Markovnikov products diene reactions

Conjugated dienes also undergo electrophilic addition reactions readily, but mixtures of products are invariably obtained. Addition of HBr to 1,3-butadiene, for instance, yields a mixture of two products (not counting cis-trans isomers). 3-Bromo-l-butene is the typical Markovnikov product of 1,2-addition to a double bond, but l-bromo-2-butene appears unusual. The double bond in this product has moved to a position between carbons 2 and 3, and HBr has added to carbons 1 and 4, a result described as 1,4-addition. 

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. 

A hydroformylation reaction in diene polymers introduces a formyl group which is an extremely reactive functional group. Sibtain and Rempel [65] carried out hydroformylation of SBR using HRh(CO)(PPh3)3 and reported anti-Markovnikov addition product. Hydroformylation takes place preferentially in the 1,2 unit. As the degree of hydroformylation increases new absorption bands appear at 1724 cm"1 due to v(C=0) and at 2700 cm 1 due to v(C-H) in CHO. Bhattacharjee and co-workers [66] carried out hydroformylation of NBR and observed new peaks at 1724 and 2700 cm"1 which are characteristics of CHO groups. 

Benzeneselenenyl chloride adds to alkynes to produce mixtures of tra/ts-alkene adducts. For example, the addition of benzeneselenenyl chloride to the alkyne (18) produces the alkene (19), which can be transformed to yield the unusual diene (20 equation 16). Alkynic alcohols give anti-Markovnikov addition products under kinetic control. The reaction is thought to proceed through the selenirenium ion (21 equation 17). Selenium electrophiles add to a, 3-alkynic cariranyl moieties to produce cis adducts in good yield (equation 18). ... 

Here catalysis involves the formation of a ruthenium vinylidene, an anti-Markovnikov addition of water (368), and cyclization of an acylmetal species onto the alkene. Although cyclization may occur via hydroacylation (Scheme 52, path A) (460-462) or the Michael addition reaction (Scheme 52, path B) (463,464), the requirement for an electron-withdrawing substiment on the alkene and the absence of aldehyde formation suggest path B to be the more likely mechanism (465,466). Trost discovered that the use of the cationic mthenium catalyst CpRu(MeCN)3+PFg is tolerant of 1,2-di-and trisubstimted alkenes and promotes cyclization of 1,6- and 1,7-enynes to five- and six-membered ring products (467). In a number of examples, the mthenium reaction is complementary to the Pd-catalyzed cyclization described above, selectively forming the 1,4-diene over the traditional 1,... 

For hydrosilylation of alkenes, the reaction rate increases with temperature and hence many of these reaetions have been performed at 100 °C. Higher reaction rates are obtained for silanes with very electronegative substituents and low steric requirements (e.g. HSi(OEt)3 > HSi(i-Pr)3). Terminal alkenes usually are hydrosilylated in an anti-Markovnikov sense to give terminal silanes. Internal alkenes tend not to react (e.g. cyclohexene), or iso-merize to the terminal alkene which is then hydrosilylated (eq 10). Conversely, terminal alkenes may be partially isomerized to un-reactive internal alkenes before the addition of silane can occur. 1,4-Additions to dienes are frequently observed, and the product distributions are extremely sensitive to the silane used (eq 11). 

With electrophilic addition to standard alkenes such as propene, the product predicted by Markovnikov s rule is also more stable. For reactions under thermodynamic (equilibrium) control, the distribution of products is determined by the relative stability of each. Thus, kineticaUy controlled and thermodynamically controlled electrophilic additions of H—to standard alkenes results in the same dominant product. This is the case with many reactions the product formed fastest is also most stable. Yet, many other reactions do not behave this way. Below we will see that the addition of HBr to conjugated dienes exemplifies reactions in which kinetic and thermodynamic control produce different dominant products. 

---