Allylceriums

Allylmetallic reagents The ally] anions obtained by reductive metallation of ally I phenyl sulfides with lithium l-(dimethy amino)naphthalenide (LDMAN, 10, 244) react with a, 3-enals to give mixtures of 1,2-adducts. The regioselectivity can be controlled by the metal counterion. Thus the allyllithium or the allyltitanium compound obtained from either 1 or 2 reacts with crotonaldehyde at the secondary terminus of the allylic system to give mainly the adduct 3. In contrast the allylcerium compound reacts at the primary terminus to form 4 as the major adduct. 

The reaction of allylcerium compounds with acrolein can be used for preparation of either (Z,Z)- or (Z,E)-1,4-dienes, equations (I) and (II). Thus the allylcerium compound generated at - 78° from 5 reacts with acrolein at the same temperature to form (Z)-6. However, if the allylcerium compound is allowed to warm to - 40° before addition to acrolein, (E)-6 is obtained in 60% yield. Similarly, the reaction of the homoallylic cerium compound from 7 with acrolein can be controlled to give either (Z,Z)- or (Z,E)-8 selectively. 

Homoalfylic alcohols. Allyl iodide reacts with Ce/Hg to form allylcerium iodide, which reacts with ketones to form homoallylic alcohols in 50-95% yield. A similar reaction is possible with benzyl iodide. Use of unamalgamated Ce results in much lower yields. 

Unlike allyllithiums, the allylceriums prefer the trans configuration, so warming the intermediate 111 after transmetallation but before electrophilic quench leads to trans alkenes such as , -112. 

A second molecule of the lithiated sulfide 62 is then used to introduce the next allylic unit, and reductive lithiation of the product 67 gives allyllithium 68. On this occasion, reaction at the less substituted terminus of the allyl system is required, and this can be achieved by transmetallation (at low temperature, to preserve the cis configuration) to the allylcerium 69. Reaction with formaldehyde followed by acetylation gives the pheromone 60 contaminated by less than 2% of the undesired E isomer. 

Cohen et al. have extensively studied the generation and reactivities of allylcerium leagents. - Allylcerium reagents react with a,p-unsaturated carbonyl compounds in a 1,2-selective fashion. It is particularly noteworthy that unsymmetrical allylcerium reagents react with aldehydes or enals mainly at the least-substituted terminus, as opposed to other allylorganometallics such as allyltitanium reagents. An example is shown in Scheme 13. ... 

Another prominent feature is that the reaction at -78 C ffovides (Z)-alkenes, while at -40 C ( )-alk-enes ate formed. Allylcerium reagents have been successMly employed in an economical synthesis of... 

Addition to the N=X bonds. Amine synthesis, particularly from chiral hydra-zones, using RCeCL as nucleophiles is well established. " The reagent formed from 1 1 RLi and CeClj is superior to RLi alone or RMgX. For conjugated aldimines, enantioselective 1,2-addition predominates. Allylcerium halide reagents can be pre-... 

In contrast to ytterbium and samarium, activation of the metal is required in the case of cerium, which is less reactive toward alkyl or aryl halides. The in situ generation of an allylcerium iodide in the presence of a ketone produces a homoal-lylic alcohol in good yield (Imamoto et al., 1981). 

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