Markovnikoff product

Wilkinson s catalyst brings about the hydrosilylation of a range of terminal alkenes (1-octene, trimethylvinylsilane) by 2-dimethylsilylpyridine with good regioselectivity for the anti-Markovnikoff product. Both 3-dimethylsilylpyridine and dimethylphenylsilane are less reactive sources of Si-H. In contrast, these two substrates are far more reactive than 2-dimethylsilylpyridine for the hydrosilylation of alkynes by [Pt(CH2 = CHSiMe2)20]/PR3 (R = Ph, Bu ). This difference was explained to be due to the operation of the two different pathways for Si-H addition—the standard Chalk-Harrod pathway with platinum and the modified Chalk-Harrod pathway with rhodium.108... 

Hydrophosphorylation has recently been extended to rhodium catalysts as a route to anti-Markovnikoff products.198 Thus, 3mol.% Rh(PPh3)3Cl affords the ( )-alkenylphosphonates (R = H, Ph, -Cr,I I ]3, CH2CH2CN, SiMe3, or cyclohexenyl) in high yields (>80%) at room temperature when 4,4,5,5-tetramethyl-l,3,2-dioxaphospholane-2-oxide (74) is used as the PH source (Equation (19)).199 The rate of reaction is highly solvent dependent... 

P. Knochel and co-workers used diphosphines as ligands in the rhodium-catalyzed asymmetric hydroboration of styrene derivatives." The best results were obtained with the very electron rich diphosphane, and (S)-1-phenylethanol was obtained in 92% ee at -35 °C, with a regioselectivity greater than 99 1 (Markovnikoff product). A lower reaction temperature resulted in no reaction, while a higher temperature resulted in lower enantioselectivity and regioselectivity. The regioselectivity was excellent in all cases. Irrespective of the electronic nature of the substituents, their position and size had a profound effect on the enantioselectivity. 

The regioselective addition of HX to alkenes produces the more substituted alkyl halide, which is known as the Markovnikov (Markovnikoff) product. Markovnikov s rule states that on addition of HX to an alkene, H attaches to the carbon with fewest alkyl groups and X attaches to the carbon with most alkyl groups . This can be explained by the formation of the most stable intermediate carbocation. 

Propanol has been manufactured by hydroformylation of ethylene (qv) (see Oxo process) followed by hydrogenation of propionaldehyde or propanal and as a by-product of vapor-phase oxidation of propane (see Hydrocarbon oxidation). Celanese operated the only commercial vapor-phase oxidation faciUty at Bishop, Texas. Since this faciUty was shut down ia 1973 (5,6), hydroformylation or 0x0 technology has been the principal process for commercial manufacture of 1-propanol ia the United States and Europe. Sasol ia South Africa makes 1-propanol by Fischer-Tropsch chemistry (7). Some attempts have been made to hydrate propylene ia an anti-Markovnikoff fashion to produce 1-propanol (8—10). However, these attempts have not been commercially successful. 

Mannig and Noth reported the first example of rhodium-catalyzed hydroboration to C=C bonds in 1985.4 Catecholborane reacts at room temperature with 5-hexene-2-one at the carbonyl double bond when the reaction was run in the presence of 5mol.% Wilkinson s catalyst [Rh(PPh3)3Cl], addition of the B—H bond across the C=C double bond was observed affording the anti-Markovnikoff ketone as the major product (Scheme 2). Other rhodium complexes showed good catalytic properties ([Rh(COD)Cl2]2, [ Rh(PPh3)2(C O )C 1], where... 

The first example of anti-Markovnikoff hydroamination of aromatic alkenes has been demonstrated with cationic rhodium complexes.170 A combination of [Rh(COD)2]+/2PPh3 in THF under reflux yields the N-H addition product as the minor species alongside that resulting from oxidative amination (Scheme 37). Hydrogenation products are also detected. 

If the more activated alkene 2-vinylpyridine is used in place of styrene with the same catalysts and the same range of substrates, anti-Markovnikoff hydroamination is also found. Thus, N-[2-(2 -pyridyl)ethyl]piperidine was isolated in 53% yield from reaction of 2-vinylpyridine with piperidine in the presence of [Rh(COD)2]+/2PPh3 under reflux. N H addition was observed with other amines, the remaining product in all cases being primarily that from oxidative amination (Table 12). When the catalytic reaction was run in the absence of phosphine, the yield of hydroamination product increased dramatically.171... 

The concerted nature of the reaction is indicated by the fact that a Markovnikoff-type directing effect is essentially absent. Secondary and tertiary products are formed in nearly equal amounts from unsymmetri-cally substituted olefins (9, 13, 14, 15, 17, 27, 34). In addition, substituents on the phenyl group of trimethylstyrene exert no effect on the product distributions (12). Furthermore, values for 2-methyl-2-pentene show only a very small solvent effect (12). All these facts suggest that little polarity is developed at the transition state and are consistent with a concerted reaction. 

A further communication from the Givaudan group reports that hydro-boration of dihydrothujopsene (117) follows an abnormal course from two standpoints. In the first place the major product is the tertiary alcohol (118), formed as a result of an atypical Markovnikoff hydration process, and secondly the minor product is the diol (119), which presumably arises by the facile formation of an intramolecular dialkylborane intermediate. 

The electrophilic addition reactions of A -unsaturated steroids and other rigid cyclohexenes are controlled mainly by the conformational preference for diaxia) addition HOBr, for example, gives mainly a 5a-bromo-6)5-alcohol. A study of similar reactions with B-nor-A -unsaturated steroids suggests that the reaction of a cyclopentene is under electronic rather than conformational control.A variety of reagents (HOBr, BrF, Brj, BrOMe, and BrOAc) gave mainly 6a-bromo-5)S-substituted derivatives (155), indicating that the initial product, a 5a.6a-bromonium ion (154). reacts further according to Markovnikoff, with attack of the anion at the tertiary 5 -position. 

Two orientations of the addition process are possible for the reaction with the unsymmetrical reagents HCl, H2O, and H2SO4. MarkovnikofFs rule tells us that the major product is the one where the H of the reagent attaches to the carbon of the double bond that has two hydrogens attached. The other doubly bonded carbon has fewer (zero) hydrogens attached ... 

Ans. The major and minor products are 2-hydroxypropane and 1-hydroxypropane, respectively. (The lUPAC names are 2-propanol and 1-propanol, as will be discussed in Chap. 13.) The mechanism for formation of each product is shown below. 2-Propanol is the major product, as predicted by the Markovnikoff rule, because it is formed by the pathway that proceeds via formation of the most stable carbocation (2 ° instead of 1 °). 

In the case of higher alkenes (more than two carbon atoms) several isomeric products are possible. The particular isomer produced depends on the stability of the alternative intermediates and this is summarized empirically by Markovnikoff s rule. See also addition reaction. 

Markovnikoff s rule /mar-koff-ni-koffs/ A rule that predicts the quantities of the products formed when an acid (HA) adds to the double bond in an alkene. If the alkene is not symmetrical two products may result for instance (CH3)2C CH2 can yield either (CH3)2HCCH2A or (CH3)2ACCH3. The rule states that the major product will be the one in which the hydrogen atom attaches itself to the carbon atom with the larger number of hydrogen atoms. In the example above, therefore, the major product is (CH3)2ACCH3. 

Remember also to assign a positive charge to the molecule before submitting it to calculation. This is usually done in the menus where you select the type of calculation. Compare your results for the two calculations. Which carbocation will lead to the major product Do your results agree with the prediction made by Markovnikoff s Rule ... 

Hydrogen sulfide adds to olefins and unsaturated compounds to give thiols in good to excellent yields. The reaction is usually very rapid and is catalyzed either by free radical initiators (peroxides or ultraviolet light) or ionic types of catalysts such as transition metals, sulfuric acid, or even free sulfur. The free radical initiated reaction addition gives anti-MarkovnikolT addition products [Eq. (10)] whereas the ionic catalyzed reaction gives Markovnikoff addition products [Eq. (11)]. Both types of reactions work well in the laboratory and are also practiced commercially. 

Markovnikoff adduct (76), although the two products are interconverted on heating (see also Chapter 4, p. 193). 

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