Allyl system cyclic

The formation of 1-and 2-aIkenes can be understood by the following mechanism. In the presence of formate anion, the 7r-allylpalladium complex 572 is converted into the 7r-allylpalladium formate 573. The most interesting feature is the attack of the hydride from formate to the more substituted side of the (T-allylic system by the cyclic mechanism shown by 574 to form the 1-alkene 575[367]. The decarboxylation and hydride transfer should be a concerted... 

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 the titanosilicate system, cyclic voltametric measurements had indicated (Section III.D) that the electron density at the tripodal sites is higher than at the tetrapodal sites. Hence, by analogy with the chromium and manganese complexes, we may expect the tripodal sites to favor hydrogen abstraction and allylic CH oxidation, although electron transfer and epoxidation occur preferentially on the tetrapodal sites. 

According to this mechanism, the first formed ion pair is 19a. Owing to dispersal of charge in the allylic system, the bond between halogen and C(2) is weakened so that an open carbenium ion (19c) readily forms, allowing for the possibility of front-side attack by the anion with the resulting formation of syn 1,2-adducts. This intermediate explains the formation of the cis-],2-adducts by chlorine addition to cyclic systems. However, syn 1,2-dichlorides can also result from linear dienes by rotation around the C(l)—C(2) bond in 19c to produce 19d, followed by back-side attack by the anion with respect to its position in 19d. Syn 1,4-adducts should instead arise by attack of the anion on C(4) in either 19a, 19c or 19d. Formation of anti dichlorides (1,2- or 1,4-) can only occur when there is appreciable translocation in the ion pair 19a to give 19b. Attack by the anion at C(2) in 19b yields anti 1,2-dichloride and attack at C(4) yields anti 1,4-dichloride. 

A number of groups have also studied alkylation in cyclic allyl systems. Selected examples of reations using 3-acetoxycyclohexene 581 are listed in Table 8.36 (Scheme Although the proline-based PhosOx ligands 545 did... 

These compounds, (2) and (3), are of theoretical interest, since they have cyclic 7r-orbital systems, and ab initio MO calculations have been performed to estimate what degree of stabilization may result. This was done by calculating the AE for their formation by an isodesmic reaction with a hypothetical cyclobutanone enol ion (19a,b), which also has a four-membered ring with an allylic system conjugated to an oxygen atom. 

In the presence of triethylamine, decomposition of the ir-allylic complexes to conjugated dienes may occur, particularly when electron-withdrawing substituents are present on a methyl group in the 1- or 3-position of the iT-allyl system.31 Cyclic alkenes and vinylpalladium chlorides also yield iT-allylic complexes in the absence of an amine. If a tertiary amine is present, however, l,4-dienes are obtained (equation 1l).32... 

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... 

The intermediate and key species proposed for the reaction in Scheme 3.2c are hypervalent silicates based on the silicon NMR spectra of (Z)-crotyltrichlorosilane in DMF. This hypervalent silicate has sufficient Lewis acidity based on the electron-withdrawing chlorine groups as well as nucleophilicity due to electron donation from the hypervalent silicon atom to the allyl systems, which enables the reaction to proceed smoothly. Thus, the high levels of diastereoselectivity can be explained by a six-membered cyclic transition state (Scheme 3.2d). 

The six overlapping p orbitals create a cyclic system of molecular orbitals. Cyclic systems of molecular orbitals differ from linear systems such as buta-1,3-diene and the allyl system. A two-dimensional cyclic system requires two-dimensional MOs, with the possibility of two distinct MOs having the same energy. We can still follow the same principles in developing a molecular orbital representation for benzene, however. 

X-Substituted Allyl Anions. Allyl anions with alkyl substituents almost always react with carbonyl electrophiles at the more substituted a position, as in the reaction of the prenyl Grignard reagent with aldehydes to give the product 4.39, presumably because the metal attaches itself preferentially to the less-substituted end of the allyl system and then delivers the electrophile in a six-membered transition structure 4.38. In contrast, alkylation of a similar anion with an alkyl halide gives mainly the product 4.40 of y attack, which is normal for an X-substituted allyl anion when a cyclic transition structure is not involved. 

Cleavage of allylic alcohols. The system selectively cleaves the double bond and the adjacent bond bearing the hydroxyl group of acyclic and aromatic allylic alcohols. Cyclic allylic alcohols are oxidized in low yield to dicarboxylic acids. 

Methyl substitution on the allylic system strongly affects the symanti ratio of adducts (Table 27). Both cyclic and acyclic transition states have been proposed to account for these observations. In general, (Z)-allylic alcohols tend to give mainly syn adducts, and vice-versa. 

Sodium cyanoborohydride (NaBHsCN) or tetrabutylammonium cyanoborohydride in acidic methanol or acidic HMPA reduces a,p-unsaturated aldehydes and ketones to the corresponding allylic alcohols. This system is limited to enones in which the double bond is not further conjugated, in which case the allylic hydrocarbon is formed in substantial amounts. Thus, reduction of chalcone gives mainly 1,3-di-phenylpropene (48%) as well as 26% of the allylic ether. Cyclic enones are also not good substrates, as competing 1,4-addition gives large fractions of saturated alcohols. ... 

The regio- and stereo-chemical outcome of reductions of cyclic allylic systems is controlled by more complex factors. Reduction of the allyl bromides (6) and (7) occurs exclusively by syn 7-attack (Scheme 9), but other structural features influence the reaction. For example, where the allylic double bond is in a thermodynamically stable position, rearrangement does not normally occur. The reaction has also been studied in complex carbohydrate alkenes. ... 

In the allyl system (Fig. 29.7) the third and fourth electrons go into a non-bonding orbital, whereas here they go into antibonding orbitals. As a result, the cyclopropenyl free radical and anion are less stable than their open-chain counterparts. For the cyclo-propeny] anion in particular, with two electrons in antibonding orbitals, simple calculations indicate no net stabilization due to delocalization, that is, zero resonance energy. Some calculations indicate that the molecule is actually less stable than if there were no conjugation at all. Such cyclic molecules, in which delocalization actually leads to destabilization, are not just non-aromatic they are u/i/iaromatic. 

Heteroatom to hydrogen transpositions in allylic systems (equation 43) can be observed in different reactions, namely Ae protonation of allyl metal compounds, in reductions of allyl heterocompounds and in retro-ene reactions. Except for the last reaction, which proceeds by a cyclic transition state, the problem of regioselectivity restricts the synthetic value of such reactions and it is only in specific cases that this can be overcome. 

According to this mechanism, the first formed ion pair is 19a. Owing to dispersal of charge in the allylic system, the bond between halogen and C(2) is weakened so that an open carbenium ion (19c) readily forms, allowing for the possibility of front-side attack by the anion with the resulting formation of syn 1,2-adducts. This intermediate explains the formation of the cw-1,2-adducts by chlorine addition to cyclic systems. However, syn... 

The reaction may occur through a six-membered cyclic transition state 12 that directs the electrophile to the other end of the allylic system. This is bad enough, but worse is to come. 

From the constitutional point of view, the employment of formic acid leads to interesting results, since hydride is site selectively transferred to the more hindered position of the allylic sulfone (Eq. 45).89 Thus, a hydride equivalent generated from formic acid is exceptional in this respect. It is proposed83 that decarboxylation and hydride transfer is a concerted process in which the hydride site selectively attacks the more substituted (more electropositive) side of the allylic system in a cyclic mechanism (SNi transfer of hydride, Scheme 1). 

Trost and coworkers [911, 1066] have examined the asymmetric allylation of sodium malonate with achiral cyclic allylic systems that lack a regioselectivity problem. Enantioselection occurs during the formation of the 7t-allyl complex. 

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