Markovnikov addition alkenes

Improved results were achieved when trichlorosilane was treated with alkenes in the presence of a ferrocenyl-palladium catalyst, [(7 )-(5)-PPFA]PdCl2 8a. In the ease of unsymmetrica alkenes, Markovnikov addition products are formed. 

Arylthiols (but not alkylthiols) add to terminal alkynes regioselectively to afford a Markovnikov-type adduct 212 in good yield using Pd(OAc)2 as a catalyst[120]. This result is clearly different from the an/i-Markovnikov addition induced by a radical initiator. The hydroselenation of terminal alkynes with benzeneselenol catalyzed by Pd(OAc)2 affords the terminal alkene 213, which undergoes partial isomerization to the internal alkene 214[121]. 

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. 

Hydration of an alkene—the addition of water—is carried out by either of two procedures, depending on the product desired. Oxymercuration involves electrophilic addition of Hg2+ to an alkene, followed by trapping of the cation intermediate with water and subsequent treatment with NaBH4. Hydroboration involves addition of borane (BH3) followed by oxidation of the intermediate organoborane with alkaline H202- The two hydration methods are complementary oxymercuration gives the product of Markovnikov addition, whereas hydroboration/oxidation gives the product with non-Markovnikov syn stereochemistry. 

Practically everything we ve said in previous chapters has been stated without any proof. We said in Section 6.8, for instance, that Markovnikov s rule is followed in alkene electrophilic addition reactions and that treatment of 1-butene with HC1 yields 2-chJorobutane rather than 1-chlorobutane. Similarly, we said in Section 11.7 that Zaitsev s rule is followed in elimination reactions and that treatment of 2-chlorobutane with NaOH yields 2-butene rather than 1-butene. But how do we know that these statements are correct The answer to these and many thousands of similar questions is that the structures of the reaction products have been determined experimentally. 

We saw in Section 7.4 that alkenes react with water in the presence of mercuric acetate to yield a hvdroxymercuration product. Subsequent treatment with NaBH4 breaks the C-Hg bond and yields the alcohol. A similar alkoxymercuration reaction occurs when an alkenc is treated with an alcohol in the presence of mercuric acetate or, even better, mercuric trifluoroacetate, (Cl CCtitiHg. Demercura-tion by reaction with NaBH4 then yields an ether. The net result is Markovnikov addition of the alcohol to the alkene. 

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 products of the above reactions are consistent with Markovnikov addition of transient Co(Por)H to the unsaturated alkene or alkyne substrate. The regio-chemistry is determined by formation of the most stable organic radical, which... 

EXERCISE 11.1 Draw the product that you would expect from an anti-Markovnikov addition of H and Br across the following alkene ... 

In other words, we must determine whether the reaction is a Markovnikov addition or an anii-Markovnikov addition. As promised, the answer to this question is contained in the mechanism. In the first step of the mechanism, a proton was transferred to the alkene, to form a carbocation. When starting with an unsymmetrical alkene, we are confronted with two possible carbocations that can form (depending on where we place the proton) ... 

Answer In order to determine whether or not to use peroxides, we must decide whether the desired transformation represents a Markovnikov addition or an anti-Markovnikov addition. When we compare the starting alkene above with the desired product, we see that we need to place the Br at the more substituted carbon (i.e., Markovnikov addition). Therefore, we need an ionic pathway to predominate, and we should not use peroxides. We just use HBr ... 

A quick glance at the products indicates that we are adding H and OH across the alkene. Let s take a closer look and carefully analyze the regiochemistry and stereochemistry of this reaction. The OH is ending up on the less substituted carbon, and therefore, the regiochemistry represents an anti-Markovnikov addition. But what about the stereochemistry Are we seeing a syn addition here, or is this anti addition ... 

Answer (a) These reagents will accomplish an anti-Markovnikov addition of OH and H. The stereochemical outcome will be a syn addition. But we must first decide whether stereochemistry will even be a relevant factor in how we draw our products. To do that, remember that we must ask if we are creating two new stereocenters in this reaction. In this example, we are creating two new stereocenters. So, stereochemistry is relevant. With two stereocenters, there theoretically could be four possible products, but we will only get two of them we will only get the pair of enantiomers that come from a syn addition, hi order to get it right, let s redraw the alkene (as we have done many times earlier), and add OH and H like this ... 

The hydrosi(ly)lations of alkenes and alkynes are very important catalytic processes for the synthesis of alkyl- and alkenyl-silanes, respectively, which can be further transformed into aldehydes, ketones or alcohols by estabhshed stoichiometric organic transformations, or used as nucleophiles in cross-coupling reactions. Hydrosilylation is also used for the derivatisation of Si containing polymers. The drawbacks of the most widespread hydrosilylation catalysts [the Speier s system, H PtCl/PrOH, and Karstedt s complex [Pt2(divinyl-disiloxane)3] include the formation of side-products, in addition to the desired anh-Markovnikov Si-H addition product. In the hydrosilylation of alkynes, formation of di-silanes (by competing further reaction of the product alkenyl-silane) and of geometrical isomers (a-isomer from the Markovnikov addition and Z-p and -P from the anh-Markovnikov addition. Scheme 2.6) are also possible. 

The hydroaminations of electron-deficient alkenes with aniline or small primary alkylamines proceed at high conversions (85-95%, nnder mild conditions, 5 mol%, rt), giving exclnsively the anh-Markovnikov addition product. Secondary dialkyl or bnlky primary amines require longer reaction times. With amines containing P-hydrogens, no imine side-products were observed. 

Although zirconium bisamides Cp2Zr(NHAr)2 do not catalyze the hydroamination of alkenes (see above), they are catalyst precursors for the hydroamination of the more reactive double bond of allenes to give the anti-Markovnikov addition product (Eq. 4.96) [126]. 

The addition of hydrogen halide to alkene is another classical electrophilic addition of alkene. Although normally such reactions are carried out under anhydrous conditions, occasionally aqueous conditions have been used.25 However, some difference in regioselectivity (Markovnikov and anti-Markovnikov addition) was observed. The addition product formed in an organic solvent with dry HBr gives exclusively the 1-Br derivative whereas with aq. HBr, 2-Br derivative is formed. The difference in the products formed by the two methods is believed to be due primarily to the difference in the solvents and not to the presence of any peroxide in the olefin.26... 

Addition of Hydrogen Halides to Alkene Markovnikov s Rule... 

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

A Summary of Markovnikov versus Anti-Markovnikov Addition of HBr to Alkenes... 

Oxymercuration-demercuration gives Markovnikov addition of H- and -OH to an alkene, yet it is not complicated by rearrangement. 

Similar to the addition of secondary phosphine-borane complexes to alkynes described in Scheme 6.137, the same hydrophosphination agents can also be added to alkenes under broadly similar reaction conditions, leading to alkylarylphosphines (Scheme 6.138) [274], Again, the expected anti-Markovnikov addition products were obtained exclusively. In some cases, the additions also proceeded at room temperature, but required much longer reaction times (2 days). Treatment of the phosphine-borane complexes with a chiral alkene such as (-)-/ -pinene led to chiral cyclohexene derivatives through a radical-initiated ring-opening mechanism. In related work, Ackerman and coworkers described microwave-assisted Lewis acid-mediated inter-molecular hydroamination reactions of norbornene [275]. 

From the results of the 1,3-diene addition reaction, the metal-catalyzed reaction of unactivated alkenes was examined, and it was found that the palladium complex effectively catalyzed the a rt-Markovnikov addition of triarylphosphines and bis(trifluoromethanesulfonyl)imide (Tf2NH).24... 

In the early days of alkene chemistry, some researchers found that the hydrohalogenation of alkenes followed Markovnikov s rule, while others found that the same reaction did not. For example, when freshly distilled but-l-ene was exposed to hydrogen bromide, the major product was 2-bromopropane, as expected by Markovnikov s rule. However, when the same reaction was carried out with a sample of but-l-ene that had been exposed to air, the major product was 1-bromopropane formed by antl-Markovnikov addition. This caused considerable confusion, but the mystery was solved by the American chemist, Morris Kharasch, in the 1930s. He realised that the samples of alkenes that had been stored in the presence of air had formed peroxide radicals. The hydrohalogenation thus proceeded by a radical chain reaction mechanism and not via the mechanism involving carbocation intermediates as when pure alkenes were used. 

Rhodium(I) and ruthenium(II) complexes containing NHCs have been applied in hydrosilylation reactions with alkenes, alkynes, and ketones. Rhodium(I) complexes with imidazolidin-2-ylidene ligands such as [RhCl( j -cod)(NHC)], [RhCl(PPh3)2(NHC)], and [RhCl(CO)(PPh3)(NHC)] have been reported to lead to highly selective anti-Markovnikov addition of silanes to terminal olefins [Eq. 

This radical anti-Markovnikov addition of HX to alkenes is restricted to HBr both HI and HCl add in a Markovnikov fashion by an ionic... 

According to the reasoning we have so far used, protonation of the double bond in this alkene should give the secondary carbocation rather than the alternative primary carbocation. Based on the information given in the question, it does not. We must get the primary carbocation, which is then quenched by bromide, i.e. anti-Markovnikov addition. It is not necessary to remember what Markovnikov or anti-Markovnikov additions mean, we just need to consider the carbocation intermediates and their relative stability. 

The general reaction for the catalytic hydration of an alkene to produce an alcohol is shown in Figure 3-6, and the mechanism is in Figure 3 7. This process is an excimple of a Markovnikov addition (as seen in Organic Chemistry 1). 

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

The reaction of / with bromine in carbon tetrachloride confirms the prediction that / is an alkene. The reaction with HBr means that G is the Markovnikov addition product, and the reaction in the presence of peroxides makes //the anti-Markovnikov product. 

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