Allyl metal compounds

Allylic metal compounds useful for further transformations can be prepared by Pd-catalyzed reactions of allylic compounds with bimetallic reagents. By this transformation, umpolung of nucleophilic 7r-allylpalladium complexes to electrophilic allylmetal species can be accomplished. Transfer of an allyl moiety from Pd to Sn is a typical umpolung. 

Many methods have been reported for the addition of allylic groups, including allyltrialkyltin compounds" (in the presence of BF3-etherate), as well as other allylic metal compounds." Allylic alcohols and homo allylic alcohols add to aldehydes in the presence of Sn(OT02 " For allylic halides, both activated... 

For enantioselective reaction of allylic metal compounds, see R. W. Hoffman,... 

In the cleavage of allyl-metal compounds, the available mechanisms of substitution are increased by the inclusion of mechanisms involving rearrangement, S E, as detailed in Chapter 3, Section 2, p. 16. 

Metal-for-metal exchanges of allyl-metal compounds... 

The discovery of bis(7r-allyl)nickel led to the recognition that most of the transition metals form such complexes in which exclusively allyl groups are attached to the central metal atom (24).4 Further investigation demonstrated very clearly that in the transition allyl metal compounds both types of metal-carbon bonds, tj and it, play important roles, and that even differences in the participation of the two types of bonds are possible. By a rather peculiar... 

Higher 43-hydrocarbyl derivatives of transition metals can be obtained by partial protolysis of the bis(7i-allyl)metal compounds formed from cyclo-octa-1,5-diene metal complexes and 1,3-butadiene with one equivalent of the Bronsted acid in the presence of the proper ligand in a polar solvent at lowered temperature (e.g. — 40°C). For instance, Ni [43-(CxHn)l and Ni—[43-(Ci2Hig)] derivatives have been yielded by the reaction of Ni(CxH 2)2 [(Ni(Cod)2] with 1,3-butadiene (C4H6), followed by treatment with a Bronsted acid, according to scheme (1) [132] and scheme (2) [37] respectively ... 

A variety of other allylic metal compounds add to aldehydes or ketones. " A variety of alkyl and allylic halides add to aldehydes or ketones in the presence of metals or metal compounds the metal or compounds based on Ti, " Mn, " Fe, " Ga, " ... 

Each starting material must now be made. The stereochemistry of the first tells us that we shc add an allyl metal compound to an epoxide. The metathesis catalyst can be any of those menticr. 

There are two main synthetic applications where the reaction of an allyl system with electrophiles is accompanied by an allylic rearrangement. One consists of the use of heteroatom-substituted allylic anions as homoenolate anion equivalents and the other represents a synthetically valuable alternative to the aldol reaction by addition of allyl metal compounds to aldehydes. 

The potential of homoenolate anions on reaction with different electrophiles (Scheme 77) is evident and explains why a great deal of effort has been expended to get high y-selectivity in reactions of such allyl metal compounds. 

Heteroatom to hydrogen transpositions in allylic systems (equation 43) can be observed in different reactions, namely Ae protonation of allyl metal compounds, in reductions of allyl heterocompounds and in retro-ene reactions. Except for the last reaction, which proceeds by a cyclic transition state, the problem of regioselectivity restricts the synthetic value of such reactions and it is only in specific cases that this can be overcome. 

Allyl metal compounds can exist in either the monohapto ti - or trihapto Ti -forms. Crotyl metal compounds that exist in the ti -form, including those classified as type I reagents, are generally sensitive to metallotropic rearrangements (sequential 1,3-shifts) which affect ( ) to (Z) isomerization via the intermediacy of the methallyl metal isomer (Scheme 6). Trihapto, or ir-bound, allyl metal reagents can exist in either of two forms the extended or ( )-isomer, and the U-shaped or (Z)-isomer. These ti -reagents can also isomerize if a pathway for interconversion with the ti -methallyl intermediate is energetically accessible. 

A number of highly enantioselective chiral allyl organometallic reagents have been described in the literature. These are of considerable interest both for the asymmetric synthesis of homoallyl alcohols as well as in double asymmetric reactions with chiral C=X electrophiles. - Two distinct groups of chiral allyl metal reagents can be identified those with conventional, easily introduced chiral auxiliaries and ones in which the center of chirality is a structural component of the reagent (e.g. allyl metal compounds with substituents at C-1). These are discussed separately in the sections that follow. 

Finally, Thomas has reported that allylstannanes (247) and (248) possessing stereocenters at C-4 undergo moderately diastereoselective reactions with p-nitrobenzaldehyde (Scheme 46). The origin of asymmetric induction in these cases is probably related to that presented in Section 1.1.3.1 for the reactions of achiral allyl metal compounds and chiral aldehydes (e.g. Scheme 27). 

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