Anti additions, of halogens

It is not normally possible to add fluorine directly to alkenes as the reaction is so exothermic that bond fission occurs. Many alkenes will not add iodine directly either, and when the reaction does occur it is usually readily reversible. Alkynes are also found to undergo preferential, though not exclusive, ANTI addition of halogens, e.g. with butyne-l,2-dioic acid (17) ... 

Halogen addition is another example of a stereospecific reaction, in which different stereoisomers of the starting material give different stereoisomers of the product. Figure 8-4 shows additional examples of the anti addition of halogens to alkenes. 

Examples of the anti addition of halogens to alkenes. The stereospecific anti addition gives predictable stereoisomers of the products. 

Similarly to allylic silanes, alkenylsilanes react with a variety of electrophiles. The anti addition of halogens to alkenylsilanes followed by the anti elimination of halosilanes yields alkenyl halides with inversion of configuration (eq (15)) [12]. 

Addition of halogens (Sections 6 14-6 16) Bromine and chlorine add to alkenes to form vicinal dihalides A cy clic halonium ion is an intermediate Stereospecific anti addition is observed... 

Generally, additions of halogens to fluoroalkenes are less stereoselective than the analogous reactions with nonfluorinated systems. The stereochemical mode of addition can be either anti or syn Partitioning between these paths is determined... 

The addition of halogen fluorides to C = C bonds in steroids is affected by steric crowding to a greater extent than by electronic factors so that the products may be of the anti-Mar-kovnikov type. In several instances the orientation of the bromine monofluoride addition was shown to depend on reaction conditions resulting cither in 6/ -bromo-5a-fluoro- or 5a-bromo-6/i-fluorosteroids.33... 

Addition of halogens to fluoroolefins is a common reaction which usually proceeds by a radical mechanism. However, some halogenations are catalyzed by Lewis acids. This reaction has been reviewed [6,14,77]. Among recently reported reactions are ionic addition of Br2 to RC6H4CF=CFX that was found not to be stereoselective [78], and the ratio of syn- and anti-addition products was thought to be determined by a combination of steric and electronic factors. 

Some examples of these addition reactions are provided in the following equations. Note that each proceeds with anti addition of the two halogen atoms. In the last example, starting with an alkyne, the two bromines end up trans. 

Two facts demonstrate that halogenation follows a different mechanism from that of hydroha-logenation or hydration. First, no rearrangements occur, and second, only anti addition of X2 is observed. For example, treatment of cyclohexene with Br2 yields two trans enantiomers formed by anti addition. 

The addition of halogens and hydroxyls across double bonds leads to halohydrins, which are useful intermediates, especially for the synthesis of epoxides. Such additions are achieved by treatment of alkenes with N-bromoacetamide [1104] or iV-brontosuccinintide [746] in aqueous media and give products of anti addition. On heating with alkalies, bromohydrins... 

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. 

In summary, the stereochemistry of the halogenation of alkenes is not easily predicted. Addition of fluorine is mostly syn, while anti addition prevails with all other halogens and interhalogens. In most cases interhalogens show Markovnikov-type regioselectivity in their reactions with unsymmetrical olefins but enantioselective methods for the addition of halogens are still rare. 

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