Markovnikov addition to alkenes

Blit amine complex.2 Reaction of BH3 C6H5N(C2H5)2 with 1.5 equiv. of iodine provides an amine complex of BI3. This complex when combined with acetic acid liberates hydroiodic acid, which undergoes Markovnikov addition to alkenes and alkynes in good yield. 

Summary This is an example of Markovnikov addition to alkenes ... 

Aikene chemistry is dominated by electrophilic addition reactions. When HX reacts with an unsymmetrically substituted aikene, Markovnikov s rule predicts that the H will add to the carbon having fewer alky) substituents and the X group will add to the carbon having more alkyl substituents. Electrophilic additions to alkenes take place through carbocation intermediates formed by reaction of the nucleophilic aikene tt bond with electrophilic H+. Carbocation stability follows the order... 

Se-phenyl areneselenosulfonates (24) undergo facile free-radical addition to alkenes to produce / -phenylseleno sulfones (25) in excellent yield86,87 (see Scheme 7). The addition occurs regiospecifically and affords anti-Markovnikov products contrary to the analogous boron trifluoride catalyzed reaction which produces exclusively Markovnikov and highly stereospecific products86 (equation 37). Reaction 36 has been shown to have the radical... 

As we saw in the previous section, Markovnikov s rule tells us to place the H on the less substituted carbon, and to place the X on the more substituted carbon. The rule is named after Vladimir Markovnikov, a Russian chemist, who first showed the regiochemical preference of HBr additions to alkenes. When Markovnikov recognized this pattern in the late 19th century, he stated the rule in terms of the placement of the proton (specifically, that the proton will end up on the less substituted carbon atom). Now that we understand the reason for the regiochemical preference (carbocation stability), we can state Markovnikov s rule in a way that more accurately reflects the underlying principle The regiochemistry will be determined by the preference for the reaction to proceed via the more stable carbocation intermediate. 

Redical Addition to Alkenes the Anti-Markovnikov Addition of Hydrogen Bromide... 

Bromine monofluoride or iodine monofluoride can be prepared from the corresponding elements in trichlorofluoromethane at — 781 and — 45 C,2,3 respectively. The compounds are unstable and decompose at low temperatures, iodine monofluoridc at — 14 C2,3 and bromine monofluoride, in part, even at — 78 C.1 Therefore, they are used, without separation and purification, in Freon solutions at low temperatures or, most frequently, as stoichiometric mixtures bromine trifluoride/bromine and iodine pentafluoride/iodine. A solution of iodine monofluoride, obtained by bubbling nitrogen-diluted fluorine gas into a suspension of iodine in trichlorofluoromethane at — 75 C, was used for addition to alkene C = C bonds. The reaction is regioselective and in most cases obeys the Markovnikov rule.4 Iodine monofiuoride prepared in situ reacts quite efficiently with steroidal alkenes.4... 

Markovnikov s rule is obeyed because the mechanism of sulfuric acid addition to alkenes, illustrated for the case of propene in Figure 6.8, is analogous to that described earlier for the electrophilic addition of hydrogen halides. 

Although HI addition to alkenes and alkynes is faster than that of the other hydrohalides and free radical anti-Maikovnikov additions are not a problem, this reaction has received less attention than the others.173 The hydroiodination of alkenes is most commonly run using concentrated HI in water or acetic acid at or below room temperature. While the early literature suggests that simple terminal alkenes afford small amounts of anti-Markovnikov products, only Markovnikov products have been reported in the more recent literature (equations 125-129).67 176-179... 

Radical addition to alkenes is strongly influenced by polar effects, and reductive radical additions are nearly always anti-Markovnikov. Resonance stabilisation of the unpaired electron in R can decrease the rate of addition, while the stabilisation of the adduct radical... 

In 1869 Vladimir Markovnikov studied the regiochemistry of a large number of these addition reactions. On the basis of his observations, he postulated an empirical rule that can be used to predict the orientation of additions to alkenes ... 

Q Use the extended version of Markovnikov s rule to predict the regiochemistry (orien- Problems 8-46, 47, and 50 tation) of electrophilic additions to alkenes. 

C. Bruneau, and P. H. Dixneuf, Metal Yinylidenes and Allenylidenes in Catalysis Applications in Anti-Markovnikov Additions to Terminal Alkynes and Alkene Metathesis, Angew. Chem. Int. Ed. 45, 2176-2203 (2006). 

Radical chloroaminations are known, using radical, transition metal ion or photochemical initiation. They also occur without overt initiation, thus anti-Markovnikov additions to terminal alkenes occur with N,N-dichlorourethane in benzene at 5-40 C (yields <= 60%)P Similar reactions occur with N,N-dichlo-roarenesulfonamides in CH2CI2 at or below room temperature (yields mostly 53-91% 10% with isobu-tylene). ° The remaining N—Cl bond is reaiUly reduced if desired with sodium sulflte. N-Halosulfoximines also add to alkenes thermally or photolytically. ... 

Diethyl N,A -dibromophosphoramidate undergoes ready Markovnikov addition to styrene or cyclohexene (92%) in ecu at -20 to -t-20 C with BF3-Et20 catalysis.Alternatively, such reactions can be carri out in refluxing CH2CI2 in the absence of catalyst, when anti-Markovnikov products (47-97%) are formed. All these compounds are versatile intermediates (Scheme 83). A few N-bromoperfluo-roamines have been added to alkenes, thermally or photolytically. ... 

Radical addition to alkenes is usually difficult, except when addition occurs to conjugated carbonyl compounds (15-24). An important exception involves radicals bearing a heteroatom a to the carbon bearing the radical center. These radical are much more stable and can add to alkenes, usually with anti-Markovnikov orienta-... 

The mechanism begins the same way as the Hg-mediated nucleophilic addition to alkenes. In the first step, an electrophilic 7r complex forms between the alkyne and Hg(II). Water attacks one of the C atoms of the ir complex in Markovnikov fashion to give a 2-hydroxy- l-alkenylmercuryfH) compound, an enol, which is pro-tonated to give a carbocation. Fragmentative loss of Hg(II) then occurs to give a metal-free enol, which tautomerizes to give the ketone product. 

Regio-selective addition to alkenes The Markovnikov s rule... 

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