Regioselectivity trisubstituted alkenes

The Pt- or Pd-catalyzed metallometallation (Sect VIL4) of alkynes with metal-metal bond-containing reagents shown in Scheme 20 provides a novel, if rather expensive, route to ay-l,2-dimetalloalkenes. Their Pd-catalyzed cross-coupUng produces regioselectively trisubstituted alkenes containing one metal group (Table 5). 

Stratakis, M., Nencka, R., Rabalakos, C., Adam, W. and Krebs, O. (2002). Thionin-sensitized intrazeolite photooxygenation of trisubstituted alkenes substituent effects on the regioselectivity as probed through isotopic labeling. J. Org. Chem. 67, 8758-8763... 

Regioselectivity with geminal dimethyl and diethyl trisubstituted alkenes... 

For a series of geminal dimethyl trisubstituted alkenes, the same trend of regioselectivity was earlier recognized by Thomas and Pawlak as well as Rautenstrauch and coworkers . By examining molecular models and assuming that the reaction is concerted, Thomas and coworkers proposed that the difference in regioselectivity could be due to the different conformations and steric repulsions in the transition states. 

However, the barrier to rotation does not always predict the regioselectivity of the ene reaction of O2 with alkenes. As shown latef, it is the non-bonded interactions in the isomeric transition states that control product formation and barriers to rotation are rather irrelevant. The calculated rotational barrier values, with the HF-STO-3G method, for the allylic methyls in a series of trisubstituted alkenes, as well as the experimentally observed ene regioselectivity of a series of selective substrates, are shown in Table 9. ... 

The regioselectivity for the hydrogen atom abstraction from each of two geminal methyl groups (twin or twix) in trisubstituted alkenes such as 34, 35, 39 and 122 (Table 17) was studied by specific deuterium labeling. Independent studies revealed that the cis effect selectivity found in solution no longer operates within the zeolite. As seen in Table 19, for the case of 124-127, the twin methyl group reactivity increases up to 14 times (see substrate 126) by zeolite confinement. 

Reactions of nitrile oxides with 1,1-disubstituted alkenes afford products in which the oxygen atom of the nitrile oxide gets attached to the most crowded carbon atom of the dipolarophile. This high regioselectivity does not seem to depend on the type of substituent present on the alkene (142-152). Some of the results cannot be satisfactorily interpreted on the basis of FMO theory (149,151). Both steric and electrostatic effects often counteract each other and contribute to the regioselectivity actually observed. With trisubstituted alkenes, the orientation of cycloaddition is apparently dominated by this phenomenon. The preference is for the more substituted carbon atom to end up at the 5-position of the heterocyclic product (153,154). However, cases of opposite regiodifferentiation are also found, in particular with donor-substituted alkenes (42,155-157) (Scheme 6.21). 

The calculated rotational barrier values for the allylic methyls, with the HF-STO-3G method [62], in a series of trisubstituted alkenes and the experimentally observed ene regioselectivity of a series of selective substrates are shown in Table 7. 

The hydroxyl group at the allylic position has a significant effect on the syn/anti methyl stereoselectivity [67,68] and the diastereoselectivity [63,64] of the photo-oxygenation ene reaction (see Sec. II.B). To assess the effect of the hydroxyl at the more remote homoallylic position, the reaction of O with the geminal dimethyl trisubstituted homoallylic alcohols (85, 86, 89) and the cis dis-ubstituted 90 was examined in nonpolar solvents [116], The regioselectivity trend was compared with that of the structurally similar trisubstituted alkenes (87, 88, 91) [105], The results are summarized in Table 12. 

In the reaction with geminal dimethyl trisubstituted alkene 104 (Scheme 27), the methylene hydrogen atoms showed little or negligible reactivity depending on the cation of the zeolite [140], These results demonstrate clearly that regioselectivity within zeolites is different to that found in solution. 

Aluminium and copper Lewis acids effect a regioselective [l,3]-rearrangement of allyl vinyl ethers in moderate to good yields (Scheme 30). The use of trisubstituted alkenes leads to depressed levels of Claisen products.53... 

These bulkier boranes enhance the regio selectivity of hydrobora tion of trisubstituted alkenes in particular and may also lead to high diastereoselectivity when there is a stereogenic centre next to the alkene. In this next examplej an allylic alcohol is hydroborated with thexyl borane. Oxidation reveals complete regioselectivity and a 9 1 stereoselectivity in favour of hydroboration on the same side as the OH group. 

Xfi-Dienes. The reagent undergoes a light-catalyzed regioselective addition to al-kenes, particularly mono-, 1,1-di-, and trisubstituted alkenes. The adducts undergo de-hydrobromination to a,p-unsaturated bromomethyl sulfones. On treatment with potassium... 

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