Mechanism of Cyclopropane Ring Cleavage

Except for some of the above reactions which proceed via radical intermediates, many of the ring cleavage reactions are likely to involve initial interaction of the metal and the cyclopropanering. The nature of this interaction between 1-siloxy-1-alkoxycyclopropane 1 with metals has been investigated by the authors in some detail, and the results are summarized below. 

The Dewar-Chatt-Duncanson model of the binding of an olefin in a transition metal complex involves two types of interactions. Transfer of electron density from the relatively high-lying olefinic ic-orbital to the metal (cf. 20) represents a Lewis acid Lewis base interaction (a-bonding). A metal-olefin jr-bond due to interaction 

There are a few points to be addressed in order to understand the mechanism of homoenolate formation. Several lines of experimental evidence for the reaction of 1-alkoxy-l-siloxycyclopropanes have provided insights into the nature of the metal interacting with the siloxycyclopropane. 

Some experimental data indicate the formation of a cationic species as postulated for path (b) and in turn show that the Si-O bond remains intact in the transition state. Evidence comes from the comparison of the reactivities of 1-trimethyl-siloxy- (1) and l-/er/-butyldimethylsiloxycyclopropanes (23) in some reactions. 

The reaction of 1-isopropoxy-l-trimethylsiloxycyclopropane (1, R = i-Pr) with TiCl yields the titanium homoenolate of isopropyl propionate 2 (R = i-Pr) and Me3SiCl (Eq. see 9). That of 23, in contrast, yields a considerable fraction of the homoenolate of silyl propionate as well, Eq. (66) [19]. Apparently, the loss of an isopropyl group from an oxygen-stabilized cationic intermediate competes with that of the silyl group with bulky substituents. 

TaWe 16. List of metals, chlorides of 1 which react with cyclopropane I to form metal homo-enolate (Ref [11])  

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Schiller JG, AE Chung (1970) Mechanism of cyclopropane ring cleavage in cyclopropanecarboxylic acid. J Biol Chem 245 6553-6557. 

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