Allyl system structure

Electron delocalization m allylic carbocations can be indicated using a dashed line to show the sharing of a pair of rr electrons by the three carbons The structural formula IS completed by placing a positive charge above the dashed line or by adding partial pos itive charges to the carbons at the end of the allylic system... 

If you see a double bond near the LG and you are not sure if it is a benzylic or allylic system, just draw the carbocation you would get and see if there are any resonance structures. 

Trifluoromethanesulfonates of alkyl and allylic alcohols can be prepared by reaction with trifluoromethanesulfonic anhydride in halogenated solvents in the presence of pyridine.3 Since the preparation of sulfonate esters does not disturb the C—O bond, problems of rearrangement or racemization do not arise in the ester formation step. However, sensitive sulfonate esters, such as allylic systems, may be subject to reversible ionization reactions, so appropriate precautions must be taken to ensure structural and stereochemical integrity. Tertiary alkyl sulfonates are neither as easily prepared nor as stable as those from primary and secondary alcohols. Under the standard preparative conditions, tertiary alcohols are likely to be converted to the corresponding alkene. 

Due to the poor nucleophilicity of aliphatic alkoxides, the intermolecular O-allylation of aliphatic alcohols has been performed, for the most part, using a large excess of structurally simple primary alcohols (Equation (37))165 and/or unsubstituted allylic substrates.166,167 When allylic systems activated with an electron-withdrawing substituent were employed, only a slight excess of the alcohol was necessary to achieve complete stereospecificity, as exemplified by Equation (38).168,169... 

A mechanism for the asymmetric induction for Pd-catalyzed allylic alkylations using chiral ligands such as 23 was proposed on the basis of stereochemical results and the X-ray structure of the intermediate Pd complex 24 <2004T2155>. The enantioselectivity of the alkylations, an example of which is shown in Equation (8), was rationalized by a conformational equilibrium that favored one of two possible 7i-allylpalladium complexes due to steric interference between the aryl substituent on the sulfmyl group of 24 and the phenyl of the 7i-allyl system. 

So a [1, 5] or larger rearrangements suprafacial shift is symmetry allowed but a [1, 3] shift would be structurally prohibited, because geometrically it will not be feasible. This would require the hydrogen to migrate to the opposite side of the allyl system and this is sterically difficult. 

Although both Sn2 and SnI mechanisms might be formnlated for such reactions, all the available evidence favours an Sn 1 process. This is rationalized in terms of formation of a favourable resonance-stabilized allylic cation by loss of the leaving gronp. In the majority of natnral prodnct structures, the nucleophile has attacked the allylic system on the same carbon that loses the diphosphate, bnt there are certainly examples of nncleophilic attack on the alternative tertiary carbon. 

Structural dependences of 4J(F1,F[) in propanic and allylic systems were recently studied by Barfield29 calculating, within the FPT-DFT approach, the FC term of such couplings. In propane Barfield carried out calculations for different values of the dihedral angles about the C3 C2 and C2 C3 bonds and for different values of the C3 C2 C3 internal angle. Using these calculated values he established a four-term trigonometric expression, which was then... 

Allylic systems have also provided fertile ground for investigation of ion-pair phenomena. Young, Winstein, and Goering established the importance of ion pairs in solvolysis of these compounds. They showed that ion pairs are responsible for the rearrangement of a,a-dimethylallyl chloride to y,y-dimethyl-allyl chloride (Equation 5.8).24 Goering s labeling methods have subsequently supplied a number of details about allylic ion-pair structure.25... 

In allylic systems, favorable overlap of the p orbitals of the n system should require a coplanar arrangement of the three sp2 carbons and their five substituent atoms evidence that such a structure is indeed preferred comes, for example, from proton magnetic resonance observations that demonstrate barriers to bond rotation in the isomeric dimethylallyl ions 21, 22, and 23. These ions form stereo-specifically from the three dimethylcyclopropyl chlorides (Section 12.2), and barriers to rotation about the partial double bonds are sufficiently high to prevent interconversion at low temperature. At — 10°C, 21, the least stable isomer,... 

As is readily noted from the results summarized in Table 8E.13, enantioselectivity is very sensitive to a variety of factors such as the nucleophile and the nature of the allylic system as well as the ligand used. As expected, the enantioselectivity varied greatly with the structure of the nucleophile. Higher enantioselectivities were consistently obtained from the reactions of 2-cyclopentenyl phenyl ether than from the corresponding reactions of 2-cyclohexenyl ether. The biphenyl-derived DiPHEMP (7a) proved to be more effective than closely related BINAP (4) for this reaction. 

Metal-n-allyl complexes are important in a number of stereoselective catalytic reactions and are therefore attractive for computational chemists (see also Chapter 13, Section 13.2). An empirical force field study based on the MM2 parameterization scheme aimed at predicting stereoselective nickel(0)-catalyzed cycloadditions was recently conducted 56. As in a similar study 57, where a force field for the structure optimization of palladium-allyl systems was developed, dummy atoms were needed to define the structural model. Based on the assumptions required to model a catalytic process, the results obtained have to be interpreted with caution. 

In MO theory a o and a n bond are both just two overlapping orbitals and a monosub-stituted bond is similar to an allyl system. Did you realize that 42 must have the same structure as the HOMO of an end (p. 29) ... 

A variety of new ligand designs and ligand combinations were used in attempts to mimic some properties of the ubiquitous bent metallocene environment at the early metal centers consequently, some of these systems were used in the further development of butadiene zirconium chemistry. The pyridine based chelate zirconium dichloride complex 43 cleanly formed the butadiene complex 44 upon treatment with butadiene-magnesium. Its structure shows that the C4H6 is arranged perpendicular to the chelate ligand plane. Complex 44 inserts one equivalent of an alkene or alkyne to form the metallacyclic 7i-allyl system 4545 (Scheme 13). 

The structure and chemical properties of metal-allyl compounds (ir-allylic, dynamic and a-allylic) which can be considered as models of a living polymer chain in butadiene polymerization have been studied. The polymerization of dienes proceeds only in dynamic allylic systems through the metal-ligand ir-bond in a-isomers. 

Let s take a closer look at the electronic structure of allylic systems, using the allyl radical as our example. One resonance form shows the radical electron on Cl, with a pi bond between C2 and C3. The other shows the radical electron on C3 and a pi bond between Cl and C2. These two resonance forms imply that there is half a pi bond between Cl and C2 and half a pi bond between C2 and C3, with the radical electron half on Cl and half on C3. 

Remember that no resonance form has an independent existence A compound has characteristics of all its resonance forms at the same time, but it does not resonate among them. The p orbitals of all three carbon atoms must be parallel to have simultaneous pi bonding overlap between Cl and C2 and between C2 and C3. The geometric structure of the allyl system is shown in Figure 15-10. The allyl cation, the allyl radical, and the allyl anion all have this same geometric structure, differing only in the number of pi electrons. 

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