Aldehydes reaction with phosphorus ylids

In the Wittig reaction an aldehyde or ketone is treated with a phosphorus ylid (also called a phosphorane) to give an alkene. Phosphorus ylids are usually prepared by treatment of a phosphonium salt with a base, and phosphonium salts are usually prepared from the phosphine and an alkyl halide (10-44) ... 

The ylid character of X5-phosphorins and their Cr(C0)3 complexes again is evident when one or both groups on phosphorus are CHR2 as one can abstract a+ protpn giving a carbanion. Reaction with electrophiles (e.g. D, CH3, and RCHO) causes side chain addition. No Wittig olefination is found with aldehydes. Instead a 1(2 -hydroxy) product 9 is formed which can be dehydrated to the X5-phosphorin derivative 10. 

How can the Z selectivity in Wittig reactions of unstabilized ylids be explained We have a more complex situation in this reaction than we had for the other eliminations we considered, because we have two separate processes to consider formation of the oxaphosphetane and decomposition of the oxaphosphetane to the alkene. The elimination step is the easier one to explain—it is stereospecific, with the oxygen and phosphorus departing in a syn-periplanar transition state (as in the base-catalysed Peterson reaction). Addition of the ylid to the aldehyde can, in principle, produce two diastere-omers of the intermediate oxaphosphetane. Provided that this step is irreversible, then the stereospecificity of the elimination step means that the ratio of the final alkene geometrical isomers will reflect the stereoselectivity of this addition step. This is almost certainly the case when R is not conjugating or anion-stabilizing the syn diastereoisomer of the oxaphosphetane is formed preferentially, and the predominantly Z-alkene that results reflects this. The Z selective Wittig reaction therefore consists of a kinetically controlled stereoselective first step followed by a stereospecific elimination from this intermediate. 

A.ii. (ElZ) Isomers in Wittig Reactions. When a phosphorus ylid reacts with an aldehyde or ketone, the alkene that is formed is a mixture of ( ) and (Z) isomers. l Initial postulations suggested that the stereochemistry of the alkene products was controlled by the stereochemistry of the betaine, which rapidly collapsed to an oxaphosphetane. Subsequent fragmentation to the alkene occurred via a syn-elimination pathway An example of this postulate is shown by addition of ylid 524 to 2-butanone to generate betaine 525 and the anti-oxaphosphetane, 527. Syn elimination led to the (El-alkene, 529. The reaction mixture also contained betaine 526, the precursor to the syn oxaphosphetane (528) which led to the (Z) alkene, 530. 

Triphenylphosphine reacts with alkyl halides to form phospho-nium salts. Organolithium bases react with alkyltriphenylphos-phonium salts to give phosphorus ylids, which react with aldehydes and ketones to give alkenes in what is known as the Wittig reaction. 

The reaction of an aldehyde with a phosphorus ylid was noted several times in previous sections. Sulfur ylids such as dimethylsulfonium methylid used here also react with aldehydes, in this case with 5.81, to give an epoxide. Subsequent opening... 

Ylids can be isolated, but are usually used in reactions immediately they are formed. They are nucleophilic species that will attack the carbonyl groups of aldehydes or ketones, generating the four-membered ring oxaphosphetane intermediates. Oxaphosphetanes are unstable they undergo elimination to give an alkene (65% yield for this particular example) with a phosphine oxide as a byproduct. The phosphorus-oxygen double bond is extremely strong and it is this that drives the whole reaction forward. 

This chapter will discuss carbanion-like reactions that utilize enolate anions. The acid-base reactions used to form enolate anions will be discussed. Formation of enolate anions from aldehyde, ketones, and esters will lead to substitution reactions, acyl addition reactions, and acyl substitution reactions. Several classical named reactions that arise from these three fundamental reactions of enolate anions are presented. In addition, phosphonium salts wiU be prepared from alkyl halides and converted to ylids, which react with aldehydes or ketones to form alkenes. These ylids are treated as phosphorus-stabilized car-banions in terms of their reactivity. 

In terms of its chemical reactivity, an ylid such as 126 may be viewed as a phosphorus-stabilized carbanion that will undergo acyl addition with an aldehyde or a ketone. When this ylid is mixed with cyclohexanone (80), there are two isolated products. The one that is more interesting to an organic chemist is methylenecyclohexane (129), formed in 52% yield the other is tri-phenylphosphine oxide, 130. It is obvious that 129 is not the expected acyl addition product. Formation of 130 indicates that the carbon atom of the ylid has been transferred to the ketone, but the oxygen atom of the ketone has been transferred to the phosphorous atom. Analysis of the reaction shows that the oxygen atom is lost from the ketone, and the CH2 unit of ylid 126 is transferred to form a new C=C bond (in green in the illustration). What is the mechanism ... 

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