Anti allyl structure

Despite the success in the 1,3-diphenylallyl system, use of many of these ligands in the alkylation of 1,3-dialkylallyl system as Equation 8E.4 has produced mixed results, as summarized in Table 8E.5. With the phosphinooxazoline-type ligands, good selectivities (>90% ee) are still obtained from the reactions of substrates possessing bulky allylic substituents such as isopropyl groups (entries 8-10), but smaller substrates such as 1,3-dimethylallyl derivatives give only a modest level of enantioselectivities (entries 1 -7). The disparity between these results appears to be sterically derived as the enhanced preference of syn versus anti orientation in the 7t-allyl structure by the bulky phenyl or isopropyl groups may not be present with the smaller substrates. 

A conjugated diene can coordinate to a transition metal by only one double bond, as an s-trans-r 2 ligand, or with the two double bonds, as an s-cis-tf or as an s-trans-rf ligand [188]. A coordinated transoid monomer (as an s-trans-rj2 or an s-trans-rf ligand) is inserted into the metal-carbon bond, acquiring the syn-/ 3-allylic structure of the growing chain end. On the other hand, when a cisoid monomer coordinates to a metal (as an s-cis-rj4 ligand), an anti-t]2-allylic structure is formed. 

The CT-allyl intermediate may be transformed to a ir-allyl structure with either the syn or anti configuration (relative to the methynyl hydrogen) as in equation (31), see also Scheme 2. The union with a second hydrogen leads to adsorbed alkenes 1-butene and either trans-2-buXtnt from the iyn-allyl intermediate or cw-2-butene from the anti form (Scheme 2). The desorption of the alkene competes with the further addition of hydrogen to form the alkane (equation 32). The reaction of a cr-allyl structure with hydrogen can yield the unbound alkene directly. The selectivity may depend upon the relative importance of these competitive reactions, which are likely to be a function of the metal as well as the reaction conditions. 

Insertion of the monomer, bonded to the metal in rf-cis fashion, into the metal-polymer bond forms a new 7t-allyl polymer end with the substituent at the anti-position. Successive insertion of the new monomer with if-cis coordination would produce cis- 1,4-polybutadiene. Insertion of if-trans-co-ordinated monomer into the metal-polymer bond leads to trans-1,4-polybu-tadiene via syn zr-allyl intermediates. The above anti Tt-allyl polymer end is often equilibrated with the thermodynamically more favorable syn zr-allyl structure via n-a-n rearrangement. Thus, the ratio of cis-1,4 and trans-1,4 repeating units of the polymer produced depends on the relative rates of the two reactions C-C bond formation between the monomer and the polymer end, and anti to syn isomerization of the zr-allyl end of the growing polymer. If the anti-syn isomerization of the anti zr-allyl polymer end occurs more rapidly than the insertion of a new monomer, the polymer with trans-1,4 units is formed even from 7j4-ds-coordinated monomer. The polymerization catalyzed by Ti, Co, or Ni complexes shows high cis-1,4 selectivity, while that with low monomer concentration results in increase of the trans content of... 

MAJOR APPLICATIONS With one chiral center, l,2-p>olybutadiene can exist in the amorphous atactic form and two crystalline forms isotactic and syndiotactic. In the formation of 1,2-polybutadiene, it is believed that the syn p-allyl form yields the syndiotactic structure, while the anti p-allyl form yields the isotactic structures. The equilibrium mixture of syn and anti p-allyl structures yields heterotactic polybutadiene. At present, the two stereo-isomers that are most used commercially are the s5mdiotactic and heterotactic structures. ... 

The recombinant enzyme has been functionally expressed to greatly facilitate the understanding of structure-function relationships. This cyclase catalyzes initial syn isomerization of FPP to NPP, with rotation about the C2-C3 bond and anti-allylic attack leading to C6-C1 bond closure to generate the bisabolyl cation. This sequence is followed by C11-C7 closure, a 1,4-hydride shift, two suc-... 

Various mechanisms for the control of the cis linkage in the propagation step are discussed [144,145]. Allyl compounds can occur in syn or anti form [Structures (13-16)], from which double bonds with trans or cis configuration are formed [146,147], respectively. Solvents or cocatalysts as ligands are of great importance for the equilibration. 

Intramolecular cycloadditions of substrates with a cleavable tether have also been realized. Thus esters (37a-37d) provided the structurally interesting tricyclic lactones (38-43). It is interesting to note that the cyclododecenyl system (w = 7) proceeded at room temperature whereas all others required refluxing dioxane. In each case, the stereoselectivity with respect to the tether was excellent. As expected, the cyclohexenyl (n=l) and cycloheptenyl (n = 2) gave the syn adducts (38) and (39) almost exclusively. On the other hand, the cyclooctenyl (n = 3) and cyclododecenyl (n = 7) systems favored the anti adducts (41) and (42) instead. The formation of the endocyclic isomer (39, n=l) in the cyclohexenyl case can be explained by the isomerization of the initial adduct (44), which can not cyclize due to ring-strain, to the other 7t-allyl-Pd intermediate (45) which then ring-closes to (39) (Scheme 2.13) [20]. While the yields may not be spectacular, it is still remarkable that these reactions proceeded as well as they did since the substrates do contain another allylic ester moiety which is known to undergo ionization in the presence of the same palladium catalyst. 

Intramolecular reactions can also occur between carbonyl groups and allylic silanes. These reactions frequently show good stereoselectivity. For example, 7 cyclizes primarily to 8 with 4% of 9 as a by-product. The two other possible stereoisomers are not observed.98 The stereoselectivity is attributed to a preference for TS 7A over TS 7B. These are both synclinal structures but differ stereoelectronically. In 7A, the electron flow is approximately anti parallel, whereas in 7B it is skewed. It was suggested that this difference may be the origin of the stereoselectivity. 

With an increase of conversion, the enantiopurity of unreacted (S)-substrate increases and the diastereoselectivity of the product decreases. Using Ru-((S)-binap)(OAc)2, unreacted (S)-substrate was obtained in more than 99% ee and a 49 1 mixture of anti-product (37% ee (2R,iR)) at 76% conversion with a higher kR ks ratio of 16 1 [46]. In the case of a racemic cyclic allyl alcohol 24, high enantiopurity of the unreacted alcohol was obtained using Ru-binap catalyst with a high kR ks ratio of more than 70 1 [Eq. (16)] [46]. In these two cases, the transition state structure is considered to be different since the sense of dia-stereoface selection with the (S)- or the (R)-catalysts is opposite if a similar OH/ C=C bond spatial relationship is assumed. 

Reactions of aldehydes with complexes 13—17 provide optically active homoallylic alcohols. The enantioselectivities proved to be modest for 13—16 (20—45% ee). In contrast, they are very high (> 94% ee) for the (ansa-bis(indenyl))(r]3-allyl)titanium complex 17 [32], irrespective of the aldehyde structure, but only for the major anti diastereomers, the syn diastereomers exhibiting a lower level of ee (13—46% ee). Complex 17 also gives high chiral induction (> 94% ee) in the reaction with C02 [32], in contrast to complex 12 (R = Me 11 % ee R = H 19% ee) [15]. Although the aforementioned studies of enan-... 

Figure 16. Optimized structures of the endo (lOa-anf and exo (IQb-anti) r -silyll allyl complexes with 4-(dimethylamino)-styrene as a substrate. Highlighted in (a) is the steric contact between the hydrogen of the benzylic carbon of the substrate and a hydrogen of the mesityl substituent 12 63 A) for the endo-anti intermediate, lOa-on/i. Highlighted in (b) is the steric contact between on hydrogen of the methyl group of the substrate and a hydrogen of the mesityl suhstituent 12 47 A) for the exo-anti silyl-allyl intermediate, Wb-anti. Hydrogen atoms are not shown except those whose interactions are highlighted.

Further structural differentiation (in the allyl group anti/meso vs. syn position) ... 

The present volume comprises 17 chapters, written by 27 authors from 11 countries, and deals with theoretical aspects and structural chemistry of peroxy compounds, with their thermochemistry, O NMR spectra and analysis, extensively with synthesis of cyclic peroxides and with the uses of peroxides in synthesis, and with peroxides in biological systems. Heterocyclic peroxides, containing silicon, germanium, sulfur and phosphorus, as well as transition metal peroxides are treated in several chapters. Special chapters deal with allylic peroxides, advances in the chemistry of dioxiranes and dioxetanes, and chemiluminescence of peroxide and with polar effects of their decomposition. A chapter on anti-malarial and anti-tumor peroxides, a hot topic in recent research of peroxides, closes the book. 

Classically, cation (3) might be anticipated to be considerably stabilized by the neighboring oxygen atom and double bond on the other hand, its 47r-electron system would suggest anti-aromaticity. The isodesmic calculations showed it to be about 200 kJ mol-1 less stable than the model cation (19b) and about 40 kJ mol-1 less stable than the allyl cation. Thus it appears that it would have an anti-aromatic electronic structure, if it was formed (80TL1807). 

It was later found that the anti selectivities could be greatly improved by using sUyl ethers and the more reactive Shi s reagent (equation 68). The transition structure in which the C—O bond eclipses the C=C bond was proposed to account for the sense of induction and is based on the work of Gung and coworkers . It should also be mentioned that Z-aUcyl-substituted allylic silyl ethers led to the anti isomer as well, and their Z-aryl-substituted analogues led to the syn isomer. 

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