Anionically Protected Lithium Homoenolate

An early effort to generate a 3-lithiated propionic acid derivative and react it with (external) electrophiles was reported in 1978 [42]. Since simple 3-lithioesters failed to undergo the required reaction, the alkyl carboxylate portion was protected by preceding conversion to the carboxylate anion. Treatment of lithium 3-bromo-propionate with lithium naphthalide generated the desired dilithiated propionic acid, which gave moderate yields of y-hydroxy acid addition products with carbonyl compounds, Eq. (45). 

Treatment of 3-stannylpropionamide with two equivalents of butyllithium at low temperature (—78 °Q generates a similar dianionic homoenolate, which reacts with standard electrophiles Eq. (46) [43], It is interesting to note that the propionate moiety rather than the butyl groups is selectively cleaved off the tin in the tin-lithium exchange. 

Dianions of the above types may not fall into the category of homoenolate in a strictly formal sense. Nevertheless the amide dianion does show a behavior typical of the homoenolate. Thus, the reaction of the isotopically labeled stannylpropionate results in scrambling of the label probably via a cyclopropane intermediate Eq. (47) [44]. As the result of such an equilibration, isomerization of a-methyl and a-phenyl substituted propionate homoenolates may occur to give the thermodynamically more favorable isomers, respectively. 

Two groups independently reported the formation of titanium homoenolates by the transmetalation reaction of 3-stannyl-propionate esters with TiCl, Eq. (48) [45, 46]. Amide homoenolates become available along this route [47], The trichlorotitanium species thus obtained have been shown ( H NMR) to be similar to that generated along the siloxycyclopropane route and indeed exhibit very similar reactivities. This method does provide a conventient alternative to the siloxycyclopropane route. 

Isotopic labelling, Eq. (49), indicated that the transmetalation occurs via direct tin-carbon bond cleavage rather than via intermediate formation of an alkoxy-cyclopropane [45], 

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