Allyllithium, exchange reactions

The lithium-titanium exchange in the allyllithium-sparteine complexes 317 by tetraisopropoxytitanium (TIPT) or chloro-triisopropoxytitanium, resulting in titanates 318, proceeds with strict stereoinversion (equation 84). We assume that—contrary to the lithium-TMEDA complexes—the lithium-(—)-sparteine complexes are weaker Lewis acids and are no longer capable of binding the TIPT in the transition state of the exchange reaction. 

Allyllithium can be made similarly. These exchange reactions take place with retention of configuration at carbon ... 

Thus, the allyllithium, -sodium or -potassium derivatives are the ones which are most easily generated (Section D.1.3.3.3.1.1.), but they are of limited value in stereoselective carbonyl addition reactions. Usually these reagents need to be tuned" in their reactivity by metal exchange prior to application in order to achieve high selectivities. 

Enantiomerically enriched l-(diisopropylaminocarbonyloxy)allyllithium derivatives (Section 1.3.3.3.1.2.) add to carbonyl compounds with syn-l,3-chirality transfer21, giving good evidence for a pericyclic transition state in the main reaction path (Section 1.3.3.1.). However, since the simple diastereoselectivity and the degree of chirality transfer are low, for synthetic purposes a metal exchange with titanium reagents or trialkyltin halides (Section D.1.3.3.3.8.2.3.) is recommended. 

Reductive lithiation of allyl phenyl sulfides.1 This reagent is particularly useful for preparation of allyllithium reagents at temperatures at which the anions are stable. Moreover, regioselectivity in reactions can be achieved by conversion to allyltitanium(IV) complexes by metal exchange with Ti(0-/-Pr)4. Thus the un-symmetrical anion formed from the allyl sulfide 1 with LDMAN reacts with cro-tonaldehyde to give a mixture of 1,2- and 1,4-adducts. The 1,2-adduct 2 can be obtained in high yield as two diastereomers (9 1) by use of the allyltitanium complex (equation I). 

Some success has been reported with Reaction 4 using low temperatures (22). We have obtained up to 61% with the procedure cited in Ref. 22 with some difficulty. A number of other routes have been found for preparing allyllithium, including the cleavage of allyl ethers with lithium metal, exchange of alkyllithium compounds with allyltin compounds, and... 

Functionalized allyllithium compounds of type XIII are also homoenolate equivalents [122,130], but in their reaction with electrophiles sometimes it is not possible to control the regioselectivity. These compounds have been prepared mainly by either deprotonation or tin-lithium exchange. Deprotonation of (F)-cinnamyl-N,N-diisopropylcarbamate 155 with n-BuLi in the presence of (-[-sparteine in toluene gave a configurationally stable lithiated O-allyl carbamate (epi-156), which equilibrates at -50 °C to give the (R)-intermediate 156. Whereas the reaction of these compounds with Mel and MeOTs gave the /-attack, however acylation, silylation and stannylation took place at the a-position (Scheme 2.21) [131]. 

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