Ketone Wittig reagent

Aldehyde or ketone Wittig reagent (an ylide) Alkene Tripheny I phosph ine oxide... 

The Wittig reaction uses phosphorus ylides (called Wittig reagents) to convert aldehydes and ketones to alkenes... 

Fluoroolefins may he prepared by the reaction of Wittig reagents and other pho sphorus-containtng y tides with fluorinated carbonyl compounds. (A discussion of the fluorinated Wittig reagents or other fluonnated phosphorus reagents with nonfluorinated carbonyl compounds is on page 581.) Tnphenylphosphoranes, derived from alkyltriphenyl phosphonium salts, react with 1,1,1-trifluoroacetone [3/] or other trifluoromethyl ketones [32, iJ] (equation 26) (Table 10). 

Allyltin compounds can be prepared by simple modifications of the usual reaction involving allyl Grignard reagents (139), by the 1,4-addition of trialkyltin hydrides to 1,3-dienes 140,141), or by the reaction of an aldehyde or ketone with the appropriate, tin-carrying, Wittig reagents (142). 

The Wittig reagent (56) is best protected as an ester and reacts chemo-selectively with the aldehyde rather than the less reactive ketone in (57),... 

Et3N, the adduct (126) yielded azulenone (127). The ketone (127) reacted with Wittig reagent to afford a [3.3 1] mixture of 1-epi-P-bulnesene (128a) and P-bulnesene (128b)A1 ... 

The Wittig reaction consists in the replacement of carbonyl oxygen of aldehydes and ketones by a methylene group with the aid of phosphine-methylenes resulting in the formation of cis or trans olefines. The reaction proceeds through the nucleophilic addition of Wittig reagent (phosphine methylene) across the > C = O bond and formation of an intermediate cyclic. 

The migration of a C=C bond to form a C=N bond was also observed with hydro-xylamine [78, 79], hydrazine [80, 81] and primary amines [82]. The /f iminylphos-phine oxide formed in the reaction may serve as a Wittig reagent in the presence of a base to react with a ketone or an aldehyde leading to ,/fun saturated alkenyl-imines 153 (Scheme 10.74). The phosphorus group can be a phosphonium salt as well as a phosphonate. 

Polymer Wittig reagents are useful for the conversion of an aldehyde or ketone to an alkene according to the sequence in Eq. 9-70 [Ford, 1986a,b],... 

Alkenes are obtained from selective hydrogenation of alkynes (see Section 5.3.1), and the reaction of a phosphorus ylide (Wittig reagent) with an aldehyde or a ketone (see Section 5.3.2). 

Macrocyclic ketones. This reagent has been used for cycloolefination of acetylenic dialdehydes (Wittig-Horner reaction) as a route to muscone, exaltone, and civetone. A synthesis of the last ketone (3) is formulated (equation 1). 

C H3MgBr, CIljLi, or related methyl nucleophiles. It also does not react with the Wittig reagent in DMSO at 70". Addition does take place if the ketone is added to an excess of reagent generated in a minimum of solvent (toluene) preheated to about 92". 

Can serve as alternative to Wittig reagent for the methylenation of hindered ketones. 

The hindered, stabilized Wittig reagent is unable to react with ketones, but reacts slowly with normal unhindered aldehydes at elevated temperature. However, it reacts at a reasonable rate with the highly reactive starting a-ketoaldehyde at 0°C. No reaction occurs on the... 

The major advantage of this reagent is its ability to react with carbonyl compounds that do not react with Wittig reagents. Thus it was used to convert the ketone group of 2 into the isomeric enol ethers (4), a reaction that failed with a number of other reagents.2... 

The Wittig reagent (C6H5)3P=CHCH2Si(CHJ)1 (9, 492) has also been used for this transformation, but this reagent does not react satisfactorily with most ketones. 

The corresponding Wittig reagent, CHj PPhj, reacts smoothly with both aldehydes and ketones to give methylenated products In high yield but with one subtle limitation. The problem cannot be detected with aldehydes because they react rapidly even at temperatures as low as -78°C, but ketones react more slowly, and an adjacent enolizable chiral center can be epimerized as a result of competitive reversible enolization. This limitation of the Wittig... 

Wittig reagents can represent enolates of unsymmetrical ketones. From Corey s work on arachidonic acid metabolites18 comes the coupling between the aldehyde 92 and the phosphonium salt 93. This is very impressive as both components have multiple functionality and there is no loss of stereochemical integrity even though the Wittig reaction is done in aqueous NaOH. 

The detailed mechanism of pyrolysis of Cp iR compounds has been studied (307—313). A useful titanocycle is formed from Cp2TiCl2 and trimethylaluminum triethylaluminum gives a different product (314). The titanocycle adds to terminal olefins in the presence of 4-dimethylaminopyridine the adduct expels olefin above 0°C to yield a bistitanocyclobutane (315). The titanocycle can also behave like a Wittig reagent, reacting with aldehydes and ketones to give olefins (314,316). 

Isomerization of the ketone precursor is brought about by a reversible reaction with the basic Wittig reagent, which yields an equilibrium mixture of two diastereomeric ketones. One of the ketone isomers then reacts preferentially with the Wittig reagent to give only the observed product. 

Ethyl perfiuoroalkanoatcs and pcrfluoroalkanoic acid anhydrides react with Wittig reagents to form either enol ethers 41 or perfluoro ketones 42. depending on the nature of the base employed to form the reactive /. -phosphane. 

The Tebbe reagent (4.85) gives better yield of alkenes with sterically hindered ketones than the Wittig reagent. ... 

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