Phosphorous ylide

The Wittig reaction, for which George Wittig received the 1979 Nobel Prize in Chemistry, is an important synthetic procedure for converting aldehydes and ketones into alkenes. The active reagent is a phosphorous ylide which undergoes nucleophilic addition to the carbonyl carbon, e.g., for addition of triphenylphosphinemethylidene to acetone. 

Phosphorous ylides such as triphenylphosphine-metJhylidene may either be represented as hypervalent species incorporating a phosphorous-carbon double bond, or in terms of a zwitterion, that is, a molecule with separated positive and negative charges. 

With the co side chain at C-12 in place, we are now in a position to address the elaboration of the side chain appended to C-8 and the completion of the syntheses. Treatment of lactone 19 with di-isobutylaluminum hydride (Dibal-H) accomplishes partial reduction of the C-6 lactone carbonyl and provides lactol 4. Wittig condensation8 of 4 with nonstabilized phosphorous ylide 5 proceeds smoothly and stereoselectively to give intermediate 20, the bistetra-hydropyranyl ether of ( )-1, in a yield of -80% from 18. The convergent coupling of compounds 4 and 5 is attended by the completely selective formation of the desired cis C5-C6 olefin. 

The adjacent iodine and lactone groupings in 16 constitute the structural prerequisite, or retron, for the iodolactonization transform.15 It was anticipated that the action of iodine on unsaturated carboxylic acid 17 would induce iodolactonization16 to give iodo-lactone 16. The cis C20-C21 double bond in 17 provides a convenient opportunity for molecular simplification. In the synthetic direction, a Wittig reaction17 between the nonstabilized phosphorous ylide derived from 19 and aldehyde 18 could result in the formation of cis alkene 17. Enantiomerically pure (/ )-citronellic acid (20) and (+)-/ -hydroxyisobutyric acid (11) are readily available sources of chirality that could be converted in a straightforward manner into optically active building blocks 18 and 19, respectively. 

Intermediates 18 and 19 are comparable in complexity and complementary in reactivity. Treatment of a solution of phosphonium iodide 19 in DMSO at 25 °C with several equivalents of sodium hydride produces a deep red phosphorous ylide which couples smoothly with aldehyde 18 to give cis alkene 17 accompanied by 20 % of the undesired trans olefin (see Scheme 6a). This reaction is an example of the familiar Wittig reaction,17 a most powerful carbon-carbon bond forming process in organic synthesis. 

The acidity of hydrogens attached to 1,2,4-trioxolane ring carbons has been highlighted in previous sections, notably Section 4.16.5.3.1 where it was described how bases will induce fragmentation of the ring via proton removal. This also forms the basis of the first step in the reaction of 1,2,4-trioxolanes with phosphorous ylides (Section 4.16.5.3.3). 

Reversely, fluoro(trifluoromethyl)methylene (3) has been observed spectroscopically during adiabatic compression of tetrafluoroethene (4).142 Interestingly, a similar process occurs on treatment of perfluoroalkenes, e.g. 5, with tributylphosphane. Fluoride ion migration occurs to form the phosphorous ylide, e.g. 6. 

Bringing together the reaction mixture, resulting from the oxidation of an alcohol with TPAP, with a solution containing a non-stabilized phosphorous ylide allows to perform a Wittig reaction with no need to isolate an intermediate aldehyde. 

It is possible to carry out an in situ Wittig reaction with a stabilized phosphorous ylide and an aldehyde obtained by a BaMnC>4 oxidation of a primary benzylic or allylic alcohol.77... 

The stable adamantylsilene 150 was found to react with the phosphorous ylide Ph3P+CH2 to form the zwitterion 405 which cyclizes to the silacyclopropane 406, as shown by NMR spectroscopy and by derivatization with methanol or acetophenone (equation 125)202. 

To avoid the introduction of soluble phosphines into a reaction mixture, Westman20 performed a one-pot phosphorous ylide formation and Wittig... 

The Wittig reaction converts a ketone to an alkene. A phosphorous ylide (pronounced "ill -id") is used. An ylide is a neutral molecule with a negatively charged carbanion. 

The reaction of aldehydes or ketones with phosphorus ylides produces alkenes of unambiguous double-bond locations. Phosphorous ylides are prepared by reacting a phosphine with an alkyl halide, followed by treatment with a base. Ylides have positive and negative charges on adjacent atoms. 

Tandem cyclization occurs in the vacuum pyrolysis of 2-methoxyphenyl and 2-methoxythiophenyl substituted phosphorous ylides. Benzothieno[3,2-b]benzofiiran has been prepared for the first time <95SL53>. The well known Pd-catalyzed benzofiiran synthesis has found application for the preparation of tricyclic systems of type 79 (benzofiiran analogue of ILV) <95JA6666>. 

The intermediacy of such oxaspiropentanes has been proposed in the addition of diazomethane to ketonesi0) and in the reaction of dimethyloxosulfonium methylide with a-haloketones55). In contrast to phosphorous ylides, sulfur ylides usually condense with carbonyl compounds to yield epoxides, thus reaction of the N,N-dimethylaminophenyloxosulfonium cyclopropylide 99 with cyclohexanone produced the dispiroepoxide 100 which rearranged to the spiro [3.5] nonan-l-one 101 upon isolation by gas chromatography, Eq. (29) S6). 

Phosphorous ylides 44 react with carbon tetrahalides (CC14, CBr4) by the replacement of the hydrogen atoms on the a-carbon with halogen atoms to form halo ylides 45 and 46, according to Kolodyazhnyi and Golokhov122 (equation 81). They stated that halogena-... 

As already mentioned, the mechanism of oligomerization is the same as discussed for polymerization, and a catalytic cycle similar to the one shown in Fig. 6.3 operates. Many nickel-phosphine complexes have been successfully used as the precatalysts 7.17 is one such example. As shown by 7.5, reaction of a phosphorous ylide with a suitable nickel-containing precursor makes this... 

Nur bedingt damit vergleichbar sind dagegen die Darstellungsmetho-den fiir die substituierten Phosphor-ylide des Typs (31 a—c). Dies gilt ebenfalls fiir die Synthese von (31 c) 109) z.B. ... 

Die Organosiliciumchemie der Phosphor-ylide ist ein neuer Zweig der Ylid-Chemie, der in den letzten Jahren rasch entwickelt wurde 121 118-125), Das folgende Schema gibt eine Ubersicht iiber die beiden bisher erarbeiteten Synthesemoglichkeiten fiir die Silazanisosteren des Typs (38) ... 

The reaction of phosphorous ylides 385 with diphenylcarbodiimides affords ionic adducts 386 which dissociate into exchange products. ... 

The phosphaalkenes 14a and b react with tertiary phosphanes even at room temperature to provide the phosphorous ylide 15a-d. ... 

In Situ Creation of a Phosphorous Ylide and Construction of Disparate Bonds... 

Fragment VI is coupled to IV at two points. In the first of these two, the a carbon of the ester must be such that a C=C bond is built. This task could be accomplished by, for example, a phosphorous ylide or phosphorane that would react with the aldehyde portion of I in a manner reminiscent of the Wittig reaction. This implies the existence of a structure such as VII at some point of the reaction scheme. 

If compound II is electrophilic, the involvement of the 7 carbon of VI now becomes clear. Thus, the sodium hydride generated anion V may be imagined as attacking nucleophilically carbon C-2 of activated cyclopropane II. The resulting anion is precisely the proposed phosphorous ylide postulated as structure VII by our previous fragmentation analysis. What follows then is an intramolecular Wittig reaction with the departure of triphenylphosphine oxide that was predicted by the atom budget procedure (see Scheme 17.2). 

There is a report describing intramolecular, stereoselective cyclopropanations by utilizing the ylide reaction Preparation of tricyclic compounds, such as 24, has been accomplished by intramolecular reaction of phosphorous ylide generated in situ by addition of enolate anion to vinylphosphonium salt (equation 82) . Optically active... 

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