Phosphonium salts allenes

The submitters have shown that these reactions proceed by dehydro-chlorination of the acid chloride to the ketene, which is then trapped by reaction with the phosphorane. The resulting betaine decomposes to the allenic ester via an oxaphosphetane. In contrast, the reaction of acid chlorides with 2 equivalents of phosphoranes involves initial acylation of the phosphorane followed by proton elimination from the phosphonium salt. ... 

Methoxy-olefins were obtained when the allenic phosphonium salt... 

Novel alkenylphosphonium salts were subjected to the Wittig reaction (Scheme 12). Allylic deprotonation took place for phosphonium salts possessing such protons, and the olefination proceeded after double bond migration. In cases where such protons were absent, allene formation was observed. 

Allylphosphonium salts are synthesized by substitution of allyl halides with PPh3. The use of allyl alcohol, allyl acetate, or nitropropene with a palladium catalyst has also been reported.19 It is shown in this study that the organophosphorous compounds can be obtained by a palladium-catalyzed addition to an allene. A notable aspect of this method is that it can control the stereochemistry of the phosphonium salt, and that (Z)-allylphosphonium salts have been obtained in pure form for the first time. 

Reaction with a first aldehyde transforms 176 into the vinylphosphonium chloride 177, which for practical reasons is subjected to an anion-exchange process, leading to the phosphonium salt 178. From this, phenyllithium treatment liberates the allenic phosphorane 179, an intermediate that has previously been used to prepare allenes from aldehydes [69], in the present case providing the products 180. The same protocol has also been applied to o-alkynylbenzaldehydes to yield allenes of interest as model compounds for the study of Schmittel and Myers-type cyclization reactions [70]. 

An intramolecular Wittig reaction of the phosphonium salt PhsP—CH2-CH2CH20C0Ph Br yields the dihydrofuran (15). Compound (16) is formed from styrene and acetone in the presence of lead dioxide by a free-radical mechanism. The A -dihydrofuran (17) is produced by the action of silver fluoroborate on the allenic alcohol HOCH2CH=C=CMeBu another compound of this type, namely (18), is the product of the reaction of the sulphurane H2C=SMe2 with salicylideneacetone. ... 

Allen et al. prepared a series of 2-(phosphonioaryl)(benz)imidazolide betaines and phosphonium salts 220 and 221 (95JCS(P1)2789,98JCS(P1)335). 

A number of cycloaddition reactions involving allene derivatives as dienophiles have been recorded. Allene itself reacts only with electron-deficient dienes but allene carboxylic acid or esters, in which a double bond is activated by conjugation with the carboxylic group, react readily with cyclopentadiene to give 1 1 adducts in excellent yield. For example, the allene 12 gave, with very high yield and selectivity, the cycloadduct 13, used in a synthesis of (-)-P-santalene (3.19). An allene equivalent is vinyl triphenylphosphonium bromide, which is reported to react with a number of dienes to form cyclic phosphonium salts. These can be converted into methylene compounds by the usual Wittig reaction procedure (3.20). 

Generation of Phosphorus Ylides and Phosphonate Anions. NaHMDS isthemostutiUzedbaseforthedeprotonation of a variety of phosphonium salts to generate the corresponding ylides, which then undergo Wittig reaction with a carbonyl compound. More recently, it was shown that such a base is compatible with a variety of other systems. For instance, it was shown that allenes and dienes could be prepared, respectively, from aromatic and alicyclic aldehydes when reacted with (Me2N)3P=CH2 in the presence of 4 equiv of NaHMDS and titanium trichloride iso-propoxide (eqs 30 and 31). ... 

Optically active, 2,5-diphenyl-tetrahydrophosphole-based phosphonium salts were utilized in the enantioselective synthesis of allenic esters (Scheme 71). ... 

Vinylphosphonium salts are reactive as dienophiles as a result of the EWG character of the phosphonium substituent. The D-A adducts can be deprotonated to give ylides that undergo the Wittig reaction to introduce an exocyclic double bond. This sequence of reactions corresponds to a D-A reaction employing allene as the dienophile.71... 

Ketoesters.—A synthesis of a-branched /5-ketoesters is provided by electrolysis of the phosphonium chloride (76), which is formed on reaction of an acid chloride with an alkoxycarbonylalkylidenephosphorane if excess ylide is present, transylidation of the salt (76) occurs, leading to formation of the allenic ester (77), as shown in Scheme 24. 

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