

You have just seen that cyclic halonmm ion intermediates are formed when sources of electrophilic halogen attack a double bond Likewise three membered oxygen containing rings are formed by the reaction of alkenes with sources of electrophilic oxygen... 

A New Improved Synthesis of Tricycle Thienobenzazepines Apphcation of chemistry recently developed by Knochel" combined with the well-described halogen dance (HD) reaction, allowed preparation of our key intermediate A in only three synthetic transformations (Scheme 6.4). In this respect, treatment of 2-bromo-5-methylthiophene with hthium diisopropylamide followed by dimethylformamide afforded aldehyde 11 in good yield, lodo-magnesium exchange with conunercial 4-iodo-3-nitro anisole followed by reaction with 11 afforded the thiophene catbinol 12. Dehydroxylation of 12 provided our key intermediate A which presented the requisite functionality to examine our approach to the construction of the seven-member ring system. 

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