Trans terminal substituents

Alkylidenemalonates were found to be excellent acceptor molecules (111). Reactions of lithium ylides with dimethyl alkylidenemalonates at —78 °C in THF in the presence of f-BuOH were diastereoselective for all the substituents R except methyl, producing Michael adducts as single diastereomers (Scheme 11.24). The only exception was dimethyl ethylidenemalonate, which produces an 86 14 mixture of diastereomeric adducts, the minor diastereomer being syn-adduct. Since dimethyl alkylidenemalonates bear two geminal methoxycarbonyl moieties, one is cis to the terminal substituent R and the other trans, so of these ester substituents can participate in chelate formation in the transition state. When the terminal substituent R is small, there is a chance for the syn-adduct to be produced, which... 

The stereochemistry of 1,3-dipolar cycloadditions of azomethine ylides with alkenes is more complex. In this reaction, up to four new chiral centers can be formed and up to eight different diastereomers may be obtained (Scheme 12.4). There are three different types of diastereoselectivity to be considered, of which the two are connected. First, the relative geometry of the terminal substituents of the azomethine ylide determine whether the products have 2,5-cis or 2,5-trans conformation. Most frequently the azomethine ylide exists in one preferred configuration or it shifts between two different forms. The addition process can proceed in either an endo or an exo fashion, but the possible ( ,Z) interconversion of the azomethine ylide confuses these terms to some extent. The endo-isomers obtained from the ( , )-azomethine ylide are identical to the exo-isomers obtained from the (Z,Z)-isomer. Finally, the azomethine ylide can add to either face of the alkene, which is described as diastereofacial selectivity if one or both of the substrates are chiral or as enantioselectivity if the substrates are achiral. 

The preferred s-trans conformation for civ-1 has been deduced from spectroscopic evidence but the s-civ conformation required for Cope rearrangement is probably disfavored only by between 2 and 4 keal/mol756. Terminal substituents on the double bonds retard the rate. 

The addition of (TMS)3SiH to a number of monosubstituted acetylenes has also been studied in some detail. These reactions are highly regioselective (anti-Markovnikov) and give terminal (TMSlsSi-substituted alkenes in good yields. High cis or trans stereoselectivity is also observed, depending on the nature of the substituents at the acetylenic moiety. For example, the reaction of the alkynes 23 and 24 with (TMSlsSiH, initiated either by EtsB at room temperature (method or by thermal decomposition of di-ferf-butyl peroxide at 160 °C... 

Nishiyama et al. introduced a new catalyst, the chiral tr<2 i -RuCl2(Pybox-i-Pr)(ethylene) complex (91), which showed for the first time both enantio- and diastereoselectivity (trans-selectivity) at excellent levels in the reactions of terminal olefins (Scheme 66).251-253 With 4-substituted Ru(Pybox-i-Pr) complexes (92), they studied the substituent effect on enantioselectivity... 

It is observed that insertion into a zirconacyclopentene 163, which is not a-substituted on either the alkyl and alkenyl side of the zirconium, shows only a 2.2 1 selectivity in favor of the alkyl side. Further steric hindrance of approach to the alkyl side by the use of a terminally substituted trans-alkene in the co-cyclization to form 164 leads to complete selectivity in favor of insertion into the alkenyl side. However, insertion into the zirconacycle 165 derived from a cyclic alkene surprisingly gives complete selectivity in favor of insertion into the alkyl side. In the proposed mechanism of insertion, attack of a carbenoid on the zirconium atom to form an ate complex must occur in the same plane as the C—Zr—C atoms (lateral attack 171 Fig. 3.3) [87,88]. It is not surprising that an a-alkenyl substituent, which lies precisely in that plane, has such a pronounced effect. The difference between 164 and 165 may also have a steric basis (Fig. 3.3). The alkyl substituent in 164 lies in the lateral attack plane (as illustrated by 172), whereas in 165 it lies well out of the plane (as illustrated by 173). However, the difference between 165 and 163 cannot be attributed to steric factors 165 is more hindered on the alkyl side. A similar pattern is observed for insertion into zirconacyclopentanes 167 and 168, where insertion into the more hindered side is observed for the former. In the zirconacycles 169 and 170, where the extra substituent is (3 to the zirconium, insertion is remarkably selective in favor of the somewhat more hindered side. 

The stereoselectivity can be rationalized by considering nonbonding interactions between R and the pyrrolidinyl substituents in TS 3, which favor TS 4 (Figure 7). Therefore, a decrease of the selectivity is observed for terminal or trans substituted oxiranes where R is a hydrogen. 

This conformational preference in conjunction with the preference of the terminal amides to exist in an i-trans conformation constrains the system such that the terminal anthranilamide substituents are positioned above and below the plane of the molecule, resulting in a local helical structure. The helical antipodes experience a... 

The trans compound, in contrast, reacts at much higher reaction temperature (190°C) and undergoes first a radical isomerization to give the cis isomer, which then rearranges to yield 1,4-cycloheptadiene.240 Substituents at the terminal vinylic carbons considerably decrease the rate of rearrangement235,241... 

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