Peterson alkenation mechanism

Mechanism The Peterson alkenation offers the synthesis of desired alkene stereoisomer by careful separation of the two diastereomeric intermediate P-hydroxysilanes and subsequently performing elimination under two different conditions. 

As mentioned in the previous section, the Peterson reaction proceeds by an irreversible addition of the silyl-substituted carbanion to a carbonyl. It has generally been assumed that an intermediate p-oxidosi-lane is formed and then eliminated. In support of this mechanistic hypothesis, if an anion-stabilizing group is not present in the silyl anion, the p-hydroxysilanes can be isolated fixrm the reaction, and elimination to the alkene carried out in a separate step. Recent studies by Hudrlik indicate that, in analogy to the Wittig reaction, an oxasiletane (304) may be formed directly by simultaneous C—C and Si—O bond formation (Scheme 43). The p-hyd xysilanes were synthesized by addition to the silyl epoxide. When the base-induced elimination was carried out, dramatically different ratios of cis- to rranr-alkenes were obtained than from the direct Peterson alkenation. While conclusions of the mechanism in general await further study, the Peterson alkenation may prove to be more closely allied with the Wittig reaction than with -elimination reactions. 

Homo-Peterson Alkenation. Styrene oxide reacts with lithio-phenylthiobis(trimethylsilyl)methane to afford cyclopropane-containing products (eq 6)7 This reaction is limited, due to the complexity of its mechanism the alkenation reagent must serve to generate both alkenic and carbenic species. For this reason, only styrene oxide and trimethylsilyloxirane undergo this transformation. 

There is another, complementary version of the Peterson reaction that uses base to promote the elimination. The starting materials are the same as for the acid-promoted Peterson reaction. When base (such as sodium hydride or potassium hydride) is added, the hydroxyl group is deprotonated, and the oxyanion attacks the silicon atom intramolecularly. Elimination takes place this time via a syn-periplanar transition state—it has to because the oxygen and the silicon are now bonded together, and it is the strength of this bond that drives the elimination forward, this diastereoisomer via this mechanism eliminates to give this alkene... 

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). 

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