Peterson olefination reaction mechanism

As is the case with the Wittig and Peterson olefinations, there is more than one point at which the stereoselectivity of the reaction can be determined, depending on the details of the mechanism. Adduct formation can be product determining or reversible. Furthermore, in the reductive mechanism, there is the potential for stereorandomization if radical intermediates are involved. As a result, there is a degree of variability in the stereoselectivity. Fortunately, the modified version using tetrazolyl sulfones usually gives a predominance of the E-isomer. 

Reaction Mechanism The mechanism of the sodium hydroxide-catalyzed elimination of hexamethyldisiloxane may easily be understood when the reaction is compared to the well-known Peterson olefination in organic chemistry [24], Provided that an enolate anion is formed as an intermediate, either directly or via a proceeding hydrolysis of the 0-Si bond with traces of water which are always present on the hot surface of the crude catalyst, trimethylsilanolate splits off readily and thus the PsC triple bond is introduced into the molecule (Eq. 5). Subsequent attack of trimethylsilanolate at the trimethylsiloxy group of the starting compound results in a formation of hexamethyldisiloxane and the initial enolate anion so that the reaction circle is closed. 

In a creative application of Peterson olefination, supersilyllithium was reacted with acetone to give a silaalkene, as shown below. Suggest a mechanism for the reaction ... 

The mechanism of formation of aromatic vinylphosphonium salts via the Peterson olefination has been investigated. It has been found that the electronic nature of p-substituents in aromatic aldehydes strongly influences the stereochemical and kinetic outcome of the Peterson olefination, whereas temperature substantially affects their Hammett correlation. This indicates that the Peterson olefination is a multistep reaction involving the formation of at least an oxyanion/betaine and a carbanion as intermediates. 

On the other hand, high Z-selectivity is seen in the olefination reactions of the carbanion 19 derived from 3,3-diethoxybutylphosphonate with aldehydes (Scheme 2.16) [41, 42]. Similarly, Z-selective Peterson reactions of the in situ generated a-phosphoryl-a-(trimethylsilyl)allyl anion 104 with aldehydes or alkyl formates to afford the 2-dienylphosphonates 105 have been reported (Scheme 2.63) [168, 169]. These methods allow access to (Z)-alkenylphosphonates, whereas Wittig-Horner reactions give the thermodynamic ( )-alkenes almost exclusively. These excellent Z-selectivities can be rationalized in terms of the chelation control mechanism (see Section 2.2.2.3). 

Due to the mechanism of action, the Peterson olefination is a very versatile method of forming carbon-carbon double bonds. In particular, the reaction can be carried out under basic or acidic conditions giving rise to either the trans or cis isomer respectively. In cases where the stereoselectivity of the new double bond is irrelevant, the ability to employ either acid or basic conditions means that potentially sensitive functionalities in the remainder of the molecule can be suitably accommodated. 

The mechanisms of these reactions bear marked similarities, in spite of the differences in their reactivities and selectivities. Thus, in certain cases, a four-membered intermediate similar to the 1,2-oxaphosphetane intermediate in the Wittig reaction appears in the Peterson reaction as a pentacoordinate 1,2-oxasiletanide. Reactions of transition metal carbene complexes with carbonyl compounds also proceed through the formation of a four-membered oxametallacycle, which was recently found to be an intermediate of some McMurry reactions. Carbonyl olefination utilizing dimetallic species of zinc or chromium is somewhat similar to the Julia reaction in that they both involve the process of ) -elimination. 

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