Allyl substituent effects

Group 10 TM complexes (110) bearing allyl ligand were synthesized by breaking the corresponding dimer [MCI (R R -allyl) (2 (109) with two equivalents of free NHC ligand (equation 15). Aryloxy dimer [Ni(allyl)(OAr)]2 (Ar = 2,6-di-i5o-propylphenyl) has also been employed for the preparation of NHC-allyl Ni complexes.Since the [(NHC)PdCl(allyl)] complexes exhibited an excellent activity in palladium-catalyzed cross-coupling reactions,this series was extensively studied with special attention to allyl substituent effects. 

The transition state involves six partially delocalized electrons being transformed from one 1,5-diene system to another. The transition state could range in character from a 1,4-diradical to two nearly independent allyl radicals, depending on whether bond making or bond breaking is more advanced. The general framework for understanding the substituent effects is that the reactions are concerted with a relatively late transition state with well-developed C(l)—C(6) bonds. 

This section will describe the Friedel-Crafts alkylation reactions of aromatic hydrocarbons with alkenylchlorosilanes containing short chain alkenyl groups such as allyl and vinyl. The reaction will be discussed in terms of the substituent effect on silicon and the arene rings. 

These substituent effects are due to the stabilization of the carbocations that result from protonation at the center carbon. Even if allylic conjugation is not important, the aryl and alkyl substituents make the terminal carbocation more stable than the alternative, a secondary vinyl cation. 

The effects of the allylic substituent, the alkene geometry, and the diene substitution as well as the influence of resident stereogenic centers incorporated in the tether connecting the 1,3-diene and the alkene subunits were totally investigated. This process has been applied to the reactions of more elaborated systems including heterocyclic structures,367 and the total synthesis of (—)-gibboside.368... 

Another advantage of this approach is that it allows the possibility of introducing substituent effects. Substituents which alter the energies of allyl n orbitals relative to the S orbital will increase the chances of interaction either with /[ or with /3. 

Similarly to the triphenylmethyl system, captodative-substituted 1,5-hexa-dienes, which can be cleaved thermally in solution into the corresponding substituted allyl radicals [15], dissociate more easily than dicaptor-substituted systems (Van Hoecke et al., 1986). Since ground-state and radical substituent effects cannot be separated cleanly, not only because of electronic but also because of steric effects, a conclusive answer cannot be provided. 

The study of substituted allyl radicals (Sustmann and Brandes, 1976 Sustmann and Trill, 1974 Sustmann et al., 1972, 1977), where pronounced substituent effects were found as compared to the barrier in the parent system (Korth et al., 1981), initiated a study of the rotational barrier in a captodative-substituted allyl radical [32]/[33] (Korth et al., 1984). The concept behind these studies is derived from the stabilization of free radicals by delocalization of the unpaired spin (see, for instance, Walton, 1984). The... 

The experimental result seems to support this model. Table 11 lists values for rotational barriers in some allyl radicals (Sustmann, 1986). It includes the rotational barrier in the isomeric 1-cyano-l-methoxyallyl radicals [32]/ [33] (Korth et al., 1984). In order to see whether the magnitude of the rotational barriers discloses a special captodative effect it is necessary to compare the monocaptor and donor-substituted radicals with disubstituted analogues. As is expected on the basis of the general influence of substituents on radical centres, both captor and donor substituents lower the rotational barrier, the captor substituent to a greater extent. Disubstitution by the same substituent, i.e. dicaptor- and didonor-substituted systems, do not even show additivity in the reduction of the rotational barrier. This phenomenon appears to be a general one and has led to the conclusion that additivity of substituent effects is already a manifestation of a special behaviour, viz., of a captodative effect. The barrier in the 1-cyano-l-methoxyallyl radicals [32]/... 

An error-propagation analysis allows the conclusion that the captodative substituent effect on the rotational barrier in this allyl radical is at least additive and perhaps slightly greater. 

Radical cyclization of oximes or oxime ethers having allylic substituents or an aldehyde group to tetrahydropyrrole derivatives was described Thus, Sm -induced 5-exo-trig radical cyclization of oxime ethers containing a formyl group was found to be particularly effective for the preparation of cyclic trans-wimo alcohols. For example, oxime 96 in the system SmE/THF/f-BuOH at 25 °C or —78°C afforded pyrrolidin-3-ols 97 and 98 in a ratio 3 2 or 9 1 (equation 41) . Cyclization of oxime ether 99 in the... 

Molander has developed effective protocols for the cyclization/hydrosilylation of 1,6-enynes catalyzed by lanthanide metallocene complexes/ For example, reaction of cyclohexyl-substituted 1,6-enyne 15a with phenylsilane catalyzed by Cp 2YMe(THF) in cyclohexane at room temperature for 2h gave silylated alkylidene cyclopentane 16a as a 6.5 1 mixture of trans. cis isomers (Table 5, entry 1). The diastereoselectivity of the reaction depended strongly on the nature of the allylic substituent. For example, yttrium-catalyzed cyclization/ hydrosilylation of the ethyl-substituted enyne 15b gave silylated cyclopentane 16b in 88% yield as a single diastereomer (Table 5, entry 2). 

The effect of the nature of the electrophile on the stereoselectivity of reactions with substrates containing a terminal alkene and an allylic substituent is dramatically illustrated by some recent results with palladium electrophiles.124 Cyclizations of 3-methyl- or 3-phenyl-5-hydroxyalkenes with palladium catalysts proceed with high selectivity (>9 1) for the 2,3-trans isomer (equation 41).50-124 It is suggested that the steric interactions of the palladium-alkene complex affects the stereochemistry of these cyclizations. In some related cyclizations to form tetrahydropyran products (equation 42 and Table 10), reaction with iodine in the presence of sodium bicarbonate gives a different major diastereomer from cyclization with mercury(II) trifluoroacetate or palladium chloride.123... 

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