Allylation reagent

In most of these examples, the chiral auxiliary is introduced to the allylic reagent at a very late stage in the synthesis of the precursor, thus providing a facile access. It is obvious that in most examples, the central metal atom is kept from becoming stereogenic, and in addition, a C2-symmet-ric cation is desirable, in order to minimize the possible number of competing transition states. 

Reactive allylation reagents were also generated in situ from allylic sulfonium salts28 or by the action of zinc on ally] acetates with palladium catalysis29. 

I.3.3.3.8.2.I.I. With Titanium(IV) and Zirconium(IV) Reagents Nonhetero-Substituted Allylic Reagents... 

Several examples for allylic reagents further substituted by heteroatoms are known. For example, a mixture of the l,l-dichloro-2-alkene and the l,3-dichloro-1-alkene leads to a homogeneous homoallylic alcohol17,18. 

Scheme 9.7. Summary of Stereoselectivity of Allylic Reagents in Carbonyl Addition... 

TS, which is usually based on the chair (Zimmerman-Traxler) model. This pattern is particularly prevalent for the allylic borane reagents, where the Lewis acidity of boron promotes a tight cyclic TS, but at the same time limits the possibility of additional chelation. The dominant factors in these cases are the E- or Z-configuration of the allylic reagent and the conformational preferences of the reacting aldehyde (e.g., a Felkin-type preference.)... 

Recently, Oshima et al. developed the conversion of acid chlorides into the corresponding homoallylic alcohols catalyzed by in r(/ -prepared hydridozirconium allyl reagents (Scheme 41),147 147a The proposed mechanism suggests an initial hydride transfer from the zirconocene crotyl hydride species, in equlibrium with its Cp2Zr(l-alkene),147a to the acid chloride with subsequent allylation to afford the corresponding homoallylic alcohols. 

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... 

Nakamura et al.100 found that in the presence of palladium catalysts imines undergo allylation readily, providing the corresponding homoallylamines with high yields. Thus, chiral palladium complex 157 has been synthesized and applied in the asymmetric allylation of imines using allyl tributyltin as the allylation reagent. In Scheme 3-54, moderate yield and up to 82% ee have been obtained with 157 as the chiral catalyst.101... 

In a similar fashion, allylboronates can be used as allylation reagents under hydroformylation conditions. Thus condensed 1,5-oxazadecalin systems are achieved via tandem hydroformylation/allylboration/hydroformylation sequences starting from an N-allyl-y-amidoallylboronate (Scheme 23) [77,78]. The aldehyde obtained from a regioselective hydroformylation undergoes diastereoselective intramolecular allylboration to give an intermediate al-lylic alcohol derivative. The reaction does not stop at this stage, since this... 

Recently, the electrochemical recycKng of allyltin reagents has been realized for the first time in protic solvents. Difhculties in the recycling of metallic allyl reagents in situ are due to the fact that reaction conditions that allow... 

Next, enantiopure silicon allylation reagent will be presented, which already inherits Lewis acidity. It is accepted that Lewis acidity of silicon, as well as its high tendency to expand valence shell, increases [59, 60] if it is tetravalent and incorporated into strained four- or five-membered ring systems (strain-release Lewis acidity) [61]. This corresponds to smaller energy gaps between sp and dsp orbitals of a strained system as compared to an acyclic species. 

Stereoselectivity diminished or disappeared altogether as the C(3) substituent was removed (allyl reagent 25) or inverted ((E)-crotylboronate 26 see Figure 10).3 ... 

One of the important advantages of the intramolecular alkene silylformylation reaction as an aldol equivalent is that the products are masked 3,5-dihydroxyalkanals, and therefore that no manipulations are required prior to iteration of the process by aldehyde al-lylation to set up the next intramolecular silylformylation. Given that allylsilanes are well-known aldehyde allylation reagents, intramolecular silylformylation employing a diallylhydrosilane would, in principle, allow for the possibility of a tandem silylformyla-tion/allylsilylation reaction. This has been reduced to practice the diaUylsilyl ethers 60 were subjected to the previously developed silylformylation conditions and the unpuri-fied reaction mixtures were subjected to the Tamao oxidation ]36] to provide triols 61... 

Table i. addition of Unsubstituted allyl reagents A. non-Aromatic Carbonyl Substrates... 

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