Metal homoenolates

Kuwajima, I. and Nakamura, E. Metal Homoenolates from Siloxycyclopropanes. 155,1-39 (1990). 

The last few years have seen the development of an entirely new methodology in which the metal homoenolate is prepared by ring opening of siloxycyclopropanes with metal halides Eq. (4). In these reactions, a very subtle balance of... 

In 1977, an article from the authors laboratories [9] reported an TiCV mediated coupling reaction of 1-alkoxy-l-siloxy-cyclopropane with aldehydes (Scheme 1), in which the intermediate formation of a titanium homoenolate (path b) was postulated instead of a then-more-likely Friedel-Crafts-like mechanism (path a). This finding some years later led to the isolation of the first stable metal homoenolate [10] that exhibits considerable nucleophilic reactivity toward (external) electrophiles. Although the metal-carbon bond in this titanium complex is essentially covalent, such titanium species underwent ready nucleophilic addition onto carbonyl compounds to give 4-hydroxy esters in good yield. Since then a number of characterizable metal homoenolates have been prepared from siloxycyclopropanes [11], The repertoire of metal homoenolate reactions now covers most of the standard reaction types ranging from simple... 

The recent surge of interests in metal homoenolate chemistry has been stimulated by the recognition that the siloxycyclopropane route can afford novel reactive homoenolate species that are stable enough for isolation, purification, and characterization. The stability of such homoenolates crucially depends on the subtle balance of nucleophilic and electrophilic reactivity of the two reactive sites in the molecule. Naturally, homoenolates with metal-carbon bonds that are too stable do not serve as nucleophiles in organic synthesis. 

The reaction of the cyclopropane 1 with ZrCU gave a complex mixture [11]. 3.1.2 Group 12 Metal Homoenolates... 

The common Lewis acids in this group, A1C13 and BX3, do not form metal homoenolates in their reaction with siloxycyclopropanes, only GaCl3 reacts with 1-alkoxy-l-siloxycyclopropanes 1 to give propionate homoenolates [11]. 

Carbon-carbon Bond Forming Reactions of Metal Homoenolates... 

Various types of carbon-carbon bond forming reactions of metal homoenolates have been reported, some of which are highly synthetically useful. Scheme 3 illustrates reaction types of zinc homoenolates (8 or 9). In this section the reactions of stable homoenolates are presented according to the reaction types. Examples, in which siloxycyclopropanes generate transient, unstable homoenolates, are described in Sect. 5. 

Internal nucleophilic cyclization leading to alkoxycyclopropanes is the most typical reaction of reactive metal homoenolates [I, 2]. The nature of stable homoenolates, however, is such that the anionic carbon C-3 forms a covalent bond with the metal atom and does not show high nucleophilic reactivity. 

Among isolable metal homoenolates only zinc homoenolates cyclize to cyclo-propanes under suitable conditions. Whereas acylation of zinc alkyls makes a straightforward ketone synthesis [32], that of a zinc homoenolate is more complex. Treatment of a purified zinc homoenolate in CDC13 with acid chloride at room temperature gives O-acylation product, instead of the expected 4-keto ester, as the single product (Eq. (22) [33]). The reaction probably proceeds by initial electrophilic attack of acyl cation on the carbonyl oxygen. A C-acylation leading to a 4-keto ester can, however, be accomplished in a polar solvent Eq. (44)-... 

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