Allyl halides with metal cations

Assuming a reactive oxonium ylide 147 (or its metalated form) as the central intermediate in the above transformations, the symmetry-allowed [2,3] rearrangement would account for all or part of 148. The symmetry-forbidden [1,2] rearrangement product 150 could result from a dissociative process such as 147 - 149. Both as a radical pair and an ion pair, 149 would be stabilized by the respective substituents recombination would produce both [1,2] and additional [2,3] rearrangement product. Furthermore, the ROH-insertion product 146 could arise from 149. For the allyl halide reactions, the [1,2] pathway was envisaged as occurring via allyl metal complexes (Scheme 24) rather than an ion or radical pair such as 149. The remarkable dependence of the yield of [1,2] product 150 on the allyl acetal substituents seems, however, to justify a metal-free precursor with an allyl cation or allyl radical moiety. 

Cycloaddition reactions of 18-electron transition metal ti -allyl complexes with unsaturated electrophiles to form five-membered rings have been extensively investigated. These transformations constituted a family of metal-assisted cycloaddition reactions in which the metal functions as an electron-donor center. These are typically two-step processes that involve the initial formation of a dipolar metal r) -alkene intermediate (2) and subsequent internal cyclization (equation 2). The most extensively investigated application of this methodology has been with dicarbonyl-ii -cyclopentadienyliron (Fp) complexes from the laboratory of Rosenblum. These (ri -allyl)Fp complexes are available either by metallation of allyl halides or tosylates with a Fp anion, or by deprotonation of (alkene)Fp cations. ... 

While y-alkylations did not occur with saturated and benzylic halides, y-selectivities in the 62-99% range were observed in reactions of dicopper(I) dianions of a variety of a, -unsaturated acids with allylic halides. y-Unsubstituted allylic halides reacted by an Sn2 mechanism, y-disubstituted compounds underwent direct 5n2 displacement and y-monosubstituted systems reacted by both 5n2 and 5n2 pathways. Scheme 72 provides an example of the dramatic reversal in regioselectivity that was observed in the allylation of the dianion (147) of tiglic acid when the metal cation was changed from lithium to copper(I). The y-alkylation product from the latter species was exclusively the ( )-isomer. 

IV. Reactions of Metalated Carboxylic Acids - Metalated carboxylic acids react with most electrophiles It the dianlon is associated with alkali metal cations. However, Cu(i) dianions22 react readily only with activated allylic or benzylic halides. 

Enantiosdective allyic substitution processes have been developed over the course of 30 years. Initial observations of the reactions of nucleophiles with paUadium-allyl complexes led to the observation of catalytic substitutions of aUylic ethers and esters, and then catalytic enantioselective aUylic substitutions. The use of catalysts based on ottier metals has led to reactions that occur with complementary regiochemistry. Moreover, flie scope of the reactions has expanded to include heteroatom and unstabilized carbon nucleophiles. Suitable electrophiles for these reactions indude allyhc esters of various types, allyhc ethers, aUylic alcohols, and aUylic halides. Enantioselective reactions can be conducted with monoesters or by selection for deavage of one of two equivalent esters. The mechanism of these reactions occurs by initial oxidative addition to form a metal-aUyl complex. The second step involves nudeophilic attadc on ttie aUyl ligand for reaction of "soft" nudeophiles or inner-sphere reductive eUmination for reactions of "hard" nudeophiles. The external nudeophilic attack typicaUy occurs by reaction of the nudeophile with a cationic aUyl complex at the face opposite to that to which Uie metal is bound. Exceptions indude reactions of certain molybdenum-aUyl complexes. Dissociation of product then regenerates the starting catalyst. Because of the diversity of the classes of these reactions, aUylic substitution—in particular asymmetric aUylic substitution—has been used to prepare a wide variety of natural products. 

It is possible to extend the synthetic prindples which were presented above to the preparation of polynuclear complexes having Cp, allyl, and CO ligands. Preliminary results from the reactions of cyclopentadienyl, allyl, and CO complexes of the transition metal halides with E(SiMc3)2 (E = S, Se, Te) indicate that MesSiQ is eliminated and a series of novel compounds are formed. The reaction between [CpFe(CO)2Br] and Se(SiMe3)2 produces a diamagnetic compound composed of the heterocubane cluster anion [Fe4Se4Br4] and two [Se Fe(CO)2Cp 3] cations (Eq. 3.52). 

Allyl silanes will also attack carbonyl compounds when they are activated by coordination of the carbonyl oxygen atom to a Lewis acid. The Lewis acid, usually a metal halide such as TiCLj or ZnCl2, activates the carbonyl compound by forming an oxonium ion with a metal-oxygen bond. The allyl silane attacks in the usual way and the (3-silyl cation is desilylated with the halide ion. Hydrolysis of the metal alkoxide gives a homoallylic alcohol. 

Silver(I) compounds are often used to generate cationic metal complexes from the corresponding metal halides. Suzuki and coworkers found that -hexylzirconocene chloride (61), derived from 1-hexene and Schwartz reagent 60, can react with aldehydes in the presence of a catalytic amount of AgAsFs to give secondary alcohols [27]. The reaction with hydrocinnamaldehyde, for example, provides the alcohol 62 in 95 % yield (Sch. 14). Allylic alcohols are also obtainable by a similar procedure using 1-hexyne as a starting material. 

Vinylsilanes react with chloral in the presence of Lewis acids (Scheme 33), but this type of reaction is little used, probably because the products are allylic alcohols, which are apt to undergo ionization in the presence of Lewis acids to give allyl cations, and hence further reaction. Reactions employing nucleophilic catalysis, although free of this problem, are also limited, only anion-stabilized systems undergoing reaction (Scheme 34). On the other hand, there is less of a problem with 3-elimination of a halide ion, as there would be with most metals 3 to a halogen. ... 

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