Group 16 Metal Homoenolates

An exothermic reaction between cyclopropane 1 and TiCl in methylene chloride produces a wine-red solution of a mixture of titanium homoenolate 2 and chlorotrimethylsilane [10, 19]. When the reaction is performed in hexane, the titanium species precipitates in the form of fine violet needles (approx 90% isolated yield, Eq. (9)). 

This complex, while sensitive to oxygen and moisture, is stable for extended periods either neat or in solution. Its carbonyl stretching band (approx. 1610 cm-) in the IR spectrum in dilute solution clearly indicates a chelate structure, which endows the carbon-titanium bond of this complex with very high stability, as compared with that of simple alkyltrichlorotitaniums [25]. 

The reaction of the cyclopropane 1 with ZrCU gave a complex mixture [11]. 

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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]. 

In view of the remarkable stability of metal homoenolates of esters, the existence of homoenolate species containing a 3-halo substituent, i.e. zinc carbenoid moiety connected to an ester group, appeared to be possible. Indeed, when a silyl ketene acetal is treated with a carbenoid generated from CHBrj and Et2Zn, two types of highly intriguing reactions ensue [58]. With a purely aliphatic substrate, Eq. (61), an alkyl cyclopropylcarboxylate due to intramolecular p-CH-insertion of the intermediate zinc carbenoid formed. When the substrate contained an olefmic double bond in the vicinity of the carbenoid function, Eq. (62), in particular an intermediate derived from an a,P-unsaturated ester, internal cyclo-propanation occurred to give bicyclic or tricyclic product (Table 15). 

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. 

A CH-group, which bears vinyl and sulfide substituents, is acidic enough to be metallated by strong bases. Other d3-synthons may contain two activating functional groups in Imposition ( homoenolate -equivalents). Only one of the a-carbons is deprotonated under appropiate conditions (e.g. succinic diesters). Ano ther possibility is an acidic carbon and a non-acidic functional group in 1,3-positions (e.g. propargyl derivatives). Silyl ethers of a, -unsaturated alcohols can also be converted to anions, which react as d3-synthons (W. Oppolzer, 1976). 

Other types of heterofunctionalized cyclopropane derivatives used as starting materials in transition metal mediated conversions are acylcyclopropanes and similar systems. These systems, being electrophilic cyclopropanes, are opened by carbanion nucleophiles (e.g. cuprates, see Section l.B.2.1.3.) or nucleophilic transition-metal centers. In combination with siloxy groups, push-pull (capto-dative) substituted cyclopropanes are available as homoenolate precursors via Lewis acid mediated ring opening (see Section 1. B.2.1.4.).. This latter procedure has been used in tetrahydrofuran synthesis. 

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