Regioselectivity hydrogen abstraction

The cycloaddition of photoenol of o-methylbenzaldehyde 66 with 5-alkyli-dene-l,3-dioxane-4,6-dione derivatives 67 is an example of a photo-induced Diels Alder reaction in which one component, the diene in this case, is generated by irradiation [48]. The yields of some cycloadducts 68, generated by photo-irradiation of a benzene solution of 66 and 67 at room temperature, are reported in Table 4.14. The first step of the reaction is the formation of (E)-enol 69 and (Z)-enol 70 (Equation 4.7) by an intramolecular hydrogen abstraction of 66 followed by a stereo- and regioselective cycloaddition with... 

Scheme 20 HOMO-LUMO interaction in the perepoxide quasi-intermediate for the cis-effect and the regioselectivity (percent) of the hydrogen abstractions...

The stereospecific labeling of the anti methyl by deuterium in compound 20 to produce substrate 21 (Table 3) was required in order to study the syn/anti regioselectivity of the ene allylic hydroperoxides. The ene products in different solvents showed a preference for hydrogen abstraction from the methyl syn to the phenyl group. The magnitude of this selectivity depends on solvent polarity. On increasing the solvent polarity, a substantial increase in the amount of syn product occurs (Table 3). 

The reaction of singlet oxygen with cis and trans alkenes shows an unexpected regioselectivity for hydrogen abstraction on the large alkyl group of the double bond . 

The transition states in the hydrogen abstraction step, as seen earlier in Scheme 13, help to explain the observed change in regioselectivity. In a transition state where there is a strengthening of the C—O bond on the tertiary carbon, a release of the 1,3 non-bonded interactions between the oxygen and the L group with respect to the intermediate perepoxide occurs. Therefore, this transition state is expected to be lower in energy than the transition state where the non-bonded interactions exist. 

Adamantane can be nitrated with N02 in the presence of ozone (Kyodai nitration) to afford 1-nitroadamantane with high regioselectivity.301 Product formation is interpreted by hydrogen abstraction by NO3 followed by rapid trapping of the resulting admantyl radical by N02. 

Extension of the above oxidation studies to alkenes such as limonene gave a complex mixture of products that resulted from all possible ene reactions to the trisubstituted double bond (Fig. 30) [165], However, use of NaY zeolite as the microreactor and in the presence of a small amount of pyridine, the photosensitized oxidation of the alkenes is regioselective, yielding only the cis and trans products that result from hydrogen abstraction from the least hindered allylic carbon center. These studies illustrated that a microreactor can provide unprecedented opportunities to conduct selective oxidation of olefins. 

Rothenberg, G., Yatziv, Y. and Sasson, Y. (1998) Comparative autoxidation of 3-carene and a-pinene factors governing regioselective hydrogen abstraction reactions. Tetrahedron, 54, 593. 

The regioselectivity observed in Eq. (6) with diprotonated quinoxaline (reaction performed in 96% H2S04) is of particular interest. Whereas in monoprotonated quinoxaline (Eq. 1) the C-2 carbon atom has the lowest electron density and substitution occurs at C-2 position only, in diprotonated quinoxaline (Eq. 6) the electron density of the equivalent C-2 and C-3 is as low as that of the equivalent C-6 and C-7 carbon atoms (NMR and IN DO calculations) [9] and substitution occurs at both C-2 and C-6 positions. To minimize polysubstitution, the conversions in Eq. (6) were limited to about 50 % (selectivity, based on the reacted bases, is >90 % in any case). Another feature of Eq. (6) is the exceptional selectivity of hydrogen abstraction from the C-5 position of w-hexyl derivatives by the aminium radical i-Bu2NH+, generated from i-Bu2NHCl+ and Fe(II) [2]. 

Wagner, P.J., Pabon, R., Park, B.-S., Zand, A.R., and Ward, D.L. (1994) The regioselectivity of internal hydrogen abstraction by triplet o-tert-amylbenzophenone. Journal of the American Chemical Society, 116, 589-596. 

Two other examples of the synthesis of bicyclic compounds whose skeleton consists only of carbon atoms are shown in Sch. 24, each of them is instructive in its own way. Upon irradiation of the cyclohexane derivative 65, not the expected product from a hydrogen abstraction from the allylic position, but the bicyclo[3.3.1]heptane 67 was obtained by cyclization of the less stable biradical 66-B [57]. The reason for this is the exclusive hydrogen back-transfer of the more stable biradical and it underlines the importance of this process in regioselectivity phenomena of the Norrish-Yang reaction. 

The photoreactive succinimide moiety was introduced in various positions of the pyrane ring of suitable protected monosaccharides. Interestingly, the regioselectivity of the hydrogen abstraction by the excited succinimide depends sensitively on the ring conformation of the pyrane ring, which is in turn influenced by the substituents and protective groups used. 

Rather regioselective oxidation of steroids is possible by such remote photo-induced hydrogen abstractions 82>. 

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