Carbonyl compounds phosphonium ylides

Wittig reactions are versatile and useful for preparing alkenes, under mild conditions, where the position of the double bond is known unambiguously. The reaction involves the facile formation of a phosphonium salt from an alkyl halide and a phosphine. In the presence of base this loses HX to form an ylide (Scheme 1.15). This highly polar ylide reacts with a carbonyl compound to give an alkene and a stoichiometric amount of a phosphine oxide, usually triphenylphosphine oxide. 

NMR spectroscopic studies f111,13C, and 31P) are consistent with the dipolar ylide structure and suggest only a minor contribution from the ylene structure.234 Theoretical calculations support this view.235 The phosphonium ylides react with carbonyl compounds to give olefins and the phosphine oxide. 

Whereas phosphonium ylides normally react with carbonyl compounds to give alkenes, dimethylsulfonium methylide and dimethylsulfoxonium methylide yield epoxides. Instead of a four-center elimination, the adducts from the sulfur ylides undergo intramolecular displacement of the sulfur substituent by oxygen. In this reaction, the sulfur substituent serves both to promote anion formation and as the leaving group. 

Among the olefination reactions, those of phosphonium ylides, phosphonate anions, silylmethyl anions, and sulfone anions are discussed. This chapter also includes a section on conjugate addition of carbon nucleophiles to a, (J-unsaturated carbonyl compounds. The reactions in this chapter are among the most important and general of the carbon-carbon bond-forming reactions. 

Fluoroolefins may be prepared by the reaction of Wittig reagents and other pho sphorus-contaimng ylides with fluorinated carbonyl compounds. (A discussion of the fluorinated Wittig reagents or other fluorinated phosphorus reagents with nonfluonnated carbonyl compounds is on page 581.) Tnphenylphosphoranes, derived from alkyltriphenyl phosphonium salts, react with 1,1,1-trifluoroacetone [3/] or other trifluoromethyl ketones [32, 33] (equation 26) (Table 10). 

TABLE 11 Comparison of Yields from the Reaction of Carbonyl Compounds Carbomethoxymethylene Triphenylarsorane and with the Corresponding Phosphonium Ylides WITH... 

Phosphonium ylides react with carbonyl compounds to give olefins whereas sulfonium ylides aflFord epoxides. In their behavior toward car-... 

Ylides of other elements have been used much less commonly than sulfur ylides in cyclopropanations. Rather, other ylides are better known for their uses in other types of reactions, the best example being the use of phosphonium ylides in the Wittig reaction with carbonyl compounds to give alkenes. Nonetheless, some cases of cyclopropanations have been reported with phosphonium ylides and the related arsenic derivatives. Examples are given in Table 9. 

Ethylene oxide reacts with phosphonium halides to give ylides, which are used to synthesize olefins from carbonyl compounds, such as aldehydes... 

Condensation of Phosphonium Ylides with Carbonyl Compounds Wittig Reaction... 

Firstly, it was necessary to develop the reaction sequence of the Wittig reaction — synthesis of the phosphonium salt, formation of the ylide and reaction with a carbonyl compound to give the olefin — into an industrial process, under the stringent criteria of safety and cost-efficiency. 

The Wittig reaction is a C,C-forming olefin synthesis from phosphonium ylides and carbonyl compounds (see also Section 4.7.4). In more than 99% of all Wittig reactions, ylides of the structure Ph3P+—CH-—X (i.e., triphenylphosphonium ylides) are used. Therein, X usually stands for H, alkyl, aryl, or C02-alkyl and seldom for other substituents. 

Reactions of Phosphonium Ylides. - 2.3.1 Reactions with Carbonyl Compounds. This year we are able to report several variations of the traditional Wittig olefination which employ the addition of catalysts to effect the reaction. For example, Lebel et al. have reported a new salt-free process for the methyl-enation of aldehydes, in which the phosphorane is generated in situ from triphenylphosphine and a diazo precursor with either a rhodium- or rhenium-based catalyst (Scheme 6). It was found that the most effective combination of catalyst and diazo-compound were Wilkinson s catalyst [RhCl(PPh3)3] and... 

In the Wittig reaction an aldehyde or ketone is treated with a phosphorus ylid (also spelled ylide and called a phosphorane) to give an alkene. The conversion of a carbonyl compound to an alkene with a phosphorus ylid is called the Wittig reaction. Phosphorus ylids are usually prepared by treatment of a phosphonium salt with a base, and phosphonium salts are usually prepared from a triaryl phosphine and an alkyl halide (10-31) ... 

Reactions of arsonium ylides with carbonyl compounds take place much more readily than with phosphonium or sulphonium ylides. The nature of the products depends upon the character of the substituents on the ylide carbon atom, where electron-withdrawing substituents favour alkene formation, and of substituents on the arsenic atom, where... 

Ylides may react with nitrosobenzene in a similar fashion to their reactions with carbonyl compounds (equation 12). Sulphonium ylides give nitrones phosphonium... 

The Wittig reaction of phosphorus ylides derived from phosphonium salts with carbonyl compounds to form alkenes is widely used in synthetic chemistry. The alkylation of phosphorus ylides offers a route to a variety of phosphonium salts which can be further manipulated. Bestman and coworkers - have been responsible for a large number of reports based upon this and other aspects of phosphorus chemistry. Their work includes many examples of the intramolecular alkylation of ylides derived from phosphonium salts to form cyclic compounds. The salts can be prepared from dihalides either with triphenylphos-... 

The key difference between the literature procedure s and the acrylate-forming reaction proved to be the use of a phosphonium bromide and BuLi to form the ylide in the latter. This gave a combination of ingredients (LiBr, ylide and/or R3PO) and conditions (benzene solvent, reflux) which caused rapid rearrangement of epoxides to carbonyl compounds. Subsequent study provided the first unequivocal evidence s that LiBr as well as some other lithium salts can cause epoxide rearrangements. 

In an attempt to improve the environmental credentials of Wittig olefination, Russell and Warren have described the synthesis of a range of water/ aqueous base soluble phosphonium salts (67-71) and assessed their reactivity towards carbonyl compounds. The use of D2O as a solvent for the reaction between stable ylides and carbonyl compounds leads to a-deuterated a, 3-unsaturated carbonyl compounds. 

---