Phosphonium ylids, reaction with aldehydes

Phosphonium ylids undergo a most important reaction with aldehydes and ketones, known as the Wittig reaction (3.111). The products of this reaction are olefinic compounds and phosphine oxides. 

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 EM Modular Reaction System can also be used to perform multi-step syntheses [83], For the production of pharmaceuticals, in this case for the synthesis of vitamin A, an ylid is formed from a phosphonium salt and a base in the first stage at 2 °C. In a second stage, the ylid reacts with an aldehyde at 60 °C in a flow-through capillary reactor. In a third stage the crude product is hydrolyzed at 20 °C in an additional micro mixer to form the target product vitamin A acetate, as illustrated. For the claimed reaction, no further experimental details were given. 

The bulky ligands PPh3 and AsPh3 add to the unsubstituted end of 31 (180,181). The resulting phosphonium salt (32) is deprotonated by butyllith-ium at — 78°C to yield an ylid (33) which reacts with aldehydes in a Wittig reaction. Deprotonation with potassium tert-butoxide followed by addition of aldehyde 34 gives the E isomer (35) only [Eq. (20)] (182). Trimethylphosphite... 

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. 

Amino aldehydes can also be condensed with tiiphenylphosphonium ylids in a Wittig olefination reaction.134 Generation of the propargyl alcohol derivative 1.228 allowed conversion of the hydroxyl group to a bromide and then to the triphenyl-phosphonium salt. Treatment with n-butyllithium gave the ylid, which reacted with... 

Over twenty years ago, Wittig found that alkyl substituted phosphonium salts could be deprotonated by strong bases to yield phosphonium ylids which in turn could react with aldehydes and ketones to yield olefins and the corresponding phosphine oxide [1]. The sequence is formulated in equations 1.1-1.3 for the reaction of methyl-triphenylphosphonium halide with cyclohexanone in the presence of base to give me thylenecyclohexane. 

The extra stabilisation makes the ylid rather unreactive and phosphonate esters 91 are often used instead of phosphonium salts in these reactions. Treatment with a base (NaH or RO is often used, BuLi will certainly not do) gives an inherently more reactive enolate anion 92 rather than an ylid. These Horner-Wadsworth-Emmons reagents (H WE as we shall call them, though they go under many other names) react with ketones as well as aldehydes and the product is normally the E-alkcnc16 93. 

The ability to control the stereochemistry of a Wittig reaction is seen in Wu s synthesis of annonacin, in which aldehyde 546 was treatment ylid 547 to give an 81 % yield of 548. A trans specific reaction is shown as a contrast in Pan s synthesis of (-)-6,7-dehydroferruginyl methyl ether. Aldehyde 549 reacted with the ylid generated from phosphonium chloride 550 to give the trans alkene (551) in 60% yield. 

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