Anti-Markovnikov rule

Hydrosilylation is also a very useful chemical modification which leads to silane modified polymers with special properties [60-62]. Silane modified polymers have improved adhesion to fillers and better heat resistance. It also acts as a reactive substrate for grafting or moisture catalysed room temperature vulcanisation. Guo and co-workers [61] carried out catalytic hydrosilylation of BR using RhCl(PPh3)3 as the catalyst. Hydrosilylation reactions followed anti-Markovnikov rule as shown in the Scheme 4.4. 

The catalytic addition of substituted silanes to olefins occurs predominantly according to the anti-Markovnikov rule, resulting in alkylsilanes with terminal silyl groups, but under some conditions (e. g., with a catalyst such as Pd, Ni complexes [1,2]) this product is accompanied by an a-adduct, i.e., one containing an internal silyl group. 

Deviation from the anti-Markovnikov rule a computational study of the regio- and stereoselectivity of diene hydroboration reactions... 

Keywords Hydroboration Anti-Markovnikov rule Markovnikov rule 1 IB NMR Lewis acid-base Frustrated Lewis pair... 

The calculated interaction energies show an excellent correlation with the dipole-dipole interactions derived from the parallel B-H and C=C bonds at the TS (Figure S5). As expected based on the anti-Markovnikov rule, the homoallyl attack corresponds to significantly stronger electrostatic interactions due to the more favorable polarization of the double bond in F6 interacting with the dipole of the B-H bonds at TS3 and TS2. 

These results lend themselves to predicting the most likely product of hydroboration reactions, as the molecule with the lowest energy transition state should yield the kinetic product. These results are also in correlation with the anti-Markovnikov regioselectivity of the reaction the electropositive boron within a hydrogen-boron bond of the borane electrostatically favors the most negatively charged sp carbon within the molecule. Intriguingly, 1,3-cyclohex-adiene presents an important exception to the anti-Markovnikov rule, whereas transition state theory continues to predict the selectivity of the reaction. 

Addition opposite to Markovnikov s rule is some times termed anti Markovnikov addition... 

Anti Markovnikov addition (Sections 6 8 6 11) Addition re action for which the regioselectivity is opposite to that pre dieted on the basis of Markovnikov s rule... 

The presence of free radicals can invert this rule, to form anti-Markovnikov products. Free-radical addition in this fashion produces a radical on the central carbon, C-2, which is more stable than the allyl radical. This carbon can then experience further addition. For example, acid-catalyzed addition of... 

Electrophilic addition of hydrogen bromide to alkenes follows Markovnikov s rule, leading to the product with halogen on the more-substituted position. However, trace amounts of hydroperoxides (among other impurities ) may initiate a reaction that gives rise to the anti-Markovnikov product, with bromine in the less-substituted position. 

The addition of hydrogen halides to simple alkenes, in the absence of peroxides, takes place by an electrophilic mechanism, and the orientation is in accord with Markovnikov s rule. " When peroxides are added, the addition of HBr occurs by a free-radical mechanism and the orientation is anti-Markovnikov (p. 985). It must be emphasized that this is true only for HBr. Free-radical addition of HF and HI has never been observed, even in the presence of peroxides, and of HCl only rarely. In the rare cases where free-radieal addition of HCl was noted, the orientation was still Markovnikov, presumably beeause the more stable product was formed. Free-radical addition of HF, HI, and HCl is energetically unfavorable (see the discussions on pp. 900, 910). It has often been found that anti-Markovnikov addition of HBr takes place even when peroxides have not been added. This happens because the substrate alkenes absorb oxygen from the air, forming small amounts of peroxides... 

The regiochemistry is important. For example, does the reaction follow Markovnikov s rule or is the reaction anti-Markovnikov Or is the substituent a meta-director or ortho-para-director You learned rules such as this for a reason make sure you continually apply them. 

Additions to carbon-carbon multiple bonds initiated by electrophiles are generally governed by the Markovnikov rule. However, the rule must be modified to accom-ipodate such substrates as vinylsilanes. The so-called anti-Markovnikov hydro-halogenation [105] is to be contrasted to the Markovnikov addition for allylsilanes. In fact, when one recognizes the acceptor role of the silicon atom and applying the polarity alternation rule, the puzzling results become self-consistent. 

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. 

Acid-catalyzed addition of water and alcohols to 4/f-chromenes gives the expected products as predicted by Markovnikov s rule (56JCS4785) an anti-Markovnikov addition of methanol followed by the reintroduction of a double bond in the alternative position gives an overall effect of substitution of hydrogen by methoxy and this is effected by treating methyl 2if-chromene-3-carboxylate (166) with triphenylmethyl perchlorate and addition of methanol to the resulting benzopyrylium salt (167) (72CR(C)(274)650). 

The addition of thiols to C—C multiple bonds may proceed via an electrophilic pathway involving ionic processes or a free radical chain pathway. The main emphasis in the literature has been on the free radical pathway, and little work exists on electrophilic processes.534-537 The normal mode of addition of the relatively weakly acidic thiols is by the electrophilic pathway in accordance with Markovnikov s rule (equation 299). However, it is established that even the smallest traces of peroxide impurities, oxygen or the presence of light will initiate the free radical mode of addition leading to anti-Markovnikov products. Fortunately, the electrophilic addition of thiols is catalyzed by protic acids, such as sulfuric acid538 and p-toluenesulfonic acid,539 and Lewis acids, such as aluminum chloride,540 boron trifluoride,536 titanium tetrachloride,540 tin(IV) chloride,536 540 zinc chloride536 and sulfur dioxide.541... 

Tho reaction follows Markovnikov s rule because the rule dictates the formation of the more stable carbocation. You should be aware that if peroxides (ROOR) are present the bromine, not the hydrogen, will add to the least substituted carbon. This is called an anti-Markovnikov addition. The other halogens will still follow Markovnikov s rule even in the presence of peroxides. 

Some reactions do not follow Markovnikov s Rule, and anti-Markovnikov products are isolated. This is a feature for example of radical induced additions of HX and of Hydroboration. 

In 1933, M. S. Kharasch and F. W. Mayo found that some additions of HBr (but not HC1 or HI) to alkenes gave products that were opposite to those expected from Markovnikov s rule. These anti-Markovnikov reactions were most likely when the reagents or solvents came from old supplies that had accumulated peroxides from exposure to the air. Peroxides give rise to free radicals that initiate the addition, causing it to occur by a radical mechanism. The oxygen-oxygen bond in peroxides is rather weak, so it can break to give two alkoxy radicals. 

This hydration of an alkene by hydroboration-oxidation is another example of a reaction that does not follow the original statement of Markovnikov s rule (the product is anti-Markovnikov), but still follows our understanding of the reasoning behind Markovnikov s rule. The electrophilic boron atom adds to the less substituted end of the double bond, placing the positive charge (and the hydrogen atom) at the more substituted end. 

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