Conjugated alkenes allylic systems

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... 

The problem with forming allyl metal derivatives by exchange of zerovalent metal with halide ions is that we often cannot be sure where the halide ion is and never sure where the metal is because of rapid [1,3] shifts, cf. compounds 21 and 23. If the allylic system has a substituent that chelates with the metal then only one derivative is likely to be formed. We need a halide at one end of the allyl group and a chelating substituent at the other. Just such compounds can be made by acylation of acrolein 72 in the presence of zinc chloride. No [1,3] shift of Br will occur as the alkene much prefers to be disubstituted and conjugated with oxygen 151 rather than unconjugated and terminal 152. 

More highly substituted alkenes react faster (trisubstituted > disubstituted > monosubstituted), as illustrated by the reaction of 4,8-dimethylnona-l,7-diene (350) to give 351. Allylic systems conjugated to a carbonyl react slower than unconjugated alkenes. Conjugated alkenes do react if there is no competi-tion.217,219... 

This chapter covers the recent literature reports (ca. 2003 onward) on asymmetric conjugate addition of triorganoaluminum reagents to enones, a,(3-unsaturated systems, nitroalkenes and also mechanistically closely related allylic alkylation of allylic phosphonates. It also includes cascade processes where the intermediate enolates (conjugate addition) and alkenes (allylic alkylation) are used for the synthesis of more complex molecules. This chapter is organized as follows. In Sect. 2 we present the results in the asymmetric conjugate addition. In this part, the... 

For acyclic allylic substrates die situation is mote complex, since a larger number of reactive conformations, and betice corcesponding transition states, compete. Hius, mediyl ciimamyl derivatives 163 tX= O.Acj, upon treatment witli litliiiim dimetliylcuprate, mainly gave tlie S 2 substitution product 166 fentry 1, Tab. 6.6 and Sdieme 6.34) [80]. Hie preference for die S 2 product is expected, since de-conjugation of die alkene system is electronically imfavorable. 

Nickel-bpy and nickel-pyridine catalytic systems have been applied to numerous electroreductive reactions,202 such as synthesis of ketones by heterocoupling of acyl and benzyl halides,210,213 addition of aryl bromides to activated alkenes,212,214 synthesis of conjugated dienes, unsaturated esters, ketones, and nitriles by homo- and cross-coupling involving alkenyl halides,215 reductive polymerization of aromatic and heteroaromatic dibromides,216-221 or cleavage of the C-0 bond in allyl ethers.222... 

If the double bonds of a polyene are not conjugated with other 7r-systems in the molecule, addition to one of the tt-bonds will proceed in a similar fashion to addition to a simple alkene. Usually addition to one of the bonds is preferred, because it is either more highly substituted and, as a result, has enhanced electron density, or because it has fewer substituents and is less hindered and more accessible to the electrophile. Alternatively, one of the double bonds of a polyene may be activated by the presence of a heteroatom at the allylic position. 

Electrochemical oxidation of alkenes results in the removal on one electron from the alkene function to give a 7t-radical-cation where the electron deficiency is delocalised over tire conjugated system. The majority of alkene radical-cations cannot be characterised because they readily lose an allylic proton in aprotic sol-... 

The ene reaction is strongly catalyzed by Lewis acids such as aluminum chloride and diethylaluminum chloride204 Coordination by the aluminum at the carbonyl group increases the electrophihcity of the conjugated system and allows reaction to occur below room temperature, as illustrated in Entry 6. Intramolecular ene reactions can be carried out under either thermal (Entry 3) or catalyzed (Entry 7) conditions 205 Formaldehyde in acidic solution can form allylic alcohols, as in entry 1. Other carbonyl ene reactions are carried out with Lewis acid catalysts. Aromatic aldehydes and acrolein undergo the ene reaction with activated alkenes such as enol ethers in the presence of Yb(fod)3 206 Sc(03SCF3)3 has also been used to catalyze ene reactions.207... 

We shall finish this chapter with some alkenes that are electrophilic, not because they are conjugated with another n system, but because they have a leaving group adjacent to them. We shall start with some substitution reactions with which you are familiar from Chapter 17. There we said that allyl bromide is about 100 times more reactive towards simple S>j2 reactions than is propyl bromide or other saturated alkyl halides. 

Dipolar cycloadditions 6.12 + 6.13 —> 6.14, however, are a large group of [4 + 2] cycloadditions isoelectronic with the allyl anion+ alkene reaction. There is much evidence that these reactions are usually concerted cycloadditions. They have a conjugated system of three p orbitals with four electrons in the conjugated system, but the three atoms, X, Y, and Z in the dipole 6.12 and the two atoms A and B in the dipolarophile 6.13, are not restricted to carbon atoms. The range of possible structures is large, with X, Y, Z, A and B able to be almost any combination of C, N, O and S, and with a double 6.12 or, in those combinations that can support it, a triple bond 6.15 between two of them. 

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