The wittig reaction in carbohydrate chemistry pdf free. The phosphawittighorner reaction proceeds through stepwise pp cleavage of an oxadiphosphetane. These compounds were further processed to provide five dipeptide vinyl sultams, which were found to be inactive against papain at concentrations up to 50. We will discuss the mechanism in greater detail in chapter 23, when we discuss the chemistry of hydrogens that are. A comparison of wittig and wittig horner wadsworth emmons reagents in reactions with some. The phosphawittighorner pwh reaction, as first coined by mathey and co workers, converts aldehydes and relatively unreactive ketones into. Fluorescent penta and hexaene fatty acids by a wittig. The geometry of the resulting alkene depends on the reactivity of the ylide. Eselective phosphonate esters are easily deprotonated and are more basicnucleophilic than stabilized ylides. Wittig reaction mechanism with reagent preparation.
The mechanism of the hwe reaction resembles that of the wittig reaction. This reaction is named after its discoverer, the german chemist georg wittig. The wittig reaction mechanism and stereochemistry youtube. Phosphorus, sulfur, and silicon and the related elements.
A comparison of wittig and wittig horner wadsworth emmons. Wittig and wittig horner reactions under phase transfer catalysis conditions article pdf available in central european journal of chemistry 14. I have also described the complete mechanism associated with this reaction. While many other routes to alkenes can proceed via elimination reactions e1 or e2. Difference between wittig and wittig horner reaction.
Convenient procedure of hornerwadsworthemmons olefination for the synthesis of simple and functionalized. Pdf wittig and wittighorner reactions under phase transfer. The wittig reaction or wittig olefination is a chemical reaction of an aldehyde or ketone with a triphenyl phosphonium ylide often called a wittig reagent to give an alkene and triphenylphosphine oxide. The results obtained by periodate oxidation corresponded to the structures proposed.
The key difference between wittig and wittig horner reaction is that wittig reaction uses phosphonium ylides, whereas wittig horner reaction uses phosphonatestabilized carbanions wittig reaction and wittig horner reactions are important synthesis reactions in organic chemistry, which produce alkenes from aldehydes or ketones. The first step involves nucleophilic attack by the phosphorus on the alkyl halide, followed by the halide ion dealkylation of the resulting trialkoxyphosphonium salt. If r is an electron withdrawing group, then the ylide is. A wittig horner reaction on unprotected carbohydrates, such as dgalactose and darabinose, leads to spontaneous oxamichael cyclization and lactonization, providing glucocfuranoside lactones 76 and 77 and mannocfuranoside esters 78, as described by mata et al. The reaction of aldehydes or ketones with stabilized phosphorus ylides phosphonate carbanions leads to olefins with excellent eselectivity. The wittig reaction was discovered in 1954 by georg wittig, for which he was awarded the nobel prize in chemistry in 1979. The reaction of a trialkyl phosphite with an alkyl halide to produce an alkyl phosphonate. Mechanism of the phosphawittighorner reaction ncbi. The reaction has become a widespread standard tool in natural products total syntheses, providing access to relatively simple as well as highly complex synthetic targets. This organic chemistry video tutorial provides a basic introduction into the wittig reaction mechanism. The aim of utilizing the wittig reaction of linking terpenoid building blocks to give vitamin a and carotenoids on an industrial scale prompted extensive research and development work of a synthetic and chemical engineering nature. The phospha wittighorner reaction proceeds through stepwise pp cleavage of an oxadiphosphetane intermediate, followed by a. Because the acidic proton on the carboxylic acid will protonate the negatively charged carbon, destroying the ylide. Norris describes the horner wadsworthemmons reaction as an alternative to the wittig reaction.
If a potassium base was used to generate the phosphine oxide anion, the reaction with a carbonyl compound proceeded as in the wittig. Organic chemistry portal, wittighornerreaction rsc ontology id, rxno. This type of carboncarbon bondforming reaction led him to the nobel prize in 1979. In 1958, leopold horner published a modified wittig reaction using phosphonatestabilized carbanions. Synthesis of stilbene by the wittig and horner wadsworthemmons reactions important concepts. Recent applications of the horner wadsworthemmons reaction to the synthesis of.
It uses a carbonyl compound as an electrophile, which is attacked by a phosphorus ylide the wittig reagent. A typical wittig reaction starts by generating a wittig reagent, known as an ylide from an alkyl halide and triphenylphosphene. Some of these chemistryrelated articles, especially on certain organicometallic reactions and reagents, are superb. The mechanism of the hornerwadsworthemmons reaction. Molecular fluorescent probes have revolutionized biochemical and biophysical studies in the last decades, but with regard to lipids there has been a lack of combining the slim shape of saturated acyl chains with fluorescent properties. Kinetics and mechanism of hornerwadsworthemmons reaction of weakly acidic phosphonate in. Despite of the synthetic versatility of the pwh reaction, it has been a greatly underexplored method for the preparation of phosphaalkenes. Wittig and wittighorner reactions are favorite tools in. The effects of a scientific discovery on industrial practice are illustrated with reference to the wittig reaction. Asymmetric epoxidation of allylic alcohol 535 derived from the wittighorner olefination of. Synthesis of alkenes intro the wittig reaction is one of the premier methods for the synthesis of alkenes. Abstract2phenylmethylene 1,3diphenylpropanedione 1 reacts with wittig horner reagent 6a in the presence of sodium hydride as a base to give the novel phosphonate product 8a.
This video gives mechanism of wittig reaction with stereochemistry and a lot of examples for csir net exam. Horner reaction of 1lithio1,3dienyl phosphine oxides with aldehydes. The phosphawittighorner pwh reaction, as first coined by mathey and coworkers, converts aldehydes and relatively unreactive ketones into. Heteroatomsubstituted aldehydes are consistently selective for cisoxaphosphetanederived products.
Pdf wittig and wittighorner reactions are favorite tools in. The wittig reaction in industrial practice pommer 1977. Our results provide unequivocal support for the generally accepted mechanism and are in very good agreement with experimental selectivities. Preparation of vinyl allenes from 1lithio1,3dienyl. Our strategy to pentaene and hexaene fatty acids builds upon commercially available 4edecenal, which is subjected to a wittig horner reaction after. Aza wittig reactions are similar to wittig reactions in that they also involve the reaction of a phosphonium ylide, in this case an iminophosphorane or phosphinimide such as 39, with a carbonyl group containing compound to form the carbonnitrogen double bond of an imine along with a byproduct phosphine oxide such as 2. When you search at that you are redirected to this page. This study shows that ez selectivity of non and semi. It is widely used in organic synthesis for the preparation of alkenes. On the other hand, the cyclic phosphonate adduct 9a was isolated from the reaction of 1 with wittig horner reagent 6a using alcoholic sodium alkoxide. Wittig reaction is an organic chemical reaction wherein an aldehyde or a ketone is reacted with a wittig reagent a triphenyl phosphonium ylide to yield an alkene along with triphenylphosphine oxide.
The reactions of wittighorner reagents and dialkyl. A variation of the wittig reaction, called the horner wadsworthemmons hwe reaction gives e alkenes. The wittig reaction was named after george wittig who converted carbonyl groups, such as aldehydes and ketones, into alkenes. Wittig and wittighorner reactions under phase transfer catalysis. A video covering the core mechanistic and stereochemical aspects of the wittig reaction for the second year module organic and biological chemistry. The hornerwadsworthemmons modification of the wittig. Water mediated wittig reactions of aldehydes in the teaching laboratory.
The most important use of ylides in synthesis comes from their reactions with aldehydes and ketones, which are initiated in every case by a covalent bonding of the nucleophilic alphacarbon to the electrophilic carbonyl carbon. For the wittig reaction follow the procedure in the book. The second direction of the wittig reaction with free and with partially protected sugars is the formation of furan derivatives, this direction being realized with pmethoxybenzoy1methylenetriphenylphosphorane only in certain. The wittig reaction chem 115 fan liu stabilized ylides are proposed to have a later and more productlike transition state with 1e thermodynamically favored over 1z. Alkenes from carbonyl compounds study questions 1 why is it important that the aldehydes added to the ylide be free of carboxylic acids. Emmons further defined the reaction in contrast to phosphonium ylides used in the wittig reaction, phosphonatestabilized carbanions are more nucleophilic but less basic. In a series of reactions between wittig or wittig horner wadsworthemmons reagents and diketones it was found that out of the five solvents tested, for the wittig reagents, namely. Solvent free method for making olefinic amino esters 205 from an aldehyde compound and a phosphonate derivative of glycine. One major milestone in this regard is the horner wadsworthemmons hwe reaction, a slightly di erent approach to the wittig reaction that is based on a modi ed wittig 1. The horner wadsworthemmons reaction has evolved in the last years as one of the most powerful and reliable method for stereocontrolled olefin synthesis.
Links for some related reactions videos are given below. The wittig reaction allows the preparation of an alkene by the reaction of an aldehyde or ketone with the ylide generated from a phosphonium salt. These reactions differ from each other depending on the. Synthesis of stilbene by the wittig and hornerwadsworth. The stereochemistry is set by steric approach control, where the antiperiplanar approach of the carbanion to the carbon of the carbonyl group is favored when the smaller aldehydic hydrogen eclipses the bulky phosphoranyl moiety. Vinyl allenes were prepared in high yields by the wittig. The reaction mechanism is similar to the mechanism of the wittig reaction. The horner wadsworthemmons hwe version of the wittig reaction focuses on the use of more stabilized phosphonate ylides 203. The wittig olefination reaction and modifications involving phosphorylstabilized carbanions. Unlimited viewing of the articlechapter pdf and any associated supplements and figures. Unequivocal experimental evidence for a unified lithium salt free wittig reaction mechanism for all phosphonium ylide types. Stereochemistry, mechanism, and selected synthetic aspects. The reaction of stabilized phosphonium ylides with aldehydes favors ealkene products.
Industrial applications of the wittig reaction springerlink. Journal of the american chemical society 2012, 4 22, 92259239. The wittighorner reaction in weakly hydrated solidliquid media. Wittig and wittighorner reactions under phase transfer. The hornerwittig reaction of the carbanion generated from the phosphonate 232 with 2,5diformylthiophene. Likewise, phosphonatestabilized carbanions can be alkylated. The wittig and horner wadsworthemmons reactions are very similar and can be used to form the same reactions. Several advantages of these reactions over the original wittig reaction have been discussed. The synthesis of phosphonate derivatives of nphenyl and nbenzyl.
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