Anti-Markovnikov addition of hydrogen

A typical example of a nonpolymeric chain-propagating radical reaction is the anti-Markovnikov addition of hydrogen sulfide to a terminal olefin. The mechanism involves alternating abstraction and addition reactions in the propagating steps ... 

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

Reaction 1 has been postulated both in oxidations of alkanes in the vapor phase (29) and in the anti-Markovnikov addition of hydrogen bromide to olefins in the liquid phase (14). Reaction 2 involves the established mechanism for free-radical bromination of aromatic side chains (2). Reaction 4 as part of the propagation step, established in earlier work without bromine radicals (26), was not invoked by Ravens, because of the absence of [RCH3] in the rate equation. Equations 4 to 6, in which Reaction 6 was rate-determining, were replaced by Ravens by the reaction of peroxy radical with Co2+ ... 

In the final step the dinitrile is formed from the anti-Markovnikov addition of hydrogen cyanide [74-90-8] at atmospheric pressure and 30—150°C in the liquid phase with a Ni(0) catalyst. The principal by-product, 2-methylglutaronitril [4553-62-2], when hydrogenated using a process similar to that for the conversion of ADN to hexamethylenediamine, produces 2-methyl-1,5-pentanediamine or 2-methylpentamethylenediamine [15520-10-2] (MPMD), which is also used in the manufacture of polyamides as a comonomer. 

The alkylating agent, 1-bromobutane, is prepared from 1-butene by free-radical (anti-Markovnikov) addition of hydrogen bromide. 

Anti-Markovnikov addition of hydrogen to the alkene forms a 1-alkyl complex from the aUc-l-ene substrate. This alkyl complex can either react with other reagents or revert to the original alkene and hydrido complex. If the reaction is carried out using a deuterio complex as shown in Scheme 2, then there is a 50% chance of isotopic exchange occurring upon reversion. 

Anti-Markovnikov addition of hydrogen bromide to alkynes occurs when peroxides are present in the reaction mixture. These reactions take place through a free-radical mechanism (Section 10.10) ... 

RADICAL ADDITION TO ALKENES THE ANTI-MARKOVNIKOV ADDITION OF HYDROGEN BROMIDE... 

The mechanism for anti-Markovnikov addition of hydrogen bromide is a radical chain reaction initiated by peroxides. 

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... 

Hexamethylenediamine is discussed in Chapter 10, Sections 1 and 8. It is produced from adiponitrile by hydrogenation. Adiponitrile comes from electrodimerization of acrylonitrile (32%) or from anti-Markovnikov addition of 2 moles of hydrogen cyanide to butadiene (68%). 

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). 

Abnormal olefin arylation reactions which are of interest mechanistically and preparatively occur with some allylically substituted compounds. The ailylic esters and ethers appear normal and produce cinnamyl derivatives exclusively while ailylic alcohols and chlorides are abnormal. Ailylic alcohols and "arylpalladium acetates form 3-arylaldehydes from primary ailylic alcohols and 3-arylketones from secondary alcohols 3°). The mechanism of reaction apparently involves anti-Markovnikov addition of the palladium compound to the double bond followed by elimination of the hydrogen atom on the hydroxyl-bearing carbon rather than the benzylic hydrogen. This again would be elimination of the more electronegative hydrogen atom. 

Hexyne has the triple bond in the middle of a carbon chain and is termed an internal alkyne. If, instead, an alkyne with the triple bond at the end of the carbon chain, a 1-alkyne or a terminal alkyne, were used in this reaction, then the reaction might be useful for the synthesis of aldehydes. The boron is expected to add to the terminal carbon of a 1-alkyne. Reaction with basic hydrogen peroxide would produce the enol resulting from anti-Markovnikov addition of water to the alkyne. Tautomerization of this enol would produce an aldehyde. Unfortunately, the vinylborane produced from a 1-alkyne reacts with a second equivalent of boron as shown in the following reaction. The product, with two borons bonded to the end carbon, does not produce an aldehyde when treated with basic hydrogen peroxide. 

Oxidation of the vinylborane (using basic hydrogen peroxide) gives a vinyl alcohol (end), resulting from anti-Markovnikov addition of water across the triple bond. This end quickly tautomerizes to its more stable carbonyl (keto) form. In the case of a terminal alkyne, the keto product is an aldehyde. This sequence is an excellent method for converting terminal alkynes to aldehydes. 

Hydroboration, followed by oxidation using alkaline hydrogen peroxide, results in overall anti-Markovnikov addition of water. 

This results in the anti-Markovnikov addition of water to the triple bond. 6.3.2.4 Addition of hydrogen (reduction)... 

As noted in (3), borane adds selectively to the less sterically hindered carbon of the alkene or alkyne, but mixtures of Markovnikov proddct (addition of boron to the most hindered carbon) and anti-Markovnikov (addition of boron to the less hindered carbon) product are usually observed. This selectivity is best understood by examining the four-center transition state required for the addition (see 4). When borane approaches the alkene unit of 2-methylpropene, two orientations are possible (8 and 9). In 8, the BH2 moiety is positioned over the less hindered carbon bearing the hydrogen atoms [ BH2 - H2C= ] because this minimizes the destabilizing steric interactions with the methyl groups on the alkene [ BH2 Me2C= ] found in the other orientation (9). In other words, this interaction destabilizes 9 and leads to selective delivery of boron to the less hindered carbon via 8. Table 5.1 2 shows the relative proportions of alkylborane formed by addition of diborane to several representative alkenes. 

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