Olefins Markovnikov selectivity

A remarkable feature of the structural work on the derivatives with unsaturated side chains is the method by which the double bonds were located. Thus a micromethod has been devised, based on the Markovnikov selectivity of olefin... 

The basic Markovnikov selectivity pattern is partially or fully overrun in the presence of neighboring coordinating groups within the olefin substrate (Section 2.2.2). Known functionalities where inversed selectivity can occur include 3-alke-noylamides (e.g. 17 reacts to give a mixture of 18 and 19, Table 3) [43], homoallyl esters and alcohols, allyl ethers (but not necessarily allyl alcohols) [44], allyl amines, allyl amides, or carbamates (cf. 20 to 21) [45], allyl sulfides [46] or 1,5-dienes [47]. As a matter of fact, aldehyde by-products are quite normal in Wacker reactions, but tend to be overlooked. 

Very recently it was shown that sulfonyl oximes 359 are suitable reagents for the addition of oxime functions to olefins 355 (entry 22) [403]. This C-C bond-forming reaction proceeded for a wide range of terminal and 1,1-disubstituted substrates with formal Markovnikov selectivity using catalytic amounts of 357a. Branched oximes 360 were isolated in 33-95% yield. 

Abstract Progress in the field of metal-catalyzed redox-neutral additions of oxygen nucleophiles (water, alcohols, carboxylic acids, and others) to alkenes, alkynes, and allenes between 2001 and 2009 is critically reviewed. Major advances in reaction chemistry include development of chiral Lewis acid catalyzed asymmetric oxa-Michael additions and Lewis-acid catalyzed hydro-alkoxylations of nonacti-vated olefins, as well as further development of Markovnikov-selective cationic gold complex-catalyzed additions of alcohols or water to alkynes and allenes. 

The bulk of the studies on intramolecular hydroamination of alkenes catalyzed by lanthanide complexes have been conducted using lanthanocene complexes or half-sandwich lanthanide complexes. The prototypical cyclizations of aminoalkenes to form five- and six-membered rings are shown in Equation 16.61. These reactions occur with exclusive Markovnikov selectivity. These reactions have also been conducted using arylamines, as shown in Equation 16.62. The intramolecular reactions of amines catalyzed by certain lanthanide complexes occur with 1,1- and 1,2-disubstituted olefins (Equation 16.63), although such reactions require high temperatures. 

Catalysts for tfie additions of amines to vinylarenes have also been developed. These catalytic reactions include some of the first hydroaminations of unstrained olefins catalyzed by late transition metals, as well as examples catalyzed by lanthanide complexes. These additions occur with Markovrukov selectivity with one set of catalysts and with anti-Markovnikov selectivity with several others. These additions occur by several different mechanisms that are presented in Section 16.5.3.2. 

Asymmetric Hydrosilylation of C=C Bond. Successful asymmetric hy-drosilylation of terminal olefins requires the catalyst ensuring both Markovnikov selectivity (unusual for platinum and rhodium) and satisfactory level of asymmetric induction. Asymmetric hydrosilylation followed by Tamao-Fleming oxidation (known to proceed with retention of configuration at the stereogenic carbon) has become one of the most useful, general methods for the preparation of optically active alcohols from alkenes (Scheme 27). 

The first successful approach was reported by Heathcock in 1969 using mercury(ll) salts. The reaction proceeds via an olefin azidomercuration followed by a reductive demercuration in the work-up. The azides derived from terminal olefins were obtained in 50-88% yield with good Markovnikov selectivity, while non-terminal olefins gave lower yields (Figure 4.2). An obvious drawback of this procedure is the use of a stoichiometric amount of mercury salts. This procedure also leads to the formation of potentially explosive Hg(N3)2. Nevertheless, this method is unique and reliable for the hydroazidation of monosubstituted non-activated olefins. 

The basis of the high normal to isoaldehyde selectivity obtained ia the LP Oxo reaction is thought to be the anti-Markovnikov addition of olefin to HRhCOL2 to give the linear alkyl, Rh(CO)L2CH2CH2CH2CH2, the precursor of straight-chain aldehyde. Anti-Markovnikov addition is preferred ia this... 

The Markovnikov regioselectivity of the gem-alkenes is associated with a chemoselectivity. in favour of methanol attack, significantly greater than that observed for the other alkenes. If no sodium bromide is added to the reaction medium, no dibromide is observed for this series. Therefore, these alkenes behave as highly conjugated olefins, as regards their regio- and chemo-selectivity. In other words, the bromination intermediates of gem-alkenes resemble P-bromocarbocations, rather than bromonium ions. Theoretical calculations (ref. 8) but not kinetic data (ref. 14) support this conclusion. 

Tucci (54), studying mainly terminal olefins, cited two reasons for the high selectivity for linear products in the phosphine-modified cobalt catalysts (a) stereoselective addition of the hydride species to the olefinic double bond, and (b) inhibition of olefin isomerization. However, the results obtained with internal olefins as substrate tended to discount the likelihood of the second reason, and it is generally accepted that selective anti-Markovnikov addition arising from steric hindrance is the principal cause for linear products from nonfunctional olefins. 

The most fundamental reaction is the alkylation of benzene with ethene.38,38a-38c Arylation of inactivated alkenes with inactivated arenes proceeds with the aid of a binuclear Ir(m) catalyst, [Ir(/x-acac-0,0,C3)(acac-0,0)(acac-C3)]2, to afford anti-Markovnikov hydroarylation products (Equation (33)). The iridium-catalyzed reaction of benzene with ethene at 180 °G for 3 h gives ethylbenzene (TN = 455, TOF = 0.0421 s 1). The reaction of benzene with propene leads to the formation of /z-propylbenzene and isopropylbenzene in 61% and 39% selectivities (TN = 13, TOF = 0.0110s-1). The catalytic reaction of the dinuclear Ir complex is shown to proceed via the formation of a mononuclear bis-acac-0,0 phenyl-Ir(m) species.388 The interesting aspect is the lack of /3-hydride elimination from the aryliridium intermediates giving the olefinic products. The reaction of substituted arenes with olefins provides a mixture of regioisomers. For example, the reaction of toluene with ethene affords m- and />-isomers in 63% and 37% selectivity, respectively. 

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. 

These catalytic reactions provide a unique pathway for addition of aromatic C-H bonds across C=C bonds. In contrast with Friedel-Crafts catalysts for olefin hydroarylation, the Ru-catalyzed hydrophenylation reactions of a-olefins selectively produce linear alkyl arenes rather than branched products. Although the selectivity is mild, the formation of anti-Markovnikov products is a unique feature of the Ru(II) and Ir(III) catalysts discussed herein. Typically, the preferred route for incorporation of long-chain linear alkyl groups into aromatic substrates is Friedel-Crafts acylation then Clemmensen reduction, and the catalysts described herein provide a more direct route to linear alkyl arenes. 

Numerous attempts to obtain the Markovnikov adduct by varying the reaction conditions, including its realization in concentrated HC1, had failed. Moreover, in a competitive reaction of a mixture of 1-heptene and styrene only the anti-Markovnikov adducts were formed for both olefins and, surprisingly, 1-heptene was found to be more reactive than styrene. This is also in agreement with the concept of two mechanisms. Here, 1-heptene assists in the formation of GeCl3 radicals and styrene acts as a radical trap, forming selectively only the anti-Markovnikov adduct. 

One simple way for arriving at a photoinduced Markovnikov-type amination of olefins is illustrated in Scheme 3.15, for the case of o-alkenylphenols (e.g., 24). Here, an ammonium salt of the o-alkenylphenolate anion (25) is prepared and irradiated this causes proton transfer within the ion pair (from the amine to the olefin), and the resulting zwitterion undergoes nucleophilic addition of the amine at the benzylic cation center. The addition is selective for N- rather than O-nucleophiles, as shown in the case of ethanolamine [34]. 

The addition of J-butyl hypoiodite to selected olefins has been investigated21. In the absence of BF3 and in the dark, /-BuOI reacts with styrene to give the anti-Markovnikov regioisomer (i.e. by a free radical mechanism). In the presence of BF3, /3-t-butoxy-/ -... 

H, C and Si NMR studies of the products suggested that the hydrosilylation of PBD occurs selectively via an anti-Markovnikov addition, i.e., the Si-atom being attached at the terminal position of olefin bonds ( -product) (Fig. 3). 

A detailed study of the reactions of trichlorogermane with unsaturated compounds was performed . It became clear that among selected olefins only 1-heptene forms the anti-Markovnikov adduct in the reaction with trichlorogermane. In contrast to the generally accepted opinion, in the reactions with 1-methylcyclohexene (equation 11), styrene (equation 12), 2,3-dimethyl-l-butene (equation 13) and isobutene (equation 14) both regioisomers 4 and 5, 6 and 7, 8 and 9, and 10 and 11 appear in commensurable amounts (together with oligomeric products see later, equation 16). 

The selective reaction of the hydridocobalt carbonyl with the olefin via Mar-kovnikov and anti-Markovnikov addition gives rise to the branched and linear alkylcobalt carbonyl isomers. It is believed that the sterically less demanding nature of HCo(CO)3 favors the formation of the branched isomer, whereas HCo(CO)4 generates predominantly the linear isomer. This is in accordance with the increased selectivity observed at higher carbon monoxide partial pressures. As HCo(CO)4 is the less reactive catalyst, the catalytic activity drops at the same time. 

In hydrocarboxylations, as in the 0x0 process, selectivity of linear versus branched products is an important issue, because (in general) mixtures of isomeric carboxylic acids are obtained, owing not only the occurrence of both Markovni-kov and anti-Markovnikov addition of the alkene to the metal hydride, but also to metal-catalyzed alkene isomerization (eq. (2)). In the case of higher olefins, Co2(CO)g as catalyst leads to a number of different carboxylic acid isomers due to the isomerization activity of the catalyst. 

Radicaloid insertions of olefins into the Rh H bond of [Rh (TPP)(H)] has been used to obtain Rh —CH2—(alkyl)—Nu// species (NuH = OH, NH) using olefins functionalized with end-on —OH and —NH functionalities. Under basic conditions, intramolecular SN2-type attack of the Nu at the a-carbon atom of Rh —CH2—(alkyl)—Nu yields [Rh (TPP)] and cyclic organic products (—CH2—(alkyl)—Nu—) (see Fig. 47). Protonation of [Rh TPP)] then allows regeneration of [Rh° (TPP)(H)]. The combination of these reactions constitutes a new method for selective intramolecular anti-Markovnikov hydrofunctionalization of olefins with O—H and N—H functionalities (150). In this way, three-and five-membered ring compounds (epoxides, furan derivatives, pyrrolidine derivatives) were readily obtained. Formation of four- or six-membered rings... 

Figure 47. Selective intramolecular anti-Markovnikov hydiofunctionalization of olefins with O—H and N—H functionalities via radical insertion of olefins in the Rh—H bond.

Hosokawa et al. clearly spelled out the heteroatom coordination theory in explaining their region-selective formation of aldehydes.20 Terminal olefin 13 was converted to a mixture of aldehyde 14 and methyl ketone 15, with aldehyde 14 as the major product (14/15 = 70/30). The authors explained that Pd(II) coordinated with both oxygen of the amide as well as the olefin as depicted in intermediate 16, which blocked the normal Markovnikov hydration position and the addition of the peroxide took place at the terminal position as... 

Mechanistic hypotheses play an important role in developing new catalytic and selective heterofunctionalizations of alkenes. Two basic reaction cycles for metal-catalyzed hydroalkoxylation (and hydration, for R = H) of alkenes can be postulated (Scheme 2). One pathway leads to Markovnikov products via activation of the nucleophile, oxy-metallation, and protonolysis (hydro-de-metallation) (Scheme 2a). Alternatively to the inner sphere syn-oxymetallation depicted in Scheme 2a, external anti-attack of the nucleophUe to coordinated olefin is plausible. The oxidation state of the metal remains constant in this cycle. The alternative hydrometallation pathway (Scheme 2b) proceeds via oxidative addition of the H-OR bond, hydrometallation of the olefin, and reductive elimination to the anti-Markovnikov addition product [3,4]. 

The Wacker reaction has been applied to numerous simple olefins such as a-olefins and cycloalkenes, or to functionalized olefins such nitroethylene, acrylonitrile, styrene, allyl alcohol, or maleic acid [3]. The carbonyl group is formed at the carbon atom of the double bond where the nucleophile would add in a Markovnikov addition. Reversal of the regioselectivity has only been observed with particular substrates such as 1,5-dienes [9]. Conversion and selectivity for the oxidation of these olefins were found to be very dependent on the water solubility of the olefin. Indeed, high molecular weight olefins do not react under the standard... 

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