Allyl system configurations

Figure 29.7. Allyl system. Configuration of tt electrons in cation, free radical, and anion.

Allylic acetates and phosphates can be readily carbonylated.248 Carbonylation usually occurs at the less-substituted end of the allylic system and with inversion of configuration in cyclic systems. 

In addition, the palladium will preferentially position itself on the sterically less demanding face of an allyl system. Both stereoselectivities are neatly demonstrated by reacting either geometric isomer of 2-ethylidenenorpinane (1) with PdCh, where only a single ir-allylpalladium complex (2) that has a syn configuration is observed and has the Pd trans to the gem-dimethyl bridge (equation 2).32... 

Another example of a reaction leading to exclusive formation of one metal configuration is (+)-bis(7r-pinenyl)nickel (163), in which the Ni atom is bonded to two 7r-allyl systems. The combination of one 7r-pinenyl system with a Ni atom is a chiral entity, in which the metal is an asymmetric center. The opposite configuration at Ni would result should the... 

The nature of this species may be different in each case. In addition to the o—allyl— w-allyl equilibrium, the various possible configurations of the bis(allyl) system need to be considered (see Section IV,B). 

The scheme shown in Eq. (46) summarizes and explains mechanistically the results so far obtained. Two butadiene molecules may be supposed to react together to form a bis(7r-allyl) C8 chain bonded to the Ni-ligand moiety. This bis(7r-allyl) system exists in two configurations, (XLIII) and (XLIV), from which DVCB and COD are formed. The nature of the ligand in these intermediates seems to be relatively unimportant. 

Electrocyclic Reactions.—Peyerimhoff, Buenker, and co-workers have carried out very detailed studies of the electrocyclic transformations159 between cyclic and open-chain hydrocarbons. The calculations employ a large GTO set of s- and p-type basis functions. In every study, the necessity of including limited configuration interaction was carefully investigated. The prototype electrocyclic transformation of cyclobutene to cis-butadiene via the thermochemical process has been studied in detail.160-161 The same authors also give an analysis of the qualitative theories for such reactions based on their ab initio calculations.163 A similar study of the electrocyclic transformations of cyclopropyl and allyl systems has also been made.163... 

Allyl sulfides are readily lithiated, and give species 113 which are configurationally stable about the allyl system at <0 °C.1... 

Extended allylic systems can be formed by deprotonation of dienes such as 142,144 and 147 with s-BuLi. The dienyllithiums 143, 146 and 148 adopt an extended W conformation, and react to give 1,3-butadienes 146 and 149 with retention of double bond geometry.19 The equivalent species 150 formed by deprotonation with LiCKOR superbases (see section 2.6) adopt a U -shaped configuration. 

A second molecule of the lithiated sulfide 62 is then used to introduce the next allylic unit, and reductive lithiation of the product 67 gives allyllithium 68. On this occasion, reaction at the less substituted terminus of the allyl system is required, and this can be achieved by transmetallation (at low temperature, to preserve the cis configuration) to the allylcerium 69. Reaction with formaldehyde followed by acetylation gives the pheromone 60 contaminated by less than 2% of the undesired E isomer. 

The reaction is stereoselective, 1,4-hexadiene being mainly obtained in the E configuration the Z-isomer is present only to a small extent and its formation can be further decreased by donor ligands such as Bu3P=0 and (Me2N)3P=0. The reaction is also regioselective the positional isomer CH2=CHCH(CH3)CH=CH2, derived from ethylene insertion into the more substituted carbon-rhodium bond of the allylic system, is present to less than 1 % when ethanol is in large excess. 

Ruthenium-catalyzed allylic alkylation falls between known catalysis with respect to regioselectivity and chirality of products. Like Pd, Mo, and W-but unlike Rh regioselectivity is not highly dependent on the nature of the starting carbonate. However, unlike Pd and Mo, but similar to Rh and W, substitution of the chiral substrate occurs with high retention of configuration, (equation 75). Hence the Ru system compliments other metal-catalyzed allylic systems well. ... 

Electron-deficient allylic systems, such as -/-hydroxy- or y-alkoxy-a,/J-unsalurated esters 1R-23, show a peculiar behavior. ( )-Alkenes afford the normal sense of diastereoface differentiation 16d 29 which dramatically increases in the presence of an additional silyloxy-substituted stereocenter at the <5 position22. On the contrary, (Z)-configurated alkenes give low or unpredictable selectivities8 20 37. 

Substitution reactions have been reported for some ring A allylic systems in contact with neutral activated alumina. The reaction of 3j0 benzoyloxy-5a Cholest i-ene (17) occurs by alkyhoxygen fission [20] and yields a mixture of the epimeric 3-hydroxycholest-i-enes (18) and cholesta-i,3-diene (19) [21], Reaction of 3j5-chlorocholest-i-ene 20) gives a similar mixture [227, suggesting a unimolecular reaction, with the nucleophile being supplied by the surface of the alumina. While some retention of configuration is noted, considerable inversion does... 

The molecular orbitals of the allyl system are formed by the overlap of three atomic p orbitals. Because there is an odd number of atomic orbitals, one of the molecular orbitals is a nonbonding orbital, whose energy is comparable to that of the isolated p orbitals from which it was derived. Note that if there were degenerate molecular orbitals in the allyl system, the electronic configurations of various allyl species would be different. For example, if P2 3 for the allyl system had identical energy levels, the allyl anion would have two unpaired electrons. 

In accordance with this model one finds diastereoselectively anti products on reaction of aldehydes with ( )-allyl compounds, whereas allyl systems with the (Z)-configuration give mainly syn products and it is even possible to effect asymmetric induction. As the double bond of the product can be oxidatively cleaved to a CW3 group, the reaction can be regarded as a stereoselective aldol reaction, an aspect which explains the widespread interest in this type of reaction. With heterosubstituted allylic anions it is sometimes possible to effect predominantly y-attack with different electrophiles by the choice of the heteroatom.2 For instance it is well known that with sulfur substituents like —SR, —SOR or —SOjR the a-attack dominates, but doubly lithiated allenethiol possesses high y-reactivity and can be used as a homoenolate anion equivalent in reaction with electrophiles such as alkyl halides (Scheme 7). ... 

On the other hand, a thermal [1,3] alkyl shift is allowed because the alkyl group can adopt an antarafacial orientation in the TS. The two lobes of the C(p) orbital of the migrating atom bond to opposite ends of the allylic system in the TS, and the configuration of the migrating group inverts. 

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