Allyl group transfer

The stereochemical outcome of allylic silacyclobutanes is explained in terms of a six-membered chairlike transition state18 (Scheme 3.2p). The aldehyde would coordinate to the Lewis acidic silicon19 to form a pentacoordinated complex, from which the allyl group transfer occurs subsequently to give homoallylic alcohols. In transition states A and B, the aryl or alkyl group (R) of the aldehydes prefers to occupy an equatorial position to produce anti- and. syn-alcohols from the (E)-and (Z)-allylic silacyclobutanes, respectively. 

Selective addition to one double bond of a cross-conjugated dienone is attributed to formation of the intermediate with the metal binding to both a Ti -boroxyallyl ligand and a Ti -allyl ligand prior to the allyl group transfer/ ... 

A palladium-catalysed 5-endo-trig heteroannulation of enones generated in situ from amino-acid-derived -keto nitriles occurs by allyl-group transfer from the carbamate protecting group to give two new contiguous stereocentres (Scheme 73) ° ... 

Another method for deallylation of ally esters is the transfer of the allyl group to reactive nucleophiles. Amines such as morpholine are used[415-417], Potassium salts of higher carboxylic acids are used as an accepter of the allyl group[418]. The method is applied to the protection and deprotection of the acid function in rather unstable /f-lactam 664[419,420]. 

Allylsilanes react with carbonyl compounds to transfer the allyl group with 1,3-transposition, in the presence of Lewis acids, typically titanium(IV) chloride47. Recently this reaction has been carried out under super-acid catalysis48. Transfer of the allyl group is also induced by tetrabutylammonium fluoride, but in this case reaction takes place regioselectively at the less substituted end of the allyl fragment49. 

It was suggested that reaction occurred by the transfer of a hydrogen atom to the y-carbon of the allyl group, i.e.,... 

In radical chain polymerisation of allylic monomers (containing the chemical allyl group CH2 = CR-CH3, like propylene, isobutylene,...), a transfer on the monomer can occur leading to a very stable allyl radical unable to propagate the chain. Thus radical chain polymerisation of such monomers, especially propylene is not feasible, as a competition exists between the propagation rate and the high rate of transfer on the monomer (see, Figure 22). 

Radiochemical experiments show that the number of polymer chains terminated by allyl groups are a minor fraction of the total, and the majority of chains derive from the realkylated hydrides. In the presence of hydrogen it is evident from Table III that chain transfer reactions dominate and some saturated polymer is formed ... 

Catalytic reactions involving addition of two molecules of butadiene and hydrogen transfer from an acidic carbon atom (adjacent to a ir-allyl group) to the nickel oxygen bond were recently described (example 12, Table IV). 

In the reaction of (R,R)-tartrate allyl-boronate with aldehydes, Si attack of the nucleophile on the carbonyl group has been observed, while Re attack occurs in (S, S )-tartrate allyl-boronate reactions. Thus, an (S )-alcohol is produced preferentially when an (R,R)-allyl reagent is used, and the (R)-product can be obtained from an (S.Sj-reagent. assuming that the R substituent in the aldehyde substrate takes priority over the allyl group to be transferred. In fact, no exceptions to this generalization have yet been found in over 40 well-characterized cases where the tartrate auxiliary controls the stereochemical outcome of the allyl or crotyl transfer.72... 

Enantioselective reactions involving q1 -allyltitanocenes are almost unknown. An attempt to realize an asymmetric transfer of the allyl group has been reported by Reetz [40], who employed a chiral titanium precursor with two different Cp groups and a stereo-genic center at the metal (CpCptBu(C6F5)Cl) [41]. However, the addition of the derived al-lyltitanium reagent to aldehydes was found to proceed with a low chiral induction (ee up to 11%) in this case. 

This chapter will begin with a discussion of the role of chiral copper(I) and (II) complexes in group-transfer processes with an emphasis on alkene cyclo-propanation and aziridination. This discussion will be followed by a survey of enantioselective variants of the Kharasch-Sosnovsky reaction, an allylic oxidation process. Section II will review the extensive efforts that have been directed toward the development of enantioselective, Cu(I) catalyzed conjugate addition reactions and related processes. The discussion will finish with a survey of the recent advances that have been achieved by the use of cationic, chiral Cu(II) complexes as chiral Lewis acids for the catalysis of cycloaddition, aldol, Michael, and ene reactions. 

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