Allylic position, metalation

The use of chiral transition-metal complexes as catalysts for stereoselective C-C bond forming reactions has developed into a topic of fimdamental importance. The allyhc alkylation is one of the best known of this type of reaction. It allows the Pd-catalyzed substitution of a suitable leaving group in the allylic position by a soft nucleophile. 

With less polar solvents and more basic allyl anions the compounds are present as ion pairs. The carbon-metal bond with the alkali and alkaline earth metals are known to have high ionic character. The allyl compounds behave accordingly as salts. The structures of allyl compounds of the alkali and alkaline earth metals are of two fundamental types, a 41 (or metal cation is associated closely with a single terminal allylic carbon, and the rf 1 (or ji) type, 15, in which the cation bridges the two terminal allylic positions. 

In 1981, it was demonstrated (70) that anions of nitro compounds can be involved in C,C-coupling with allyl acetates at the allylic carbon atom with the use of metal complex catalysis. For many years, this observation did not come to the attention of chemists interested in the synthesis of cyclic nitronates. However, Trost demonstrated (71) that this process can be used in the synthesis of five-membered cyclic nitronates from olefins (18) containing two acyl groups in the different allylic positions (Scheme 3.21). 

The latest addition to the theories of cationic polymerisation is the self-initiation theory of Kennedy [12]. According to this theory, initiation can take place in certain systems by a reaction between the metal halide and the olefin in which a hydride ion is abstracted by the metal halide from an allylic position in the monomer ... 

This can then dehydrogenate by being metalated in an allylic position followed by elimination of a hydride ion (F). 

The allyl alkali-metal compounds give 2 line C-13 nmr spectra. The terminal positions are equivalent either because of rapid equilibrium between two covalent structures, or because the structure is a delocalized symmetrical ion. The chemical shifts and C-C coupling constants are recorded in table I. Reasonable agreement with literature 6 values are found (8, 2). 

The two mechanisms may result in substantial and characteristic differences in deuterium distribution. The metal hydride addition-elimination mechanism usually leads to a complex mixture of labeled isomers.195 198 208-210 Hydride exchange between the catalyst and the solvent may further complicate deuterium distribution. Simple repeated intramolecular 1,3 shifts, in contrast, result in deuterium scram-bling in allylic positions when the ir-allyl mechanism is operative. ... 

Allylic metals, in propargylic alcohol alkylation, 11, 129 ir-Allylic palladium complexes, and carbocyclization, 11, 426 Allylic position, alkenes, dienes, polyenes, metallation, 9, 6 Allylic selenides, [2,3]sigmatropic rearrangement, 9, 481 Allylic substitution reactions for C-N bonds via amination... 

Benzyl methyl ether or allyl methyl ethers can be selectively metalated at the benzylic/allylic position by treatment with BuLi or sBuLi in THF at -40 °C to -80 C, and the resulting organolithium compounds react with primary and secondary alkyl halides, epoxides, aldehydes, or other electrophiles to yield the expected products [187, 252, 253]. With allyl ethers mixtures of a- and y-alkylated products can result [254], but transmetalation of the lithiated allyl ethers with indium yields y-metalated enol ethers, which are attacked by electrophiles at the a position (Scheme 5.29). Ethers with ft hydrogen usually undergo rapid elimination when treated with strong bases, and cannot be readily C-alkylated (last reaction, Scheme 5.29). Metalation of benzyl ethers at room temperature can also lead to metalation of the arene [255] (Section 5.3.11) or to Wittig rearrangement [256]. Epoxides have been lithiated and silylated by treatment with sBuLi at -90 °C in the presence of a diamine and a silyl chloride [257]. 

Members of a Japanese group (101, 102), using polybutadiene as their base polymer, were able to metalate the allylic position of the polybutadiene chain. The allylic anion was then allowed to react with additional monomer to produce a grafted polybutadiene copolymer, Eqs. (33) and (34). 

Considering the mechanistic rationales of the transition metal-catalyzed enyne cycloisomerization, different catalytic pathways have been proposed, depending on the reaction conditions and the choice of metal catalyst [3-5, 45], Complexation of the transition metal to alkene or alkyne moieties can activate one or both of them. Depending on the manner of formation of the intermediates, three major mechanisms have been proposed. The simultaneous coordination of both unsaturated bonds to the transition metal led to the formation of metallacydes, which is the most common pathway in transition metal-catalyzed cycloisomerization reactions. Hydrometalation of the alkyne led to the corresponding vinylmetal species, which reacts in turn with olefins via carbometalation. The last possible pathway involves the formation of a Jt-allyl complex which could further react with the alkyne moiety. The Jt-allyl complex could be formed either with a functional group at the allylic position or via direct C-H activation. Here the three major pathways will be discussed in a generalized form to illustrate the mechanisms (Scheme 8). 

Alkenes lacking hydrogen atoms at allylic positions are much less acidic than ordinary unsaturated hydrocarbons. Superbases regioselectively exchange allylic protons in alkenes whenever there is a choice. However, a few examples of metallation of alkenic C-H bonds with superbases are known and a compilation of them is reported in Table 1. Ethylene itself has been deprotonated by the superbasic mixture constituted by butyllithium, potassium tert-butoxide, and TMEDA.41... 

Metallation of Alkenes, Dienes, and Polyenes at the Allylic Position... 

The selective and efficient metallation of alkenes in the allylic position by the Schlosser s base is certainly one of the major achievements in the field of superbase chemistry. A complete list of successful alkene metallations by superbases would be too long for the purpose of this chapter but some representative examples are shown in Tables 2 and 3 as a general trend for the unsubstituted and heterosubstituted alkenes, respectively. 

Table 2 Metallation of propene and alkylsubstituted derivatives thereof at the allylic position Compound R References...

Table 3 Metallation of heterosubstituted alkenes in the allylic position ...

The superbase metallation of alkenes in the allylic position followed by reaction with electrophiles is one of the best methods to access Z-alkenes with new functional groups. 

Conjugated and homoconjugated dienes can also be easily metallated by superbases at the activated allylic position, to give rise to pentadienyl type organopotassium species. Their conformation has been the object of careful NMR and... 

In indium-mediated reactions, a catalytic use of expensive indium is desirable from an economical standpoint, and it has been achieved in combination with more electron-positive metals. Thus, allylation of aldehydes and ketones, as well as prenylation of 2-chlorobenzoquinone, has been performed by using metallic aluminum or zinc in combination with a catalytic amount of indium(m) chloride.63 Also, allylation of aldehydes and ketones with allyl bromide is carried out with a catalytic amount of indium powder (from 0.01 to 0.1 equiv.) in THF in the presence of manganese and chlorotrimethylsilane as the reducing and oxophilic agents, respectively (Scheme 2). 64,65... 

The more elegant route—which we call for simplicity the method of [1 + l]-addition —may also be used to prepare other binuclear complexes containing either two allyl groups, or one allyl group and one halogen in the bridging positions. The results so far obtained are summarized in Eqs. (9) and (10). Compounds 45-47 are the first allyl-transition metal complexes in which not one metal atom but a dimetal unit is coordinated sandwich-like by two i73-bonded allylic ligands (75, 79). 

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