Markovnikov addition of water

Hydration of alkynes (Section 9.12) Reaction occurs by way of an enol intermediate formed by Markovnikov addition of water to the triple bond. 

One of the features that makes the hydrobora ( ion reaction so useful is the regiochemistry that results when an unsymmetrical alkene is hydroborated. For example, hydroboration/oxidation of 1-methylcyclopentene yields trans-2-methylcydopentanol. Boron and hydrogen both add to the alkene from the same face of the double bond—that is, with syn stereochemistry, the opposite of anti—with boron attaching to the less highly substituted carbon. During the oxidation step, the boron is replaced by an -OH with the same stereochemistry, resulting in an overall syn non-Markovnikov addition of water. This stereochemical result is particularly useful because it is complementary to the Markovnikov regiochemistry observed for oxymercuration. 

ANSWER In this case, H and OH are added across the pi bond in an anti-Markovnikov addition. No stereocenters were formed, so the stereochemical outcome is not relevant. We have only seen one way to achieve an anti-Markovnikov addition of water across a pi bond hydroboration-oxidation. Therefore, our answer is ... 

In 1993, ten challenges faced the catalysis research community. One of these was the anti-Markovnikov addition of water or ammonia to olefins to directly synthesize primary alcohols or amines [323]. Despite some progress, the direct addition of N-H bonds across unsaturated C-C bonds, an apparently simple reaction, stiU remains a challenging fundamental and economic task for the coming century. 

In 1998, Wakatsuki et al. reported the first anti-Markonikov hydration of 1-alkynes to aldehydes by an Ru(II)/phosphine catalyst. Heating 1-alkynes in the presence of a catalytic amount of [RuCljlCgHs) (phosphine)] phosphine = PPh2(QF5) or P(3-C6H4S03Na)3 in 2-propanol at 60-100°C leads to predominantly anti-Markovnikov addition of water and yields aldehydes with only a small amount of methyl ketones (Eq. 6.47) [95]. They proposed the attack of water on an intermediate ruthenium vinylidene complex. The C-C bond cleavage or decarbonylation is expected to occur as a side reaction together with the main reaction leading to aldehyde formation. Indeed, olefins with one carbon atom less were always detected in the reaction mixtures (Scheme 6-21). 

Recently, on the basis of the Markovnikov addition of water to alkynes, Trost et al. developed a three-component addition reaction of terminal alkynes, water, and methyl vinyl ketone, affording 1,5-diketones in DMF/water in the presence of ruthenium and indium catalysts (Eq. 4.38). 

The rate-determining step in the hydration mechanism is step 1 the formation of the carbocation => accounts for the Markovnikov addition of water to the double bond. 

The net result of hydroboration-oxidation is an anti-Markovnikov addition of water to a double bond. 

Mechanism of the Markovnikov addition of water to an alkene to yield an alcohol. 

Hydration of alkenes can also be achieved either by oxymerciiration-reduction (Markovnikov addition of water) or hydroboration-oxidation (a ti-Markovnikov addition of water). Addition of water by oxymercura-tion-reduction or hydroboration-oxidation has two advantages over the acid-catalysed addition of water. These procedures do not require acidic condition, and carbocation rearrangements never occur. Thus, they give high yields of alcohols. 

In contrast, a Markovnikov addition of water was reported in the irradiation of a variety of o-hydroxystyrenes, again in aqueous acetonitrile, with the formation of 2-(2-hydroxyphenyl)ethanols. In this case, an intramolecular proton transfer from the excited state of the styrene was envisaged as the first step of the reaction [51]. A similar mechanism was postulated in the photohydratation of m-hydroxy-1,1 -diaryl alkenes that gave the corresponding 1,1-diarylethanols, although direct protonation of the P-carbon by water competed in some cases [52]. 

The epoxidation-epoxide opening sequence with this reagent provides a convenient access to the products of an //-Markovnikov addition of water to olefins. Interestingly, the Cp2TiCl/H20 couple combination shows anti stereoselectivity in the reduction step [73, 74], which is complementary to the hydroboration-oxidation method (Scheme 32). 

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-Oxidation of Alkynes Hydroboration-oxidation of an alkyne gives anti-Markovnikov addition of water across the triple bond. Di(secondary isoamyl)borane, called disiamylborane, is used, since this bulky borane cannot add twice across the triple bond. On oxidation of the borane, the unstable enol quickly tautomerizes to an aldehyde. (See Section 9-9F.)... 

Hydrogen peroxide, H202 Oxidizes organoboranes to yield alcohols. Used in conjunction with addition of borane to alkenes, the overall transformation effects syn Markovnikov addition of water to an alkene (Section 7.5). 

Oxymercuration-demercuratiori is highly regiospecific, and gives alcohols corresponding to Markovnikov addition of water to the carbon-carbon double bond. For example ... 

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

The addition of water to alkenes, to produce alcohols, requires the presence of either (i) a strong acid or (ii) mercury(II) acetate (in an oxymercuration reaction). In both cases, the reactions involve the Markovnikov addition of water (i.e. the OH becomes attached to the more substituted carbon). 

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