Regioselectivity olefins

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

Points b to d should be explained in more detail for intermolecular cycloaddition reactions of acyclic nitronates A with monosubstituted olefins. Regioselectivity of the process is determined by the character of the approach of olefin to the dipole (head-to-head or head-to-tail, (Chart 3.16, part (1)). In the former case, the substituent R is bound to the C-5 atom in the latter case, to the C-4 atom. 

On the other hand, 4-vinylidene-l,3-oxazolidin-2-one 26 undergoes a facile [2 + 2]-cycloaddition with electron-deficient olefins regioselectively at the terminal double bond to furnish methylenecyclobutane derivatives [25]. 

From the Hg-containing alcohol in Figure 3.38 and NaBH4 one can obtain the Hg-free alcohol. The overall result is a hydration of the C=C double bond. According to the nomenclature of Section 3.3.3, its regioselectivity corresponds to a Markovnikov addition. It is complementary to the regioselectivity of the reaction sequence hydro-boration/oxidation/hydrolysis (Table 3.1). The latter sequence would have converted the same olefin regioselectively into the primary instead of the secondary alcohol. 

Preparatively it is important that mineral acids, carboxylic acids, and ferf-carbenium ions can be added to olefins via carbenium ion intermediates. Because of their relatively low stability, primary carbenium ions form more slowly in the course of such reactions than the more stable secondary carbenium ions, and these form more slowly than the even more stable tertiary carbenium ions (Hammond postulate ). Therefore, mineral and carboxylic acids add to asymmetric olefins regioselectively to give Markovnikov products (see Section 3.3.3 for an explanation of this term). In addition, these electrophiles add most rapidly to those olefins from which tertiary carbenium ion intermediates can be derived. 

The magnitude of the LCAO MO coefficients of the interacting orbitals permits a prediction of the expected oxetane regiochemistry. Both in the HOMO of a donor-substituted olefin and in the LUMO of an acceptor-substituted olefin, the coefficient of the unsubstituted carbon atom is the larger one in absolute value. Therefore, electron-poor olefins regioselectively afford the oxetane with the substituted carbon next to the oxygen. (Cf. Barl-trop and Carless, 1972.) In contrast, an electron-rich olefin predominantly yields the oxetane with the unsubstituted carbon next to the oxygen. (Cf. Scheme 22.)... 

A variety of silyi enol ethers and silyi dienol ethers are employed in cycloadditions as electron-rich olefins. Regioselective Diels-AIder reactions using these functionalized dienes are used in the syntheses of various natural products (eq (29)) [26]. 

Methylimidazo[l,2-(j]pyridines can be olefinated regioselectively at C-3 with acrylates in the presence of a ruthenium catalyst (Scheme 45).The products form in high yields and with selectivity for the E-stereochemistry. 

The NO2 radicals obtained from nitrite ions in acidic solution add to olefins regioselectively. The resulting radical is oxidized in situ by CAN to provide nitroolefins, even labile ones, in high yield and short reaction times.32 The second reaction in Fig. 5 shows another preparation by the addition of nitryl iodide, from potassium nitrite and iodine, followed by a base-induced elimination of hydrogen iodide. A radical mechanism is probably not involved. This mild one-pot method, a valuable alternative to other published techniques, is accomplished in a non-aqueous medium and gives good yields and regioselectivities even with sterically hindered substrates. 

With olefins, regioselectivity is governed by the Markownikov rule, but there are examples of anti-Markownikov additions, with hydroboration [18] being the most prominent one. 

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