Markovnikov additions exception

Exceptions to the Markovnikov rule when hydrogen bromide reacts with unsym-metric alkenes have long been known.117,118 The reaction for this anti-Markovnikov addition was explained as being a chain reaction with the involvement of bromine atoms influenced by the presence of peroxides.119-121 Both added peroxides and peroxides formed by the action of oxygen (air) on the alkene are effective. 

The rate of acid-catalyzed hydration of styrenes can be enhanced photochemically (Scheme 2).293,294 Except for nitro-substituted styrenes, such photohydrations undergo exclusive Markovnikov addition. 

Organopalladium complexes appear to undergo addition-elimination reactions in the same way as the inorganic complexes except that anti-Markovnikov addition is preferred. The reaction, of course, is limited to organopalladium compounds which lack hydrogens in positions to the palladium atom. If the j8 hydrogens are present, the organopalladium compounds decompose by hydridopalladium elimination more rapidly than they add to olefins and useful reactions are not obtained. 

In most cases the addition is anti-Markovnikov. An exception is the reaction of 35, R = H with in which 6.5% of the other isomer was also obtained... 

For electrophilic attack, Markovnikov addition is that in which the positive portion of the reagent adds to the least substituted carbon atom of the double bond undergoing reaction.) This may result from a steric preference for the least-substituted metal alkyl intermediate formed by insertion of olefin into the metal hydride bond . Vinylarenes comprise an exception, where interaction of nickel with the aromatic ring stabilizes the precursor of the branched nitrile, leading primarily to a Markovnikov addition product . 

Reaction of hydrogen sulphite ions with alkenes, in the presence of either oxygen or peroxides170-177, produces a reasonable yield of the sulphonic acid salt (equation 24), formed by anti-Markovnikov addition. Alkynes also undergo a similar reaction178, except in this case a disulphonate salt is formed (equation 25). 

It should be noted that uncoordinated allylphosphine does not cyclise when treated with initiator, This gives evidence for the kinetic coordination template effect operating in this process. In conditions of kinetic control (enforced by spatial effects resulting from coordination to the metal), macrocyclic product formation by addition of P-H to the C=C double bond of a neighbouring coordinated allylphosphine proceeds contrary to Markovnikov s rule (phosphorus as a more electronegative element bonded to a less substituted carbon atom). This template process is notable for its exceptional regiospecific character in that the formation of Markovnikov addition products is not detected. 

Exceptions with an anti-Markovnikov addition are observed in the hydrocarboxylation of octyne [427], nonyne-2 [345], phenylacetylene, 1-phenylpropyne-l [345] and propargyl alcohol [428]. 

Additions to cyclopropanes can take place by any of the four mechanisms already discussed in this chapter, but the most important type involves electrophilic attack. For substituted cyclopropanes, these reactions usually follow Markovnikov s rule, though exceptions are known and the degree of regioselectivity is often small. The application of Markovnikov s rule to these substrates can be illustrated by the reaction of 1,1,2-trimethylcyclopropane with The rule predicts that the... 

The uncatalyzed hydroboration-oxidation of an alkene usually affords the //-Markovnikov product while the catalyzed version can be induced to produce either Markovnikov or /z/z-Markovnikov products. The regioselectivity obtained with a catalyst has been shown to depend on the ligands attached to the metal and also on the steric and electronic properties of the reacting alkene.69 In the case of monosubstituted alkenes (except for vinylarenes), the anti-Markovnikov alcohol is obtained as the major product in either the presence or absence of a metal catalyst. However, the difference is that the metal-catalyzed reaction with catecholborane proceeds to completion within minutes at room temperature, while extended heating at 90 °C is required for the uncatalyzed transformation.60 It should be noted that there is a reversal of regioselectivity from Markovnikov B-H addition in unfunctionalized terminal olefins to the anti-Markovnikov manner in monosubstituted perfluoroalkenes, both in the achiral and chiral versions.70,71... 

Markovnikov s rule was empirical that is, it was based on observation only. At the time it was proposed, the concept of organic reaction mechanisms had not yet been imagined so it was impossible to provide a theoretical basis for why the H added to one carbon and the X to the other. Now that we know the mechanism for the reaction, it is easy to understand why these reactions are regiospecific. In fact, if we write the mechanism for the additions to give both possible products, we can predict the preferred product simply on the basis of what we already know about carbocations. It is much better to make predictions based on the mechanism of the reaction because cases that are exceptions to the empirical rule will be readily apparent. 

Hydroboration-Oxidation In Section 8-7 we saw that hydroboration-oxidation adds water across the double bonds of alkenes with anti-Markovnikov orientation. A similar reaction takes place with alkynes, except that a hindered dialkylborane must be used to prevent addition of two molecules of borane across the triple bond. Di(second-ary isoamyl)borane, called disiamylborane, adds to the triple bond only once to give a vinylborane. (Amyl is an older common name for pentyl.) In a terminal alkyne, the boron atom bonds to the terminal carbon atom. 

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-... 

Opening of the three-center bond affords the carbenium ion of greatest stability according to the well-known Markovnikov rule. It should be pointed out that except in extremely crowded cases such as adamantylideneadaman-tane, there is no direct evidence that the three-center n-bonded complex is an intermediate, and thus it is only a depiction of the transition state or high-lying intermediate of the addition reaction. 

Haloboranes add to terminal alkynes in a cis /in-Markovnikov manner however, the bromoboration of acetylene itself exceptionally provides a trans adduct [28] (eq (17)). A sequence of haloboration with boron tribromide and then cross-coupling with organozincs is useful for a formal carboboration of alkynes with various organic groups. Addition of boron triboromide to allene is convenient for the synthesis of (2-bromo-2-propeny])borate [29] (eq (18)). 

Alcohols can be prepared by the acid catalysed addition of water to alkenes. Concentrated H2S04 is used as catalyst. The addition follows Markovnikov s rule. Secondary and tertiary alcohols can be produced by this method, but primary alcohols cannot (except for ethanol). 

In Section 10.9 we shall study an exception to Markovnikov s rule. This exception concerns the addition of HBr to alkenes when the addition is carried out in the presence of peroxides (i.e., compounds with the general formula POOR). 

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