Cyclopropanes metal homoenolate reaction

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

Next to the cyclopropane formation, elimination represents the simplest type of a carbon-carbon bond formation in the homoenolates. Transition metal homoenolates readily eliminate a metal hydride unit to give a,p-unsaturated carbonyl compounds. Treatment of a mercurio ketone with palladium (II) chloride results in the formation of the enone presumably via a 3-palladio ketone (Eq. (24), Table 3) [8], The reaction can be carried out with catalytic amounts of palladium (II) by using CuCl2 as an oxidant. Isomerization of the initial exomethylene derivative to the more stable endo-olefin can efficiently be retarded by addition of triethylamine to the reaction mixture. 

Among isolable metal homoenolates only zinc homoenolates cyclize to cyclopropanes imder 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 CDCI3 with acid chloride at room temperature gives O-acylation product, instead of the expected 4-keto ter, as e single product (Eq. (22) [33]). The reaction probably proceeds by initial e ectro-philic attack of acyl cation on the carbonyl oxygen. A C-acylation ... 

Internal nucleophilic cyclization is one of the most typical reactions of reactive metal homoenolates (equation 3 and Scheme and provides a convenient route to silyloxycyclopropanes (e.g. 8) through cyclization of 3-halo esters. Zinc homoenolates (9) also cyclize to cyclopropanes under suitable conditions. Treatment of the zinc homoenolate in CHCb with an acid chloride at room temperature gives an 0-acylation product (Scheme 6), instead of a 4-keto ester (see Section 1.14.7.1). The reaction of the zinc homoenolate wifo MesSiCl in a polar solvent gives a silyloxycyclopropane (Scheme 6), providing a very mild route to silyloxycyclopropanes. ... 

The cyclopropane 1 reacts with none of the group 1 and 2 metal chlorides. Among early transition metal chlorides, NbCl reacted with i in moderate yield to give the same homoenolate obtained by the reaction of equimolar amounts of titanium homoenolate 2 and NbCl (Scheme 2). TaCl5, CrCl3, MoCls, and WC15 did not give any characterizable products. 

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