Regioselectivity conjugated diene reactions

The coordination of the dienophile to a Lewis acid (in the calculations a proton was used as the Lewis acid) leads also to an increase in regioselectivity. The re-gioselectivity of reactions of electron-rich, or conjugated dienes, with electron-deficient dienophiles is also controlled hy the diene HOMO-dienophile LUMO interaction. From Fig. 8.2 it appears that the difference in magnitudes of the LUMO coefficients at carhon atoms 1 and 2 of acrolein (Ci -C2 = 0.20) is smaller than the same difference for protonated acrolein (Ci -C2 = 0.30-0.43) so that the reaction of the latter should he considerable more regioselective than the former in accordance with the experimental results [3]. 

Grubbs et al. reported that the ruthenium-catalyzed RCM of a conjugated diene proceeds in such a way that the less hindered olefin moieties participate in the reaction. Consequently, RCM of 115 gives exo-methylene compound 116, and not exo-vinyl compound 117 (Scheme 24) [105]. This regioselectivity is complementary to that observed for enyne metathesis of 118, which gives exclusively 117 (Scheme 24) [106a]. 

The chemo- and regioselectivities of hydroformylation reactions of open chain, conjugated dienes using the usual catalyst are, in most cases, rather low [36]. The rhodium/ mesitylene co-condensate (catalyst A), in the presence of bis(diphenylphosphino)ethane, DPPE, catalyses the hydroformylation of 1,3-butadiene, isoprene, and E,Z)-, 3-pentadiene to the corresponding p,y-unsaturated monoaldehydes, with unusually high chemo- and regioselectivities (Scheme 17). 

Asymmetric monoepoxidation of conjugated dienes has been accomplished via (salen)Mn(III)-catalyzed [salen = A,Ar/-bis(salicylidene)ethylenediamine] oxidation. The reaction exhibits regioselectivity for attack at cis double bonds of cA.traws-conjugated dienes, and affords trans epoxides as the major products from cis olefins33. Thus, diene 14 gave optically active fraws-vinylepoxide 15 as the major product with 87% ee as shown in equation 16. 

Scheme 4.1. Regioselectivity in conjugate addition reactions to acceptor-substituted dienes.

Wilkinson s catalyst [RhCl(PPh3)3], a standard catalyst for this reaction, is reported to give lower yields with less regioselectivity in these reactions. Conjugated dienes gave mixtures of 1,4- and 1,2-addition products in the presence of rhodium-NHC systems, whereas [RhCl(PPh3)3] leads to selective 1,4-addition. 

The reaction has been applied successfully to the synthesis of a precursor of provitamin D. 520, which has a homoannular conjugated diene in the B ring[340]. Treatment of the 7 -carbonate 518 with Pd catalyst at 40 °C afforded the 5.7-diene 520 regioselectively in good yield. No heteroannular diene 521... 

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